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

Unless some leakage

is

acceptable

and the

head

is low

(say,

6 m or 20

ft), another type

of

valve should

be

selected.

The

knife gate

is

adapted

for

pressures below

approximately

170 to 350

kPa

(25 to 50

lb/in.

2

).

Pinch Valve

A

pinch valve

is

essentially

a

rubber

or

elastomer tube

closed pneumatically

or by a

screw, wedge,

or

lever.

The

valve

is

leakproof

and

requires

no

packing,

but it

is not

suitable

for

high pressures

and the

tube weakens

where

it is

compressed. When closed pneumatically,

it

is

an

inexpensive

and

suitable valve

for a

bypass pipe

around

a

positive displacement pump

for

guarding

against

damage

due to a

blockage

or to an

inadvert-

ently

closed valve.

But it is

limited

in

size,

the

tube

is

relatively expensive

and

difficult

to

replace,

and the

laying

length

is

greater than

for

other valve types.

Plug

Valves

(Lubricated

and

Nonlubricated)

A

plug valve contains

a

cylindrical

or

tapered plug with

an

opening cast

or cut

into

it. A 90°

turn

of the

plug

fully

opens

or

fully

closes

the

valve. Hence, plug valves

are

considered

to be

quarter-turn valves.

The

design

of the

plug

seat

is

such that solids

do not

accumulate

and

cause

the

plug

to jam or

bind. Plug valves

can be

obtained with

full-ported

plugs

or

with reduced port areas.

Lubricated plug valves (Figure 5-9) contain

a

lubri-

cating

system

in

which

a

nearly solid lubricant

is

forced

into

the top of the

plug

and

through

a

series

of

Figure

5-9.

Lubricated plug

valve.

Courtesy

of

Rockwell

International.

grooves into

the

bottom

so

that

the

faces

of the

plug

and

seat

are

wiped with

the

lubricant, which

functions

as

a

deformable

sealant each time

the

valve

is

open

or

closed.

This feature

is

attractive

in

applications where

the

valve

may be fixed in the

open

or

closed position

for

a

long time.

The

valve

is

less

likely

to

freeze

in

position, and,

if

necessary,

a

small amount

of

lubricant

can be

forced into

it to

unfreeze

it.

Many

different

lubricants,

which allow

the

valve

to be

used

in

differ-

ent

fluid

services,

are

available. Lubricant

can

also

be

used

in

slurry service because there

are no

spaces

that

can

become packed with

solids.

On the

other hand,

some

fluids can

dissolve

the

lubricant

off

the

plug

face,

which

would cause

the

valve

to

seize

and

gall,

but

this

usually

is no

problem with

the fluids

encountered

in

water,

waste

water,

and

sludge pumping stations.

A

plug valve provides

a

very tight

seal

and is

especially

useful

in

service pressures greater than 1000

kPa

(150

lb/in.

2

).

In the

cylindrical style, excess lubricant

is

forced

outside

of the

valve body where

it can be

exam-

ined

for

contamination, less torque

is

required

to

turn

it,

and the

rotor

has a

100%

bore.

Both styles

are

excel-

lent

for

pump control service. Lubricated plug valves

require occasional

but

simple maintenance.

Nonlubricated plug valves have

become

unpopular,

and

several former manufacturers

no

longer make

them. They seem

to

have

no

advantages over

the

lubri-

cated types.

5-3.

Sluice

Gates,

Shear

Gates,

Flap

Valves,

and

Stop Plates

The

devices

in

this section

are not

actually valves,

but

they

are

used

as a

means

of flow

isolation

in

channels

and

on

ends

of

pipes such

as

those entering

wet

wells.

Sluice

Gate

Sluice gates (Figure 5-10)

are

used against

a

wall

between

two

basins

or

between

a

pipeline

and an

open

channel. Sluice gates

are

usually

located

where

the

influent

channel

or

sewer enters

the wet

well

and can

be

used

if the

pumping station

• is

subject

to flooding due to

excessive

influent

flow

or

pumping failure;

• is

critical, that

is, if any

major

emergency repairs

must

be

done

as

quickly

as

possible;

• has two wet

wells;

or

• has

submersible pumps

and the wet

well must

be

dewatered when

(or if) the

pump

lifting

mechanism

jams.

Figure

5-10.

Sluice

gate.

Courtesy

of

Rodney Hunt

Co.

Stems

and

actuators must

be

sized

to

overcome

large

friction

forces

for the

following reasons:

•

Breakaway forces

are

sometimes greater than

the

manufacturer's

typical recommendation, particu-

larly

if the

gate remains closed

for a

protracted

period.

•

Power actuators

can

develop very large torques

and

thrusting

forces. Therefore,

it is

prudent

to

size

operating stems

in a way

that will more than ade-

quately

sustain those forces, especially when

the

fluid

contains

debris that

can be

trapped underneath

the

sluice gate leaf when closing.

Once

the

gate

has

been broken loose,

the

operating

forces

are

reduced greatly.

Shear

Gates

Shear gates

are

mounted

on the

ends

of

open pipes.

Unlike conventional valves, they cannot

be

mounted

in

a

pipeline.

One

side

of the

shear gate contains

a fixed

bolt;

on the

other side

is a

lever, which

is

used

to

lift

and

rotate

the

gate about

the fixed

bolt

on the

other side.

