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

Discharge

case

and

adapter.

The

discharge case

(Figure

11-24)

or

discharge adapter

is

mounted

on

the top

bowl. Both

of

them

contain

the

threaded

or

bolted connection

for

attachment

of the

bowl

assembly

to the

column.

The

discharge case con-

tains

a

center

hub to

which

the

enclosing tube

can

be

connected.

The

discharge adapter

is

fully

open

on

the

inside

and

does

not

accept

an

enclosing tube.

Column

and

Lineshafting

The

column supports

the

bowl assembly, ducts

the

liq-

uid

to the

discharge head,

and

encloses

the

lineshaft-

ing. Column pipe

is

available with threaded

or

flanged

connections (Figure

11-27).

The

threaded column

has

a

smaller outside

diameter,

which

is

important

in

well

installations.

The

flanged

column

is

easier

to

assemble

and

disassemble.

The

lineshafting transmits

the

torque

from

the

driver

to the

pump rotor.

The

lineshafting sections

are

connected

to

each other with threaded couplings

or,

for

larger diameter

shafts,

with sleeve couplings.

Open lineshafting

is

exposed

to the

pumped liquid

and

is

guided

in

lineshaft bearings, which

are

sup-

ported

in

bearing retainers. Lineshaft sleeves protect

the

shafting

from

wear

in the

bearings. With

the

enclosed lineshafting (Figure

ll-27b),

the

enclosing

Figure

11-26.

Horizontally

mounted

axial-flow

pumps,

(a)

Bearing frame (after

the

Hydraulics Institute

[1]);

(b)

sub-

mersible

motor

pump.

tube

isolates

the

shafting

from

the

pumped liquid

and

provides

a

channel

for the

lubricating liquid. Exter-

nally

threaded enclosing-tube connector bearings

serve

as

tube couplings

and

shaft

bearings

at the

same

time.

Enclosing-tube stabilizers support

the

tube

in

the

column

and

protect

it

from

lateral vibration.

Discharge

Head

The

discharge head directs

the

pump

flow

from

the

column

to the

piping system, provides

for

sealing

of

the

shafting

and the

enclosing tube,

and

provides

a

base

from

which

the

pump

is

suspended

and on

which

Figure

11-27.

Column

and

lineshafting.

(a)

Open;

(b)

enclosed.

After Fairbanks

Morse

Pump

Corp.

the

driver

is

mounted (Figure

11-24).

The

discharge

head

may be a

casting

or a

fabricated component.

The

discharge head

usually

contains

a

separate

packing

box

(Figure

ll-28a)

for the

packing, which

seals

the

lineshaft.

Modified

packing boxes

for

mechanical

seals

are

also available (Figure

ll-28b).

The

packing boxes

for

enclosed lineshafting

are

called

tension nuts (Figure

ll-28c).

The top

section

of the

enclosing tube

is

threaded into

the

tension nut, which

is

tightened

to

keep

the

tube under tension

and to

hold

it

straight.

Some pump installations require underground dis-

charge piping. Underground discharge heads have

an

elbow mounted below

the

motor pedestal.

The

dis-

charge head

in

submersible pumps

is

replaced

by a

simple discharge elbow (Figure

11-25).

Driver

The

driver

of

lineshaft pumps

is

mounted

on the

dis-

charge head.

It can be an

electric

motor (Figure

1

1-24)

Figure

11-28.

Head

accessories,

(a)

Packing

box;

(b)

packing

box

with

mechanical

seal;

(c)

tension

nut;

(d)

packing

box

(water

flush).

After

Fairbanks

Morse

Pump

Corp.

or an

angle gear with

an

engine. Because

the

pump

bowl

assembly

or

column contains

no

thrust bearings,

the

driver must

be

equipped with

a

thrust bearing

capable

of

supporting

the

hydraulic axial thrust

of the

pump

and the

weight

of the

pump rotor with

its

line-

shaft.

Solid

shaft

drivers require

an

axially adjustable

coupling

for the top

section

of the

lineshaft (Figure

11-29).

The

adjusting

nut of the

coupling

is

threaded

Figure

11-29.

Barrel

or

"can"

pump.

