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THE SEW
AG
EPU
MPIN
GH
AN
DBO
OK
THE SEWAGE PUMPING HANDBOOK
Foreword
3
The use of submersible pumps in sewage anddrainage pumping applications has increasedgreatly in the last decades since they entered themarket. The introduction of heavy-duty submers-ible pumps with motor power ratings exceeding500 kW has also made them available for centralmunicipal pumping duties. The good servicerecord and high quality standard attained bythese pumps has all but excluded the use of con-ventional pumps in municipal service.
By the same token, the special characteristics ofsubmersible pumps have also required the devel-opment of new knowledge on their implementa-tion, such as the design of pumping stations. Thiswork has been advanced by both pump manu-facturers and municipal engineers. The intention of this book is to bring the newestinformation on both submersible pumps andpumping stations to the use of all concernedprofessional people in a concise form. The book isdivided into Sections according to the relatedtopics.
Basic pump theory is described in Section 1, pro-viding a reference background for the assessmentof pump performance. Submersible pump design
and construction is described in Section 2. Pumpperformance is dealt with in Section 3, offeringmethods for the calculation of pump performancein various installations. Factors affecting pumpselection are also discussed. Section 4 offers infor-mation on pump testing. Basic design of pumpingstations is discussed in Section 5, offering designinformation for both large and small applications.Continuous regulation of submersible pump oper-ation by frequency control is described in Section6. The concept of whole-life cost for pumps andpumping installations is presented in Section 7.Matters relating to pump commissioning are pre-sented in Section 8, whereas pump operation andservicing is described in Section 9. Section 10 dealswith pumping station control and monitoring.Appendix A offers information on the hydrauliccharacteristics of common pipe components forpipeline loss calculations. Appendix B presents amethod for the determination of sewage pump-ing station capacity and pump starting frequency.
One objective of the book has been to make thecontents easy to read and comprehend. The pre-sentation is therefore enhanced with a large num-ber of illustrations, providing examples of andcomplementary information on the matter athand.
Foreword
Table of Contents
5
Table of Contents
1 Pump Theory ............................................. 71.1 The Head Equation .........................................71.1.1 Flow with Losses or Addition of Energy ....71.1.2 Fluid Flowing from a Container .................. 81.2 The Basic Pump Equation ............................. 81.3 Pump Curve and Losses ...............................101.3.1 The Effect of Finite Number of Vanes ......101.3.2 Friction Losses Hf ..........................................101.3.3 Discontinuity Losses Hs ...............................101.3.4 Leakage Losses Hv .........................................101.3.5 Other Losses ....................................................111.4. Cavitation and NPSH .....................................111.4.1 Definition of NPSH ....................................... 121.4.2 Reference Plane ............................................. 121.4.3 Required NPSH ............................................... 121.4.4 Available NPSH ..............................................141.4.5 NPSH Safety Margin ..................................... 151.4.6 Damming up of Suction Wells................... 15
2 Pump Construction ................................. 162.1 General ............................................................162.2 Pump ................................................................182.2.1 Impellers ..........................................................182.3 Motors ............................................................. 272.3.1 General ........................................................... 272.3.2 Explosion-proof Motors .............................. 272.3.3 Motor Cooling ............................................... 272.3.4 Motor Tightness ...........................................292.3.5 Motor Bearings ..............................................312.3.6 Motor Protection Devices .......................... 322.4. Pump Connection ........................................ 342.5 Construction Materials,
Corrosion and Wear .....................................362.5.1 Corrosion Resistance ...................................362.5.2 Wear Resistance ........................................... 372.5.3 Abrasive Liquids ............................................ 37
3 Pump Performance .................................. 383.1 Pump Head .................................................... 383.1.1 Submersible Pumps ..................................... 383.1.2 Dry-installed Pumps ....................................393.2 Pump Performance Curves ........................393.2.1 H Curve ...........................................................393.2.2 Efficiency Curves ..........................................403.2.3 Power Curves .................................................403.2.4 NPSH Curve ....................................................403.3 Pipe Losses and Rising Main
Characteristic Curves ...................................413.3.1 Friction Losses ................................................41
3.3.2 Local Losses ....................................................433.3.3 Rising Main Characteristic Curve .............433.4 Rising Main Size ............................................443.4.1 Economy .........................................................443.4.2 Free Passage for Solids ................................453.4.3 Avoiding Settling of Solids and Sludge ...453.4.4. Water Hammer ..............................................453.4.5 Avoiding Water Hammer ........................... 473.5 Pump Duty Point ..........................................483.5.1 Single Pump Operation ...............................483.5.2 Parallel Operation, Identical Pumps ........483.5.3 Parallel Operation, Different Pumps ........483.5.4 Serial Operation ........................................... 493.5.5 True Duty Point ............................................ 493.6 Sludge Pumping ........................................... 493.7 Complex Rising Mains ................................ 493.7.1 What Goes on in a Complex
Rising Main? ................................................. 493.7.2 Determination of Head ................................513.7.3 Pipe Size and Flow Velocity .........................513.7.4 Choice of Pump ..............................................513.7.5 Confirming Measurements ........................513.8 Duty Point Evaluation for
Parallel Pumping Stations .......................... 52
4 Testing of Pumps .....................................544.1 Testing Arrangements ................................. 544.1.1 Production Testing ....................................... 544.1.2 Field Testing, Duty Point .............................564.2 Acceptance Tests .......................................... 574.2.1 Testing Standards ......................................... 57
5 Pumping Stations ....................................595.1 Pumping Station Basic Design ..................595.1.1 Wet Well Volume and Surface Area ........595.1.2 Pumping Station Inlet Pipe ....................... 605.1.3 Wet Well Floor Shape ................................. 605.1.4 Stop Levels ...................................................... 615.1.5 Start Levels .....................................................625.1.6 Suction Pipe Dimension and Design .......625.1.7 Pumping Station Internal Pipework ........635.1.8 Flushing Devices ...........................................635.1.9 Odour Problems in Pumping Stations .... 645.1.10 Pumping Station Design Examples ......... 645.1.11 Dry-installed Pump Positions ....................675.2 Package Pumping Stations ........................ 685.2.1 Out-of-doors Pumping Stations .............. 685.2.2 Indoor Pumping Stations ...........................705.3 Pumping Stations with
Column-installed Pumps ............................705.4 Pumping Station Dimension Selection ... 725.4.1 Regular Sewage Pumping Stations .......... 72
Table of Contents
6
5.4.2 Stormwater Pumping Stations ..................725.4.3 Combined Sewage Pumping Stations
and Retention Basins ...................................735.5 Pump Selection ..............................................745.5.1 Pump Selection Based on Pump Curves ..745.5.2 Observing Pump Efficiency .........................745.5.3 Number of Pumps .........................................755.6 Special Considerations ................................765.6.1 Pump Vibrations ............................................765.6.2 Pump Noise ....................................................77
6 Frequency-controlled Sewage Pumps ........... 786.1 General ........................................................... 786.1.1 Pump Motor Selection ................................ 786.1.2 Maximum Frequency .................................. 786.1.3 Minimum Frequency and Minimum
Performance.................................................. 796.1.4 Pump Frequency Curves ............................. 796.1.5 Pump Clogging .............................................806.1.6 EMC Cable Requirement ............................806.1.7 Bearing Currents ..........................................806.1.8 High Tension ..................................................816.1.9 Explosion-proof Motors ..............................816.1.10 Guaranteed Values .......................................816.1.11 Tests with Frequency Controller
(String Tests) ...................................................816.1.12 Collaboration with the Pump
Manufacturer .................................................81
7 Pump Whole-life Cost Evaluation .......... 827.1 General ........................................................... 827.2 Calculation Period ........................................ 827.3 Investment Costs ......................................... 827.4 Energy Costs .................................................. 837.4.1 Efficiency Over Time .................................... 837.4.2 Energy Usage Calculations ........................ 847.5 Maintenance Costs ...................................... 847.6 Cooperation With Pump Suppliers .......... 857.7 Life Cycle Cost Publication ......................... 85
8 Commissioning ....................................... 86
9 Operation and Service ............................ 879.1 Safety .............................................................. 87
10 Pumping Station Control and Condition Monitoring ............................. 88
10.1 Local Control Methods ............................... 8810.1.1 Manual Control Units ................................. 8810.1.2 Relay-based Control Units ......................... 8810.1.3 Programmable Logic Controllers .............. 8810.2 Sensors for Pump Control and
Condition Monitoring .................................89
10.2.1 Wet Well Water Level Sensors ..................8910.2.2 Current Sensor ............................................. 9010.2.3 kWh Meter .................................................... 9010.2.4 Phase Failure Relay ...................................... 9010.2.5 SARI 2 Monitoring Device .......................... 9010.2.6 ASM 3 Alarm Status Module ......................9110.3 Pump Control Units ......................................9110.3.1. Control Features ............................................9110.3.2 Condition Monitoring Features ................9210.3.3 Parameters and Signals ..............................9210.3.4 Data Logging and Analysis .........................9310.3.5 User Interface ................................................9310.4 Remote Control and Monitoring System 9310.4.1 Different Levels for Remote Control ........9310.4.2 Software and Hardware .............................9410.4.3 Data Transmission ....................................... 9510.4.4 Alarm Transfer ............................................... 9510.4.5 System Integration ......................................9610.5 Internet & WAP Based Remote Control
and Monitoring .............................................96
Symbols .............................................................98
APPENDIX A ...................................................... 100
APPENDIX B....................................................... 108
Pump Theory 1
7
1 Pump Theory
This section is a primer of fluid pumping theoryand provides the reader with the theoretical back-ground knowledge essential for deeper under-standing of the pumping process.
1.1 The Head Equation
Figure 1 shows part of continuous fluid flow in aduct. Between the two observation sections 1 and2 no energy is transferred to or from the fluid andthe flow is assumed to be frictionless. Thus thetotal energy of the fluid relative to a horizontalreference plane T at the two sections must beequal. The total energy comprises components forpotential energy, pressure energy and kineticenergy, and for a fluid particle with a mass m theenergy at the observation sections is as follows:
where ρ is the fluid density and g the accelerationof gravity.
For a flow without losses the total energy in sec-tion 1 and 2 will be equal, thus
.
Dividing both sides of the equation with the termmg it is obtained
(1)
This equation is called Bernoulli's equation afterthe engineer who first derived it. The terms of theequation are expressed as heads, and the terms
are consequently called static head, pressure headand kinetic head, respectively. The equation is essential for fluid mechanics andcan be used to account for many hydrodynamicphenomena, such as the decrease in pressure thataccompanies a reduction in a flow cross sectionarea. In this case the fluid velocity increases, andfor the total head to remain constant and assum-ing the potential head remains unchanged, thepressure term or static head, must decrease.
1.1.1 Flow with Losses or Addition ofEnergyIf there are losses in the flow between section 1and section 2 in Figure 1, the head equation 1 canbe written
(2)
where Hr is the head loss.
If energy is added to the flow by placing a pumpbetween section 1 and section 2 in Figure 1, theequation 2 can be written
(3)
where H is the pump total head.
Section 1 2
PotentialEnergy
PressureEnergy
KineticEnergy
mgh1 mgh2
mgp1
ρg------ mg
p2
ρg------
12---mv1
2 12---mv2
2
mgh1 mgp1
ρg------ 1
2---mv1
2+ + mgh2 mg
p2
ρg------ 1
2---mv2
2+ +=
h1
p1
ρg------
v12
2g------+ + h2
p2
ρg------
v22
2g------+ +=
v2
p2
p1
v1
h2
h1
2
Q
T
1
Fig. 1
Section showing flow of liquid through two obser-vation cross sections. T is a reference plane for the po-tential heads h1 and h2, p1 and p2 are the prevailingpressures and v1 and v2 the fluid velocities at sections1 and 2.
h1
p1
ρg------
v12
2g------+ + h2
p2
ρg------
v22
2g------ Hr+ + +=
h1
p1
ρg------
v12
2g------ H+ + + h2
p2
ρg------
v22
2g------ Hr+ + +=
1 Pump Theory
8
1.1.2 Fluid Flowing from a ContainerAn example of the application of the Bernoulliequation is the calculation of the flow rate of afluid flowing freely from an open container.
Figure 2 shows an open container with an outletorifice near the bottom. For practical purposes thearea A1 is assumed much larger than the orificearea A2, and the atmospheric pressure p1 in thecontainer is equal to that outside the orifice, p2.
Choosing the centre line of the orifice as the refe-rence plane T, the term h2 is equal to zero and h1equal to h. Because A1 is much larger than A2, the
kinetic head can be assumed as zero. Thus
the head equation 1 can be written
(4)
whence
(5)
For volume flow without losses is obtained
(6)
To accommodate for losses present, a flow coeffi-cient µ is added to equation 6, whence
(7)
The flow coefficient µ is dependent on the shapeof the orifice, and can be obtained from textbooks on the subject. If the fluid level in the con-tainer is allowed to recede, the level height h willchange, which will have to be accommodated forin calculations.
1.2 The Basic Pump Equation
The basic pump equation is used to calculate anddesign geometrical shapes and dimensions ofcentrifugal pumps. The basic pump equation isalso used to deduce the pump Q/H curve.
A pump impeller vane and its associated velocityvectors are shown in Figure 3.
v = absolute fluid velocityw = velocity relative to the vaneu = perimeter velocityvu = tangential component of absolute velocityvm = radial component of absolute velocity
The relative velocity is parallel to the vane at anygiven point.
Also and
Assuming the flow to be without losses and thenumber of vanes infinite (∞), the familiar basicequation of pump theory can be derived using thelaws of mechanics. This relationship is known asthe Euler equation and is expressed as:
. (8)
where the index t refers to a flow without lossesand ∞ refers to the assumption of infinite numberof vanes ensuring complete fluid direction.
In an actual pump neither of these assumptionscan be satisfied, as friction losses are alwayspresent, and the finite number of vanes will notdirect the flow entirely in the direction of thevane.
p2 = p2
p1
v1
h
T
v2
A1
A2
Fig. 2
Section of a fluid container with an outlet orifice nearthe bottom. A1 and A2 are the cross section areas of thesurface and the outlet orifice, h the height differencebetween surface and orifice centre line, v1 surface re-cession velocity and v2 liquid outlet velocity throughthe orifice. Ambient pressure is constant.
v12
2g------
hv2
2
2g------=
v2 2gh=
q1 A2 2gh=
q1 µA2 2gh=
vu1 v1 α1cos⋅= vu2 v2 α2cos⋅=
Ht∞1g--- u2vu2 u1vu1–( )=
Pump Theory 1
9
The reduction in head caused by losses in the flowis taken into account by the hydraulic efficiency ηh,and the reduction due to the deviation of the flowfrom ideal angle β2 is accounted for by a vanecoefficient k. With these modifications, the Eulerequation for an actual pump reads as follows:
(9)
It can be shown that both ηh and k are less thanunity. They will not be discussed in further detailhere.
Centrifugal pumps are normally designed with α1 = 90 °, whence vu1 = 0.
Thus the basic pump equation is simplified to
(10)
Hηh
g------ ku2vu2 u1vu1–( )=
H kηh
u2vu2
g---------------=
u2
v2
vm2
2
2
vu2
1
w1
12d
dvm 1
11
v1
u1
vu
Fig. 3
Pump impeller vane with the velocity triangles at leading and trailing edges. Fluid absolute velocity v, relative velocityw, vane perimeter velocity u, liquid absolute velocity tangential component vu and radial component vm.
1 Pump Theory
10
1.3 Pump Curve and Losses
The ideal head obtained from the Euler equationis independent of the volume rate of flow Q. If theQ/Ht∞
curve is plotted, Ht∞ is indicated by astraight line. The real Q/H curve is derived fromthis by subtracting the effects of the finite num-ber of vanes and various other losses that occurwithin the pump. Please refer to Figure 4.
1.3.1 The Effect of Finite Number of VanesAs noted earlier, a finite number of vanesdecreases the head by the vane factor k. Takingthis into account, the theoretical head Ht isobtained. It can be written:
(11)
Ht is not perfectly linear because the vanecoefficient is slightly dependent on the volumerate of flow Q. The head reduction from Ht∞ to Ht
is not caused by flow losses, but by deviation ofthe fluid from the ideal flow angles due to thefinite number of vanes.
1.3.2 Friction Losses Hf
Friction losses occur as the fluid flows through thepassages of the impeller and the pump casing.They increase approximately with the square ofthe flow rate Q.
1.3.3 Discontinuity Losses Hs
Discontinuity losses are generated in the follow-ing areas:• At the vane leading edge where the fluid hits
the vane tip. The loss is smallest at the pumpdesign point, where the fluid contacts the vaneat the vane angle β1. The losses increase withincreasing deviation of the contact angle fromthe vane angle β1, see Figure 5.
• At the vane trailing edge losses occur due toeddies shed by the vane. These increase ap-proximately with the square of the flow rate.
• In the pump casing at flow rates other thanthe design value, when the flow velocity at thecasing differs from that at the impeller perime-ter. The effect is shown in Figure 6. The velocitydifferences create turbulence leading to losses,growing with increasing difference of actualflow from design flow.
The effects of the discontinuity losses are shownin Figure 4.
1.3.4 Leakage Losses Hv
Leakage losses occur at the clearance betweenimpeller and pump casing. Even if the clearance iskept as small as possible, a small backflow passesfrom the high pressure area at the impeller rim tothe low pressure area of the impeller eye. Thusthe flow through the impeller is slightly largerthan the flow out of the pump casing, and thepump head is met with a reduced flow, the differ-ence being the leakage loss Hv. The effect of theleakage loss is shown in Figure 4. As the pumpwears out, this loss will increase.
Ht kHt∞=
Reduction of flow, Q
Effect of finite number of vanes Ht
Friction losses Hr
Discontinuity losses HsHN
H
QN Q
caused by leakage losses, Hv
Fig. 4
True pump Q/H curve (H) reduction from theoreticalpump head Ht∞ .
losses
Q > QN
Q = QN
Q < QN
w1'
v1'
v1
w1
v1"
w1"
u1
1
Fig. 5
Vane leading edge relative velocities (w) and lossesat various flow rates. Minimum losses occur at thepump nominal flow when the fluid angle of attackis equal to the vane leading edge angle β1.
Pump Theory 1
11
1.3.5 Other LossesThere are further losses in a centrifugal pump, notaffecting the Q/H curve, but that will increase themotor shaft power requirement. These include:• friction losses at the impeller outside surfaces• shaft seal friction losses• bearing friction losses
For submersible pumps, the last two items areincluded in the motor losses.
1.4. Cavitation and NPSH
Cavitation is caused by the formation and collapseof vapour bubbles in a liquid. Vapour bubblesform when the local static pressure in a flowingliquid decreases to or below the liquid vapourpressure at ambient temperature. When the bub-ble, or void, moves with the flow to an area with ahigher pressure, it will rapidly collapse. The implo-sion causes a transitory, extremely high localshock wave in the fluid. If the implosion takesplace near a surface, the pressure shock will, ifoccurring repeatedly, eventually erode the surfacematerial.
The cavitation phenomenon will typically occur incentrifugal pumps at a location close to the impel-ler vane leading edge, see Figure 7. Cavitation mayalso lower the pump Q/H curve and efficiency. Acavitating pump emits a typical rattling noise, likesand being pumped through the pump. No pumpmaterial will completely withstand cavitation, socare should be exercised if the pump operatingconditions present a risk of cavitation.
Wear marks from cavitation typically occur locallyand consist of deep pittings with sharp edges. Thepittings can be several millimetres deep, pleaserefer to Figure 8.
Normally pump curves published for submersiblepumps are drawn so that a pump in normal sub-merged installation will not cavitate as long asthe duty point is on the allowed section of the Q/H curve.
Velocity in casing
Absolute velocity after impeller (vu)
Resulting losses
Q<QN Q=QN Q>QN
Fig. 6
Effect of the difference of velocities in the pump cas-ing and at the impeller perimeter. Pump casing di-mensions are designed to accommodate nominalflow at the perimeter speed, leading to losses at otherflow rates.
Vapour bubbles
Imploding�vapour bubbles
( Q > QN )
Fig. 7
Pumped fluid hitting the vane leading edge at anangle different than the vane angle. Eddies and lowpressure zones will form on the other side of thevane. If the pressure falls below the vapour pressure,vapour bubbles will form. Moving entrained in theflow to an area with higher pressure, they will even-tually implode. The consequent high pressure im-pact may lead to pitting and erosion of the adjacentstructure.
Fig. 8
Typical cavitation pitting in impeller.
1 Pump Theory
12
If the submersible pump is installed dry with asuction pipe, the installation must be checked forcavitation. In these cases the concept of NPSH isused.
1.4.1 Definition of NPSHNPSH is the acronym for Net Positive SuctionHead. The following pressure heads are used forthe calculation of NPSH:
ht = inlet geodetic headhA = height difference between reference
plane and tip of vane leading edge.Hrt = flow losses in inlet pipe
= pressure drop caused by inlet velocity
∆h = local pressure drop at vane leading edgepb = ambient pressure at liquid levelpmin = minimum static pressure in pumppv = liquid vapour pressure at prevailing
temperature
The pressure heads are shown in Figure 9.
In order to avoid cavitation, the minimum staticpressure in the pump (pmin) must be larger thanthe liquid vapour pressure, or
Figure 10 shows the principle of static liquid pres-sure distribution in inlet pipe, pump and pressurepipe of a dry pump installation.
1.4.2 Reference PlaneThe reference plane is the plane on which theNPSH calculations are performed. It is the hori-zontal plane through the centre point of the circledescribed by the tip of the vane leading edge. Forhorizontal pumps the reference plane coincideswith the shaft centre line. For vertical pumps thelocation of the reference plane is stated by thepump manufacturer.
1.4.3 Required NPSHThe required NPSH is obtained from the followingequation:
(12)
This is also called the pump NPSH value. It can bepresented as a function of flow as shown in Figure11. It is independent of temperature and type ofliquid being pumped. The pump manufacturer isrequired to state NPSH as a numeric value orcurve.
Any pump will actually have different NPSH-val-ues, depending of definition of occurrence, as can
hA
HORIZONTAL PUMP VERTICAL PUMP
Reference Plane
Minimum PressureNPSH
pming
Hrt
h
v02
2g
ht pb
pbg
required
Fig. 9
Dimensions and reference pressures for NPSH calculations.
v02
2g------
pmin pv>
NPSHrequired hA
v02
2g------ ∆h+ +=
Pump Theory 1
13
be seen in Figure 12. According to the testing stan-dards used by pump manufacturers, the NPSHr isdefined as the situation where pump head isdecreased by 3% due to cavitation. This value isdefined as NPSH3.
Light cavitation can be harmless to the pump ifthe vapour bubbles do not implode near thepump structural parts, such as the impeller vane.
The difference between the various NPSH valuesis greater in pumps with impellers with fewvanes. Thus single-vane impellers have the great-est differences in NPSH values with the differencebeing caused by the NPSH3 curve dropping, andthe tests thus giving too favourable readings.Therefore an NPSHr curve based on the 3% rule ofthe standard is a poor base for a cavitation riskassessment in pumps with few vanes. The NPSHr
Liquid static pressure
Vapour pressure
Absolute 0 pressure
Lowest pressure in pump
pbg
pming pv
g
pb ht
ht
Fig. 10
Pressure variation in a dry pump installation. Distribution of static liquid pressure in inlet pipe, pump and pressurepipe.
NPSH required
(m)
Q N Q
Fig. 11
Typical variation of required NPSH with pumpflow rate.
1 Pump Theory
14
curve published by a pump manufacturer shouldin principle guarantee that no damages will occurin the pump if the pump is operated above it. Thisis especially the case for wastewater pumps,which have a low number of impeller vanes. Theproblem is that there is no accurate way of testingand establishing such an NPSH value.
1.4.4 Available NPSHThe available NPSH indicates the pressure availa-ble for the pump suction under the prevailingconditions. This may be called the pumping sta-tion NPSH.
(13)
The term ht is positive when the reference plane isabove the liquid surface and negative if below it.Available NPSH is determined by the pumpingstation designer.
Figure 13 shows vapour pressure for water as afunction of water temperature.
Figure 14 shows atmospheric pressure as functionof elevation above sea level.
NPSHavailable
pb
ρg------ Hrt– ht–
pv
ρg------–=
Fig. 12
Different NPSH curves.
NPSH NPSHF (Cavitation free)
NPSHonset of noise
NPSHonset of material loss
NPSH0 (0% Head drop)
NPSH3 (3% Head drop)
Q
Temp ( C) Head (m)
pvg
10010
10
20
30
40
50
60
70
80
90
9
8
7
6
5
4
3
2
1
0,5
Fig. 13
Vapour pressure for water as a function of temper-ature.
Barometric
Altitude km
m H2Opressure
Fig. 14
Atmospheric pressure as function of elevation abovesea level.
Pump Theory 1
15
1.4.5 NPSH Safety Margin
NPSHavailable ≥ NPSHrequired + Safety margin
The NPSH margin must be sufficient to allow forvariations in a situation where the real conditionsmay differ from those theoretically calculated.The suction pipe flow losses may be inaccuratelyestimated and actual pump operation point maydiffer from the theoretical because of variations inthe Q/H curve and inexact pressure pipe resis-tance calculations. Harmful cavitation may occurearlier than expected, or at greater NPSH valuesthan NPSH3 (Figure 12). Manufacturing technicalvariations of the shape of the vane leading edgemay affect cavitation behaviour. The requiredNPSH may also be affected by the inlet pipeshape.
For horizontally installed pumps with straightsuction pipes, a safety margin of 1 to 1,5 m is suit-able.
For vertically installed pumps the safety marginshould be set at 2 to 2,5 m, provided that a reduc-ing bend is used before the pump inlet. Bend cen-treline curvature radius should be at least D1 + 100mm, where D1 is the diameter of the larger open-ing.
The matter of NPSH, safety margins and measur-ing methods for NPSH are discussed in details inthe EUROPUMP publication “NPSH FOR ROTODY-NAMIC PUMPS,REFERENCE GUIDE ”(1997).
1.4.6 Damming up of Suction WellsIn practical installations situations may arise,where the liquid level on the suction side risesand the pump head decreases so that the pumpduty point moves to a sector where NPSHr > 10 m.No cavitation will occur, however, since NPSHavail-
able will also rise and still be larger than NPSHre-
quied. Typical installations where this situation willarise are dry dock drainage pumping, sewer block-age situations and drainage pumping with vary-ing suction liquid levels.
2 Pump Construction
16
2 Pump Construction
This section describes the construction of modernelectric submersible pumps. Various designs andthe main parts of the pumps are discussed as wellas topics concerning pump operation and mainte-nance. The study is limited to pumps for munici-pal sewage, drainage and raw water.
2.1 General
The submersible pump is a unit combining apump and an electrical motor to an enclosed unit,suitable for submersible installation in a wet wellholding the liquid to be pumped. The submersiblepump may be connected to the pressure pipingwith a special baseplate connection at the bottomof the wet well for ease of installation andremoval, or it can be installed connected with aflexible hose or other arrangements with riserpipes. Power to the unit is fed through one ormore flexible cables, supplied with the pump inlengths suitable for the installation.
