Pump principles

PUMPS and

PIPINGBy. Engr. Yuri G. Melliza

PUMPS: It is a steady-state, steady-flow machine in which mechanicalwork is added to the fluid in order to transport the liquid from one point to another point of higher pressure.

LowerReservoir

Upper Reservoir

Suction GaugeDischarge Gauge

Gate Valve

Gate Valve

CLASSIFICATION OF PUMPS

1. Centrifugal: It consist essentially of an impeller arranged to rotate within a casing so that the liquid will enter at the center or eye of the impeller and be thrown outward by centrifugal force to the outer periphery of the impeller and discharge into the outer case. It operates at high discharge pressure, low head, high speed and they are not self priming.

Centrifugal Mixed Flow

single stage multi stage Propeller or axial flow Peripheral

2. Rotary:It is a positive displacement pump consisting of a fixed casing conta- ining gears, cams, screws, vanes, plungers or similar element actuated by the rotation of the drive shaft. A rotary pump traps a quantity of liquid and moves it along toward the discharge point. For a gear type rotary pump the unmeshed gears at the pump provides a space for the liquid to fill as the gears rotate. The liquid trapped between the teeth and the pump casing is eventually released at the discharge line. It operates at low heads, low dis- charge and is used for pumping viscous liquids like oil.

cam gear screw vane

3. Reciprocating: It is a positive displacement unit wherein the pumping action is accomplished by the forward and backward movement of a piston or a plunger inside a cylinder usually provided with valves.

Piston Direct Acting

single duplex

Crank and Flywheel Plunger Power Driven

simplex duplex triplex

4. Deepwell Pumps: It is used when pumping water from deep wells. The pump is lowered into the well and operated close to water level. They are usually motor driven with the motor being at the ground level and connected to the pump by a long vertical line shaft.

Turbine Ejector or centrifugal reciprocating Airlift

For a final choice of a pump for a particular operation the following data are needed. Number of units required Nature of liquid Capacity Suction conditions Discharge conditions Intermittent or continuous service Total dynamic head Position of pump, vertical or horizontal Location, geographical, indoor, outdoor, elevation Type of power drive

Centrifugal Pump

Rotary Pump (Gear Type)

Reciprocating PUmp

impeller

eye

discharge

Gear

PistonValves

Cylinder

FUNDAMENTAL EQUATIONS

1. TOTAL DYNAMIC HEAD

meters HZZ2g

vvPPH L12

2

1

2

212t

γ

2. DISCHARGE or CAPACITY Q = Asvs = Advd m3/sec

3. WATER POWER or FLUID POWER WP = QHt KW

4. BRAKE or SHAFT POWER

KW 60,000

TN2BP

π

5. PUMP EFFICIENCY

100% xBPWP

P η

6. MOTOR EFFICIENCY

100% xMPBP

mη

7. COMBINED PUMP-MOTOR EFFICIENCY

mPC

C

ηηη

η

100% xMPWP

8. MOTOR POWER

KW 1000

)(cosEMP

θI

For Single Phase Motor

For 3 Phase Motor

KW 1000

)(cosE 3MP

θI

where: P - pressure in KPa T - brake torque, N-m v - velocity, m/sec N - no. of RPM - specific weight of liquid, KN/m3 WP - fluid power, KW Z - elevation, meters BP - brake power, KW g - gravitational acceleration, m/sec2 MP - power input to HL - total head loss, meters motor, KW E - energy, Volts I - current, amperes (cos) - power factor

PIPES and FITTINGS

Nominal Pipe Diameter: Pipe sizes are based on the approximate diameter and are reported as nominal pipe sizes. Regardless of wall thickness, pipes of the same nominal diameter have the same outside diameter. This permits interchange of fittings. Pipe may be manufactured with different and various wall thickness, so some standardization is necessary. A method of identifying pipe sizes has been established by ANSI (American National Standard Institute). By convention, pipe size and fittings are characterized in terms of Nominal Diameter and wall thickness.For steel pipes, nominal diameter is approximately the same as the inside diameter for 12" and smaller. For sizes of 14" and larger, the nominal diameter is exactly the outside diameter.SCHEDULE NUMBER: The wall thickness of pipe is indicated by a schedule number, which is a function of internal pressure and allowable stress. Schedule Number 1000P/S where P - internal working pressure, KPa S - allowable stress, KPa Schedule number in use: 10,20,30, 40,60, 80, 100, 120, 140, and 160. Schedule 40 "Standard Pipe" Schedule 80 " Extra Strong Pipe"

FITTING: The term fitting refers to a piece of pipe that can: 1. Join two pieces of pipe ex. couplings and unions 2. Change pipeline directions ex. elbows and tees 3. Change pipeline diameters ex. reducers 4. Terminate a pipeline ex. plugs and valves 5. Join two streams to form a third ex. tees, wyes, and crosses 6. Control the flow ex. valves

VALVES: A valve is also a fitting, but it has more important uses than simply to connect pipe. Valves are used either to control the flow rate or to shut off the flow of fluid.

