Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

is

avoided;

and (3) the

change

of flow is

gradual

and

does

not

upset primary sedimentation tanks

or

other

processes

in a

wastewater treatment plant.

The

disadvantages

of a

variable-speed drive

are

that

(1)

it

does

not

provide protection

from

power

fail-

ure,

(2)

such

a

drive

for a

motor

is

expensive

(it

costs

more than

the

motor),

(3) it may

require special train-

ing for

maintenance workers,

(4) it

adds complexity,

and

(5) it

reduces reliability somewhat.

If

surge con-

trol

is the

only objective, variable-speed drives

for

electric motors

are

seldom

the

best answer.

Engines

can be

easily used

for

variable-speed

drives unless there

is

prolonged operation

at low

loads. Note that engines

are

more reliable than

elec-

tric

power, although

the

maintenance required

is

high.

7-3. Control

Tanks

Control tanks range

from

the

simple, reliable stand-

pipe

to

hydropneumatic tanks (air chambers).

If all

else

fails,

they are, when

properly

maintained,

a

posi-

tive

and

reliable means

of

control.

Avoid

using surge tanks

for

sewage.

Air

cham-

bers

are

sometimes used with sewage,

but

they

should

be flushed

intermittently

or

continuously.

Some installations

are

successful

if

well maintained,

but

some experienced engineers will

not use air

chambers

for

sewage under

any

circumstances.

Air

chambers

are

worse than useless

if not

maintained,

because they become inoperable

yet

nevertheless

give

a

false

sense

of

security.

The

consequences

of

inadequate

maintenance should

be

dramatized

in the

O&M

manual.

Standpipes

or

open-end surge tanks (Figure 7-4)

provide

a

free

water surface

at

atmospheric pressure

and

act as

small reservoirs

to

accumulate

or

supply

water

temporarily

to

control pressure variations.

They

are

useful

at

high points where

the

hydraulic

grade line

is

within

20 or 30 ft of the

ground. They

are

simple

to

construct, easy

to

maintain, require

no

power

or

other utilities,

and are

completely auto-

matic

in

operation.

Overflow

drain, piping,

and a

water disposal area

must

be

provided because upsurges

often

cause over-

flow. The

height

of the

tanks

may be

aesthetically

unacceptable. They cannot

be

used

for

pipelines

with

a

large variation

in

HGL,

and

they must

be

protected

from

freezing

in

cold climates.

One-Way

Surge

Tanks

One-way

surge tanks (Figure 7-5)

are

somewhat simi-

lar to

Standpipes, and, because they

can be

pressure

vessels

as

well

as

gravity tanks, they

are

especially

useful

if the

original

HGL is too

high

to

allow

the use

of

a

standpipe.

If

used

as a

pressure vessel,

the

tank

must

be

closed

with

an

airtight cover containing

air

release

and

vacuum relief valves.

But

unlike stand-

pipes, they permit water

to flow

only

from

the

tank

into

the

pipeline. Therefore,

a

water-disposal system

or

storm drain (which

is

needed

for a

standpipe)

is not

required. One-way tanks

are

sometimes used

at

pump-

ing

station discharge headers

in low

head

(15-m

or

50-ft)

applications. They

are

simpler

in

design

and

operation than hydropneumatic tanks,

but are

more

complex than Standpipes.

Electrical power

is

required

to

operate

the

solenoid

valve

on the fill

line. Special attention

to the

selection

and

specification

for the

connecting check valve

is

required.

The

valves

are

usually

in

low-pressure ser-

vice

and

require

a

soft

seat material

to

prevent water

from

leaking past

the

disc.

Two-Way Surge

Tanks

Two-way

surge tanks

are

similar

to

one-way surge

tanks except that they have

no

check valve

and no

external

fill

line

(see

Figure

7-6). Ordinarily,

the

tank

is

full

of

water.

In

this condition

it is

ineffective

for an

upsurge,

but on a

downsurge

it

releases water into

the

pipeline while

a

large vacuum relief valve allows

air

to flow

into

the

plenum

to

avoid

a

vacuum.

If

there

is a

return upsurge, water

flows

into

the

tank against

the

increasing pressure

of air in the

plenum, which

can

escape (but only slowly) through

a

small

air

release

valve.

In

this operational sequence,

the

two-way tank

behaves much like

an air

chamber. Eventually,

the

surges

dissipate

and the

tank gradually

fills

with water

again.

Air

Chambers

An

air

chamber

(or

hydropneumatic tank)

is a

pres-

sure

vessel about half

filled

with

air and

half

filled

with

water

and

connected

to the

piping system

on the

discharge side

of the

pumps (see Figure 7-3).

The air

in

the

tank

is

compressed, which stores energy avail-

able

to

sustain

flow

after

power

failure.

Pipe inlets

to

these

tanks

are

usually

fitted

with

differential

orifices

that

allow water

to

exit

the

tank with

low

loss

but

cause high energy dissipation

for flow

into

the

tank.