Flap

Valves

Flap valves (see Figure

5-11)

for

pump discharge

are

substitutes

for

check

and

isolation valves

and are an

economical, reliable method

of

preventing

backflow

through

out-of-service pumps. With this type

of

device,

no

isolating valve

is

installed.

Rather,

the flap

valve

is

installed

on the

individual pump discharge

piping

at the

point

of

discharge

to the

receiving sewer,

channel,

or

discharge structure.

A

prudent engineer

makes some provision (slide gate

or

bulkhead slots)

for

isolating

the flap

valve

for

maintenance purposes,

but

no

expensive, heavy-duty isolating valves

are

required.

Be

sure

to

provide

a

vent just upstream

from

the flap

valve

to

drain

the

pump discharge

and

prevent

slam.

Use flap

valves with

a

cushion design

specifi-

cally intended

for

pump discharge service.

Stop

Plates

A

stop plate

is a

thin, vertical, rectangular plate used

to

form

a

temporary

dam in

open channel

flow. It is

Figure

5-11.

Flap

valve designed

for use

with pump dis-

charge. Courtesy

of

Rodney Hunt

Co.

sometimes used

in the wet

wells

of

pumping stations

to

block

flow to

part

of the wet

well

so

that

a

pump

and its

suction piping

can be

dewatered

for

mainte-

nance.

The

plate

may

have

its own

actuator

or may be

lifted

by

hand. Large plates

can be

lifted

with

a

crane

or

hoist

and

stored

on a

rack

or in a pit

when

not in

service.

The

plate

is

usually

aluminum,

but

wood,

fiber-

glass, stainless steel,

and

other materials

are

some-

times

used.

A

local fabricating shop

can

make stop

plates

if

supplied with detailed design information.

Alternatively,

a

somewhat more sophisticated plate

can be

obtained

from

manufacturers, which means

the

engineer need

not

design such details

as

reinforcing.

Except

for

very small units, stop plates cannot

be

moved

up or

down when there

is a

substantial

difference

(more than about

0.2 m or 6

in.)

in

water level across

the

gate.

If it is

necessary

to

move

the

plate under

such

conditions,

(1) a

sluice gate

may be

used instead

or (2) a

valve

(typically

a

100-

to

200-mm

[4- to

8-in.] gate

or

butterfly)

or

small stop plate

can be

mounted

in a

larger

plate

to

allow equalization

of

water levels before

the

larger plate

is

moved. Stop plates

are

inexpensive,

are

simple,

are

suitable

for

local fabrication,

and

take

up

lit-

tle of the

valuable space

in a wet

well,

but

moving them

is

awkward

and the

leakage

is

high.

5-4. Check

Valves

A

check valve

is

usually

(but

not

always) required

to

(1)

prevent reverse

flow and

prevent runaway reverse

pump

speeds when

the

pump

is

shut off,

(2)

keep

the

pipeline

full

of

water

to

prevent

the

entrance

of

air,

and

(3)

minimize water hammer

and

surges

for

pump

start-up

and

shut-down.

Vertical

pipelines

are

poor

locations

for

check

valves

if the

water contains grit

or

solids.

For

verti-

cally

placed valves

in

clean water service, special

springs

or

counterweights

may be

needed. Manufac-

turers that state that

a

check valve

can be

placed

in a

vertical

pipeline

are

referring only

to the

springs

or

counterweights

and

ignoring

the

danger

of

deposited

grit

and

solids, which

can

(and will)

jam the

valve.

The

designer's responsibility

is the

selection

of a

valve

that

will

give good service

in

keeping with

the

pump selection, hydraulics,

and

size

of the

system.

The first

decision

is

whether

a

check valve will serve,

or

whether

a

more

sophisticated

pump-control valve

is

necessary

to

limit surges. Some insights

for

this deci-

sion

are

contained

in

Chapters

6, 7, and 26 as

well

as

in

Parmakian

[12],

but

only

a

sophisticated mathemat-

ical

model

of the

system solved

by

means

of a

com-

puter

can

provide

a

rational analysis. Unfortunately,

such modeling

is too

time consuming

and

expensive

for

common use.

Valve

Slam

Check valves

can be

divided into

two

broad classifica-

tions:

(1)

those that

are

closed

by the

static pressure

of

water above

the

valve (mechanical checks)

and (2)

those held shut

by an

external actuator (pump control

or

controlled check valves).

The

latter

do not

slam,

but

swing checks

do if,

before

the

valve

is

fully

closed,

any

substantial reverse velocity catches

the

valve disc

and

accelerates

it

until

it

strikes

the

body seat

abruptly.

The

sudden stop

of

disc, lever,

and

counter-

weight

(if

there

is

one) plus

the

violent impact

of the

disc

on the

body seat (especially

if the

contact

is

metal

to

metal) causes

an

explosive noise

and

vibrations that

shake

the

pipe

and may

shake

the

whole building.

The

real

problem

is the

water hammer that results

if the

water column

is flowing

backward

at a

significant

velocity when

the

valve

closes.

However, valve slam

can

occur without water hammer

and

vice versa.

At

worst, valve slam

can

rupture water

lines

and

pump casings.

At

best,

it is

annoying.

In

between,

it

pounds

the

system,

can

overstress pipes

and

joints,

and

may

well result

in

eventual leaks

and

greatly

increased

maintenance.

