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

tive

and

emergency unscheduled maintenance, which

may

require working crews

of

several persons

for

sev-

eral days.

To

avoid problems later, discuss

the

station

layout

with

the

owner's maintenance personnel before

beginning

the

design.

Convenience

Items providing

for

convenient operation

and

mainte-

nance include

the

following:

1.

The

building:

•

Materials

on the

walls,

floors, and

ceilings easy

to

maintain with

no

rough finishes

•

Coves

at the

base

of all

walls

•

Good

floor

drainage (slope

the floor to the

drains

so

that

water cannot

form

puddles)

•

Easy access

to all

windows, louvers,

and

equipment

to

be

serviced

•

Sufficient

space

to

service

and

maintain equipment

•

Loading platform

and

ramps

•

Restrooms,

a

slop sink,

and a

janitor's

closet

for all

but

the

smallest stations

•

Room

for

storing miscellaneous small tools

and

maintenance items such

as

impellers,

wearing

rings,

packing,

and

other materials

as

specified

by the

owner

or

manufacturer

•

Workbench, tools,

and log

books (for

all but the

smallest

stations).

2.

Access:

•

Convenient roads

•

Convenient hatches

for

equipment removal

•

Pipe basements instead

of

crawl spaces

3.

Equipment:

•

Cranes

or

eyes

for

handling

the

debris

from

screen-

ing

equipment

•

Drainage

to

sump pumps

for

dewatering

and

wash-

down

•

Eyes, monorails,

or

cranes

for

equipment handling.

4.

Miscellaneous:

•

Hose valves (bibbs) with

a

backflow

preventer

for

cleaning

and for

yard irrigation

(freezeproof

them

if

necessary); check state regulations

for the

separa-

tion

of

potable water

from

wash water

• A

desk, chairs,

and file

cabinets

for

storing records,

maintenance

manuals,

and

plans

• An O&M

manual

for

reference

and

troubleshooting

•

Special tools

•

Site maintenance equipment

and

storage rooms

• A

drinking water

fountain

or

bottled water

and

drinking cups.

Safety

considerations include

the

following:

•

Nonslip walkways

of

checker

or

diamond plate

or

covered with nonskid

"sanded"

paints

•

Handrails

and

kick plates around openings

and

stairways

•

Safety

cages

on

ladders

and

intermediate platforms

for

ladders longer than

3.6 m (12

ft); straight stair-

ways

are far

more convenient

and

safe

than

are

lad-

ders

•

Air-purging systems

•

Chlorine detectors

and gas

masks

• A

self-contained air-breathing tank

and

mask appa-

ratus

•

Explosive

air

detectors

•

Hydrogen

sulfide

detectors

• Eye

washers

and

showers

if

hazardous chemicals

are

used

•

Personnel equipment such

as

hard hats,

safety

har-

nesses,

first-aid

equipment, ropes,

and

life

vests

or

rings

•

Fire

extinguishers

•

Telephone

or

two-way radio

communication

—

unnecessary

if

service trucks

are

always

so

equipped

•

Grounding

of all

electrical

equipment including

GFCI receptacles

for

hand tools

•

Backflow

(cross-connection) preventers

or an air

gap

between

the

potable water supply

and the

pumping

station water supply systems

• No

open-

sty

Ie

personnel

lifts;

all

lifts

and

elevators

to

have solid walls, ventilation, roof escape hatch,

telephone,

and no

gates.

Security

Consider

the

following

as

security items:

•

Fencing

and

exterior lighting

•

Unauthorized entry alarms with signal transmission

to

some authority (e.g.,

the

police).

25-4.

Future

Expansion

Designers must have

the

vision

and

presence

of

mind

to

plan

for

expansion beyond

the

design

life.

If a

future

demand

for

increased

flow is

likely (which

is

nearly

always true), consider provisions

for

future

expansion

and

weigh

the

cost

of

such provisions

against

the

future

cost

of

remodeling

or

abandonment.

The

size

and

importance

of the

pumping station,

the

owner's requirements,

and the

uncertainties

of

need,

time

lapse,

and

expected

flow

increase must

be

taken

as

intangible

but

essential

and

sometimes overriding

considerations.

The

method

for

achieving

the

future

increase

in

capacity

should

be

included

in the

specifications

and

the

plans. Future expansion

can be

facilitated

in a

number

of

ways

that include:

•

Pumps with medium-sized impellers that

can be

changed

for

larger ones

•

Generous space

for

each pump

so

that

a

larger

pump

can be

installed;

the

extra space will

be

appreciated

by the

maintenance

staff

•

Suction

and

discharge piping large enough

to

per-

mit

the

increased

flow

•

Space

for

additional pumps; terminate suction pip-

ing

and

pipe connections

to the

header with valves

and

blind

flanges

•

Provision

for

enlarging

the

building

by

adding steel

waterstops

and

dowels

for

connecting

future

walls

and

floors;

bend

the

dowels

to the

face

of the

con-

crete

and

protect exposed steel with lean mortar

(note

that rubber

or

plastic waterstops

can be too

easily

damaged

by

future

construction)

• An

electrical system designed

so

that

the

entire sys-

tem

will

not

have

to be

rebuilt;

use

larger conduit,

for

example,

and

leave extra space where

needed.

Plan

the

operations necessary

to

allow

the

changes

to

be

made while

the

station remains

in

operation.

Consider

the

following:

• The

pump manufacturer should know whether

larger impellers

are to be

used

so

that bearing

frames,

bearings, motor,

and

other

associated

parts

are

sized

properly.

• The

removal

of

blind

flanges on the

discharge side

of

a

future

pump requires either

(1) a

means

for

quickly

draining pressurized pipelines

or (2) a

valve

at

the flange.