The

operation during

a

downsurge-upsurge sequence

is as

follows:

•

Upon pump shut-down

after

power failure,

the air

pressure forces water

out of the

tank

and

into

the

pipeline.

As

water leaves

the

tank,

the

volume

of air

increases

and the

pressure decreases.

• The

decreasing pressure causes

the flow

into

the

pipeline

to

decrease gradually. Eventually,

the flow

in

the

pipeline stops

and

then reverses,

and the

downsurge

gives

way to an

upsurge.

• As

water enters

the

tank,

the

volume

of air

decreases

and the

pressure increases.

• The

increasing pressure causes

the flow to

decrease.

Eventually,

the flow in the

pipeline stops

and

then

reverses.

The

cycle then repeats until friction gradu-

ally

halts

it.

Because

the

tank must

be

large enough

for a

reserve

of

water

to

remain

at the end of the

downsurge

and for a

reserve

of

compressed

air to

remain

in the

plenum

at

the

height

of the

upsurge,

the

computer modeling must

be

accurate.

Air

chambers

are

very

effective

and

reli-

able

for

controlling both upsurge

and

downsurge

and so

versatile that they

can be

used

in

almost

any

water

pumping

system, although they

are

unusual

in

systems

with

force mains smaller than

250 mm (10

in.).

They

are

commonly used

on the

pumping station headers

and

at

high points.

Air

chambers require

a

fair

amount

of

complex auxiliary equipment

and

controls and, hence,

need

frequent

maintenance.

For

long, large transmis-

sion

mains,

the

site area

may

need

to be

enlarged.

The

relationship between

air

pressure

and air

vol-

ume

within

the air

chamber

can be

described

by the

equation

P

lV

\-

2

=

P

2

v

l

2

(7-1)

The use of the

empirical exponent

1.2 is

standard

practice

and

describes

a

gaseous expansion between

isothermal (exponent

=

1

.0)

and

adiabatic (exponent

=

1.43). Some analysts

use an

exponent

of

1.3.

The

following

are the

recommended design criteria

and

accessory equipment

for air

chamber systems:

• An air

compressor

to

supply

air

automatically

to the

chamber. Oil-free

air

compressors

are

required

for

potable water systems

by

some health departments.

• A

solenoid valve

to

vent excessive

air

from

the

chamber.

•

Level probes,

float

switches,

or

capacitance probes

in

the

chamber

or in a

separate probe well

to

start

and

stop

the

compressor

and

open

and

close

the

solenoid valve.

•

Liquid-level gauges

or

sight glass

to

observe water

level

in the

chamber. Equip

the

sight glass with

stopcocks

and

cleanout plugs

so

that

the

glass

can

be

cleaned while

the

tank

is in

service.

• A flanged

access opening

for

inspection

and

main-

tenance.

• A flanged

connection

to the

main pipeline.

•

Design

the air

chamber

in

accordance with

the

ASME Boiler

and

Pressure Vessel Code, Section

VIII

[4].

The air

chamber should bear

the

ASME

Code stamp. Code vessels are, however, very

expensive, especially

in

large sizes.

•

Provide

a

poppet safety-relief valve

on the air

chamber.

•

Specify slosh plates

in

horizontal

air

chambers.

Use

horizontal

air

chambers only

for

clean

water

—

never

for

sewage.

• The

ratio

of

air:

water

at

normal operating (steady-

state) pressure should

be

about

1:1.

• If an air

chamber

is

used

for

sewage

or

dirty water

service,

use

only

a

vertical tank with

a

hopper bot-

tom

(conical

or

elliptical)

and a

fresh

water supply

(for

flushing the

tank) pumped through

a

small (25-

or

38-mm

[1- or

I

1

I

2

-In.])

pipe.

The

pump

can run

either constantly

or

intermittently. However, read

the

warning

in

Section

7-1

before deciding

to use an

air

chamber.

7-4. Valves

for

Transient Control

Valves

can be

used

effectively

to

control both upsurge

and

downsurge

at

pump start-up

and

shut-down,

but

they

cannot prevent downstream column separation

if

that

is the

problem

(as in

Figure 6-8),

nor can

they

prevent downsurge upon power failure.

Air and

Vacuum

Control

Vacuum

Relief

Valves

The

only vacuum relief valve unequivocally recom-

mended

for

wastewater service

is

shown

in

Figure 7-2.

Although

it is

reasonably well protected

from

scum,

grease,

and

stringy material,

it

must nevertheless

be

serviced

on a

regular

schedule

—

at

least once every

four

months.

Vacuum

relief valves

for

water service

are

usually

accompanied

by air

release

valves,

as

described

in the

next subsection.

Air

Release

Valves

For

bleeding

air

from

pump casings

or air

pockets

from

other high points within

a

pumping station,

a

simple hand-operated cock

is

sufficient

in

small sta-

tions.

For

large stations

and for

long,

flat

pipelines,

use

air

release valves that close automatically when

air

is

expelled.

Add a

check valve

so

that

air

cannot

be

sucked

back into

the

valve.

