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

Table

25-5.

Continued

Advantages

Disadvantages

Pneumatic

ejectors

Capacity range

is 0.5 to 140

m

3

/h

(2 to 600

gal/min).

High

head.

Automatically

variable volume.

Batch

discharge velocity scours force main.

Operation

is

usually trouble

free

in

small sizes even without

screening

or

comminution.

Rugged,

reliable, long

life.

Conditions

favoring

use

Where centrifugal pumps unsuitable.

An

air

supply

is

already available.

Low

(~1

1

m

3

/h

or 50

gal/min), inconsistent

flow and

high

head.

Single receiver

is

acceptable

and a

standby

is not

needed (but

avoid

unless

fully

justified).

Maximum

efficiency

is

60%

—

typically

less

due to

compressor

efficiency

and

blow-down

losses.

Some designs

are

unsuitable

for raw

sewage

due to the use of

conventional swing check valves.

No

suction

lift;

fills by

gravity alone.

Maintenance (for some makes) includes unclogging check

valves

(which

are

susceptible

to

plugging)

and

cleaning

electrodes

for

level controls.

Noisy

and

creates high ambient temperatures

in an

enclosed space.

High capital cost, especially

in

large sizes where

a

standby

receiver

is

included.

Settling

in the

discharge pipeline

may

occur unless

the

unit

is

sized properly.

Electrode-controlled units

corrode.

Two

compressors

and two

pressure vessels

are

needed

to

provide

a

semblance

of

continuous

flow.

Flow cannot

be

increased

in the

future

except

by

adding ejectors.

Adequate basement required

for

installation.

Grinder

pumps (centrifugal

or

positive-displacement types)

Many

pumps

can be

connected

to a

single, small force main.

Positive-displacement types

can

produce

flow at

heads

to 36

(120

ft) or

more with capacities

up to 1.8

m

3

/h

(8

gal/min)

and

motors

up to 7.5 kW (10

hp).

Conditions

favoring

use

Residences below

the

sewer main with

a

long force main.

High

head,

low flow.

Costly.

Unsuitable

for

municipal application.

Usually

installed

one per

residence, which mandates prompt

repair service

at any

hour

in any

weather.

Reliability

is

improved

and

high pump cost

is

slightly reduced

by

a

single sump containing

two

pumps serving

two or

more

residences.

Centrifugal

types

may not

pump when

the

force main

is

pressurized.

Cutter

pumps

Sizes available

are

75-150

mm

(3-6

in.).

Capacities

are

11-340

m

3

/h

(50-1000

gal/min.

Power range (dry

pit)

is

2.3-30

kW

(3^0

hp).

Power range (submersible)

is

2.3-15

kW

(3-20 hp).

Comminutors

and bar

screens

may be

eliminated.

Very

effective

on all

(e.g., overalls

and

disposable diapers)

but

the

most unusual solids (e.g., panty hose, which

are too

thin

to

cut).

Long

life

(many years)

in

severe service.

Effective

with relatively

low

horsepower motors.

Prior

to

selecting

the

pumps,

a

basic

decision

must

be

made

on the use of the

following:

•

Vertical

wet

well pumps

•

Horizontal

dry

well pumps

in a dry

well-wet

well

design

•

Vertical

dry

well pumps

in a dry

well-wet

well

design

•

Horizontal pumps that

are floor-mounted

above

the

suction well, with

a

priming

system.

Only

three-phase motors

are

available

at

1

150

or

1750

rev/min.

Cost

is

double that

of a

comparable centrifugal pump.

Efficiency

is low (30 to

60%) because

of the

cutting required.

Maintenance

of

submersible styles requires disconnection

and

removal,

or

work

must

be

done

in a

hazardous area.

In

some installations,

the

most

economical

design

is

obvious,

but in

others,

alternative designs must

be

prepared

on the

basis

of first

cost

and

operation

and

maintenance

costs.

• If a

deep

pump setting

is

required

to

reach

the

water

surface

and if

construction

is

difficult

due to

poor

soils

or

ground water problems,

a wet

well station

design with vertical pumps

is

always

the

most cost

effective.

• If the

setting

is

shallow

and if the

design

of the

intake works eliminates

the

need

for a

separate suc-

tion

well,

a

horizontal pump installation

with

the

pumps

(below

low

water)

in a dry

well

is

usually

the

most economical.