Note that valves

are

likely

to

become

inoperative unless regularly exercised. Gate valves

are the

worst, lubricated plug valves

are

good (and

the

most expensive),

and

stop plates

or

slide gates

are

the

best,

the

most reliable,

and the

least expen-

sive

when they

can be

used

for

isolating pump inlets.

• The

removal

of

blind

flanges on

suction

pipes

requires

dewatering

the wet

well

(or a

portion

thereof)

in a way

that minimizes

(or

eliminates)

downtime

or

prevents

overflows

in

wastewater

pumping stations.

The wet

well

may be

compart-

mentalized

by

inserting stop plates into grooves

provided

for

that purpose.

•

Considerable storage

is

available

in

sewers

upstream

from

most wastewater pumping sta-

tions

—

storage

that

is

sufficient

to

make

the

needed

connections

if

planned properly

and

executed

at the

lowest flow.

•

Because blind

flanges are

heavy, provide

lifting

handles

to

speed removal.

•

Ensure convenience

for the

lifting

equipment needed

to

install

new or

replacement facilities,

and

provide

access

for the

installation

of new

equipment.

The

capacity

of an

existing pumping station

can

often

be

increased.

To

avoid blunders,

first

make

field

measurements

of flow,

static head,

friction

head, true

pipe diameter,

and

roughness

coefficient.

Trust

no

plans

—

not

even record drawings. Measure

the

station,

especially

the

piping

connections,

so

that

the new

facilities

will

fit

exactly. Various methods

of

increas-

ing

the

capacity

are

•

Cleaning

and

relining transmission pipelines

in

place

•

Laying

new

transmission lines

or

force mains

•

Substituting larger impellers

in

existing pumps

•

Substituting larger pumps

and

motors

if the

suction

and

discharge piping permits

•

Adding submersible pumps

in the

existing

wet

well

•

Converting

the dry

well

to a wet

well

and

adding

submersible

or

VTSH pumps

•

Adding

a new wet

well with submersible pumps

•

Constructing

a new

pumping station

•

Adding

a

booster pumping station.

25-5.

Hydraulic

Constraints

The

hydraulic constraints include

the flow

capacity

(and

the

range

of

capacities),

the

discharge character-

istics

required

of the

pumps (the

range

of

both

the

flow

and

the

head),

the

suction head,

and the

type

of

fluid

pumped.

Wastewater

Pumping

Pumping stations should

be

designed

to

handle

flows

from

the

entire service area. Obtain owner input

regarding

the

size

of the

service area. Install

a

pump-

ing

capacity that

is

adequate

for the

fully

developed

service

area,

or

install

a

pumping

and

force main

capacity that

can

handle both

the

current needs

and

those

of the

immediate

future

with

an

adequate provi-

sion

for

economically adding

to the

capacity

in the

future.

Consider both average

and

peak

flows,

which

may

be

based

on (1)

existing

and

future

land use,

(2)

per

capita

flow

estimates,

or (3)

actual

field flowrate

measurements. Consult

the

literature before making

flow

projections

[1-5].

•

Choose

a

realistic

design period,

but

make

it

possi-

ble

to

meet

flow

demands beyond

the

design period.

•

Design

the

station layout

to

allow

for

planned

increases

in the

capacity

at

specific times within

the

design period.

•

Always

use the

larger pump size when choosing

between

two

sizes.

•

Optimize

the

size

of

force mains.

The

velocity

should

be at

least

0.76

m/s

(2.5

ft/s)

to

keep grit

moving.

At

such

a low

velocity,

a

daily

flush at

1

.2

m/s

(4.0

ft/s)

for an

hour

or

more

is

desirable.

The

maximum

velocity should

not

exceed about

2.4 m/s

(8

ft/s) because

of

high head losses

and the

possibil-

ity

of

water hammer.

Water

Pumping

For the

probable population

to be

served

and the

esti-

mated

flow

needs, choose

a

realistic design period

and

allow

for

future

expansion beyond

the

design period.

• The

station layout should permit plans

for

periodic

increases

in flow

capacity.

•

Carefully

consider

the

maximum daily

and

hourly

flowrates,

the

requirements

for fire flow, and a

com-

bination

of the

two.

• If

head

and flow are

expected

to

change

signifi-

cantly

over

the

years, choose pumps with medium-

sized

impellers

so

that impellers

only

—

not

pumps

—

need

to be

changed

to

match

the flow.

•

Choose

the

correct

pipe

roughness

(or

range

of

roughnesses)

and not a

single "conservative" value

(see Section

3-2).

•

Optimize

the

size

of

transmission mains. Where

velocities exceed about

2.4 m/s (8

ft/s),

water ham-

mer

may

become

a

concern

and may

require addi-

tional, expensive control equipment

and

structures.

Sludge

Pumping

Sludge

is

usually

confined

to

treatment plants

and

transported

for

only short

distances

—

a

few

hundred

meters

or

yards

at

most.

Centrifugal

pumps

can be

used

for

thin sludges

in

large quantities.

But

positive-

displacement pumps

are

usually required

for

thick slud-

ges, because these sludges

are

thixotropic

and,

after

immobilization, high pressures

are

required

to

start

their movement.

For

wastewater sludges, consider:

•

Sludge properties, which

are

entirely

different

from

those

of

water

(see

Chapter

19)

•

Sludge pipelines longer than

1.6 km (1 mi)

must

be

designed

on the

basis

of

measured friction.

(The

associated problems

are

difficult

and

beyond

the

scope

of

this book.

See

Section 19-5.)

