TEXT & REFERENCE Carrier

•• TECHl\llCAL DEVELDPMEl\IT PRDliRAM

· • DESIGN FACTORS • SIZING PROCEDURES

• • PUMP SELECTION & APPLICATION ·

Copyright ,c Carrier Corporation 1965, 1986 Printed in U.SA 791-033 Section T200-33

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T200-33

TECHNICAL DEVELOPMENT PROGRAM

WATER PIPING SYSTEMS AND PUMPS

CONTENTS

INTRODUCTION ...........................................• Page 1

FORMUI..AS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

TYPES OF PIPING SYSTEMS ........ " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

MATERIALS • . . • . . • • . . • • . • . • • • • • • . . . • . . • • • • • • . • . . . . . • • . • • • . . . . . • . • • 6

SUPPORTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . 7

VALVES . • . . • • . • . • • • • • • • • . • • . • . • • . . • . • • • • • • . • . • • • • . . . • . . • • • . • • . • . • • 9

1. 2. 3.

Starting and Stopping Flow • • • 9

Regulating or Throttling Flow • • • • • • 11

Preventing Back Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 12

STRAINERS ............... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

EXPANSION TANKS . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

AIR VENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

OTHER ACCESSORIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

PIPE SIZING .................................................. ·. . . . . 2 2

1. 2. 3.

Pipe Sizing Example ..........•.•..........•..........•.•.

Total Head on Pump •••.••• Direct Return System Sizing

25 26 27

PUMPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1. 2. 3.

Pump Terms Capacity

............................................. .............................................

Head .••.•••.•.•••••.••.•.•••••••••••.••.••.••••.•••• • •

28 28 28

4. 5. 6. 7. 8. 9.

Suction Head •• Discharge Head Total Head ••••

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Page

. .............................. . . ...................................... . Liquid Horsepower •.......•.•..........................• Brake Horsepower •• . .............................. . Net Positive Suction Head . .............................. .

DETERMINING PU MP HEAD ........................................ .

TYPES OF CENTRIFUGAL PUMPS •••••••••••••••••••••••••••••••••••••

PUMP PACKING ......•.................•................•.........

PU MP 'MATERIALS .....•.•.•......•..•••••.•.•..•••....•••••..••.••

PUMP RATINGS ..............•....................................

NOISE IN PUMPING SYSTEMS •••••••••••••••••••••••••••••••••••••••

CONCLUSION ................................................... .

WORK SES SI ON .......••.•.•.•.........•.••..•........••.•.•......

28 29 29 • 29 30 30

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34

36

37

38

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INTRODUCTION

TECHNICAL DEVELOPMENT PROGRAM

WATER PIPING SYSTEMS AND PUMPS

In air conditioning work, water is often used to carry heat from a

point of generation, such as a cooling coil to a point where it can

be rejected to some other medium such as the refrigerant in a water

chiller. The water is often recirculated so that the pick up and re­

jection of heat is a continuous process.

The piping and pumping systems used to transport the water to and from

the various heat exchangers are usually relatively simple and straight

forward, and are complete in themselves, that is they do not function

as part of a big piping network. The water is usually at temperatures

between 40F and lOOF, although year-round air conditioning may require

hot water piping for heating. The motive force for circulating or

"pushing" the water through the piping system is almost invariably

furnished by a centrifugal pump.

The design of piping systems is an old art and much has been written

about it. This presentation will review overall design considerations,

with emphasis on those points which we consider especially applicable

to the piping and pumps used in air conditioning work. Frequent

• reference will be made to the Carrier System Desig_n Manual Part 3 -

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Piping Design for illustrations and amplification of material presented

here.

FORMULAS

A review of some of the formulas pertaining to the heat carrying

capacity of water is in order. £ A/C !7 JXAN HllN,; PGt. I 67 I

1. BTU/HR. = GPM x 500 x water temperature change in °F

2. Tons of refrigeration effect = GPM x temperature change in °F

24

If we use chiller tons as a base, we can arrive at approximate condenser

water gpm's and/or temperature changes as follows.