• If the

distance between

the

pump room

floor and the

low

water

surface

is

such that

a

horizontal pump

can

handle

the

required suction

lift,

an

economical

design

is

horizontal pumps installed

at the

ground-

floor

level.

Complete reliability depends

on the

proper

functioning

of the

pump priming

system

—

particularly

in an

unattended station.

A

small pumping station

may

well

use a

single

operating pump and,

to

reduce

the

spare parts inven-

tory,

an

identical standby unit.

The

electrical

design

should

preclude

the two

pumps operating together

and

should

also ensure that each operates alternately.

In

multiple

pump stations, pumps should

be

started

one

at

a

time.

The

hydraulic transient

or

surge analysis

should

be

based

on the

simultaneous shutdown

of all

operating

units

as a

result

of

power

failure.

Most solutions

can be met

with

the

variety

of

pumping

stations compared

in

Table 25-6

and

pumps

compared

in

Table 25-7.

Well

Water

Pumping

Pumps used

for

well water

are

almost always vertical

mixed

flow or

radial

flow

(compared

in

Table 25-8).

If

flooding

can

occur

and

station reliability

is

essential,

all

electrical machinery must

be

above grade, which

might

require vertical pumps.

If

space

is

limited, ver-

tical

turbines

can be

mounted directly above

a

clear

well

or

reservoir, although protection against contami-

nation

is a

significant

factor

for

pumps

in a

clear well.

The

types

of

wells include

•

Shallow

or

caisson wells

for

depths

up to 60 m

(200

ft)

•

Horizontal collectors

and

infiltration

galleries

•

Deep, cased (drilled) wells

for

depths

of 6 to 600 m

(20 to

2000

ft) and

diameters

of 100 to 750 mm

(4

to 30

in.).

Table 25-6.

Comparison

of

Water

Pumping

Station

Types

Advantages

Disadvantages

Vertical

wet

well pumps

Wide selection

is

available: axial

flow

(single stage),

mixed

flow

(single

and

multistage), Francis turbine (single

and

multistage),

and

radial

flow

(turbine

in

single

or

multistage).

Small

floor

area

and

small superstructure.

Ground

floor

pump motors above

flooding.

Pump suction always

flooded; no

priming.

NPSH easily met.

Ideal

for

deep installation

and

large variations

in

water level.

Vertical

turbines

are

especially

flexible in

meeting head

requirements

by

adding

stages.

High superstructure

to

pull pumps

or, at

greater expense, pumps

can

be

pulled through

a

hatch

in the

roof

of a low

superstructure

by a

truck crane parked

outside

—

a

method

that

may be

cheaper than inside crane.

Requires disconnecting

the

motor

and

pulling

the

pump

for

inspection

or

repair.

Requires more

shaft

bearings.

Priming

may be

required

if air in the

pump column causes

problems.

If

idle pumps collect

air in

pump column, either

(1)

an

automatic

air

vent valve

at the

discharge elbow

or (2)

priming must

be

installed.

Wet

well-dry

well,

horizontal

pumps

below

water

level

(flooded suction)

Pump types available include split-case centrifugal,

end-suction,

overhung-shaft,

and

horizontally mounted axial-flow

(propeller

and

mixed-flow) types.

Eliminates priming

and air

problems.

Low

headroom requirement.

Easy maintenance

and

accessibility.

Service

can be

accomplished without disconnecting motor.

Large pump room

floor

area.

Greater excavation.

Motors

are

subject

to flooding.

Longer

electric

conduits.

Pumping stations

are

more costly

due to

additional

floor

area,

access,

lighting, ventilation, etc.

Greater forced ventilation needed

to

cool motors.

Horizontal

pumps

on floor

above

suction

well

Easy maintenance

and

accessibility.

Short electric conduits.

Motors above

flood

level.

Ventilation minimized.

Reduced excavation

and

below-ground construction. Similar

to

wet

pit.

Requires priming equipment.

Dependability (due

to

priming equipment)

is

reduced.

Limits

the

choice

of

pumps

to

those

for

negative suction.

Booster Pumping

Selecting

the

type

of

booster pump

is

somewhat com-

plicated.

The

following

discussion

is

oversimplified

and

should

be

supplemented

by

reviewing Chapter

18.

Booster

pumping stations

fall

into

two

general

classes:

(1)

distribution boosters, which increase pres-

sure

to

serve

a

water distribution system

at a

higher ele-

vation,

and (2)

in-line boosters

on a

transmission main.

