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

fuel

oil

pipelines. They

are

used

in

applications

requiring throttling, such

as

pressure

or flow

control.

Globe valves

are

suitable

for

clear liquid service,

but

not for fluids

containing grit

or

abrasives, which cause

severe seat erosion. Never

use

globe valves

for

sewage

or

sludge service because they

are

prone

to

becoming

plugged

with

solids.

Globe

or

Piston

Valves

with

Vee-

Ports

Globe

or

piston valves containing

vee-ports

are

made

to

eliminate seat wear

by

cavitation

and to

allow

the

flow

to

start

in a

controlled manner. Throttling

is by

the

vee-ports

as

shown,

for

example,

in

Figure

5-15.

The first 50% of the

stroke allows only about

20% of

full

flow

—

a

feature

that minimizes

the

effects

of

surge

caused

by

opening

and

closing pump-control valves.

Needle

Valves

A

needle valve

is a

special variation

of a

globe valve.

The

plug

is a

slender tapered

needle.

The flow

annulus

is

easily

fouled

by

particulates;

otherwise,

its

charac-

teristics

and

advantages

and

disadvantages

are

much

the

same

as

those

of a

globe valve.

A

needle valve

offers

very

fine

pressure

and flow

control, even

in a low flow

range with

the

valve almost

completely

closed.

It is

often

used

in

seal-water piping

and,

to

control

the

speed

of

operation,

in the

control

piping

of

other valve types.

Special

Control

Valve

Functions

Some control valves regulate parameters such

as

pres-

sure rather than

flow, and

these valves

may

operate

by

being either

fully

open

or

fully

closed. Most special-

purpose control valves

are

built

on a

single body

design. Only

the

exterior piping

to the

hydraulic actu-

ator (diaphragm

or

piston)

in the

bonnet

is

changed

to

effect

the

type

of

control wanted, whether

it be

con-

stant

flow,

constant pressure,

or

proportional

flow.

Make

it

easy

to

service these valves

by

incorporat-

ing

enough isolating valves

to

close

off the

water

sup-

ply

to

them.

Altitude

Control

Valves

Altitude control valves

are

used

to add

water

to

reser-

voirs

and to

one-way tanks used

in

surge control

(see

Chapter

7). The

body design

is

usually

of the

globe

type. Altitude control valves

are

made

in

many varia-

tions

of two

functional

designs:

• One

design

in

which

the

valve

closes

on

high water

level

in the

tank

and

does

not

open again until

the

water leaves through

a

separate line

and the

water

level

in the

tank

falls.

• A

second design

in

which

the

valve closes

on

high

water level

in the

tank

and

opens

to

allow water

to

flow out of the

tank when pressure

on the

valve inlet

falls

below

the

reservoir pressure

on the

down-

stream side

of the

valve.

Figure

5-15.

Vee-ported globe valve

with

differential piston. Flow

is

from right

to

left. Courtesy

of GA

Industries,

Inc.

Pressure

Relief

Valves

Pressure relief valves

are

often

of the

globe type

in

terms

of

body design.

A

control system

is

added

to

establish

how the

valve operates.

In the

angle type

(Figure 7-8),

a

direct-acting, adjustable spring

is

pro-

vided

to

open

the

valve

and

permit

flow

when

the

inlet

pressure exceeds

the

spring setting.

A

common appli-

cation

is to

place

one of

these valves

on the

branch

of

a tee on a

pump discharge line.

The

valve then serves

to

release

the fluid

before

a

high pressure

can

develop

and

overstress

the

piping

and

valves.

There

is an

inherent time

lag in the

opening

of any

valve, how-

ever,

so the

valve lags

to

some degree behind

the

actual

pressure rise

due to

surges.

Surge

Anticipation

Valves

The

entire surge anticipation system consists

of a tee

on

the

header,

an

isolation valve

(to

permit servicing

the

surge anticipation valve),

the

surge anticipation

valve

itself,

a

vent pipeline

to

waste,

a

pressure-sens-

ing

pipeline connected

to the

pump discharge,

and a

pilot system with

an

electronic timer. Following

pump

power

failure,

the

pressure

in the

pump dis-

charge drops, which opens

the

valve

and

vents water

in

anticipation

of the

subsequent high-pressure wave.

The

control system should

be

designed

so

that

the

valve

does

not

open immediately

after

power failure

but

only

after

a

timed delay

(i.e.,

not

until

the

pres-

sure wave

in the

pipeline approaches

the

pumping

station).

The

electronic timer keeps

the

valve open

for

a

short period

and

then closes

it

slowly.

If a

second

pressure wave follows,

the

valve reacts like

a

surge

relief valve.

Although

this type

of

valve

can

significantly

reduce

the

return upsurge

or

high pressure

after

a

pump

power

failure,

it

does nothing

to

control

or

reduce

the

effects

of the

initial downsurge

or

low-

pressure wave (see Chapters

6 and 7).

5-6.

Valve

Actuators

(also called

"operators")

for

valves

can be

manual

or

electrically, hydraulically,

or

pneumatically

powered.