Courtesy

of

Dresser

Pump

Division.

on

the

lineshaft

and is

used

to set the

pump rotor

in the

proper axial relation

to the

pump bowls.

In

hollow-

shaft

motors,

the top

section

of the

lineshaft extends

through

the

hollow

shaft

to the top of the

motor.

An

adjusting

nut

(Figure

1

1-24)

with

a

coupling, which

is

installed

at the top of the

motor, provides

for the

proper

setting

of the

rotor

and for the

transmittal

of

thrust

and

torsional

loads.

All

submersible motors that drive submersible

pumps

are of a

special

hermetic design with

a

small

outside diameter

to fit the

well.

The

motor

may be

filled

with

air, oil,

or

water.

The first two

types

of

motors

are

usually sealed with

a

double mechanical

seal with

an

oil-filled chamber between them.

The

thrust bearing

of the

motor must support

the

axial thrust

of the

pump

as

well

as the

weight

of the

motor rotor.

Lubrication

of

Vertical

Pumps

In

open lineshaft pumps,

all

lineshafting

and

bowl

assembly bearings

are flooded and

lubricated

by the

pumped liquid. Enclosed lineshaft bearings require

external lubricant supply

and can be

lubricated with

water, oil,

or

grease. With water-lubricated

and

enclosed lineshaft pumps, clean water

is

injected

under pressure into

the

tension

nut

(Figure

ll-28d)

below

the

stuffing

box and

forced through

the

line-

shaft

bearings.

It

then

flows

through

the top

bowl

bearing

and

mixes with

the

pumped liquid.

The

injected

water lubricates

and

cools

the

lineshaft bear-

ings

and

protects

them

from

any

abrasives

in the

pumped liquid.

It

should

be

noticed that

the flushing

water does

not

reach

the

bowl bearings, which are,

therefore, lubricated

by the

pumped liquid.

With oil-lubricated pumps,

a

drop-feed oiler

is

connected

to the tap in the

tension

nut and

feeds

oil at

a

slow rate

and at

atmospheric pressure into

the

enclosing tube (Figure

ll-28c).

To

allow

the oil to

flow

through

all of the

lineshaft bearings,

the

dis-

charge case

of the

bowl assembly

is

provided with

two

pressure-relief passages (Figure

11-24),

which vent

the

cavity below

the

lowest connector bearing

to the

well

and

allow

the oil to

escape into

the

well. Here

again,

the

bowl bearings remain lubricated

by the

pumped liquid.

With

grease-lubricated pumps, external lines feed

all of the

pump bowl bearings. Individual grease-

admission passages

may be

provided

to

each bowl,

or

the

pump

shaft

may be gun

drilled with cross bores

to

each

bearing.

The

grease

is

injected under

pressure

by

a

manual

or

automatic lubricator.

The

grease also pro-

vides protection against

the

intrusion

of

abrasives.

1 1

-8.

Types

of

Vertical Pumps

The

types

of

vertical pumps

are

shown

in

Figure

11-lb.

Deep

Well,

Lineshaft

Pumps

A

typical

lineshaft-driven

deep well pump

is

shown

in

Figure

11-24.

The

bowl assembly

of the

pump

may

contain

as

many

as 60

stages, depending

on the

well

setting

and on the

discharge pressure requirements.

The

bowls

may be

equipped with enclosed

or

semi-open

impellers

and

optional wear

rings or

wear plates. Semi-

open impellers

offer

the

advantage

of

end-clearance

adjustability

for

pumping liquids with

a

higher abrasive

content,

but

they

are

difficult

to

adjust

for

deep settings.

Lineshafting.

The

lineshafting

is

usually open

for

settings

up to 75 m

(250

ft) and is

usually enclosed

for

deeper settings. Enclosed

shafting

is

drip

fed

and

lubricated with oil. Open

shafting

is

lubricated

with

pumpage

but

requires prelubrication with

water

before starting, because

the

lineshaft bearings

must

be

provided with lubricant

to

avoid

dry

run-

ning

until they become

flooded

with

the

pumped

liquid. Grease lubrication

is not

very practical

due

to the

requirement

for

long grease lines.

Column

and

head.