For mechanical refrigeration assuming a heat rejection factor of 1. 25

(1. 18 BHP per ton):

3. Chiller tons = GPM Condensing Water x Temperature Change in °r

30

For absorption refrigeration assuming a heat rejection factor of 2. 55

(19. 6 lbs. of 12 psig steam per hour per ton of refrigerating effect).

4. Chiller tons = GPM Condensing Water x Temperature Change in °r

61

Present design practice uses a chilled water temperature change of

about 1 OF; a condenser water temperature change of about 1 OF for

mechanical refrigeration with cooling tower; a condenser water temper-

ature change of about l 7F for absorption refrigeration with cooling tower;

and a condenser water temperature change of about 20F when using

city water at 70 or 75F.

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• These values are assumed to result in reasonable economic balance

among first cost, operating costs, and energy requirements. We

believe that this assumption is being challenged more often than it

was. Higher temperature changes result in less gpm, smaller pip~

sizes, lower operating costs, and lower energy requirements. For

instance, chilled water temperature changes of 20F or more can be

used without incurring any great problem in the selection of water

chillers and water cooled coils.

TYPES OF PIPING SYSTEMS

For our purpose, water piping systems can be classified as follows:

• 1. Once through type, where water flows from a source through

the system and out to waste. Examples are a city water

condensing system and a well water chilling system. A

pump may or may not be required.

2. Open recirculating type, where water is pumped from a

reservoir through the system and back to the reservoir for

reuse, with the water being brought into intimate contact

with air somewhere in the circuit. Examples are chilled

water systems using washers for cooling and dehumidifi-

cation and condensing water systems which use cooling

towers •

•

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3. Closed recirculating type, where water is simply circu-

lated through a closed system of piping and equipment,

without coming in close contact with air, except at the·

expansion tank, whose area of contact is negligible. An

example is a chilled water system using coils for cooling

and dehumidifying.

4. Recirculating Piping systems are further classified as either

direct return or reversed return. A direct return system is

illustrated in Figure 1 on the left. The same units are

shown on the right piped with a reversed return system.

• SUPPLY

RETURN

RETURN

SUPPLY

UNITS PIPED VERTICALLY UNITS PIPED VERTICALLY

n~BB I

RETURN. UNITS PIPED HORIZONTALLY

RETURN UNITS Pl PED HORIZONTALLY

Direct Return Piping System Reverse Return Piping System

FIG. 1 •

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If the pressure drops through the units are identical, then in the

case of the direct return system, each of the first five units will

required a balancing valve and means to measure flow plus balancing

time to insure the same flow through all units. In the reverse return

system, however, the pressure difference from supply line through the

unit to return line is the same for all units. Each unit will, therefore,

take an equal share of the total flow and no balancing is required.

The cost of the extra length of return pipe is probably less than the

cost of valves and balancing, and much time and trouble will be saved.

A reversed return should always be used in a multi-room system which

uses a large number of under the window units of identical pressure

drop.

GENERAL CONSIDERATJONS

Water piping systems should be as direct and uncomplicated as possible.

Offsets, bends, and changes in elevation should be kept to a minii:num.

Any fitting or valve that is omitted, represents a reduction in first cost,

operating cost, and maintenance cost.

On the other hand, condenser and chiller tubes must be periodically

cleaned, and cooling coils, control valves and pumps will eventually

require repair or servicing. All of these operations must be preceeded

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by draining the water out of the equipment involved. It is convenient

and economical to be able to isolate such parts by means of shut off or

isolating valves so that the entire system does not have to be drained

and refilled. It may also be important that any piece of equipment

can be isolated and worked on while the remainder of the system con­

tinues to operate normally. In addition to shut off valves, unions or

pairs of flanges are required at strategic locations so that the piping

can be easily dismantled for the possible removal of such things as

coils, and control valves. Judgment and imagination are needed to

balance convenience in servicing against first cost and maintenance

cost of the system.

Let us examine the various parts of a piping system in some detail.

MATERIALS

The usual piping materials are black steel for the large sizes say

1 1/2" and above and hard copper for the smaller sizes. Galvanized

steel may be used, but usually only for drainage lines. It is now

customary to specify water treatment to control corrosion and galvanized

or wrought iron is not generally required.