Many submersible pumps can also be installeddry like conventional pumps. This type of installa-tion ensures uninterrupted operation of theinstallation in case of flooding of the dry well.
Submersible pumps are available for a number ofapplications with different requirements, and dif-ferent designs for various special applicationshave been devised.
A submersible pump comprises a waterproofmotor and matching pump components. Thepump components include the impeller, the pumpcasing and the required connection parts for dif-ferent installation alternatives. These include aguide shoe for submersible installation onto amatching connecting baseplate, a stand for porta-
Fig. 15
Section of a GRUNDFOS 2,4 kW submersible pumpshowing details of motor and pump. The pump is fit-ted with a guide shoe for use with a submerged base-plate in the wet well, facilitating easy pumpinstallation and removal.
Section of a GRUNDFOS 17 kW submersible pumpshowing details of motor and pump. The pump is fit-ted with a guide shoe for use with a submerged base-plate in the wet well, facilitating easy pumpinstallation and removal. The pump casing is adjust-able with set screws for maintenance of impeller suc-tion clearance.
Fig. 16
Pump Construction 2
17
ble pumps and the necessary connection flanges,stand for dry-installed pumps and seat ring forcolumn-installed pumps.
The motor is a dry squirrel-cage electric motormatching a range of pump parts for variousduties. Motor and pump have a common shaftwith the bearings and shaft seals housed in themotor. The motor also includes watertight cableinlets and a handle for lifting the unit.
Figure 15 shows a modern small submersible sew-age pump and Figure 16 a medium-sized submers-ible sewage pump. Submersible sewage pumpsare available with motors rated from under 1 up to500 kW for duties ranging from light portable useto large city sewerage system main pumpinginstallations. A submersible pump for dry installa-tion is shown in Figure 17.
a b c
S
SS
closed semi-open open
Fig. 18
Different impeller designs. The closed impeller has integral shrouds on both sides of the vanes, whereas the semi-open impeller incorporates only one shroud on the back side. An open impeller consists of only a hub and vanes, re-lying on close clearances (s) to the pump casing for its function.
Fig. 17
Section and outline of a 160 kW submersible pump.The pump is intended for horizontal dry installationand connects with integral flange joints to both suc-tion and pressure pipework. The submersible designpermits flooding of the installation without risk ofdamage to the pump.
2 Pump Construction
18
2.2 Pump
The pump comprises the impeller and the pumpcasing as well as ancillary devices and fittings.
2.2.1 ImpellersSubmersible pumps are fitted with differentimpeller designs depending on intended use. Thevarious impellers can be classified as• impellers for sewage pumps• impellers for macerating pumps• propellers for axial pumps
Impellers can also be classified according to con-struction as closed, semi-open or open impellers.These are shown in Figure 18. Semi-open impellersand open impellers rely on the close clearancebetween impeller and pump casing (about 0,5mm) for their function. The efficiency of theseimpellers is very sensitive to wear and willdecrease rapidly as the clearance increases. Figure19 shows the effect on pump efficiency from wearon closed and open impellers. The open and semi-open impellers are also susceptible to impuritiesbecoming jammed between impeller and suctionplate, slowing down or completely stopping thepump.
Impellers for Sewage PumpsIn order to avoid pump blockage, or clogging, spe-cial impellers have been developed for sewagepumping. These include single-channel impellers,double-channel impellers and vortex impellers.The design principles of these are shown in Figure20. For very large sewage pumps, impellers with amultitude of vanes may also be used.
Free Passage The concept of free passage is of special relevanceto sewage pumps. It refers to the ability of thepump to let solids in the pumped liquid pass
Closed Impeller
Open Impeller
Operating time
Fig. 19
Test results from a comparison of the effect of wearon pump efficiency for different impeller types.
Winglets
Counterweights
Vortex impeller 1-channel impeller� 2-channel impeller�
Fig. 20
Impeller types for sewage pumps.
Pump Construction 2
19
through, and thus to the capacity of being non-clogging. The dimension of the free passage usu-ally refers to the largest spherical object that maypass through the impeller and the casing open-ings. If the free passage is described with twonumbers, it refers to the largest oblong objectthat can pass crosswise through the pump.
The ability of a pump to operate without cloggingrelates strongly to the free passage, as can beseen in the diagram in Figure 21. Normally a freepassage of 80 mm will be sufficient forunscreened sewage in small and medium-sizedpumps. In larger pumps (flow > 100 l/s) the mini-mum free passage should be at least 100 mm.
Free passage alone does not ensure good proper-ties against clogging in a pump. Impeller and vanegeometry must also have features that preventblockages. Pumps from different manufacturershave varying qualities in this respect. There arecases where a pump clogging problem has beensolved by changing pump make, even with thepumps having equal free passages, same numberof vanes and pump speed. The tendency of sew-age to choke a pump may vary from one locationto another, with “easy” and “difficult” pumpingstations. The design of the sewer line leading upto the pumping station is important for the func-tion of the pumps, because they must be able tohandle any agglomeration of solids originatingthere. Real conditions in sewage systems cannotbe simulated in laboratories, and the good proper-ties of the Grundfos sewage pumps against clog-ging are based on long-term practical experience.
Single-channel ImpellersA single-channel impeller is shown in Figure 22.The single vane is designed as long as possible forbest efficiency within the limits set by the require-ment of free passage. The impeller having onlyone route of passage for the pumped liquidensures good inherent characteristics againstclogging. The asymmetric shape requires theimpeller to include integral counterweights forbalance. Highest attainable efficiency is 70…75%.
Clogging probability
Free passage (sphere) [mm]
40 60 80 100 120
Fig. 21
A diagram showing the relation between probabilityof clogging and pump free passage. Good safetyagainst clogging is achieved with 80 mm free pas-sage.
Fig. 22
A GRUNDFOS S-1 single-channel impeller for sew-age use. The impeller is semi-axial in design withone long continuous vane, ensuring good proper-ties against clogging. The asymmetric design re-quires the casting to include counterweightmasses to facilitate static and dynamic balancingof the impeller
2 Pump Construction
20
Double-channel ImpellersA double-channel impeller is shown in Figure 23.The inherent difficulty with double-channelimpellers is that long, fibrous impurities mayenter both channels and get caught by the vaneleading edges, causing the pump to clog. This sit-uation can be alleviated by good vane leadingedge design, and this can be found only by devel-opment work under real conditions in difficultpumping stations. With the right design and afree passage of at least 100 mm, double-channelimpellers can be designed to handle unscreenedsewage without clogging. Highest attainable effi-ciency is 80…85% for double-channel impellers.
Three- and Four-channel ImpellersIn very large pumps impellers with three or fourvanes can be used and still have a free passage ofat least 100 mm and an impeller with good prop-erties against clogging. Also for these impellersthe design of the vane leading edge is decisive.Highest attainable efficiency is 82…86% for theseimpellers.
Vortex ImpellersThe principle of the vortex impeller is to induce astrong vortex in the open pump casing. Thepumping action of the vortex pump is thereforeindirect, with the impeller being situated outsidethe main liquid flow. Vortex impeller pumps haveinherently excellent properties against clogging,and the pumps run very smoothly. The use ofsmall vortex impeller pumps for sewage isincreasing largely due to improved design andefficiency in later years. They are also used as sandseparation pumps in sewage treatment plants. Avortex impeller is shown in Figure 24. Highestattainable efficiency is around 50% for vorteximpellers. It is essential to note that the efficiencyin the flow range 3…15 l/s of vortex pumps isroughly equal to that of single-channel pumps.
Flow and Head (Q/H) Ranges for Differ-ent Impeller and Submersible PumpTypesFigure 25 shows the typical application areas fordifferent sewage pump and impeller types of theGRUNDFOS range. It can be seen that withincreasing flow and pump size the number ofvanes of the impeller also increases. The diagramalso shows the Q/H area for which submersiblepumps are available for sewerage use. The largestpump in the Grundfos range has a motor of 520kW power.
Fig. 23
A GRUNDFOS S-2 double-channel impeller. Goodproperties against clogging are achieved with re-cessed vane leading edges and a semi-axial design.The symmetric design is inherently in balance.
Fig. 24
A GRUNDFOS SuperVortex impeller. The design in-cludes patented vane winglets. The winglets pre-vent the formation of secondary eddies over thevane edges, greatly improving pump efficiency.
Pump Construction 2
21
Fig. 25
Vortex 1-channel 3-channel 4-channel2-channel
Flow and head (Q/H) ranges for different impeller types.
Fig. 26
GRUNDFOS Grinder pump. The macerating unit is made of hardened stainless steel.
2 Pump Construction
22
Impellers for Macerating PumpsFor installations with very small amounts of sew-age, macerating pumps have been developed.Typical applications are pumping stations for sin-gle homes, small developments or camping areas.The required flow is very small, sometimes lessthan 1 l/s, but the total head may be high becauseof long and narrow rising mains. The flow for amacerating pumps are typically 1…5 l/s, withheads as high as 50 m.
In macerating pumps the solids are shredded intosmall fragments of around 10 mm, which makes itpossible to use rising mains of small dimensions,usually DN 40…DN 80. For the very small flowsfrom single pumping stations even smaller pipingwill be used, in order to attain a flow velocity of atleast 0,5 m/s.
Macerating pumps may not be used for sewagewith sand content, since the shredding unit is verysusceptible to wear. Where macerating pumps areconsidered for larger installations comprising sev-eral buildings, it is always advisable make a tech-nical and economic comparison with a solutionbased on conventional pumps.
Figure 26 shows a GRUNDFOS macerating pump.Outside of the impeller is a macerating unit withsharp cutting elements installed. The maceratingunit is made of hardened stainless steel.
Propellers for Axial PumpsAxial pumps, using the submersible motors fromsewage pumps, have been developed by manymanufacturers. Figure 27 shows a GRUNDFOSaxial-flow pump with adjustable-pitch propeller.The design incorporates trailing fixed vanes thattransform the rotating movement of the waterinto pressure energy, increasing pump efficiency.Propeller pumps are normally column-installed.
Propeller pumps are used for stormwater andflood water pumping, drainage, irrigation and rawwater pumping as well as for effluent pumping insewage treatment plants. Propeller pumps are notsuitable for unscreened sewage because of risk ofclogging. Small and medium-sized propellerpumps are not suitable for sewage treatmentplant internal circulation pumping of e.g. returnsludge, since they may clog and get jammed bythe fibres present in these fluids. Highest attain-able efficiency for propeller pumps is 75…85%.
The operating range (Q/H area) of the GRUNDFOSpropeller pumps is shown in Figure 28. Part of therange is also covered by channel-impeller pumpsfor column installation, which may be a more suit-able choice for many applications. The final choicebetween these should be based on desired dutypoint and application. The pump manufacturershould be consulted in the selection process indifficult projects.
Fig. 27
GRUNDFOS propeller pump. Blade angle isadjustable for best efficiency.
Pump Construction 2
23
For recirculation pumping in sewage treatmentplants special axial pumps have been developedas shown in Figure 29. They are intended to oper-ate at very low heads, only 0,3… 1,0 m, and highflow rates, up to 2000 l/s. These pumps aredesigned non-clogging with back-swept blades,large (10 mm) clearance between blade tips andcasing, and by omitting the lead vanes. Highestattainable efficiency for circulation axial pumps is35…50%. The delivery loss at the exit of the thim-ble is significant for the head. Additional head canbe attained by using a conical thimble, loweringlosses.
10
0 100 200 500 1000 2000 4000 5000
8
6
4
2
0
Fig. 28
GRUNDFOS propeller pump flow and head (Q/H) range.
Fig. 29
Submersible recirculation pump for sewage treat-ment plant. The pump is lowered in place alongguide rails.
2 Pump Construction
24
Impeller Auxiliary VanesAuxiliary vanes on the outside of the shrouds arean important feature of the impellers of smallsewage pumps. The auxiliary vanes increase thevelocity of the flow of fluid in the space betweenthe impeller and the pump casing. Figure 31shows the location of auxiliary vanes on a single-channel pump impeller.
Auxiliary vanes assist the pump operation byperforming the following functions:• Decrease axial loads on bearings, particularly if
semi-open impellers are being used• Reduce impeller and casing wear at the suction
clearance• Prevent the wedging of fibres in the suction
clearance• Prevent fibres and rags from wrapping around
the pump shaft behind the impeller.
The use of auxiliary vanes extending to theshroud perimeter is not possible on large impel-lers, since at high flow rates they could cause apressure drop below the vapour pressure of theliquid, leading to cavitation. Large pumps are,however, less prone to jamming because of highmotor torque. Auxiliary vanes are therefore notincluded on the inlet side of large impellers.
Suction ClearanceThe clearance between the impeller and thepump casing should be as small as possible inorder to reduce leakage losses. The suction clear-ance is in the order of 0,5...1,0 mm for most cen-trifugal pumps. The clearance can be designedcylindrical or axial as shown in Figures 32 and 33.
Fig. 30
GRUNDFOS recirculation pump
Auxiliary
Pressure distribution
Pressure distribution
ps
p's
S
with auxiliary vanes
without auxiliary vanes
vanes
Fig. 31
The effect of the auxiliary vanes on the suctionshroud is a lowered pressure difference p's over thesuction clearance. With less back-flow the suctionclearance will last longer and the risk of jamming isreduced.
Pump Construction 2
25
Pump performance and efficiency over time aredependent on the suction clearance being keptwithin specified limits. The lowering effect of thesuction clearance on pump efficiency and headcan be calculated with the following empiricalequation:
(14)
where
Q= flow [l/s]H= head [m]s= clearance [mm]∆η and ∆H are proportional.
For semi-open impellers the effect is increased bythe factor 1,5.
Figure 34 shows the results of a test where apump was operated with varying suction clear-ance.
If the suction clearance widens to 2...3 mm forimpellers without auxiliary vanes and to 4...5 mmfor impellers with auxiliary vanes, it is necessaryto restore the clearance in order to retain pumpperformance. If the suction clearance is madeadjustable, this procedure is easily done by servicetechnicians in the field, whereas a pump with
Fig. 32
Cylindrical suction clearance. The design is suscepti-ble to jamming, since fibres that get wedged in thespace between impeller and casing may accumulateand drag down the pump. In case of wear, a wearring on the suction cover and the impeller need to bereplaced or re-machined.
Fig. 33
Axial suction clearance. This design is less prone tojamming, since drag forces will remove wedged ma-terial towards the pump suction. The clearance canbe made adjustable for ease of maintenance andwear compensation.
∆η ∆H K2
K+ K–≈ ≈
K 0 008 s2 H
Q----⋅ ⋅,=
Fig. 34
Effect of different suction clearance dimensions onpump curve and efficiency.
2 Pump Construction
26
fixed impeller suction clearance will have to bebrought to the shop for overhaul, or worse,scrapped for high costs of spare parts and work.
In pumps with adjustable axial suction clearanceperformance can always be guaranteed by check-ing and adjusting the suction clearance duringroutine maintenance. Figure 35 shows a submers-ible pump design, where the suction clearance isadjusted with the help of three set screws.
For dry-installed pumps GRUNDFOS has devel-oped a patented design (SmartTrim) allowing thesuction clearance to be adjusted and restoredwithout the need of removing the pump or open-ing pipe connections. Adjustment does not affectpipe connections or require re-alignment of these.Figure 36 show the principle. The adjustment isdone by first closing the clearance and then back-ing up the adjustment screws 1 mm, after whichthe suction cover is tightened against the setscrews with the fastening bolts.
The adjustment margin on the GRUNDFOS pumpsis 10...15 mm, depending on pump size. It is dimen-sioned to last the lifetime of the impeller.
Impeller AttachmentThe attachment of the impeller onto the shaftmust be both reliable and easy to dismantle.Removal is necessary for shaft seal maintenance,and for impeller replacement if the pump is usedfor pumping abrasive materials. The impeller mayhave either a cylindrical or tapered fit onto theshaft end.
A shaft joint tapered to the right angle is easy todismantle. The tapered joint is additionally tight-ened with a screw, which makes the joint free ofplay and rigid.
The joint is keyed for transmission of torque. Solidimpeller mounting is a key component in pumpoperational reliability, and great care shouldalways be exercised when the impeller is disman-tled. It is good practice to always use a torquewrench when setting the impeller screw. Thepump manufacturer issues correct tighteningtorque information and possible recommenda-tion of screw lubricant in each case.
Fig. 35
Suction clearance adjustment system with threeset screws.
Fig. 36
Suction clearance external adjustment system ondry-installed pumps.
Pump Construction 2
27
2.3 Motors
2.3.1 GeneralSubmersible pump motors are squirrel-cage elec-tric motors wound for regular three-phase or sin-gle-phase alternating current supply. Single-phase pump motors are available only for smallpumps (2 kW or less). Motors are available for 50or 60 Hz supply and a number of voltages. Themotors are built for submersible operation, ClassIP 68 according to IEC. The electrical features ofsubmersible motors are described in detail later inthis book.
The submersible pump is a fixed combination of amotor and a pump with a common shaft andbearings. The motor is short-coupled to the pump,and some of the pump parts, such as the volutecover may be integral with the motor attachmentflange. For best results the pump and motor aredesigned together, with one motor frame fitting arange of pump parts for different duties and dif-ferent operational ranges by the same manufac-turer. Motor and pump sections are selected anddesigned so as to exclude overloading at any dutypoint on the pump curve.
Submersible motors are normally air-filled. Smallmotors (1,5 kW and less) are also made oil-filled.The oil used in these is low-viscosity oil used alsoin transformers, in order to keep the rotor frictionlosses a small as possible. The growing losses andlower efficiency prevent manufacturers frommaking larger oil-filled motors. Oil-filled motorsare cheaper than air-filled motors because ofsmaller number of parts.
2.3.2 Explosion-proof MotorsSubmersible pumps are available in explosion-proof versions for use in environments where thepumped liquid or ambient atmosphere may con-tain explosive gases. This condition may exist, forexample, at or near petrochemical works but aspace can also be defined as explosive elsewhere,if deemed necessary for safety reasons.
The principle of explosion-proof motors is theirsafety against causing a potentially explosiveatmosphere from igniting. The following twoalternative technical solutions are available to fillthe requirement:• The motor is designed so that the enclosure
can withstand any internal explosive blaze andprevent it from spreading into the explosivesurroundings. This is referred to as Class D.
• The motor is designed so that no sparking orhigh temperatures may occur inside the motor.This is referred to as Class E.
An explosion-proof motor is designed and builtaccording to the rules set forth by internationalgoverning bodies (for example, Euronorm 50014and 50018). The requirements for class D motorsare detailed, involving among others the selectionand gauge of construction materials, casing jointdesign and manufacturing tolerances, motor inte-rior volume utilization as well as strength of thestructure and fasteners. The essential require-ment for the joints is that the mating surfaceshave to be longer, as they are supposed to serve as“extinguishing” gaps. Certification and approvalof a design is always subject to extensive tests,where the actual ability to withstand internalexplosions, is determined.
Class E explosion-proof motors do not requireextensive structural modifications, but are testedfor internal temperature rise at certain loads. Alsointernal sparking must be prevented by adequategaps between rotating and stationary parts.
Usually explosion-proof motors are based on theregular designs of a manufacturer, and form acomplement to these. The power characteristicsare normally not altered, and the pump parts arecommon for both. The structural requirements onexplosion-proof motors make these more expen-sive than regular motors.
2.3.3 Motor CoolingMechanical and electric losses in the motor areconverted to heat, which must be dissipated. In aregular submersible pump motor (see Figure 14)the heat is transferred from the stator casing tothe liquid through submersion. For cooling pur-poses it is normally sufficient if the motor is sub-merged to about half of the motor depth. Theliquid level may be pumped down all the way forshort periods without risk of overheating themotor.
A motor operating in water this way is in fact veryefficiently cooled, since cooling continues afterthe motor has stopped. Thus it is possible to allowfrequent starts and stops of submersible motors,
2 Pump Construction
28
which is beneficial for the design of pumpinginstallations.
Allowed Water TemperatureCooling of submersible motors relies on thepumped liquid, either by submersion or other-wise. Water temperature is therefore essential.Usually the motors are rated for +40 °C liquids.Higher liquid temperatures may be allowed, butthen the pump selection should be referred to thepump manufacturer. Also the cavitation risk mustbe assessed for higher temperatures with anNPSH analysis, because of higher vapour pressureof the liquid.
Submersible Motor Cooling in Dry Instal-lationsMany submersible motors are installed dry forvarious reasons. Adequate cooling of thesemotors must be ensured, and it can be arranged ina number of ways:
With a cooling water jacket that encases themotor or parts thereof. Part of the pumped liquidis diverted through channels from the pump cas-ing into the cooling jacket where it recirculatesafter the casing has filled up. The water enters thespace behind the impeller through a filteringclearance (about 0,5 mm) and is circulated by theauxiliary vanes on the back of the shroud aroundthe motor stator housing inside the jacket. Excessheat is conveyed to the water through forced con-vection, ensuring efficient cooling. The principle isshown in Figure 37. The usage of a filtering clear-ance and wide enough cooling channels hasensured that the system is non-clogging also inpractice. A cooling jacket is often optional onsmall and medium-sized pumps for dry installa-tions, whereas very large pumps are oftenequipped with a cooling jacket as standardregardless of installation method.
In some cases, where the liquid being pumped isunsuitable for circulation in the water jacket,external cooling water may be used. In thesecases the pump is modified with external waterconnections in the jacket and by plugging theentrance channels from the pump casing. A safetycircuit is necessary to protect the pump fromoverheating due to disruption of the externalcooling water supply.
With thick stator housing walls. This design, suit-able for small submersible pumps, employs athickened stator housing that conveys the heatfrom the stator to the pumped liquid. In this con-struction the stator housing flange may contactthe pumped liquid directly or through the oilhousing cover flange. The flange can be shapedwith a recess or channel for good contact with theliquid. The stator casing may also be made of alu-minium in dry-installed pumps to furtherenhance heat dissipation. Figure 38 shows theconstruction.
For dry-installed pumps only a cooling waterjacket offers equal or even superior motor coolingto submergence. Other motors may have to bedown rated for dry installations, limiting theselection of pump components from the match-ing range.
Fig. 37
A dry-installed GRUNDFOS submersible pump withmotor cooling jacket. Part of the pumped liquid isfiltered through a gap of about 0,5 mm and remainsrecirculating in the cooling jacket, circulated by thepumping action of the impeller back shroud auxilia-ry vanes. Efficient cooling is provided by heat con-vection from the stator through dissipation into thepumped liquid.
Pump Construction 2
29
With an internal cooling circuit, where a coolingfluid, e.g. glycol, is circulated by a separate smallimpeller on the pump motor shaft. The pumpincorporates a heat exchanger between pumphousing and the motor, where the cooling fluidyields heat into the pumped liquid. System com-plexity may pose problems.
2.3.4 Motor TightnessWater intrusion in the motor leads invariably todamage, or, if detected by safety devices, at leastto pump outage. The chief requirement anddesign consideration of submersible pumpmotors is therefore complete integrity againstleakage. Motor tightness is ensured with gooddesign and continuous quality control includingtests during manufacturing.
All submersible motor joints are machined to fit,and O-rings are used throughout. The O-rings are
renewed each time a joint is opened for service toensure tightness.
The electrical cable inlet to the motor must bereliably watertight. A good design uses compress-ible rubber grommets that match both cable andthe inlet recess. The grommet is compressed toprescribed tightness by the shape of the matchingparts when assembled. A cable clamp external tothe sealing carries all outside tensile loads on thecable, preventing pulling at the seal.
The possibility of water intrusion through thecable is a reality. If the cable free end is allowed tobe submerged, water may travel by capillaryaction between the copper strands of the leads tothe motor. This action is enhanced by the temper-ature changes of the motor, and water may thisway enter an otherwise undamaged motor. Thecondition can arise in new pumps that have beenstored outdoors prior to installation with thecable free end unprotected.
Most pump manufacturers deliver their pumpswith protecting sleeves on the cable free ends.Warning labels are attached to warn the storageand installation personnel of the risk of submerg-ing the cable free end.
Securing submersible motor tightness requiresspecial knowledge and special tooling, and it istherefore advisable to return the pump to anauthorized shop for repairs. Pump manufacturesoffer training and special tools to their customers.For owners of large numbers of submersiblepumps an in-house authorized shop may be war-ranted.
Shaft SealsThe shaft seal, providing safety against leakage ofthe pumped liquid into the motor, is one of themost important elements in a submersible pump.
Modern submersible pumps almost exclusivelyuse a shaft sealing arrangement with doublemechanical seals separated by an oil-filled cham-ber. The arrangement, developed and refined overthe years, provides adequate protection againstleakage and motor damage in most cases.
Figure 39 shows a mechanical seal arrangementused in submersible pumps. There is a lower orprimary seal and an upper or secondary seal. The
Fig. 38
A GRUNDFOS submersible pump suitable for dry orsubmerged installation. The thick-walled lower sec-tion of the motor serves as a heat conduit to thepumped liquid. The stator casing may be made ofaluminium to further enhance the effect.
2 Pump Construction
30
seals, being separated by an oil bath, operateunder different conditions. This is reflected intheir construction with different materials. Bothseals comprise two contacting slip rings, one sta-tionary and one rotating with the shaft. The ringsare pressed against each other by spring forceand, for the primary seal, in addition by the pumppressure.
Sealing between the slip rings is based on theextremely smooth and flat contact surfaces of theslip rings. The surfaces are in such close contactthat no or only a very minute leakage can passbetween them. The flatness and smoothness ofthe rings are in the magnitude of 0,0005 mm andthe faces are finished by lapping. The slip ringsseal against the stationary seat or shaft with O-rings. The material of the O-rings is selected towithstand high temperature and the corrosiveand dissolving action of the seal oil and the impu-rities in the pumped liquid.
Notches in the stationary slip rings of the primaryseal secure them in the seat against turning. Therotating rings are locked similarly with drive pins.Spring clips or washers keep the stationary ringsin their seats during abnormal pressure situa-tions.
The material of the primary seal faces is normallyhard, because of the abrasive action of the
pumped liquid. The material used today is siliconcarbide (SiC), which has a hardness around 2000on the Vickers scale and ranks next to the dia-mond. The silicon carbide rings can be either solidor converted. Converted carbide rings are sinteredto SiC to a depth of approximately 1 mm, leavingthe ring interior unchanged. SiC also has verygood resistance against corrosion, and can beused in all wastewater and dewatering applica-tions.
If the secondary seal is oil lubricated, a combina-tion of materials may be used. A stationary ring ofa softer material with good friction properties incombination with a hard rotating ring provides forlow seal rotation resistance. The oil lubricationprotects the seal against wear. Modern secondaryseals normally have faces of silicon carbide andcarbon .
Modern submersible pumps utilize mechanicalseals custom-designed for the purpose. Gooddesigns have been developed by most major man-ufacturers. A proprietary design combining pri-mary and secondary seal is shown in Figure 40.