Valve

design (quarter-turn

or

lift

type), valve

size, operating pressures,

and

special requirements

such

as

operation

on

loss

of

power

or

control

of

surge

pressures determine

the

type

and

complexity

of the

valve

actuating system.

A

comparison

of

valve actua-

tors

is

given

in

Table

5-5.

Manual

Actuators

Plug

and

butterfly

valves

150 or 200 mm (6 to 8

in.)

in

diameter

and

smaller

and

ball valves

100 mm (4

in.)

in

diameter

and

smaller

can be

actuated with

a

simple

lever attached

to the

plug

shaft.

Some lever-type actu-

ators

can be fitted

with adjustable stops

for

balancing

or

throttling service. This feature, however,

is

rarely

necessary

for

isolating valve service. Quarter-turn

valves larger than

the

sizes indicated here should

be

fitted

with

geared manual actuators

for two

reasons:

• The

torque required

for

actuation

is too

great

for

direct operation.

•

Valves equipped with geared-type operators close

slowly

and

thereby reduce

the

potential

for

damag-

ing

surge pressures.

Gate valves

up to 300 mm (12

in.)

in

diameter

can

be

actuated manually

by

handwheels acting directly

through

threaded nuts bearing

on

threads

cut

into

the

valve

stem. Larger, manually operated gate valves

should

be fitted

with gear reducers

to

reduce

the

force

required

to

move

the

valve disc

or

plug

to

within rea-

sonable limits. This accommodation

is

always pro-

vided

at the

expense

of the

time required

to

operate

the

valve. Thus, powered actuators

are

recommended

for

valves larger than

450 mm (18

in.)

in

diameter.

Manually

actuated valves installed more than

2.1

m

(6 ft 9

in.) above

the

operating

floor

should

be

equipped with chain actuators

to

permit operation

without

the

need

for a

ladder, which

is

both inconve-

nient

and

dangerous. Small

(100

mm [4

in.]

in

diame-

ter and

less) plug

and

butterfly

valves

can be

equipped

with

extension arms

and

chains. Larger valves should

be

equipped with chain-wheel actuators. This type

of

actuator

can be

obtained with

a

hammer-blow feature

to

break

loose

valves that

are

hard

to

start.

The

following

are

miscellaneous

but

important

notes

on

specifying valves

and

designing installations:

• To

avoid

safety

problems, valve stems

and

actuators

should

not

protrude into walkways.

• If a

valve

or

gate

is

below

floor

level

and

needs fre-

quent

operation,

a floor

stand

is

appropriate.

A

less

expensive square nut,

to be

turned

by a

wrench,

is

sometimes

a

suitable substitute. Nuts should

be

standardized and,

in the

United States,

the

dimen-

sions

are

given

in

AWWA

standards.

•

Ordinary chain wheels should

not be

used

in a wet

well

because

of the

possibility

of

sparking. Non-

sparking

chain-

wheel

designs

are

available.

•

Specifications

and

purchase orders

for

valves

should state

the

direction

of

opening.

The

usual

standard

is for

valve stems

to

turn counterclockwise

to

open. However, some valves open clockwise;

if

these must

be

used, paint handwheels, nuts,

and

levers

red

and/or mark

the

directions

for

operating

them

plainly.

•

Valves

are

made with both rising

and

nonrising

stems.

The

rising style

is

advantageous

for

indicat-

ing

the

valve opening. Geared actuators should

incorporate position indicator dials.

• In

some valves (especially

butterfly

valves),

the

flow

through

the

valve tends

to

move

the

disc

and

may

cause

flutter.

Such valves require locks. Worm

gearing

can be

designed

to be

self-locking.

•

Butterfly,

plug,

and

ball valves

200 mm (8

in.)

and

larger

in

nominal diameter should

be

equipped with

some type

of

actuator employing

a

mechanical

advantage.

Some engineers

and

pumping station

workers prefer mechanical actuators

for

150-mm

(6-in.) valves

as

well. Worm gearing

is the

best

because

it is

usually manufactured with greater pre-

cision than other types

of

actuators. Pantograph

and

traveling sleeve type actuators

are

often

trouble-

some.

Powered

Actuators

Powered actuators

can

operate directly

on the

valve

shaft

or

stem

or

through gear reducers

and

special

drive linkages.

For

rotary motion valves, such

as

ball

or

cone designs intended

for

surge control service

and

some

butterfly

valve actuating mechanisms, these spe-

cial linkages serve

two

purposes:

(1)

conversion

of

linear motion

to

rotary motion

and (2)

special closing

and

opening characteristics

to

control pressure tran-

sients

on

pump start-up, shut-down,

and

power failure.

Electric Actuators

Electric actuators generally consist

of

electric motors

driving through

a

gear train

to

power

the

valve stem

or

shaft.

In

general,

the

speed

of

operation

and

differen-

tial pressure

at

specified conditions determine motor

power requirements. Motor operators have

a

hammer-

blow

feature

to

start hard-to-open valves (e.g., gate,

nonlubricated plug).