Fittings for black steel pipe are usually welded for larger sizes or

malleable iron screwed type for smaller sizes. Hard copper fittings

are wrought copper or brass. •

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The weight of the pipe and fittings will depend on the pressures and

temperatures encountered in the system. Various codes will have

something to say about this and the ones that apply must be.

consulted.

Special conditions may required special materials, but this is beyond

the scope of this presentation. We refer you to Chapter 1 of the

Design Manual, Part 3, for additional information on materials, pages

3-2 and 3-3.

SUPPORTS

Hangers or supports are required at intervals ranging from 8 ft. to

20 ft. depending on the pipe size. See tables 7 and 8 in the Design

Manual. Except at anchor points, pipe supports should not be rigid

but allow some movement of the pipe. In many systems, with

several changes of direction, the expansion due to temperature

changes can be taken care of in this way. The hangers for chilled

water piping are installed outside the

insulation to prevent sweating, and

some means must be provided to prevent

crushing the insulation. This is usually

a metal plate of suitable length and

thickness curved in cross section to

fit around the bottom part of the

insulation and mounted between the

FIG. 2

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• I insulation and the hanger. For hot water service, the hanger is

usually around the pipe1

and pipe and hanger are insulated together.

At the flanges of pumps or heat exchangers, the piping should be

supported so that no stress is imposed on the pump casing or the

heat exchanger itself. At pump suction and discharge, flexible

rubber connectors are sometimes used to prevent stress, and also

to compensate for slight misalignment. They are also supposed to

prevent the transmission of vibration into the piping system, but they

are not very effective for this purpose. Water is an incompressible

liquid and vibrations from the pump are transmitted through the water

column with very little reduction in intensity. Pump vibration problems • are best handled with spring type pipe hangers and by providing a good

mass under pumps or other vibrating components in a piping system.

Refer to pages 7 and 8 of the Design Manual. In large and important

systems, it may be desirable to consult vibration experts.

Often it becomes necessary to support a long straight vertical riser

which goes from the bottom to the top of the building. In large sizes,

the pipe and water represent a very large weight. The accompanying

sketch shows a method of support which is inexpensive and provides a

dirt leg with cleanout and a solid support with a minimum of hangers

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and special fittings. Note that

the weight of the water is carried

directly into the foundation. The

joint between the horizontal main

and the riser should be made with

a forged welding tee and not by

TEE

HORIZONTAL MAIN

field cutting and welding. Ex-

-CLEANOUT

...,.-DRAIN

,..r=::i:::~-- STEEL PLATE -,:,...::;.s=-.• =,. . .,.=~,:el;,

FOUNDATION

pansion of the pipe due to BASEMENT FLOOR /

temperature changes in the riser, may FIG. 3

need special attention in this case •

VALVES

There are hundreds of types of valves, each one of which best

suits a particular application, but for our purposes only five or

six types need consideration. We refer you to the Design Manual,

pages 10 to 15, and also to the catalogs and other publications of

the valve manufacturers.

Valves perform one of three basic functions.

1. Starting and Stopping Flow.

Gate valves are usually used for this function, because when wide

open water flows straight through with a minimum of pressure drop.

They are not practical for throttling flow •

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This illustration shows an OS and

Y gate valve with rising stem. This

is the type usually used for isolating

pumps, coolers and condensers. A

quick glance at the stem reveals

whether the valve is open or closed

and the stem threads are outside the

valve where they are free from cor-

rosion and can easily be lubricated.

With stem extended, these valves

are very tall and it is often difficult

RISING STEM (OUTSIDE SCREW

AND YOKE) --------

BOLTED

GLAND~

SOLID WEDGE

FLOW

/

HANDWHEEL (DOES NOT RISE WITH STEM)

BOLTED

BONNET

FLANGED ENDS

Gate Valve - Rising Stem

FIG. 4

to install them in such a position in the piping that the stem does

not interfere with fixed objects or block off passage.

A plug cock has the same low

pressure drop, when open, as

a gate and is also excellent

for throttling service or as a

balancing valve. It can be

used to perform both functions

simultaneously.

Plug Cock

FIG. 5

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2. Regulating or Throttling Flow

Globe valves or angle valves are usually used for this service.