Fig. 39
A GRUNDFOS double mechanical seal with primaryand secondary seals.
Fig. 40
A GRUNDFOS integrated double mechanical seal.
Pump Construction 2
31
All mechanical seals used in submersible pumpsmust allow rotation in either direction, sincepumps frequently get started in the wrong direc-tion or may be turned backwards by back-flowingwater in installations without check valves.
All submersible pumps with double mechanicalseals have an oil space between the seals. The oilserves the following functions vital to the func-tion of the seals and the pump:• Seal lubrication, especially of the secondary
seal• Seal cooling• Emulsification of possible leakage water, thus
rendering it less harmful• Seal condition monitoring. By checking the
seal oil during maintenance, the seal conditionand rate of leakage can be estimated.
Overfilling of the seal oil chamber should beavoided in order for the oil to be able to absorbleakage water by emulsification and to preventpossible overpressure due to thermal expansionof the oil. The pump manufacturer provides infor-mation on oil quantity and filling and monitoringmethods.
In special applications, where the pumped liquidcontains very fine materials, the primary seal mayopen due to material build-up on the slip ringfaces. In these cases it may be warranted toarrange for continuous external flushing of theseal. These installations are always consideredseparately for each case by manufacturer and cus-tomer.
Mechanical seal life expectance cannot be deter-mined theoretically or even by lab tests. Perfor-mance over time is also difficult to predict. Seallife varies greatly from case to case, with servicerecords from a few years to over 15 years reported.
2.3.5 Motor Bearings
Bearing loadsThe submersible pump bearings carry the com-bined load of the pump and motor as exerted onthe common shaft. The following forces act on thebearings, either radially or axially:• hydrodynamic radial force• hydrodynamic axial force• magnetic radial force• the weight of the rotating parts
The significant forces acting on the bearings arethe hydrodynamic forces.
The hydrodynamic radial force is the resultant ofthe pressure distribution at the impeller perime-ter in various relative positions to the pump cas-ing. The radial force is dependent on a number ofdesign factors as well as on the pump operatingpoint.
The hydrodynamic axial force is the resultant ofthe forces induced by the impeller diverting theflow from axial suction to radial discharge, andfrom the pressure difference between suction andpressure side of the impeller. The axial force isalso strongly related to the pump flow and oper-ating point.
BearingsRolling bearings are used throughout in submers-ible pump motors. Ball bearings are used for theirability to carry both axial and radial loads. In verylarge motors a combination of ball bearings androller bearings are used because of the largeforces on the components.
To allow for heat expansion of the shaft and formanufacturing tolerances, the shaft upper bear-ing is allowed axial movement, whereas the lowerbearing is locked axially.
Bearing selection is governed by internationalstandards with regard to bearing life. According tothe pump standard ISO 5199, “Bearing rating life(B10)” should be at least 17500 hours.
Submersible pump bearings are normally lubri-cated for life at the pump factory, using specialgrease suitable for the high operating tempera-tures allowed in submersible motors.
2 Pump Construction
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2.3.6 Motor Protection DevicesSubmersible motors are equipped with variousprotection devices for prevention of damages forthe following reasons:• overheating• water intrusion• seal failure• bearing failure• winding insulation deterioration
Some protection devices are standard issuewhereas others may be available as extra equip-ment on request only. Large pumps need betterprotection devices because of the greater eco-nomic values of these pumps.
The protection devices can be divided into inter-nal devices with sensors inside the motor andexternal devices in the pump motor control panel.
Internal Protection DevicesThe following protective devices are mountedinside the motor:• Thermal switches in stator windings. These are
normally bimetal miniature switches thatopen at a fixed, preset temperature, pleaserefer to Figure 41. Three switches, one in eachphase, are used in three-phase motors. Theswitches are connected in series in the controlcircuit, which is wired to stop the motor whenopening. The switches reset upon cooling, clos-
ing the circuit, making re-start of the pumppossible. Thermal switches in the windingsprotect the motor against overheating frominsufficient cooling, and are especially impor-tant in pumps depending on submergence forcooling.
• Water intrusion into the sealed motor can bemonitored with a moisture switch that reactsto excess moisture. Normally the moistureswitch is connected in series with the thermalswitches in a circuit that disconnects the cir-cuit breaker coil and stops the motor uponopening. Figure 42 shows a moisture switchthat operates when the humidity reaches100%. The moisture switch is non-reversingand does not reset after tripping. In a commoncircuit with moisture and thermal switches, itcan be determined which device has opened,since only the thermal switches close againafter cooling. The motor must be opened anddried out before any attempts to restart it afterthe moisture switch has tripped.
• Water intrusion into the sealed motor past theshaft seals can be monitored with a leakagedetector sensor in the seal oil chamber. Regu-lar motor oils used as seal oil in submersiblepumps can form an emulsion with up to 30%water content. The leakage detector eitherreacts on a water content exceeding 30% (con-
Fig. 41
Thermal switch. The unit consists of a miniature bi-metal switch that opens according to the switchtemperature rating. The switch is connected in thecontrol panel to break the current in case the motoris overheating.
Fig. 42
A GRUNDFOS moisture switch. The unit consists of anumber of moisture-sensitive disks stacked onto anactuating rod, and a micro switch. The hygroscopicdisks expand upon contact with excess moisture,pulling the actuating rod. A cam at the rod end tripsthe micro switch and breaks the circuit. The unit isnon-reversing and must be replaced after use.
Pump Construction 2
33
ductive detectors) or monitors the water con-tent continuously (capacitive detectors). Thelatter may be calibrated to trip at any watercontent and used to indirectly observe primaryseal condition by monitoring water intrusionover time (leakage rate). Leakage detectors areusually not standard, but available as extraequipment.
• Water intrusion into the sealed motor throughcapillary flow through the supply cable beforepump installation can be prevented by fitting atight protective sleeve over the cable free endat the factory. The sleeve is not removed untilthe cable is connected at the control panel.
• The condition of the bearings and/or bearinggrease can be monitored with thermal sensorsin the bearing bracket. These are installedclose to the bearing outer race, and calibratedto register bearing temperature. Thermal sen-sors are extra equipment.
External Protection DevicesThe following protective devices are mounted inthe motor control panel:• Short-circuit protection is accomplished by
means of fuses, circuit breakers or electronicmotor protectors. Fuses and circuit breakersshould be dimensioned to withstand themotor starting current, but the rating must notexceed that of the supply cable or switchgear.Where fuses are used, these should be of theslow type.
• Overload protection is required in a suddenoverload situation, such as when the impeller
develops operational difficulties or getsjammed, when the pump becomes clogged orduring loss of phase in the mains supply. Over-load protection is frequently provided by over-load relays coupled to the motor contactors.These consist of ambient temperature-com-pensated bimetal elements, that trip the cur-rent to the contactor coils in case the currentexceeds the set specified value. Overloadrelays provide good protection against loss ofphase in the supply. The overload relay shouldbe set according to the motor nominal current.When star delta start is used, the currentthrough the overload relay is reduced by thefactor 0,58 (1/√3), which must be taken intoaccount when setting the relay. Figure 43shows an overload relay.
• The stator winding insulation is monitored byan automatic resistance measuring device thatmeasures the resistance between the phasesand between phases and earth each time thepump stops. Alarm levels for resistance can beset, preventing damages short circuits anddamages to windings.
Thermal overload relay. The relay connects to themotor contactor and breaks the current in case of theelectric load exceeding the set value.
Fig. 43
2 Pump Construction
34
2.4. Pump Connection
A submersible pump, when installed submerged,is connected only to the discharge pipe. For fixedinstallations a self-connecting baseplate arrange-ment is normally used.
Submersible BaseplateThe concept of the submersible baseplate hasbeen developed over the years for use with sub-mersible pumps. The arrangement allows for thepump to be lowered into the pump well andfirmly connect to the discharge pipework withoutthe need of the operating personnel to enter thewell. Likewise the pump can safely be hoistedfrom the well for service. The system includes railsor pipes that guide the pump down onto thebaseplate. A special flange, or guide shoe, on thepump discharge mates with the joining surfacesof the baseplate for a firm connection. Well-designed systems are made to precision and havemachined surfaces and rubber seal rings for a
sturdy and tight connection. The pump is kept inplace by its own weight. Figure 44 shows a sub-mersible pump baseplate and guide rails.
Figure 45 shows a flexible seal designed in a waythat the seal action is further enhanced by pumppressure, ensuring a tight connection at all times.
Some pump manufacturers offer conversion kitsfor the connection of pumps to older baseplatesor as replacement pumps to some other manufac-turer's baseplate. Thus the upgrading or conver-sion of existing pumping stations may be donewith a minimum of work and costs.
Fig. 44
A GRUNDFOS submersible baseplate. When seated,the weight of the pump keeps it firmly in place. Pre-cision-machined connecting surfaces and a rubberdisk seal ensures tightness. Clearance between theguide shoe and the rails ensures unobstructed hoist-ing of the pump even in fouled conditions
Fig. 45
Flexible seal between pump pressure flange andconnector. The seal is designed in a way that theseal action is further enhanced by pump pressure,ensuring a tight connection at all times.
Pump Construction 2
35
Hose ConnectionFigure 46 shows a submersible pump installationwith hose connection. It may be used for tempo-rary installations or in applications where thepump is shifted around the wet well for sludgepumping.
Column InstallationThe concept of column installation of submersiblepumps has been developed during the past fewyears. The pump is lowered into a vertical pipe orcolumn, where the circular pump casing fits ontoa seat ring installed at the lower end of the col-umn, please refer to Figure 47. The pump stays inplace by its own weight and from the pressureforce from the pumping action. The pump casingis special-designed for the installation, and is fit-ted with trailing vanes. The seat ring is conical,ensuring a tight connection between pump andcolumn. The tight connection and dowels preventthe pump from spinning loose at start-up
Column installation is ideal for submersible pro-peller pumps, but also for sewage pumpsintended for large flows and low to moderateheads. Figure 48 shows the Q/H area on whichcolumn-installed Grundfos pumps are available.For this range column installation is likely to leadto lower investment costs, but each projectshould be assessed individually. Column-installedpumps have the same efficiency as pumpsintended for other installation modes, but thepump curves will differ slightly because of theopen pump casing. Column installation is verysuitable for return sludge pumping in sewagetreatment plants. The column pipe may be madeof stainless steel or hot dip galvanized steel.
For seawater installations a column made ofstainless steel may create a strong galvanic ele-ment between pump and column, leading topump corrosion. Especially galvanized pump partswill rapidly corrode from the galvanic action ofthe large cathode area of the column surrounding
Fig. 46
Submersible pump on stand with hose connection.This installation version is used for temporary orshifting installations.
Seat ring
Fig. 47
Pump column installation. Pump rests on conicalseat ring installed at the bottom of the column
2 Pump Construction
36
the pump. A lifting chain left in place, for instance,will have to be made of stainless steel. The castiron pump should be protected by sacrificinganodes that are replaced at regular intervals.Painting the column with a paint layer of at least200 µm thickness prevents the cathode surfacefrom forming and thus pump corrosion.
2.5 Construction Materials, Corrosion and Wear
2.5.1 Corrosion ResistanceCast iron is the main construction material in sub-mersible sewage pumps, with fasteners and hard-ware made of stainless steel. The pump shaft iseither made entirely of stainless steel or pro-tected against contact with the pumped media.Where the pump or baseplate includes fabricatedsteel parts, these are hot-dip galvanized. Thesematerials will last for decades in regular sewerageduty.
In cases, where the pumped liquid contains indus-trial effluents, the corrosion resistance of cast ironmay not be sufficient, especially for parts subjectto fast flow velocities, such as impellers and pumpcasings, which will be subjected to erosion corro-
sion. In these applications the natural corrosionlayer, providing the underlying material with nat-ural protection becomes scrubbed away, leadingto rapid corrosion. The use of stainless materialsfor these vulnerable parts may be warranted.
Corrosion in seawater is dependent on a numberof factors, such as salinity, oxygen content, pollu-tion and temperature, and the right materialselection must be considered for each case. Sacri-ficial zinc anodes may offer protection againstcorrosion in certain cases.
The supply cable sheath material must be able towithstand oils and other pollutants present insewage. Other rubber parts, such as O-rings areusually made of Nitrile or Neoprene for resistanceagainst sewage, oil and chemicals.
Submersible pumps are also available madeentirely of stainless steel for use in highly corro-sive liquids, such as process industry effluents.Stainless steel submersible pumps are 3…4 timesas expensive as pumps made of regular materials.In difficult applications the pump manufacturermay not be able to guarantee the corrosion prop-erties for a specific case, but will cooperate withthe client to find the right solution for the case.
Fig. 48
GRUNDFOS column-installed pump flow and head (Q/H) range.
Pump Construction 2
37
2.5.2 Wear Resistance The sand content in sewage is on averagebetween 0,002 and 0,003 % (in volume). The con-tent may periodically, e.g. during heavy rainfalland snow melting be much higher in areas withcombined sewage and stormwater drain systems.Regular cast iron will last in most applications foryears, but special material may have to be consid-ered for highly abrasive effluents, such as sewagetreatment plant sand trap pumping.
2.5.3 Abrasive LiquidsPump performance in an abrasive liquid isstrongly dependent on the content of abrasives inthe liquid. The standard abrasive is commonquartz or silicon sand, to which the following canbe applied directly.
The sand content is either expressed as volume orweight content, which are related as follows:
pm ≈ 3·pv (15)
where pm is weight content and pv is volume con-
tent in %. Thus pv = 5% equals pm =15%.
With increased sand content the density of theliquid/sand mixture increases. Since requiredpump power is directly related to the density ofthe pumped liquid, required power will have tobe checked separately in each case to ensurepump performance, whenever liquids with highsand content are being pumped. For sand trappumps in sewage treatment plants, a powerreserve of 30% has proven adequate.
The density of a mixture water and sand can bewritten
ρ = 1 + 0,007pm (16)
where pm is expressed in %.
Thus, if pm = 15%, ρ = 1,1 kg/l.
The following factors affect the wear of a pump:• sand content• sand quality• pump material• pump head• type of impeller
Figure 49 is a diagram showing the relationsbetween the pump wear rate and the sand con-tent and pump head. High sand contents in theliquid will have a dramatic effect on pump servicelife. The effect of the sand content is exacerbatedby high pump head.
Pump wear can be minimized using suitablewear-resistant materials and through appropriatedesign. For best results, materials with a hardnessover 500 HB should be used. The difficultmachineability of hard materials, such as Nihardand some alloyed irons, may require specialimpeller and pump casing designs where machin-ing is minimized.
The use of submersible pumps in abrasive envi-ronments must be considered separately on acase by case basis and using sound engineeringjudgment.
Sand content�Pm [%]
Pump head H0 [m]
Pump service life [h]1
0,110000100010010
0,2
0,5
1
5
10
20
50 20 10 5
Fig. 49
Pump wear rate as a function of sand content andpump head. H0 is pump head at Q=0. Wear rate isexpressed as expected service life of a cast iron im-peller and is strongly dependent of sand content andpump head. The graph is based on experiments, andcan be used generally.
3 Pump Performance
38
3 Pump Performance
Pump performance is the result of the interactionbetween pump and rising main or pressure pipe-line. An introduction to pump selection and thecalculation of rising main resistance characteris-tics are presented.
3.1 Pump Head
3.1.1 Submersible PumpsIn the following the concept of head is applied tosubmersible pumps. For practical reasons thepressure in the pump well, or lower well, isassumed to be equal to the pressure prevailing inthe receiving, or upper container. Should thesecontainers be under different pressure, the pres-sure difference would have to be taken intoaccount. The difference in atmospheric pressurecan also be disregarded in all practical installa-tions, since the difference in atmospheric pres-sure between a receiving container situated, forinstance, 100 m above the pump well is only 0,001bar or 0,01 m of water.
Figure 50 shows how the head is defined in asubmersible pump installation. The followingunits are used:H = pump total head (m)Hst= pump static head (m)
Hd = pump dynamic head (m)
Hgeod= geodetic head (m)
HJ = pipeline losses (m)
pL = atmospheric pressure in pump well
pU = atmospheric pressures in upper container
v2 = flow velocity at outlet (m/s)
g = acceleration of gravity (9,81 m/s2)
If an observation pipe is installed at the pumpoutlet flange, the pumped liquid will rise in it to aheight Hst from the well level. This height repre-
sents the pump static head. In addition, the liquidhas a velocity v2 at the pump discharge, which
can be converted to pressure or dynamic head Hdwith the following equation:
(17)
The sum of the static head and the dynamic headis the pump total head, thus
H = Hst + Hd (18)
According to international agreement (StandardISO 2548), the total head H according to equation18 is used when plotting characteristic curves forsubmersible pumps.
The total head H is thus available to pump the liq-uid through the rising main. The pressure or headrequired to pump a given flow through a pipelineis made up by the geodetic head and the flowlosses. Thus can be written:
H = Hgeod + HJ (19)
The geodetic head Hgeod is the actual physical
difference in height between the liquid levels inthe pump well and the receiving container. Pipe-line flow losses consist of pipe friction losses, locallosses from various fittings in the pipeline
Hd
v22
2g------=
Fig. 50
Head components in submersible pump installations.
Pump Performance 3
39
(elbows, valves, etc.) and the outlet loss at thepoint of discharge.
Losses due to liquid flow in the well to the pumpare considered as pump losses in submersiblepump installations. If a suction pipe is installedbefore the pump, it will have to be taken intoaccount when calculating pipeline losses.
3.1.2 Dry-installed PumpsWhen calculating heads of dry-installed pumps,the situation before the pump will also have to beconsidered. Figure 51 illustrates the situation.
In this case it is assumed that the suction well andthe receiving container are open to the atmo-sphere and that the pressure at the liquid surfacesis constant. Thus the pump head is the sum of thegeodetic head and the flow losses in the suctionand pressure pipelines. Thus
H = Hgeod + HJt + HJp (20)
where HJt represents flow losses in the suctionpipeline and HJp flow losses in the pressure pipe-line.
3.2 Pump Performance Curves
Centrifugal pump characteristics are normallypresented as a set of curves, where the data hasbeen established through the testing of thepumps or assessed by the manufacturer for e.g. aspecial impeller diameter. For submersible pumpsthe following important information is normallyplotted as curves against the flow rate Q:• H head curve• η efficiency curve(s)• P power curves
Figure 52 shows a typical pump performancecurve sheet with information important for theuser.
3.2.1 H CurveThe head or H curve gives the pump total head asa function of the flow Q. The curve may containadditional information on pump usage, such aslimits due to cavitation, vibration or motor over-load.
Fig. 51
Pipeline loss components for dry-installed pumps.
Fig. 52
Typical pump performance curve sheet for submers-ible pump. The dashed sections of the curves indicateareas, where pump prolonged use is prohibited. Thereasons for the limitations may be cavitation, vibra-tions or motor overload.
3 Pump Performance
40
3.2.2 Efficiency CurvesPump efficiency η is also a function of the flowrate Q. The efficiency can be indicated as a ratio orpercentage. For submersible pumps both thepump efficiency η and the overall efficiency ηgr are
defined, where ηgr includes motor losses. It isimportant to distinguish between these defini-tions for efficiency, especially when comparingpump performance. The losses leading to thepump efficiency are discussed in Section 1 of thisbook. Thus it can be written:
ηgr = ηmot · η (21)
where ηmot is motor efficiency.
The efficiency can also be marked on the headcurve, with numbers indicating different effi-ciency values. If several head curves for variousimpeller diameters are plotted in the same graph,these markings can be connected to formisograms, or operating areas with the same effi-ciency. The pump performance diagram will thenassume its typical appearance as shown in Figure53.
3.2.3 Power CurvesThe pump required power is also a function of theflow rate Q. Figure 52 contains both the pumppower curve and the motor power curve. Themotor power is the electric power drawn by themotor and measured at the cable junction box atthe motor. According to international standardson pump testing the pump power is designated Pand the power absorbed by the motor Pgr. The
required power can also be calculated using theequation
(22)
whereP = power (W)ρ = liquid density (kg/m³)Q = volume flow (m³/s)g = acceleration of gravity (9,81 m/s² )H = pump head (m)η = efficiency
3.2.4 NPSH CurveSince NPSH calculations are performed only fordry-installed pumps, the NPSH curve is not usuallyincluded on submersible pump data sheets. It willbe provided by the manufacturer on request ifcavitation is feared in a dry installation, or ifotherwise required by the client.
Results from tests performed with clean water areapplicable as such on normal municipal sewageand most industrial effluents, since the low solidscontent in sewage (less than 0,05%) does not sig-nificantly affect pump performance.
Fig. 53
Pump performance sheet with curves for differentimpeller diameters, where the efficiency is indicatedas points directly on the head curves. Impeller selec-tion is facilitated by connecting the points with thesame efficiency.
P ρQgHη
----------------=
Pump Performance 3
41
3.3 Pipe Losses and Rising Main Characteristic Curves
In the following the theory for calculation of flowlosses in pipelines is presented. Practical calcula-tions can be made with the help of the detailedinstructions with calculation diagrams and nomo-grams presented in Appendix A, or with a com-puter program.
Flow velocities used in sewage pumping are highenough to ensure uniform turbulent flow in thepiping. Flow losses therefore increase with thesquare of the flow velocity. The flow loss of a ris-ing main is the sum of the friction loss of the pipe-line constituent parts and the local losses fromthe various components and fittings.
3.3.1 Friction LossesPipe friction losses depend on the following fac-tors :• pipe length• pipe internal diameter• flow velocity• pipe wall relative roughness• fluid kinematic viscosity.
A dimensionless relation, Reynold's number isintroduced:
(23)
whereRe = Reynold's numberv = flow velocity (m/s)D = pipe internal diameter (m)ν = kinematic viscosity (m²/s)
The kinematic viscosity for water is dependent ontemperature:
The equation for pipeline losses can be written:
(24)
where HJp= pipeline loss (m)
λ = friction factorl = pipeline length (m)v = flow velocity (m/s)g = acceleration of gravity (9,81 m/s²)D = pipeline internal diameter (m)
Obtaining the friction factor λ from the diagramin Figure 54, equation 24 can be solved. Surfaceroughness values (mm) presented in the followingtable can be used:
The surface of an old pipe material becomesrougher from erosion. Corrosion and sedimentlayers forming on the pipe surface may decreasethe pipe diameter, also leading to higher flowlosses.
The effect of pipe diameter change can be calcu-lated with the following relation:
(25)
Thus an increase of pipe diameter from, forinstance, 100 mm to 108 mm decreases the flowloss by 30%.
Equation 25 is sufficiently accurate for practicalpurposes when comparing flow losses in risingmains of different diameter, particularly sinceaccurate surface roughness values are seldomavailable.
t °C 0 20 40 60 100
ν 10-6 m²/s 1,78 1,00 0,66 0,48 0,30
Re vDν
-------=
HJp λ lv2
D2g-----------=
Pipe material k new k old
Plastic 0,01 0,25
Drawn steel 0,05 1,0
Welded steel 0,10 1,0
Drawn stainless steel 0,05 0,25
Welded stainless steel 0,1 0,25
Cast iron 0,25 1,0
Bituminized cast iron 0,12
Asbestos cement 0,025 0,25
Concrete 0,3...2,0
H'Jp HJpD'D-----
5≈
3 Pump Performance
42
Rising main flow losses are frequently calculatedwith the help of proprietary computer programs,also available from some pump manufacturers.These programs may also suggest some pumpselections from the manufacturer’s range to bestsuit the purpose. It is advisable to take a cautiousview on the pump selection suggested by a pro-gram only, and always contact the pump manu-facturer in dubious cases.
The rising main is sometimes divided into twoseparate parallel pipelines. They have the samelength but may have different diameters or bemade of different materials. The distribution offlow between the two lines and the ensuinglosses in these lines can be difficult to determine.Grundfos has developed a method for this, wherethe two lines are substituted with a single virtualrising main. An equivalent diameter is deter-mined for this so that the resulting flow losses are
equal to the resultant losses of the two true risingmains.
The equivalent diameter is calculated with thefollowing equations:
A. Both rising mains have the same diameter
(26)
where D = diameter of the two parallel risingmains
B. The rising mains have different diameters
(27)
where D1 and D2 are the different diameters of theparallel rising mains.
TRANSITION ZONE
SMOOTH PIPE
TURBULENT FLOW
RELATIVE SURFACE ROUGHNESS K/d
REYNOLD'S NUMBER Re=
Fig. 54
Moody diagram for establishing the friction factor λ. The value of λ is obtained using Reynold's number and the relativeroughness number k/D as parameters, where D is pipe internal diameter in mm and k equivalent surface roughness inmm. Completely turbulent flow can be assumed in wastewater applications.
De 1 3 D⋅,=
D D12 65,
D22 65,
+( )0 3774,
=
Pump Performance 3
43
The volume rates of flow for the two rising mainsare calculated wit then following equations:
A. Both rising mains have the same diameter
(28)
B. The rising mains have different diameters
(29)
(30)
The equations above are valid for turbulent flow,which is normal for water pumping. The equa-tions require that both pipelines have the samesurface roughness.
3.3.2 Local LossesChanges in pipeline internal diameter and shape,bends, valves, joints, etc. as included in the risingmain cause additional losses that comprise both afriction and turbulence component. The followingequation is used to calculate the losses:
(31)
whereHJn= local loss (m)
ζ = local resistance factorv = flow velocity (m/s)g = acceleration of gravity (9,81 m/s²)
Local resistance factors for different pipeline ele-ments and fittings are presented in Appendix A.The friction loss of these are not included in thelocal resistance factor, but is calculated as part ofthe rising main friction loss by including theirlength and internal diameter when calculatingpipeline length.
Pipe expansion discontinuity loss can be calcu-lated using the Borda equation:
(32)
whereHJn= local loss (m)
v1 = flow velocity 1 (m/s)
v2 = flow velocity 2 (m/s)
g = acceleration of gravity (9,81 m/s²)
If the pipe expansion is designed with a conicalsection with an expansion angle of 10°, the loss isreduced to 40% of the value calculated with equa-tion 32. This fact is important when expandingthe pipe section right after the pump pressureflange, where the flow velocity can be quite high.By designing the transition with a 10° gradualexpansion joint, energy can be saved. In a con-tracting pipe section the losses are much smaller,and the conical section can be built much shorter.
Losses in a section with velocity reduction aregenerally much greater than in section withincreasing velocity.
The final component of pipeline loss is the outletloss at the end of the rising main. If no expansionis provided, the loss equals the velocity head or v²/2g.
Loss coefficients for different valves are providedby the manufacturers. Guide values for the mostcommon valves used in sewage installations arepresented in Appendix A.
