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

Close-Coupled

Pumps

The

impeller

of a

close-coupled pump

is

mounted

on

the

driver

shaft.

There

is no

separate pump

shaft,

and

no

coupling

is

required between

the

driver

and the

pump

(see Figure

11-11).

Submersible

Pumps

Submersible pumps

are

close-coupled pumps driven

by

a

submersible motor

and

designed

for

submerged

installation

in a wet

well (see Figure

11-12).

Impeller-between-Bearings

Pumps

In

impeller-between-bearings pumps,

the

bearings

are

mounted

on

each side

of the

impeller (see Fig-

ure

11-13). Only

the

axial split-case design

is of

interest here.

Axial

Split-Case

Pumps

Axial split-case pumps have

a

casing that

is

split

along

the

(usually horizontal) centerline

of the

shaft.

The

impellers

can be

readily exposed

for

inspection

and

service

by

removing

the

upper half

of the

casing

(see Figure

11-13).

11-3.

Construction

of

Centrifugal

Pumps

A

representative centrifugal pump

and its

basic com-

ponents

are

shown

in

Figure

11-10.

The

function

of

the

components

of the

pump

and the

different

options

available with these components

are

discussed

in the

following

subsections.

Figure

11-10.

An

overhung-impeller,

separately

coupled,

end

suction,

clear-liquid

pump.

Courtesy

of

Fairbanks

Morse Pump

Corp.

Impeller

The

impeller increases

the

velocity

of the

liquid

and

raises

its

pressure.

The

impellers

of

centrifugal

pumps

may

be of the

radial-

or

mixed-flow

type.

The

pump

characteristics

are

determined

by the

impeller type.

The

relationship between impeller type, specific

speed,

and

pump characteristics

is

discussed

in

Chap-

ter 10 and

illustrated

in

Figure

10-8.

Centrifugal

pump

impellers

may be

enclosed,

semiopen,

or

open (Figure

11-14).

Enclosed impellers

are by far the

most common. Semiopen impellers

are

used

less

frequently;

they have,

in

theory, slightly bet-

ter

efficiency

and are

easier

to

cast,

but

they require

close tolerance

of

axial clearances between

the

blades

and

the

front

cover.

The

third type,

the

open impeller,

requires close clearances

on

both sides simulta-

neously. Because such tolerances

are

difficult

to

main-

tain,

open impellers

are

seldom used

in

centrifugal

pumps.

The

pressure

of the

liquid

at the

impeller discharge

is

higher than

at the

impeller inlet,

so the

liquid tends

to

circulate back

to the

impeller suction. Some provi-

sion must

be

made

to

contain this recirculation. This

is

the

function

of the

impeller

and

casing wear rings

in

enclosed impellers

and the

wear plates

in

semiopen

impellers.

The

impellers

are

subjected

to

radial

and

axial

thrust

forces. Radial thrust

is

caused

by

unequal pres-

sure distribution

in the

volute

due to

volute asymmetry.

The

theory

of

radial thrust

is

presented

in

Section 10-6.

The

axial thrust

is

caused

by the

pressure

differen-

tial between

the

front

and

back sides

of the

impeller.

Axial

thrust

in

multistage high-pressure pumps

can

reach high values. These pumps

are

sometimes

equipped with hydrodynamically balanced impellers,

Figure

11-11.

An

overhung-impeller,

close-coupled,

end

suction

pump.

After Fairbanks

Morse

Pump

Corp.

as

shown

in

Figure

1

l-14e.

This type

of

impeller

has a

balancing wear ring

on its

back shroud, which, com-

bined with pressure relief

(or

pressure balancing)

holes, equalizes

the

pressure

on

both sides

of the

back

shroud

and

reduces

the

axial thrust. Although

the

back

shroud

wear rings reduce

the

axial thrust, they also

have

disadvantages:

(1)

they complicate

the

pump

design,

and (2) the

pressure relief holes cause continu-

ous

recirculation

of the

pumped liquid with

a

resulting

loss

of

efficiency.

Consequently,

hydro-dynamically

balanced impellers

are

rarely used

in the

pumps dis-

cussed

in

this book. Instead,

the

thrust bearings carry

Figure

11-12.

Submersible

pump

(pull-up

design)

in the

Lancaster

Reclamation

Plant,

Los

Angeles

County

Sanitation

District

No. 14.

the

full

axial load, which,

from

practical experience,

has

been shown

to be the

simplest technical solution.

