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

Figure

11-17.

A

submersible

vortex

pump.

Courtesy

of

WEMCO.

required

for

different

applications.

The

types

of

great-

est

interest

to

pumping station designers

are

pumps

for

clear liquids; nonclog pumps;

wet

well volute pumps;

vertical

turbine, solids-handling (VTSH®) pumps;

self

-priming

pumps; vortex pumps; cutter pumps;

and

grinder pumps (see Figure

11-la).

Some

of

these

pumps

are

available

in

separately coupled, close-cou-

pled,

and

submersible designs.

Clear-Liquid

Separately

Coupled

Pumps

The

pump shown

in

Figure

11-10

is a

clear-liquid,

end-suction, separately coupled pump with

an

over-

hung

impeller

and is

available

in the

horizontal model

shown

or in a

vertical model.

In

vertical models,

the

driver

may be

mounted either

on the

frame

(Figure

1

1-1

8a)

or on a floor

above

the

pump (Figure

1

1-1

8b)

with

a

long line

shaft.

Line

shafts

usually require

intermediate supports

and

bearings.

Impeller.

The

impeller

of the

pump typically

has

seven

to

nine vanes,

the

optimum

for

high hydraulic

efficiency.

Usually,

the

impeller

is of the

enclosed

design.

Wear

rings. Both types

of

wear

rings

—

radial

and

axial

—

are

common.

Shaft

seal. Packing,

as

well

as

mechanical seals,

are

used

for the

shaft.

Customer preference

is the

deciding

factor.

ANSIB73.1

andB73.2

pumps. ANSI B73.1,

"Hori-

zontal End-Suction Centrifugal Pumps

for

Chemi-

cal

Process,"

covers separately coupled pumps

and

basic design requirements. Most important, pump

mounting

dimensions

are

standardized,

so

that

pumps

made

by

different

manufacturers

can be

interchanged without piping modifications.

ANSI

B73.2, "Vertical

In-Line

Centrifugal Pumps

for

Chemical

Process,"

covers vertical, separately

coupled,

and

close-coupled

pumps.

It is

similar

to

B73.1

in all

other aspects. ANSI pumps

can be an

eco-

nomical selection

for

clear liquids when head

and

capacity requirements

fall

within

the

ANSI standard

range

of 2.3 to 220 L/s (37 to

3500

gal/min)

and 9.8 to

61m

(32 to 200 ft)

head.

Clear-Liquid,

Close-Coupled

Pumps

Clear-liquid, close-coupled pumps

are

popular, espe-

cially

for

smaller sized pumps (Figure

11-19).

These

pumps

have

the

advantages

of

relatively lower cost,

compactness,

and

ease

of

installation. Close-coupled

pumps

are

hydraulically similar

or

identical

to

sepa-

rately

coupled pumps

and

have

the

same options with

respect

to

seals, wear rings,

and

materials. They

are

available

in

horizontal

or

vertical models. Both close-

coupled

and

separately coupled pumps

are

described

inANSIB73.2.

A

decided disadvantage

of

some close-coupled

designs

is

that

the

motor must

be

removed

to

repack

or

replace

mechanical

seals,

and the

extra cost

for

maintenance, therefore,

is

likely

to

offset

the

slightly

greater cost

of a

separately coupled unit.

Separately

Coupled,

Nonclog

Pumps

Nonclog

(or

solids-handling) pumps were developed

to

pump liquids containing

soft

solids

and

stringy

material without plugging

or

needing

frequent

service

and

cleaning. Separately coupled, nonclog pumps

are

available

for

vertical mounting (see Figure

11-15)

or

for

horizontal mounting (similar

to the

pump

of

Fig-

ure

11-10).

Vertical pumps

may be

driven

by a

motor

installed

on the

pump

frame

(Figure

U-ISa)

or by a

line

shaft

to a

motor installed

on a floor

above

the

pump

(Figure

U-ISb).

Externally,

separately coupled, nonclog pumps

are

similar

to

clear-liquid pumps. Internally, there

are

some

significant

differences (compare Figures

11-15

and

11-19).