3.3.3 Rising Main Characteristic CurveIn sewage installations the pump sump and thedelivery well are open to the atmosphere, and therising main characteristic curve will contain thegeodetic head and the flow losses only. Figure 55shows the general shape of the characteristicresistance curve for a pipeline. Since the flow isturbulent at the flow velocities in consideration, itcan be assumed that the flow loss varies in pro-portion to the square of the flow rate. Thus, if theflow loss at one flow rate is calculated with themethod described above, the other points of thecurve are obtained sufficiently exactly with thefollowing equation:
(33)
Q1Q2----=
Q1
D1
De------
2 65,Q⋅=
Q2 Q Q1–=
HJn ζ v2
2g------=
HJn
v1 v2–( )2
2g-----------------------=
H'J HJQ'Q-----
2=
3 Pump Performance
44
3.4 Rising Main Size
Rising main size is selected based on the follow-ing factors:• economy• required internal diameter for the application• required smallest flow velocity for the applica-
tion.
3.4.1 EconomyThe economy of an installation is made up byboth procurement costs and operational costsduring its lifetime. A number of installation andoperational costs are directly dependent on risingmain size, and will react to changes in pipelinesize as follows:
With decreased pipeline diameter • Piping and pipework component procurement
prices will decrease.• Pumping station procurement cost will
increase due to increased flow losses with con-sequent requirement for larger pumps andcontrol equipment. Costs for increased electri-cal supply systems, such as substations mayincrease significantly.
• Operating costs will increase due to higherenergy costs because of pipeline losses.
With different costs having opposite relations torising main size, an optimal pipeline size may befound. Figure 56 shows the relation. The selectionof an optimal pipeline diameter may be based onFigure 57, which shows the optimum flow velocityfor different installations, and is based on severalstudies.
Where possible, a more detailed study can, andshould, be conducted.
Fig. 55
Characteristic resistance curve for a pipeline. Pipelosses (HJ) are plotted against flow rate (Q) and addedto the geodetic head, which is constant.
Fig. 56
The relation of key cost components for a pumpinginstallation as related to rising main size. With costshaving opposite relations to pipeline size and flowvelocity, an optimum can be found.
0 20 40 60 80 100 120 140 160 180 200
Fig. 57
Guideline values for economically optimal flow veloc-ities for submersible pump installations. The figure isbased on a study of submersible pump installationsusing geodetic head, pipeline length, yearly operationhours and energy cost as parameters.
Pump Performance 3
45
Pumping station internal piping should beselected so as to minimize component costs with-out unduly increasing the flow losses in the sta-tion. Figure 58 shows the flow loss in the internalpipework in a pumping station with two pumps induty-standby operation as well as the economicalpipe dimensions, based on several studies.
3.4.2 Free Passage for SolidsFor untreated municipal sewage the smallestallowable free passage of the rising main is gener-ally 100 mm in order to allow passage of solidswithout clogging. In pumping stations with smallflows the internal pipework may have a free pas-sage of 80 mm, especially when the pump freepassage is also 80 mm.
3.4.3 Avoiding Settling of Solids andSludgeIf the flow velocity in a rising main is too low, sandor sludge may have time to settle, which increasesthe risk of clogging. Settled sludge may hardenand form a crust on the pipeline wall that perma-nently decreases the diameter, leading toincreased flow losses. Larger sludge clots movingwith the flow may block bends or other fittings inthe rising main.
For municipal sewage a minimum flow velocity of0,7 m/s is recommended. Where only domesticsewage is pumped, the minimum flow velocitymay be as low as 0,5 m/s, but if sand is found inthe sewage, this lower value is not endorsed. Ininstallations with varying flow, e.g. where fre-quency converters are used, the flow velocity maytemporarily be lower.
Where settling is known to occur, flushing out ofthe system with all pumps running simulta-neously at intervals is recommended. The shapeof the pipeline is also important, and sedimenta-tion is likely to occur in rising mains having a pro-nounced low, such as pipelines laid underneathwaterways, e.g. a river. In these cases it is recom-mended to select a higher flow velocity.
3.4.4. Water HammerOscillating pressure waves are generated in a liq-uid being pumped through a pipeline duringstarting and stopping of the pumps. This phe-nomenon is called water hammering, and, ifsevere, may lead to pipeline and equipment dam-age. The severity of the phenomenon is depen-dent on a number of variables, such as change ofvelocity during the reflection cycle, pipe materialcharacteristics as well as liquid characteristics.
When the liquid is accelerated or decelerated, atransitory pressure wave oscillates back and forthuntil dampened. The oscillating frequency can becalculated with the following equation:
(34)
whereµ = reflection cycle duration, during which
the pressure wave oscillates back and forth once (s)
L = pipeline length (m)a = pressure wave velocity (m/s)
Total
Size recommendation
Fig. 58
Flow loss in the internal pipework in a pumping sta-tion for each of two submersible pumps in duty-standby operation as a function of flow. Each indi-vidual pipe installation includes a baseplate withbend, valves, an upper bend and a branch pipe.
µ 2La
-----=
3 Pump Performance
46
Pressure wave velocities in clean water in pipes ofdifferent materials can be obtained from the fol-lowing table:
Sewage and sludge often contain insoluble air orgas, which has a significant effect on the pressurewave velocity, as can be seen from the followingtable, where the pressure wave velocity isexpressed as a function of the quantity of insolu-ble air in the liquid:
Dissolved air has no practical effect on pressurewave velocity.
The pressure transient resulting from a change inthe flow velocity within a reflection cycle can becalculated with the following equation:
(35)
where∆h = pressure change (m)a = pressure wave velocity (m/s)∆v = flow velocity change (m/s) during one
reflection cycleg = acceleration of gravity (9,81 m/s²)
Since it is difficult to establish the change in flowvelocity when the pump starts or stops, exact cal-culations of the pressure transient cannot be eas-ily performed. Only if, for example, a valve isclosed within the reflection cycle, and the flowvelocity change ∆v is equal to the flow velocity v,can the pressure change be accurately calculated.Because the pressure fluctuates symmetrically,the pressure may fall below vapour pressure,causing cavitation with resulting high pressuretransients and noise. Potential locations for theseare pump, valve and pipeline high point. The highgrade vacuum may also cause the pipeline to col-lapse.
In sewage pumping the water hammer pressuresinduced during pump stop are higher than thoseinduced at pump starting. In theoretical computa-tions the objective is to calculate the water retar-dation immediately after pump stop and thepressure transient induce at that instance. Themost uncertain and significant factors to find outare the pump flow resistance and lowest pressuregenerated in the pump, after the supply currenthas been cut off. This information is not readilyavailable from the pump manufacturers.
Another uncertain factor is the air or gas contentin the water or pipeline. The solution here is toanalyze for different concentrations in order tofind out the effect of the gas content.
Figure 59 shows the outcome when observing thewater hammer phenomenon in a twin pumpinstallation. The following is noted:• Measured reflection cycle duration is 45 sec-
onds. Theoretical calculations for a fluid with-out gas or air indicated a duration of only12…20 seconds. The difference between thetwo values shows that gas is present in thewater.
• Immediately after pump stop the pressure inthe pipeline falls to vacuum. Because thepressure was measured at the dischargeflange, the pressure inside the pump musthave been even lower. It is likely that the pres-sure inside the pump fell below cavitationpressure (-10 m).
Significant for the water hammer phenomenon isthat it cannot be heard, since the pressure surge isfairly slow, but it can be observed with a pressure
Pipe material Velocity (m/s)
Steel 900...1300
Cast iron 1000...1200
Reinforced concrete 1000...1200
Plastic 300...500
Head = 15 m
Quantity of insolubleair as volumetric ratio
Velocity ratio ofpressure wave
0 1,0
10-6 1,0
10-5 0,96
10-4 0,73
10-3 0,32
10-2 0,11
∆ha ∆v⋅
g--------------±=
Pump Performance 3
47
gauge. Noise will be emitted only in case of cavi-tation or if a valve closes rapidly.
Water hammer is not a common problem in sew-age installations. Theoretical description of theproblem is difficult because of the large numberof unknown entities.
3.4.5 Avoiding Water HammerIf water hammer occurs in sewage installations,the situation can be alleviated with one or severalof the following measures:• Preventing of simultaneous stopping of two or
more pumps.• Installing automatic valves with closing times
of 20…30 seconds instead of regular checkvalves. Pump stops after valve has closed.
• Stopping pumps slowly with frequency con-trol.
• Using soft start equipment also for stopping ofthe pumps. Complete control of stoppingsequence not always possible.
• Installing automatic air relief valves at pointswhere negative pressure occurs.
• In cases of cavitation in pump during the stop-ping cycle, the installation of a by-pass suctionline with check valve from the wet well to the
rising main will prevent the pressure fromdropping inside the pump. Dimension of by-pass pipe should be selected one size smallerthan pump pressure flange size.
• Using heavier pipe components that will with-stand water hammer pressure. Vacuum tran-sients may be more critical to the pipeline andequipment than pressure surges.
Head [m]
Pum
p 1
sta
rts
Pum
p 2
sta
rts
Du
ty p
oin
t
Pum
ps
1 &
2 s
top
Pres
sure
su
rge
Time [s]
Fig. 59
Water hammer sequence pressure measurementsas function of time.
3 Pump Performance
48
3.5 Pump Duty Point
3.5.1 Single Pump OperationBy adding the geodetic head (Hgeod) and the pip-ing loss (HJ), the rising main head is obtained. Thegeodetic head is a constant independent of theflow, whereas the losses increase with approxi-mately the square of the flow rate Q (see Figure55). If a pump head curve drawn to the same scaleis overlaid or plotted on the rising main character-istic curve, the point of operation will be the inter-section of the curves. At this point the pump headequals the head required by the rising main. Thepump flow rate Q can then be read directly off thediagram as illustrated in Figure 60.
3.5.2 Parallel Operation, Identical PumpsParallel operation is the situation where the com-bined flow of two or more pumps is directed intothe same rising main. The shape of the character-istic curve for the rising main will change slightlywith the different numbers of pumps operating,since each pump has its own discharge line up tothe common point, and the rising main constitu-tion will therefore vary.
Assuming two identical pumps with identical sep-arate pipework combined by a branch or headerto the rising main operating in parallel, we obtainthe characteristic rising main curve as illustratedin Figure 61. The duty point for both pumps isobtained by plotting the sum of two pump headcurves at constant head onto the rising main char-acteristic curve for two pumps.
3.5.3 Parallel Operation, Different PumpsWhen calculating the point of operation for twodifferent pumps operating in parallel, differentcharacteristics for the separate pipework up tothe header should be assumed. The followingmethod for obtaining the points of operation canbe used.
The losses for each pump in their separate pipe-work before the common header are checked first.These can be plotted in the graph as reductions ofthe heads, reducing the pump curves. The combi-nation of these reduced curves at constant headgives the combined head curve for the pumps. Theintersection of this curve and the rising maincharacteristic curve is the combined point of oper-ation. By drawing backwards from this point at
Pump point of operation (D) obtained by plottingpump head curve onto the rising main characteris-tic curve. Total head is the sum of the geodetic head(Hgeod) and the pipeline loss (HJ).
Fig. 60
Fig. 61
Operating points for two identical pumps operat-ing singly (B) or in parallel (D). Since the pipeline re-sistance increases with the flow rate, the combinedoutput of two pumps (QD) is always less than twotimes the output of a single pump. For practicalpurposes a single pump can be assumed to have theoperating point C.
Pump Performance 3
49
constant head to the reduced pump curves, theindividual pump operating points can be read atthe original pump curves straight above theseintersections. Likewise, the individual operationpoints with the pumps running singly areobtained by reading from the head curves abovethe intersection of the rising main curve and thereduced pump curves. The method is illustrated inFigure 62.
3.5.4 Serial OperationIt is possible to connect a number of pumps inseries in order to increase head. The combinedhead is obtained by adding the individual headsat constant flow. The complexity of the arrange-ment makes it warranted only in rare instances,and it is nearly always advisable to use a largerpump from the manufacturer’s range that can dothe job alone.
Submersible pumps can be connected in seriesonly if the boosting pumps are installed dry, thusmaking them different from the lead pump.
Another risk involving pumps connected in seriesis the possible failure of the lead pump, whichmay lead to cavitation in the booster pump due to
increased suction losses or loss of suction head.The designer should design the pumping plant sothat serial connection of pumps can be avoided,and make certain that there are pumps availablefor the intended duty point.
3.5.5 True Duty PointThe true pump duty point will almost always dif-fer from calculated. The reason for this is the inac-curacies in all numeric methods for calculation ofrising main losses, as well as the tolerancesallowed in the published pump performancecurves. Furthermore, the properties of the pumpwill change with use due to wear, and corrosion orsedimentation will change the rising main withage. Figure 63 shows the relation between perfor-mance tolerances. Pump inherent performancetolerances are discussed in detail in Section 4 ofthis book.
If the duty point is located on the low flow seg-ment of the pump Q/H curve, and the rising maincharacteristic curve is steep, the flow tolerancerange can be very large in proportion to designedduty point. This fact should be taken into accountwhen selecting the pump.
Fig. 62
Operating points for two different pumps discharg-ing into a common rising main. Losses in the indi-vidual pipework are reduced from the pump headcurves plotted to scale in the graph. The combinedoutput curve is obtained using the reduced headcurves, giving the combined operating point D. Theindividual operating points are A and B. For thepumps operating singly, the points of operationwill be C and E, respectively.
Fig. 63
Tolerance area for the duty point. The true duty pointfor an installation may lie within the shaded arealimited by the allowed tolerance zones for pumphead curve and rising main characteristic curve.Pump output Q may vary largely.
3 Pump Performance
50
3.6 Sludge Pumping
Sludge of varying consistence is frequently beingpumped by submersible pumps in sewage treat-ment plant duty. With increasing solids content inthe sludge, the rising main flow losses willincrease while pump performance decreases.When selecting a pump for sludge duty these twofactors must be considered. The effect is illus-trated in principle in Figure 64.
The situation is complicated by the fact that notenough is known about the behaviour of sludge incentrifugal pumps. Treatment plant sludge mayhave high gas content, either dissolved orentrained, and this will have a profound effect oncentrifugal pumps. As a rule, sludge with high sol-ids content also has a high content of gas, whichwill lower pump performance significantly. Inextreme cases the pump will stop pumping whenthe separated gas accumulates in the impellereye, preventing it from developing the necessarycentrifugal force.
As a precaution when pumping dense sludge, thepump should be placed as low as possible, toensure positive suction head. The use of long suc-tion pipes should also be avoided, since the pres-sure drop in these is also increased by the solidscontent.
With a sludge solids content less than 1%, it is usu-ally safe to assume that the rising main character-
istic curve is the same as for water. With greatersolids contents the characteristic curve will behigher, but lack of data on the sludge makes theestablishment of a correct curve difficult. Anotherpractical problem is the fact that the solids con-tent of the liquid in the pump and rising main canmomentarily considerably exceed the mean ordesign value. In thicker sludges the pump motorcooling may become a problem, depending oncooling method.
Generally submersible sewage pumps are suitablefor pumping of treatment plant sludges with asolids content of maximum 3%. These sludgesinclude primary sludge, return sludge and excesssludge, whereas for denser sludges, such as thick-ened sludge and digested sludge, positive dis-placement pumps are preferred. For these thickerliquids the pumped volumes are relatively small.
Propeller pumps are not recommended in sewagetreatment plant duty because of the risk of clog-ging. For return sludge pumping a channel typepump in vertical column installation is a goodsolution.
3.7 Complex Rising Mains
Long transfer sewer lines frequently have complexprofiles, with low and high turning points. Air orgas trapped at the high turning points increasespump head, whereas the low turning pointincreases the risk of sedimentation. There arecases where a selected pump has proven to beinadequate, and cases of sedimentation are alsoknown. Exact forecasting of sewer main perfor-mance is difficult because of the intermittentpumping action of the pumps. Water in the mainmay move as little as 100 metres during onepumping cycle, and air or gas in the pipeline willnot be removed and the flow will not stabilize inthat period.
3.7.1 What Goes on in a Complex RisingMain?In Figure 65 the section YK-VP contains air. As thepump starts, the liquid level VP begins to slowlyrise and the air pressure in the section YK-VPincreases and a flow develops from point VP andto point PK (v2). As the pump stops, the flow fromVP to PK continues for some time, slowly decreas-ing. Because the duration of flow from VP to PK is
The effect of solids content in sludge on pump headcurve and rising main characteristic curve. The graphshows the principle only, and cannot be used for nu-meric evaluations.
Pump H curves
Rising main curves
H
Q3% Q2% Q1% Q0% Q
3%2%
2%
3%
1%
1%
0%
0%
Fig. 64
Pump Performance 3
51
longer than pump running time, the maximumflow velocity v2 is slower than v1. The low velocityof v2 and the rising section after point AK mayincrease the risk of sedimentation. The air or gasin the section YK-VP prevents the siphoning effectfrom forming, leading to increased geodetic head.
The exact location of point VP is difficult to esti-mate accurately. If the amount of air were con-stant, the location of point VP could be calculatedas a function of time. In practice the amount of airin the pipe will change, and the location of pointVP cannot be calculated. If YK is located lowerthan PK, the air could in theory be removed withan automatic air valve. If point YK is situatedhigher than PK, the air will flow back into the pipeafter the pump has stopped. Automatic air valvesare prone to clog up in sewage. A solution couldbe a hand-operated air valve that is opened at cer-tain intervals according to information on air orgas accumulation gathered over time.
3.7.2 Determination of HeadFor a rising main with a profile similar to Figure66, the pump total head is difficult to estimateexactly. An estimation of magnitude can be made,however. The minimum Head (Hmin) is deter-mined with the rising main completely filled andthe maximum head (Hmax) as a situation with alldownward sloping sections air or gas-filled. Thus
Hmin = Hgeod + pipe flow friction losses for totallength of rising main
Hmax = h1 + h2 + h3 + +hn + pipe flow friction lossesfor total length of rising main
Real total head is a value between the maximumand minimum value. A useful estimation may bethe mean value of Hmax and Hmin.
3.7.3 Pipe Size and Flow VelocityAs noted above, the air or gas collected in the ris-ing main will even the flow velocity in the follow-ing section, causing lower flow velocity in the lowpoints of the pipeline. This gives reason to choosea rising main of a dimension small enough toensure that the flow velocity does not fall too low.Minimum pipe dimension is DN 100, however.
A smaller pipe has also a smaller volume, mean-ing that the water moves a longer distance witheach pumping cycle, increasing the flow velocitiesat the low point of the pipeline. From an odourpoint of view a smaller pipe dimension is better,since the sewage stays a shorter time in the risingmain. A higher flow velocity may also carry outsome of the air in the pipe. In these cases thedimensioning flow velocity (v1) should be at least0,8 m/s, in more difficult cases even higher.
3.7.4 Choice of PumpIn a complex rising main the true head may differconsiderably from calculated. If the calculatedduty point is situated near either end of the pumpQ/H curve allowed section, this pump should notbe considered. A pump with a Q/H curve passingabove the calculated duty point should also beconsidered, since it offers security of choice andthe flow velocity increases.
3.7.5 Confirming MeasurementsSince true duty point may differ considerably incases with a complex rising main, it may be usefulto measure the volume rate of flow a few weeksafter pumping station commissioning, using thevolumetric method. Comparing measured valueswith calculated will show deviations and indicatetrue state of the rising main. The measurementscan be repeated a few times during the first yearof operation, since gas or air content in the risingmain may change.
Fig. 65
Rising main conditions.
Hgeodh1
h2 h3 hn
Fig. 66
Determination of head.
3 Pump Performance
52
Control measurements are necessary after com-missioning. All rising mains laid out in difficultterrains require careful planning and site-specificconsiderations and technical solutions.
3.8 Duty Point Evaluation for Parallel Pumping Stations
The combined output of two or more pumpingstations discharging at different points into thesame common rising main may be determinedusing a graphical method. The method isdescribed below.
Figure 67 presents graphically the situation wheretwo pumping stations operate in parallel dis-charging into a common rising main. When bothpumping stations are operating, the pump oper-ating points are governed by the pressure at thejunction point 3, where the outputs of the pump-ing stations merge in the common rising main.The total heads for the individual pumping sta-tions can be separated into components as isshown in Figure 68. The heads comprise the fol-lowing components.
HJ 3-4 = Pipe loss in the common main
between sections 3 and 4Hgeod 1= Geodetic head for pumping station 1
Hgeod 2= Geodetic head for pumping station 2
HJ 1-3 = Pipe loss in separate portion of rising
main between points 1 and 3HJ 2-3 = Pipe loss in separate portion of rising
main between points 1 and 3
The loss in the common main HJ 3-4 is equal for
both pumping stations.
The combined output of two pumping stations isgraphically determined by following the steps ofthe procedure shown in Figure 69:1. The geodetic head Hgeod and the pipe loss in
the separate portion HJ are subtracted from
each of the pumping station H curves. The Hcurve is taken for one pump or two pumpsoperating, as the case may be. The pipe loss HJis also determined accordingly.
2. The head loss curve HJ 3-4 for the common
main is plotted
3. The reduced H curves 1 and 2 obtained in step 1is plotted onto the head loss curve both com-bined and separately (1+2).
Separate pipelines
Common pipeline Discharge
Hgeod 1
Hgeod 2
1
2
3
4
Fig. 67
Pumping stations in parallel operation. Defini-tions and heads.
Station 1 Station 2
HJ3-4
Hgeod 1
Hgeod 2
HJ1-3
HJ2-3
H2
H1
H
Fig. 68
Head components.
Pump Performance 3
53
4. The intersection point A between the com-bined pumping stations H curve 1+2 and thehead loss curve HJ 3-4 represents the combined
output Q1+2 at the discharge point.
5. A horizontal line is plotted through point A,intersecting the separate head curves 1 and 2in points C and B respectively. The correspond-ing flow rates at these points, Q1 and Q2, repre-
sent the pumping station individual outputs.6. Plotting the individual outputs Q1 and Q2 onto
the individual head curves for each pumpingstation, the operating point for each pump isobtained as the intersection points T1 and T2.
The operating points for the pumping stations
working singly are the intersection points C’ andB’ of the reduced individual head curves 1 and 2and the head loss curve HJ 3-4 as plotted in step 3above.
The procedure can be extended for installationswith even more pumping stations in a commonmain. Working out the various operating pointsbecomes, however, an arduous task. Large sewer-age systems comprise collection wells and gravitysewer sections, breaking the network into sepa-rate pressurised sections that each can be deter-mined exactly. It is therefore unlikely that verycomplex combined calculations will have to beperformed.
H curve of pumps H curve of pumps
H-curve H-curve
Station 1 Station 2
Hgeod 1
HJ 1-3H1 T1
Q1 Q
H
Hgeod 2
HJ 2-3
H2 T2
Q1Q
H
1
2
1
1
2
2
H
B C A
B'
C'HJ 3-4
QJ1+2 QQ'1Q1Q2 Q'2
in Point 3in Point 3
Fig. 69
Establishing operating points for pumping stations discharging into a common rising main.
4 Testing of Pumps
54
4 Testing of Pumps
Actual pump performance is established or con-firmed through testing. Tests may be conductedfor a number of different reasons and at variouslocations. In order to reach unambiguous results,testing standards have been developed andagreed upon. The standards require controlledand calibrated testing circumstances and aretherefore generally not applicable for field or sitetesting.
A pump manufacturer tests his pumps at theworks for both production development purposesand quality control. Corroborated tests may berequired to confirm that pump performance isaccording to the terms of purchase or to settleperformance disputes.
Field testing of pumps in actual installations willnot yield exact data on pump performance,because the precision of the testing arrange-ments cannot meet the terms of the testing stan-dards. These tests provide useful information onpumps and pumping stations, however, and maybe used for pumping station monitoring, if per-formed periodically or if suitably automated.
Testing standards are conventions that are agreedfor use as a gauge for pump performance evalua-tion. The presentation below offers methods fortheir interpretation. It has been kept brief for clar-ity, and therefore it can and should be used withthe appropriate standard text as immediate refer-ence.
4.1 Testing Arrangements
4.1.1 Production TestingTesting of submersible pumps under controlledcircumstances requires a testing facility built andcalibrated to the standards governing the testing.Testing facilities in a production line should alsobe designed for efficient handling and connectionof the pumps so as not to slow down the produc-tion process. Figure 70 shows the principle of asubmersible pump testing facility. The test rigincludes the necessary pipework and instrumentsfor pressure and flow rate measurement. The test-ing facility may also include various measure-ments recording devices as well as computingequipment for the processing and presentation ofthe measured data. For pump head the total head,including both the static and dynamic componentis used.
Pressure gaugeFlow meter Control valve
Q [l/s]
H [m]Z1.2
P2
2D2
D2
M
Fig. 70
Principle of a submersible production pumps testing facility, where water is circulated. All pipework is designed to pro-vide ideal and known operating conditions for the pressure gauge and flow meter for unambiguous readings.Obtained data are fed into computer for speedy results and evaluation service.
Testing of Pumps 4
55
Pump total head was established in equation 18and can be written:
(35)
where
= pressure gauge reading changed to head
Z1.2 = pressure gauge height above water level
= pump dynamic head at pressure measure-ment point
HJ = Head losses between point of measure andpump flange (calculated).
According to the testing standards, the point ofmeasure of pressure shall be at a distance 2 x D2
from the pump pressure flange. The distance shallconstitute of a straight pipe section.
4.1.2 Field Testing, Duty PointTesting of pumps in actual installations is usefulwhen information on pump performance withreasonable accuracy is required, or when pumpingstation performance over time is monitored.
The pump flow rate can be accurately estimatedwith the volumetric method, where the change ofwet well level in a pumping station with knowndimensions is measured against time. If theincoming flow in the pumping station cannot bestemmed for the duration of the measurements,the effect of this must be checked separately withthe pumps stopped. The pump flow rate can thenreadily be calculated.
Pump static pressure is measured with a pressuregauge connected to the submersible pump nearthe outlet flange. The pump total head can thenbe calculated using equation 35(HJ = 0). The testarrangement is shown in Figure 72.
If an accurate head curve for the pump is avail-
able, the pump operating point can be deter-mined without an estimation of the flow rate. Thesum of the static head and the gauge height Z1-2 is
Hp2
ρg------ Z1 2,
v22
2g------ HJ+ + +=
p2
ρg------
v22
2g------
v24 Q⋅
π D22⋅
--------------=
H
D
QQD
Z1,2
v22
2 g
p2 g
v22
2 g
Fig. 71
Using a pump head curve sheet for operating pointestimation. Measured static head and dynamichead function against flow rate are plotted. Pumpoperating point D is obtained graphically.
Pressure Gauge
Hose
Z1,2
D2
Fig. 72
Field testing of a submersible pump. The pressuregauge is connected to the pump pressure flange witha flexible hose. The height of the gauge above thewater level in the well during the event, Z1.2, is re-corded. Pump outlet diameter D2 is used for calcula-tion of dynamic head if flow rate is known. Air in theflexible hose must be removed after the pump isstarted.
4 Testing of Pumps
56
measured onto the H axis of the pump head curvesheet. The function of the dynamic head against anumber of flow rates is then plotted onto thesheet. Pump operating point will be in the inter-section of this curve and pump head curve. Theprinciple is shown in Figure 71.
Site conditions do not fulfil the testing rig require-ments of the testing standards. The results cantherefore not be used for pump acceptance tests.