Wear

Rings

The

wear rings control

the

liquid recirculation

between

the

impeller discharge side

and the

inlet side.

The

wear

of

wear rings

is

caused

by

grit

or

other abra-

sive

matter

in the

pumped liquid. Rubbing

of

wear

rings also causes wear,

but in

well-designed pumps,

the

shaft

deflection

does

not

exceed wear ring clear-

ance unless

the

pumps

run

outside

of

their design

operating range

(or

close

to

shut-off).

The

wear rings

may

be

separate

or

integral. Separate rings

are

machined independently

from

the

impeller

and the

inlet

cover

and

then pressed

or

fitted

in

place

and

locked

by

mechanical devices such

as

setscrews, pins,

or

locking compounds.

The

phrase

"integral

wear

rings"

means that

no

separate wear rings

are

installed,

but

that

a

close

clear-

ance

is

arranged between

the

impeller

and the

inlet

cover. Enough material

is

usually provided

in the

cover

and on the

impeller

so

that

after

the

wear ring

clearance exceeds

the

recommended limits, both

can

be

remachined

and

separate replacement wear rings

can

be

installed.

Wear

rings

can be of

axial, radial,

or

some special

design. Axial

or

"face-type"

wear

rings

rely

on the

impeller setting relative

to the

suction head

to

main-

tain

the

wear ring clearance. They require

an

axially

adjustable

impeller

or an

axially adjustable suction

cover (see Figures

11-11

and

ll-15b).

Axial wear

rings

have

the

advantage

of

being adjustable

for

wear

in

the field

without

the

need

for

pump disassembly

or

wear ring replacement. They

do,

however, complicate

the

pump design. They

are

used primarily

on

pumps

smaller than about 0.25

m

3

/s

(4000

gal/min).

Radial-type wear

rings are not

sensitive

to the

exact

axial

impeller

location

and, hence,

do not

require

axial

adjustment

of the

impeller (see Figures

11-10,

1

l-14a,

ll-15a,

and

ll-15c).

They cannot

be

readjusted

for

wear, however,

so

they must

be

replaced when leakage

caused

by

wear becomes objectionable.

Some pumps designed

for

gritty liquids

and

very

long wear

ring

life

are

equipped with

flushed

wear

rings (see Figure

U-ISc).

Flushing liquid,

free

of

abrasive

particles

and

injected under pressure into

the

wear ring clearance, prevents

the

entry

of the

grit.

Figure

11-13.

An

impeller-between-bearings,

axial

split,

single-stage

pump.

Courtesy

of

Dresser

Pump

Division.

L-shaped wear rings (Figure

ll-15d)

are

another

option designed

to

increase

the

length

of the

leakage path

and

to

reduce

the

leakage rate.

For

gritty applications,

they

may be

combined with

the

external

flush

option

to

conserve

on flushing

water requirements. They

do

require

the

same axial adjustment

as the

facetype

rings.

Because

the

wear

rings may

contact each other under

extreme operating conditions,

the

wear

ring

material

must

be

both

nongalling

and

compatible with

the

pumped

liquid.

A

difference

in

ring hardness

of

Brinell

hardness number

50 on the C

scale

(50

BHN)

is

gener-

ally

recommended especially

for

stainless-steel rings.

Front head

rings are

usually made

of the

harder material.

Wear

Plates

Semiopen impellers have

no

wear rings

but

rely

on the

close clearance between

the

vane

tip and the

suction

cover

to

control

the

leakage around

the

vane tips

(see

Figure

ll-14b).

When gritty liquids

are

pumped, wear

of

the

blade tips

and of the

casing

is to be

expected.

Hence,

the

suction cover

is

often

equipped with

a

wear plate, which

has a

contour matching

the

blade

tip

contour

and can be

axially readjusted

to

compensate

for

wear.

The

wear plates

are

usually made

of

abra-

sion-resistant material such

as

Type

416,

heat-treated

stainless steel (e.g., ASTM

A

743-CAIS,

350

BHN).

Figure

11-14.

Impeller

designs,

(a)

Enclosed

impeller;

(b)

semiopen

impeller;

(c)

open

impeller,

radial

flow;

(d)

open

impeller,

axial

flow;

(e)

hydrodynamically

balanced

impeller.

After Fairbanks Morse Pump

Corp.