Impeller

A

good, nonclog impeller

has

vanes with

a

hydraulic

foil

cross-section and,

to

prevent catching stringy

material, blunt, well-rounded leading edges. Small

pumps

have only

one or two

vanes. Larger pumps

are

built

with

two or

three vanes.

A

reduced number

of

vanes increases

the

size

of flow

passages

and

allows

larger solids

to

pass.

The

solids-handling capability

of the

pump

is

cus-

tomarily

defined

as the

solid-sphere size that will pass

through

the

pump.

It is

accepted

in the

industry that

passing

a

sphere

75 mm (3

in.)

in

diameter

is

required

for

smaller pumps

and

that

a

100-mm

(4-in.) sphere

is

acceptable

for

virtually

all

capacities.

It

should

be

noted that

a

restricting area

may

exist

in the

pump cas-

ing as

well

as in the

impeller,

and the

pump specifica-

tion should clearly

indicate

the

minimum

sphere

diameter

for the

pump,

not

just

for the

impeller.

Wear

Rings

Pumps

can be

equipped with axial, radial,

or

special

design wear rings. Because grit

is

frequently present

in

wastewater, wear ring

life

is an

important consider-

Figure

11-18.

Vertically

mounted,

overhung-impeller

pumps,

(a)

Frame-mounted

motor;

(b)

pump

with

extension

shaft.

After Fairbanks

Morse

Pump

Corp.

ation. Optional design wear rings

are

sometimes spec-

ified

for

larger pumps

to

extend their

life.

These

designs

may

comprise

flushed

rings, L-shaped

rings,

or

rings

made

of

special, abrasion-resistant ring mate-

rials, including special (sometimes ceramic) coatings.

A

typical mechanism

for the

axial

ring

adjustment

is

shown

in

Figure

11-15.

The

thrust bearing housing

is

equipped with jackscrews, which allow

the

bearing

housing

to

move axially

so

that shims

can be

inserted

between

the

frame

and the

housing

flange.

This moves

the

rotor assembly

and

sets

the

clearance

of

wear

rings.

Cleanouts

The

pump casing

and

suction nozzle

are

usually

equipped

with

hand-hole covers

for

inspection

of

the

impeller

and

easy removal

of any

trash caught

in

the

pump. Cleanouts

in the

volute provide access

to

the

volute cutwater,

and

cleanouts

in the

front

head

provide access

to the

impeller

inlet

—

the

two

critical

areas.

The

cleanout covers should

not

obstruct

the

flow

and

should

be

designed

to be flush

with

the

sur-

face

of the

water passage (Figure

11-18).

Seals

The

packings

of

solids-handling pumps should have

a

water

seal

ring

with clean-liquid injection

to

prevent

grit

intrusion

and

shaft

abrasion. Grease injection

is

sometimes used when clean water

is not

available.

Mechanical seals,

of

either single-

or

double-face

design,

are

also

frequently

specified

for

nonclog

pumps. Double-face mechanical

seals

usually have

carbon

and

ceramic faces. They require clean-liquid

injection

into

the

seal cavity

to

prevent contamination

of

the

seal

faces

with abrasive particles. When single-

face

mechanical seals

are

used, particular attention

must

be

given

to

abrasion resistance

of the

seal faces,

and

high-grade

and

high-hardness materials should

be

chosen. Ceramics

or

tungsten carbide

are

frequently

used. Seals must

be

arranged

in the

housing

so

that

adequate

flushing and

cooling

of the

seal

faces

is

ensured.

Close-Coupled,

Nonclog

Pumps

Close-coupled sewage pumps

are

designed

for low

initial cost

and for

compactness (Figure

11-11).

The

motor

shaft

supports

the

impeller just

as in the

close-

coupled, clear-liquid pump shown

in

Figure

11-19.

The

impeller

and

volute, however,

are

designed

as in

Figure

11-15,

so the

hydraulic components

of

close-

coupled pumps

are

identical

or

very similar

to

those

of

separately coupled pumps.