Air

Release

and

Vacuum

Relief

Valves

Air

release

and

vacuum relief valves

(or

"air/vac

valves")

are

needed

to

remove

all of the air

during

pump start-up

and to

introduce

air to

prevent

a

vac-

uum

after

pump shut-down.

A

critical consideration

is

sizing

the air

exhaust

to

prevent excessive shock when

the

water columns meet

after

column separation. Flex-

ibility

can be

obtained

by

adding

a

throttling device

for

optimizing

the

release

of

air.

Air/vac valves

may not be

able

to

prevent vapor

pockets

at

high points, because

air may not be

admit-

ted

quickly enough.

The

location

of a

vapor pocket

in

a

pipeline

is

critical

and

cannot always

be

predicted

accurately,

so

valves cannot ensure complete protec-

tion. Furthermore,

the

valves

do

nothing

to

relieve

high

upsurge pressures.

If

air/vac valves

are

used

on

sewage force mains,

specify

stainless-steel trim

and

provide quick-connects

for

freshwater

flushing.

Instead

of

cleaning

the float and

linkage mechanism

by flushing the

valves

in the field, it

is

preferable

to

replace

the

interior mechanism with

a

clean

one and

take

the

dirty

one to the

shop

for a

thor-

ough

cleaning

and

overhaul. According

to

Murphy's

law,

however, someone will eventually

forget

to

open

the

isolation valve

and

render

the

valve useless unless

the

isolation valve

functions

like

the one

shown

in

Fig-

ure

7-2.

It

should

be

possible

to

design most

force

mains

so

that air/vac valves

are not

required. Some experi-

enced engineers

forbid

the use of

such valves

for

sewage

under

any

circumstances (see Section 7-1)

and

have

always

managed

to find a

different

control strategy.

Check

Valves

Substantial

control

of

surges

is

obtainable

by

selecting

the

correct valve. Some valves shut

off

very

fast

and

might

be

preferred

for a

short

(1-km

or

l

/

2

-mi.)

trans-

mission line. Some

can be

adjusted

to

close very

slowly

or to

close

at

different

rates

in

three stages.

Always

choose

a

valve that closes automatically

on

stored energy when power

fails.

Swing

Check

Valves

Swing

check valves should

be

supplied with

an

out-

side lever with

a

weight

or a

spring adjusted

to

close

the

valve just before

fluid

reversal occurs following

pump

shut-off

or

power failure. Such check valves

are

especially

useful

in low

head

(15-m

or

50-ft)

pumping

stations. Note that head loss depends less

on flowrate

than

it

does

on the

adjustment

of the

weight

or

spring

(see Figures

B-2 and

B-3).

Cushioned

Check

Valves

An

alternative

to

quick closure

is

cushioned closure.

Cushioned

check

valves have

either

an

air-filled

or an

oil-filled

dashpot that

can be

adjusted

for

rate

of

clo-

sure.

The

oil-filled dashpot

is

much more positive

in

its

action

and

more easily adjustable than

the

air-filled

type.

Check valves

for

sewage must

not

obstruct

the flow

and

must

not

have projections that could accumulate

stringy

materials. Consequently, such check valves

are

of

the

single-disc, top-pivot type.

The

angle-seated,

rubber-flapper

type closes

the

quickest,

but its

lack

of

an

external indicator

of

valve position

is a

disadvan-

tage.

A

variety

of

check valves, such

as the

double leaf

and

slanting disc types (which

are

suitable only

for

water),

have varying degrees

of

resistance

to

slam.

If

an air

chamber

is

installed

at a

pumping station,

the

check valves must

be

compatible.

The

stored

energy

in the air

chamber moves

a

short water column

quickly,

so a

check valve must close very

fast.

Rubber

seats

or an

oil-filled

dashpot

to

cushion

the

seating

may

be

helpful

in

preventing slam. Cushioned check

valves

are

effective

in

limiting surges

in

both pump

start-up

and

shut-down,

but

these valves cannot begin

closing until

after

the

pump stops.

Surge

Relief

Valves

Surge relief valves

act to

reduce upsurges. They

do

not

control

the

initial

downsurge that occurs

on

pump

shut-down

or

power failure. Hence, they

are

most use-

ful

in

short, steep pipe

profiles

where reversal

of flow

quickly follows power failure.

There

is a

wide variety

of

valves with guided discs,

pistons,

flappers, or

membranes available

in

spring-

actuated

and

diaphragm-actuated designs (controlled

by

springs,

air

pressure,

or

hydraulic pressure) that

are

kept closed until

an

upsurge arrives.

The

spring-actu-

ated type

is

shown

in

Figure 7-8. Surge relief valves

open quickly, remain open until

the

surge dissipates,

and

some then close slowly. Because upsurges travel

at

elastic wave speed, such

a

valve

may not

open

quickly enough

to

prevent

a

very short surge

of

high

pressure.

Hence,

insert

the

characteristics

of a

pro-

posed valve into

a

computer model

of the

system

to

determine what pressure rise

to

expect.