•

Comparisons

of

plunger, progressive cavity,

and

lobe pumps based

on

both

first

cost

and

mainte-

nance

•

Pipelines

of no

less than

150 mm (6

in.)

in

diame-

ter,

although glass-lined piping down

to 100 mm (4

in.)

may be

suitable

•

Glass lining

for

sizes below

250 mm (10

in.)

and

cement-mortar lining

for any

size

(but

particularly

for

larger pipe).

Water

plant sludges

are of two

general types:

(1)

those resulting

from

sedimentation

and flocculation

and

(2)

those resulting

from

lime softening.

The

former

are

gelatinous, bulky,

and

difficult

to

dewater.

It may be

desirable (although

difficult)

to

decant

them,

but

they

are not

difficult

to

pump.

The

charac-

teristics

of

softening sludges vary greatly,

and it is

difficult

to

predict

a

conservative design that oper-

ates well over

the

range

of

necessary conditions.

It is

wise

to

•

Choose pumps that

can be

easily

and

relatively

quickly

disassembled

and

cleaned

and

locate them

for

ready accessibility.

• Use

straight

pipelines

that

can be

rodded

out,

or,

better,

use

open troughs that

are

easily accessible

for

cleaning.

• For a

more extensive discussion

of

sludge pumping,

refer

to

Chapter

19.

25-6.

Types

of

Pumping

Stations

The

following discussion about

the

types

of

pumping

stations

is

necessarily abridged. Greater detail

is

given

in

Chapters

11-15,

17-19,

26, and 29 of

this volume,

so

this section

is

limited

to a few

selected

essentials

relating

to

choice. Develop alternatives only

to the

point where realistic, comparative cost estimates

can

be

made (e.g.,

as

shown

in

Example

29-1).

Wastewater

Pumping

Stations

Decisions should

be

based

on the

required capacity,

cost, reliability,

the

owner's preference,

and

aesthetic

considerations. Reliability

is

more important than

effi-

ciency. Sewage pumps must pass stringy materials

and

other

solids,

and it is

frequently specified that they

must

be

able

to

pass

a

75-mm (3-in.) sphere.

But

because many relatively large pumps contain restric-

tions, always require certification

on the

size

of

solids

to be

passed

entirely

through

the

pump.

Before

choosing

the

type

of

pumps

and

pumping

stations, consider

the

following:

•

Minimum

100-mm

(4-in.) piping

for

sewage

may

dictate

the

pump size required

to

maintain mini-

mum

velocities. Many authorities

and

some codes

allow

75-mm

(3-in.) piping,

but if

this small

pipe

is

used, make certain

the

pumps

(or

screens) cannot

pass solids 75-mm (3-in.)

in

diameter.

•

Provide enough headroom,

floor

space,

and

access

to

replace

the

pumps, motors, valves,

and

piping.

•

Single volute pumps

and

single vane impellers

are

asymmetrical

and

cannot

be

hydraulically

balanced.

Many

manufacturers cannot trim single-vane impel-

lers.

If a

single-vane impeller

is

trimmed,

it

should

be

dynamically balanced

(often

by

using attached

counterweights)

after

trimming.

• A V/S

drive allows substitution

of a

small pump sump

for

a

large

wet

well,

but V/S

pumping

may

increase cap-

ital

cost, does increase complexity

and

maintenance,

and

may

decrease

efficiency.

Many

V/S

installations

seem

to

have

been misapplied,

so

read Sections

12-5

and

15-1

and

Example

29-1

before

deciding

to use

V/S.

• For flat

system head curves, multiple

C/S

pumps

alternated

"first

on-first

off'

may

have most

of the

advantages

of

both

V/S and C/S

pumping.

•

Provide

one

spare standby unit

of the

largest capac-

ity,

and

consider

the

need

for

other spares depend-

ing on the

critical

effects

of

station outage.

•

Supply standby power.

(See

Section

8-4 for

engine-

generator requirements

and

Example

9-10

for

engine-generator sizing. Engine requirements

are

discussed

in

Chapter

14.

Note that

a

separate elec-

trical

utility

service

can

also

be

used

for

standby.)

Submersible

Pumps

Large

(75 to 750 kW or 100 to

1000

hp)

submersible

pumps

are

gaining market acceptance, although

a

num-

ber of

utilities seem

to

agree that

(due

to

unfortunate

experiences

and

possibly inadequate facilities) they

should

be

installed only

in dry

pits.

In one

large utility,

no

submersible pumps larger than

75 kW

(100

hp) are

allowed

in wet

wells.

In

another large utility,

it

required

a

dozen

people working nearly

a

whole morning

to

remove

a

large submersible pump

from

a

storm water

pumping

station because

it was

thought necessary

to

dis-

connect

the

power cable

from

the

motor before pulling

the

pump. Hence,

two

workers were required

to

enter

a

wet

well

—

a

task that

is not

recommended

by any

pump

manufacturer.

As the wet

well

is a

confined

space,

the

large crew

was

necessary

for

safety.

Contrast

the

above scenario with

the

experience

of

still

another large utility that likes large submersible

pumps.

The

pump maintenance supervisor

at

that utility

stated that

190 kW

(250

hp)

submersible pumps

can be

lifted

and

removed

in

half

an

hour

by a

crew

of

three

(boom operator, electrician,

and

mechanic) using

a

truck-

mounted

boom. Larger pumps require

a

mobile crane,

but

the job is

otherwise just

as

easy. (Workers never enter

the wet

well except

to

work

on

guide rails

or

brackets.)

The

pumps

are

cradled

on

their sides

for

access

to

impel-

lers.