DESIGN OF A PIPING SYSTEMThe following items should be considered by the engineer when he is developing the design of a piping system.1. Choice of material and sizes2. Effects of temperature level and temperature changes. a. insulation b. thermal expansion c. freezing3. Flexibility of the system for physical and thermal shocks.4. Adequate support and anchorage5. Alteration in the system and the service.6. Maintenance and inspection.7. Ease of installation8. Auxiliary and standby pumps and lines9. Safety a. Design factors b. Relief valves and flare systems

HEAD LOSSES HL = Major loss + Minor lossesMajor Loss: Head loss due to friction and turbulence in pipesMinor Losses: Minor losses includes losses due to valves and fittings, enlargement, contraction, pipe entrance and pipe exit. Minor losses are most easily obtained in terms of equivalent length of pipe "Le". the advantage of thisapproach is that both pipe and fittings are expressed in terms of "Equivalent Length" of pipe of the same relative roughness.

Darcy-Weisbach Equation

meters 2gDLvf

h2

f

meters 2gD

v)eL(Lfh

2

f

Considering Major Loss only

Considering Major and Minor Losses

Where; f - friction factor from Moody's Chart L - length of pipe, m Le - equivalent length in straight pipe of valves and fittings, m v - velocity, m/sec D - pipe inside diameter, m g - gravitational acceleration, m/sec2

REYNOLD'S NUMBER: Reynold's Number is a non dimensional one which combines the physical quantities which describes the flow either Laminaror Turbulent flow. The friction loss in a pipeline is also dependent upon this dimensionless factor

vD

vD

NR νμ

ρ

where; - absolute or dynamic viscosity, Pa-sec - kinematic viscosity, m2/secFor a Reynold's Number of less 2100 flow is said to LaminarFor a Reynold's Number of greater than 3000 the flow is Turbulent

Moody’s Chart

f

NR

D

where - absolute roughnessD - inside diameter/D - relative roughness

VALUES OF ABSOLUTE ROUGHNESS FOR NEW PIPES

Type of Material Feet MillimeterDrawn tubing, brass, lead, glass    

centrifugally spun cement, bituminous    

lining, transite 0.000005 0.0015Commercial Steel, Wrought iron 0.00015 0.046Welded steel pipe 0.00015 0.046Asphalt-dipped cast iron 0.0004 0.12Galvanized iron 0.0005 0.15Cast iron, average 0.00085 0.25Wood stave 0.0006 to 0.003 0.18 to 0.9 

Concrete 0.001 to 0.01 0.3 to 3 

Riveted steel 0.003 to0.03 0.9 to 9 

For Laminar flow:

RN64

f

Centrifugal Pumps

1. TOTAL HEADHt = nH

where:n - number of stagesH - head per stage

2. SPECIFIC SPEED: Is the speed in RPM at which a theoretical pump geometrically similar to the actual pump would run at its best efficiency if proportion to deliver 1 m3/sec against a total head of 1 m. It serves as a convenient index of the actual pump type.

43S

H0.0194

QNN

where: Q - flow in m3/sec for a single suction pumpH - head per stageN - speed, RPMNS - specific speed, RPM

3. SUCTION SPECIFIC SPEED

43

S

43

HNPSH

SN

NPSH0.0194

QNS

where: NPSH - Net Positive Suction Head

4. NET POSITIVE SUCTION HEAD: The amount of pressure in excess of the vapor pressure of the liquid to prevent cavitation. NPSH = Hp Hz - Hvp - hfs , meters where:

Hp - absolute pressure head at liquid surface at suction, m Hz - elevation of liquid surface at suction, above or below the pump centerline, m (+) if above PCL (-) if below PCL

Hvp - vapor pressure head corresponding the temperature of the liquid,m hfs- friction head loss from liquid surface at suction to PCL.5. CAVITATION: The formation of cavities of water vapor in the suction side of the pump due to low suction pressure.

CAUSES OF CAVITATION Sharp bends. High temperature High velocity Rough surface Low atmospheric pressure

EFFECTS OF CAVITATION Noise Vibration Corrosion Decreased capacity

6. CAVITATION PARAMETER

34

S

SN

HNPSH

δ

7. IMPELLER DIAMETER

meter 2gH60

DNπ

where: - peripheral velocity factor whose value ranges from 0.95 to1.09

8. AFFINITY LAWS OR SIMILARITY LAWS FOR CENTRIFUGAL MACHINES a. For Geometrically similar pumps Q ND3 Power N3D5

H N2D2 T N2D5

b. For pumps with Variable Speed and Constant impeller diameter Q N Power N3

H N2 c. For pumps at Constant Speed with Variable impeller diameter Q D Power D3

H D2

RECIPROCATING PUMPS

Specification: Ds x Dw x L where: Ds - diameter of steam cylinder Dw - diameter of water cylinder L - length of stroke

