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

•

Headloss:

Specify

a

price penalty based

on

life-

cycle energy costs

for

more headloss than

a

stated

value.

•

Proof-of-design

tests:

Require certification

of

suc-

cessful

completion

of

proof-of-design testing con-

ducted

on a

6-in.

(or

larger) valve

in

accordance with

AWWA

C504, Section 5.5, altered

as

necessary

to

apply

to the

valve specified (e.g.,

"disc"

in the

stan-

dard

means

"plug"

in a

specification

for a

plug valve).

•

Massiveness

of

construction

and

conservatively

designed

bearings,

shafts,

and

other moving parts:

Shafts,

especially

in

cushioned-swing check valves,

should

be

very large; compare

the

various makes

and

models

to

specify

a

high-quality product.

•

Service

records:

Find

a way to

specify

features that

eliminate valves with poor records.

•

Complexity:

Specify

valves

and

actuators that

are

simple, trouble

free,

and

require minimum mainte-

nance

or the

kind

of

maintenance within

the

capa-

bility

of the

workforce.

•

Resilient seat material that will

not

cold

flow

under

differential

pressures: Look

for

well-designed

mechanisms

to

retain seats

in

place (see Table

5-1);

for

wastewater

and

sludge, seat materials must

be

resistant

to

oils

and

solvents.

•

Responsibility: Include

a

clause that involves

the

manufacturer

in the

responsibility

for

valve (and

valve actuator system) performance. Warning:

In

some instances, manufacturers have inserted addi-

tional

rubber

shims

or

seals

to the

seats

during some

tests

to

meet

the

C504 testing requirement that

the

valve

be

drop-tight

in

both directions. Such prac-

tices

should

not be

accepted.

Life-Cycle

Cost

Quality might

be

said

to be an

inverse

function

of

life-

cycle cost, which

is a

combination

of

capital, mainte-

nance,

and

energy costs. Headlosses

can

result

in

mind-boggling energy costs,

as

demonstrated

in

Example

5-1.

Example

5-1

Energy

Penalties

for

Three

Valves

Problem: Compare cone,

butterfly,

and

globe valves

for

life-cycle costs

of

energy. Assume

(1)

electric power

at

$0.05/kW

• h, (2) a flow

velocity

of

3.05

m/s (10

ft/s),

(3)

headlosses based

on

the K

factors

of

Table B-7,

(4) a

wire-to-water power

efficiency

of 75% for the

pump,

(5)

inter-

est at 8%, (6) a

life

of 20 yr, and (7)

300-mm

(12-in.)

valves wide open.

Solution: Calculate

the

annual cost

of

electric power, using headloss data

from

Table B-7,

Equation 10-7

for the

relation between head, power, pump

efficiency,

and flowrate, and

Equa-

tion 29-4

for the

present worth

of an

annual expenditure. From

the

results,

it is

evident that

headloss

is an

important cost factor.

Headloss

Energy

cost

(in

dollars)

Valve

m

ft

Annual

Present

worth

Cone 0.02 0.06

24 235

Butterfly

0.15

0.5 211

2,070

Globe

2.3 7.5

3,010

29,600

Location

Quality

is a

continuing concern and, hence, also

a

function

of

maintenance. Maintenance costs

are

elu-

sive, but, whatever they are, they

can be

reduced

by

placing valves

and

actuators

in

locations that

are

eas-

ily

accessible

for

servicing. Make

it

convenient

to

iso-

late

and

drain separate parts

of the

system,

and

write

maintenance

and

exercising programs into

the O&M

manual. Note that, because

of

savings

in

parts

and

operation

and

maintenance labor,

it may be

more cost

effective

to

install

an

expensive valve (such

as a

plug

or

a

valve

of

corrosion-resistant construction) that

works when needed than

to

install

a

cheap valve (such

as

a

gate

or a

valve

of

lesser quality) that

may not

work

until repaired.

High quality

is

achieved

as

much

by

good location

and

good piping layout

as by

specifying high-quality

hardware.

A

good valve

is

cheapened

by

misapplica-

tion

or

poor location. Some considerations

for

proper

location

are

• In all but

clean water service, install valves (such

as

gate

or

swing check)

with

bonnets upright

so

that

(1)

the

bonnet cannot become

clogged

with debris

that

can

render

the

valve inoperative

and (2) the

removal

of the

bonnet

is

easier

and

maintenance

is

less expensive (this means valves should

not be

located

in a

vertical pipe).