A pressure gauge connected to the piping of apumping station can be used for an approximatedetermination of the pump duty point. The heightof the pressure gauge above the water level in thepump suction sump, the calculated pressurelosses between pump flange and point of mea-
sure as well as the dynamic head (v2/2g) shall beadded to the reading. For dry-installed pumps thelosses in the suction piping shall be deducted.These losses are usually minimal.
4.2 Acceptance Tests
Pump acceptance testing is the procedure withwhich a pump is confirmed to have the propertiesset out in the manufacturer’s sales literature orcontract specifications. Acceptance tests can beroutinely carried out by the manufacturer as partof the manufacturing process or may be executedin the presence of the customer or his representa-tive.
The testing standards contain two major princi-ples:• The pump is tested at a duty point agreed
upon at the time of purchase.• The pump is tested at any point along the
curve published for the pump. This practice isintended for pumps in serial production, andthe allowed tolerances are larger than thosefor custom-made pumps.
In the case of pumps in serial production, thepump manufacturers perform production tests oftheir pumps either at a multitude of points alongthe curve or at three distinct selected points.These three points are selected at both ends ofthe allowed portion of the pump curve, and at apoint at the middle of the curve.
The essence of the testing standards is to governhow the tests should be conducted technicallyand what are the allowed performance toler-ances, unless otherwise agreed. The testing stan-dards do not regulate actions to be taken in case apump fails to perform according to the tolerances,or the consequences thereof. The parties shouldseparately agree on these issues at time of pur-chase or later.
4.2.1 Testing Standards The purpose of the testing standards is to definein detail how the tests are performed and how thetest results shall be technically compared with theguaranteed values. The content of the standardsis mainly as follows:• terms, definitions and symbols• organizations of tests• test arrangements• measuring uncertainties• verification of guarantees
Unless otherwise agreed, the testing standardsstipulate the following guarantee values to becompared:
Testing Standard ISO 9906 (Grade 1 and 2)Pumps over 10 kW power:• Q/H duty point• Efficiency η or ηgr
Pumps less than 10 kW power:• Q/H duty point• Efficiency η• Motor power input Pgr (over the range of oper-
ation)
Pumps produced in series with selection madefrom typical performance curves (Annex A):• Q/H duty point• Efficiency η
• Pump power input P• Motor power input Pgr
Testing Standard ISO 2548 (Class C)• Q/H duty point• Efficiency η or ηgr
Pumps produced in series with selection madefrom typical performance curves (Annex B):• Q/H duty point• Motor power input Pgr
Testing of Pumps 4
57
Testing Standard ISO 3555 (Class B)• Q/H duty point• Efficiency η or ηgr
These standards contain performance values forthe tolerances of the measured variables.
If guarantee values are required in specificationsor sales contracts, the following variables accord-ing to the testing standards are suitable:• Q/H duty point• Efficiency ηgr or η
The desired duty point and testing standard to beused must also be specified.
The testing standards do not require testing ofthe pump NPSHr value unless specificallyrequested. NPSH tests are difficult and time-con-suming, and do not provide absolute informationon the possibilities of cavitation, as can be seen inSection 1.4.3. The benefits of NPSH tests are thusquestionable. The testing standard ISO 9906 pro-vides tolerance factors for the NPSHr value. In thetesting standards ISO 2548 and ISO 3555 no toler-ance factors for NPSHr are provided.
Testing Standard ISO 9906 Grade 1 and 2The new testing standard ISO 9906 was publishedin 2000 and is scheduled to replace the older test-ing standards ISO 2548 and ISO 3555.
Grade 1 requires higher accuracy, whereas Grade 2allows larger tolerances. Because sewage pumpsusually operate in intermittent duty, Grade 2 issuitable for these pumps. Grade 1 is intended forthe testing of fine-tuned process pumps in contin-uous duty. For the verification of guaranteed val-ues, a crosshair method is used. The principle isshown in Figure 73.
The verification principle as shown in Figure 73works in the following way:
A tolerance cross with the horizontal line ±tQ·QG
and the vertical line ±tH·HG is drawn through theguarantee point QG, HG.The guarantee on the head and flow rate has beenmet if the measured Q/H curve cuts or at leasttouches the vertical and/or horizontal line.
The efficiency shall be derived from the measured
Q/H curve where it is intersected by the straightline passing through the specified duty point QG,HG and the zero of the Q, H axes and from where avertical line intersects the η curve.
The guarantee condition on efficiency is withintolerance if the efficiency value at this point ofintersection is higher or at least equal to ηG (1-tη).
Testing Standards ISO 2548 (Class C) andISO 3555 (Class B)The ISO 2548 (Class C) corresponds generally tothe standard ISO 9906 Grade 2 and the ISO 3555(Class B) to the standard ISO 9906 Grade 1. Thestandard ISO 2548 is suitable for sewage pumps.
In these standards an elliptic graphic method forthe verification of guaranteed values is used. Theprinciple is shown in Figure 74. The efficiency veri-fication is performed in the same way as in stan-dard ISO 9906.
The verification principle as shown in Figure 74works in the following way:
An elliptic tolerance zone with semi-axis QGXQ
and HGXH is drawn with the guaranteed dutypoint QG, HG as centre point.
The guarantee on the head and flow rate has beenmet if the measured Q/H curve cuts or at leasttouches the ellipse.
Measured curve
QG Q
H
HG
G
+ tQ QG- tQ QG
+ t H
XG
. .
.
- tH XG.
Gt
Fig. 73
Verification of guarantee on flow rate, head andefficiency according to ISO 9906.
4 Testing of Pumps
58
The efficiency shall be derived from the measuredQ/H curve where it is intersected by the straightline passing through the specified duty point QG,HG and the zero of the Q, H axes and from where avertical line intersects the η curve.
The guarantee condition on efficiency is withintolerance if the efficiency value at this point ofintersection is higher or at least equal to ηG (1-tη).
Grundfos has developed an application method ofthe elliptic tolerance zones of the ISO 2548 stan-dard, making it more convenient to use innumeric calculations. The method uses the slopeof the tangent to the Q/H curve at the point ofexamination, and makes it possible to numeri-cally determine Hmin and Hmax at guaranteed rateof flow, so that the ellipse condition is met.
Other Testing StandardsMany countries have issued national standardsequivalent to the ISO standards. In the U.S. anational testing standard issued by HydraulicInstitute is frequently used. This standard differsfrom the ISO standards with regard to the toler-ance system.
Allowed True Performance Deviations The maximum possible deviation from the guar-anteed duty point is composed of inaccuracies ofthe measurement technology and the allowedtolerances. The testing standards specify theaccuracy requirements of the measuring instru-ments and guide values for the allowed tolerances.
The maximum possible true deviation fromdesired volume of flow is also dependent on theshape of the rising main characteristic curve andlocation of duty point on the pump Q/H curve.According to the standards ISO 9906, Grade 2 andISO 2548, the deviation near the optimal pointmay be ± 3 …10%, depending on main curve shape.For the standard ISO 9006, Grade 1 and ISO 3555,the corresponding deviation is ± 2…6%. If the dutypoint is in the low volume flow range, and the ris-ing main characteristic curve is flat, the deviationsabove may be much greater.
The normal tolerances for pump efficiency accord-ing to the ISO testing standards are as follows:• ISO 9906, Grade 2 -5%• ISO 2548 -5%• ISO 9906, Grade 1 -3%• ISO 3555 -2,8%
These are proportional values, not percentagepoints.
For sewage pumping the ± tolerances of the stan-dards ISO 9906, Grade 2 and ISO 2548 are quiteacceptable. They are also compatible with normalproduction variations in manufacturing. Morestringent requirements may cause extra costs inproduction and delivery delays. The publishedcurves of sewage pumps are also based on thesestandards, a fact stated on these curves.
Sometimes a condition stipulating that negativetolerances are unacceptable may be raised by cus-tomers. Problems and misunderstandings are acommon consequence of this, with difficulties forboth manufacturer and customer. The ISO testingstandards do not recognize asymmetric tolerancesystems, and the published curves of the manu-facturers are based on the symmetric tolerancesystems of the ISO standards. If the customerfinds that a true volume flow lesser than indi-cated by the published curve is unacceptable, abetter solution than requesting no-negative toler-ances would be to increase the required volumeflow by 3…10% and then select a pump based onthis figure.
Requiring no-negative tolerances for the pumpefficiency does not make much sense, since pumpmanufacturers would then be forced to lower thepublished nominal figures. Too low, conservativefigures would not be representative of the major-ity of the pumps and lead to misunderstandingsand confusion.
�
Guaranteed point QG, HG
Measured curve
HG XH
QG XQ
QG Q
Q
HG
H
H
Fig. 74
Verification of guarantee value on flow rate andhead according to ISO 2548 and ISO 3555.
Pumping Stations 5
59
5 Pumping Stations
The working environment for submersible pumps,regardless of size, is the pumping station. Pump-ing station design and construction is decisive forthe performance of the pumps, and care and dili-gence should therefore be exercised wheneverspecifying them. The following is a primer ofpumping station design offering hints and advicefor the design engineer and the operator ofpumping stations. Some aspects of pump opera-tion and interaction with the pumping stationpipework is also discussed.
5.1 Pumping Station Basic Design
The decisive factor for pumping station operationis a good hydrodynamic design. A bad pumpingstation design may lead to pump malfunction,uneconomical pumping and frequent needs forpumping station service and cleanout.
Modern sewage pumping stations are designedfor pumping of unscreened sewage, and thedesign criteria for these differs from those forclean water. In the following the design and spe-cial requirements of sewage and stormwaterpumping stations are discussed.
5.1.1 Wet Well Volume and Surface Area The wet well effective volume should be of correctsize. Too large a volume may lead to accumulationof sludge in the well, whereas too small a volumeleads to too frequent starting and stopping of thepumps. The use of modern submersible pumps,with high allowable starting frequency, leads tosmaller and more efficient pumping stationdesigns.
The effective sump volume is the volumebetween pump start and stop levels, and it can bedetermined with the use of nomograms as a func-tion of allowed starting frequency. A method forthe calculation of the effective sump volume ispresented in Appendix B of this book.
In reality the incoming volume in a pumping sta-tion varies greatly over time, and the mean start-ing frequency will therefore be lower thantheoretical.
In a good design the start and stop levels shouldbe relatively close to each other for the followingreasons:• Pump starting frequency becomes high
enough to prevent sludge an impurities fromsettling onto well floor.
• Pumping station inlet should stay low relativeto the liquid level in the wet well.
A guideline maximum value for the effective vol-ume height in small pumping stations is approx. 1m and 2 m in larger pumping stations.
The effective volume can be substituted with thewet well surface area using the following equa-tion:
(36)
whereAW = wet well surface area in m²
Q = pumping station total flow rate, l/s
For small pumping station flow rates, however,the surface area will be limited by the physicaldimensions of the pumps when submersiblepumps are used. The surface area will then be
AWQ20------=
B = 1,5 DC = 0,8 D
Fig. 75
Recommended pump installation dimensions forsubmersible pumps.
5 Pumping Stations
60
larger than obtained with equation 36. Recom-mended pump installation dimensions are shownin Figure 75.
For larger flows the direction of approach towardsthe pumps should be square on. If the flow comesfrom behind, the submersible baseplates disturbthe flow causing eddies to form. These impedepump operation, lowering pump performanceand efficiency and increasing the risk of cavitationand pump vibrations.
5.1.2 Pumping Station Inlet PipeLocation and size of the pumping station inletpipe is important for the function of the pumpingstation. Problems encountered in operation of thepumps are frequently caused by bad inlet pipedesign.
An inlet pipe located too high relative to the liquidsurface or with a high flow velocity may causeentrainment of air and the formation of eddies inthe water when splashing into the well. Air mixedinto sewage water has a tendency to remainbecause of the possibility of air bubbles to adhereto the solid particles present. A separate calmingchamber may therefore not alleviate the situationat all.
Inlet fall height should always be minimized andshould not exceed 1 m with the water level down,regardless if the pumping station has a separatecalming chamber or not. The effect of a high inletfall height cannot be effectively alleviated withbaffles.
Air entrained in the water has a tendency toremain inside the pump impeller, where the cen-trifugal force makes it accumulate around theimpeller hub. This may lead to increased powerrequirement and lowered performance and effi-ciency. The risk of cavitation and pump vibrationsalso increases. If the amount of air in the pump isvery high, the pump may cease to function alto-gether.
Air is frequently a problem for pumps drawingdirectly from aeration basins in treatment plants,because of the high content of air. If a pump isplaced in an aeration basin it should be placed aslow as possible, with the suction pipe near thebottom.
Location of the inlet pipe should be as remote aspossible from the pump inlets. Figure 76 showsdesigns to be avoided.
Flow velocity at the inlet should not exceed 1,2 m/s so as to avoid the formation of eddies in the wetwell.
5.1.3 Wet Well Floor ShapeThe shape of the wet well floor is important forthe functioning of a sewage pumping station. Agood design prevents bottom sedimentation, butmay also assist in the prevention of scum forma-tion and the accumulation of flotsam on the sur-face. The following principles should berecognized in a good bottom design:
All corners should be benched to a minimumbench angle of 45°, in small pumping station thebench angle may be as high as 60°. The anglemay be smaller, if the section is flushed by a flow.
Fig. 76
Inlet locations to be avoided. Too high an inlet fallheight may lead to entrained air reaching the pumpinlet directly or along the bench surface, with con-sequent operational problems in the pumps.
Pumping Stations 5
61
Bottom area should be minimized and liquid vol-ume below pump stop level should be kept to aminimum.
Minimizing the bottom area and the residual vol-ume, the flow velocities near the inlets willincrease, flushing out possibly settled sludge. Asurface area decreasing with the falling waterlevel leads to less accumulation of surface debris.
5.1.4 Stop LevelsThe start and stop levels are specified at thedesign stage. They should always be checked forfunction and possibly altered at commissioning inorder to secure good operation.
The stop level should be as low as possible, so thatthe flow velocity increases toward the end of theworking cycle. Limits for the stop level are set bythe required motor cooling submergence or bythe level when air becomes sucked into the pumpintake. The latter level cannot always be foreseen,but must be confirmed at trials during pumpingstation commission.
In pumping stations with two submersible pumpsin duty-standby configuration the stop level cannormally be set below the motor even if themotor is cooled chiefly by submergence, see Fig-ure 77. The identical pumps are selected so as tobe able to cope with the pumping station flowalone, and the risk of the liquid level remainingfor long near the stop level is slight. Submersiblepumps also have protective devices against over-heating that stop the pump in case of inadequatecooling conditions.
In pumping stations with a multitude of sub-mersible pumps running under varying condi-tions, the stop level must be set so that the pumpmotors always have enough submergence foradequate cooling. Pumps with cooling water jack-ets or other means of heat dissipation indepen-dent of submergence are preferred in suchinstallations.
The stop level setting for dry-installed pumps isdependent on the suction pipe inlet height, shapeand flow velocity. 200 mm above the suction pipeinlet is a good rule-of-thumb for this height, anduseful for the designer. The shape of the suctionpipe inlet is important, and good designs areshown in Figures 78 and 79. For this inlet shape aprovisional pump stop level height can be calcu-lated using the following equation:
(37)
hs1 = E+a
hs1
hs2
K
E
hs2 = E+k/2a = 100-300mm
Fig. 77
Recommended stop levels at the design stage. hs1 = stop level for two submersible pumps in duty-standby operation or pumps with cooling indepen-dent of motor submergence. hs2 = stop level for mul-tipump installations with motors cooled bysubmergence. Final stop level settings should be de-termined during commission trials.
Reducing
Dp
D2
D1L R
F
hs
0,2 m
G
45bend
Fig. 78
Recommended installation dimensions for verticaldry-installed submersible pumps. F = 0,5 · D1, v1max = 2,0 m/s, G = Dp, L ≥ D1 + 100 mm, R ≈ L.
hs 0 04 Q 0 2,+,=
5 Pumping Stations
62
wherehs = stop level height, m
Q = pump flow rate, l/s
In pumping stations with several different stoplevels, such as in frequency-controlled installa-tions, it is important to program the controlsequence to pump down to lowest stop level atleast once per day to clean out the bottom.
5.1.5 Start LevelsIf the wet well surface area AW is dimensionedusing equation 36, the first start level in a pump-ing station with two submersible pumps in duty-standby configuration can be set 1 m above thestop level. Where small inflows are encountered,the start level may be lower. The second startlevel, can be set 0,2...0,3 m above the first.
In pumping stations with more than two pumpsthe starting levels should be considered from caseto case. If the pumps have a common stop level, asuitable design would be with the first start level 1m above stop level and the following start levelsat 0,3 m intervals from this. If the pump stop lev-els are staggered should the start levels be set ator near equal intervals.
In pumping stations with dry-installed pumps thestarting levels have to be set above the pump cas-ing in order to ensure that the casings fill up andthe pumps start pumping. For vertical pumps, thisheight may be considerable and should be setwith a margin according to Figure 78.
Horizontal pumps do not normally require specialconsiderations for the start levels, if the suctionpipe is designed to prevent air pockets from form-ing, see Figure 79.
5.1.6 Suction Pipe Dimension and DesignDesign and dimensioning of the suction pipe isimportant, with bad designs possibly causingvibrations, lowered pump efficiency and risk ofcavitation.
The suction pipe should be dimensioned so thatthe flow velocity does not exceed 2,0 m/s for ver-tical pumps and 2,5 m/s for horizontal pumps.When new, bigger pumps are installed in oldpumping stations, these figures may have to beexceeded. The situation must then be considered
from case to case. Wider NPSH safety marginsmay be warranted.
Recommended suction pipe inlet designs areshown in Figures 78 and 79. The downward suc-tion exerts a cleansing flow on the pumping sta-tion floor, and is less prone to suck air from thesurface.
In vertical pumps the suction pipe will have toturn 90° to reach the pump suction cover. Thebend before the pump suction inlet is crucial forthe function of the pump, since it causes the flowto be irregular. Too sharp a bend may cause impel-ler cavitation, lower pump efficiency and causevibrations. If the pump suction inlet is smallerthan the suction piping, a reducing bend shouldbe used, minimizing the interference. Figure 78shows suction bend dimension recommenda-tions.
The contraction of the straight inlet pipe to a hori-zontal pump should be eccentric so as to avoid airfrom collecting and possibly blocking the impeller.
An inlet design with unfavourable flow charac-teristics may cause a pressure drop large enoughto spend the available NPSH and lead to pumpcavitation. The recommended NPSH marginshould be observed in installations where the suc-tion pipe geometry gives reason to concern. Theconcept of cavitation and NPSH and recommen-dations for NPSH margins are presented in detailin Section 1 of this book.
Eccentric
D2
D1
F
hs
0,2 m
45
reducer
Fig. 79
Recommended installation dimensions for horizon-tal dry-installed submersible pumps. F = 0,5 · D1,v1max = 2,5 m/s.
Pumping Stations 5
63
5.1.7 Pumping Station Internal PipeworkThe internal pressure pipework in a pumping sta-tion should be selected for a flow velocity of 2...3m/s. Especially if the sewage contains sand shouldthe flow velocity be at least 2 m/s, in order for thesand to be carried with the flow out of the pump.In frequency-controlled installation this require-ment may cause problems at low frequencies. Fig-ure 58 in Section 3 shows size recommendationsand losses typical for piping. The pipework shouldhave a dimension of at least 100 mm but can be80 mm in small pumping stations, provided thatthe pump free passage is 80 mm.
The use of flexible joints in the internal pipeworkis not recommended, since most pipe vibrationsare pressure-induced by the flowing liquid, andcannot be avoided by the use of flexible joints.When installing flexible joints the pipe is cut, andthe section will be subject to a separating forcewith a magnitude of pump pressure x area. Thepressure near the pump is pulsating at a fre-quency determined by pump speed and numberof impeller channels, causing the piping and jointto vibrate. The vibration is more pronouncedwhen flexible joints are installed. Flexible jointsare also susceptible to damage.
The pressure pipework is normally expanded afterthe pump, and in order to save energy these tran-sition pieces should have a conical shape with amaximum shank angle α of 10°. Please see Figure79.
For vertical dry-installed pumps and submersiblevortex pumps the check valve should be installedas far away as possible from the pump in order toalleviate possible problems with air in the pumpat start-up.
For horizontally installed large pumps, where theshaft bearings include separate bearings for radialand axial forces, the check valve must not beplaced directly in the vertical pipe from the pumpdelivery flange. Possible shocks from a rapidlyclosing valve may pound at the pump hardenough to gradually damage the radial bearings.
In multipump installations the pump pressurepipes should be joined by a branch designed toprevent settling of solids during pump stoppageinto the individual pipes, which may lead to valveblockage. Good branch designs are shown in Fig-ure 80.
5.1.8 Flushing DevicesPumping station flushing devices consist of aremote-controlled by-pass valve mounted on thesubmersible pump before the connection to thepipework. When the valve is opened, the pumpflow is directed back into the wet well, agitatingthe liquid and causing settled sludge and scum todisperse. The suspended matter will then bepumped out with the liquid when the flushingvalve is closed.
The flushing valve should be of the normallyclosed type (e.g. a spring-loaded pneumaticdevice) so that in the case of a malfunction thepumping action will be able to proceed.
In a correctly dimensioned and shaped pumpingstation wet well flushing devices are normally notneeded. They serve a function in old, large wet
Fig. 80
Pressure pipework branch designs. The designshould emphasize smooth transition and preventsludge in rising main from settling on valves inpump risers when the pumps are stopped.
5 Pumping Stations
64
wells and in special situations, where the sewagecontains large amounts of e.g. grease. A flushingdevice can also be retrofitted without changes towet well structures. The flushing devices arebrand specific and detailed information is avail-able from the pump manufacturer.
5.1.9 Odour Problems in Pumping StationsA sewage pumping station may cause odour prob-lems in its immediate environment. Many factorsaffect the situation, such as pumping stationlocation, sewage quality, situation before thepumping station and wet well dimensions anddesign. If the pumping station is fed by another,remote pumping station, the sewage transfertime between the pumping stations may be soextensive that the sewage turns septic by anaero-bic action. Septic sewage produces Hydrogen-Sul-phide (H2S) that, apart from being toxic, alsocreates a typical foul odour.
The occurrence of odour problems is practicallyimpossible to predict. In case of severe problems,they may be attempted to be corrected by the fol-lowing measures:• Lowering start and stop levels, in order to cut
the retention time in the wet well and preventsludge from forming.
• Installing a submerged inlet bend in the wetwell, in order to convey the incoming sewagebelow the surface, thus preventing aerosolsfrom forming.
• Installing air filters in the wet well ventilators.• Dosing odour-preventing chemicals into the
sewer upstream from the pumping station.
5.1.10 Pumping Station Design ExamplesWet well design will depend on pumping stationsize and flow volume. Figures 81…84 show princi-ples for wet well design for various cases andpumping station sizes. A pumping station withsubmersible pumps for large flows can bedesigned according to Figure 83. If the pumpsrequire it, the stop level can be set at height hs2.
The flow velocity vD in the expanding section of
the wet well must be high enough to avoidsludge settling. A suitable value for vD is 0,1...0,3
m/s when the liquid is at stop level. The dimen-sion D can be calculated using the relation
(38)
whereQ = pumping station flow rate, l/s
D Q1000 VD C⋅ ⋅--------------------------------=
Fig. 81
Pumping station design for submersible pumpsand relatively small flows (Q = 4...50 l/s). Thepreferred cross section of small pumping sta-tions is circular, which minimizes liquid surfacearea and avoids corners where sludge could ac-cumulate. Minimum diameter 1,5...2 m to facil-itate service workovers.
Pumping Stations 5
65
vD = flow velocity in expanding section,
0,1...0,3 m/sD, C = pumping station dimensions, m
Fig. 82
Pumping station design for two submersible pumps and moderate flows (Q = 50...2000 l/s). The elongated wet wellshape is an important feature that places the inlet pipe away from the pumps and prevents the build-up of sludge onthe wet well floor.
5 Pumping Stations
66
stopvmax = 1,2 m/s
Fig. 83
Pumping station design for several submersible pumps and large flows. If the pumps are depending on submergencefor cooling, the stop level hs2 is chosen accordingly.
Pumping Stations 5
67
5.1.11 Dry-installed Pump PositionsFor dry installation most manufacturers can offerpumps for both vertical and horizontal installa-tion. Usually a pump in horizontal position offersadvantages, such as:• simplified piping with less bends• suction flow to the impeller is even• lower pump position.
For larger pumps the NPSH safety margin require-ment may not be met for pumps in vertical posi-tion, because of pump location and greatermargin requirement, whereas a horizontal pumpwill be acceptable. All possible pump duty pointsmust be considered when performing NPSH cal-
culations for installations where more than onepump is operated simultaneously.
Large pumps for horizontal installation are fittedwith slide bars for easy removal of pump motorfrom the pump housing. Please refer to Figure 17in Section 2.
vmax = 1,2 m/sstop
Fig. 84
Wet well design for a pumping station with multiple dry-installed pumps. Flow velocity across the suction bends vo =0,3...0,4 m/s with the liquid at stop level. Pump internal distance B can be selected as for submersible pumps, whereasthe distance M should be selected according to inlet fall height, and should ensure an even flow at the suction inlets.
5 Pumping Stations
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5.2 Package Pumping Stations
5.2.1 Out-of-doors Pumping StationsPackage pumping stations are made ready at afactory for installation on site. The material usedis glass-fibre reinforced plastic (GRP) or, forsmaller pumping stations, Polyethylene (PE), andthe stations are made complete with all internalpipework and other components in place. Thusthe installation is reduced to the excavation of thesite, laying of a foundation and connecting thestation to the incoming sewer and rising main,and connection of the control panel to the powersupply and possible telemetry connections.
The buoyancy of the pumping station whenempty requires it to be anchored to a foundationor concrete slab, which also may be prefabricatedand matched to the pumping station foundationbolts. The concrete slab mass can be calculatedusing the following equation:
(39)
whereMB = concrete mass (kg)
VG = volume of pumping station below water
table (m³)
The pumping station must be vented to preventthe build-up of toxic or explosive gases. If there isrisk of freezing, the upper part of the pumpingstation can be insulated.
Packaged pumping stations are fitted with accesscovers that may be made of aluminium or galva-nized steel and moulded into the structure. Theinternal pipework can be either cast iron or thin-walled stainless steel with fabricated bends andbranches. Valves should be cast iron and suitablefor use in either horizontal or vertical position.Figures 86…88 show typical package pumping sta-tion arrangements.
Wet well-dry well pumping station outlines. Pumpscan be installed vertically (A) or horizontally (B). Sub-mersible pump construction is protected against ac-cidental flooding of the dry chamber. A separatesump pump is provided in the dry well for drainageof leakage water. The pumping station control panelcan be installed on the top or in the dry well spaceabove flood level.
Fig. 85
MB 2000 VG⋅=
Pumping Stations 5
69
Fig. 86
Package pumping station with separate above-ground service building. Wet well collar serve asfoundation for the building.