The

column

and the

head must

be

designed

for the

anticipated pressure

and for

high

axial loads.

For

column diameters

up to 300

mm

(12

in.),

columns

are

usually threaded. Larger

columns

are

usually

flanged

for

ease

of

assembly

and

disassembly.

Axial

Thrust

The

thrust

due to the

total dynamic head (including

the

pumped liquid contained

in the

column) combined

with

the

weight

of the

bowl assembly,

the

lineshaft,

and the

column causes significant stretching

of

both

the

lineshaft

and the

column. This must

be

considered

in

component selection.

The

column

and the

lineshaft

stretch

by

different

amounts,

and the final

axial setting

of

the

rotor requires particular attention with deep

well

pumps.

Deep

Well

Submersible

Pumps

Submersible motor-driven pumps (Figure

11-25)

are

another

popular

configuration

of

deep well pumps. They

have

no

lineshafting and, therefore,

no

related problems.

The

overall

efficiency

is

improved somewhat because

there

are no

lineshafting

friction

losses. These advan-

tages become greater

as the

well depth increases.

On the

other hand, they must

be

equipped with

a

special sub-

mersible motor

and a

long power-supply cable.

The

motor

is not

accessible

for

inspection

or

maintenance

without

pulling

the

entire pump

from the

well (see Table

25-8).

The

reliability

and

life

of a

submersible pump

depend greatly

on the

motor design

and

quality.

All

bowl

and

column components

of

submersible

pumps

are

very similar

to

those

of

lineshaft pumps.

Discharge

Elbow

The

discharge elbow (Figure

11-25)

replaces

the

dis-

charge head

of

lineshaft pumps.

It is

rather simple

in

design

but

must

be

capable

of

supporting

the

bowl

assembly with

the

motor

and the

column.

Lubrication

The

pump bearings

are

lubricated with pumpage

because there

is no

practical

way to

supply lubricants

of

any

kind

from

an

outside source.

The

motor bear-

ings

are

sealed

in the

motor and, depending

on the

motor design,

are

lubricated with water

or

oil.

The

hydraulic axial thrust

and the

weight

of the

pump

and

the

motor rotor

are

carried

by the

motor thrust bearing.

Short-Setting

Lineshaft

Pumps

The

basic components

of a

short-setting pump

are the

same

as

those

of a

deep well pump.

The

major

differ-

ence

is in the

length

of the

column

and in the

light-

weight

design

of the

column

and the

head.

The

column

of a

short-setting pump

is

often

limited

to one

section; occasionally,

it is

eliminated altogether with

the

bowl mounted

to the

head directly.

Bowl assembly.

The

bowl assemblies

are

one-

or

two-stage propeller bowls (Figure

11-30)

or

single-

and

multistage

mixed-flow

bowls.

Lineshafting

and

options.

The

lineshafting

can be

open

or

enclosed.

The

lineshafting

and

bowl assem-

bly

can be

lubricated with pumpage, water

flush,

oil,

or

grease.

The

wear ring

or

wear plate, packing

or

mechanical seal,

and

different

bearing material

options

are

available

and are of

greater importance,

because short-setting pumps

are

used

to

pump

a

great variety

of

liquids.

Installation.

Short-setting pumps usually take their

suction

from

a wet

well.

The

performance

of

vertical

pumps

is

rather sensitive

to

inlet

flow

conditions.

The wet

well should, therefore,

be

designed

by a

Figure

11-30.

Propeller

pump

bowl

assembly.

Courtesy

of

Fairbanks

Morse

Pump

Corp.

competent

engineer. Particular attention

must

be

given

to (1)

inlet

flow

velocities,

(2)

freedom

from

excessive turbulence

and

unsymmetrical

flow, and

(3)

sufficient

submergence

to

suppress

vortexing

and

the

intake

of air

(see Equation

12-1).

Short-Setting

Submersible

Pumps

Submersible motors

are

also used with short-setting

pumps,

but

less frequently. They eliminate

the

need

for

a

superstructure

to

protect

the

motor.

Deeper

sumps

are

required, however,

to

accommodate

the

motor length. Their main advantage (the elimination

of

the

lineshafting)

is of

less significance.