As

with other actuators,

a

hammer-blow feature

is

important

to

start

the

valve

in

operation

in

either direction. Usually, this type

of

Table

5-5.

Comparison

of

Various

Types

of

Valve

Actuators

Operating

Type

Cost

characteristics Operation

Remarks

Manual,

direct

Least

Relatively

smooth.

Operator must

be

Suitable

for

smaller

valves.

in

attendance.

Manual, geared

Low

Smooth, slow.

As

above. Suitable

for

valves

up to 450

mm

(18

in.)

in

diameter.

Electric

Moderate

to

Smooth;

can

Avoid unless

Can be

expensive depending

expensive position only

in

stored-energy

on

size,

functions,

and

depending

on

increments (batteries) number

of

valves,

options

unless

provided

reserve

system

with

electronic

is

provided,

positioner.

Pneumatic Moderate Tends

to be

jerky, Easily provided

Corrosion,

caused

by

water

in

difficult

to

with

local

compressed air,

can be

position. receiver. troublesome.

These

systems

can

freeze

up at

exhaust ports.

Hydraulic, water Moderately Very smooth. Easily provided Corrosion

and

freezing

can be

expensive with

local

problems.

hydropneumatic

tank.

Hydraulic,

oil

Expensive Very smooth. Good, with System

can be

complex.

precharged

gas

Reliability achieved only

accumulators.

by

using

first-class

components; recommend

system pressures

less

than

14,00OkPa

(2000

Ib/in.

2

).

actuator

is

equipped with

a

handwheel

for

manual

operation should

the

motor

be

disabled.

It is

important

to

specify

a

declutching mechanism that disengages

the

motor

from

the

power train whenever

the

hand-

wheel

is

being

used

—

it

can be

motor preference

or

handwheel preference.

Electric

motor actuators

can be

specified,

however,

to

accept remote commands,

to

telemeter position

to

remote locations,

and to

function

with

remote reversing starters. Electronic, modulating

positioners

are

available,

but

these rarely

are

used

in

pumping station designs.

To

provide safeguards

against

potential damage,

specify

(1)

torque-limiting

switches

for

both open

and

closed positions

and (2)

four

train limit switches

to

position

the

valve

for

seat-

ing.

Specify

integral, independent

safety

overrides.

Direct current power with battery support

is

rec-

ommended

for all

system control

and

monitoring

functions

where

the

actuating system must

function

during

power

failure.

Batteries should

be

constantly

trickle charged

at low

input with automatic switching

to

fast

charging

if the

battery charge

is

low.

Hydraulic

Actuators

Hydraulic actuators

use fluid

under pressure

as a

source

of

power,

and

both linear

and

rotary actuators

are

avail-

able. Hydraulic actuators

(fluid

power actuators)

can be

designed

to use

either

oil

under pressure

from

a

self-

contained system

or

water

from

the

local potable sup-

ply,

wherein

the

water

is

usually

run to

waste. However,

because potable water supply systems must

be

designed

to

resist corrosion,

specify

stainless steel, bronze,

or

chrome-plated construction. Hydraulic actuators should

be

selected

to

provide

sufficient

power

to

break

the

valve

loose. Once

the

valve

is in

motion, depending

on

the

actuator linkage,

a

lower pressure

differential

may

be

required

to

move

it

from

one

position

to

another.

One of the

advantages

of fluid

power actuators

is

that

fluid

can

be

stored

in

pressure-charged accumulators

or

hydropneumatic

tanks

to

provide

a

source

of

power

under

emergency conditions, such

as

commercial

power failures. Another advantage

is the

ease

of

chang-

ing

the

speed

of

opening

or

closing

the

valve. Pressure

should

not

exceed 14,000

kPa

(2000

lb/in.

2

)

in all fluid

power

systems

to

limit leaks

and

joint failures,

and a

limit

of 75% of

that

pressure

is

better.

Specify

premium

components

for fluid

power systems.

If

operation

of the

equipment

during emergencies

is a

prime concern,

retain

a

specialist

to

design

the

system.

Pneumatic

Actuators

Pneumatic actuators

are

available

for

both linear

and

rotary

motions.

The

disadvantages

of

pneumatic oper-

ators include

(1)

noise;

(2)

poor operating characteris-

tics because

the

powering

fluid, a

gas, expands

on

change

of

pressure;

(3) a

tendency

to

freeze

because

of

expansion

on

release

to

atmospheric pressure;

and

(4)

corrosion (with compressed

air

systems) because

of

water entrained

in the

gas.

A

pneumatic actuator system generally

has a

lower

initial installed cost than

a

motorized actuator system.

However,

the

maintenance costs

for the

pneumatic

actuators

and

associated equipment (compressors,

receivers, traps, separators,

filters,

and

piping)

are

usually

much higher than they

are for a

motorized

actuator system.

Pneumatic actuator systems

are

especially attrac-

tive

for

pumping stations because they

can

actuate

valves when

a

power failure occurs.

A

receiver (tank)

provides

the

compressed

air to

operate

the

actuator.