In

general, they have

the

same design

and

performance features

and

offer

the

same options

as

separately coupled pumps.

Customer resistance

is

sometimes encountered

in

proposals

for the use of

close-coupled pumps

in

sew-

age

service because

of the

reputation

of

some

of

these

pumps

for

being lightly built.

Low

initial cost

may not

Figure

11-19.

Close-coupled,

clear-liquid

pump.

After

the

Hydraulic

Institute

[1].

be

true economy

if the

life

of

various pump compo-

nents

is

short

and if

downtime

and

maintenance costs

are

unusually

high. Therefore, check

the

bearing

life

of

the

pumps

for the

actual operating conditions

and

compare

the

life

with

the

anticipated service require-

ments

(e.g., 24-h

duty

or 8-h

duty). Also check

the

shaft

stresses

and

shaft

deflection

both

at the

seals

and

at

the

wear rings

to

confirm

that rubbing

and

rapid

wear

will

not be a

problem. Check

the

design

of the

stuffing

box;

it

should

be

practical

to

remove

the

seal

without

disturbing either

the

motor,

the

volute,

or the

discharge piping.

The

maintenance cost

and the

down-

time

can

thus

be

kept competitive

with

those

of

sepa-

rately

coupled pump designs.

Submersible

Nonclog

Pumps

Close-coupled, submersible nonclog pumps

are

equipped

with

motors designed

to

operate immersed

in

the

wastewater. Because they

are

installed directly

in

the wet

well (see Figure

11-12),

pumping stations

with

submersible pumps require

no dry

well. They

may

have

no

aboveground construction whatever

except

for a

concrete slab

and a

small housing

for the

control center.

The

construction cost

is

therefore less

than

that

of the wet

well-dry well stations

as

shown

in

Figure 29-9.

On

the

other hand, submersible pumps

are not

readily accessible

for

inspection

and

service, give lit-

tle

warning

of

incipient problems,

and

require ship-

ment

to

qualified

service centers

for any

kind

of

motor

or

seal repairs. This tends

to

lead

to

higher service

and

maintenance

cost

and

longer turnaround times. State-

ments

about

low

maintenance costs

can be

misleading

when

based

on

short-term (e.g., 2-yr) experience.

Pumps

with

externally cooled submersible motors

must

be

immersed

for

continuous operation

and can

run

only

for

short periods

of

time

if

exposed

to

air.

Pumps

with

motors internally cooled

by the

pumped

liquid

can be run

continuously

at

full

load without

immersion.

The

same

is

true

for

some (but

not

all) oil-

filled

submersible motors. Pumps that

can

operate

dry

continuously

at

full

load

are

also available

for dry pit

installation. They

are not

damaged

if the dry

well

is

accidentally

flooded, but

this advantage comes

at the

expense

of

higher cost

of the

pump

and

motor,

a

slightly

greater energy requirement (because

of

less

efficiency),

and a

larger structure (because

of the

added

dry

pit). Submersible, nonclog pumps

are

fre-

quently

used

for

sump pumps.

Submersible pumps

can be

installed with

fixed-dis-

charge piping

and can be

supported

by a

tripod

or a

similar device mounted

on the wet

well

floor.

This

type

requires draining

of the wet

well

for any

kind

of

inspection, service,

or

maintenance.

A

more popular

method

for

installing submersible pumps

is the

pull-

up

design (Figure

11-12)

in

which

the

discharge pip-

ing

is

connected

to a

special

elbow that

is

permanently

mounted

on the wet

well

floor. The

elbow

and the

pump

discharge nozzle

are

equipped with

a

self-lock-

ing

coupling. Because

the

pump-mounting bracket

slides

up and

down

on two

rails,

it can be

raised

from

the wet

well

or

lowered onto

the

elbow

by

means

of a

crane

and a

cable with

no

need

for

personnel

to

enter

the

wet

well

or to

drain

it. In

some designs,

the

self-

locking suction elbow

of the

pull-up design

can

allow

considerable leakage, which reduces

efficiency

and

capacity. Hence,

it is

advisable

to

specify

that pump

tests

include

the

suction elbow. Such pump tests, how-

ever,

do not

guarantee

efficient

field

performance

because

the

joints

can be

subject

to

deterioration

under actual operating conditions.