Fig. 87
Typical package pumping station. The wet well iscomplete with folding work platforms for valve ac-cess and service.
5 Pumping Stations
70
5.2.2 Indoor Pumping StationsPumping stations for very small capacities can beinstalled indoors, for instance in basements ofbuildings close to the source of the effluent. Thesemay be designed as containers with the pumpsintegrated or mounted externally. Figure 89shows a typical arrangement.
5.3 Pumping Stations with Column-installed Pumps
Pumping stations with column-installed pumpstypically have large pumping capacity, andespecially axial pumps are sensitive to the condi-tions in the suction chamber. Figure 90 showsrecommended distances between pumps andbetween pumps and wall sections. It is veryimportant that the flow feeding the pumps iseven and that the flow velocity at this point doesnot exceed 0,5 m/s.
Fig. 88
A package wet well-dry well pumping station. Roundwet well shape adds strength and facilitates manu-facturing. Dry-installed submersible pumps are safeagainst flooding and are easy and clean to maintain.Intermediate platform offers access to control panel,mounted above flood level.
Fig. 89
Pumping station for small flows. The pump may beintegrated into the structure of the container andcan easily be removed. Air-tight construction is suit-able for indoor installation, and the unit can be in-stalled near the effluent source.
Fig. 90
Pump distance and suction flow velocity recom-mendations for column-installed pumps.
Pumping Stations 5
71
Pump immersion must be sufficient in order forsuction vortices to be avoided. Figure 91 shows adimensioning recommendation diagram accord-ing to the proposed new CEN standard (draft). Theappearance of suction vortices is still impossibleto completely predict beforehand. Pump charac-teristics and flow conditions in the suction cham-ber influence the development of suction vortices.The suction chamber shape may induce vortices
to appear unexpectedly. A vortex can be pre-vented by placing a float on top of it, if possible.
Pumps are frequently installed in columns in amanner where the water exits straight upwardsthrough the column. In these cases the pumphead and energy use can be changed by the topdesign. A good working design is shown in Figure92. The pump column is terminated well below
10 50 100 500 1000 5000
3
2
1
3
2
1
Fig. 91
Pump immersion depth recommendation according to proposed CEN (draft) standard.
1
Top outlet design and head determination for column-installed pumps.
Fig. 92
5 Pumping Stations
72
the weir, allowing the flow to smoothen beforeflowing over the weir. In this design the pumphead can be calculated with workable accuracyusing the equations in Figure 92. Losses in the col-umn can be disregarded in practical examina-tions.
5.4 Pumping Station Dimension Selection
Pumping station dimensioning is based on theexpected incoming flow, which usually must beestimated without the use of collected data.Guidance values cannot always be applied, sinceflow rates depend on a great number of variables.Figures are available from the sewerage systemsdesigners or, less accessible, from technical litera-ture on the subject. The possibility of seweragesystem future expansions must also be consid-ered as reserve capacity or flexibility in pump sizeinstallation. Sewage pumping station incomingflow is also typically greatly varying with time,both in short and long cycle.
Incoming flow estimation always starts with theanalysis of the possible constituent parts. Theseare normally classified as• residential sewage• industrial effluent• stormwater (rain and melting snow)• leakage waterOf these, leakage water is water entering the sew-erage system from ground water leaks, leakingwater mains or stormwater inadvertently enter-ing a separate sewage system through manholesor other entrances, such as worksite excavations.
To correctly dimension a pumping station, thetype of use must be known. Sewerage systems areclassified as • sewers for regular sewage, receiving domestic
and/or industrial effluents only• stormwater sewers, handling stormwater only• combined sewers, handling both regular sew-
age and stormwater in various proportions.
5.4.1 Regular Sewage Pumping StationsFlow rate estimation of residential sewage is nor-mally based on population numbers. The flowrate varies in daily and weekly cycles, the variationbeing in the range of 0,5...1,5 times average flow.Industrial effluent must be estimated on a case bycase basis, depending on the plant type in ques-tion. The amount of leakage water presentdepends on a number of variables, such as watertable level, local rainfall and soil characteristicsand general condition of the subterranean pipe-work. It can be estimated as units per pipelinelength unit, e.g. kilometre or as a ratio related toquantity of the sewage.
The possibility of flooding at the pumping stationwith consequent environmental damage must betaken into account. For this reason sewage pump-ing stations have two pumps in duty-standby con-figuration, with each pump capable of handlingpeak flow. Thus flooding will not occur in situa-tions when one pump is out of order or being shutdown for service. If pumping station capacity isbased on two pumps operating in parallel, a thirdpump should be provided as standby. Estate orother private pumping stations may be equippedwith a single pump, since the incoming sewageflow can easily be controlled by restricting theusage of facilities.
5.4.2 Stormwater Pumping StationsRain water flow rates are considerably larger thanother stormwater sources, such as melting snow.Dimensioning of the system is based on the larg-est anticipated amount that will reach the pump-ing station. This amount may not necessarily bethe most severe torrent, for stormwater sewersare allowed to flood under heavy rain circum-stances because of the relative harmlessness ofrainwater. The design values are also affected byflood tolerance of the area and the type of urbanenvironment in question. Leakage water additionmust also be considered.
Controlled flooding at the pumping station isarranged with overflow weirs, discharging in asuitable direction, such as a ditch or canal.
Stormwater pumping stations do not have thesame required reliability factor as sewage pump-ing stations, and they can be designed to handlethe maximum flow with all pumps running inparallel duty.
Pumping Stations 5
73
5.4.3 Combined Sewage Pumping Sta-tions and Retention BasinsThe dimensioning flow rate for combined sewagepumping stations is the sum of the estimatedsewage, stormwater and leakage water flowrates. Reliability requirement is the same as forsewage pumping stations, making it necessary todimension them with at least one pump asstandby. Combined sewage pumping stationscombine in an unfavourable way the properties ofregular sewage and stormwater pumping sta-tions, and their use is therefore discouraged.
In conjunction with both stormwater and com-bined sewage pumping stations, retention basinsmay be used for temporary storage of incomingsewage that exceeds the installed pumpingcapacity. As the flow decreases (such as afterheavy rainfall), the basin is emptied by pumpingor through gravity sewer, and normal pumpingstation operation can be resumed. Retentionbasins may also be used to even out fluctuationsof the incoming sewage flow to a treatmentplant. Essential in a retention basin is to preventsolids from settling onto the basin floor whenemptying. This can be accomplished by designingthe basin shape “self-cleaning” or by agitatingand mixing the basin content. Special ejector mix-ers have been developed by pump manufacturers,
consisting of an ejector drawing air from the sur-face combined with a submersible pump. Thedesign is shown in Figure 93.
The air provided by the ejector makes the mixingmore effective at low water depths. The requiredpumping power can be estimated at approxi-
mately 70 W per m2 basin bottom area. The ejec-tors should be placed and directed so that the jetsflush the solids towards the basin drain.
Fig. 93
Ejector and pump for retention basins.
5 Pumping Stations
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5.5 Pump Selection
5.5.1 Pump Selection Based on PumpCurvesThe pumps for a pumping station project are ini-tially selected using the methods described inChapter 3 of this book. It is good practice to con-sider a number of pumps from a manufacturer'srange that have curves passing near the desiredoperation point.
Pumps having curves both above and below theinitial requirement should be included, becauseother considerations, such as pump efficiency andcost may be economically decisive factors.
The intermittent character of sewage pumpingstation operation allows a wide margin for thepump selection, giving the designer freedom ofchoice beyond a fixed nominal point of operation.Theoretically calculated operating points areuncertain in any case, since actual head may varydue to changing start and stop levels broughtforth by programmed level control, pump wearand tolerances in pipeline and pump characteris-tic curves.
For instance, a pump with a head curve higherthan originally desired may offer better overalleconomy, especially if the pipeline characteristiccurve is flat, or the dynamic losses are small com-pared to geodetic head.
The pumping station designer is therefore welladvised to select a pump from the manufacturer'sstandard range and to refrain from requestingexactly tuned pumps. The use of standard pumpswill also simplify pump spare part service andlater pump replacement, if needed.
The pump selection should be checked so as tomake sure that the operating point under any cir-cumstances does not fall outside allowable rangeof the pump curve. Operating range restrictionscan be imposed for a number of reasons such asrisk of cavitation or vibration, or overloading. Thefollowing should be checked:• Single pump duty points in installations with
several pumps pumping in parallel into com-mon rising main. Duty points for situationswith one, two and more up to and including allpumps operating in parallel.
• Effect of liquid level variations on pump dutypoint. Liquid level may vary in suction sump aswell as in discharge reservoir. If the pump dutypoints moves into the cavitation area (NPSHr>10 m) because of rising suction level and thusincreased suction head, the pump can nor-mally be used without consequences, becausethe NPSHA will increase correspondingly. Pumpcavitation is thus prevented, and only pumppower requirement and available motor powermust be confirmed. It is recommended thatthe pump manufacturer is consulted in uncer-tain cases. It is especially important to checkall possible level combinations for propellerpumps, since these have very narrow allowedQ/H bands, because of strongly varying powercurves.
5.5.2 Observing Pump EfficiencyWith larger pumps, the pump efficiency becomesincreasingly important for the pump selection.When warranted, whole life costs calculationsshould be performed for a number of alternatives.Please refer to Section 7. All duty points at differ-ent duty situations should be taken into account.The following three different cases should beexamined separately:A. Two pumps installed in duty/stand-by configu-
ration, or all pumps have separate risingmains. In these cases the pumps have only one dutypoint (if the variations in suction liquid levelare disregarded), and the situation is fairlyeasy from an efficiency point of view. Pumpselection should not be based on having apump Q/H curve passing near the desired duty,if the pump best efficiency point falls far awayfrom it. Another pump with a Q/H curve pass-ing above the desired duty point, but with a farbetter efficiency may be found in the sameprice range, and be a far better choice.
B. Several duty pumps with common rising main. In this case the pumps may have several dutypoints, depending on number of pumps inoperation. Normally the duty point is selectedfor the situation when all duty pumps are inoperation at the same time. In order for thepump efficiency to be as good as possible withless pumps running, the pump should be cho-sen so that pump best efficiency point lies tothe right of the main duty point, please seeFigure 94.
Pumping Stations 5
75
C. Pumps used with frequency control.In order for the efficiency to be acceptable alsoat low frequencies, and for the Qmin to besmall enough, the best efficiency point shouldlie to the left of the main duty point. Pleaserefer to Figure 95.
D. Several duty pumps with common rising mainand frequency control.For this case it is likely that the best choice is apump with the best efficiency point coincidingas closely as possible with the main duty point.
Frequently more than one pump, even from thesame manufacturer, may be considered for adesired duty point. One alternative may offerlower costs but have lower efficiency thananother. The decision between these pumpsshould in principle be based on a whole life costanalysis. This assessment will frequently have tobe performed by the customer or his consultant,since the pump manufacturer usually does nothave all relevant information. The position of thebuyer may also be significant, since a contractormay stress purchasing price over operating costs,whereas the owner will look at total costs.
Unfortunately, the pump manufacturer fre-quently has to select and offer pumps with verylittle or no information on the project, and pumpselection may therefore not be optimal, or it may
even be incorrect. In particular, information onthe following items is important:• Will more than one pump use a common rising
main? In this case the rising main characteris-tic curve or the number of duty pumps andvalue of geodetic head are needed.
• Information on frequency control usage.• For pumps in column installation information
on nature of liquid is needed, in order to ascer-tain the possibility of using axial propellerpumps.
5.5.3 Number of PumpsSewage pumping station pumps are selected sothat at least one pump always is on standby. Espe-cially in larger pumping stations, the number ofpumps should be selected so as to optimize pumpusage and investment cost. The cost of pumpingcapacity, or pump power in kW, decreases withincreased pump unit size. On the other hand, therequirement of one standby unit will increase thecost of redundancy if few very large units areused. Installation costs are therefore almost con-stant for a given capacity, regardless of number ofpumps used to meet the requirement, at leastwithin a reasonable range. Likewise, the energycost will remain almost constant, if the pumpsconsidered can run near the optimal operatingpoint.
Selection point
max
Fig. 94
Several duty pumps with common risingmain. Pump ηmax should lie to the right of the selec-tion point.
Selection point
max
Fig. 95
Frequency control operation. Pump ηmax should lieto the left of the selection point.
5 Pumping Stations
76
Factors affecting pump number selection mayalso be the requirement of even or continuousoutput, which is easier to accomplish with a largenumber of pumps.
Unless special requirements are put forth, theoptimal number of pumps for most small tomedium size pumping stations is two.
In pumping stations with several pumps it is nor-mally good practice to select identical pumpsonly. In some special cases where the incomingflow fluctuates randomly and to a great extent,e.g. as a consequence of rain storms, it may besensible to install larger pumps that run only dur-ing peak situations.
The effect on investment cost by variations in thenumber of pumps installed for a given pumpingrequirement may vary from one manufacturer toanother, since pump size increments are differentfor different manufacturers. Thus increasing thenumber of pumps may lead to a more favourableinstallation for one manufacturer and to a moreexpensive one for another. Where a multitude ofpumps are required for the operation of a pump-ing station, the final number could be left openfor the bidding manufacturers to decide withingiven limits. Thus a larger number of bidders arelikely to be able to offer competitively.
5.6 Special Considerations
5.6.1 Pump VibrationsMost sewage pumps vibrate, at least to someextent. Vibrations are caused by residual mechan-ical unbalance of the rotating parts, pressure pul-sations incited by the impeller vanes and thehydrodynamic radial force caused by the fluidmass rotating with single-vane impellers. Vortexpumps vibrate much less, since they do not inducepulsating pressures. For pumps with volute cas-ings the residual imbalance is negligible as com-pared with the other vibration factors. Improvingalready good balancing procedures by the manu-facturer does not have a measurable effect onvibrations in the pump.
Sewage pump impellers (except for vortex impel-lers) induce higher rates of vibrations than impel-lers for clean water, because of the small numberof vanes and large channels. Pump installationmethod also has an important impact on thevibration level. A submersible pump resting on abaseplate stays in place by its own weight only,which increases vibrations as compared to a fixedinstallation. A vertically installed dry pump mayvibrate more than a horizontally installed pumpbecause of a different support structure. The suc-tion bend required may also enhance vibrationlevels.
Vibration prediction and calculation informationfor sewage pumps is available in the EuroPumppublication “Guide to Forecasting the Vibrationsof Centrifugal Pumps”, 1992 EuroPump. The valuespresented in this book are guide lines and validwhen measured at the main bearing closest tothe pump impeller. Any vibration velocity above10 mm/s (RMS) measured at this point indicatesan abnormal situation in the pump. The reasoncan be clogging of pump, operation outsideallowed section of pump Q/H curve, severe cavita-tion, and high content of air in liquid or damagedimpeller. Possible mechanical imbalance can becontrolled by running the pump out of water,when the vibration reading should be less than 2mm/s (RMS).
Vibration frequency for pumps with volute pumpcasings equals rotational speed times number ofimpeller vanes. If the pump or piping is supportedin such a way that the natural frequency of these
Pumping Stations 5
77
elements is near the exciting frequency from thepump, the system resonance will increase vibra-tions. In these cases the support structures mustbe stiffened. For a frequency controlled pump thesystem may vibrate more at some frequencybecause of resonance.
The pressure pulsation induced by the pumpmoves ahead in the pressure piping with the liq-uid flow several metres, causing vibrations in thepipe wall. Normal piping vibration levels arebelow 10 mm/s (RMS). Greater levels may lead topipe failure. The reason can be inadequate pipesupport or resonance.
Column-installed pumps have low vibration levelsbecause of trailing vanes in the casing, effectivelydampening the pressure pulsation. A single-vaneimpeller may still cause pressure pulsationsbecause of the strong hydrodynamic forcesinduced.
5.6.2 Pump NoisePumping station noise level is affected by the fol-lowing elements:• pump vibration noise• piping vibration noise caused by pressure pul-
sation from the pump or other transmittedvibrations
• flow in piping. Pipe bends, tee branches andvalves cause disturbances in the flow, emittingnoise
• pumping station acoustic characteristics• inlet stream in wet well• pump cavitation.
The sound level in the pumping stations com-posed by all the above constituents, and soundinformation on the pump alone is not very useful,and cannot be accurately measured on site. Noiseemitted from the piping is usually decisivebecause of the large vibrating emission surface.Correct pump sound level measurement wouldrequire the pump to be located in sound insulatedspace, with the piping on the outside. There areno standards available for allowed sewage pumpsound levels. Sound level measuring proceduresfor submerged pump units is difficult to definebecause of the practical difficulties involved.
Pumping station noise is not a common problem.A pumping station built in connection with a resi-dential or office building may in some cases cause
noise problems. In severe situations the pipingand dry-installed pump motors can be clad withsound-proofing insulation.
6 Frequency-controlled Sewage Pumps
78
6 Frequency-controlled Sewage Pumps
6.1 General
The reasons for using frequency control are inprinciple as follows: • levelling of flow for process-related technical
reasons• energy savings made possible by favourable
rising main characteristic curve.
Process-related technical reasons for frequencycontrol are found in the following applications:• return sludge pumping applications• recirculation pumps in nitrogen reduction pro-
cesses• treatment plant incoming pumping stations.
Alternatively the incoming flow can be regulatedby increasing the number of pumps in the finalpumping station. With smarter pump control andincreased starting frequency the output can belevelled. Treatment plant basin and channeldesign can also be used to level the flow in theplant. In other pumping stations the use of fre-quency control should be considered only if majorenergy savings can be expected.
Frequency control brings energy savings only ifthe rising main is long and the geodetic portion oftotal head is less than 40%. In installations with ahigh geodetic head the energy consumption isbound to rise with frequency control, since thepump duty point will move to a section of thepump Q/H curve, where pump efficiency is lower.Losses are experienced in the frequency controlunit and it lowers the pump motor efficiency.Whenever a frequency controller is being consid-ered for the objective of energy saving only, thepay-back period of the investment in the controlsystem should be calculated separately. For thisthe variations in flow, pump efficiency at differentfrequencies must be known. The latter is alsodependent on the rising main characteristic curve.Also the efficiencies of the frequency controllerand the pump motor at various frequencies mustbe known.
Frequency control increases risk of pump clog-ging. Should frequency control be selected, anumber of conditions and facts should be consid-
ered when designing the pumping station andchoosing the pumps.
6.1.1 Pump Motor SelectionThe supply current modulated by frequency con-trollers is not perfectly sine wave shaped, causingthe motor efficiency to decrease slightly. Nor-mally, however, considering pump duty point andoperation conditions, the pump standard motorcan be used, provided that nominal supply fre-quency (50 or 60 Hz) is not exceeded. The choiceof motor should be confirmed with the pumpmanufacturer, since he will have complete infor-mation on the motor power and heating charac-teristics.
6.1.2 Maximum FrequencyIn installations with one frequency-controlledpump and one or more unregulated pumps in par-allel operation, output control will be irregular atthe point where a fixed-speed pump is added toor taken off duty, unless the frequency-controlledpump is allowed to operate at higher than nomi-nal frequency at that point. Typically the requiredover frequency is 53 Hz for 50 Hz pumps. The situ-ation is shown in Figure 96 below.
H2
H
HJpipe
fN fmax fmin
(fN)
(fmin)
Q1 Q2Q
Fig. 96
Parallel operation of speed-controlled pumps. Onlyone pump is speed-regulated at the time. Q1 = Nomi-nal output of one pump, Q2 = Regulated output ofone pump at minimum speed. Regulated output ofone pump is extended to Q1 + Q2, preventing repeat-ed starting and stopping of pumps at full speed. fN =Nominal frequency (50 or 60 Hz), fmax = Maximumfrequency, fmin = Minimum frequency, η = efficiency.
Frequency-controlled Sewage Pumps 6
79
If all pumps in a parallel installations are fre-quency-controlled, the situation with irregularcontrol will not arise, and regulation above nomi-nal frequency will not be necessary.
If regulation above nominal is needed, this mustbe stated on all inquiries, in order for the pumpmanufacturer to be able to allow for it in thepump and motor selection process. In some casesthe use of a standard 60 Hz pump for a 50 Hzinstallation with frequency control is favourable,but this solution should be weighed against fol-lowing drawbacks:• If the frequency controller is out of order and
the pump can be run at nominal 50 Hz only,the output of a 60 Hz pumps falls to 50…80%of that of a 50 Hz pump, depending on risingmain characteristics and friction losses.
• The motor is likely to have special windingsbecause of the differences in voltages in stan-dard 60 Hz pumps as compared to standard 50Hz pumps, which will impede future spareparts service.
• Pump efficiency may be lower, since some 60Hz pumps are converted from 50 Hz by reduc-ing impeller diameter.
6.1.3 Minimum Frequency and MinimumPerformanceAllowed minimum frequency for a specific pumpis often inquired. A comprehensive answer to thisquestion requires information on the installationand rising main, since the shifting of duty pointwhen reducing operating frequency is dependenton pump Q/H curve shape and rising main charac-teristics. It is recommended that the minimumfrequency is worked out from required minimumflow with the help of pump Q/H curve and risingmain characteristics.
Pumping sewage at too low flow (too low fre-quency) may lead to excessive pump wear fromsand or other abrasive matter remaining in thepump instead of being pumped out with the liq-uid. Too small a flow may also lead to clogging ofthe pump. If the geodetic portion of pump head ishigh (above 40%), the duty point will move to theleft of the pump Q/H curve, where pump effi-ciency is lower, and energy costs may increase. Asa general rule, minimum performance can be lim-ited to 25% of flow at best efficiency at nominalfrequency (Qopt).
If the rising main characteristic curve is steeplyrising, and several pumps may be running simul-taneously, it may be necessary to define severalminimum pump flow levels depending of numberof pumps in use at the same time. The pumpsmust then be controlled by a suitable, program-mable logic device. A minimum frequency deter-mined for the maximum number of pumpsrunning simultaneously, and then used in all situ-ations, with varying minimum performance asresult, depending on pumps in use.
6.1.4 Pump Frequency CurvesThe pump Q/H curves for different frequenciesare necessary in order to determine pump perfor-mance at various speeds against a given risingmain characteristic curve. Minimum frequencymust be determined and also pump efficiency atvarious frequencies. Pump curves for different fre-quencies are easily drawn based on affinity rulescalculations, but possible limitations on curveusage can be determined by pump manufactureralone. It makes sense to request frequency curvesfrom the manufacturer, with Q, H and η for differ-ent frequencies at e.g. 5 Hz intervals.
Frequency curves can be calculated based on theaffinity rules using the following equations:
(40)
(41)
(42)
(43)
(44)
The above equations are valid simultaneously forthe change of a given Q/H duty point and with anaccuracy acceptable for practical considerations.
Q' Qf'f---=
H' Hf'f---
2=
P' Pf'f---
3=
η' η=
NPSHR' NPSHRf'f---
2=
6 Frequency-controlled Sewage Pumps
80
6.1.5 Pump CloggingIn frequency control operation the pump cloggingrisk increases for the following reasons :• Liquid level in pumping station stays station-
ary from accurate pumping control, leading toaccumulation of debris on the surface and/orpit bottom. These may be larger than the freepassage of the pump, blocking the impeller atpump-out.
• The pumps run continuously for too long peri-ods, preventing the backwash at stoppingfrom clearing pump from debris accumula-tions.
• The pumps stop slowly as controlled by the fre-quency controller, preventing the backwashfrom clearing pump from debris accumula-tions. Smooth starting also prevents cleansingaction.
• Low speed in combination with rising maincurves and losses may lead to complicatedpump internal flow patterns, increasing clog-ging susceptibility.
Frequency control installations differ from eachother and a comprehensive prediction of pumpbehaviour is impossible. The risk of clogging maybe reduced with the following actions:• After pump start-up the frequency is con-
trolled in a manner that the suction well levelfalls steadily and reaches pump stop level in anhour under normal conditions, after whichpump is stopped.
• As above, in addition pump is programmed torun at nominal speed for 30 seconds beforestopping at stop level. This increases flow inpump sump and rising main, flushing out pos-sible debris accumulations.
• Pump is programmed for 1…2 flush-outsequences per hour by increasing pump speedto nominal for 20 seconds each, after whichpump is stopped without frequency control orthe frequency is lowered as quickly as possibleto minimum frequency, where it is allowed toremain for approximately 20 seconds, afterwhich normal operation is resumed.
• Pump is programmed to run in the wrongdirection for some time before each start. Thiswill remove any beginning clogging remainingin the pump from the previous running inter-val. The frequency when running the pumpbackwards should be lower than nominal inorder to avoid vibrations, i.e. 30 Hz for a 50 Hzpump.
• Using an automatic valve in lieu of non-returnvalve, and programming it to remain open forsome time at certain intervals after the pumphas stopped, allowing the back flow to washout the pump.
When the pump stops suddenly, the amount ofwater in the rising main continues to flow, effi-ciently flushing the freely rotating impeller, clear-ing possible beginning clogging.
The measures above can be accommodated in thepumping station planning stage but taken intouse only if necessary.
The use of frequency control for return sludge andrecirculation pumps does not increase the risk ofclogging. This is also the case for the final pump-ing station before the sewage treatment plant, ifthe sewage screening takes place prior to thesepumps.
6.1.6 EMC Cable RequirementAccording to the EU Council Directive on Electro-magnetic Compatibility (EMC), frequency control-ler manufacturers may require that pump motorcables are replaced with EMC compatible andapproved cables. The requirement of EMC cablesfor submersible pumps may complicate manufac-turing and add to costs. EMC cables also compli-cate pump handling, because these cables are lesssupple than regular supply cables. For submers-ible pumps the choice of frequency controller maybe dictated by requirement of EMC cables or not.Use of EMC cables may be avoided with the use ofemission suppressing filters in the frequency con-troller.
6.1.7 Bearing CurrentsIn some instances frequency control causes inter-ference currents through the bearings of large,air-cooled squirrel-cage motors, causing bearingdamage. It is likely that submersible motors areless susceptible to these currents, because theyare well grounded by the pipework and submer-gence in water, and thus protected. This assump-tion is supported by the Grundfos experience thatsubmersible motors have been free from bearingdamages to date. Adding insulation to the bear-ings would require extensive redesigning of themotors and add to costs.
Frequency-controlled Sewage Pumps 6
81
6.1.8 High Tension For supply voltages above 500 V, frequency con-trol may cause too high voltage fluctuations forstandard motors. In these cases the motors mayhave to be redesigned with special winding insu-lation and insulated bearings. The use of voltageshigher than 500 V are therefore discouraged incombination with frequency control.
6.1.9 Explosion-proof MotorsIn frequency control the motors may operate at ahigher temperature than normally. Thus an explo-sion proof certification of a motor at nominal fre-quency may be void for frequency controloperation. The ex-proof certification of a pumpcable is likely not to be valid for an EMC cable.
If an explosion-proof motor is intended for fre-quency control, this must be clearly stated in theinquiry documents, in order for the manufacturerto be able to correctly assess pump and motorsuitability. Adding frequency control to an exist-ing installation also warrants contacting the man-ufacturer for clearance.