The

same utility

has

mechanics

who are

certified

for

repairing submersible pumps

and who can

even balance

impellers

to

tolerances more

precise

than

factory

specifi-

cations. Minor servicing (such

as

changing

the oil and

checking

the

shaft

and

bearings

for

play)

can be

per-

formed

while

a

pump

is in the

cradle. When

a

pump

is to

be

sent

to the

shop

for

major

repairs, such

as

replacement

of

seals

and

bearings,

the

power cable

is

disconnected

at

the

motor starter

and not at the

motor.

The

cable

is

pulled

out

of the

conduit

and

accompanies

the

pump. This

pro-

cedure allows

the

connection

to the

motor

to be

tested

for

shorts.

After

repairs

and

before approval

for

returning

the

pump

to the wet

well,

the

motor

is run for a final

check.

Thus, large submersible pumps

in wet

wells

can

provide

good service when

the

facilities

are

specifically tailored

for

their

use and

maintenance crews

are

well trained

and

motivated. Handling these large pumps

is,

however,

entirely

different

from

handling small ones because

of

their weight

and the

weight, size,

and

stiflhess

of

their

electrical power cables. Both cable

and

pump must

be

maneuvered

with

the

assistance

of

cranes, whereas

the

cables

for

smaller pumps

can be

handled manually.

Special care must

be

taken

in

both

the

design

and

handling

of

large submersible pumps

to

prevent cables

from

being

cut or

rubbed

on

projections.

At

least

one

edge

of the

hatch should

be

round

and

smooth

so the

pump

can be

swung inboard without damage

to the

cable.

Cables

should

not be

allowed

to

swing

freely

in

currents, because

the

movement

may

eventually crack

the

insulation

or

wires. Pulling

the

slack

out of the

cable

is

usually

sufficient,

but

discuss

the

problem

with

the

pump manufacturer,

One

technique

for

removing large submersible

pumps

is to

lift

only

the

pump

and

allow

the

cable

to

hang

freely

in a

"!!"-shaped

loop like

an

elevator

cable.

After

the

pump

is

swung inboard

and

placed

on

the

floor (or in a

cradle),

the

cable

can be

retrieved

with

the aid of the

crane

or

boom.

If the

cable

is to be

disconnected

at the

motor

cap

(cable entry),

the

dis-

connected

end can be

held

at floor

level with,

for

example,

a

Kellems® Support Grip. This method elim-

inates

the

need

for

separate handling

of the

cable

and

for

special arrangements

for

storing

the

cable while

the

pump

is

being

repaired.

On the

other

hand,

disconnect-

ing

the

cable

at the

motor starter does

not

disturb

an

attachment that must remain absolutely

waterproof.

When

a

pump

is

pulled

up in the

above manner,

the

cable bends

in an arc

above

the

motor cap. This bend-

ing

does

not

harm short cables

for

small pumps,

but it

may

overstress long, heavy cables

for

large pumps.

The

cable

can be

protected

(1) by

means

of

semiflexible

stainless-steel cable sheathing designed

to

control

the

radius

of the

curvature,

(2) by the use of

cable saddles,

or (3) by

emerging vertically downward

from

the

motor

cap.

The

need

for

special protection should

be

dis-

cussed with pump manufacturer.

An

alternative method

is

to

lift

both pump

and

cable together, thereby elimi-

nating

bending near

the

motor cap.

A

motor-driven

cable

reel

mounted close

to the wet

well

is of

great help

for

handling

and

storing cable.

The

cable

is

unwound

from

the

reel when

the

pump

is

installed,

and a

prede-

termined tension keeps slack

out of the

cable.

When

the

pump

is

removed, both

it and the

cable should

be

washed

with

a

high-pressure hose

for

sanitation.

The

tension

in

long, heavy

electric

power

cables

for

large pumps

may

require relief.

One

relieving method

is

to

clamp

the

cable

at

intervals

to a

stainless-steel

wire rope that

carries

the

weight

of the

cable. Another

method

is to use a

Kellems® Support Grip

to

attach

the

midpoint

of the

cable

to a

stainless-steel wire rope

held

by a

hook device

at the

hatch

of the wet

well.

The

unprotected

end of the

power cable

is not

sub-

mersible

and

should

not

even

be

exposed

to

water.

Submersible pump control

and

power cables should

never

be

spliced.

The

pump should

be

ordered with

sufficient

cable

to

avoid

the

need

to

splice.

Water that

does enter

the

ends

of a

control cable

can

cause

false

signals

in the

control panel

and

possibly enter

the

motor itself. Connecting

a flexible

cable

to

another

in

a

junction box, however,

is

acceptable

if the

junction

box

is

properly located. Water

has

entered power

cables through junction boxes located improperly (for

example, recessed into

floors or

mounted

in

regions

of

condensing humidity).

Electric power cables

for

submersible pumps

in

trench-type

wet

wells

are

particularly vulnerable

to

movement, because

the

pumps (being

on the

axis

of

the

trench)

are

located directly

in the

path

of the

strong currents

from

the

inlet.

The

cables will also

collect

stringy material. Exposure

to

currents

and

rags

can

be a

serious problem that deserves

careful

evalua-

tion

by

both

the

designer

and the

pump manufacturer.

If

the

distance

from

the top of the

motor

to HWL is

short,

a

rigid conduit

rigidly

fastened

to the

motor

and

extending

to HWL

would shield

the

cable

from

all

currents.

The

cable emerging

from

the

conduit must,

however,

be

protected

from

excessive bending.

The

above method does

not

apply

to

deeply sub-

merged

pumps.