The

pump ends

of

submersible pumps

are

very

similar

to

those

of the dry

well solids-handling pumps

described

previously. Some manufacturers

offer

the

same pump ends

for dry

well

and wet

well applica-

tions.

Wet

Well

Volute

Pumps

The wet

well volute pump

is

mounted

on top of the

wet

well with

the

pump itself immersed

in the

liquid

(Figure

1

1-20).

The wet

well pump concept eliminates

the

need

for a dry

well

and

provides

significant

sav-

ings

in the

pumping station cost

and

surface area

requirements. Immersion

of the

pump

in the wet

well

makes pump priming unnecessary

and

reduces NPSH

requirements.

Impeller

The

pump impeller design

may be

single

or

double

suction (Figure

11-20),

closed

or

semiopen.

In all

other respects,

the

impeller

is

similar

to the

clear-liq-

uid

or

solids-handling pump impellers described pre-

viously.

Volute

Well-designed

wet

well volute pumps

are

equipped

with

double volutes.

As

discussed

in

Chapter

10, a

standard volute with

a

single cutwater exerts radial

thrust

on the

impeller, especially when

the

pump oper-

ates

off the

bep.

The

journal bearings

of wet

well

pumps

are

lubricated

by the

pumped liquid and, there-

fore,

have

a

very limited radial load capability.

A

dou-

Figure

11-20.

A wet

well

volute

pump.

Courtesy

of

Dresser

Pump

Division.

ble

volute balances virtually

all of the

radial loads

and

resolves

the

bearing overload problem.

The

drawback

of

a

double volute

is

that

it

significantly reduces

the

flow

passage area, which except

for

large pumps nor-

mally

prohibits using

it for

pumping wastewater.

Shafting

Wet

well volute pumps

are

driven

by

lineshafting

in

an

enclosing tube, which

is

similar

to the

shafting

of

vertical pumps (refer

to

Section

11-8

for a

more

detailed description).

Bearings

Journal-type,

water-

,

oil-

, or

grease-lubricated bear-

ings

are

used

in a wet

well volute pump. They

are

sim-

ilar

or

identical

to

vertical pump bearings (for

additional information,

see

Section

11-8).

Vertical

Turbine,

Solids-Handling

(VTSH®)

Pump

The

VTSH(D

pump

is a wet

well, solids-handling

pump

that combines

the

advantages

of the

classical

solids-handling pump with

the

well-proven vertical

pump

concept (Figure

11-21).

The

VTSH® pump

is

the

latest arrival

in the

nonclog pump technology.

It is

a

proprietary, patented design.

The

pump

is

installed

on top of the wet

well

and

requires

no dry

well.

The

driver, either

a

motor

or an

angle gear,

is

mounted

on top of the

pump discharge

head. Standard drivers with

any

desired design

or

con-

trol options

are

used.

The

driver

is

readily accessible

for

service

or

maintenance.

The

symmetrical bowl

design eliminates radial thrust forces. Although

the

pump

itself

is

expensive,

the

total cost

of a

pumping

station

may be

reduced

by

using

it

because

the

pump

concept eliminates

the

need

for a dry

well.

Impeller

The

pump impeller

is an

enclosed,

mixed-flow,

solids-

handling

design with

two

vanes

and a

large solids-

sphere diameter.

Bowl

The

pump bowl replaces

the

volute

of a

standard non-

clog pump.

It

contains

a

symmetrical, two-vane, non-

clog-design axial

diffuser

that balances

any

radial

loading

on the

impeller.

Bearings

The

pump

shaft

is

supported

by two

bearings located

in

the

bowl.