It is

difficult

to

give advice

on

the

best kinds

of

valves

to

specify

because valve slam

depends

on

many interrelated factors

in

addition

to

valve

design. Other factors that

are

just

as

important

include static head,

friction

head,

the

inertia

and

spin-

down characteristics

of the

impeller

and

motor, size

of

pipe,

and

velocity

of flow.

Generally, valve slam

is

caused

or

aggravated

in the

following ways.

Low flywheel

effect.

The

principal cause

of

valve

slam

is

quick

deceleration

of the

pump

due to low

angular momentum

of the

impeller,

the

driver,

and

the

water within

the

casing. With enough inertia,

valve slam

can be

prevented,

but the

necessary

fly-

wheels

may be

large

and

costly.

High proportion

of

static head.

If the

headless

is

70%

static

and 30%

dynamic (due

to

friction),

valves slam worse than

if the

headloss

is 50%

static

and 50%

dynamic.

A

simple vertical

lift

(e.g., into

an

adjacent elevated tank)

is

especially

prone

to

valve

slam.

Frequency

of

valve slam. Valve slam

may

stress

material beyond yield strengths

and

cause perma-

nent deformations.

A few

deformations

of a

given

intensity

may be

acceptable,

but

numerous defor-

mations eventually cause leakage

or

rupture. Even

a

single slam,

if

severe,

is

dangerous.

Large

pipe diameter.

As

valve size increases,

the

resulting time

to

close

it

increases,

the

disc velocity

increases,

and the

energy

in the

system increases.

Parallel pumps.

If two

pumps

are

connected

to a

header

and

pump

1

shuts

off

while pump

2 is

oper-

ating, pump

2 may

cause

the

short water column

between

the two

pumps

to

reverse very quickly and,

thus,

cause

the

check valve

of

pump

1 to

slam.

Column separation. Water column separation

can

cause valve slam

in two

different

ways:

(1)

rapid

reversal

of flow

through

the

check valve, even

though

flow in

most

of the

pipeline

reverses slowly,

and

(2) a

fast-rising positive pressure surge

due to

the

collapse

of a

vapor cavity

if the

surge arrives

at

the

valve when

it is not

closed

fully

(see Chapter

6).

Air

chambers

and

surge

tanks. These units

can

pre-

vent

column separation,

but at the

same time they

can

cause rapid

flow

reversal

at the

valve

and

thus

aggravate valve slam (see Chapter

7).

Insufficient

closing force.

If the

closing force

due to

the

disc weight

and

spring

or

counterweight

is

low,

the

valve operates

too

slowly.

But the

closing force

should

not be so

high that

the

valve does

not

open

fully

under

steady-

state

pumping conditions.

If the

valve

is not

fully

open,

the

headloss increases

and

debris

is

more likely

to

hang

up in the

valve. Also,

excessive closing force

can

cause

the

disc

to

bounce

off

the

seat

so

that valve slam recurs, sometimes

two

or

three times.

Constant-

speed

pumps. Constant-speed pumps have

two

features that aggravate valve slam:

(1)

they

must

be

turned

on and off at

full

capacity (unlike

variable speed pumps),

and (2) as

they

are

turned

off,

their speed cannot

be

ramped down gradually.

With

variable-speed pumps,

the

speed

can be

ramped down during normal shutdown (although

not

when

the

power

fails).

Friction

in the

hinge

bearings. Friction

is

increased

by

dirt

and

corrosion.

If the

disc hesitates

before

moving, valve slam

is

almost certain

to

occur

—

mostly

significant with tilting disc check

valves.

Body

shape. Details

of

check-

valve

design

influ-

ence

the

closure operation. Because

the

disc must

open wider

in a

valve with

a

straight body than

in a

valve

with

a

bulbous body (Figure

5-12),

the

move-

ment upon closing

is

correspondingly greater

and

the

valve slam

may be

greater.

Inertia. Closing time increases with inertia

of

mov-

ing

parts.

A

counterweight

in a

valve without

a

dashpot

may

therefore cause valve slam,

and

replacing such

a

counterweight with

a

spring some-

times minimizes slam

if the

pumps have

no

signifi-

cant spin-down time.

Figure

5-12.

A

swing

check

valve

at

full

flow.

After

GA

Industries,

Inc.

Preventing

Valve

Slam

Valve

slam

can be

prevented,

or at

least kept within

bounds,

by

•

using

a

valve that closes

quickly

—

before

the flow

can

reverse

by

adding

a

heavy counterweight

or a

stiff

spring

to the

external lever;

•

adding

a

dashpot

or

buffer

to

make

the

disc seat

gently

but

before

the flow can

reverse;

or

•

closing

the

valve with

an

external actuator

so

that

the

water column

is

gradually brought

to

rest with-

out

a

significant

increase

in

pressure.

The first two

methods

may

prevent valve slam

but do

not

necessarily prevent pressure surges.

Small Valves

For

small valves, either

confine

swing checks

to

pipe

less than, say,

250 mm (10

in.)

in

diameter

or

precede

the

valve with

a

reducer

and

follow

it

with

an

expander. Ordinary swing check valves

are

manufac-

tured

in

large

sizes,

but

there

is a

potential

problem

in

using them

in a low

friction head system because they

cannot

close

quickly enough.