A

solenoid valve, energized

to

close (deenergized

to

open)

is

placed

in the air

line connecting

the

receiver

to the

pneumatically actuated valve. Upon power fail-

ure,

the

solenoid valve opens

and the

pneumatic actu-

ator causes

the

valve

to

close. This system allows

some control over

the

time

of

closure

of the

valve

so

that

excessive surge pressures

can be

avoided. Size

the

receiver

to

hold twice

as

much

air as

needed

to

operate

all of the

valves through

one

cycle.

In

most pumping stations requiring only

a few

powered valves

(no

more than three

or

four),

an

elec-

tric actuator system generally

has the

lowest installed

cost. Hydraulic systems

are

usually

the

most expen-

sive, with pneumatic systems

in the

middle.

The

cost

of

the

hydraulic

and

pneumatic actuators themselves

may

be

cheaper than

the

electric

actuators,

but the

cost

of

the

necessary auxiliary

equipment

—

such

as

receiv-

ers, compressors, dryers,

filters,

and

relief

valves

—

rapidly increases

the

cost

of

small pneumatic systems.

However, self-contained actuators that

use the

pumped water

for

power

(so

that auxiliary equipment

is

not

required)

are

relatively inexpensive

and low in

maintenance labor.

Similarly, electric actuators require less mainte-

nance than pneumatic

and

hydraulic actuators. Again,

it

is the

maintenance associated with

the

auxiliary

equipment that usually causes electric systems

to be

selected.

5-7.

Air and

Vacuum

Valves

Air

release

and

vacuum relief valves

are

often

needed

along transmission mains

and may

sometimes

be

unavoidable

in

sewage force mains.

Air

must

be

bled

slowly

from

high points

to

prevent

(1)

"air binding"

and (2) the

reduction

of the

cross section

of the

pipe

at

high

points.

Vacuum

conditions must

be

prevented

when

the

pump head drops quickly

(as in

power

fail-

ures)

to

prevent column separation.

Vacuum

relief

valves

can be as

large

as

one-sixth

of the

diameter

of

the

transmission main, whereas

air

release

valves

may

be as

small

as

one-fiftieth

of the

diameter

of the

pipe.

Although

most valves such

as

this

are not

within

the

pumping

station, their presence

in the

transmission

main

has a

profound

effect

on

surge and,

hence,

on the

whole

system.

A

pipeline designed

for

velocities high enough

to

scour

air to the

exit

is an

alternative approach that

does

not

require

the use of air

release

valves. Such

velocities

are

within

the

normal design range

for

pipes

300 mm (12

in.)

in

diameter

or

smaller (see Table

B

-9).

Air and

vacuum

valves

are not

objectionable

and

may

be of

some

benefit

(by

eliminating

air

bubbles

altogether),

but

only

if

there

is

assurance that cata-

strophic

failure

is

precluded

by

air-

scouring

velocities

in

pipes

on flat or

downward

slopes.

Excessive head-

loss

can be

prevented

by the use of

larger

pipe

for

upward

slopes.

Note, however, that air-scouring

velocities

must

be

reached

frequently enough

to

pre-

vent

large

air

bubbles

from

forming. Also note that

large pockets

of air may

greatly increase

the

head

on

the

pumps. Design such combination systems

on the

assumption

that

the air

release valves will sometimes

fail

to

operate.

Air

and

Vacuum

Valves

in

Water

Service

Air

release

and

vacuum

relief valves (called

"combi-

nation

air

valves") must

often

be

used (sometimes

at

frequent

intervals) along transmission

pipelines.

Wherever possible, select

a

pipeline

profile

that mini-

mizes

the

number

of

these

valves because they consti-

tute

an

onerous maintenance problem.

In

water

service,

the

short valve body (Figure 5-16)

is

appro-

priate

for

both

air and

vacuum valves. Lescovich

[14]

has

discussed

the use of air

release

valves

in

transmis-

sion mains.

Air and

Vacuum

Valves

in

Wastewater

Service

Sewage grease, corrosive gases, solids,

and

scum

in

sewage combine

to

aggravate

the

maintenance prob-

lem and to

reduce

the

reliability

of

valves

in

wastewa-

ter

force mains (see Section 7-4).

Use

other control

strategies, such

as

rerouting force mains,

to

avoid high

points

and the

need

for air or

vacuum valves.

If, for

some reason, such valves

are

unavoidable,

use the

tall

form

or

long body constructed

of

stainless steel

or

epoxy-lined

iron

in

styles that permit

the

entire inside

mechanism

to be

replaced quickly

and

easily

in the

field

so

that repair

and

cleaning

can be

done

in the

shop.

Because

a

valve that

fails

to

operate might cause

a

pipeline rupture, reliability

is

mandatory. Some manu-

facturers

recommend

an

overhaul every

6

months,

but

failures

have occurred with such

a

schedule.

To be

safe,

count

on

inspection and/or overhaul

at

frequent

intervals (twice

per

week

to be

conservative

or

once

per

month

for

greater risk)

and

install both

air

valves

and

vacuum valves

in

pairs

so

that

there

is one for a

backup.