•

Make sure that

the

connecting piping

is

large

enough

for

projecting

butterfly

valve discs

to

rotate.

•

Place

a

spool

(at

least

two or,

better, three

pipe

diameters long) between

a

butterfly

valve

and an

elbow

to

prevent

the

diagonal streamlines

from

causing

the

vane

to flutter,

excessively wearing

the

bearings

or

even locking

the

vane; vane shafts

should

be

horizontal

so

there

is no

bottom bearing

to

collect grit.

•

Place

all

valves

in

locations such that operation,

maintenance,

and

repairs

can be

comfortably

accomplished.

•

Locate sleeve, grooved-end,

or flexible

couplings

nearby

so

that

(1)

valves

can be

easily removed

for

replacement

or for

factory

repair

and (2)

move-

ments

and

strain

in the

piping

are not

carried

through

the

valve body (unless

the

valve body

is

designed

to

resist

such

loads).

Check

Valves

In

comparison with choosing other valves,

the

judi-

cious selection

of the

right valve

for

check service

is

by

far the

most

difficult

and

frustrating. Because there

are

many kinds

and

makes

of

check valves, choose

the

type

and

style only

after

careful

study

of

Sections

5-4

and

7-1

and the

literature

of

various manufacturers.

Location

is

particularly important

for

check valves;

for

example, check valves

on

manifolds usually

behave

quite

differently

from

isolated check valves.

Check valves should,

if

possible,

be

placed

no

closer

than

four

or five

pipe

diameters

from

a

pump; other-

wise

turbulence

from

the

pump tends

to

cause

the

valve

disc

to flutter and

wear

the

bearings. (Flutter,

however,

is of

less concern

if the

spring

in a

spring-

loaded lever

is

stiff

enough

to

prevent slam.) Further-

more, such

a

separation

of

pump

and

check valve

allows

the

discharge pressure gauge

to be

placed far-

ther

from

the

pump where pressure

fluctuation is

much

less violent, gauge wear

is

reduced (this

is not a

concern

if the

gauge piping contains

a

normally

closed spring-loaded

shutoff

valve),

and

accuracy

is

greatly improved.

The

cost

of the

several

fixed

pres-

sure gauges needed

for

several pumps

can be

greatly

reduced

by

using

a

pair (one

for

suction,

one for

dis-

charge)

of

portable gauges.

Safety

No

automatic control system

for

valves

is

complete

if

it

does

not

incorporate safety features

to

prevent dam-

age

from

malfunctioning equipment.

These

safety fea-

tures

can be

arranged

to

prevent

the

operation

of an

unprimed pump,

to

prevent

the

operation

of a

pump

against

a

closed control valve that does

not

open

on

schedule,

or to

limit excessive pumping when dis-

charging

to a

broken pipe. Most

safety

features

include pressure-sensing devices

and

relays with tim-

ers

that operate valves only when pressures

are

nor-

mal. When abnormal conditions occur,

the

valve

should remain closed

(or

should

be

closed

if

open),

and

pumps should stop

(after

a

suitable delay)

to

pre-

vent

damaging pumps

and

motors.

5-2. Isolation Valves

Isolation

valves

are

either

fully

closed

or

fully

opened.

Valves

that remain

in one

position

for

extended peri-

ods

become

difficult

—

even

impossible

—

to

operate

unless they

are

"exercised"

from

time

to

time. Valves

should

be

exercised

at

least once each year (more

often

if the

water

is

corrosive

or

dirty),

and the

required exercise routine should

be

emphasized

in the

O&M

manual.

Isolation

Valves

for

Water

Service

Isolation valves likely

to be

used

for

water service

include

•

Ball (these

are

expensive,

so in

sizes larger than,

say,

100 mm [4

in.]

they

are

used rarely

for

isola-

tion only)

•

Butterfly

(these

are

popular

in all

sizes)

•

Cone (these

are

also expensive,

so in

sizes larger

than,

say,

300 mm [12

in.]

they

are

also used rarely

for

isolation only)

•

Eccentric plug (these

are

excellent

and

useful

in all

sizes)

•

Gate (these

are

popular

in all

sizes)

•

Plug (these

are

either lubricated

or

nonlubricated,

both

of

which

are

rarely used

for

only isolation

in

sizes larger than approximately

50 mm [2

in.];

the

lubricated version

is

frequently

used

if the

valve

is

to be

closed

for

extended periods

of

time).