Spring-Loaded

Levers

Many engineers advocate

the use of

springs instead

of

counterweights

to

reduce

the

inertia

of

moving parts

and

thus speed

the

closure. However,

as the

valve

closes,

the

spring tension relaxes

and the

torque

on the

disc

shaft

may

decrease enough

to be

insufficient

to

prevent valve slam,

so

choose

a

design

in

which

the

combination

of

spring tension

and the

lever

arm

between

the

spring

and

shaft

creates high torque

at

closure.

A

resilient seat aids

in

minimizing contact

noise. These

are the

least expensive check valves.

Counterweight

and

Dashpot

A

counterweight

has the

advantage

of

providing max-

imum

torque

on the

disc

shaft

at

closure,

but it

does

not

close

the

valve

as

quickly

as a

spring because

of

the

inertia

of

moving parts. Some (especially manu-

facturers)

think

the

valve should

be

equipped with

either

(1) a

side-mounted

or

top-mounted

oil-filled

dashpot

to

cushion

the

movement

of the

lever

at

shut-

off

or (2) a

bottom-mounted piston-type shock

absorber that engages

the

disc before

it

closes.

Air-

filled

dashpots

are

difficult

—

occasionally

impossi-

ble

—

to

adjust

to

prevent valve slam.

The

required

massiveness

of

construction

needed

to

resist

the

high

force

of

water against

the

disc

and the

dashpot mecha-

nism

makes this valve more expensive,

but it is the

recommended style when

the

spring-loaded lever type

is

inadequate.

But

note, however, that

a

heavy coun-

terweight

or a

stiff

spring, properly adjusted,

is

cush-

ioned

by the

water

in the

valve.

Pressure-Regulated

Bypass

Dump

A

spring-loaded, pressure-actuated surge relief valve

(Figure 7-8) with

a

pipeline

returning

the

wasted

water

to the wet

well

can

reduce

the

surge

to an

acceptable, preset level,

but it

does nothing

to

mitigate

valve

slam.

Actuator-Controlled

Plug

or

Ball

Valve

An

actuator

can be

programmed

to

both open

and

close

the

valve slowly enough

to

prevent water ham-

mer

(see Figure 7-7).

A

stored energy system

is

needed

to

operate

the

valve when power failures

occur.

Summary

Selecting

a

proper type

of

check valve

and

control

mechanism

is

more

art

than

science.

Experience,

not

analytical

theory,

is a key

consideration.

Of

course,

a

simple method

to

determine whether

a

conventional

swing

check valve

can be

used without excessive

valve

slam would

be

desirable,

but

unfortunately

the

complexity

of the

problem precludes

a

simple, accu-

rate procedure. Complex computer programs

can be

used

to

predict with

fair

accuracy whether valve slam

will

occur.

The

cost

of the

analysis

may be

discourag-

ing

if the

system

is

small,

but

large systems should

always

be so

analyzed.

If any

general statement

on

check-

valve

selection

can be

made,

it is

probably this:

use

a

swing check valve with

an

outside lever

and

spring.

If

that

is

inadequate,

use a

valve with

a

cush-

ioned closure system such

as a

dashpot

or

bottom

buffer.

As a

last resort,

use a

powered actuator.

But

note that even

the

experts

disagree,

and

some prefer

counterweights

to

springs.

Check

Valves

for

Water

Service

Check valves

useful

for

water service include

•

Swing check valves

•

Center-post guided

(or

silent) check valves

•

Double leaf

(or

double door, double disc,

or

split

disc) check valves

•

Foot

valves

•

Ball

lift

valves

•

Tilting

(or

slanting) disc check valves.

The

several styles

of

swing check valves

can be

divided into those with

and

those without

an

outside

lever. Outside levers

can be

equipped with either

springs

or

counterweights,

and the

levers

can be

cush-

ioned

or

noncushioned. Bottom

buffers

can be

used

instead

of

dashpots

affixed

to the

outside lever.

Check

Valves

for

Wastewater Service

Valves

for

wastewater

service

must

be

capable

of

passing large solids and,

as

with isolation valves,

must

have

no

obstructions

to

catch stringy material.

Valves

likely

to be

used

for

wastewater

are

essentially

limited

to

•

Swing check valves

•

Flap valves, which might

be

used

in

special circum-

stances (for example, with combined sewers that

contain storm water

and

wastewater)

•

Ball

lift

valves, which

are

useful

in

positive

dis-

placement sludge pumps

as the

ball

can be

lifted

completely

out of the flow

path.

The

rubber clapper swing check valve

has no

outside

lever,

is not

fully

ported, and, hence, should

not be

used

for raw

sewage

or

sludge.

Description

of

Check

Valves

The

following descriptions

of

check valves

for

water

and

wastewater

offer

some guidance

and

suggestions.

A

summary

of

recommendations

for use is

given

in

Table 5-3.

Ball

Lift

Check

Valves

A

ball

lift

check valve contains

a

ball

in the flow

path

within

the

body.

The

body contains

a

short length

or

guide piece

in

which

the

ball moves away

from

the

seat

to

allow

the

passage

of fluid.

Upon reverse

fluid

flow, the

ball rests against

an

elastomeric seat.

These valves

are

often

encountered

in

pumping

stations

in

sizes

of 50 to

150

mm (2 to 6

in.)

or

smaller

for

pump seal water

or for

wash water supply piping.