1. VOLUMETRIC EFFICIENCY

100% xVQ

ηD

V

where: Q - discharge , m3/secVD - displacement volume, m3/sec

2. DISPLACEMENT VOLUME

For Single acting

secm

4(60)

Nn')L(DV

32

DW

For Double acting without considering piston rod

secm

4(60)

Nn')L(DV

32

DW

For Double acting considering piston rod

secm

d-2D 4(60)LNn'

V3

22

D W

where: N - no. of strokes per minute L - length of stroke, m D - diameter of bore, . d - diameter of piston rod, m n' - no. of cylinders n' = 1 (For Simplex) n' = 2 (For Duplex) n' = 3 (For Triplex)

3. PERCENT SLIP% Slip = 100 - V

4. SLIPSlip = VD - Q

5. THERMAL EFFICIENCY

100% x)h(hm

3600(WP)e

ess

where: hs - enthalpy of supply steam, KJ/kg he - enthalpy of exhaust steam, KJ/kg ms - steam flow rate, kg/hr WP - fluid power, KW

6. FORCE PRODUCED and ACTING ON THE PISTON ROD

KPa 4

)P(PDF es

2

Ss

where: Ps - supply steam pressure, KPa

Pe - exhaust steam pressure, KPa Ds - diameter of steam cylinder, m (Ps - Pe) - mean effective pressure

7. FORCE TRANSMITTED TO THE LIQUID PISTON

)P(Pe)P(P

DD

KPa 4

)P(P(Dw)F

FeF

esm

sud

w

s

sud2

w

smw

where: em - mechanical efficiency Psu - suction pressure of water cylinder, KPa Pd - discharge pressure of water cylinder, KPa

8. PUMP DUTY: Work done on the water cylinder expressed in Newton-meter per Million Joules

Joules Millionm-N

)h-(h1000m

10 x)H(H9.81mDuty Pump

ess

6sudw

where: mw - water flow rate, kg/hr Hd - discharge head of pump, m Hsu - suction head of pump, m

9. PUMP SPEED

V = 43.64(L)1/2(ft), m/min

secm

4(60)

Vn')(DV

32W

D

where: ft - temperature correction factor L - length of stroke, m

10. TEMPERATURE CORRECTION FACTOR

ft= 1 For cold water

  = 0.85 for 32.2C water  = 0.71 for 65.5C water  = 0.55 for 204.4C

water11. For Indirect Acting pumps

L

907N tf

Example no. 1 A mechanical engineer of an industrial plant wishes to install a pump to lift 13 L/sec of water from a sump to a tank on a tower. The water is to be delivered into a tank 105 KPa. The tank is 18 m above the sump and the pump is 1.5 m above the water level in the sump.The suction pipe is 100 mm in diameter, 8 m long and will contain 2 - standard elbows and 1 - Foot valve. The discharge pipe to the tank is 65 mm in diameter and is 120 m long and contains 5 - 90elbows, 1 - check valve, and 1 - gate valve. Pipe material is Cast iron. Determine the KW power required by the pump assuming a pump efficiency of 70% and motor efficiency of 80%. Other Datam = 0.001569 Pa – sec = 1000 kg/m3

At Suction

At Discharge

Using point 1 and 2 as reference pointP1 = 0 gageP2 = 105 KpaZ1 = 0Z2 = 18 mHL = 0.71+45.634 = 46.34 metersPump efficiency = 70%Motor efficiency = 80%Overall Efficiency = 0.70(0.80)=0.56

A centrifugal pump design for a 1800 RPM operation and a head of 61 m has a capacity of 190 L/sec with a power input of 132 KW. What effect will a speed reduction to 1200 RPM have on the head, capacity and power input of the pump? What will be the change in H, Q and BP if the impeller diameter is reduced from 305 mm to 254 mm while the speed is held constant at 1800 RPM. Neglect effects of fluid viscosity.Given:N1 = 1800 RPM N2 = 1200 RPMH1 = 61 m H2 = Q1 = 190 L/sec Q2 = BP1 = 132 KW BP2 =

For N1 = N2 = 1800 RPM D1 = 305 mm ; D2 = 254 mm

FROM AFFINITY LAWS OR SIMILARITY LAWS FOR CENTRIFUGAL MACHINES a. For Geometrically similar pumps Q ND3 Power N3D5

H N2D2 T N2D5

b. For pumps with Variable Speed and Constant impeller diameter Q N Power N3

H N2 c. For pumps at Constant Speed with Variable impeller diameter Q D Power D3

H D2

PREPARED BY: ENGR. YURI G. MELLIZAXAVIER UNIVERSITYATENEO DECGAYAN