Boosters

should

be

constructed only when justi-

fied

by

sound economic

and

operational reasons.

The

advantages

and

disadvantages

of

booster pumping

stations

make

it

necessary

to

conceive alternative

system

layouts, some

of

which might exclude

a

booster. Make

a

choice

on the

basis

of a

present-

worth

analysis

of

capital

and O&M

costs

as

well

as

adaptability

for

meeting

the

requirements

of the

situ-

ation.

Table

25-7.

Comparison

of

Water

Pumps

Advantages

Disadvantages

Axially

split-case pumps vertically

or

horizontally mounted

Easy

to

maintain

and to

inspect.

Axially

balanced,

so

there

is no

axial thrust.

Requires

a

housing

to

prevent freezing

and

protect bearings

and

packing

from

dust.

Vertical

axial

flow,

mixed

flow, and

radial

flow

(turbine)

pumps

Can be

housed

in

low,

flat-roofed

buildings with roof hatches

for

pulling

the

pump with

a

crane parked outside.

Quiet, especially with submersible motors.

Submersible motor style needs

no

housing except

for the

control panel

and

valve vault.

Virtually

no

maintenance except

for

pulling

the

entire unit

every

2 to 5 yr.

If

outdoors, standby units

may

freeze, noise

is a

problem

in

residential areas,

and the

station

is

easier

to

vandalize.

Table

25-8. Comparison

of

Well Pumps

Advantages

Disadvantages

Self-priming centrifugals

Capacity

to

0.25

m

3

/s

(4000

gal/min).

Limited

to

caisson,

gallery,

or

wells with

a

water table less than

4.6 m (15 ft)

below

the

pump

Vertical

turbines

(V/T)

Most common type

of

well pump.

Efficiency

as low as 50%

depending

on the

size

and

application.

Motor

or

engine driven.

Diameter:

50 to

>1200

mm (2 to >48

in.).

Capacities exceeding

0.6

m

3

/s

(10,000

gal/min).

Use for

finished water

and

booster pumping

is

increasing.

Can be

tailored

for

specific head

by

adding bowls.

Vertical

turbines,

Hneshaft

Minimum

maintenance. Unsuited

to

crooked wells.

Driver accessible. Somewhat noisy.

Excellent

for

wells less than

90 m

(300

ft)

deep.

Suitability

is

marginal

at

depths over

300 m

(1000 ft). Even

at

depths less than

18Om

(600 ft),

shaft

stretch wears impellers

and

bowls unless

the

thrust bearing

is

kept carefully adjusted.

Vertical

turbines,

submersible

Quiet,

so

they

are

suitable near hospitals, schools,

and

residences.

Only

solution

for

crooked wells.

Practical

at

depths over

21Om

(700 ft).

Water cooling

is

very effective.

No

long-shaft problems.

Maintenance requires pulling

the

entire unit.

Regular maintenance

is

required.

Seal problems

may be

severe.

Long

electric

cables.

The

comparative advantages

of the

different

types

of

pumps

for

booster stations

are the

same

as for raw and

finished

water stations.

In

general, most booster pumps

are

best served

by

either

a

horizontal split-case double

suction

pump

or a

vertical turbine

can

pump.

The

per-

sonal preference

of the

engineer

and

owner

is an

impor-

tant

factor.

During

the

past three decades,

the

trend

has

been toward

the

increasing

use of

turbine pumps, espe-

cially

in the

western United States. However,

the

selec-

tion

of the

pumping units requires considerable analysis

because

of the

many factors that must

be

weighed

against each other. Some

of

these

are as

follows:

• A

smaller number

of

larger pumps

is

less

costly.

•

More favorable

flowrates

with less storage

are

obtained with more small pumps.

•

Pumps

of

different

sizes (proportional

to flow

ratios

of

1:2:4)

give

the

greatest number

of flow

combina-

tions,

but

they

(1)

violate

the

desirability

of

stan-

dardization,

(2)

cause

one

pump

to do

most

of the

work,

and (3)

increase

the

cost

of the

standby pump.

• The use of V/S

pumping improves

flow

matching,

but it (1)

adds complexity,

(2)

increases

the

diffi-

culty

of the

engineering,

and (3)

reduces reliability.

•

Before resorting

to

V/S, explore

all

other possibili-

ties

first,

such

as (1)

balancing storage

at

either

or

both ends

of

system,

(2)

using complementary

C/S

pumps

of two or

more sizes,

(3)

bypassing

a

portion

of

the

discharge,

or (4)

adding

a

throttling valve.