Pump

Shaft

In

transmitting power

from

the

driver

to the

impeller,

the

pump

shaft

must support

all

radial

and

axial impel-

ler

forces.

It is

subject

to

torsion, bending,

and

tension.

Good

shaft

design

is

needed

to

ensure that

the

endur-

ance limit

of the

shaft

material

is not

exceeded under

any

anticipated operating conditions. Furthermore,

the

deflection

of the

shaft

must

be

controlled

to

avoid

wear

ring rubbing

and

damage

to the

packing

or the

mechanical seals.

A

deflection

of

0.05

mm

(0.002

in.

or 2

mils)

at the

shaft

seal

is

often

given

as a

recom-

mended limit. Deflection

at

radial wear rings should

be

compatible with wear ring nominal clearance.

Shaft

Sleeve

Most

shafts

are

equipped with

a

replaceable

shaft

sleeve

to

protect

the

shaft

from

wear

in the

packing area (see

Figure

11-10).

Typical sleeve material

is

heat-treated

type

416

stainless

steel

(ASTM

A

754-CAIS,

300 to 350

BHN).

The

sleeve must

be

sealed

to the

shaft

to

stop

leakage

of the

pumped liquid,

and

either

O-rings

or

anaerobic sealing

and

locking compounds (which harden

in

the

absence

of

oxygen)

are

used

for

this purpose.

Positive locking devices (such

as

pins, keys,

or

notches)

are

sometimes used

to

prevent sleeve rotation

on

the

shaft,

but the use of

sealing compounds

is

also

a

successful locking method.

Figure

11-15.

A

solids-handling

pump,

(a)

Pump;

(b)

impeller

axial

adjustment;

(c)

wear

ring

external flush;

(d)

L-shaped wear rings;

(e) lip

seal. Courtesy Fairbanks

Morse

Pump

Corp.

Pump Casing

The

pump casing

or

pump volute

encloses

the

impel-

ler and

contains

the

discharge and, sometimes,

the

suction

nozzles (see Figure

11-10).

The

casing col-

lects

the

high-

velocity

flow

from

the

impeller

and

con-

verts some

of the

velocity energy into pressure.

The

casing

may be one

piece

or of

axial-

or

radial-split

design.

Suction

Cover

The

suction cover

(or

front

head) encloses

the

suction

opening

of the

casing

and

also contains

the

suction noz-

zle

of the

pump.

On

some designs,

the

suction cover

is

an

integral part

of the

pump casing (see Figure

11-10).

Stuffing

Box

Cover

The

stuffing

box

cover (also called

the

"back

head"

or

"adapter")

encloses

the

inboard opening

of the

casing

and

also contains

the

stuffing

box

(see Figure

11-10).

Frame

and

Bearing

Housings

Separately coupled pumps

with

overhung impellers

have

a

frame

that contains

the

pump bearings (see Fig-

ure

11-10).

The

bearings

may be

mounted directly

in

the

frame

or in

bearing housings mounted

in the

frame.

Bearing housings facilitate

the

assembly

of the

bearings

and are

used

in

larger pumps. With horizon-

tal

pumps,

the

frame

frequently

supports

the

entire

pump

(see Figure

11-10).

Bearings

The

pump bearings carry

the

shaft

and

absorb

all of

the

radial

and

axial forces acting

on the

shaft.

A

great

majority

of the

centrifugal

pumps

for

pumping sta-

tions

are

equipped with

antifriction

bearings (rollers,

tapered rollers,

or

balls

in

single

or

multiple rows).

Journal

bearings

are

used predominantly

for

high-

speed

and

high-load applications (e.g.,

boiler

feed

pumps

and

multistage high-pressure chemical pumps)

and

are of

little interest

to

pumping station designers.

The

thrust

bearing

of the

cantilever-type pump

carries

not

only

all of the

thrust load

but

also

its

share

of the

radial load.

The

thrust bearing must

be

capable

of

supporting axial loads

in

both directions

because

thrust

reversal

can be

expected with most

pumps,

especially during

the

starting

process.

In

some designs,

a

separate (usually smaller) thrust

bearing

is

installed

for the

thrust reversal. Typically,

the

thrust bearing

is

pressed

on the

shaft

and

locked

in the

bearing housing

(or in the

frame); thus,

the

thrust bearing determines

the

axial location

of the

shaft

and the

impeller.