Very

short

surges

are

less important than longer ones

and are

dif-

ficult

to

model.

Surge

Anticipation

Valves

Surge anticipation valves

(Figure

5-14)

overcome

the

disadvantage

of the

surge relief valve

by

beginning

to

open before

the

upsurge arrives.

The

initial reduction

of

pressure following

a

power

failure

is

sensed

and a

timer

is

then actuated

to

open

the

valve

and

release

water

before

the

anticipated high pressure arrives.

The

sequence

of

operation

is as

follows:

•

Pump power failure occurs. Water continues

to flow

into

the

pipe away

from

the

pump,

but the

pressure

downstream

of the

pump drops.

• The flow in the

pipe halts, then reverses

and flows

back toward

the

pump

and

closes

the

check valve.

Pressure begins

to

rise

in the

pipe next

to the

pump

because

the

check valve acts

as a

closed-end

pipe.

• The

surge anticipation valve

has

already opened

to

release water

and

prevent high pressure.

The

valve

then

closes over

a

period

of

three

to ten

times

t

c

.

This type

of

valve reduces only

the

high transient

pressures;

it

cannot control

the

initial

low

pressures

that

occur

in the

pump discharge piping upon pump

power

failure

or

shut-down.

The

conditions under

which such valves

can

operate

are

restricted,

so

con-

sult

the

valve manufacturer. Pressure relief valves

without

the

"anticipation"

feature

may be

preferable.

Surge

anticipation valves

can be

used

for

water

but

not for

sewage.

7-5. Containment

of

Transients

One

method

of

coping with transients that

is

espe-

cially

useful

for

pipes

200 mm (8

in.)

or

less

in

diame-

ter is to

specify

thick-walled pipe

and

heavy valves,

pump

casings,

and

accessory equipment with higher

pressure ratings than necessary

for

normal operating

or

test pressures.

The

disadvantage

of

such

a

method

(beyond

the

obvious extra cost incurred)

is

that many

pipe materials

and

valves

may

eventually

fatigue

and

fail

after

several years

of

being subjected

to

intense

cyclic pressures. Hence, portions

of the

force main

may

have

to be

replaced eventually, which would

make this method costly indeed.

On the

other hand,

it

is a

simple method

to

use,

and the

short-term mainte-

nance cost

is

zero. This solution

is

common

for

small

pumping

systems.

For

larger pipes, other devices

are

more

effective

and

less costly.

An

overview

of the

various devices

is

given

in

Table

7-1.

7-6.

Surge

Control

for

Water

Pumping

Stations

The

installations discussed

in

this section

are

more

or

less typical

(or at

least common),

but

they

are by no

means universal.

Deep

Well

Pump

The

start-up

of a

deep well pump with

a

high static

head

poses

a

special

problem. With

the

pump turned

off,

the

pipe column between pump

and

check valve

is

empty. Unless

air is

admitted

to the

column,

the

fast-

rising water

may

slam into

the

closed check valve

(which

is

holding tons

of

water

at

rest) sometimes

with

a

tremendous force that

can

displace motors

and

pipes.

If

such

a

problem occurs,

it can be

controlled

by

•

admitting

air

into

the

column (through

an

orifice)

after

shutdown

so

that, upon start-up,

the air is

exhausted slowly

and

acts

as a

cushion; plus

•

adding

a

surge relief valve

to

bypass water until

a

pump-control valve

(or a

check valve) opens; plus

•

constructing stout well system piping.

A

deep well pumping system

is

shown

in

Figure

7-9

together with

a

typical arrangement

of

valves.

It is

assumed that

a

hydraulic transient analysis

has

been

performed

and the

need

for a

surge relief valve estab-

lished

for the

particular application.

For

clarity,

the

bypass piping containing valves

E and D is

shown

schematically

as

vertical. Physically, this equipment

would

probably

be

installed horizontally.

The

function

of

the

valves

is

discussed

in the

following subsections.

Air

release

and

vacuum

relief

valve. Valve

B

removes

air

during start-up

and

introduces

air

after

shut-down

to

avoid

a

vacuum.

The

valve must

be

sized

so

that optimum

air

cushioning prevents slam.

A

good

way

is to add a

throttling device

for flexibility in

adjustment.

Check

valve. Valve

C

prevents reverse

flow if (1)

the

pump

shaft

breaks,

(2) a

failure

in the

electrical

system shuts

off the

pump with

the

pump-control

valve

energized

and

open,

or (3) the

power

fails.

Applicable types

of

check valves include silent, dou-

ble

door, cushioned swing,

and

tilting disc.

By

placing

the

check valve

at the

location

shown,

the

transmis-

sion main

is

protected

from

water hammer

by the

relief valve.

Table

7-1.

Comparison

of

Devices

for

Controlling

Hydraulic

Transients

Method

and

application

Pump-control

valve (electric, hydraulic,

or

pneumatic);

for

water

and

sewage

Air

chamber; primarily

for

water

Standpipe;

for

water only

One-way

tank;

for

water only

Air

release

and

vacuum relief valve;

primarily

for

water, risky

for

sewage

Advantages

Effective

in

controlling surges

due to

pump

start-up

and

shut-down.