Globe valves have a relatively high pressure drop when open, but

give good throttling characteristics, that is, the percent flow is

nearly proportional to the per cent of opening.

If a globe valve is used for balancing

flow, the wheel should be removed

after adjustment to prevent accidental

readjustment. An angle globe valve

can be advantageously used for

throttling. It's use saves one elbow

and the pressure drop when open is

less than half that of a straight

through globe valve.

If a plug cock or butterfly valve

is used as both shut off and balancing

valve an indicator should be provided

so that after use as a shut off, the valve

can be reopened to the original position.

Control valves are usually auto-

matically controlled globe valves I

although butterfly valves are be-

RING BONNET

Globe Valve

FIG. 6

RISING STEM (INSIDE SCREW)

SCREWED THREADED BONNET

NARROW SEAT DISC (CONVENTIONAL)

FLOW

Angle Valve

FIG. 7

SCREWED

ENDS

(RISES WITH STEM)

SCREWED ENDS

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coming popular for this use, especially in the larger sizes.

3. Preventing Back_ Flow.

Check valves perform the single function of checking or preventing

the reversal of flow in piping. The 15° swing check is the usual type,

with lift checks often used at pump discharge. If a pump operates

between two water levels· in an open system, the water will surge

back from the top level through

the pump to the bottom level at shut-

down causing it to run backwards,

damage seals, or even completely

drain the pump if it is above the

lower res evoir. To prevent back

flow, a check valve is placed near

BOLTED BONNET-~-!

FLOW q

COMPOSITION DISC

the pump discharge. This should be Swing Check Valve

of the non-slam type, to prevent FIG· 8

water hammer as the check closes. Most non-slam checks are

expensive and must be installed in a vertical riser.

It is possible to use a pneumatically operated butterly valve at the

pump discharge to perform all three functions so far considered. For

this multiple use, the valve is a normally closed type so that when air

is bled from the branch air line the valve goes to the closed position.

Air can be bled manually, which performs function #1.

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If a mechanical stop is provided at the valve open position which gives

rated flow in the piping system, then after use as a shut off, the

valve automatically returns to the correct

degree of opening, and function #2 is

accomplished. A; bleed valve can also

be installed in the branch air line, FLOW

which will automatically open whenever

the pump shuts down. By adjusting

the bleed rate of this valve to accom-

plish a reasonably fast closing of the

valve, function #3 is attained .

Finally, if several pumps are installed

in parallel, a check valve should be

installed in each pump discharge to

prevent water being bypassed back

through an idle pump. See Figure 10.

SCREWED UNION

RING BONNET

COMPOSITION DISC

'-SCREWED END

Lift Check Valve

FIG. 9

Multiple Pump Piping

FIG. 10

Pressure reducing valves are occasionally used in water piping. One

instance is when supplying clean city water to the lantern ring in a

pump seal, where the pressure must be regulated to about 5 psi above

suction pressure at the pump. Such valves should be sized to suit

the downstream flow rate and not to suit the pipe size •

- 14 -

Pressure relief valves are also sometimes required. For instance, in

a chilled water system, there may be a stretch of pipe which can be

accidentally valved off at each end. The water trapped between the

valves can exert a high pressure when warmed up and it may be

desirable to supply a relief valve to prevent damage. Also, in an

extensive system in which all the control valves are the throttling

type rather than three-way valves, the pump can build up a high

pressure when all valves are nearly closed, and a relief valve is often

installed at the pump discharge to relieve the excess pressure into the

pump suction.

STRAINERS

In many piping systems, a certain amount of finely divided "trash"

can circulate without doing any great harm. In such cases, the need

of a strainer is doubtful, and if one is used it should be no finer than

20 mesh to prevent its becoming clogged too rapidly. Operators are

human like the rest of us and if the task of cleaning strainers becomes

too irksome, the strainer basket will get punched full of holes or be

removed.

In a condenser water system using a cooling tower and not using control

valves, the strainer provided at the tower suction connection is usually

considered sufficient. A mud ring is sometimes installed around the

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intake as shown in the illustration

to hold back heavy dirt so that it

accumulates in the bottom of the pan

where it can be easily removed at the

yearly maintenance period. Spare

tower sump screens should be provided

so that when the dirty screen is

removed, it can be replaced immediately

without shutting down the tower.