One

type

of

vacuum relief valve that

is

entirely

appropriate

for

wastewater pipelines

is

discussed

in

Section

7-1 and

shown

in

Figure 7-2.

Description

of

Air and

Vacuum

Valves

Air

Release

Valves

Air

release

valves slowly

release

the

pockets

of air

that accumulate

at

high points

in

piping systems.

In

pumping stations, they

are

recommended

on the

dis-

charge

of

vertical turbine pumps, especially when

pumping

from

wells

and

sumps. This type

of

valve

has a float

that

falls

to

vent

air as the air

accumulates

in

the

body.

Valves

smaller than

19 mm

(

3

/

4

in.) usu-

ally have

a float-activated

compound lever with

a

link-

age

mechanism

to

provide

a

tight closure.

The

valve body contains

an

orifice,

usually

5 mm

(

3

I

16

in.)

or

smaller, through which

the air

escapes.

Figure

5-16.

Combination

air

valve

for

water

ser-

vice.

After

APCO

Valve

&

Primer

Corp.

Combination

Air

Valves

A

combination valve (Figure

5-16)

consists

of an air

release

and

vacuum relief valve with

an air

release

valve

attached

to it. It

allows

the use of one

valve

and

one

connection

to the

piping instead

of two

connec-

tions,

and it

fulfills

two

functions.

Slow-Closing

Air and

Vacuum

Valves

This type

of

valve

has a float

assembly

and

large vent-

ing

orifice

to

exhaust large quantities

of air

from

pipe-

lines when they

are

being

filled

and to

admit large

quantities

of air

when

pipelines

are

being drained.

The

valve assembly includes

a

perforated water

diffuser

on

the

inlet

to

prevent

the

water column

from

rapidly

entering

the

valve

and

slamming

the float

shut, which

could possibly cause

a

severe water hammer problem.

Slow-Closing

Combination

Air

Release

Valves

A

slow-closing

air and

vacuum

valve with

an

attached

air

release valve allows

the use of a

single valve unit

with

one

piping connection.

5-8. Materials

of

Construction

Bodies

Most valves

in

water, wastewater,

and

sludge pumping

stations

are not

exposed

to

severely corrosive condi-

tions. Bodies

are

usually cast iron (ASTM

A48 or

A126), cast steel (ASTM A216),

or

ductile iron

(ASTM A395)

for

valves

100 mm (4

in.)

or

larger

and

bronze (ASTM

B62 or

B584,

for

which several alloys

are

available),

for

valves

75 mm (3

in.)

and

smaller.

Fabricated steel (ASTM

A36 or

A516)

is

sometimes

used

in

valves larger than 1800

mm (72

in.),

espe-

cially

in

butterfly

valves.

In

many locales, however,

water

and

wastewater

are

indeed corrosive

to

iron

and

steel.

For

such liquids,

the

iron

and

steel bodies

should

be

lined with epoxy (such

as a

product com-

plying

with

AWWA

C550)

or

other products. Also,

some waters attack bronzes that contain high percent-

ages

of

zinc

and

cause

dezincification

[15].

There

is

no

universal agreement

on an

acceptable level

of

zinc

in

bronzes

and

other copper alloys.

For

example,

AWWA

C504

allows bronzes with zinc contents

of up

to

16%,

but

many engineers believe this

is too

high

and

allow

no

more than

5 to 7%

zinc

[16].

Some cop-

per

alloys (bronzes) frequently used

in

valves

and

having

zinc contents

of no

more than

7% are

alloys

C83600,

C87600,

C90500,

and

C93700,

defined

in

ASTM

B

584.

A

common

phrase

encountered

in

specifying

valves

is

"iron

body, bronze mounted (IBBM)."

There

is

no

universally accepted definition

of

this phrase.

Some manufacturers provide

a

bronze disc

and

seat,

while others provide only

a

bronze seat. Others fur-

nish

a

bronze disc

and

seat

up to a

certain size,

and

then provide only

a

bronze seat

in

larger sizes.

Be

careful

to

define

exactly what

is

meant when using

a

phrase such

as

"iron

body, bronze mounted."

Seats

General

Seats

in

isolation

and

control valves

are

more subject

to

erosion

and

corrosion than bodies because

the fluid

velocity impinges most noticeably here. Bronze

on

bronze

is the

cheapest,

and

seats

in

metal valves

75

mm

(3

in.)

and

smaller

are

frequently

bronze (note

the

of the

zinc content

in

bronzes

for

valves used

for

some waters). Most seats

or

seat reten-

tion devices

in

valves

100 mm (4

in.)

and

larger

are

some grade

of

stainless steel. Stellite

facing

is

much

harder

and

more erosion resistant than stainless steel,

but

it is

also much more expensive.

Do not

specify

an

exotic material unless there

is a

clear need

for it. The

most

common materials

for

resilient seats

are

listed

in

Table

5-1,

but

there

are

numerous plastics available

with

special qualities. Buna

N can be

attacked

by

industrial

chemicals,

but

unless there

is an

excess

of

illegal dumping

of

such solvents,

it is a

nearly ideal

seat material.

Teflon®

is

more resistant

to

attack,

but

it

creeps.