The

gate valves most likely

to be

used

are

double

disc

or, for raw

water with grit, resilient seat,

but

solid

wedge,

knife,

or

even sluice gates

may be

useful

in

some circumstances.

The

plug valve most likely

to be

used

is the

nonlubricated type with either

a

rectangu-

lar or a

round port,

but a

lubricated plug valve would

be

used

for

higher pressures. Lubricants approved

by

the FDA are

available

for

water service. Globe valves

are not

normally used (because

of

their high headloss)

except

in

piping

50 mm (2

in.)

and

smaller

in

which

an

ability

to

regulate

flow is

desired.

For

clean water,

the

double disc gate

and

butterfly

valves

are the

most

frequently

used.

The

more expensive ball

or

cone

valves

are

used

for flow

control, pump control,

or

powered check service, usually

in

conjunction with

another valve

for

isolation

so

that

the

control

or

check

valve

can be

repaired.

Isolation

Valves

for

Wastewater

Service

Valves

for

wastewater service

are

more limited

in

type

because stringy materials catch onto

and

build

up on

any

obstructions

to the flow and

sticky grit tends

to

lodge

in any

pocket, such

as a

valve seat. Depending

on

the

quality

of

treated wastewater, however, valves

for

water service might

be

used, albeit with some risk.

Isolation valves likely

to be

used

for

wastewater

service include

•

Ball

(in

unusual circumstances only)

•

Cone

(in

unusual circumstances only)

•

Gate (popular

in all

sizes)

•

Eccentric plug (popular

in all

sizes).

The

specific

styles most

often

used

are

eccentric plug,

knife

gate,

and

resilient seat gate. Solid wedge gate

valves with

nonresilient

seats might

be

used,

but

grit

that

can

collect

in the

seat

is

often

troublesome. Lubri-

cated

and

nonlubricated plug valves

and

ball valves

are

used,

especially

if flow

control

is

needed.

Description

of

Isolation

Valves

The

following descriptions

are for

both water

and

wastewater valves.

The

types

of

valves that

are

recom-

mended

or

could

be

used

for

isolation service

are

given

in

Table

5-2.

Ball Valves

The

rotor (round plug)

in a

ball valve rotates

90°

from

fully

closed

to

fully

open.

A

ball

valve

has

very

low

Table

5-2.

Recommendations

for Use of

Isolation

Valves

Service

usage

3

Water Wastewater

Type

of

valve

Raw

Clean

Raw

Treated

Slurry

Gas

Fuel

oil

Angle

GG

XGXGG

Ball

EE

EEEEE

Butterfly

GG

XGX--

Cone

EE

— — —

Diaphragm

— — —

GG

— —

Gate

Double

disc

G E X-F

GXGG

Knife

F-G

F-G F-G F-G

FXX

Sluice

GG

G G — — —

Resilient

seat

G G

F-G

FXX

Solid

wedge

GG

FGXGG

Globe

XG

XFXGG

Pinch

GG

GGGGG

Plug

Eccentric

EE

EEF

— —

Lubricated

GG

GGGGG

Nonlubricated

GG

—

FF

a

E,

excellent;

G,

good;

F,

fair;

X, do not

use;

—,

use is

unlikely.

headless

in the

fully

open position because

the

bore

of

the

pipe

is

carried straight through

the

ball, which

results

in

headloss nearly

the

same

as in a

straight

piece

of

pipe

of the

same laying length

as the

valve.

Ball valves

are of two

basic types:

(1)

seat supported,

usually

for

valves smaller

than

150

mm (6

in.)

and (2)

trunnion

supported, usually

for

valves

150 mm (6

in.)

in

size

or

larger

as

described

in

AWWA

C507.

Ball

valves

in

water, wastewater,

and

sludge pump-

ing

stations

are not

ordinarily used

as

isolation valves.

Their laying length, weight,

and

cost

are

much greater

than

those

of

gate

and

butterfly

valves. Seat-supported

ball valves

are

widely used

in

auxiliary piping

in

such

services

as

seal water,

fuel

oil,

and

natural

gas as

well

as in

isolating pressure gauges

and air and

vacuum

valves

because such piping

is

smaller than

75 mm (3

in.).

An

advantage

of a

ball valve

is

that

it

offers

a

rel-

atively

leak-free seal.