•

Because

of

maintenance, capital cost,

and

ineffi-

ciencies,

V/S is

justified

only

(1)

when feeding

large systems with inadequate storage

and

large

demand

fluctuations or (2) in an

intermediate zone

where both

the

upper

and

lower zones contain

sources

of

supply

and

variable speed

is

needed

to

balance pressures.

• Two

philosophies

in

selecting

the

size

of

booster

pumps

are (1) the use of

pumps

of the

same size

(which

is

often

better

for

small stations),

and (2) the

use of two

groups

of

pumps: small ones

for

average

demands

and

large ones

for

maximum demand

(which

is

often

better

for

large stations with heavy

fire

demands).

A

summary

of

advantages

and

disadvantages

of

booster pumping stations

is

given

in

Table 25-9.

Sludge

Pumping

Sludge pumping

is

usually

a

part

of a

water

or

wastewater

treatment plant. Hence, sludge pumping stations

per se

are

uncommon

and

pumps

are

usually housed

in the dry

well. Pumps

are

compared

in

Table

25-10.

Sludge pump-

ing

evaluation

and

design

is

complex

and not

straightfor-

ward

due to

variations

in the

characteristic behavior

of

sludge

as a

non-Newtonian

fluid in

addition

to

other fac-

tors

that

render traditional design approaches inappropri-

ate.

For an

extensive discussion,

refer

to

Chapter

19.

25-7. Power,

Drivers,

and

Standby

Early

decisions about drivers

and

standby units

affect

the

type,

configuration,

and

physical size

of the

pumping sta-

tion.

The

preliminary design considerations should include

plans

for the

electrical system, such

as

load estimating

(power

and

lighting), load data collection, load character-

istics, selection

of the

power source, plans

for

load growth

and

change,

selection

of the

best voltage,

and

selection

of

the

best distribution system

for

reliability,

flexibility,

safety,

and

maintainability (see Chapters

8 and 9).

Motors

A

generalized guide

for

selecting

C/S

drivers

is

pre-

sented

in

Table

25-11,

and a

guide

for

selecting

V/S

drivers

is

given

in

Table 15-2. About

90% of all

driv-

ers are

electric

motors,

and the

squirrel-cage induction

motor,

by far the

most common,

is

available

in a

wide

variety

of

casings,

windings, insulation, allowable

Table 25-9.

Comparative

Advantages

of

Booster

Pumping

Stations

Advantages

Disadvantages

Allows suction side

pipeline

to be

designed

at a

lower pressure Additional construction costs, O&M,

and

replacement

costs.

rating

and may

reduce pipeline construction cost. Increases operational complexity.

May

avoid designing primary pumping station

for

abnormally

Offsite

facilities (access roads

and

power lines)

may be

required.

high

discharge pressure

with

resultant cost savings. Complicates analysis

and

control

of

water hammer.

May

reduce

maximum

system pressures over large service areas Requires matching

the flows of

primary

and

in-line booster

and

reduce energy costs

and

leakage. stations. Complicates design because head-capacity curves

Useful

in

increasing

flow in

existing pipeline systems. cannot

be

independently established

for

either station.

Useful

in

eliminating substandard pressures

in

zone extremities. Large

fire flow

requirements

and

small minimum domestic

flow

needs require

a

wide range

in

pumping capability.

temperature

rise,

shafts,

and

bearings.

Synchronous

motors

are

occasionally

used

for

very

low-speed

applications

or for

drivers larger than

375 kW

(500

hp).

Wound-rotor

motors

are

sometimes

used

for

V/S

applications,

but

they

are

expensive

and

mainte-

nance

is

greater

than

with

squirrel-cage motors. Direct

current

machines

are

rarely

used

in

pumping

stations.

NEMA

Designations

NEMA

design

letters

A, B, C, and D are

used

for

des-

ignating

the

starting

torque

and

starting

current

of

general-purpose

motors

up to 150 kW

(200 hp).

•

Design

A

(never

used

in

pumping

stations)

is

nor-

mal

starting

torque

and

normal

starting current.

Table

25-10.

Comparison

of

Sludge

Pumps

Advantages

Disadvantages

Air

lifts

Suitable

for

return activated sludge,

raw

sewage,

and

sandy

waters.

Also used

for

pumping shallow wells.