A

large

(150-mm

or

6-in.)

plastic conduit

buried

in the

concrete

wall

or a

stainless-steel conduit

attached

to the

wall

and

ending near

the

discharge cou-

pling

is a

possible

solution

if the

cable

can

slide easily

up

and

down

the

conduit. Again, excessive cable

flexure at

the

ends

of the

conduit must

be

prevented. Note that

the

cable would have

to be

very long

and be

payed

out from

a

cable reel

as the

pump

is

lifted.

Note also

that

this

method

is not

cheap. Another method

is to

attach

the

power cable

at

intervals

to

sliders

or

rollers held

in a

track

(like fastening sails

to a

mast) that

is

bolted

to the

wet

well wall

out of the way of

strong currents. These

methods have

not

been tried, however,

and

there

is no

guarantee

of

success,

so a

full-scale

test should

be

made.

Designers

who

have

any

doubts should describe

the

cable environment

to

manufacturers

of

cables

and

seek their advice. There

may be no

problem

at

all,

or

perhaps simple measures will

suffice.

Otherwise

a

summary

of

options

is (1) to

employ

one of the

meth-

ods

above;

(2) to

invent some adequate alternative

way

to

protect

the

cable;

(3) to do

nothing

special

and

replace

cables

from

time

to

time;

(4) to

locate

the

pumps

in a dry

pit

—

expensive

in first

cost

but

less

costly

to

maintain,

so the

life

cycle

cost

may be

rea-

sonable;

or (5) to

substitute

a

different

type

of

self-

cleaning

wet

well wherein cables

are not

subjected

to

strong currents.

Refer

to

Tables 25-3

and

25-4

and

Sections 24-10

and

27-6

for

further

discussion.

Comparison

of

Different Types

of

Wastewater

Pumping

Stations

Different

types

of

pumping stations (Table 25-3)

are

interrelated with

different

types

of

pumps (Tables 25-4

and

25-5),

so all

three tables must

be

considered

together.

Be

alert, because

a

comment

in

Table 25-3

is

not

repeated

in

Table 25-4.

Water

Pumping

Stations

All

pumping stations (except wells) require redundant

pumping

equipment

so

that while

the

largest pump

is

removed

for

maintenance,

the

remaining pumps meet

all the

demands

on the

station.

Raw

Water

Pumping

Some

influencing

considerations that

are not

under

the

designer's control include variation

in the

water level,

potential

flooding,

trash

and

debris,

fish

protection,

aggressive water, silt

and

turbidity, microorganisms

and

slimes,

and

ice

—

frazil,

surface,

and

anchor. Head

and

discharge requirements

influence

the

type

of

pump

and, perhaps,

the

type

of

station.

Table 25-3.

Comparison

of

Wastewater

Pumping

Station Types

Advantages

Disadvantages

Dry

well-wet

well

Easy access

for

maintenance.

Wider range

of

head

and

capacity.

Wider choice

of

driver arrangements.

Possible

to use flood-protected

motors.

Flooded suction improves reliability.

With long

shaft

and flood-protected

motors

at

grade, electrical

leads

are

short.

Greater

cost

due to

excavation

and

building below

grade

—

expensive

if

groundwater

is

high,

if

soils

are

very poor,

or if

blasting

is

required.

Greater risk

of

outage

due to flooding; dry

well must

be

kept dry.

Flood-protected motors

(in the dry

well)

are

expensive.

Long leads

to the

motor

(in the dry

well)

from

the

control panel

if

the

motors

are

frame-mounted

to the

pumps.

Wet

well

pumps

with

above-grade

drivers

Less excavation (eliminates

the dry

well).

Small superstructure.

Above-grade drivers protected

from

flooding.

Difficult

to

remove

for

servicing, especially

the

pumps with

separate discharge

pipe.

Difficult

to

service

in

place,

wet

well must

be

drained

or

pump

removed

for

major disassembly

to

reach many parts.

High superstructure needed

to

permit

the

removal

of the

pump.

Submerged bearings

are

subject

to

overloads

and

frequent

failures.

Difficult

to

keep lubricated

and

protected

from

grit.

Should

not be

used

in raw

sewage (clogging) applications.

Wet

well with self-priming

pumps

above

grade

Least construction cost, easiest maintenance.

Convenient access

to

pumps above ground.

Eliminates

dry

well.

No

flood

hazard

to

motors.

Self

-priming

pumps

do not

have trimmable

impellers,

so

they

are

usually

belt driven

to

reach

the

design point.

Belt

drives

are

easily changed

to

meet

a new

condition point,

but

V-belts

require proper

adjustment

and

periodic replacement.

Reduced reliability

due to the

need

for

priming.

Distance

from

the low wet

well level

to the

pumps

is

limited

by

NPSH

and the

priming system

to

about

8 m (25

ft).

If

operated infrequently, putrefaction with

gas

production

can

occur

to

break

prime

and/or

to

make sewage

boil

at

6O

0

C

(14O

0

F),

so an

external priming system must then

be

used.

Wet

well

submersible

pumps

and

motors

No

dry

well; excavation

and

concrete reduced.

No

superstructure required except

for

engine-generator

or

cabinet

for

motor controls.

No

seal water system,

no

long

shafts

with steady bearings

required.

Reduces

the

land area needed.

All

of the

above reduces construction costs (see Figure 29-9).

Quick removal

and

replacement

in

emergencies.

Well

adapted

for

increasing

the

capacity

of a

pumping station

using

existing

wet and dry

wells.

No

daily

nor

weekly maintenance (but overhaul needed every

1 to 5

yr).

Units

removable

for

shop servicing, minimizes

field

work.

Quiet operation.

Safety

from

flooding.

On

a

life-cycle basis, balance

the

lower

first

cost

of

submersible

pumping

station with

its

lack

of

regular,

frequent

maintenance

chores against

the

cost

of

complete overhauls

by

specially trained mechanics

(or at

manufacturer's service

center).