They have also been

successfully

used

by at

least

one

major

pump manufacturer

for raw

wastewater pump

discharge piping

up to 600 mm (24

in.).

Except

for

small sizes, bodies

are

made

of

ductile iron.

The

ball

is

hollow with

an

external rubber coating resistant

to

grease

and

dilute concentrations

of

petroleum prod-

ucts,

acids,

and

alkalies.

The

specific gravity

of the

balls

can be

adjusted

to

suit

a

wide range

of

operating

conditions.

The

valves

are

said

to be

self-cleaning, rugged,

reliable, nonclogging

and to be

able

to

withstand

repeated cycling, because each time

the

ball

is

reseated,

a

different

part

of the

surface rests

on the

seat.

In

larger sizes

(100

mm [4

in.]

or

more)

for

sludge pumping service, ball

lift

checks

are

part

of the

mechanism

in

plunger (piston) sludge pumps.

As

check valves

for

wastewater pump discharge

piping,

the

valves have these advantages:

(1)

the

head-

loss

is

lower than

it is for

other types;

(2)

there

are no

external penetrations

and no

leakage

to the

outside

(although

good swing check valves properly

set up do

not

leak either);

and (3)

stringy

materials

have nothing

to

wrap around

and do not

foul

the

valve.

On the

other

hand,

the

standard valve (unlike swing check valves)

gives

no

indication

of

whether water

is flowing

—

a

serious disadvantage. Ball

lift

check valves

are, how-

ever, available with

ball

position indicator-proximity

switches.

Decisions

to use a

ball

lift

check valve instead

of,

say,

the

faster-closing swing check valve with

a

spring-loaded lever should

be

based

on a

computer-

aided dynamic hydraulic analysis

of the

system.

Center-Post

Guided

Check

Valves

Center-post guided check valves

are low

cost

and are

called

"silent

check

valves"

by

some manufacturers.

They close more rapidly than

any

other check valve.

As

shown

in

Figure

5-13,

the

disc

is

held closed

by a

spring until

the

pump

is

started.

The

spring selection

is

very critical;

it is the

differential

pressure across

the

valve (difference between static head

and

TDH)

that

must

be

specified

and not the

safety

pressure rating

of

the

system.

An

incorrect specification results

in

valve

slam.

Three disadvantages

of

this valve type

are

that

(1)

the

operating mechanism

is

enclosed

so the

valve

must

be

removed

for

servicing,

(2)

there

is no

external

indicator

of the

position

of the

disc,

and (3) the

head-

loss

is

high.

Double

Leaf

Check

Valves

Double leaf (also double door, double disc,

or

split

disc) check valves contain

two

hinged half-discs

in a

short

body.

The two

half-discs

are

hinged

in the

mid-

dle and

contain

a

spring that forces them

closed.

This

type

of

valve

has no

connecting

flanges of its

own.

Table

5-3.

Recommendations

for Use of

Check

Valves

3

Water

Waste

water

Type

of

valve

Raw

Clean

Raw

Treated Sludge

Ball

EEEEE

Ball

lift

F-G F-G F-G F-G G

Center-post guided

PFXXX

Double

door

XGXXX

Flap

— — G — —

Foot

F G — — —

Swing

No

outside lever

PPPPP

Outside lever

and

counterweight

F-G

F-G F-G F-G G

Outside

lever

and

spring

F-G

F-G F-G F-G F

Outside lever

and air

cushioned

FFFFP

Outside

lever

and oil

cushioned

GGGGG

Slanting disc

GGXFX

a

E,

excellent;

G,

good;

F,

fair;

P,

poor;

X, do not

use;

—,

use is

unlikely.

Figure

5-13.

Center-post

guided

"silent"

check

valve.

Courtesy

of

APCO

Valve

&

Primer

Corp.

Instead,

it is

inserted between

two

adjacent pipe

flanges.

It

can be

installed

in

either

the

horizontal

or

vertical position.

These valves should never

be

used

in

sewage

or

sludge

service

or in

abrasive conditions because

the

hinge

and

discs

can

catch solids

and the

seat

and

discs

would

wear

in

abrasive service. Double leaf check

valves

are

small, light,

and

inexpensive

and

have

a

short

laying length. They close very quickly

but

they

cannot

be

adjusted

from

the

outside,

nor can

they

be

cushioned,

so a

sudden

flow

reversal

can

cause slam

and

water hammer. Shut-off

is not

leakproof

.

Other

disadvantages

are (1) the

valve must

be

removed

to

service

the

mechanism,

(2)

there

is no

external indica-

tion

of

whether

the

valve

is

open

or

closed,

and (3)

they

are

subject

to a fluttering

motion caused

by

vor-

tex

shedding

as the fluid

moves past

the

valve plates.

If

the fluid

velocity

is

less than

3.4 m/s

(11

ft/s)

and

the

valve

is at

least eight pipe diameters downstream

from

any

source

of flow

disturbance such

as a

pump

or

a fitting, the

problem

is

reduced

[13].

Foot

Valves

A

foot

valve

is a

special design

of a

lift

check valve.

It

is

used

in the

suction line

of a

sump pump

to

prevent

loss

of

prime.

It is

designed

for

upflow

and is

attached

to the

bottom

of a

pump suction pipe.