Allows

automatic control system.

Can

control surges

at any

operating

pressure

for the

pump

and

pipeline

system.

Pressurized vessel

can be

used

in

almost

any

pipe

and

pumping system

encountered

in the

waterworks

industry;

can be

sized

to

control

pressure changes

to

prescribed

ranges; commonly used

on

pumping

station

discharge header, high points,

or

knees

in

pipeline;

reliable.

Can

control both upsurge

and

downsurge.

Nonpressurized tank

or

tower.

Simple

to

construct

and

operate.

Used

at

high points

in

pipelines

to

supply

water

to

pipe

to

prevent

column separation.

Does

not

require electrical power

at

site.

Similar

to

standpipe

in

function;

contains check valve

on

connecting

pipe

so

tank

can be

used

in

system

in

which

the HGL is

much above

the

top of the

tank; used

at

high points

in

pipelines

to

supply water

to

pipe

to

prevent column separation.

Also sometimes used

at

pumping station

discharge header

in low

head

applications (usually discharge head

< 15

m

or < 50 ft);

simpler

in

design

and

operation than

a

pressurized

air

chamber.

Relatively simple

devices

to

install;

used

at

high point

in

pipeline

to

allow

air

to

enter pipeline

to

prevent large

negative (vacuum) pressures

or

reduce

effects

of

column separation.

Disadvantages

Ineffective

in

controlling surges

due to

power

failure.

Must

have auxiliary power source (e.g.,

batteries

or

compressed air)

to

operate

valve when power fails; even then,

it

may

not

protect against downsurge

and

column

separation.

Requires

a

fair

amount

of

complex

auxiliary

equipment

and

controls.

Requires

frequent

maintenance like

any

other

mechanical-electrical

system;

large tanks

can be

expensive; additional

land area required

at

site, especially

for

large tanks.

Not

recommended

for

sewage

due to

buildup

of

grease

and

settleable solids

and

production

of

gases;

if

used,

specify

frequent

blow-down

to

minimize

sewage

in

tank.

Cannot

be

used

if

normal

HGL at

high

point

is

more than about

9 m (30 ft)

above ground level because tank would

be

uneconomical.

Overflow

drain, piping,

and

water disposal

area must

be

provided

if

pressure cycles

in

pipe system

can

cause water

to

overflow

standpipe.

Requires tall tower, which

may not be

aesthetically acceptable

at

site; impractical

in

a

pipeline

that

experiences large

variations

in the

HGL.

Not

useful

usually

on

pump discharge

if

discharge head

is

greater than about

15 m (50

ft).

Requires tall tower, which

may not be

aesthetically acceptable

at

site.

Requires

special

attention

to

selection

and

specification

of the

connecting check

valve; requires separate

fluid

supply

to

maintain water

in the

tank.

Requires

regular

maintenance

so float and

linkage mechanisms

do not

hang

up or

corrode.

Should

not be

trusted

to

provide complete

protection

from

column separation,

because location

in the

pipeline

is

critical

and

cannot always

be

predicted

accurately;

does

nothing

to

relieve high

transient pressures (upsurges).

Surge

relief

valve. Valve

E

limits excessive pres-

sure during start-up

or

when

the

check valve closes

with

the

pump-control valve open. Hence, lighter

weight

equipment

and fittings can be

used.

The

surge

relief valve allows

the first

part

of the

well water

(which

may

contain sand

and

entrapped air)

to be

exhausted.

The

valve supplies

a

free

flow of

water

for

cooling submersible motors

if the

pump-control valve

fails

to

open,

and

also prevents

free

discharge

and

extremely high head, both

of

which

are

detrimental

to

the

pump.

The

pressure setting should

be

about

10%

higher than

the

normal pumping

pressure,

and the

valve

should

be

sized

to

handle

the

full

pump

flow.

Either globe

or

angle body valves

are

suitable.

Pump-control

valve. Valve

F

opens

and

closes

slowly

to

limit surges

in the

transmission main.

An

electric timer

(or an

alternate system) should open

it

only

after

all of the air (or

undesirable water)

has

been

wasted.

On

shut-down,

the

pump

is

kept running until

the

valve

is

about

95%

closed.

A

globe valve provides good protection against

surge

and is

easy

to

adjust

and to

service,

but it has the

greatest

headless

(although

it can be

sized

for

"accept-

able"

headloss).

A

butterfly valve

has low

headloss

and is

available

in

large sizes.

If

repairs

are

required, however,

it

must

(unlike

the

globe valve) usually

be

removed

and

sent

to the

factory.

Butterfly

valves

are not

often

used

for

pump-control

service.

Because

the

curve

of flow

ver-

sus

stroke

is

poor,

eccentric plug valves should

not be

used

for

controlled closure. Ball valves

are

excellent

because

of

their good throttling characteristics

and

low

headloss when wide open,

but in

large sizes they

are

expensive.