HANDLES

COOLING TOWER TANK

STRAINER BASKET

-::-it--SUMP

lTO Cl!::O=ND=E=N=S:::!JER PUMP SUCTION

FIG. 11

If a strainer is used it is usually placed at the pump suction and

a "y" pattern strainer as shown in Figure 12 is often used.

In some instances, continuous

and positive cleaning is absolutely

necessary. Perhaps dirty river water is

being used in a once through system

for condensing purposes. In such

cases, a double basket type strainer

such as the type illustrated in Figure

FOR BLOW OUT 13 should be provided. It should be

installed in a location that is easily FIG. 12

accessible, and everything should be

done to make the task of routine cleaning as easy and convenient as

- 16 -

possible, otherwise the cleaning

will be neglected. These strainers

contain two strainers baskets and an

easily operated transfer valve diverts

flow from the dirty basket to the

clean one. The dirty basket can

then be removed and a spare clean

basket installed. The cleaning

can then be done at a convenient

time and place, and the piping system

is not shut down for even a short

time. These strainers are expensive

,, but in an extreme case, their cost

is well justified. Self-cleaning

strainers are also available, for use

in extreme cases where dirt accumu-

lates rapidly or for remote locations

which cannot be conveniently serviced

at frequent intervals.

FIG. 13

EXPANSION TANKS

Every closed recirculating system needs an expansion tank to take

care of expansion and contraction of water due to temperature change; .lii • and to provide a place to automatically replace water lost through pump

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gland leakage and other losses. An open tank is preferred, and is

usually placed at the top of the return main closest to the pump so

as to maintain a positive suction pressure at the pump intake. (See

Figure 14). If it is impractical to install an open tank at the top of

the system because of difficulty in protecting the tank from freezing,

obtaining city water supply, or providing overflow drains, a closed

expansion tank may be installed at any convenient point in the

system . It should. be tied in as close

as possible to the lowest pressure point,

and may need a vacuum breaker to prevent

collapse of the tank if the system is

drained. See Figure 15.

The variation in volume of the water can

be calculated by obtaining the total inter-

nal volume of the system including

piping, heat exchangers, pumps, etc.

and multiplying the volume by the change

in specific volume of water between the

highest and lowest temperature expected.

This change in volume is usually about

GATE VALVE-....

QUICK FILL LINE

DRAIN VALVE-.... (GATE VALVE)

TO DRAIN

TRAP

ENLARGED PORTION OF RETURN LINE TO PERMIT

AIR SEPARATION~ (NOTE 2)

RETURN

LINE

FLOAT VALVE

GAGE GLASS

EXPANSION LINE

(Ir MIN.)

ENLARGED TEE FOR AIR SEPARATION

L:..-1 AT LEAST 4d .J NORMAL LINE SIZE

f--1~1,_J i "-CIRCULATING PUMP

FIG. 14

1 % for chilled water systems and 3% for hot water systems . (See Table 15,

page 3-31 of the Design Manual). A safety factor of about 25% should

- 18 -

be added. Note that the volume calculated is not the volume of

the expansion tank but the volume of the space above the normal water

level in the tank.

The equalizer line from the expansion

tank to the system should be at least

1 1/2" size and should' not be pro-

vided with a shut off valve. In a

chilled water system, the sides

and bottom of the tank and the equal-

izer line may have to be insulated to

prevent sweating.

SIGHT

GL~

GATE VALVE_/

/

GATE VALVE

FIG. 15

The connection of potable water to any system is usually restricted

by water department rules. These lex:: al codes should be consulted

to be sure that the method of connecting is approved.

AIR VENTS

When properly placed, an open expansion tank acts as an air vent.

Entrained air in water can be expected to collect as bubbles of air

where the water stream reduces velocity, changes direction, or is

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heated. Any such point in

the system should have either

a manual or an automatic air

vent. Figure 16 shows how an

automatic air vent is installed.

The outlet should be piped to

a drain. Manual vents should

be provided at heat exchangers

and cooling coils.