Foot valves

are

prone

to

leakage, especially when

used

in fluids

containing abrasives

and

solids,

and

they

are

difficult

to

service. Foot valves decrease

the

net

positive suction head available

(NPSH

A

,

see

Sec-

tion

10-4).

In a raw

sewage pumping station,

a

better

choice would

be to use a

self

-priming

pump

if a

con-

ventional

wet

well-dry

well pumping station cannot

be

used

or is not

feasible.

Lift

Check

Valves

The

body

of a

lift

check valve

is

similar

to

that

of a

globe valve.

A

plug

or

stem moving within

a

guide

lifts

upward

and

allows

fluid to

pass through

the

valve.

The

plug seats when

the flow

reverses.

A

lift

check valve does

not

provide

a

tight

shutoff.

It

cannot

be

used

in fluids

containing solids

or

abra-

sives,

and

gum-forming

fluids can

cause

the

stem

to

stick. Sudden

flow

reversal

can

cause water hammer.

This type

of

valve

is

normally encountered

in

pumping

stations only

in

sizes

50 mm (2

in.)

and

smaller

and in

services such

as

utility water

and

com-

pressed air.

Swing

Check

Valves

A

swing check valve (Figure

5-12)

contains

a

hinged

clapper

or

disc that rests

on a

seat

and

prevents

fluid

from

flowing

backward through

the

body.

A

disadvan-

tage

of

metal-to-metal

design

is the

lack

of a

tight seal

when

the

disc

is

seated,

so a

rubber seat

is

better.

The

disc

is

usually

affixed

to a

hinge

pin by

means

of an

arm.

The pin and arm

allow

the

disc

to

move

up and

out

of the flow

path

in the

direction

of fluid flow.

Swing

check valves

can be

installed

in

both hori-

zontal

and

vertical positions.

In a

horizontal position,

the

valve bonnet must

be

upright.

In a

vertical pipe,

the

valve must

be

installed

so

that

fluid flow is in the

upward

direction,

but

never install swing check valves

in

vertical pipes

in

sewage, sludge,

or

slurry service

because rags, debris,

and

grit would settle against

the

disc

and

eventually prevent functioning. Clearing

the

valve

in

this position

is a

messy, disagreeable task.

Slamming when

a

pump stops

and the fluid

reverses direction

is a

significant

problem when using

swing

check valves

of

some designs.

In

general, swing

check valves larger than

150 mm (6

in.) should have

an

outside lever

and

spring

to

close

the

disc quickly

before

the fluid can

reverse direction. Note, however,

that

a

commonly used check valve standard,

AWWA

C508,

does

not

cover

the

outside lever

and

spring

design.

A

disadvantage

of

this type

of

valve

is

that

the

outside lever

and

spring

or

counterweight

can

prevent

the

disc

from

opening

fully,

especially

at low flow

velocities

—

less

than about

3 m/s (10

ft/s)

—

with

a

consequent

increase

of

headloss.

The

swing check

valve

in

Figure

5-12

is

fully

ported when open 20°,

but

most designs require

a

swing

of 60° to

open

fully.

Headless

through

the

valve

at low

velocities

is

gener-

ally

higher than

the

manufacturer's data, which

are

usually

based

on a

fully

open valve

disc.

If the

valve

is

properly chosen

for the

specific application

and the

spring tension

or the

counterweight properly adjusted,

however,

the

headloss should agree with

the

manufac-

turer's data. Headloss increase

is

often

caused

by

increasing spring tension

or the

weight

on the

lever

arm

to

reduce

slam

—

a

direct result

of

improperly

selecting

the

valve. Swing check valves

in

pipes larger

than

400 or 450 mm (16 or 18

in.) should

be

specified

with

caution, especially

if the

head exceeds about

15

m

(50

ft), because

the

force

on the

disc

is

enormous.

Cushioned

Swing

Check

Valves

Some check valve manufacturers

offer

pneumatic and/

or

hydraulic dashpots attached

to the

valve

to

regulate

the

speed

of

closure

of the

disc upon water column

reversal.

The

cushioning system consists

of

• a

weighted lever

arm

attached

to the

disc

pin or

axle

• a

piston mounted outside

of the

valve body

and

contained

in a

cylinder

(dashpot)

attached

to the

weighted arm.

As

the

velocity decreases,

the

weighted lever

arm

forces

the

disc

to

close,

and the

piston moves down-

ward

in the

cylinder.

The

piston compresses

the air (in

a

pneumatic system)

or

displaces

oil

through

an

ori-

fice

(in a

hydraulic system).

Adjusting

the

valves

on

the

pneumatic

or oil

lines

(or the

orifices

in the

dash-

pot) controls

the

rate

of

closure.

The

hydraulic system

offers

better control than

the

pneumatic

system, which

often

does very little

to

reduce

the

slam. Sturdy valves

can be

closed quickly

or

slowly

and can

even

be

closed

in two or

three

stages, such

as

quick closure

to

50%, moderate speed

of

closure

to

95%,

and

slow closure

to

shut-off.

Be

very

careful

in

selecting applications

for

these

valves;

close coordination with

the

manufacturer

is

necessary.

In

addition,

field

adjustment

after

installa-

tion

is

needed

to set the

closing controls properly.

Rubber

Flapper

Check

Valves

The

rubber

flapper

swing check valve

is a

swing check

that

is

entirely enclosed.