Maximum

flow is

about

550

m

3

/h

(2500

gal/min).

Simple

and

cheap.

No

moving parts, very little maintenance.

Conditions

favoring

use

Return

activated sludge.

Decanting digester supernatant.

Sometimes

for

shallow wells.

Close control

of flow is not

critical.

Unusual

cost

for

alternative system.

Air

supply

is

already available.

Digester

gas

mixing systems.

Limited

to low

lifts

—

about

2 m (7

ft).

Does

not

pressurize

fluid, so

they

are

limited

to

free

discharge.

Maximum

efficiency

is 35% or

less.

Difficult

to

regulate discharge,

and

pumping

is

unreliable

if air is

throttled

for low flowrates.

Requires

a

large submergence.

Sensitive

to

small changes

in

head

and

viscosity.

Suitable only

for

thin sludges without large

solids.

Use for flow

measurement

is

unwieldy

and

inaccurate.

Requires

air

compressor

or

blower.

Plunger

pumps

(positive

displacement)

The

most reliable

of the

positive-displacement types.

Discharges

to 120

m

3

/h

(540 gal/min)

at

pressures

up to

13,800

kPa

(2000

lb/in.

2

)

or

more.

Can

pump very thick sludge, large solids, stringy materials,

and

grit.

Slow

speed

and

long

life.

Suitable

for

suction

lift.

Major

repairs

can be

made

in a

local machine shop.

Capacity

is

constant despite large changes

in

head.

Conditions favoring

use

Sludge

pumping

for

digesters.

High

and/or variable head.

High

suction

lift.

Viscous fluids.

Maximum

efficiency

is

typically

40 to 50%

depending

on the

type.

Needs extra attention

for

lubrication

and

routine maintenance.

Requires sizable

floor

space.

Solids size

is

limited

by

check valve clearance.

Check valves must

be

dismantled

to

locate trapped solids; dual

checks

are

helpful.

Check valve spheres

are

short lived.

Low

capacity; primarily

useful

only

for

sludge.

Air

chamber

is

usually required

to

prevent water hammer.

Requires bypass pressure relief

or

shear

protection.

Head depends

on

peak

of

sinusoidal

flow

characteristics.

Air

chambers

are

needed

to

smooth

out flow

pulsations.

Rotary

lobe pumps (positive

displacement)

Efficiency

near maximum

(-75%)

at all

speeds

and

pressures.

Quick,

easy replacement

of

moving parts

(in the

cantilever type).

No

check valves

are

required.

Pressures

to 690 kPa

(100

lb/in.

2

).

Capacity

to

about

450

m

3

/h

(2000

gal/min).

High

tolerance

for

rags

and

stringy materials

and can

pass solids

to

120 mm (4

in.)

in

diameter; easy access

for

cleanout.

Long

life

at low

speed.

Meters

the flow

accurately despite changes

in

TDH.

Good

for

viscous sludge; creates

the

least shear rate.

Can run dry or in

reverse without damage.

Smooth, relatively nonpulsating

flow.

Conditions favoring

use

Where trouble-free operation

and low

maintenance

are

more

important than capital cost.

Where

floor

space

or

building cost

is at a

premium.

High

first

cost (about

130%

of the

cost

of a

progressive cavity

pump).

Efficiency

is low for

thin liquids.

Comminution, cutting,

or

grinding solids

and

grit removal (for

moderate

to

high grit content)

is an

essential pretreatment.

In

usual service, replace lobes yearly, seals every

2 yr at an

approximate

annual cost, depending

on the

model,

of

$1200

to

$3000

(in

1997).

Suction

lift

due to

slip

is

limited

to 3 m (9

ft).

Side clearances

are

abraded

by

grit,

so

avoid where grit loads

are

heavy.

Bypass

relief pipe

is

required

to

prevent damage

if

discharge

is

plugged

or

valve

is

closed.

Limited

to a

maximum solids content

of

about

5%.

(Continues

on

Table

25-10.

Continued

Advantages

Disadvantages

Diaphragm

pumps

(positive

displacement)

Simple

and

easy

field

repairability.

High pressure.

Suitable

for

suction

lift.

Moderate shear rate.

Low

cost.

Can

meter

flow.

Handles viscous liquids, sandy

and

muddy waters.

Conditions favoring

use

Temporary use, especially with

a

gasoline engine outdoors.

Low flows.

Dewatering

excavations.

Slurry

pumping.

High

grit loads.