Ball valves

for

severe duty service (wastewater,

storm

water, surge control,

and

pump control)

are

usually

of the

trunnion type

and

should

be

selected

with

a

great deal

of

care, especially with respect

to

materials

for

seats

and

bearings, because ball valves

vary

widely

in

quality. Seats

are

subjected

to

wear

and

tear

from

grit

and

tramp iron (nuts, bolts, scraps)

in

wastewater

and

storm water service, particularly

in

check

and

surge control usage. Bearings

and

shafts

must

be

designed

to

withstand unbalanced forces dur-

ing

rapid closure

on

pump failure

and

during surge

control

episodes.

Ball valves

are

often

reserved

for

severe service conditions,

and

some engineers have

experienced

difficulties

with

cold-flow

of

resilient

seat materials under high

differential

pressures.

In

some seat designs

or

under some operating condi-

tions, those materials tend

to

escape

the

mechanism

intended

to

retain them

in the

plug

or

body castings.

Stainless steel

and

stainless steel against

monel

metal

are

excellent alternatives

for

seats

in

such

service

—

although

these

are not a

universal panacea,

and

prob-

lems have been reported with metal-seated ball valves

as

well.

There

are two

kinds

of

mechanisms

for

operating

ball valves.

In

most valves,

a

shaft

allows

the

ball

to

rotate about

a fixed

axis

so

that

the

seat

is

wiped

by

the

turning

of the

ball.

In

some metal-seated valves

(e.g., Figure

5-

Ia),

a

loose-fitted trunnion allows

pressure

from

the

force main

to

push

the

ball against

its

seat. When

the

pump starts against

the

closed

valve,

the

pressure moves

the

ball

away

from

its

seat

and

allows

the

valve

to

open

freely.

If the

valve

is

closed before

the

pump

is

stopped,

the

action

is

reversed,

and

when

the

pump stops,

the

manifold

pressure again seats

the

ball tightly.

In

effect,

the

action

is

like that

of a

cone valve

but

without

its

complex mechanism.

The

seat

is

thus never wiped

except

on

power failure.

An

alternative mechanism

for

accomplishing

the

same

purpose (avoiding wiping

the

seat when

the

valve

is

opened

or

closed)

is a

movable retainer ring

for

holding resilient seat material.

The

inside

surface

of

the

ring

is

always exposed

to the

pressure

in the

force

main. When

the

pump

is

started

and its

pressure

exceeds force main pressure,

the

retainer ring retracts

away

from

the

seat

and

allows

the

valve

to

open

freely.

When

the

valve

is

closed

and the

pump subse-

quently

stopped, pressure

from

the

force main closes

the

seat

and

makes

it

driptight.

The

retainer ring

mechanism

can be

adjusted

for a

wide range

of

differ-

ential pressures. Valves

in

service

for 20 yr

show

no

signs

of

seat wear.

Designers should

carefully

investigate

the

perfor-

mance history

of the

ball valve under consideration

and

be

satisfied that

the

valve will

be

satisfactory

under

the

conditions that will prevail

in

service.

In

addition

to

considerations

of

cost

and

trouble-free

operating

life

under imposed conditions,

the

cost, dif-

ficulty,

down

time,

and

probable frequency

of

repairs

should

be

weighed. Some valves

can be

repaired

in

place, whereas others must

be

removed

and

disman-

tled

for

repairs.

Trunnion-supported ball valves should

be

installed

with

the

shaft

horizontal

and

should

be

located

in

hor-

izontal

—

not

vertical

—

pipes.

A

ball valve

for

pump, check,

and

surge control

in

water

or

wastewater service

is

usually

fitted

with

a

worm

gear

and

compound lever

(or

pantographic)

operating mechanism,

as

shown

in

Figure 5-1.

The

operating mechanisms

may be fitted

with

a

variety

of

actuators. Such

a

valve

is

superior

to all

others

in

pro-

viding

ideal opening

and

closing characteristics

for

minimizing

surges caused

by

pump start-up

and

shut-

down

or

loss

of

power.

As

shown

in

Figure 5-2,

the

last

10% of Cv (a

measure

of flow)

requires about

50% of the

stem travel,

so the

last portion

of flow is

choked

off

very slowly.

The

parameter

Cv is

more pre-

cisely

defined

by

Equations

5-1

and

5-2.

In

SI

units

,2

Cv

=

2.919-yr

(5-la)

*]K

where

d is the

diameter

of the

valve (approach pipe,

actually)

in

meters

and K is the

dimensionless valve

coefficient

in

Table

B

-7.