Elimination

of (1) the dry

well,

(2)

frequent

preventive

maintenance,

and (3) the

seal water systems that, coupled

with

simpler design

and low first

cost, lead some engineers

and

many operators

to

prefer submersible pumps.

Valves

and

headers must

be

accessible

(1)

in an

adjacent

vault,

(2)

in a

small above-grade superstructure,

or (3) by

exposing

the

header above grade.

Pump must

be

removed

and

disassembled

for

inspection

and

maintenance. Requires

a

hoist

or

crane

and

specially trained

mechanics

Hazard

of

pumps jamming

on

guide rails

or not

seating properly.

Often

more

difficult

to

remove pumps than manufacturers admit.

Pumps larger than

75 kW

(100

hp)

especially

difficult

to

handle

(see text above).

Special motors, seals,

and

moisture monitoring required (but

moisture probes

are

useless

for

leaks

via

power cable).

Makes

differ

greatly with respect

to

satisfactory performance,

and

quality

of

pump/driver unit

has a

very

high impact

on

maintenance

and

life-cycle cost.

Vibration

has

occurred with some makes

of

pumps larger than

22

kW (30

hp).

Guarantees

often

are

valid only

if

repairs

are

made

by

authorized

service

centers.

At

least

one

uninstalled

spare unit

(in

addition

to

standby units)

in

each size

is

needed

to

permit shop servicing, which adds

to

the

cost trade-off

for

submersibles.

Pumps

and

motors

are

generally

not

suited mechanically

for V/S

operation (although AFDs

can be

used).

Inaccessibility

of the

pump

and

motor

for

routine preventive

maintenance

and the

need

for

occasional

expensive overhauls

lead some engineers

to

refuse

to

countenance their use.

Table

25-3. Continued

Advantages

Disadvantages

Wet

well

submersible

pumps

and

motors

(continued)

Large units tend

to

break down more

often

than

do

small ones

and

tend

to

require high maintenance costs.

Choice

of

pumps

for wet

well service should depend almost

entirely

in the

utility

itself

—

attitude,

experience,

and

commitment

to

train

and to

support maintenance workers

(see

Sections

1-13,

24-11,

and

27-6).

Horizontal

vs.

vertical

configuration

Horizontal

pumps

Some double-suction pumps available

in

large

sizes.

Easy

access

for

maintenance.

Motors less costly.

More convenient

for

belt drive.

High

headroom

not

required.

Permits

a

high head design with

a

double-ended

shaft

motor

and

two

pumps coupled with

series

piping.

Requires

a dry

well.

Requires

a

large

floor

area.

Long leads

from

the

control panel

to the

motor.

Direct-connected

vertical pumps

Less

floor

space

and a

smaller superstructure

(see also Table

25-4).

Same

as for

horizontal pumps (above) plus.

Double suction

not

available.

High

headroom required

for

lifting

motor.

Supporting

the

motor

by the

pump casing

is

poor practice

in

seismic areas.

Extended

shaft

vertical pump

The

same

as

direct-connected vertical pumps plus

Avoids

flood

hazard

to

motor.

Permits work

on

pump without disturbing motor.

Short electrical leads.

Long

shaft

requires

a

structure

to

support

the

intermediate

bearings plus added lubrication

and

maintenance. Consider

a

stiff,

hollow

shaft

without intermediate bearings (which

may

be

impractical because limitations

are

severe).

High

headroom needed

for

replacing motors,

shafts,

and

pumps.

Flexible

couplings required

at

pump

and

motor.

Must ensure that manufacturer analyzes

shaft

for

torsional

vibration

—

especially

for

variable-speed drivers.

Field

construction

vs.

package

plants

Field

construction

No

limitations

of

head

or

capacity.

Complete

flexibility of

layout.

Special features

can be

included, such

as bar

screens,

comminutors,

better access

and

working room, overhead

hoists,

and

spare parts storage.

In

smaller sizes,

may be

faster

to

build than package stations.

Greater ease

of

maintenance

due to

less

crowding.

Higher construction

cost.

More engineering time

and

skills.

More

difficult

to

avoid blunders.

Package

types

—

general

Low

construction cost (see Figure

29-9).

Consulting

engineers' cost

is

lower

for

"off-the-shelf

units.

Equipment

can be

factory assembled

and

tested

prior

to

shipment.

Standardization reduces chances

for

blunders.

Especially suitable

for

small stations.

Available

in

following types: underground

wet

well-dry

well,

wet

well with suction pumps, submersible pumps

in wet

well,

and

pneumatic ejector.

Alarming

number

of

fatalities

and

injuries

in

can-type prefabs

due

to

asphyxiation,

H

2

S,

flooding, and the

difficulty

of

escape.

Access

is

often

poor

and

rescue almost impossible.

Ventilation

is

usually

woefully

inadequate.

Layout

design

is

inflexible

and

special features

are

limited.

Cramped working space makes maintenance

and

repairs

difficult

and

more costly because

of

added labor.

Capacity

is

usually limited

to

about

110

m

3

/h

(500

gal/min),

although larger

ones

have

been

built.

Corrosion

and

buckling

are

potential problems with steel shells,

which

often

corrode

quickly

(in 2 yr) due to

stray

electrolytic

currents,

and

hence cathodic protection

is

required.

FRP is

difficult

to use

where soil loadings

are

significant;

it is

also

vulnerable

to

corrosion

if the

protective resin coating cracks.

Discuss hazards

and

disadvantages

with

owner.

Table

25-3.