Teflon®

(polytetrafluoroethylene,

or

TFE)

is

suitable

for

both water

and

wastewater.

It is

hard, strong,

and

impervious

to

attack

by

nearly

all

chemicals,

but it has

shortcomings such

as

cold

flow and

creep. Other

materials

are

usually more suitable.

Elastomers

Elastomers (natural

or gum

rubber

and the

synthetic

rubbers, such

as

neoprene, Viton,

and

Buna

N) are

used

for

resilient seats, O-rings,

and a few

other parts.

Natural

rubber

is

suitable

for

clean water,

but

waste-

water contains

oil and

grease

products

and

organic

solvents that attack

it. The

synthetic rubbers usually

give good service

in

both water

and

wastewater sys-

terns.

Elastomers

are

subject

to

wear

in

grit slurry ser-

vice

and

where they

rub

against iron tubercles, hard

scale deposits,

or

corroded surfaces. They are, how-

ever, suitable

if the

grit

has

been removed.

Engineers should

be

aware that rubber com-

pounds

—

both

natural

and

synthetic

—

are

not

uni-

formly

and

consistently resistant

to

some disinfecting

agents commonly used

in the

water works industry,

such

as

chloramines. There appear

to be

wide varia-

tions

in

resistance

to

attack

on

various elastomers

and

on

differing

formulations

of the

same

elastomer.

It

should

also

be

noted that various standards pertaining

to

valves (such

as

AWWA

C500,

C504,

and

C509)

do

not

address

the

issue

of

resistance

of the

rubber seat-

ing

material

to

either chloramines

or

free

chlorine.

The

resistance

to

attack

by

disinfecting agents

is

addressed

by

Reiber

[19].

As of

1995,

the

issue

of

developing

a

testing standard

to

specify

the

resistance

of

elastomers

to

disinfecting agents

was

being dis-

cussed

and

investigated

by

several

AWWA

standards

committees.

Packing

Valve

packing

is as

important

in a

valve

as the

packing

gland

is in a

pump.

It

prevents leakage past

the

valve

stem

and

damage

to the

valve housing.

The

seal

is

normally

made

by

placing packing material around

the

valve stem

and

compressing

it

with

a

follower

or

gland, which

is

tightened

by a

packing nut. Although

asbestos

has

historically been used

as a

packing mate-

rial, more

and

more valve manufacturers

are

discon-

tinuing

its use and are

switching

to

nonasbestos

materials such

as

Teflon®,

aramid

fiber,

acrylic

fiber

bound with

nitrile,

and

Buna

N.

Note that some com-

mon

standards

specify

packing material that

is no

longer even made.

For

example,

AWWA

C500

for

gate valves specifies

flax

conforming

to

Federal

Spec-

ification

HH-P- 106d,

or

asbestos conforming

to

Fed-

eral Specification HH-P34c.

The flax

specification

HH-P-106d

was

discontinued

in

1978

by the

federal

government

because

of

insufficient

usage.

AWWA

C504, pertaining

to

butterfly

valves, still requires

packing

for

stuffing

boxes

to be

asbestos

or flax.

Most

valve

manufacturers

do not

comply (and,

in

fact,

can-

not

comply) with these

AWWA

standards

on

packing

materials.

Stems

Gate, globe,

and

needle valves have stems that

rise in

the

body

to

seat

or

unseat.

In

both water

and

wastewa-

ter

service, these stems

are

usually bronze,

and the

problem

of

dezincification

is

especially acute (see

the

discussion

on

zinc content

in

"Bodies"

of

this section).

5-9.

Installation

of

Valves

Warping

of the

valve body

due to

pressure

and

ther-

mal

stresses

in the

connecting

pipelines

or

lack

of

proper valve support

can

damage

the

valve enough

to

prevent

it

from

functioning.

The

valve body should

not be

supported

by the

adjacent piping,

nor

should

it

support

the

piping.

The

following

are

some sugges-

tions

for

valve support:

• For

piping

and

valves supported

on floors,

provide

separate bases

or

supports

for

valves

100 mm (4

in.)

or

larger.

• For

piping

and

valves suspended

from

ceilings, pro-

vide separate hangers

at

valves

—

one

hanger

or

sup-

port

at

each

end of the

valve body

or on the

connecting pipe within

one

pipe diameter

of the

valve

end.

•

Provide enough

flexibility in

connected piping

so

that thermal strains

in the

piping

do not

stress

the

valve.

•

Install piping without springing, forcing,

or

stress-

ing

the

pipe

or any

connecting valves.

•

Some valves (such

as

AWWA

C507 ball valves)

are

intended

to be

supported

in a

certain

manner.

Be

sure

to

read

the

relevant standard before designing

the

support system.

•

Always consult

the

valve manufacturer about

proper support.

End

Connections

End

connections

for

valves

can be

•

screwed (ANSI

B

1.20.1);

• flanged

(ANSI

B

16.1

for

cast-iron valves, ANSI

B

16.5

for

steel valves);

•

grooved

end

(AWWA

C606);

•

butt welded (ANSI

B

16.34);

or

•

socket welded (most commonly

found

in

plastic

valves).