At any

differential pressure

across

the

valve

Q

=

0.3807CvVAP

(5-2a)

where

Q is in

cubic meters

per

second

and AP is

dif-

ferential

pressure

in

kilopascals.

In

U.S. customary units

,2

Cv

=

29.854=

(5-lb)

JK

where

d is the

diameter

of the

valve

in

inches

and K is

the

valve

coefficient

in

Table

B

-7.

Flow through

the

valve

is

given

by

Q = C v

JAP

(5-2b)

where

Q is in

gallons

per

minute

and AP is in

pounds

per

square inch.

One can

think

of Cv as the

gallons

per

minute

of

water

at

6O

0

F

that

flow

through

the

valve

at a

pressure

difference

of 1

lb/in.

2

An

inconsequential cor-

rection

for

density

at

other temperatures

is

omitted

from

Equation

5-2

because

the

correction applicable

in

pumping

stations would never reach

2%. As the

valve

is

closed,

Cv is

reduced,

as

indicated

in

Figure 5-2.

Figure 5-1. Ball valves

with

link

and

lever

motion,

(a) A

metal-seated valve. Courtesy

of

APCOAViIlamette

Valve,

Inc.

(b) A

resilient-seated valve.

Note

that

removing

the

cover

allows

the

seal

ring

to be

replaced

with

the

valve

in

situ. Courtesy

of GA

Industries,

Inc.

Butterfly

Valves

A

butterfly

valve (Figure

5-3)

is a

quarter-turn valve

in

which

a

disc

(or

rectangular vane

for

square channels)

is

rotated

on a

shaft

so

that

the

disc seats

on a

ring

in

the

valve body.

The

seat

usually

is an

elastomer

bonded

or

fastened either

to the

vane

or to the

body.

Figure

5-3.

Butterfly

valve.

Courtesy

of

Henry

Pratt

Co.

Most,

if not

all,

manufacturers have

now

standard-

ized

on the

short body style

(see

AWWA

C504).

In the

long body style,

the

vane

is

contained entirely within

the

body when

the

vane

is in the

fully

open position.

In

the

short body style,

the

vane protrudes into

the

adjacent

piping when

in the

open position.

The

wafer

style, which

is

very thin, requires instal-

lation

of the

valve between pipeline

flanges

bolted

across

the

valve body. When planning

the use of

short

body

or

wafer-

sty

Ie

butterfly

valves, make sure

the

pipe

on

either side

is

large enough

to

accept

the

vane.

Wafer

valves

are not

recommended

for

isolating

pur-

poses where

it may be

necessary

to

remove

the

adja-

cent, connecting pipe spools. Some consultants

refuse

to use

wafer

valves under

any

circumstances.

Butterfly

valves

are

used

in

both isolation

and

lim-

ited throttling service.

Butterfly

valves

can be

designed

for

leakproof

shut-off,

but

leakage

is

significant

with-

out

a

resilient seat. Solids, wear,

and

scale buildup

cause leakage even with resilient seats.

In

throttling

service,

the

control range

for the

vane angle

is

about

15

or 20° to 60 or

70°.

Because most

butterfly

valve

designs

are

entirely unsuitable

for

throttling

and

some

are

prone

to

failure, select only valves recommended

by

the

manufacturer

for

throttling. Then

be

suspicious.

Investigate installations

of

valves proposed

for

throt-

Figure

5-2.

Stroke versus

Cv for

various valves. After

GA

Industries,

Inc.,

DeZurik,

A

Unit

of

General

Signal

Corp.,

and

Willamette

Valve,

Inc.

tling

service.

Pay

particular attention

to the

seat

design.

The

AWWA

C504

standards alone

do not

ensure

butterfly

valve seats that

are

adequate

for

severe

throttling

(as in

pump-control valves) where seats must

be

very rugged

for

longevity. Some valves have their

rubber

seats vulcanized into

the

body. Such valves

are

reliable,

but

when

the

seat needs

to be

replaced,

the

valve

must

be

shipped

to the

manufacturer

for

repairs.

Other valves have seats that

are

intended

to be

replace-

able

in the field, but

after

several years

of

service, cor-

rosion

may

make

it

impossible

to

unscrew studs

or

nuts,

and the

valve must

be

sent

to the

shop

for

repairs

anyway.

So

beware

of

sales claims concerning

"easy

replacement"

in the field.