Continued

Advantages

Disadvantages

Package

type:

wet

well-dry

well

The

same

as the dry

well-wet

well type above plus

it can be

less

conspicuous

with

only hatches aboveground.

Inconvenient access.

Layout design

and

special features

are

limited.

Crowded space adds maintenance

difficulty.

Forced

draft

ventilation

is

often

inadequate,

so the

atmosphere

is

somewhat corrosive.

Package

type:

wet

well with

self-priming

pumps

Same

as the wet

well with suction pump above plus

it is

less

expensive than

the

package

wet

well-dry

well.

Maximum

capacity

is

about

90

m

3

/h

(400 gal/min).

Package

type:

pneumatic

ejector

Suitable

for

basements

and for

small capacities

and

high heads.

Can

provide scouring

of

force main

at low flows.

Relatively

high construction cost.

Low

efficiency.

Consider cutter pump (for

low

head) followed

by a

centrifugal

pump

(for medium head)

or by a

positive displacement lobe

or

progressive cavity pump (for high head).

Maintenance

of

auxiliary equipment

is

expensive.

Package

type:

submersible pumps

in wet

well

Low

cost. Pump

characteristics

somewhat

limited.

See wet

well submersible pumps

and

motors above.

Table

25-4.

Comparison

of

Non-Clog

Wastewater

Pumps

Advantages

Disadvantages

Self-priming centrifugal pumps

Sizes

to

250-mm

(10-in.)

suction/discharge.

Capacities

to

0.15

m

3

/s

at 24 m

(2400

gal/min

at 80 ft)

TDH.

Heads

to 38 m

(125

ft) at

less discharge.

Low

required NPSH.

Suction

lift

to 7.6 m (25

ft).

Dry

well

is

unnecessary

if

suction

lift

is not

excessive.

Quick, easy access

for

impeller

cleanout.

Typical

efficiencies

are

10-20%

below other non-clog pumps.

Priming depends

on the

volume

of

suction line, which must

be

short.

High dynamic suction

loss.

Close-coupled

centrifugal

pumps

(common

pump

and

motor

shaft)

Lowest

first

cost.

Most compact.

Conditions

favoring

use

Limited

finances.

Temporary stations.

Operating time less than

2000

hours/yr.

Motor bearings must carry

the

radial

and

thrust loads

of

pumping.

Some pumps require special motors with

a 3-

month delay

after

ordering.

Maintenance problems arise because

the

motor must

be

removed

to

replace packing

or

mechanical seals (not true with some

models).

Horizontal

frame-mounted centrifugal

pumps

Pump bearings

can be

selected

for

long

life

and

suitability

for

continuous

operation

with

high radial

and

thrust loads.

Does

not

need

a

special motor.

Coupling between

the

motor

and

pump reduces lateral vibrations,

permits

work

on the

pump

without

disturbing

the

motor.

Wide range

of

pump sizes

and

characteristics

is

available.

Higher cost than close-coupled pumps.

Vertical frame-mounted centrifugal pumps

in dry

well

Same

as for

horizontal frame-mounted pumps

(see also Table 25-3).

Same

as for

horizontal frame-mounted pumps plus motor must

be

lifted

to

work

on the

pump

or to

replace mechanical seals

(but

not to

replace seal packing).

Table 25-4.

Continued

Advantages

Disadvantages

Vertical

frame-mounted

centrifugal

pumps

in dry

well

with

long

shafts

for

motors

on floor

above

Same

as

vertical frame-mounted pumps (see also Table 25-3).

Highest

cost;

shafting adds 5-7%

to

pump

cost.

Must

analyze

the

shaft

for

torsional vibration.

Impractical

for

large (1.3

m

3

/s

or

20,000

gal/min) pumps.

Wet

well

submersible

pump

and

motors

Wide

range

of

sizes

and

characteristics

—

up

to 5

m

3

/s

(80,000

gal/min)

for a

vertical

propeller

pump.

Conditions

favoring

use

Need

for low first

cost.

Many

other

units

in

same system.

See

also Table

25-3.

V/S

control limited

to

AFD.

Pump

and

motor

are

more expensive than

dry

well types.

Maintenance requires removal

(a

messy operation)

from

wet

well

and

disassembly

by

trained workers,

so

factory service

may

be

required

at

substantial expense.

Dry

well

submersible

pumps

Excellent reliability.

Maximum

protection against

dry

well

flooding.

No

seal water.

No

intermediate

shafting

to

disassemble

for

work

on

pumps

and

no

steady bearings

nor

structure

to

support

or

gain access

to

them. Eliminates maintenance

of

such bearings.

No

daily maintenance.

No

oiling, greasing,

or

leaking packing

glands. Reduced housekeeping.

Air

fin or

water jacket cooling permits much greater pump-

cycling

frequency.

Above

advantages

may

make submersibles

the

most cost

effective.

Minimal

or no

vibration. Quieter than ordinary

dry pit

pumps.

No

long

shafts

to

interfere with cranes.

Becoming popular.

Conditions

favoring

use

High

risk of dry

well

flooding.

Deep stations.

Reduced maintenance requirements.

More costly than pump

and

separate motor, although

elimination

of

seal water systems

and

reduction

of wet

well

size because

of

more frequent allowable starts

may

make

price competitive.

Water

jackets require

frequent

flushing,

occasional disassembly,

and

they

are

often

clogged with solids

in

pumped

fluid at low

speeds. Fresh water cooling expensive.

Repairs

in

place

may

void manufacturer's guarantee.

If

delays

are

likely

for

shop repairs, must have uninstalled spare

unit.

Air

cooling

is

inadequate

for

many models (requires cooling

water

jacket,

fins, or

large

fan on

motor).