Shut-off

valves. Always install enough

shut-off

valves

(A, D, and G in

Figure 7-9)

so

that other equip-

ment

can be

repaired

without draining

the

entire sys-

tem. However, valves

A and D in

Figure

7-9 are

unnecessary because, whenever

the

pump

is

off,

all

the

piping upstream

from

the

transmission main

can

be

drained

by

stopping

the

pump

and

closing valve

G.

Valves

A and D are

actually hazardous (which should

be

noted

in the O&M

manual) because,

if

valve

A is

inadvertently

left

closed,

the air and

vacuum

relief

valve cannot

function,

and if

valve

D is

closed,

there

is

no

protection against water hammer.

Safety

features.

Provide

controls

(1) to

shut

off the

pump

if the

pump-control valve,

F,

does

not

open

and

Method

and

application

Swing

check valves;

for

water

and

sewage

Cushioned

swing

check valve;

for

water

and

sewage

Surge

relief valves; primarily

for

water

Higher

pressure rated piping, valves,

and

equipment

Advantages

Can

dramatically reduce

effect

of flow

reversal

in

pipeline

after

pump shut-

off

or

power failure.

Especially

useful

in low

head pumping

stations

in

which

the

discharge head

is

15m

(50 ft) or

less.

No

electrical

power needed.

Simple device

to

install

and

maintain.

Same

as

swing check valves, except: slam

can

be

reduced

or

eliminated; upsurge

can

be

partially controlled.

Effective

in

reducing positive surges

caused

by

reversal

of

water column

in

the

pipeline

after

power

failure.

Simple

to

use.

Used

in

many

small

raw

sewage pumping

stations

and

force

main systems;

short-term maintenance cost

is

zero.

Disadvantages

Cannot prevent downstream column

separation

in the

pipeline

if

that

is the

problem.

Slam

can be a

severe problem (see

Sections

5-4 and

7-1).

Does

not

control downsurge.

May

cause pump

to run

backward.

Cannot prevent downstream column

separation

in

pipeline

if

that

is the

problem.

Requires drain piping

and

disposal area

to

dispose

of the

water that drains

from

the

pipeline.

Pipe

and

other accessories

may

eventually

fatigue

after

several years

of

being subjected

to

cyclic surge

pressures.

In

the

long term, portions

of

force

main

may

fail

and

have

to be

replaced.

Cost

effectiveness

may be

poor.

Table 7-1.

Continued

(2)

to

close

the

pump-control valve

and

shut

off the

pump

if

there

is

sustained

low

pressure, which would

indicate

a

pipe break. Always include manual controls

to

override electrical malfunction

of the

automatic

equipment.

Control-valve

bypass.

The

valve arrangement

of

Figure

7-10

is

superior

because

it

provides

better

pro-

tection against upsurge than

the

valving

of

Figure 7-9.

The

surge protection

on

pump start-up

is

equally

effec-

tive,

and

because

the

pump-control valve

closes

in

anticipation

of

pump shut-down,

the

bypass arrange-

ment

limits

the

transmission main pressures

to the

relief

valve

setting.

The

configuration

of

piping

and

valves

in

Figure

7-10

is

particularly preferred

if the

pump

is to

start

and

stop

frequently

(several times

per

day).

The

pump-control valve,

E,

must have

a

fast-clos-

ing

feature

so

that

it

acts

as a

check valve

if the

power

fails

or a

pump

shaft

breaks.

If a

downsurge could

occur

as a

result

of

power failure, another vacuum

relief valve (with

a

large opening)

may be

needed

between valves

C and F. The

orifice

in the air and

vac-

uum

valve

A

must

be

carefully

sized

to

prevent exces-

sive

upsurge during pump start-up.

The

waste pipeline must have

a

positive

air

break

before

discharging into

a

sewer

or

drain line

to

pre-

vent

a

cross-connection

and

possible contamination

of

the

well.

The

surge relief valve should

be

protected

from

excessive wear.

So, if the

well water contains sand,

the

system shown

in

Figure

7-11

is

advantageous because

the first

portion

of the

discharge (which usually car-

ries

the

most sand)

is

vented through valve

B. As the

surge relief valve,

D, is

never opened (unless

a

surge

occurs

as a

result

of

power failure),

the

valve must

be

exercised periodically (say, weekly).

The

exercising

should

be

part

of the

pump monitoring program

and

so

specified

in the O&M

manual.

The

blow-off valve,

B,

can be a

manual type,

or a

small

pump-control

valve

can be

substituted

if

pump starts

are to be

auto-

matic.

The

pump-control valve,

C,

must

be

equipped

with

a

fast-closing feature

to

prevent reverse

flow

fol-

lowing power failure.

A

still simpler system

is

shown

in

Figure 18-13.

Whether

the

protection provided

by the

valves

of

Fig-

ures 7-9, 7-10,

and

7-11

is

required depends

on

the

static head,

the

length

and

size

of the

force main,

the

depth

to

groundwater,

and the

inclination

of the

designer toward conservatism.