The

radial bearing

is

also

pressed

on the

shaft,

but is

free

to

slide

in the

frame

or

in the

bearing housing. This protects

the

bearings

from

excessive axial

loads

generated

by

unequal

thermal expansion

of the

shaft

and

frame.

The

antifriction bearing design

is of

little conse-

quence

as

long

as the

bearings have been selected

by

the

manufacturer

to

deliver adequate bearing

life

and

the

design provides

for

proper bearing installation

and

lubrication.

The

bearing

life

can be

expressed

in

statis-

tical terms only. Commonly,

the

L-IO

bearing

life

rat-

ing is

used.

It is

defined

by the

AFBMA

as the

number

of

operating hours

at a

given load that

90% of a

group

of

bearings will complete before

the first

evidence

of

fatigue

develops. Average

life

is

statistically three

to

five

times

the

L-IO

life

(depending

on

whether

the

rat-

ing is for

degassed

or

nondegassed steel). Average

life

is

defined

as the

operating hours

at

which

50% of the

group

of

bearings

fail

and the

rest continue

to

operate.

The

L-IO

life

must

be

specified

and

computed

for a

selected pump operating condition.

The

bearing

life

is

a

function

of

bearing load

and

speed.

The

impeller

radial loads (and,

to a

lesser

degree,

the

axial loads)

vary

considerably between

the

best

operating point

(bep)

and

shut-off

of the

pump (see Figure

10-18).

As

a

result,

the

computed

L-IO

life

at

shut-off

for

a

typi-

cal,

single volute pump

can be as

little

as

one-hun-

dredth

or

less

of

the

life

at

bep.

A

frequent

specification

is

40,000

h of

L-IO

life

at

the

operating point

for

pumps

in

continuous, 24-h ser-

vice. Specification

of

bearing

life

at

shut-off

or for the

entire head-capacity range

of the

pump (which

is the

same

as

shut-off)

should

be

avoided. Such

a

specifica-

tion leads

to

oversized

shafts

and

bearings

and

does

not

ensure

the

computed bearing life because,

at

shut-

off,

the

bearings

are

exposed

to

shock loading,

the

effect

of

which cannot

be

readily predicted

nor

pro-

vided for. Operation

at

shut-off

can be

damaging

to

the

shaft,

impeller,

and

other structural components

of

the

pump

as

well and, therefore, operation

at

shut-off

for

prolonged periods

of

time should

be

avoided.

L-IO

life

in

excess

of

100,000

h is of no

practical

value.

It can be

taken only

as an

indication that

bear-

ing

fatigue

failures

are

unlikely. Bearings

fail

anyway

(as

shown through experience)

due to

other causes

such

as

corrosion caused

by

water contamination

or

deteriorated lubricants, rolling

surface

damage

due to

abrasive particles,

and

lack

of

lubricant.

Grease

is the

most common lubricant

for

pump bear-

ings.

It

requires relatively little attention

and

service

and

usually

gives long bearing

life.

Oil-bath lubrication

is

used

much less

frequently

because retaining

the oil in

the

bearing housing poses some technical

difficulties,

especially

on

vertical pumps. Force-fed

oil

lubrication

is

used

very

infrequently.

Although

it

ensures

the

lowest

bearing operating temperatures, provides

the

best protec-

tion

from

bearing contamination,

and

gives

the

longest

bearing

life,

it

requires expensive auxiliary equipment

(which

also needs service

and

attention).

In the

experi-

ence

of

pump operators,

the

benefits

gained

from

force-

fed

lubrication

do not

justify

the

additional expense.

Bearing Seals

The

bearings must

be

protected

from

contamination

by

abrasive particles

or

corrosive liquids (including

water).

Consequently,

the

bearing

frame

must

be

sealed.

The

following

are

typical seal arrangements.

Close-Clearance

Seals

Close-clearance seals consist merely

of a

close clear-

ance

between

the

shaft

and the

frame

or

bearing hous-

ing

(see Figures 11-10

and

11-15). They

are

very

simple, have

an

unlimited

life,

and

require

no

mainte-

nance.

However, they give only limited protection

from

water

spray (e.g., hosing down

the

pumps)

or

from

condensation during shut-down. Still, close-clearance

seals

are

very

successful

on

continuously operating

and

well-maintained

pumps where

the

normal bearing

operating

temperature prevents

any

water condensation

and

grease acts

as a

barrier

to

outside contaminants.