OTHER ACCESSORIES

VENT VALV~

FIG. 16

MAIN

Piping systems are meant to circulate definite gpm's at specified

points in the system. After the system is in operation, it often

becomes necessary to find out if the specified quantities are in fact

being delivered. Pressure gages, thermometer wells, and possibly

flow meters should be provided at all necessary locations so that

this may be done and to assist the operator in trouble shooting .

Each pump should be furnished with a certified characteristic curve or

plot showing head versus gpm and gages should be provided as close

as possible to the pump suction and discharge flanges so that total

- 20 -

pressure rise across the pump can be found. By referring to the charac-

teristic curve, the gpm can then be read from the chart. This will

give a fairly accurate reading.

Flow meters can be installed which give a continuous reading of the

gpm flowing but the expense is usually not considered justified.

Often times 1 it is only necessary to accurately determine the flow at

very infrequent intervals, or perhaps only once to demonstrate that

specified gpms are being delivered. In such cases, a standard ASME

orifice plate installed between two flanges, with the necessary auxiliary

tappings can be provided.

Figure 17 shows a rough sketch of a concentric orifice plate with its f pressure tappings. For accurate

readings of gpm flows, the orifice

plate must be made, installed, and

used in strict accord with the

specifications of the ASME Power

Test Code. For information, refer

to the 11 Flow Meter Computation

Handbook 11 or the supplement to FIG. 17

the Power Test Code, Chapter 4,

Flow Measurement, Part 5 -

Instruments and Apparatus. Both publications are available from the

/ ~

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ASME. Note that the orifice plate forms a dam across the pipe and

dirt can collect and restrict flow. This orifice type of flow meter

also imposes a fairly substantial additional pressure drop in the system.

For these reasons, it is often removed after use and replaced with a

flat disc which has a hole equal in diameter to the inside diameter

of the pipe.

Thermometers or thermometer wells should be installed to assist the

system operator in routine operation and troubleshooting. Permanent

thermometers of correct scale range and with separable sockets should

be used at all points where temperature readings are regularly needed •

Thermometer wells only, should be installed where readings will be

needed during start up and infrequent troubleshooting.

Gage cocks should be installed at points where pressure readings will

be required. It should be remembered that gages installed permanently

in the system will deteriorate rapidly due to vibration and pulsation

and will not be reliable for use in troubleshooting when needed. For

this reason, gages should be removed from the system except when readings

are being taken. Good practice is to install gage cocks and provide the

operator with (or request that he obtain) several good quality gages for

troubleshooting.

Sleeves are usually provided at points where piping passes thru walls

and floors. In finished areas, sleeves are fabricated by cutting a

- 22 -

length of pipe of sufficiently large diameter for pipe and insulation

to pass thru. In unfinished areas, sleeves may be fabricated from

sheet metal. Wall sleeves are generally flush with both sides of

the wall. Floor sleeves in equipment rooms usually project several

inches above the floor to prevent water leakage around the pipe in

case of flooding.

Pages 33 and 42 of the Design Manual give detailed information on

the recommended accessories, and method of piping around various

pieces of equipment, such as are found in air conditioning systems.

PIPE SIZING

After a piping system has been laid out and the gpm figured it

becomes necessary to size the pipe and determine the total resistance

in the system so as to know what head the pump must work against.

Pipe size is limited by the maximum velocity permissible. Table 13

on page 21 of the Design Manual gives some recommended water

velocity limits 1 based on noise considerations and the effect of water

and entrained air wearing away or eroding the pipe. Erosion is, of

course, increased with high velocity but it is also affected by the

number of hours of operation per year. Table 14 on page 21 of the

Design Manual gives some recommended velocity limits, which are

based on experience and are designed to give a good balance between

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• pipe size (or cost) and a reasonable life before the pipe is eroded

away.

Pipe velocity may also be limited by the total head available or

desirable. For instance, in a city water system, the total head

(including static lift; pressure drops through meters, condensers,

control valves; and pipe friction) cannot exceed the total pressure

available in the city main. Economic considerations such as high

pumping costs, may also place a ceiling on velocity.