6.1.10 Guaranteed ValuesThe essential requirement of pump performanceis that pump volume rate of flow matches thespecified demand and that pump energy costs areunder control.
In order to secure total output, the guaranteedduty point should be according to parallel opera-tion of pumps. If the rising main characteristiccurve is flat (high Hgeod) or each pump has its ownindividual rising main, the same duty point is alsosuitable for pump efficiency guarantee evalua-tion. On the other hand, if the rising main charac-teristic curve is steep or if the geodetic head isfluctuating, the determination of a rational guar-antee point for pump efficiency gets difficult. Theguarantee duty point of η may be different fromthat of volume rate of flow and head. It makessense to separately agree what duty point shall beused for evaluation of pump efficiency. This pointcould be the maximum efficiency point or theintersection point of pump Q/H curve at nominalfrequency and the rising main characteristiccurve. A duty point at other frequency than nomi-nal may also be chosen as guarantee point. Itshould be noted that, according to the test stan-dards, test pump speed may differ by ±20% from
specified pump installation duty speed, and thattest bed results are converted to duty point datausing the affinity law equations.
6.1.11 Tests with Frequency Controller(String Tests)If the overall efficiency of the combination ofpump and frequency controller shall be verified,the situation of the pump manufacturer is diffi-cult. This situation requires exact information onfrequency controller efficiency and pump motorefficiency at modulated current, when voltagealternation is different from unmodulated sinewave. These data are device-specific and almostimpossible to get accurate information on before-hand, and they must therefore be postulated. Thetest standard also do not specify tolerance valuesfor total efficiencies measured under these condi-tions. The frequency controller must also be madeavailable in advance to the pump manufacturerfor testing, further complicating things and add-ing costs. String tests are of little practical value.
6.1.12 Collaboration with the Pump Man-ufacturerDesigning and executing an installation with fre-quency control of pumps is much more compli-cated than simple fixed-speed pump installations.The close collaboration between pump manufac-turer and client is therefore important already atthe planning stage. Guaranteed duty points andtesting standard usage should also be agreedupon beforehand at contract negotiations when-ever possible.
7 Pump Whole-life Cost Evaluation
82
7 Pump Whole-life Cost Evaluation
The pump selection process should comprise apump life time cost evaluation, including the esti-mation of all costs of acquiring, operating andmaintaining the pumping plant over its fore-casted life span. The importance of life cycle costevaluations and comparative calculationsincreases with increasing pumping plant size. Forinstance, the energy costs of operating a mid-size(30 kW) sewage pumps over three years are equiv-alent to the original pump procurement costs.
7.1 General
Pumping plant whole-life costs are needed forproject financial and investment feasibility calcu-lations. For instance, in a pumping station renova-tion project, where old pumps are replaced withnew ones, the chief investment assessment crite-rion is life cycle cost evaluation. Correct long-termcalculations will have to take into account pre-dicted energy cost changes, inflation and interestrates in addition to pump life cycle costs. Thesecalculations require financial and project man-agement skills in addition to solid pump knowl-edge.
Life cycle cost calculations are commonly used forcomparison of pumps during purchasing. Thealternatives to consider are either different makesor different models from the same manufacturer.In these comparisons the financial elements nor-mally have the same proportional magnitude forthe various alternatives. As future changes inenergy costs and maintenance labour costs aredifficult to forecast, it makes good sense to sim-plify the comparative calculations to comprise lifecycle costs calculations at present-day cost level,without financial analysis. Thus the analysis canbe based on two different approaches:• The whole-life costs are calculated for the dif-
ferent alternatives at present-day cost level,and compared.
• A comparison based on the most inexpensivealternative is performed, where the pay-backperiods for those alternatives with lower oper-ating and maintenance costs are calculated.
It should be noted that these methods have fairlylarge error margins for energy and maintenancecosts because these items are based on forecasts,such as future pumped volume and wear rateestimations.
The decision may also be based on reasons ofprinciple or commercial grounds. Environmentalaspects may stress energy use and costs. If thepumps are part of general contract and purchasedby a contractor, purchase price alone may be deci-sive.
7.2 Calculation Period
The useful life time of modern sewage pumps is inthe magnitude of 25 years. A pumping stationmay become due for renovation much earlier, e.g.if changes in the neighbourhood lead to increasedpumping needs or zoning measures call for itsabolition or relocation. Also the unavailability ofspare parts may cause early pump obsolescence. Asuitable period for whole-life economic calcula-tions is therefore 8…10 years from commission.
7.3 Investment Costs
Pump purchase prices are obtained from the man-ufacturers by inquiry or negotiations. Final pricemay also include other commercial and purchas-ing costs, such as transportation. Also the effectsof different pump specifications on other acquisi-tion costs must be considered. A larger motormay, for instance, require a frequency converter ormains supply fuse of higher rating, adding invest-ment costs.
Figure 97 shows the proportional effect of pumpsize on pump cost for pumps of 1500 1/min nomi-nal speed. A pump with lower nominal speed willgenerally be more expensive than a pump of thesame rating running at higher speed, because oflarger size. The figure shows that for small pumpsof less than 10 kW rating, purchase price will bedecisive for the life cycle costs.
Pump Whole-life Cost Evaluation 7
83
7.4 Energy Costs
The energy requirement is correctly calculatedusing the efficiency (ηgr), since efficiency is sub-ject to the manufacturer’s guarantee according tothe testing standards, whereas the informationon power is not. It is important that the testingstandard to be used is agreed upon at this stage,since different testing standards have differenttolerances for pump efficiency, which can affectthe efficiency figures reported by the manufac-turer. Please refer to Section 4, Testing of Pumps,for more information.
7.4.1 Efficiency Over TimeWith the exception of vortex pumps, the pumpingefficiency of sewage pumps deteriorates overtime because the clearance between impeller andsuction cover widens from wear. This changeshould be taken into account when performingenergy usage calculations. Based on tests andexperience, the following efficiency reductions
factors can be used when performing calcula-tions:
• Closed impeller with adjustable suction clear-ance: -1,5% (ηgr points)
• Semi-open impeller with adjustable impellerclearance: -3,0% (ηgr points)
• Closed impeller without adjustable clearance:-3,0% (ηgr points)
• Semi-open impeller without adjustable impel-ler clearance: -5,0% (ηgr points)
The efficiency reduction factors above suggestthat in practise sewage is pumped with pumpshaving significantly lower efficiency than newpumps. The higher values for pumps withoutclearance adjustment possibilities are based onthe fact that these pumps run longer intervalsbetween clearance restorations, because partshave to be replaced in shop conditions. The effectis greater on pumps with semi-open impellers,since these pumps wear faster and the efficiency
Price/PN
200
100
010 50 100
PN (kW)5002
(%)
Fig. 97
Effect of pump size on pump specific cost Price/PN when pump nominal speed is 1500 1/min relative to 10 kW pump.
7 Pump Whole-life Cost Evaluation
84
is more sensitive to changes in the clearance (seesection 2.2.1 Impellers).
7.4.2 Energy Usage CalculationsThe energy calculation can be performed usingtwo different methods:
• Appraisal using yearly pumped water volume,first computing specific energy, using the fol-lowing equation:
(45)
where
H = pump head at duty point [m],
g = 9,81 [m/s2],
ηgr = overall efficiency (pump + motor) at dutypoint [decimal value],
Liquid density is assumed to be 1000 kg/m3.
The energy consumption is calculated usingspecific energy and estimated yearly pumpedvolume.
• Appraisal based on operating hours, first com-puting power at guaranteed efficiency, usingthe following equation:
(46)
where
Q = pump volume flow at duty point [l/s],
H = pump head at duty point [m],
g = 9,81 [m/s2],
ηgr = overall efficiency (pump + motor) at dutypoint [decimal value],
Liquid density is assumed to be 1000 kg/m3.
The energy consumption is calculated usingthe power obtained and estimated yearly
operating hours. If the pump Q/H curve passesabove the desired duty point the pump outputwill be higher and consequently, the operatinghours fewer. This must be considered whenusing the pump operating hour method.
The energy calculation methods are fairly simplewhen the pump is operated in a single duty point.The situation gets more complicated with pumpsin parallel duty and if the pump is used with fre-quency converter. In parallel duty, calculationsshould be performed separately for the differentduty points, and then by approximating thepumped volumes or operating hours accruing ineach of these.
With frequency converter the pump has an infi-nite number of duty points. A duty point, repre-sentative of the average pump duty should beselected for the calculations in these cases.Another uncertainty factor when calculating theenergy consumption for frequency-controlledpumps is the fact that the overall efficiency of thesystem is difficult to accurately determine. Forcomparative calculations the pump efficiencywithout frequency controller may be used.
7.5 Maintenance Costs
Normally submersible pumps are recommendedroutine maintenance on a yearly basis. Mainte-nance includes seal oil control, motor insulationcontrol with resistance meter, suction clearancecheck and, if necessary, adjustment, and generalsurface inspection. Most manufacturers recom-mend very similar routines. Distinctions betweenpumps from different manufacturers are mostclear in the possibilities of maintaining andrestoring pump efficiency.
If a pump is equipped with an adjustable suctionclearance mechanism, the costs for maintainingpump efficiency does not add to costs, since theadjustment can be performed during normal rou-tine maintenance on site. If, on the other hand,pump efficiency maintenance requires that spareparts be used or the pump brought to shop, thecosts for these measures will have to be takeninto account when the whole-life costs of thepump are calculated.
Espg H⋅
ηgr 3600⋅-------------------------- kWh m
3⁄[ ]=
Pgrg Q H⋅ ⋅
ηgr 1000⋅------------------------- kW[ ]=
Pump Whole-life Cost Evaluation 7
85
7.6 Cooperation With Pump Sup-pliers
Whole-life cost calculations and comparisons areseldom completely unambiguous, and it is there-fore reasonable and fair to perform these openlyand in cooperation with the pump vendors, atleast when considering a pump from that sup-plier. This way possible misunderstandings can beavoided, and suggestions and alternatives as pro-posed by the supplier can be taken into accountfor best possible selection.
7.7 Life Cycle Cost Publication
The pump manufacturers associations Europump(Europe) and Hydraulic Institute (USA) have jointlypublished a guide for pump life cycle cost (LCC)evaluation:PUMP LIFE CYCLE COSTSA GUIDE TO LCC ANALYSIS(ISBN 1-880952-58-0)
This publication deals with the complete pump-ing system from the design stage, existing pump-ing systems and examples of implementedimprovements.
8 Commissioning
86
8 Commissioning
During pump commissioning, the following itemsshould be inspected:• Check duty point(s) using pressure gauging
and possibly flow metering, using the volu-metric method, for comparison of these withprojected values and to confirm that true dutypoint lies within allowed limits of pump Q/Hcurve.In long rising mains with several high and lowpoints on the way to the discharge point, thetrue situation may take some time to stabilize.The measurements should therefore berepeated after some time after commissioningto confirm duty point.
• Check pump operation for vibrations andnoise. Check for signs of cavitation.
• Compare start and stop levels with projectedvalues and adjust if necessary. Lowest possiblestop level for dry-installed pumps should befound by trials, observing suction of air intoinlet pipe.
For submersible pumps combined with large vol-ume rates of flow, check for surface vortices atlow level. Adjust stop level where necessary.
Operation and Service 9
87
9 Operation and Service
Submersible sewage pumps should be subjectedto routine inspection and service yearly. Thescheduled maintenance should be performed onsite and includes• Oil check and replacement when necessary.• Suction clearance (impeller/pump housing)
inspection and adjustment, if the clearancehas widened to 2 mm or more from wear. Forpumps without adjustment possibility the res-toration of the suction clearance and pumpperformance requires installation of newparts.
• Motor insulation resistance metering at con-trol panel.
• Checking of lifting chain and lifting eyes andlugs.
• General pump inspection and operation con-trol.
The pump operator’s manual provides thoroughinformation on maintenance.
Routine maintenance can be performed by theowner or by contract service company. Impellerreplacement should be possible on site duringmaintenance, if necessary. Shaft seal replacementand other work on the watertight pump motorenclosure should always be referred to an autho-rized workshop.
Spare part availability is not a problem for sub-mersible pumps made by recognized manufactur-ers. The pump manufacturing series are long andparts kept in stock both for assembly of newpumps and for spare parts service. Pre-stocking ofspare parts is normally not warranted.
9.1 Safety
The most important risk factors associated withthe operation of sewage pumps relate to the fol-lowing:• electricity• lifting and handling of pumps• hot surface temperatures of dry-installed
pumps• handling of pump parts during service and
repairs• incidents of fire and explosions in hazardous
environments• health risks from human contact with sewage.
The following international standards deal withpump and pumping safety issues:
• EN 809 (1998) Pumps and pump units for liquids – commonsafety requirements
• prEN 13386 (1999) Liquids pumps – submersible pumps andpump units – particular safety requirements (proposal 2002)
10 Pumping Station Control and Condition Monitoring
88
10 Pumping Station Control and Condition Monitoring
All sewage pumping stations, either working indi-vidually or as a part of a sewage network com-prised of several pumping stations, should bereliably controlled in order to provide safe andefficient operation. Modern electronic controltechnology offers possibilities to design and buildversatile control and condition monitoring sys-tems to reduce long-term operation costs and toincrease operational reliability.
Unreliable sewage pumping stations represent anecological as well as economical risk in the form ofwaste water overflowing into the environment orbasements of buildings. Reliability is therefore theprime concern in design of a control unit for wastewater pumping station.
This chapter describes the sensors the reliablepumping station control is based on, differentcontrol methods concentrating on the modernstate-of-the-art control technology and finally anetwork level remote control and monitoring sys-tem and its future possibilities combining Inter-net and WAP technology.
10.1 Local Control Methods
Local control is always needed at site at the pump-ing station to control the operation of the pumps.The local control unit can be built to differenttechnical levels according to requirements of con-trol features as well as costs.
10.1.1 Manual Control UnitsManual control is the simplest control method. Itconsists merely of a switch (normally manual-off-auto) with the necessary relays and circuit break-ers to start and stop the pumps. Manual control isnormally never used as the primary pump control,but is used as a backup control method duringmalfunctions of the regular controls and duringpump repair and maintenance work to check thepump operation. Possibility for manual controlshould always exist.
10.1.2 Relay-based Control UnitsIn case pump condition monitoring is notrequired, automatic relay-based control units forlocal control purposes can be used. Relay-basedcontrols are simple units with fixed or adjustablestart and stop levels. They may include sequenc-ing of multiple pumps, or this may be accom-plished with additional pump sequencing units.
In case continuous level measurement is used,these control units may have freely adjustablestart and stop levels and local level display. Inmost cases, however, relay-based control unitsuse preset or manually adjustable level switches,such as float switches.
Relay-based control units are both easy to use andreliable due to simplicity of design. They are suit-able for small or secondary pumping stationswhere little or no operational flexibility isrequired.
10.1.3 Programmable Logic ControllersPump control units based on programmable logiccontrollers (PLCs) offer extensive possibilities forpump condition monitoring, data logging andanalysis as well as flexible pump control. Design-ing of a good pump control unit based on PLCs isdemanding and always requires solid knowledgeof the operation and requirements of a sewagepumping station, in addition to programmingskills. Selection of control and measurement sig-nals, pump and pumping station analysis andchoice of level measurement sensors are amongthe things that has to be considered.
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10.2 Sensors for Pump Control and Condition Monitoring
The pump control unit, simple relay-based or ver-satile PLC, requires different sensors to gatherinformation from the operation of the pumps aswell as the whole pumping station as shown inFigure 88. Such sensors provide information onwet well water level, current consumption of thepump, condition of the pump primary seal as wellas motor coil insulation and so on.
10.2.1 Wet Well Water Level SensorsThe basic information required by any automatedpump control system is the water level in the wetwell of the pumping station. There are multipleways to provide this information as there are dif-ferent types of information available as well.Depending on the sensor, the current water levelis given as continuous analog signal or on/offinformation as the water level passes certain, nor-mally preset, height positions.
By experience, pressure transmitters offer themost reliable and economic way for continuouslymeasuring the water level in sewage applications.Especially, a piezo-resistive pressure sensor, eitherembedded in a stainless steel enclosure or inte-grated in a sealed liquid-filled rubber construc-tion, is excellent for use in waste water. Pressuretransmitters provide continuous analog current(0…20 mA or 4…20 mA) or voltage (0…45 mV) sig-nal proportional to the water level.
The function of pressure transmitters is sensitiveto sedimentation, but this can be avoided by cor-rectly installing the transmitter inside a protectivepipe as shown in Figure 89.
Ultrasonic devices are the only choice if the levelsensors cannot be in contact with the liquid. Mod-ern ultrasonic sensors have built-in programma-ble functions for various operating conditions andranges. Ultrasonic sensors are also quite expen-sive.
Ultrasonic sensors are normally accurate and reli-able. On the other hand, in waste water applica-tions steam and foam on the liquid surface maycause level indication errors or complete loss ofecho, which can lead to interrupted level monitor-ing. Problems arising from such situations can beavoided with the installation of backup devicesfor the most vital functions, such as a float switchfor high level alarm.
Some earlier level sensors were based on capaci-tive sensors. This kind of sensor is also installed in
Fig. 88
Multiple sensors provide accurate information forthe pump control and condition monitoring unit.
Fig. 89
Correct installation of the level transmitter is essen-tial. The completely sealed transmitter is suspendedin the wet well and its piezo-resistive element trans-mits the level signal to the control unit. The sensor isused with electronic pump control units and pro-vides continuous level reading.
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the wet well and consists of a rubber or plasticbladder connected to a vertical pipe and contain-ing a reference liquid. A wire is suspended in thepipe and connected to a signal transmitter. Thelevel of the reference liquid in the pipe rises andfalls with the level of the content in the wet well.The capacitance of the wire-pipe elementchanges accordingly and the signal is transformedin the transmitter to a suitable signal for thepump control unit.
Capacitive devices are normally reliable butexposed to malfunctioning due to accumulationof sediments on the bladder at the bottom of thewet well. The device also needs careful installa-tion and more service than pressure transmitters.
Float switches have been used for level control inwaste water pumping applications for manyyears. They provide the simplest means for levelcontrol at fixed levels, but do not provide any pos-sibility for continuous level control. In multiplefloat switch installations there is always a risk forthe control wires getting entangled in each otheror the pump cables. Float switches, together witha relay-based control unit, are used today mainlyin small installations.
On the other hand, due to their simplicity and reli-ability float switches are even today quite oftenused as a backup or emergency level control sys-tem in larger units, too. This usage provides emer-gency operation in case of the main levelmeasuring equipment failing.
A level bell is another very simple level sensingdevice. It consists of an upside-down positionedplastic cone with air tube between the upper endof the cone and the controller. As the water levelreaches the cone, air inside the cone and the tubeis compressed causing pressure against a switchin the controller. As the pressure rises, the switchcontacts and the pump starts. With this kind ofdevices the pump is usually stopped after a pre-set delay.
10.2.2 Current SensorThe pump input current is monitored by a currenttransformer through which one of the threemains phase cables is routed. Each pump requiresone current transformer for adequate monitoringreliability. Current transformers provide an analogsignal (0…20 mA or 4…20 mA) proportional to thepump input current.
10.2.3 kWh MeterPumping stations with a modern electronic con-troller should always be equipped with a kWhmeter, which has a potential free pulse output. Asthe meter provides a certain amount of pulses foreach kilowatt hour used by the pumping station,the energy consumption can be monitored.
10.2.4 Phase Failure RelayAll the three mains phases are connected to thephase failure relay. This device provides an alarmsignal in case loss of power in the pumping sta-tion.
10.2.5 SARI 2 Monitoring DeviceThe Grundfos SARI 2 is a combined monitoringdevice for motor insulation resistance and seal oilwater content. The motor insulation resistance ismeasured between one of the mains phases andground when the pump is stopped and discon-nected from the mains supply. Low insulationresistance indicates moisture inside the motor,which could lead to the motor burning and expen-sive repair work.
In case the pump is equipped with a GrundfosOCT 1 oil condition transmitter, the SARI 2 alsocontinuously monitors the water content in theseal oil chamber. As the primary shaft seal wearsin time and water leaks into the oil chamber, theOCT 1 probe indicates the water content in theseal oil. This information is routed to the SARI 2monitoring device, which raises an alarm.
Fig. 90
Grundfos SARI 2 monitoring device. The SARI 2 ismounted on a DIN rail in the control panel. The SARI2 is intended for both stand-alone alarm use and forinterfacing with a remote monitoring system.
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10.2.6 ASM 3 Alarm Status ModuleAll the Grundfos submersible pumps areequipped with an internal moisture switch as wellas temperature switches embedded in eachmotor coil. These safety devices are connected isseries, and if any one of them trips, the controllerstops the pump and raises a safety device failurealarm. With the ASM 3 module these two alarms(moisture or overheat) can be separated to pro-vide accurate alarm information.
10.3 Pump Control Units
A modern and versatile pump control unit isbased on the use of microprocessors and controlsoftware. The unit is likely to be a PLC with a built-in application software for pump control and con-dition monitoring. The user interfaces with theunit to access necessary control parameters, suchas start and stop levels that can easily be checkedand adjusted. The complete control unit consistsof the electronic controller and an array of auxi-liary equipment, such as level sensor, currenttransformers, and phase voltage relays, etc., form-ing an integrated package. Figure 92 shows anintelligent electronic pump control unit.
10.3.1. Control FeaturesThe main parameter to measure is the water levelin the wet well. A continuous level indicator isalways used in this type of control unit. Severaltypes of sensors are available, such as a sealedpressure transformer and ultrasonic devices.
The pump control sequence is normally quite sim-ple. In a regular duty-standby application, the pre-set operation levels are stop level, start level andsecond start level. The duty pump starts when thewater in the wet well reaches the start level, andstops when the water has been pumped down tothe stop level. The duty pump is alternated ateach cycle in order to ensure even distribution ofusage and wear between the pumps. The standbypump starts at the second start level in a situationwhere the incoming flow is larger than the capac-ity of one pump. If more than one standby pumpis installed, these may be started at the same levelsimultaneously or at adjustable intervals, or atdifferent levels.
All the running pumps are stopped simulta-neously when the level reaches the stop level or atadjustable intervals. In some multi-pump installa-tions all pumps may have different start and stoplevels. This, however, makes pump conditionmonitoring calculations more complicated andless reliable.
In some cases a separate overflow pump withdifferent characteristics may be installed to han-dle large flows. This pump does not participate inthe sequencing and should be controlled by a sep-arate unit independently from the other pumps.
Fig. 91
Grundfos ASM 3 alarm status module.
Fig. 92
The GRUNDFOS PumpManager electronic pumpcontrol and condition monitoring unit. The unitcontrols all pumping station functions with soft-ware stored in a PLC circuitry. Operational parame-ters are set using the keypad and the LCD display ofthe unit. Pumping station data logged and calcu-lated by the unit are readable on the display by en-tering codes on the keypad or from a remotemonitoring system. A scanning function allows theoperator to get all important data with a minimumof keystrokes.
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Flow measurement is possible without a separateflow meter. It is done according to the volumetricmethod, where the changes of levels in a wet wellof known dimensions are measured against time.The unit software calculates both incoming andpumped flows with the same accuracy as that of amagnetic flow meter. The volumetric method isalso the basis for the measurement of the pumpcapacity, which is continuously measured as arunning average of ten latest pump actions.
In case of an overflow from a pumping station, itshould be possible to estimate the volume accu-rately and unambiguously in order for the opera-tor to handle possible claims of damage. Whenthe incoming flow at the time and the duration ofoverflow is known, the volume is estimated by theunit software for authority reporting.
Pump motor current measuring is necessary forprotection and condition monitoring purposes.With adjustable over and under current limits, theunit is set to protect the pump motor in abnormalsituations. In case the input current rises abovethe over current limit, where a burnout of themotor becomes a risk due to possible pump fail-ure or clogging, the pump is automaticallystopped. Together with embedded thermal relaysor electronic motor protectors in the pump motorstarter this offers a very reliable motor protection. An abnormally low input current indicates thatthe pump is not pumping normally, which may bedue to pump impeller wear or failure or gasentrapment.
Pump running hours and number of starts areinformation needed for scheduling of pumpmaintenance. These are also important informa-tion in verifying the pumping station operationaldesign and when determining the correct startand stop levels during the commissioning.
All the features described above are available inthe GRUNDFOS PumpManager control and condi-tion monitoring unit, and readable from the inter-face display. This enables the motor control panelto be simple without separate ammeters, hourcounters and sequencing relays, features that areall incorporated within the PLC.
With the control units entirely controlling thepumps by software, it is simple to embed specialfeatures within the program. Thus it is possible,for example, to let the pumping station pump
below the stop level to allow pump snoring (con-trolled dry running) at given intervals. This is toprevent sludge accumulation and cake formationon the wet well surface. In this manner possiblepump blockages can be avoided with savings inunscheduled maintenance costs. This has alsobeen found, by experience, to be an effective wayto prevent odour problems.
Another possible advantageous operational fea-ture is to let the pump starting level intentionallyfluctuate around its setting; this is to prevent asludge rim from forming on the wet well wall atthe start level.
The pump control unit is also programmed toindicate all operation failures in the station, suchas alarms for high level, low level, pump powerfailure, and other alarms based on settings ofparameter limits.
10.3.2 Condition Monitoring FeaturesThe pump control unit performs automatic pumpcondition monitoring based on the parameterslogged and analyzed. When the rate at which thewater level in the wet well rises and falls duringthe pumping cycle is monitored, pumping capac-ity of each pump can be calculated. The unit thencompares these values to the pump nominalperformance data stored in the memory andraises an alarm in case the performance is outsideset tolerance limits.
The benefit of such a system lies in its capabilityto give early warning for slowly developingdefects that ultimately could lead to sudden andunexpected pump failure and consequent envi-ronmental damage. Also developing pressure pipework problems can be detected by closely analyz-ing pump performance. Another benefit of such asystem is the monitoring of the operation fromthe economical point of view, where maintenanceactions can be planned and executed according toneed. This finally leads the pump servicing fromrepair-on-failure to preventive and even predictivemaintenance.
10.3.3 Parameters and SignalsThe pump control unit needs a number of param-eters in order to operate as required. The parame-ters are entered into the unit based on actualpumping station dimensions and units takenfrom the plans or measured at site. For calibration
Pumping Station Control and Condition Monitoring 10
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either actual dimensions or percentages of refer-ence values can be used. Values to be entered areusually various operating levels such as pumpstart and stop levels, low and high level alarm aswell as overflow levels, which all correspond tocertain water level in the wet well. Other parame-ters usually required are wet well dimensions andpump nominal values for the input current andcapacity, which are available from the pump datasheets.
Several signals are necessary for the pump controlto operate as planned. These are either digital oranalog. Digital signals are either input or outputsignals and indicate an ON or OFF status. Neces-sary digital input signals are pump running orstandby indication from the circuit breakers aswell as potential free contact signals from thephase voltage relay and energy meter when avail-able. Digital output signals are needed for start-ing and stopping the pumps.