Elastomer Seals

Elastomer (lip-type) seals

are the

most

frequently

used

seals

in

current designs. They give good protec-

tion

from

contamination

and

moisture,

but

they also

generate

friction

heat

and

cause higher bearing-oper-

ating

temperatures with potentially reduced lubricant

life

(Figure

ll-15e).

Labyrinth Seals

Labyrinth

seals

offer

good bearing protection

and

gener-

ate

no

heat. They require more space

and are

relatively

expensive.

At

present, they

are

offered

as

optional equip-

ment

only. They

are

suitable only

for

horizontal

shafts.

Pump

Shaft

Seals

To

contain

the

liquid

in the

pump,

the

pump

shaft

must

be

sealed against

the

pressure

in the

pump.

The two

basic types

of

seals

are

packings

and

mechanical seals,

and

both have advantages.

The final

selection

of

seal

type

depends

on

installation requirements

and

owner

preferences,

but

packing

is

recommended

for

water

and

wastewater

service

in any

installation where leakage

from

the

stuffing

box is not

objectionable

or on

vertical

turbine

pumps that require dismantling

for

seal removal.

Packing

is the

simplest

and

most frequently used

seal.

The

packing

is

usually made

of

braided synthetic

fibers

impregnated with special lubricating com-

pounds.

The

packing rings require continuous lubrica-

tion

and

cooling, accomplished

by

using

the

pumped

fluid

or

an

external clean water source,

so a

properly

adjusted

packing must continuously leak

and

drip.

All

packings require periodic inspection

and

occasional

adjustment

of the

gland (see Figure

11-10).

Packings

are

frequently

equipped with

a

water seal

ring

(see Figure

11-10)

so

that

a

buffer

liquid

from

an

outside source (potable water

or

chlorinated

final

efflu-

ent for

wastewater pumps)

or

from

the

pump discharge

(for

clean water pumps)

can be

injected into

it.

This

ensures that

the

packing gets

an

adequate supply

of

lubricating

and

cooling liquid under

all

operating con-

ditions, particularly when

the

pressure

on the

pump

side

of the

packing drops below

the

atmospheric pres-

sure.

The

injection

of

clean liquid

is

also used

to

pre-

vent

contaminants

and

gritty materials

in the

pumped

liquid

from

entering

the

packing

and

causing acceler-

ated

shaft

sleeve wear.

If

potable water

is

used

as the

buffer

liquid,

an air gap

must

be

provided between

the

fresh

water source

and the

packing

to

prevent contami-

nation

of the

water supply with

the

pumped liquid.

Although most types

of

packing

in

wastewater ser-

vice must

be flushed

with clear water

to

avoid long-

term

scoring

or

abrasion

of

shaft

sleeves

due to

grit

in

the flow

stream, some types

of

braided, graphite-lubri-

cated

and

impregnated

or

Teflon-impregnated, non-

asbestos packing have proven

to

work

effectively

in

such

service without

any

external clear water

flushing.

Mechanical

Seals

Mechanical seals have

the

advantages

of no

leakage

and

no

need

for

periodic

maintenance,

but

they

are

rel-

atively

expensive

and

require care

in

installation. Some

are

subject

to

catastrophic failure.

For

mechanical seal

replacement,

the

pump must

be

partially disassembled,

which

is

costly. Mechanical seals should therefore

not

be

specified indiscriminately. Note that

API 610 has an

excellent discussion

of

mechanical seals.

A

typical, simple, single-face mechanical seal

is

shown

in

Figure

1

l-16a.

Mechanical seals rely

on

very

Figure

11-16. Mechanical

seals,

(a)

Single-face mechanical

seal;

(b)

double-face mechanical

seal.

Courtesy

of

John

Crane,

Inc.

close clearance between

the

stationary

and

rotating

seal

faces

for

sealing. Both

faces

are

lapped

flat to

within

approximately

a

micron. They

are

lubricated

and

separated

from

each other

by an

extremely thin

film

of

the

pumped liquid.

The

most frequently used

sealing

face

materials

are

ceramic

for the

stationary

seal

and

carbon (sintered graphite)

for the

rotating seal.

The

secondary seals that prevent leakage around

the

sealing

faces

are

rubber

or

synthetic elastomer com-

pound

O-rings

or

bellows.

Double-face

mechanical seals (Figure

ll-16b)

are

installed

when

the

pumpage contains abrasives

(e.g.,

wastewater

containing gritty material). Clean liquid

is

injected

into

the

cavity between both seal faces

at a

recommended pressure

of 70

kPa

(10

lb/in.