The

bearings

are

rubber

or

bronze

and are

lubricated with water, oil,

or

grease. Because

the

radial thrust

of the

impeller

is

balanced

and the

axial

load

is

carried

by the

motor bearings, there

is no

dan-

ger of

bearing overload even with water lubrication.

Column,

Lineshaft,

Head,

and

Lineshaft

Bearings

The

remaining pump components

are the

typical verti-

cal

pump components discussed

in

Section

11-7.

Figure

11-21.

A

water-lubricated,

vertical

turbine,

sol-

ids-handling

(VTSH®)

pump.

After

Fairbanks

Morse

Pump

Corp.

Self-Priming

Pumps

Self-priming

pumps

are

designed

for

automatic start-

ing

with suction

lifts

up to 7.6

m

(25

ft). They

require

no

external priming system. They

are

used

for

waste-

water

pumping

in wet

well stations where they

can

be

installed above

the wet

well.

No dry

well

is

needed. Another popular application

of

self-priming

pumps

is the use of

portable units

for

construction

site dewatering.

Self-Priming

Feature

A

cross-section

of a

typical self-priming pump

is

shown

in

Figure

11-22.

The

pump

has a

large casing,

which completely surrounds

the

volute.

The

lowest

point

of the

volute contains

an

opening

to the

casing.

The

inlet

and

outlet openings

of the

casing

are

arranged above

the

impeller

so

that when

the

pump

stops,

the

casing remains partially

filled

with

the

pumped liquid.

A

vent line

to the wet

well

is

provided

in

the

discharge

line

(not shown).

At

start-up,

the

liquid contained

in the

impeller

inlet

is

displaced

by the

impeller into

the

volute

and

discharged into

the

casing. Some

of the

liquid returns

through

the

volute opening into

the

impeller periphery

and

continues

to

circulate back

to the

casing.

The

recirculation

of the

liquid

creates

reduced pressure

in

the

impeller eye, which draws

the air

into

the

suction

line

of the

pump

and

mixes

it

with

the

recirculating

liquid.

The air

separates

from

the

liquid

in the

casing

and

is

vented

to the wet

well

or the

atmosphere

through

the

vent

line.

The

priming action continues

until

all of the air is

evacuated

from

the

suction line

and

the

liquid rises

to the

pump inlet level,

at

which

time normal pumping

operation

begins.

The

casing contains

an

inlet check valve, which

helps

to

keep

the

suction

and

discharge lines

filled

with

liquid upon shut-down

and

reduces

the

priming

time

for

subsequent start-ups.

Impeller.

The

impeller

of the

pump

is of the

typical

nonclog design.

The

pump characteristics with

respect

to

performance

and

solid spheres

are the

same

as for a

standard nonclog pump

of the

same

size.

The

pump

efficiency

is

somewhat lower,

due to

the

continuous loss

of

liquid through

the

vent line

and due to the

effect

of the

volute opening.

Wear

rings.

The

impeller

is

usually equipped with

axial design wear rings.

The

axial clearance

of the

wear

rings

is

adjusted

with shims

at the

thrust bear-

ing.

Hand

holes.

Two

hand holes with quick-access cov-

ers are

provided

for

inspection

and for

removal

of

any

debris

from

impeller intake

and

from

the

check

valve.

Pump

seals. Pump seals

are

usually

of the

double-

face

mechanical

seal

type. They

are

filled

with fil-

tered liquid

from

the

pump discharge

or

with

oil

from

an oil

reservoir. This arrangement prevents

dry

seal operation

and

intrusion

of air

during

the

prim-

ing

cycle.

Vortex

Pumps

Vortex

pumps (Figure

11-17)

are

designed

to

pump

large

solids

and

gritty sludge

or

slurry.

The

impeller

of

the

vortex pump

is

recessed

in the

stuffing

box

cover,

and

the

impeller vanes

do not

extend into

the

pump

casing. They induce

a

strong vortex

in the

space

between

the

impeller

and the

suction cover.