The

seat

is on a 45°

angle

and

the

steel-reinforced

flapper

need travel only about

35°

to

reach

the

fully

open position.

The

short stroke

and

light

weight

of the flapper

make

it

capable

of

very

fast

shut-off,

which, combined with

the

resilient seat,

reduces slam.

The

construction

of the

valve

is

simple,

as is

maintenance.

There

is no

outside lever,

no way of

adjusting

the

closing force,

and no way of

determining

whether

the

valve

is

open

or

closed.

This type

of

valve

should

not be

used

in raw

sewage service because

debris

can

pack above

the

disc

and

prevent

the

disc

from

opening.

Slanting Disc

Check

Valves

A

slanting disc check valve contains

a

disc balanced

on

a

pivot. Instead

of

being perpendicular

to the

longi-

tudinal axis

as in

conventional swing check valves,

the

seat

is at an

angle

of 50 to 60°

from

the

valve longitu-

dinal axis. Slanting disc check valves should only

be

used

in

water

service;

rags

and

solids present

in raw

sewage

and

sludge would hang

up on the

disc.

The

advantages

of

this type

of

valve

are (1)

head-

loss

is low

(although

not as low as in a

swing check

valve)

in the

open position because

the

vane

or flapper

is

designed

as an air

foil,

(2)

various pneumatic

and

oil-filled

dashpots

can be

used

to

control

the

opening

and

closing speeds,

and (3) the

performance

of

these

controls

can be

adjusted

in the field. The

disadvan-

tages

are (1)

velocities less than

1.5

m/s

(5

ft/s)

do not

fully

open

the

vane;

(2) the

disc oscillates

in the flow

and

the

bearings wear

on the

bottom,

so the

valves

begin

to

leak;

and (3) the

valve

is not

fully

ported.

The two

controls most frequently encountered

are

bottom

buffers

and

top-mounted dashpots.

These

two

systems

are

sometimes mounted together

on one

valve.

The

bottom

buffer

consists

of an

oil-filled cylinder

in

which

a

piston

is

moved

by the

closing disc

or

vane.

The

disc moves

freely

for the first 90% of its

closure,

then strikes

the

buffer

piston, which

can be

adjusted

in

the field to

control

the

last

10% of

disc travel.

The

top-mounted

oil

dashpot system allows both

the

opening

and

closing speeds

of the

disc

to be

adjusted

over

the

full

range

it

travels. This adjustment

can

be

especially valuable with pump start-up because

the

opening speed

can be

regulated

to

open

the

valve

slowly,

which greatly reduces hydraulic transient

effects

caused

by

pump start-up.

A

disadvantage

is the

high

load exerted

on the

mechanical linkage when

the

pump

reaches

shut-off

head. However,

no

electrical

interconnections between

the

pump motor control

center

and

check valve

are

needed.

5-5. Control

Valves

Control valves

are

used

to

modulate

flow or

pressure

by

operating

in a

partly open position, thus creating

a

high headloss

or

pressure differential between

upstream

and

downstream locations. Such operations

may

create cavitation

and

noise.

If

there

is a

large

pressure

differential

and the

limits

of

operation

are

approached

or

exceeded,

the

discs tend

to

flutter

and

bearings

may

wear quickly. Valve seats

are

especially

vulnerable

to

wear because,

if the

pressure

differential

is

high across

the

seat, small channels

may be cut

(called

"wire

drawing"), which prevents

a

tight seal,

aggravates

the

wire drawing,

and

makes frequent

replacement necessary.

To

minimize wasting energy

and to

increase

the

life

of the

valve,

it is

desirable

to

minimize

the

time

of

operation

at

partly open positions.

If the

valve must

throttle

flow for

extended periods, choose

a

style well

adapted

for the

purpose

and

select

hard materials

for

those parts that wear quickly.

Some control valves

may be

manually operated

(for

example, needle valves used

to

control

the flow of

a fluid in a

valve actuator). Most control valves, how-

ever,

are

power-operated

by

programmed controllers.

These valves

are

used

for a

variety

of

purposes: pump

control, check valve control, control

or

anticipation

of

surges,

or

control

of

pressure

or flow. The

power

source

can be (1)

hydraulic (usually oil),

(2)

pneu-

matic,

(3) a

combination

of

pneumatic

and

oil,

(4)

electric,

or

even

(5) the

pressure

of the

pumped water.

All

control methods feature some kind

of

adjustable-

speed actuator, sometimes with three electric speeds

that depend

on the

position

of the

valve mechanism.

Whatever

the

power source,

a

backup

is

needed

for

power outages.

The

backup

can be a

pressure tank

for

pneumatic

or

hydraulic actuators

or

trickle-charged

batteries

for

electric actuators (see Section 5-6).

Control valves

are

selected

on the

basis

of the

requirements

of the

hydraulic system

and the

charac-

teristics

of the

pump.

A

major decision

is

whether

to

use

a

check valve that

is

controlled

by the flow or a

more sophisticated valve that itself controls

the flow.

The

characteristics

of the

type

—

and

even

the

brand

—

of

pump-control

or

check valves

are

important. Every

type

of

valve used

as a

check valve

suffers

some

of the

effects

of

cavitation, noise,

and

vibration while open-

ing

and

closing,

and

some types

are

more vulnerable

than

others. Cavitation occurs

at

regions

of

large pres-

sure drops.