2

)

above

the

sealed liquid pressure.

It

lubricates

and

cools both

seals

and

prevents intrusion

of the

pumped

liquid

between

the

inner seal

faces.

A

wide variety

of

seal

designs, arrangements,

and

materials

is

available

for

corrosive

or

toxic liquids, high pressures,

and

high

surface

velocities,

but

these exotic designs

are not

required

in

water

and

wastewater service.

Separately

Coupled

Pump

Design

A

pump designed

for a

separately coupled driver

is

shown

in

Figure

11-10.

The

pump itself

is

totally

self-

sufficient

and

directly mounted

on a

base

or a

founda-

tion.

The

driver

is

either

separately mounted

on the

same base

or

foundation,

or it can be

mounted directly

on

the

pump

frame.

A flexible

coupling

is

usually used

to

connect

the

pump

to the

driver.

Close-Coupled

Pump

Design

Close-coupled pumps have

the

pump impeller

installed

on the

motor

shaft

and the

pump casing

bolted directly

to the

motor (see Figure

11-11).

Com-

pactness

and low

initial cost

are the

prime objectives

of

this concept.

The

motor

shaft

of

close-coupled

pumps

must

resist

all

impeller radial

and

axial thrust

forces.

Hence,

the

motor bearings must carry

the

pump

impeller loads

in

addition

to

normal motor elec-

tromagnetic

and

torsional loads. Standard

flange-

mounted

motors

are

used

for the

close-coupled pumps

when

the

shafts

have adequate strength

and

rigidity

and

when

the

bearing

life

meets

the

specified

require-

ments.

Special motors

with

bigger bearings

and

shafts

must

be

supplied when

the

loading exceeds standard

motor capabilities.

Submersible,

Close-Coupled

Pump

Design

Submersible,

close-coupled

pumps

operate

while

immersed

in the

pumped liquid.

The

submersible

motors

are

single-purpose machines designed

specifi-

cally

for

this

application

(see Figures

11-12

and

11-17).

The

motor housing

is

hermetically sealed

to

prevent

the

intrusion

of the

pumped liquid.

The

motor

is

usually equipped with

two

mechanical seals,

and

the

space between

the

seals

is filled

with oil.

To

com-

pensate

for the

thermal expansion

of the

oil,

the oil

chamber must contain

a

pressure-limiting device,

which

is

usually

an air

cushion

or a

sealed bladder.

The

pressure

-limiting

device

is

important. Several

serious accidents have occurred

in

which

a

pump

exploded

when

lifted

from

the wet

well soon

after

being

stopped.

The

cause

of the

explosion

was the

expansion

of the oil due to

stored heat that

was no

longer dissipated

by

water.

The

motor housing itself

may be filled

with

air or

with

oil.

The

oil-filled motor

is

less sensitive

to the

intrusion

of

pumped liquid,

but the

churning

oil

increases friction

and

lowers

efficiency.

Oil-filled

motors

are

generally

not

supplied with larger pumps.

The

motor junction

box

must also

be

hydraulically

sealed

at all

joints, especially

at the

cable entry. Fur-

thermore,

the box is

usually equipped with

a

hydrauli-

cally tight bulkhead

for

further

protection

of the

motor

windings

from

leakage.

Submersible motors should always

be

equipped

with moisture-sensing

devices.

These

devices

are

fre-

quently installed

in the

seal

oil

chamber

and can be

used

to

initiate

an

alarm when

the

outer mechanical

seal starts leaking. Some manufacturers install mois-

ture probes

in the

motor itself

to

indicate moisture

intrusion

from

any

source.

The

motors

may be

externally

or

internally

cooled.

Externally cooled motors rely

on

liquid

in the wet

well

for

cooling

and

must

be at

least

partially

immersed

for

continuous operation

at

full

load. They

can

be

operated

in air

only

for

short periods. Other-

wise, they must

be

derated

for

continuous operation.

Internally cooled motors circulate some

of the

pumped liquid through

the

motor-cooling passages

or

through

a

special motor-cooling jacket

and can

thus

operate

in the air

without

any

restrictions.

11-4.

Overhung-Impeller

Pumps

Overhung-impeller pumps

are the

most

frequently

used type

in

pumping stations

for

pumping clear water,

wastewater,

and

thin sludge.

Different

designs

are

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

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