The

pumped liquid does

not

have

to

pass through

the

impeller itself,

but flows

directly

to the

volute

and

dis-

charge nozzle. Solids with

a

sphere size slightly

smaller than

the

suction

and

discharge nozzle diame-

ter

will pass through

the

pump unimpeded. Vortex

pumps

also have good abrasion resistance

due to the

flow

pattern through

the

pump and, therefore,

are

ideal

for

pumping gritty

and

abrasive sludge.

The

cas-

ing

can be

made

of

abrasion-resistant material such

as

Ni-Hard (ASTM

A

532, Class

I,

Type

A) or

conven-

tional cast iron

(ASTMA

126).

Vortex

pumps

are

frequently

used

for raw

sewage

in

residential areas where

the flows are low and

where

even

the

smallest single-vane, nonclog pump

can no

longer

be

selected

to

pass

the

required sphere size

without

being

too big for the

required

flow and

with-

out

being forced

to

operate

in the

low-flow,

reduced-

efficiency

range near

shut-off.

Unlike other pumps,

vortex pumps

can be

operated

at

shut-off

without

damage.

The

hydraulic

efficiency

of

vortex pumps

is

low

(about 35%), which restricts their

use to

small

capacities.

The H-Q

performance curves

of

vortex

pumps

are

very

flat, and

they

can

develop only

low

heads.

In

other respects,

the

design

of the

vortex pump

resembles

the

standard nonclog pump design. Vortex

pumps

are

available

for wet

well installations

(as

shown

in

Figure

11-17)

with submersible motors,

but

dry

well installations

are

more prevalent.

Cutter

Pumps

A

cutter pump

is a

small overhung-shaft, close-cou-

pled centrifugal pump

in

which

an

impeller

is fitted

with

a

special steel cutter cone whose

two

vanes

are

an

extension

of the

impeller vanes (Figure

1

1-23).

The

cutter vanes

and a

cutter

knife

(both with tungsten car-

bide cutting edges)

are

adjusted

to

clear

the

vanes

by

0.05

mm

(0.002

in.) and, thus, create

a

shearing

device that easily chops rags, disposable diapers, sani-

tary napkins, overalls,

or

even wire coat hangers into

Figure

11-22.

A

self-priming

pump.

After Fairbanks

Morse

Pump

Corp.

Figure

11-23.

Impeller

and

cutter

from

a

cutter

pump.

After

Prosser-Enpo

Industries,

Inc.

25-mm

(1-in.)

pieces. Objects such

as

blankets, how-

ever,

go

about

halfway

through before

the

motor

stalls,

and

pantyhose

can

clog

the

pump because

the

threads

are too fine to be

completely cut. Both sub-

mersible

and dry

well types with explosion-proof

enclosures

are

available.

These

units

are for use in

applications containing

wastewater

heavily laden

with

debris where some

form

of

comminution

is

indispensable. Examples

include

raw

household sewage; wastes

from

hospitals,

prisons,

and

asylums;

and

industrial wastes

from

fab-

ric

mills,

pulp

and

paper mills,

and

poultry-,

fish-, and

vegetable-processing plants. Cutter pumps

do not

develop high heads,

but

they

can be

used

to

precede

a

positive displacement pump (see also Section

19-2)

to

obtain

a

very high head.

Grinder

Pumps

Grinder pumps

are

another solution

to the

clogging

problem

of

small-capacity solids-handling pumps.

The

inlet

of the

grinder pump

is fitted

with

a

station-

ary,

serrated, steel cutting

ring. A

close-fitting,

lobed

rotor

is

mounted

in

front

of the

impeller

and

shreds

any

solid material

in raw

sewage.

The

pump impeller

and

pump discharge

are

designed

to

match

the flow

and

head requirements

and are not

affected

by

solids

size.

The

pump

can be

selected

to

match

the

application

flow

requirement

and to

operate

at or

near

its

bep.

It

is,

therefore,

of

relatively small size

and

higher

effi-

ciency.

It has the

drawback

of

higher complexity

and

short

life.