In

general, valves

75 mm (3

in.)

and

smaller should

have

screwed ends, whereas larger valves have

flanges

or

grooved ends because,

in

larger sizes,

assembling

pipes

or

valves with threaded ends becomes very labo-

rious.

Furthermore,

bolted

connections

are

much eas-

ier to

disassemble than screwed connections, even

if

unions

are

installed

at

moderate spacing.

Butterfly,

gate,

and

eccentric plug valves with

grooved-end

or flanged

connections

are

probably

the

most

commonly encountered valves

in a

pumping

sta-

tion.

Butterfly

valves must

be of

long body

(not

short

body)

style

per

AWWA

C504

to

accommodate

grooved ends.

The use of

grooved-end connections

provides greater ease than

do flanges for

removing

valves

from

a

piping manifold. Sleeve

(Dresser®)

couplings need

not be

provided adjacent

to

grooved-

end

valves, although such couplings

may be

needed

for

other reasons, such

as

thermal expansion, align-

ment,

and

differential

settlement.

Locate valves

and

design

the

surrounding piping

to

prevent

clogging with grit. Except

for

clean water

ser-

vice, bonnets must

be

within

45° of

upright

to

keep

out

grit that

can

build

up and

prevent operation.

If

possible, avoid installing valves

on

risers, especially

if

there

is a

long section above

the

valve.

If a

valve must

be

placed

on a

riser,

use a

ball

or

plug valve that

is

wiped clean

by

operation

and has no

crevices

to

col-

lect grit. Design risers

to

enter

a

header horizontally

from

an

elbow

to the tee so

that grit

and

solids cannot

block

the

riser. Even

if

valves

are

installed

on

horizon-

tal

pipes, locate them

at

least three

(or,

better,

five)

pipe diameters

from

the

riser. Even

in

clean water

ser-

vice,

butterfly

valves should

be at

least

two

(three

is

better) pipe diameters

from

an

elbow

so

that stream-

lines, entering

from

an

angle,

do not

make

the

valve

difficult

to

open

or

close

and do not

cause

the

vane

to

flutter.

If

the

valve must

be

close

to an

elbow,

orient

the

valve

so the

vane

is not

subjected

to

dynamic load-

ing

from

the flow

through

the

elbow.

5-10.

Corrosion

Protection

When used

in

water lines, sewage lines, sludge piping,

or any

other service

in

which

the fluid is

particularly

corrosive, metal valves

can be

lined with epoxy. This

epoxy lining

can

only

be

applied

to

valves

100 mm (4

in.)

and

larger.

In

areas

of

corrosive soil, buried valves should

be

coated

with

coal

tar,

coal-tar epoxy,

or a

high-solid

(usually

70%

solids

by

volume

or

higher) epoxy.

Pay

particular attention

to the

need

for

stainless-

steel

bolts

in

buried

and

submerged valves

and

valves

exposed

to

H

2

S

atmospheres.

In

some soils

and

waters,

galvanized steel bolts

and

nuts (ASTM A307)

corrode readily. Therefore, type

304 or 316

stainless

bolts (ASTM

A193,

Grade

B8 or

B8M)

and

nuts

(ASTM A194, Grade

8 or

8M)

may be

required.

Low

solids

and low pH

waters

are

very corrosive

to

brass

[15],

iron,

and,

to

some degree, cement liners.

Such

waters

can be

rendered

benign

by

chemical

treatment

[16,

17,

18].

Seats must neither corrode

nor

erode.

The

corro-

sion

and

erosion resistance

of

several seat materials

are

compared

in

Table

5-1

(see

also Lyons

[6]).

5-11.

References

1.

Cook,

D.

T.,

"Selecting

hand-operated valves

for

process

plants,"

Chemical Engineering,

89,

126-140

(1982).

2.

O'Keefe,

W.,

"Pump

controls

and

valves,"

in

Pump

Handbook,

I. J.

Karassik,

W. C.

Krutzsch,

W. H.

Fraser,

and J. R

Messina,

Eds.,

McGraw-Hill,

New

York

(1976).

3.

Deutsch,

D.

J.,

et

aL,

Process

Piping

Systems,

pp.

193-

357, McGraw-Hill,

New

York

(1980).

4.

AWWA

Mil,

Steel

Pipe—

A

Guide

for

Design

and

Installation,

2nd

ed.,

American Water Works

Association, Denver,

CO

(1985).

5.

Hutchison,

I. W.

(Ed.),

ISA

Handbook

of

Control

Valves,

2nd

ed.,

Instrument Society

of

America,

Research Triangle Park,

NC

(1976).

6.

Lyons,

J.

L.,

The

Valve

Designer's Handbook,

Van

Nostrand Reinhold,

New

York

(1982).

7.

Lyons,

J.

L.,

and C. L.

Askland,

Jr.,

Lyon's

Encyclopedia

of

Valves,

Van

Nostrand Reinhold,

New

York

(1975).

8.