Analog input signals from additional sensors areused for various continuous measurements. Thesesignals are, for example, pump motor windingand bearing temperature measurements, pumpsealing house oil condition information, datafrom an additional flow meter or frequency con-verter etc. The use of these signals may require anadditional extension card and special version ofthe application software.
10.3.4 Data Logging and AnalysisThe pump control unit must have sufficient mem-ory capacity to log and analyze data over a certainperiod of time. The unit has to log at least thehours run, number of pump starts and incidentsof abnormal pump motor current. The unit has toanalyze and calculate flow, pump capacity andoverflow from the logged data. The logged datacan be collected and further analyzed by down-loading the data at intervals to a portable PC withsuitable software, or continuously by an auto-matic remote control system.
Even in case the pump control unit operates as anoutstation of a network level control and monitor-ing system, it must have memory capacity to storethe logged and analyzed data for several days.This is due to the fact that vital data must not belost even during possible communication break-down between the outstation and control center.
10.3.5 User InterfaceTo access the data and to enter parameters, theoperator needs to interface to the pump controlunit. The interface must be at least a small LCDdisplay and a keypad. Using the keypad the usermust be able to enter all necessary parametersand read logged and calculated data. The use ofsuch interface must be simple and logical.
Normally some helpful features like an automaticscanning function, makes the routine checking ofthe data easy and fast. Separate LED signal lightsare used for indicating alarms and pump runningstatus.
10.4 Remote Control and Monitoring System
Waste water pumping stations are designed toincorporate extra capacity in case of too large aninflow or a pump failure. This reserve volume doesnot, however, prevent overflow in case the faultgoes unnoticed for a longer period. Scheduledmaintenance visits alone cannot prevent all con-tingencies possible in pumping stations; there-fore systems for remote control and conditionmonitoring as well as for alarm relaying havebeen developed.
The visible and audible alarms located outside ofthe pumping station used in the earlier years havebeen developed into sophisticated and decentral-ized remote control systems. These latest systemsconsist of PLC based pump control units control-ling and monitoring the local processes in thepumping stations. These control units also func-tion as telemetry outstations and are remotelyconnected to a central computer where specialnetwork level administration software is running.
10.4.1 Different Levels for Remote ControlThe modern pump control units enable theremote control and condition monitoring systemto be tailored according to the features requiredby the customer in comparison to the availableinvestment funds.
In case a very simple automatic alarm transfer ispreferred, the control unit can be equipped with aGSM modem, with which the alarms generated bythe control unit will be transferred to the GSMphone of the person on-duty as an SMS message.
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Such a system offers highly increased operationalreliability with modest investment costs sincethere is no control center at all. On the otherhand, the modern control unit is capable of utiliz-ing the whole SMS message by adding the mostimportant logged and analyzed information intothe alarm message. Such information could bethe pump running hours, number of starts, energyconsumption, pumping station inflow and pumpcapacities, for example. In case such a system cre-ates automatic reports and transfers them to theperson on duty on a weekly basis even withoutany alarm situations, the normal driving aroundto the pumping stations can largely be avoided.
In case a network level remote control and moni-toring system is introduced, there are severalways to build the communication link betweenthe outstations and the control center asdescribed in the following sections.
10.4.2 Software and HardwareThe control center consists of a standard PC work-station, a printer for report printing and the spe-cially designed administration software. The userinterface of the software must be mouse-con-trolled and menu-driven for flexible and easy use.Depending on system configuration, theoreticallyan unlimited number pump control units can becontrolled and monitored by a single control sta-tion. Practically the number is limited to about200 outstations by the time required to gatherthe observation data from the outstations duringnight time.
The central control station performs remote con-trol and monitoring, by which real time condi-tions at the pumping stations can be viewed atany time. This feature largely substitutes for sitevisits by the operating personnel. Pumps can bestarted and stopped; levels and other parameterscan be changed, and so forth. The system gathersall pumping station observations on a daily basisand stores the data into the databases from
Fig. 93
The GRUNDFOS solution for the network level remote administration. The local control units at the pumping stationsare connected via normal telephone (PSTN), radio, GSM modem network or any combination of them to the centralcontrol station. Also fixed (leased) line cable pairs can be used, but are nowadays getting rare.
Pumping Station Control and Condition Monitoring 10
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which e.g. the pump and flow data for many yearsbackwards can be monitored for further analysis,if desired. The software also generates numericand graphic reports on flows, pump data, alarmsand other parameters. Figure 93 shows a remotecontrol arrangement.
10.4.3 Data TransmissionAlthough the pump control units operate com-pletely independent, the transmission of data iscrucial for the remote control systems to work.The time needed for data transfer can bedecreased, if the pump control unit performs alldata analysis locally and stores the results in itsmemory. Only the calculated results, instead of alllogged data, need to be transferred to the controlcenter. This also enables the pump control unitsto operate independently without having to beconstantly connected to the control center.
The results can also be stored at the outstation forsome period of time, usually one week, before it isautomatically sent as a package. This is an impor-tant feature in case there are indefinite break-downs in the communication link.
Data transmission is always configured to suit theindividual needs. The communication link mustbe flexible and normally the public switched tele-phone network, radio modems, GSM modems orany combination of them can be used. Also fixedcable pairs can normally be used, but they havelately got quite rare due to increasing monthlyfees and uncertain reliability. A modem is neededin both ends of the communication link to modu-late the data for transfer. The choice between thedifferent transfer methods must be made by thecustomer keeping in mind the building costs, datatransmission costs and features both required andoffered by each method.
In general, radio modems and fixed cable pair(leased lines) are used over short distances andalways in case there is a need for continuous com-munication such as with control loops betweenfresh water reservoir and water intake station. Ifconnected through the public telephone network,the pumping station and the central control sta-tion can be situated at a practically unlimited dis-tance from each other. The public telephonenetwork also makes it possible to authorize thirdparties, such as equipment vendors and service
companies, to access a pumping station for spe-cial purposes.
Modern GSM telecom technology offers an attrac-tive solution for remote control and monitoringfor far-off outstations with long distances. GSMoften offers the best alternative for retrofit instal-lations, since PSTN line access installation after-wards is expensive and availability may belimited. It is a clear trend that GSM modem con-nections are getting increasingly popular in thefuture.
10.4.4 Alarm TransferAlarms raised at an outstation are transferred tothe control center, where all incoming alarms arestored in the database. The administration soft-ware running on the control center computerincludes an automatic categorization of alarms aswell as a calendar of the service personnel onduty, according to which it transfers the alarm tothe right person at the right time (in case thealarm is categorized to be transferred). Occasion-ally, the control center computer is also equippedwith a separate alarm printer, which prints out allthe alarms for later analysis.
The alarms are normally transferred to the GSMphone of the service person on duty as an SMS(text) message. The message may include, in addi-tion to the alarm text and the station name, moredetailed information on the pump status (run-ning/off/failure), station inflow, pumped volumeduring the day, other active alarms (which arestated as not to be transferred), etc.
Another way to transfer the alarms is by pager.The control center computer creates the alarmreport text, contacts the pager operator and sendsthe message to be displayed on the pager. Typi-cally the message contains coded information onthe station identity and type of alarm issued.
If so required, the control center can also transferalarms via voice message. A text corresponding tothe alarm and stored vocally on the computerhard disk, is retrieved by the software and used fortransmission over telephone to the operator. Thecontrol center can be programmed to call differ-ent numbers, until the alarm is acknowledged byan operator.
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10.4.5 System IntegrationA wastewater pumping control and monitoringsystem can be integrated into some other controlsystem, such as a treatment plant control system,or an integrated water company control system, ifcombined control is preferred.
Integration does not mean that all systems run inthe same computer and with the same software.It is normally useful to pick the best system foreach application and to combine them at a suit-able level. This could, for example, mean commonsoftware for alarm transfer and reporting. Tomake integration possible, the systems should bedesigned using standard procedures such as PCoperating systems, standard data transmissionand signal input and output protocols.
10.5 Internet & WAP Based Remote Control and Monitoring
Alarm messages transferred to the service per-sons as SMS messages are purely one-way infor-mation. If the service person would have thepossibility to control the system and change somevital parameters from his mobile phone whilebeing on the field, the total flexibility by themeans of a mobile control center can be achieved.
The latest improvements in remote control andmonitoring techniques involve Internet and WAPtechnology to overcome the limitations of tradi-tional monitoring systems described above. TheInternet/WAP control and monitoring systemsalso enable remote monitoring to be offered asservice to the municipalities. Figure 94 shows anInternet or WAP based remote control arrange-ment.
Fig. 94
The GRUNDFOS model for the Internet/WAP based control and monitoring system offers completely mobile controlcenter from the WAP cellular phone. In addition, the system utilizes Internet for data storage and the customers canmonitor the outstations and create reports from any computer with Internet access. The system also enables the wholeremote control and monitoring to be offered as a contract service.
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The Internet based control and monitoring systemallows historical data from the outstations to beviewed and reported from multiple locations, thusenabling the information to be used whereverneeded. After typing in the user identification theoperation personnel, persons on duty, decision-makers, sewage system engineers, etc. are able tobrowse detailed historical data from the outsta-tions for years backwards e.g. from their ownoffice computers. Figure 95 shows the interfacepage of the Grundfos Web based control service.
Fig. 95
From the Internet the customers can browse the history data of their own outstations independent on the location. In-formation can also easily be shared inside the organisation – operation personnel, duty persons, decision-makers aswell as sewage system designers and engineers.
Symbols
98
Symbols
A areaAW wet well surface area
a pressure wave velocityD pipe internal diameterFa axial force
Fr radial force
fH head measurement uncertainty factor
fmax maximum frequency
fmin minimum frequency
fN nominal frequency
fQ flow rate measurement uncertainty
factorg acceleration of gravity, 9,81 m/s²H pump total head (head)H0 head at zero flow rate
Hd dynamic head
Hf head friction losses
HG guaranteed head
Hgeod geodetic head
HJ head loss in pipeline
HJn local head loss
HJp head loss in pressure pipeline
HJt head loss in suction pipeline
Hmax maximum allowable head
Hmin minimum allowable head
Hr head loss
Hrt head losses in inlet pipe
Hs head discontinuity losses
Hst static head
Ht theoretical head
Ht∞ ideal head
Hv head leakage losses
h heighthA height difference between reference
plane and tip of vane leading edgehs pump stop level
ht inlet geodetic height
I electric currentIl line current
Iph phase current
k coefficientL lengthl length
MB concrete mass, kg
MH head measurement uncertaintyMQ flow rate measurement uncertaintym massNPSH net positive suction headn rotational speednN nominal rotational speed
P pump power inputPgr motor power input
p pressurepb ambient pressure at liquid level
pL atmospheric pressure in pump well
pm sand content by weight
pmin minimum static pressure in pump
pU atmospheric pressure in receiving well
pv liquid vapor pressure, sand content
by volumeQ volume rate of flowQ0 volume rate of flow at zero head
QI volume rate of flow, one pump
QII volume rate of flow, two pumps
Qin incoming flow
QN pump nominal flow rate
q volume flowRe Reynold’s numberS curve slopeT cycle durationt pump running timetH head tolerance factor
tQ flow rate tolerance factor
Ul line voltage
Uph phase voltage
u perimeter velocityV volumeVG pumping station volume below
water table, m3
VH effective wet well volume
Vh effective wet well volume
v true fluid velocityv2 flow velocity at pump outlet
vm radial component of true velocity
vu tangential component of true velocity
w velocity relative to the vaneXH head tolerance factor
XQ flow rate tolerance factor
Z starting frequency
Symbols
99
Z1.2 pressure gage height above water level
ZImax maximum pump starting frequency
ZIImax maximum pump starting frequency
Zmax maximum pump starting frequency
β vane edge angle∆h local pressure drop at vane leading edge
∆h pressure (head) change∆HL lower allowable head deviation
∆HT allowable head deviation∆HU upper allowable head deviation
∆QL lower allowable flow rate deviation
∆QT allowable flow rate deviation∆QU upper allowable flow rate deviation
∆v flow velocity changeζ local resistance factorη pump efficiencyηgr overall efficiency
ηh hydraulic efficiency
ηmot motor efficiency
λ friction factorµ reflection cycle durationν kinematic viscosityρ fluid density
Appendix A
101
Local Resistance Factors
Branches
Qh vh
Qs
vs v
Q
Diverging flows
Qh/Q α = 90° α = 45°
ζh ζs ζh ζs
0,0 0,95 0,04 0,90 0,04
0,2 0,88 -0,08 0,68 -0,06
0,4 0,89 -0,05 0,50 -0,04
0,6 0,95 0,07 0,38 0,07
0,8 1,10 0,21 0,35 0,20
1,0 1,28 0,35 0,48 0,33
Qh vh
Qs
vsv
Q
Merging flows
Qh/Q α = 90° α = 45°
ζh ζs ζh ζs
0,0 -1,00 0,04 -0,90 0,04
0,2 -0,40 0,17 -0,38 0,17
0,4 0,08 0,30 0,00 0,19
0,6 0,47 0,41 0,22 0,09
0,8 0,72 0,51 0,37 -0,17
1,0 0,91 0,60 0,37 -0,54
Appendix A
102
Qhvh
Qs
vs v
Q
45
Qh/Q Diverging flows
ζh ζs
0,0 0,92 0,06
0,2 0,97 -0,06
0,4 1,12 0,00
0,6 1,31 0,09
0,8 1,50 0,20
1,0 0,30
Qhvh
Qs
vs v
Q
45
Qh/Q Merging flows
ζh ζs
0,0 -0,82 0,06
0,2 -0,30 0,24
0,4 0,17 0,41
0,6 0,60 0,56
0,8 1,04 0,80
1,0 1,38 1,13
Appendix A
103
Bends
D
R
D
R D⁄ 1 5 ζ;, 0 4,= =
D
R
D
R D⁄ 1 5 ζ;, 0 7,= =
D
R
90
R/D 1 2 3 4 6
ζ 0,36 0,19 0,16 0,15 0,21
R/D 8 10 12 16 20
ζ 0,27 0,32 0,35 0,39 0,41
Appendix A
104
D
R
ζ
α R/D
1 2 4
20° 0,07 0,03 0,03
40° 0,13 0,06 0,06
60° 0,20 0,10 0,09
80° 0,27 0,13 0,12
90° 0,32 0,15 0,13
120° 0,39 0,19 0,17
140° 0,46 0,23 0,20
160° 0,52 0,26 0,23
180° 0,60 0,30 0,26
α 20° 40° 50° 70° 80°
ζ 0,03 0,12 0,24 0,54 0,74
α 90° 120° 140° 180°
ζ 1,00 1,86 2,43 3,00
Appendix A
105
Expansions and Contractions
v1 v2
HJn
v1 v2–( )2
2g-----------------------=
HJn ζv1
2
2g------= ζ k 1
A1
A2------–
2
=
v1
A1
A2
v2
β° k β° k β° k
5 0,13 45 0,93 100 1,06
10 0,17 50 1,05 120 1,05
15 0,26 60 1,12 140 1,04
20 0,41 70 1,13 160 1,02
30 0,71 80 1,10
40 0,90 90 1,07
Friction drag not included
HJn 0≈
Appendix A
106
A1
A2
v2v1
HJn ζv2
2
2g------=
A2/A1 0 0,1 0,2 0,3 0,4
ζ2 0,50 0,46 0,41 0,36 0,30
A2/A1 0,5 0,6 0,7 0,8 0,9
ζ2 0,24 0,18 0,12 0,06 0,02
HJn 0 5v2
2
2g------,= HJn
v12
2g------=
v1v2<<v1
v2v1<<v2
Appendix A
107
Bend Combinations Suction Inlets
ζ 2 ζ90°×=
ζ 3 ζ90°×=
ζ 4 ζ90°×=
ζ 3 0,=
ζ 0 2,=
ζ 0 05,=
Appendix A
108
Valves
ξ-values depend strongly on shape. Factory values should be used when available.
ξ-values above are valid for fully open valves. In partly open position, ξ may be 1,5-2 times as high. Depend-ing on shape and position, a certain minimum flow velocity through the valve is required for it to be regardedas fully open. Exact information on each valve is available from the manufacturer or supplier.
Gate valves without narrowing: ξ = 0,1…0,3Gate valves with narrowing: ξ = 0,3…1,2
Ball non-return valves ξ ≈ 1,0 (fully open)
Flap non-return valves ξ = 0,5…1,0 (fully open)
Appendix B
109
Pumping Station Starting Frequency and Pumping Capacity
In a pumping station the water volume comprisesthe volume below the lowest pump stop level andthe pumpable volume above this level, fluctuat-ing with pump usage and water incoming flowrate. The starting frequency of the pumpsdepends on the available pumpable volume andthe incoming flow rate.
The following different cases are investigated:• single pump pumping station• pumping station with two pumps in duty-
standby operation• pumping station with more than two pumps.
Single Pump StationIncoming water during one unit of time (cycle)can be expressed as:
(B1)
whereQin = incoming flow rateT = duration of cycle
The same volume must be removed by the pumpduring the cycle, whence
(B2)
whereQ = pump capacityt = pump running time
Combining equations B1 and B2 is obtained
(B3)
When the pump is stopped, the volume betweenthe start and stop levels Vh fills up during the timeT - t, whence
(B4)
Substituting with the expression B3 for t in equa-tion B4:
(B5)
Solving equation B5 for T is obtained:
(B6)
Starting frequency is the inverse value of T, hence:
(B7)
The starting frequency Z is a function of the ratioQin/Q and is shown in Figure B1.
Differentiating the equation B7 over Qin isobtained:
(B8)
Equation B8 equals 0 when Qin = ½Q
Substituting Qin = ½Q to equation B7:
(B9)
V Qin T⋅=
V Q t⋅=
tQinT
Q------------=
Vh Qin T t–( )⋅ Qin T Qin t⋅–⋅= =
Vh QinT Qin
QinT
Q------------–=
TVhQ
QinQ Qin2
–----------------------------=
ZQinQ Qin
2–
VhQ----------------------------=
Zmax
Qin/Q
Z [%]
Fig. B1
Starting frequency curve Z for a single pump pump-ing station as a function of the ratio between incom-ing flow rate Qin and pump capacity Q.
dZdQin------------
Q 2Qin–
VhQ----------------------=
ZmaxQ
4 Vh⋅--------------=
Appendix B
110
From this the pumping station capacity Vh isobtained:
(B10)
The solution to equation B10 is shown graphicallyin Figure B2.
In practice there may be situations where theincoming flow to a pumping station is very smalland only momentary, for instance in pumping sta-tions serving a few households only. In such casesthe selected pump capacity should be selectedmuch larger, in order to attain high enough a flowvelocity in the rising main to prevent sedimenta-tion. In this situation the Qin/Q ratio remainssmall, and the Zmax value is not reached at all orvery seldom only.
VhQ
4 Zmax⋅---------------------=
1000
100
10
Vh [m3]
2 10 100 1000
Q [l/s]
0,12000
1
Q = Pump capacity, l/s
Vh = Effective wet well volume, m3
Zmax = Maximum starting frequency, 1/h
Diagram for the determination of the effective wetwell volume Vh for a single pump pumping station.
Fig. B2
A AB
t
t
T
t Qin /Ql < 1
AB t
Tt1 Qin /Ql > 1
BA
t2
Fig. B4
Operation time diagram of the duty and standbypumps in a pumping station for an incoming flow(Qin) both smaller and larger than the capacity ofone pump (QI).
Start level 2
Start level 1
Stop level
Qin
Ql or Qll
Hh
A B
Fig. B3
Pumping station with two pumps in alternatingduty. The lead pump starts when the water levelrises to start level 1. If the incoming flow exceedsthe capacity of one pump, the lag pump will startat start level 2. Pumps alternate between lead andlag positions with each running cycle.
Appendix B
111
Two Pumps in Duty-Standby ConfigurationThe principle of operating a pumping station withtwo identical pumps is shown in Figure B3. Thepumps assume alternately the positions of duty(lead) and standby (lag) pump with each runningcycle. When the water level in the wet wellreaches the first start level the duty pump starts.The water level is pumped down to the stop level,and the pump stops, allowing the water level torise again to the first start level, completing thecycle.
The duty pump alone is able to handle most regu-lar incoming flow situations, and the standbypump will start only if the incoming flow rate(Qin) is larger than the capacity of one pump (QI),in which case the water level continues to rise tothe second start level, starting the standby pump.If the combined capacity of two pumps (QII) islarger than the incoming flow, all pumps stopwhen the water reaches the stop level.
Figure B4 shows a time diagram of the runningcycle of two pumps in alternating duty, furtherexplaining the principle.
Qin = Incoming flow rate, l/s
Qin/Ql <1 Qin/Ql >1
Ql (l/s)
Ql
Vh
(m3)
Qll
VH = Effective wet well volume to starting level 2, m3Vh = Effective wet well volume to start level 1, m3Zlmax = Maximum pump starting frequency for Qin>Ql, 1/hZlmax = Maximum pump starting frequency for Qin<Ql, 1/hQll = Pumping capacity two pumps when Qin/Ql >1, l/sQl = Pumping capacity when Qin/Ql <1, l/s
Fig. B5
Nomogram for the determination of the effective wet well volume Vh and the starting frequency Z for a pumpingstation with two pumps in duty-standby configuration.
Appendix B
112
Equations B9 and B10 can be used in the situationwhere the incoming flow is smaller than thecapacity of one pump for the calculation of start-ing frequency for each pump. With two pumpsstarting alternately, the expressions are dividedby two, whence
(B11)
(B12)
The solution to equations B11 and B12 are showngraphically in Figure B5.
In the case when the incoming flow is larger thanthe capacity of one pump, two additional factorsmust be considered. These are the ratio of thepumping station capacity to the first start level,Vh, and the second start level, VH, and the com-bined capacity of the pumps QII. The followingequation for the starting frequency can then bederived:
(B13)
The expression for ZIImax can be solved by differ-entiation, but the expression is very complex. Agraphic presentation of the solution is presentedin Figure B5.
Figure B6 shows the relation between starting fre-quency and the Qin/QI ratio. The starting fre-quency rises sharply at conditions requiringparallel duty. The diagram shows a marked peakvalue ZIImax.
The diagram in Figure B7 shows the effect of theratio Vh/VH on Z for constant VH and varying Vh. Inthis case the ratio QII/QI is 1,6. The conclusionfrom Figure B7 is, that ZIImax is reduced and ZImax
increased with lower start level 1.
The diagram in Figure B8 shows the effect of theratio QII/QI on Z for a constant Vh/VH ratio of 0,8.Increasing rising main losses, decreasing QII/QI
also decreases ZIImax.
If the pumps are selected so that one pump canhandle all incoming flows, ZIImax loses signifi-cance.
Pumping Stations with more than TwoPumps Pumping stations with a multitude of pumps canbe divided into the following two design catego-ries:• Stations with common stop level for all pumps• Stations with different or stepped stop levels
for each pumpThe starting cycle of the pumps are normallyalternated between the pumps in order to ensureeven distribution of wear.
Qin Ql<
Zlmax
Ql
8 Vh⋅--------------=
Vh
Ql
8 Zlmax⋅----------------------=
Qin Ql>
Zll
Ql VH Vh–( )
Qin2
QlQin–------------------------------
QllVH
QinQll Qin2
–-------------------------------
1–
+=
Qin/Ql < 1 Qin/Ql > 1
Qin/QlQll
Zllmax
Zlmax
Z
Fig. B6
Starting frequency curve Z for one pump and twopumps in pumping station with two pumps in duty-standby configuration as function of the ratio be-tween incoming flow rate Qin and pump capacity QI.
Appendix B
113
Pumping Station Capacities and StartingFrequenciesWith several pumps installed in a pumping sta-tion, the starting frequency changes dramaticallywith variations in the incoming flow. The startingfrequency will vary between zero and peak values,of which there are several.
Great flow fluctuations are typical for sewagepumping, and it becomes impossible and alsorather unnecessary to numerically calculate start-ing frequencies for each pump. With the aid ofdesign nomograms total pumping capacities andaverage starting frequencies, on which pumpingstation further design in all practical cases can bebased, can be determined.
For the different design categories the followingnomograms can be used.
Common Stop LevelFigure B9 shows a diagram from which VH or Z
can be selected as functions of overall flow rateQoverall.
For both of these categories it is good practice todivide the total pumping volume (VH) by the start-ing levels at approximately equal intervals if allthe pumps are identical. If the pumps have differ-ent capacities, the pumping volume may bedivided into intervals proportional to the pumpcapacities. The use of modern electronic level con-trol equipment facilitates the optimization of thestart levels either manually or automatically.
Qin/Ql
Z
0
2,0
1,0
Qll/Q=
1,81,6
1,41,2
Fig. B8
Starting frequency curves for different QII/QI ratiosand a Vh/VH ratio of 0,8.
Qin/Ql
Vh/VH = 0,4 Vh/VH = 1,0
Z
0
1,0
0,6
0,8 0,4
0,6
0,8
1,0 1,6
Fig. B7
Starting frequency curves for different Vh/VH ratioswith constant VH and a QII/QI ratio of 1,6.
Appendix B
114
Stepped Stop LevelsFigure B10 shows a diagram from which VH or Zcan be selected as functions of overall flow rateQoverall.
Recommended Starting FrequenciesPump and control equipment operation and wearis significantly related to the number of starts andstops over the long period, such as a year, sincevery high starting frequencies can be allowed inthe short term. If peak starting frequencies areused for dimensioning, the occurrence of thesemust be investigated. As shown earlier, the peakstarting frequency for one pump ZImax may neverbe attained in reality. Likewise, the peak startingfrequency for two pumps in parallel operationZIImax is usually much higher (1,5...2 times) thanthe ZImax value and only occurs occasionally.
PumpPump Pump Pump
Start
Start
Start
Start
Stop
For similar pumps:etc.
Qoverall = Flow rate, l/s
Qoverall [l/s]
VH [m3]
V1 V2 V3
V1
V2
V3
V4
1 2 3 4
= Pumping volume to top level, m3= Approx. average starting frequency of pump, 1/h
VH
Z
Fig. B9
Starting frequency nomogram for pumping station with more than two pumps and common stop level.
Appendix B
115
Starting frequencies selection should be checkedagainst pump and control equipment manufac-turers' recommendations. The following guide-lines for mean allowable starting frequencies forsubmersible pumps may be used:
Start
Start
Start
Stop
For similar pumps:etc.
Qoverall = Flow rate, l/s
VH [m3]
Qoverall [l/s]
Start
Stop
Stop
Stop
V4
V3
V2
V1
VH
V1 V2 V3
PumpPump Pump Pump1 2 3 4
= Pumping volume to top level, m3= Approx. average starting frequency of pump, 1/h
VH
Z
Fig. B10
Starting frequency nomogram for a pumping station with more than two pumps and stepped stop levels.
Pump power Allowable Z
0...5 kW 25 1/h
5...20 kW 20 1/h
20...100 kW 15 1/h
100...400 kW 10 1/h
THE SEW
AG
EPU
MPIN
GH
AN
DBO
OK