The

pumps

are

usually submersible

and

motor

driven,

and

their installations resemble those

of

small

vortex

or

small standard nonclog pumps, except that

they

are

seldom used

as

pull-up designs.

11-5.

Impeller-between-Bearings

Pumps

The

impeller-between-bearings pumps have

one

bear-

ing

arranged

on

each side

of the

impeller,

so the

radial

load

of the

impeller

is

shared equally

by

both bear-

ings.

The

loads

on the

bearings

are

less,

and the

shaft

has

lower bending moments than

has an

overhung

shaft.

On the

other hand, impeller-between-bearings

pumps

require

shaft

seals

at two

locations

and two

separate bearing housings.

Impeller-between-bearings pumps

are

available

in

axial-split

and

radial-split designs. Because radial-

split pumps

are not

used

in

pumping stations, they

are

not

considered here.

Axial-Split

Pumps

A

typical axial-split pump (also called "horizontal

split"

or

"horizontal split-case pump")

is

shown

in

Figure

11-13.

The

pump casing

is

split along

the

cen-

terline

of the

shaft.

The

lower half

of the

casing sup-

ports

the

entire pump

and

also contains

the

suction

and

discharge nozzles.

The

upper half

of the

casing

can be

removed

for

inspection,

and the

pump rotor

can

be

removed

for

repairs without disturbing

the

suction

or

discharge piping. Axial-split pumps

may be

single

stage

(as in

Figure

11-13)

or

multistage

for

higher

pressures.

The

pumps

are

usually mounted with

shafts

in

the

horizontal position,

but

vertically mounted

pumps

for

reduced

floor

space

are

also available.

Impeller.

Single-stage pumps

are

equipped with

double-

suction

impellers (Figure

11-13).

Double-

suction impellers

are

inherently axially balanced

and, therefore, exert very

low

axial forces. They

also have relatively large impeller intake

eye

areas

and, therefore,

low

NPSH requirements. Multistage

pumps usually have single-intake impellers that

are

arranged, when possible,

in a

back-to-back

fashion

to

reduce

the

axial thrust.

The

impellers

are

equipped with radial wear rings.

Bearings.

The

pump bearings

are

mounted

in two

separate bearing housings.

One of the two

bearings

is

locked

in the

housing

and

carries

the

radial

as

well

as the

axial loads.

The

other bearing

is for the

radial load only.

The

bearings

may be

lubricated

with

grease

or

oil.

Seals.

The two

stuffing

boxes contain either packing

or

single-face mechanical seals.

For

gritty liquids

or

for

installations

with

low

suction pressure,

the

packings

are

provided with lantern rings

and

buffer

liquid

injection.

11-6.

Classification

of

Vertical

Pumps

Vertical

pumps were originally developed

for

well

pumping.

The

bore size

of the

well limits

the

outside

diameter

of the

pump

and so

controls

the

overall

pump design. Vertical pumps

are

very versatile

and

are

often

used

for

installations

not

well

pumping.

Vertical

pumps

can be

subdivided into three

major

categories:

(1)

lineshaft pumps,

(2)

submersible

pumps,

and (3)

horizontally mounted

axial-flow

pumps (Figure

11

-Ib).

Lineshaft

Pumps

In

lineshaft

pumps (Figure 11-24),

the

driver

is

mounted

on the

discharge head.

The

lineshafting

extends through

the

column

to the

bowl assembly

and

transmits torque

to the

pump rotor. Lineshaft pumps

can be

further

subdivided into

the

following three cat-

egories:

•

Deep

well

lineshaft

pumps, which

are

used

for the

pumping

of

deep water wells

•

Short-setting lineshaft pumps, which

are

used

for

the

pumping

of

shallow wells

or

pump sumps

•

Barrel pumps, which

are

short-setting pumps

equipped with their

own

barrels

or

"cans"

in

place

of

pump sumps.

Submersible

Pumps

Submersible pumps (Figure

11-25)

are

driven

by a

submersible motor.