Friction Loss rate in pipe may be found by using Charts 3, 4, and 5 on

pages 22, 23, and 24 in the Piping Design Manual.

• Chart 3 applies to new, smooth, clean standard weight steel pipe and

can be used to determine the friction loss rate in a closed piping

system, such as a chilled water recirculating system.

Chart 4 applies to steel pipe which has been subjected to scaling and

corrosion for 15 to 20 years. This chart is used to determine the

friction loss rate in open recirculating type systems such as condenser

water systems using cooling towers.

Chart 5 is used to determine friction loss in copper tubing which can be

expected to stay clean throughout its normal life •

•

- 24 -

Note that the friction loss or head is given in feet of water per

100 ft. of straight pipe.

Friction loss in valves, fittings, or obstructions can be evaluated by

assigning an equivalent length of straight pipe to each size and type

of fitting as shown in Tables 10, 11, and 12 in the Piping Design

Manual. For instance, in Table 10, we note that a 4" globe valve

has an equivalent length of 120 feet. In other words, the friction

loss in one 4" globe is the same as that through 120 feet of 4"

straight pipe. Note that a 4" angle valve is equivalent to only 4 7 feet

of 4" pipe, yet both valves perform the same function.

In a closed system, friction is the only loss or head which the pump •~. '

'

has to overcome. The height of water on the suction side of the pump

is always exactly equal to the height on the discharge side. In open

systems, this is not true, and there is always a difference in head on

the two sides of the pump. In a cooling tower, for instance, the

height between the water in the pan and the exit from the distribution

at the top of the tower constitutes an unbalanced 'head which must be

overcome by the pump. If the distribution system consists of spray

nozzles which require a pressure behind them to force water through the

nozzles, this pressure must be added to the unbalanced static head.

•

•

•

- 25 -

The total head on the pump will consist of the following:

pipe friction head, including entrance and exit losses; losses through

fittings, valves, and accessories; pressure losses through equipment,

such as coolers, condensers, cooling coils, etc.; any unbalanced

head between reservoirs and at the base of cooling towers; and

pressure drops through nozzles or similar equipment.

Pipe Sizing Example.

COND.

211

COOLING T TOWER 12

1

l 99 1 10

1

GATE VALVE

1--121--i i-12'-I

FIG. 18

851

Tower Nozzles Require - 8 psi

Pressure drop thru cond. = 13 psi (Includfog ent. & exit losses)

Pressure Drop thru Strainers = 4 psi

GPM = 1200

Max. Velocity = 8 ft. per second

Pump - 8 1 suction connection

6" discharge

All elbows are long radius

Since this is an open recirculating system, we will use Chart 4 on

page 23 of the Piping Design Manual, to determine friction losses .

Referring to the Chart, we find that an 8 11 pipe size gives 7. 7 ft.

per sec. velocity and 3.8 ft. per 100 ft. friction loss.

- 26 -

Total equivalent length of straight pipe (Tables 10, 11, and 12)

Straight pipe = 10 + 85 + 12 + 12 + 3 + 2 + 8 + 20 + 99 + 21 + 8 + 3 = 283'

Equivalent lengths Exit loss - sump = Ells = 10 @ 13' = Gate Valves = 2 @ 9 Lift Check =

Friction Loss = 675 x 3.8 100

=

283' 24'

130' 18'

220' 675'

26'

Note that if we had used an angle lift check instead of one elbow

we could have reduced the total equivalent lengths by the following:

Deduct one ell = 13'

Diff. in checks = 220' - 85' = 135'

148'

This is 5. 61 friction head or ..hQ. x 100 = 21% of the total friction head. 26

Total Head on Pump

Friction Head = 26'

Unbalanced head at base of tower = 12'

Pressure drop in strainer = 4 psi

Pressure drop in condenser = 13 psi

Pressure drop in nozzles = 8 psi

TOTAL 25 psi

25 psi x 2.31 = 58'

TOTAL HEAD ACROSS PUMP 96'

•'

)

'

- 27 -

• At this point, some designers add a safety factor of 5 to 10%. We

have neglected the loss in the 6" to 8" increaser at the pump dis-

charge. On the other hand, we are starting out with clean pipe

and it is unknown whether the pipe will "age" to the predicted

condition. At the start, the pump has a tendency to pump more

gpm than needed which might overload the pump motor. Note that the

reduction in head due to clean pipe would be (refer to Chart 3 page

22 of the Design Manual).