In all

other

respects,

the

short-

setting

submersible pumps resem-

ble

deep well submersible pumps.

Barrel

Pumps

Barrel

or

"can"

pumps

are

short-setting pumps

arranged

in a can or

barrel (Figure

11-29).

Used pri-

marily

for

booster service, they

are

particularly attrac-

tive

for

installations where NPSH

can be

increased

as

needed

by

making

the

barrel deeper

and

lowering

the

bowl

assembly.

The

expense

of

constructing

a

sump

is

thereby avoided.

By

specifying unit responsibility

for

pump,

motor,

and

barrel,

the

manufacturer thus

becomes responsible

for

barrel dimensions

and

hydraulic design.

The

barrel inlet

may be

arranged

in

line with

the

pump

discharge nozzle,

as

shown

in

Figure

11-29,

or

the

inlet

can

intersect

the

barrel

at any

elevation.

Underground

or

aboveground discharge heads

are

also

available. Barrel pumps

are

otherwise very similar

to

short-setting

pumps.

Submersible motors

are

rarely

used.

Sometimes rotating

flow

between

the

barrel

and

the

pump causes poor performance.

A

plate between

pump

and

barrel, extending

from

the

bottom

of the

barrel

to the

water surface

or to the

inlet pipe, prevents

rotation.

A

cross between pump bell

and the

bottom

of

the

barrel

is

also

helpful.

A

cone with

fins

might

be

better

than

a

cross,

but a

cone

is

inappropriate

if the

bottom

of the

bowl assembly

is

like that shown

in

Fig-

ure

11-30.

Horizontally

Mounted,

Axial-Flow

Pumps

Horizontally mounted,

axial-flow

pumps

are

typically

low-head

pumps,

and

larger sizes

can

have very high

capacity

—

up

to 30

m

3

/s

(500,000

gal/min)

or

more.

Smaller pumps frequently have

a

bearing

frame

(Fig-

ure

ll-26a)

that contains radial

and

thrust bearings

to

support

the

pump

shaft.

The

propeller

is

cantilever

mounted without additional bearings.

The

frame

bear-

ings

are

lubricated with

oil or

grease.

Larger pumps

are

usually custom engineered,

and

their design varies

from

installation

to

installation.

The

thrust bearing

is

usually arranged outside

of the

pump

and is

lubricated with oil. Radial bearings

can

be

mounted within

the

pump casing

on one or

both

sides

of the

propeller. They

are

usually

of

underwater,

grease-lubricated, journal-bearing design.

High-capacity pumps operating with suction

lift

and

located above

the

high-water level

may

have

a

center

hub

that

is

equipped with water

shaft

seals

and

is

accessed through

a

manhole.

The hub is

never

flooded

with

water.

The

bearings

are

mounted inside

the

hub,

are

lubricated with

oil or

grease,

and are

always

kept dry. They

can be

either

of

journal

or

anti-

friction

design.

Submersible motor-driven, axial-propeller pumps

are

used

in

Europe

for flood-control

applications.

The

motor

of

these pumps

may be filled

with water

and the

bearings lubricated with water.

The

pump

and

motor

are

inherently

flood

resistant. Such pumping stations

are

compact,

low in

cost,

and

require

no

pump house.

Only

the

controls

and the

power supply

are

installed

above

the

surface

(Figure

ll-26b).

Submerged hydraulic motor-driven, axial-propel-

ler,

and

mixed-flow

pumps

are

also available.

In

these

units

the

hydraulic

fluid

power unit

is

electric motor

or

engine driven

and

located away

from

the

pump.

The

only connection

to the

pump

is a

pair

of

hydraulic

hoses.

11-9.

Positive-Displacement

Pumps

When compared with kinetic pumps, positive-dis-

placement pumps

are

inherently low-capacity, high-

discharge-pressure pumps. Although they

are

used

to

pump

a

great variety

of

liquids, they

are

also capable

of

pumping slurries

in

small volumes

or in

consisten-

cies that cannot

be

handled

by

centrifugal

pumps

—

the

basic reason

for

using positive-displacement pumps

in

water

and

wastewater pumping

and

treatment stations.

Only

designs suitable

for

sewage, slurries, sludge,

and

similar liquids

are

discussed

in

this chapter.