Variable-speed control limited

to

AFD, which

may

increase

cooling problems especially

at

lower speeds

(see Section

15-11).

Access

to the

pump (the part

of the

unit requiring

the

most

maintenance)

is

costlier than with

a

conventional pump.

Propeller

pumps

High

efficiency.

Large discharge,

up to 57

m

3

/s

(2000

ft

3

/s).

Low

cost,

low

maintenance.

Available

with variable pitch blades, which reduces motor

horsepower

and

starting torque.

Conditions favoring

use

Large discharge

at low

head

(9 m or 30 ft) per

stage

for

axial

flow and up to 24 m (80 ft) per

stage

for

mixed

flow.

Screened sewage

or

storm water.

Unsuited

for raw

sewage

flows

smaller than

1

m

3

/s

(15,000

gal/

min)

and

unsuitable

for

high heads.

Sewage should

be

screened

for

small pumps.

Variable pitch blades

may not be

reliable. Check with users.

Vertical

column,

wet

well

pumps

Axial-

or

mixed-flow

pumps permit high capacity and, with

multiple

stages, reach high head.

Avoid

centrifugal

volute-type pumps

for wet

well applications

and

use

submersible

or

vertical turbine, solids-handling

pumps

instead.

Conditions

favoring

use

Large

flows

following primary treatment

or

screening.

Propeller pumps

for

intermittent service

on

screened storm

water.

Non-clog volute pumps limited

to

small capacities.

Radial loads

on

submerged bearings limit bearing

life.

Diffusion

vaned

pumps cannot pass stringy solids,

not

useful

for

raw

sewage

in

smaller

sizes.

Limited stable capacity range.

Entire pump

with

column must

be

removed

for

service.

Available

in

diameters

of 0.6 to 3.7 m (2 to

12

ft) and flows to

3.8

m

3

/s

(60,000

gal/min).

Maximum

typical

efficiency

is 75%

over wide delivery range.

Constant-speed drive produces equivalent

of V/S

pumping;

self-

adjusts

to

match

inflow.

Conditions

favoring

use

Low

lift,

as at

treatment plants.

Little variation

in flow

level

at

discharge.

Can

handle large solids

so

screening

and

grit removal

unnecessary.

Need

for V/S

pumping with

C/S

drive.

Adequate

area outdoors.

Does

not

pressurize

fluid.

Pumps only

to a fixed

discharge level.

Requires critical clearance between trough

and

screw.

Efficiency

(1)

depends

on

slip

due to

clearance

(so it

cannot

be

pretested),

(2) is

reduced

by

cascade

at

exit receiver,

and (3)

decreases

at

lower

flows due to

slip.

Maximum

lift

9 m (30

ft).

Lower bearing

is

submerged

in the

sump

and

inaccessible unless

the

sump

is

drained.

Concrete trough

is

difficult

to

construct.

Large

units

and

high

lift

are

subject

to

binding

and

excessive

backflow.

Requires more space than other

lift

stations.

Ice is

troublesome

at the

inlet. Pump

may

require

an

insulating

cover

in

cold

climates.

Releases odor

if

sewage contains

H

2

S.

High

cost.

Enclosed

Archimedes

screws

Same

as

open screws except

Guaranteed

efficiency

is 85% and

only decreases

to 80% at

low flow.

No

concrete trough.

No

leakage.

All

bearings

are in the

open

and

readily accessible.

Flowrates

from

1

15

to

8000

m

3

/h

(500

to

35,000

gal/min).

In

freezing weather

the

pump must

be run

nearly

dry (in

reverse)

if

it

goes

on

standby.

Largest size

is 3 m (10 ft) in

diameter.

High cost.

(Continues

on

Table 25-4.

Continued

Advantages

Disadvantages

Vertical

turbines

for

handling solids (VTSH®)

Eliminates need

for a dry

well. Pump

is

expensive.

True

non-clog design; reliably pumps long stringy solids

and

Sizes smaller than

750 mm (10

in.)

not

available.

passes

75-mm

(3-in.) spheres (see Section

11-4).

Repairs

to

pump require

lifting

entire unit

out of wet

well.

Flowrates

of

0.13-1.3

m

3

/s

(2000-20,000

gal/min.)

at

6-21

Hence,

the

superstructure must

be

very high

or a

hatch must

(20-70

ft)

TDH.

be

located

so the

unit

can be

lifted

by a

mobile crane.

Motor above ground

floor is

protected

from

flooding.

When

a

pump

is

removed

for

service,

the wet

well

is

Capacity

of

existing pumping stations

can be

increased

by

interconnected with

the

pump room which, depending

on the

converting

dry

wells

to wet

wells. ventilation system (see

NEC

Art. 500), becomes

a

Class

1,

At

least

two

manufacturers. Group

D,

Div.

1 or 2

hazardous space.

High

efficiency

over

a

broad operating range. TEFC motors

are

acceptable

if the

room

is

ventilated

at

15

air

Ideal

for

increasing

the

capacity

of

existing pump stations

and

changes

per

hour.

At

lower ventilation

rates,

explosionproof

for

V/S. motors

are

strongly recommended (and

may be

required

in

Can

operate

at

very

low

speed.

the

future).

All

electrical

equipment should

be

protected

against

corrosion

and

labeled

for

hazardous environments.

All

electrical

panels must

be in a

separate room.

As

with

any

column pump,

the

column must

be

vented;

a

conventional

air

release

valve

is

unreliable.

Use

open vent

with

return

to the

sump

or a

solenoid-controlled, pilot-

operated sleeve

or

diaphragm valve.

Table

25-5.

Comparison

of

Special

Design

Wastewater

Pumps

Advantages

Disadvantages

Open

Archimedes screws

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

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