26' - 675 2 2 = 26 - 15' = 11' 100 x •

or 11 1/2% of the total head.

Let us be conservative and select a pump to handle 1200 gpm at 100'

total head.

Direct Return System Sizing

In any direct return system which contains several parallel circuits,

the pressure drop through each circuit at its rated flow must equal

the available difference in pressure between the supply and return mains

at the circuit connections. Since the available pressure differences will

vary with the distance from the pump, balancing valves may be required

in. some of the circuits to insure rated flow in that circuit.

At the end of this presentation, a work session is included to illustrate

problems encountered in designing a direct return system.

- 28 -

PUMPS

After the designer has laid out the piping system and figured out the

total pumping head, he must select a pump. The pump (or pumps)

must fit into the space available, and should be easy to service,

and be able to pump the necessary gpm against the existing head

at the lowest possible horsepower.

Information about pumps is found in the manufacturers catalogs.

Such information is usually based on pumping clear water at 60F

and is applicable without correction to most conditions found in air

conditioning work. • • .

PUMP TERMS

Capacity is usually given in gallons per minute, although other units

may be used. In any case, the basis is volume per unit of time.

Head is a form of energy and is usually expressed in feet of the

liquid being pumped or pounds per sq. in. Total head against which

a pump must work consists of suction head (or lift); discharge head;

friction head; and velocity head.

Suction Head is the total pressure at the pump suction nozzle and

includes; static suction head (or lift); entrance loss and friction head

T I I ' ).

fl .• . I

!

~· I

•

- 29 -

in the suction line; velocity head; and any positive pressure which may

exist on the suction reservoir. With the system in operation, a

pressure gage at the pump suction would indicate a positive static suction

head minus suction line friction head; minus velocity head. A vacuum

gage would read suction lift plus friction head plus velocity head.

Discharge head is the total pressure at pump discharge and includes

static discharge head; plus any positive pressure existing at the discharge

reservoir; discharge pipe friction loss; plus equipment pressure drop;

and velocity head. A discharge pressure gage would indicate total

discharge head but not including velocity head.

Total head is the Jischarge head minus the suction head, or discharge

head plus suction lift Nhere suction pressure is below atmospheric

pressure.

Velocity head is usually neglected in pump calculations because it is

a very small part of the total head (at 8 fps velocity it equals v2 /2g

or 64/64. 4 = 1 ft.) Calculations of pumping head are not sufficiently

accurate to warrant concern with velocity head. It should be

remembered, however, that the pump must furnish the additional energy

represented by the velocity head, and in open systems the velocity

head is lost when the water is discharged to atmosphere .

Liquid horsepower is obtained by the following formula -

Liq. HP= GPM x 8.33 (lb./gal) x Hd. (ft.) 33,000

= GPM x Hd. 3960

!1''

- 30 -

Brake Horsepower - is the power required to drive the pump and equals

Liquid Horsepower divided by the overall efficiency of the pump.

Overall efficiency is determined by test measurement and includes

mechanical as well as hydraulic losses.

Net Positive Suction Head (NPSH)

If the pressure anywhere in a piping system falls below the vapor

pressure of the liquid, vapor bubbles will form. When moved into

a higher pressure area, these bubbles will collapse. This is called

cavitation and is most apt to occur at the inlet to the pump impeller.

It causes noisy pump operation, rapid erosion and wear, or in extreme

cases, violent water hammer, and it must be avoided. Pump ma nu-

facturers give the NPSH required by their pumps, usually on the

characteristic curves, at various capacities (see figure 35 & 36).

NPSH is equal to the pressure drop in feet of liquid from suction flange

to the point inside the impeller where pressure starts to rise. Available

NPSH at the pump suction must always be greater than NPSH required by

the pump.

To find the available NPSH in a given system at the pump suction

flange, use the following formula:

NPSH = h - h + (h - h ) a vp e f

' '

• •