As

shown

in

Figure

11

-Ic,

positive-displacement

pumps

are

subdivided into three

categories:

recipro-

cating pumps, rotary pumps,

and

pneumatic (ejector)

pumps.

Of all the

various reciprocating pumps, only

the

plunger pump

is of

interest

for

pumping sludge

and

slurries.

Two

types

of

rotary pumps

are of

inter-

est: lobe pumps

and

progressive cavity pumps.

The

only type

of

pneumatic positive-displacement

pump

of

interest

is the

pneumatic ejector.

It is

used

for

pumping

raw

sewage

in

small volumes and, some-

times, high heads.

Plunger

Pumps

A

representative plunger pump

is

shown

in

Figure

11-31.

It can be

used

for all

types

of

sewage, sludge,

scum, slurries,

and

clarifier

and

thickener

underflow.

It

can be

applied

for

transfer

and for

metering service.

Such pumps

are

available

in

single-

and

multicylinder

models.

The

plunger contains

the

crosshead, driven

by

a

camshaft

arrangement.

The

capacity

of the

pump

can

Figure

11-31.

A

plunger

pump.

Courtesy

of

Komline-Sanderson

Engineering

Corp.

be

adjusted

by

changing

the

stroke,

the

rotating speed

of

the

pump,

or

both.

The

stroke

of the

pump

is

changed

by the

eccentric

setting.

A

plunger pump

is

equipped with single

or

dual

ball

lift

check valves.

The

dual design contains

two

ball check valves

in

series

for

each plunger

on

both

the

suction

and the

discharge

side.

Two

valves rarely

hang

up on

foreign matter

at the

same time,

so if one

valve

is

unseated

the

other continues

to

operate prop-

erly until

the

foreign matter

is

flushed

through without

affecting

the

pump

operation.

"Quick-opening"

covers

provide

for

easy access

to the

ball checks

and

seats

for

servicing

or

replacement.

The

pump bodies

and

plunger housings

are

sepa-

rate components,

and the

plunger

can be

removed

for

replacement without disturbing

the

shaft

assembly,

pump

body,

or

piping.

As

with

all

positive-displace-

ment pumps, plunger pump capacity

is not

altered

by

a

changing discharge head.

The

positive pressure

exerted

by the

plunger clears plugged lines.

The

pumps

are, therefore, well suited

for

metering applica-

tions.

The

pumps

are

driven

by

constant-

or

variable-

speed motors

with

gear reducers.

Rotary

Lobe

Pumps

A

representative rotary lobe pump

is

shown

in

Figure

11-5.

It

contains

two

elastomer-coated rotors that

are

driven

by an

integral

gear

box and

synchronized

by

timing

gears.

The

rotors

run

without touching each

other

or the

casing.

The

liquid

is

drawn through

the

inlet port into

the

pockets between

the

lobes

and

chamber walls. Because liquid cannot escape between

the two

rotors,

it

discharges

in the

direction

of

rotation

of

the

outer lobes through

the

discharge nozzle.

The

rotors

may be

cantilever mounted

(as

shown

in

Figure

11-5)

or

supported

by

bearings

in

each cover.

Cantilever mounting allows

a

hinged cover assembly,

which

provides easy access

to the

pumping chamber

for

inspection

and

replacement

of

rotating compo-

nents.

The

pump discharges

at a

continuous

and

smooth

flowrate

and

is

relatively nonagitating

and

nonshear-

ing.

The

pump

is

self-priming

and can be run dry

without

damage

from

blocked

or

starved suction

inlets.

It is

suited

for

handling

a

wide range

of

sludge

viscosities

and

types.

The

smoothly rounded contours

of

the

rotor

and

their

full

sweep within

the

pumping

chamber provide

a

high tolerance

to

raggy sludges.

No

check valves

are

required (provided

the

gearing

prevents backward rotation),

and the

pumping

is not

susceptible

to rag

buildup. Elastomeric coatings

for

the

lobes have been developed

to

pass hard solids

up

to 120 mm (4

in.)

in

diameter,

and the

coating

has

good wearing

life

in

mildly abrasive duty. Where there

is

a

high content

of

debris,

an

automatic reverse mech-

anism

can be

provided

to

reduce

operator

attention.