Pump-Control

Valves

Pump-control valves

can be any

type

—

angle,

ball,

butterfly,

cone, globe,

or

plug

—

suitable

for the

liquid

being pumped.

Use

angle

and

globe valves where high

headloss

can be

tolerated

or is

desirable

(as in

bypass

pipelines);

and use

ball,

butterfly,

cone,

or

plug valves

where energy costs

are

important (see Example 5-1).

Controls

and

electrical

interlocks

are

provided

so

that

the

valve

is

closed when

the

pump starts.

After

the

pump starts,

the

main valve opens slowly

at an

adjust-

able rate. When

the

pump

is

signaled

to

shut off,

the

valve slowly

closes

at an

adjustable rate. When

the

valve

is 95 to 98%

closed,

a

limit switch assembly

shuts

off the

pump.

Surges induced

by

start-up

and

shut-down

of

constant-speed water pumps

can be

effectively

con-

trolled

by

diaphragm-

or

piston-operated globe-type

valves utilizing differential

pressure

to

open

and

close

the

valve. Operation

is

usually initiated

by

activating

solenoid valves that

act on the

trim piping controlling

pressure

on the

diaphragm.

The

initiation

of

solenoid

operation

is

usually linked electrically

to the

pump

motor control circuit,

and the

speed

of

operation

is

controlled

by

adjusting needle valves

in the

trim pip-

ing. Variations

of

this basic type

of

valve include

straight-

through

or

angle bodies, surge relief valves,

and

head sustaining valves.

To

provide some assur-

ance

of

reliability,

the

trim piping

to the

power side

of

the

diaphragm must

be fitted

with

a fine

strainer

to

remove particulate material that might otherwise

interfere with valve operation.

Piston-operated globe valves have

an

advantage

over diaphragm-operated valves

in

that leakage

from

the

valve occurs long before failure. Diaphragm-

operated valves

are

completely sealed

and do not

leak, but,

on the

other hand, they give

no

warning

of

impending diaphragm rupture, which puts

the

valve

out

of

service. Both valves

are

very

effective

in

reducing surges

due to

pump start-up

and

normal

pump

shutdown,

but

they cannot prevent surges

caused

by

power failure.

Power-actuated ball,

butterfly,

cone,

and

plug

valves

are

more expensive

to

install but, when

fully

open, cause less headloss than other valves.

Control

Valves

for

Water

Service

The

control valves likely

to be

used

for

water ser-

vice include angle, ball,

butterfly,

cone, globe, nee-

dle

(for

fine flow

regulation

in

control piping),

and

eccentric, lubricated,

or

nonlubricated plug valves.

See

Figure 5-14.

Control

Valves

for

Wastewater

The

only valves suitable

for

control

of

wastewater

are

ball, cone, long radius elbow,

and

eccentric,

lubricated

or

nonlubricated plug valves.

Figure

5-14.

Control

valve

for

water

service

with

exter-

nal

piping

arranged

for

surge

anticipation.

Courtesy

of

CIa-VaI

Co.

Description

of

Control

Valves

Except

for

globe

and

needle valves,

all of the

valves

that

can be

used

for

control

are

described

in

Section

5-2. Recommendations

for

their

use are

given

in

Table 5-4.

Angle

Valves

Angle valves

and

globe valves

are

similar

in

construc-

tion

and

operation except that

in an

angle valve,

the

outlet

is at 90° to the

inlet

and the

headloss

is

half

as

great

as it is in the

straight-through globe valve.

An

angle valve

is

useful

if it can

serve

the

dual purpose

of

a 90°

elbow

and a

valve. Conversely,

an

angle valve

should

not be

used

in a

straight piping run; instead,

use

a

globe valve.

As

with globe valves, angle valves

are

best used

in

clear liquid service because

fluids

containing

grit

or

abrasives cause severe

seat

erosion.

Globe valves must never

be

used

in

sludge

or raw

sewage service because they

are

prone

to

becoming

plugged with solids.

Globe

Valves

As

in the

angle valve,

a

globe valve

has a

disc

or

plug

that moves vertically

in a

bulbous body. Flow through

a

globe valve

is

directed through

two 90°

turns

—

upward

and

then

outward

—

and

is

controlled

or

restricted

by the

disc

or

plug.

The

pressure drop

or

headloss

is

higher than

in

angle, gate,

butterfly,

or

ball

valves.

Because

of

this high pressure

drop

—

even

in the

wide-open

position

—

globe

valves

are not

ordinarily

used

as

isolation valves except

in

seal water, gas,

and

Table

5-4.

Recommendations

for Use of

Control

Valves

3

Water

Wastewater

Type

of

valve

Raw

Clean

Raw

Treated

Angle

FGXX

Ball

EEEE

Butterfly

FGXF

Cone

EEEE

Globe

Diaphragm

GGXF

Differential

piston

GGXF

Surge

relief

Diaphragm

or

piston

GGXX

Angle

valve (for water)

GG

— —

Long

radius elbow valve (designed

for

sewage)

GGFG

Surge

anticipation

Diaphragm

or

piston

GGXX

Angle valve (for water)

GG

— —

Long radius elbow valve (designed

for

sewage)

GGFG

a

E,

excellent;

G,

good;

F,

fair;

P,

poor,

do not

use;

—,

use is

unlikely.

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

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