Some (but

by no

means all)

replaceable body seats

are

satisfactory.

Butterfly

valves

are

used most

frequently

in

water

and

air

service. They

are not

suitable

for

sewage, sludge,

or

grit service because

the

disc collects stringy solids

and

the

seating edge

is

easily abraded. When used

in a

"dirty"

service, such

as raw

water, install

the

valves

with

the

vane axles horizontal

to

prevent grit

from

set-

tling

in the

shaft

bearings

and

oriented

so

that

the

bot-

tom

part

of the

vane moves

in the

direction

of flow to

wash

solids through

the

valve when

it first

opens.

Butterfly

valves should

be

located

at

least three (and

preferably

five)

pipe diameters

from

an

upstream bend

and

at

least

two

pipe diameters

from

a

downstream

bend

to

place

the

valves

in

regions

of

approximately

symmetrical

velocities

and

coaxial streamlines. Valves

that

are

closer

to

bends

may

chatter,

and if

they

are

very

close, excessive forces

may be

required

to

operate

them.

The

excessive forces

can be

eliminated

by

mounting

valve

shafts

in the

plane

of the

bend,

but it is

best

to

orient

shafts

horizontally wherever possible.

Cone

Valves

Cone valves, sometimes called "lift-action" plugs,

are

essentially plug valves that

are

positioned

by first

lift-

ing

the

plug

in the

body before

the

operating mecha-

nism moves

the

plug

to its new

position (see Figure

5-4).

After

reaching

the new

position,

the

plug

is

then

reseated

to

provide

a

seal.

The

valve

has

excellent

characteristics

for

surge-control

and

pump-control

service. Cone valves have been used successfully

in

large water

and

wastewater

applications.

Compared

with

a

ball valve,

the

axial motion

in a

cone valve

reduces operating forces

and

seat wear

and

produces

a

tighter

shutoff.

However, because

of the

unusual

stroking requirements, cone valve operating mecha-

nisms require skilled maintenance personnel

and can

be

complex

and

prone

to

failure.

A

variant

of the

cone valve

is the

small taper-

ground cock

in

which

the

plug

is

axially forced into

tight

contact

by a

spring. There

is no

mechanical

lift,

but

a

slight axial movement occurs anyway

as the

valve

is

turned. Cocks

are

useful

for the

isolation

of

taps, pressure gauges,

and

sampling ports.

Diaphragm

Valves

(Not

Stem

Guided)

A

diaphragm valve contains

an

internal

flexible

elas-

tomer diaphragm that presses down against

the

inte-

rior body wall (or, sometimes, against

an

internal weir

Figure

5-4. Cone valve. Courtesy

of

Allis-Chalmers

Corp.

that

is

part

of the

body).

The

diaphragm seals

the

valve

body

from

the

stem,

so the

valve

is

leakproof

and

the

stem requires

no

packing. This type

of

valve

is

useful

in

sludge

and

grit service because there

is

little

obstruction

through

the

body that could

collect

grit

or

solids. Cleaning tools such

as

pigs, however, cannot

pass through them. These valves

can be

lined with

various

plastics such

as PVC or

polyethylene

and

are,

therefore,

often

used

in

chemical piping (e.g., chlorine

solution

piping). They

are not

often

encountered

in

raw

sewage, water,

and

sludge pumping stations

because other types

of

valves

are

cheaper. Sometimes

a

diaphragm valve

or its

close relative,

the

pinch

valve,

is

used

as a

safety

device

on a

bypass

pipe

around

a

positive displacement pump

to

prevent

destruction

of the

pump

or

main pipeline

if a

down-

stream valve were

to be

mistakenly

closed.

The

dia-

phragm

or

pinch valve

is set to

open automatically,

either

by a

preloaded spring

or by air

pressure,

at a

suitable

overpressure.

The

lack

of a

visible position

indicator

is a

shortcoming.

Another type

of

diaphragm valve

is a

stem-guided

globe valve

with

a

diaphragm

in the

bonnet

to

actuate

the

valve (see Section 5-5).

The two

types

are in no

way

similar

and

should

not be

confused.

Eccentric

Plug

Valves

In

an

eccentric plug valve (Figure 5-5), both

the

body

and

the

plug seat

are

offset

from

the

center

of

rotation

so

that when

the

valve

is

opened

the

plug-seat surface

rotates away

from

the

body-seat

surface;

this move-

ment

minimizes

the

scraping

and

deterioration

of

seats common

in

other valves.