The

motor

is

mounted below

the

bowl assembly

and is

directly coupled

to the

pump

rotor

shaft.

The

lineshaft

is

eliminated altogether

and

is

replaced

by a

cable, which supplies power

to the

motor. Submersible pumps

are

available

as

•

Deep well submersible pumps, particularly

useful

for

very deep (more than

180

m

or 600 ft)

settings

or for

crooked

wells.

•

Short-setting submersible pumps, used

for

pumping

shallow wells

and

sumps

or

where

the

noise

of a

motor would

be

objectionable.

Figure

11-24.

A

lineshaft-driven vertical turbine

pump.

After

the

Hydraulic Institute

[1].

Horizontally

Mounted,

Axial-Flow

Pumps

Horizontally mounted,

axial-flow

pumps (Figure

1

1-26)

are

high-capacity pumps

and are

typically used

for flood

control

and

similar applications. They

are

often

engi-

neered

for

each particular installation

and may

have

many

different

driver

and

bearing arrangements.

Figure

11-25.

A

vertical

turbine

pump

driven

by a

sub-

mersible

motor.

After

the

Hydraulic

Institute

[1].

11-7.

Construction

of

Vertical

Pumps

A

typical vertical pump consists

of

four

major

compo-

nents:

(1)

the

bowl assembly,

(2) the

column,

(3) the

discharge head,

and (4) the

driver.

The

function

of

these components

and

their design features

are

dis-

cussed

in the

following

subsections.

Bowl

Assembly

The

bowl assembly

of a

vertical pump does

the

actual

pumping.

A

typical multistage bowl assembly

is

shown

in

Figure

11-24.

Each stage

consists

of one

bowl with

its

impeller

and

bearing.

Shaft.

All of the

impellers

of the

bowl assembly

are

mounted

on one

common pump

shaft.

Impellers.

The

impellers

may be of the

radial,

mixed-flow

(shown),

or

axial design.

The

impeller

design determines

the

pump characteristics.

The

relationship between

the

impeller design,

specific

speed,

and

pump characteristics

is

discussed

in

Sec-

tion 10-3

and is

illustrated

in

Figure 10-8.

The

impeller design

may be

enclosed, semiopen,

or

open (Figure

11-14).

The

latter

is

typical

of

axial-

flow

or

propeller

pumps.

The

comments about

impellers near

the

beginning

of

Section

11-3

apply

to

vertical pumps

as

well.

Pump

bowl.

The

pump bowl contains

the

axial dif-

fuser.

Spiral

diffuser

vanes straighten

the

discharge

swirl

into axial

flow and

convert

the

kinetic velocity

energy into pressure. Axial discharge

from

the

bowl

makes staging

of the

bowls possible without

hydraulic losses.

The

bowls

may be flanged and

bolted together

or

machined

for

threading

and

screwed into each other.

Wear

rings

and

wear

plates.

The

bowl wear rings

or

wear plates, which

are

also called

"liners"

(Figures

1

1-14

and

1

1-24),

are

also mounted

in the

bowl (for

the

description

of

both

and

their

function,

see

Sec-

tion

1

1-3).

The

wear rings

may be of

integral

or of

separate design.

Bowl

bearings. Each bowl also contains

a

bowl

bearing. Because impellers with

a

diffuser

are not

subject

to

radial thrust,

the

bearings

are not

required

to

support

any

significant

loads

and act

primarily

as

guide bushings

for the

shaft.

Suction case

or

suction bell.

The

suction case

or the

suction

bell (Figure

11-24)

is

attached

to the first

bowl.

The

suction case

is

designed

for the

mounting

of

a

suction

pipe.

The

suction

pipe

is

required

in

well

pumping when

the

water level

in the

well

is

expected

to

drop below

the

bowl assembly inlet.

The

suction bell

is

commonly used

in

open

pit

installations

and

provides

for an

hydraulically

smooth, unobstructed

flow

into

the first

stage. Both

types

of

pump inlets contain straight axial inlet

vanes, which help

to

guide

the flow of

liquid.

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

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