Piping

flexibility.

Although

not

shown

in

Figures

7-9, 7-10,

and

7-11,

a

sufficient

number

of flexible

couplings specific

to the

installation

are

necessary

for

properly joining

fittings and

connections. Consider-

ations include:

•

Adjustment

for

potential misalignment

and for

pre-

venting

strain

in

rotating equipment

•

Sufficient

freedom

to

permit convenient removal

of

the

pump

and

other high-maintenance equipment

without

the

necessity

for

removing other valves

or

fittings

•

Resolution

of

hydraulic thrusts generated during

various operating modes.

Figure

7-9. Valves

for a

deep

well

pumping

station

with

an

upstream bypass. After

Hy/Con

Valve

Co.

[5].

Figure

7-10. Upstream

and

downstream

bypass

of a

pump-control

valve. After

Hy/Con

Valve

Co.

[5].

Figure

7-11.

Bypass

of

pump-control valve

for

sandy water applications. Gate valves

are

preferred over butterfly

valves

in the

smaller

sizes

(say,

<250

mm or 10

in.).

Butterfly valves should have gear actuators

to

prevent exces-

sively

rapid closure. Install enough flexible couplings

for

fitting

up.

After

Hy/Con

Valve

Co.

[5].

Flexible piping connections that

can

serve

the

above purposes include sleeve couplings (such

as the

Dresser® Style 38),

the flanged

coupling adapter

(such

as

Dresser® Style 128),

and the

grooved-end

coupling (such

as

Victaulic® Style 77). Designers

should

be

aware that

two

couplings

are

usually

required

for

effectiveness

in

coping with misalign-

ment

and

eliminating piping strain.

Low

Suction-Lift

Turbine

Pump

If

the

depth

to

water level

is

only

a few

feet

(as it

might

be for a

shallow well

or for

pumping

from

a

clear well

or

forebay),

the

same arrangement

of

valves

shown

in

Figure

7-9 can be

used with

the

following

modifications.

Air

release

and

vacuum

relief

valve. Valve

B can

be

smaller

and the

throttling device

can be

omitted

because

the

volume

of air to be

exhausted

is

small.

In

fact,

an air

release valve might

be

used

in

lieu

of the

air/vac valve.

Pump-control

valve.

The

opening

of

valve

F

need

not

be

delayed because

the

volume

of air to be

exhausted

is

small

and (if the

supply

is a

clear

well

or

forebay)

the

initial quality

of

water

is

good.

Safety

feature.

A

low-level cutout switch should

be

located

in a

clear well

or

forebay.

Air

Chamber

with

Turbine

Pump

For

wells without troublesome sand

or

silt

in the

dis-

charge,

an air

chamber

may

sometimes

be the

appropri-

ate

surge-arresting facility.

On

pump start-up,

air flows

through

an air

vacuum

valve

and

into

the air

chamber

from

which excess

air is

vented through

an air

release

valve.

When

the

pump

is

shut off,

a

vacuum breaker

allows

the

water

in the

pump

to

drain into

the

well.

If

the

depth

to the

water table

is

large enough,

the air in

the

pump column

can be

used

to

maintain

the air

charge

in

the

hydropneumatic tank, which eliminates

the

need

for

an air

compressor

and

level controls.

Turbine

Booster

Pump

Because

the

major

difference

between

a

well pump

and

a

booster pump

is the

continuously

flooded

suc-

tion

of the

booster pump,

no air

needs

to be

exhausted

upon

start-up.

Air

release valve. Vacuum relief

is not

needed,

so

valve

B in

Figure

7-9 or

7-10

need only

be a

small

air

release valve.

Check

valves.

The

purpose

of the

check valve

is to

prevent reverse

flow

through

the

pump

in the

event

of

a

power failure while

the

pump-control valve remains

open.

However, pump-control valves that close

quickly

in the

event

of a

power

failure

can

also serve

as

check valves.

Relief

valve.

The

relief valve

is

still needed

for

sev-

eral

of the

reasons given

in the

subsection entitled

"Deep

Well

Pump"

in

this section,

but

some

of

those

reasons obviously

do not

apply

to

booster pumping.

Pump-control

valve.

It is

necessary

to

open (and

close)

the

pump-control valve slowly but, because

the

system

is

always

full

of

water, unnecessary

to

delay

its

opening.

Safety

features.

In

addition

to

those listed

in the

subsection entitled

"Deep

Well Pump,"

a

pressure

switch

may be

desirable

to

keep

the

pump-control

valve

shut until

the

pressure developed

by the

pump

exceeds

the

pressure

in the

transmission main.

Centrifugal

High

Service

or

Booster

Pump

The

same valve arrangements

can be

used with cen-

trifugal

pumps with

flooded

suctions

for

both high

service

and

booster pumping. However,

the air

release

valve

should

be

located

at the top of the

pump casing

and

should

be

equipped with

a

check valve

to

prevent

air

from

returning

to the

casing.