In the

open position,

at

a

quarter-turn

from

closed position,

the

plug rests

against

the

side

of the

valve body.

In

some makes

of

valves,

the

port

is

either rectangular

or, at

least,

not

full

ported.

In

others,

the

port

is

circular

and the

same

size

as the

pipe

so

that

a pig can

pass

through

the

valve.

Some styles

or

makes

are

difficult

to

service

in

the field.

Plug valves

are

especially attractive

in

wastewater

and

sludge applications because there

is

nothing

to

become jammed

or

clogged with solids.

The

plug

is

usually coated with

an

elastomer, such

as

neoprene,

to

obtain

a

resilient seating.

Eccentric plug valves used

in

sewage

and

sludge

service,

or for any fluid

containing

solids

or

grit,

should

be

installed

so

that

the

plug rotates about

a

hor-

izontal axis.

The

plug should

be

stored

in the top

when

the

valve

is in the

open position

and

should seat

in the

direction opposite

the

high-pressure side

so

that

the

pressure

of the

water forces

the

plug against

the

seat

for

a

tighter

seal.

Because some

designs

hold pressure

in

only

one

direction, they must

be

installed

to

hold

Figure

5-5.

Eccentric

plug

valve.

Courtesy

of

DeZurik,

A

Unit

of

General

Signal

Corp.

against

the

applicable pressure, which

is

often

oppo-

site

to the

direction

of flow

(as,

for

example,

in the

isolation valve

on the

discharge side

of the

pump).

Debris problems

are

less severe

if the

body seat

is

upstream.

The

valve body

may be

marked

to

install

the

other way,

so

both

the

specifications

and the O&M

manual

must clearly state both which

way the

valve

is

to be

installed

and the

reason

for the

orientation.

When these valves

are

installed, tighten

the flange

bolts

only enough

to

stop

leaks.

Excessive tightening

squeezes

the

gasket material against

the

plug

and

causes

it to

bind. This cautionary note should also

be

placed

in

both

the

specifications

and in the O&M

manual.

Add the

warning that

all

valves (not just

eccentric

plug valves) should

be

exercised

on a

regular

schedule.

Gate

Valves

A

gate valve

has a

disc sliding

in a

bonnet

at a

right

angle

to the

direction

of flow.

Gate valves

are

further

divided into several subtypes:

•

Double disc (Figure 5-6)

•

Solid wedge

resilient

seated (Figure 5-7)

or

metal

seated

•

Knife

(Figure 5-8)

•

Rising stem (shown

in

Figure 5-7)

•

Outside screw

and

yoke

(OS&Y),

which also

is

often

a

rising stem design

(as

shown

in

Figure 5-7)

•

Nonrising

stem (NRS)

(as

shown

in

Figure 5-6)

•

Inside screw

and

outside stem thread

•

Bolted, screwed,

or

union bonnet

A

rising stem

(as

shown

in

Figure 5-8) allows

the

operator

or

observer

to

determine easily

if the

valve

is

open

or

closed.

An NRS (as

shown

in

Figure 5-6)

allows

the

valve

to fit

into cramped areas where

a

ris-

ing

stem would strike

a

ceiling

or

wall.

The NRS

style

should

be

avoided wherever possible because when

it

is

used there

is no

indication

of

whether

the

disc

is

fully

seated.

If a

hard object

is

caught

on the

seat,

workers

may

assume

the

gate

is

closed. Dismantling

a

pressurized pipe, thought

to be

isolated,

can

lead

to

flooding

and

even loss

of

life.

Hence, some indication

of

valve position

is

desirable

for all

valves. Further-

more,

in the O&M

manual, caution workers always

to

back

off

nuts

by two or

three threads

and

crack

the

joint

to

determine whether there

is

pressure.

If

there

is, the

nuts

can be

retightened until

the

trouble

is

cor-

rected.

Gate valves

are

suitable only

for

isolation (open/

closed) service. They should

not be

used

for

throttling

service because

a

relatively small valve opening

Figure

5-6.

Double disc

NRS

(nonrising

stem)

gate

valve. Courtesy

of

Mueller®.

allows

a

high

flow

capacity

[I].

Furthermore, gate

valves

are

subject

to

damaging vibration when partly

open.

Double

Disc

Gate

Valve

The

double disc gate valve equipped with

a

rising

stem

is one of the

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