British Valve Manufacturer's Association,

Technical

Reference

Book

on

Valves

for the

Control

of

Fluids,

2nd

ed.,

Pergamon, London

(1966).

9.

Schweitzer,

P.

A.,

Handbook

of

Valves,

Industrial

Press,

New

York (1972).

10.

Pearson,

G.

H.,

Applications

of

Valves

and

Fittings,

Pitman, London (1968).

11.

Zappe,

R. W,

Valve

Selection Handbook, Gulf

Publ.,

Houston,

TX

(1981).

12.

Parmakian,

J.,

"Water hammer," Section

9.4 in

Pump

Handbook,

I. J.

Karassik,

W. C.

Krutzsch,

W. H.

Fraser,

and J. P.

Messina,

Eds.,

McGraw-Hill,

New

York

(1976).

13.

Collier,

S.

L.,

et aL,

"Behavior

and

wear

of

check

valves,"

Journal

of

Energy

Resources Management,

Transactions

of the

American Society

of

Mechanical

Engineers,

105,

58

(March 1983).

14.

Lescovich,

J.

E.,

"Locating

and

sizing valves

in

transmission mains," Journal

of the

American

Water

Works

Association,

64,

457^61

(July 1972).

15.

Jester,

T.

C.,

"Dezincification

update," Journal

of the

American

Water

Works

Association,

77,

67-69

(October

1985).

16.

"Valve

dezincification

prevented,"

Heating/

Piping/

Air

Conditioning,

53(11),

30

(November

1981).

17.

Singley,

J.

E.,

et aL,

Corrosion Prevention

and

Control

in

Water

Treatment

and

Supply

Systems, Noyes Data,

Park Ridge,

NJ

(1985).

18.

Merrill,

D.

T.,

and R. L.

Sanks,

"Corrosion prevention

by the

deposition

of

CaCO

3

films,"

Journal

of the

American

Water Works

Association, Part

1, 69,

592-

599

(November 1977); Part

2, 69,

634-643

(December

1977); Part

3, 70,

12-18 (January 1978).

19.

Reiber,

S. H.

Chloramine

Effects

on

Distribution

System

Materials,

American Water Works Association,

Denver,

CO

(1993).

The

purpose

of

this chapter

is to

provide

an

overview

of

the

problems caused

by

hydraulic transients

and an

insight into

the

circumstances that make

a

more thor-

ough

analysis necessary.

The

fundamental theory

of

hydraulic

transient analysis

is

described simply, with

no

attempt

to

present rigorous mathematical

or

analyt-

ical methods. Simple numerical examples include

surge

pressure calculations, attenuation

of

surge pres-

sure

by

programmed valve

closure,

and the

design

of

pipe

to

resist upsurge

and

downsurge pressures.

For

more complete discussions

of

hydraulic tran-

sients,

see

Parmakian

[1],

Rich [2],

Wy

lie

and

Streeter

[3], Waiters [4],

and

Chaudhry

[5].

6-1.

Introduction

Hydraulic

transients

are the

time-varying phenomena

that

follow

when

the

equilibrium

of

steady

flow in a

system

is

disturbed

by a

change

of flow

that occurs

over

a

relatively short time period. Transients

are

important

in

hydraulic systems because they

can

cause

(1)

rupture

of

pipe

and

pump casings;

(2)

pipe

collapse;

(3)

vibration;

(4)

excessive pipe displace-

ments,

pipe-fitting,

and

support deformation and/or

failure;

and (5)

vapor cavity formation (cavitation,

column

separation).

Chapter

6

Fundamentals

of

Hydraulic

Transients

BAYARD

E.

BOSSERMAN

Il

WILLIAMA.

HUNT

CONTRIBUTORS

Robert

C.

Glover

Joseph

R.

Kroon

M.

Steve

Merrill

Gary

Z.

Waiters

Some

of the

primary causes (and

frequency

of

occur-

rence)

of

transients

are (1)

valve

movements

—

closure

or

opening

(often),

(2) flow

demand changes (rarely),

(3)

controlled pump shutdown (rarely),

(4)

pump

failure

(often),

(5)

pump start-up (rarely),

(6) air

venting

from

lines

(often),

(7)

failure

of flow or

pressure regulators

(rarely),

and (8)

pipe rupture (rarely).

The

identification

and

calculation

of

pressures,

velocities,

and

other

abnormal behavior

resulting

from

hydraulic

transients make possible

the

effective

use of

various control strategies, such

as the

•

selection

of

pipes

and fittings to

withstand

the

antic-

ipated pressures;

•

selection

and

location

of the

proper control devices

to

alleviate

the

adverse

effects

of

transients;

and

•

identification

of

proper start-up, operation,

and

shutdown

procedures

for the

system.

The

analysis

of

unsteady

flow in

pipe systems

is

generally divided into

two

major

categories.

Rigid

water

column

theory

(surge

theory).

The fluid

and

pipe

are

inelastic,

and

pressure changes propagate

instantaneously.

These

flow

conditions

are

described

by

ordinary

differential

equations.

•

Solution: closed-form integration

or finite

differ-

ence numerical integration.

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

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