At

the

head

of the

treatment plant (where sludges

contain

a low

percentage

of

solids

but a

high grit con-

tent) urethane-coated rotors

are

recommended,

and

the

pumps should

be run at

reduced speeds. Other-

wise, Buna

N-coated

rotors

are

used.

The

pumps

are

used

for

pumping sludge with

as

much

as 6%

solids.

In

summary,

the

initial

cost

is

relatively high,

but

the

advantages

of (1)

quick, easy, inexpensive replace-

ment

of

moving parts,

(2)

compactness

and

space sav-

ing,

(3)

high tolerance

for

rags

and

large

solids,

(4)

long

life

at low

speeds,

and (5)

self-priming make

the

overall

life

cycle cost attractive (see also Section 19-2).

Progressive

Cavity

Pumps

A

progressive cavity pump

is

designed

specifically

to

transfer

abrasive

and

viscous

fluids

with

a

high solid,

fiber,

and

air

content.

A

hard steel screw rotor rotates

and

orbits within

an

elastomer stator,

as

shown

in

Fig-

ure

11-6.

The

pitch

of the

stator

is two

times

the

pitch

of

the

rotor.

As the

rotor turns,

it

contacts

the

stator

along

a

continuous sealing line, creating

a

series

of

sealed cavities that progress

to the

discharge end.

The

cavity

fills

with liquid

as it

gradually opens

and

expands

at the

suction

end of the

rotor.

The

trapped

liquid

is

transported

to the

discharge

end and is

then

gradually

discharged

in an

axial direction. Multistage

pumps

of up to

four

stages

are

available

for

reduced

wear

from

abrasives. Some engineers require

a

mini-

mum

of two

stages

for

pumping wastewater sludge.

The

pump

driveshaft

is

supported

by

bearings con-

tained

in the

bearing

frame.

The

shaft

is

sealed with

a

packing

or a

mechanical seal.

The

bearings

are

usually

lubricated with

grease.

A

Cardan

shaft

with universal

joints

of

various designs turns

the

rotor

and

allows

for

orbiting motion

at the

same time.

The

universal joints

may

be

lubricated either

by the

pumped liquid

or by

grease.

Progressive cavity pumps

are

used

in

wastewater

treatment plants

for

transferring

all

types

of

slurries

and

sludge,

and

they

can

pass solids with

a

sphere

diameter

of up to 50 mm (2

in.).

A

"bridge

breaker"

can be

added

at the

front

end of the

pump

to

reduce

the

size

of

large solids. These pumps

are

self-priming

up to

lifts

of 8.5 m (28

ft),

but

they cannot

be

operated

dry.

The flow is

even

and the

shear

is

low.

The

pump

is

relatively easy

to

service,

but

sufficient

clear

floor

space must

be

provided

for

dismantling

and

access

to

the

rotor

and the

Cardan

shaft

(see Figure

1

1-32).

Progressive cavity pumps

are

relatively

low in

cost,

but

stators

and

rotors

may

have

to be

replaced fre-

quently,

especially

if

grit

is

present

in the fluid. To

reduce wear, pump speed should

be low (no

more than

400

rev/min)

when pumping sludge

or raw

sewage.

Pneumatic

Ejectors

Pneumatic

ejectors

are

used

for

pumping

low flowrates

of

wastewater

at

high heads

(flowrates

to 140

m

3

/h

[600

gal/min]

at

heads

to 90

m

[300

ft]),

especially

if

the flowrates are

highly variable.

The

pump consists

of

a

pressure vessel that

is

allowed

to fill by

gravity until

a

predetermined level

is

reached (Figure

11-8).

Controls

are

then operated

to

admit compressed

air to the

ves-

sel.

The

high pressure moves

the

liquid into

the

force

main. When

the

chamber

has

been emptied,

the

con-

trols close

the air

supply valve

and

vent

the air in the

tank

to the

atmosphere, which allows

the

next cycle

to

begin.

The

compressed

air may be

supplied

from

a

plant

air

system

or

from

compressors installed

on

location.