A

manual petcock

on

the

pump casing

is

usually adequate

if the

suction

is

always flooded.

7-7. Surge Control

for Raw

Sewage

Pumping

Stations

The

strategies

for

controlling surges

in raw

sewage

pumping

are

limited

to

those given

in

Section

7-1.

Those

transient control strategies are, however, ade-

quate

for

nearly

all

situations.

Of

course,

any

pro-

posed solution should

be

checked

by a

computer

analysis.

Some additional comments

on

surge control

are

presented below.

Quick-Closing

Check

Valve

If

the

pump spin-down time

as

determined

by

com-

puter analysis

is

sufficiently

long (say, more than

four

or

five

t

c

)

and

there

are no

knees,

a

quick-closing

check valve

is

usually

sufficient

to

prevent surges.

Such

conditions

are

common with force mains

not

much

longer than

1 km

(0.6

mi) and

with

uniform

gra-

dients

of

less than about

4%.

Pump-Control

Valve

If

a

pump-control valve

is

provided

so

that

the

pump

starts

and

stops against

the

closed valve,

it can

only

be

operated slowly

on

both closing

and

opening.

A

com-

puter

analysis should

be

used

to

program

the

closure

time.

The

pump-control valve

can be an

eccentric plug

or a

lubricated plug,

but

cone

and

ball valves have

the

best characteristics

for

this service

and are

preferred

even

at

their higher cost.

If

a

power

failure

occurs,

a

stored-energy system

must

be

used

to

activate

the

pump-control valve.

But

unless

the

valve closes quickly,

the

water runs back-

ward

through

the

pump,

so the

pump, impeller,

and

wet

well systems must

be

able

to

handle

the

back-

ward-flowing

sewage.

Increasing

Rotational

Inertia

If

column separation

can

occur,

first

investigate

the

increase

of

inertia

(WR

2

)

of the

moving parts.

If the

required additional

WR

2

is not too

great, this

is a

solu-

tion with

the

utmost reliability

(see

Section 7-2).

Other

Control

Strategies

If

none

of the

foregoing

can be

used

to

control surge,

consider

the

other strategies outlined

in

Section

7-1.

7-8.

Pipeline

Design

Selecting

the

wall thickness

to

withstand

the

pressures

expected

in the

operation

of a

pipeline

is

crucial. Wall

thickness depends

on the

pressures

due to

normal

operation, upsurge,

and

downsurge

as

well

as the

pipe

material

and its

safety

factors.

The

safety

factors

are

governed

by

user groups (such

as the

American Water

Works

Association), industries (such

as the

American

National Standards

Institute,

Inc.),

or by

manuals

of

practice.

In

some standards,

a

specific

overpressure

allowance

is

required, whereas

in

others

a

safety

fac-

tor

to be

applied

to the

yield strength

of the

pipe mate-

rial

is

given.

Upsurge

The

required wall thickness

for

positive line pressures

for

normal upsurge

is

•

-

'ST

ZS

y

£j

where

e is

pipe wall thickness

in

meters (inches),

P is

internal pressure

in

newtons

per

square meter (pounds

per

square inch),

D is

outside diameter

in

meters

(inches),

SF is the

safety

factor,

s

y

is the

yield strength

in

newtons

per

square meters (pounds

per

square

inch),

and

E

}

is

longitudinal joint

efficiency

associated

with

the

effective

strength

of the

weldment.

The

allowable stress,

s, may

either

be

specified

as a

percentage

of the

yield strength

or as

s

y

/SF.

Typical

values

of SF for the

water industry

are

given

in

Table

7-2.

The

longitudinal joint

efficiency,

E^

is

deter-

mined

by the

type

of

weld

or it may be

considered

to

be

accounted

for in the

allowable stress specification

(see ANSI

B3

1

.

1).

The

joint

efficiency

factors

in

Table

7-3 are

used primarily with welded steel pipelines.

Standards

for

materials

of

construction

often

include

an

allowance

for

pressure

due to

hydraulic

transients.

For

example,

"ordinary"

surge pressures

are

incorporated

in the

safety

factor

of 4.0 for

ACP,

which

conforms

to

AWWA

C400

for

internal pressure

in

combined loading,

but the

term "ordinary surge

pressure"

is not

defined.

A

maximum value

of 350

kPa

(50

lb/in.

2

)

in

addition

to the

design operating pres-

sure

is

typical.

For

higher surge pressures,

the

pipe

wall

thickness should

be

increased.

A

design proce-

dure

for

including such exceptional surge pressures

in

the

design

of ACP is

described

in

AWWA

C401.

Table

7-2.

Typical

Safety Factors (SF)

for

Pipe

3

Type

of

pipe

Pressure

condition Ductile iron

Steel

PVC AC

Maximum

operating

2.0 2.0

2.0-2.5

2.0-4.0

Upsurge transient

a 1.5 a a

Downsurge collapsing

4.0 4.0

None None

a

Include

the

upsurge

transient

in

maximum

operating

pressure.

Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

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