Air

receivers

of

adequate capacity

for

sev-

eral cycles

are

sometimes installed

to

provide

for

lim-

ited continuing operation

of the

system during power

outages.

Figure

11-32.

Installation

of a

progressive cavity

pump.

Courtesy

of

Netzsch Inc.

Mechanical controls

for the

system have proven

to

be

more durable than electronic controls.

The

check

valves

should

be

specifically designed

for

passing sol-

ids and

stringy material. Because

of the

danger

of

freez-

ing,

the

pneumatic ejector

and the air

supply system

should

not be

exposed

to low

ambient temperatures.

11-10.

Special

Pumps

The

pumps

in

this section

are for the

limited purpose

of

producing either high pressures

at low flowrates or

moderate

to

large

flowrates

at low

lifts.

Regenerative

Turbine

Pumps

Regenerative turbine pumps derive their name

from

the

many buckets machined into

the

periphery

of the

impeller.

They have long been recognized

for

effi-

ciently producing

low flows at

pressures much higher

than

those

of

centrifugal pumps

of

comparable size.

They

are

excellent

for

seal water

and

washwater

pumps.

Regenerative turbine pumps

are

unique

in

opera-

tion.

The

liquid circulates

in and out of the

impeller

buckets many times

on its way

from

the

inlet

to the

outlet

of the

pump. Both centrifugal

and

shearing

action combine

to

impart additional energy

to the

liq-

uid

each time

it

passes through

the

buckets. Pressure

from

the

inlet

in

Figure

11

-3

3

a

increases

linearly

to

the

discharge.

The

impeller runs

at

very close axial

clearances with

the

pump channel

rings to

minimize

the

recirculation

losses,

so

these pumps

can

only

be

used with clean liquids.

The

pump

is

damaged

if

oper-

ated against

a

closed valve,

so a

spring-loaded relief

valve

is

always installed

on the

pump discharge.

A

seal water pumping unit normally consists

of a

liquid holding tank with

a float-operated

makeup valve

connected

to the

central water supply.

To

ensure reli-

ability,

two

pumps, each alternating

as

duty

and

standby

units,

are

connected

to the

tank.

An air gap

between

the

incoming water supply

and the

tank

overflow

prevents

any

possibility

of

backflow.

Complete

"package

units"

are

available. Pumps

are

made with capacities ranging

from

0.13

L/s to

more than

4.4 L/s (2

gal/min

to

more

than

70

gal/min)

at

heads ranging

from

3 m to

more

than

120

m (10 ft to

more than

400

ft).

Multistage

Centrifugal

Pumps

A

multistage centrifugal pump

is a

viable alternative

to a

regenerative turbine

for

supplying seal water

or

Figure

11-33.

A

regenerative

turbine

pump,

(a)

Impel-

ler;

(b)

Section

A-A.

After

Aurora

Pump,

a

Unit

of

Gen-

eral

Signal.

wash

water.

The

shaft

and

impellers

in the

pump

shown

in

Figure

11-34

are

fabricated

from

stainless

steel.

Sizes

are

available

for

pumping

from

0.08

to

over

22 L/s

(1.2

to

over

350

gal/min)

at

corresponding

heads

of 260 to 150 m

(600

to 350

ft).

An

advantage

of

the

multistage configuration

is

that pressures

do not

change very much with

a

change

in flowrate.

Screw

Pumps

Screw pumps

(or

Archimedes' screws)

are

high-vol-

ume, nonclog,

atmospheric

-head

devices that

can

pump

a

variety

of

solids

and

debris

in raw

wastewater

without

screening. There

are two

general types:

(1)

the

open screw, which rotates

in a

trough (Figure

11-7),

and (2) the

enclosed screw,

in

which both

the

screw

and an

enclosing cylinder rotate (Figure

1

1-35).

A

major advantage

of

these pumps

is

variable pump-

ing at

constant speed, because

the

output

(up to

design

capacity)

is

controlled

by the

sump level

and

equals

the

influent

flowrate. The

disadvantages

are the

inducement

of

turbulence,

the

release

of

odors

and

other volatile substances

in

wastewater,

and the

rela-

tively

high initial cost.

But

when comparing costs

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

Подождите немного. Документ загружается.