Instant shut-down

on

component failure. Check valves

are

required.

Solids handling ability

is

limited

by

ball check. Poor

for

rags,

sticks,

and

string.

Progressive

cavity pumps (positive

displacement)

Typical

maximum

efficiency

is

~75%

.

Good solids handling

for

abrasive

and

viscous liquids.

Generates pressures

to

7000

kPa

(1000

lb/in.

2

).

Maximum

capacity

is

1

10

m

3

/h

(480

gal/min).

Rotor acts

as a

check valve.

Smooth, relatively nonpulsating

flow.

Low

capital cost.

Conditions favoring

use

High

head, variable head.

High suction

lift.

Grit-free

sludge with

a

high solids content.

Handles

gas

inclusion.

High

maintenance cost. Typically, stators replaced yearly, rotors

rebuilt every

2 yr at

about

$400/yr

for 20

gal/min pump

to

$3300/yr

for 450

gal/min pump

in

1997.

Large

floor

space

and

clearance

are

required

—

especially

to

pull

rotor.

Unsuited

to

heavy grit loads.

Cannot pump solids larger than

45 mm

(1.8

in.).

May

require

in-

line grinding

as

pretreatment.

Cumbersome piping.

Instant failure when

run dry or if the

discharge plugs. Safety

devices

for (1)

high pressure

and (2) no flow are

recommended.

Elastomer (material

in

stator) limits

the

type

of

liquids pumped.

Slow speed drive required.

Non-clog centrifugal

Wide selection

of flow and

head applications.

Vertical

and

horizontal configurations

are

available.

If

applied accurately, pump

efficiency

is

high

and

pumping

is

economical.

Conditions favoring

use

Thin sludges without debris (waste

and

return-activated

sludge).

High

volume

of

sludge

to be

pumped.

Need

for

economic pumping outweighs maintenance costs.

NPSH considerations

may be

critical.

May

be

difficult

to

size large enough

to

pass

solids

without

clogging,

yet

small enough

to

avoid dilution

by

drawing

in

overlying

liquid (rat holing).

Requires capacity adjustment

(variable-speed

drives).

Needs separate

flowmetering for

process control

and

speed

control.

Pump clearances must

be

adequate

to

pass

the

material typically

contained

in

sludge.

Vortex

pumps

(recessed

impeller,

centrifugal)

Readily passes solids (debris, grit, rags, stringy materials).

Efficiency

only

35 to

50%.

Good longevity. Very

flat H-Q

curve.

Materials

for

gritty slurries available. Usually requires variable-speed drive.

Excellent reliability. Must

be

accurately sized

to

avoid excessive recirculation.

Conditions favoring

use

Cannot trim impellers

of

U.S. makes.

Primary sludges.

Increase

of

head greatly reduces capacity

Undegritted

sludges.

Need

for

compactness.

•

Design

B is

normal

starting

torque

and low

starting

current

(the

most

common

type

in

pumping

sta-

tions).

•

Design

C is

high

starting

torque

and low

starting

current.

•

Design

D is

high

starting

torque,

low

starting

cur-

rent,

and

high

slip.

NEMA

code

letters

are

different

from

NEMA

design

letters.

The

code letter

identifies

the

starting

conditions

in

"kilovolt-amperes

per

horsepower"

when

the

motor

is

started

on

full

voltage.

These

data

are

primarily used

to

determine

if the

capability

of the

electric supply

or

on-site generator

is

sufficient

for

starting

and

picking

up the

load.

Starting

in-rush

current requirements

are

needed

for

sizing

the

emergency generators, conductors,

and

controllers

and are of

interest

to the

power company.

The

motor controls should permit starting only

one

unit

at a

time,

and if

drivers

of

different

sizes

are

used,

the

emergency generator size

can be

minimized

by

starting

the

largest motor

first.

Hazardous Locations

A

Class

1,

Division

1

wastewater

wet

well requires

full

containment

of

electrical conductors

and

equip-

ment.

A

non-explosionproof

submersible motor

can

be

made compliant

by

using

a

redundant low-level

float

switch

to

disconnect

the

motor

and

sound

an

alarm

when

the

liquid level

falls

to the top of the

motor.

Class

I,

Division

2

allows

the use of

non-explo-

sionproof wiring

and

equipment provided

it

does

not

produce arcs (see

NEC

501-3).

Close-Coupled Motors

Close-coupled motors have

a

long

shaft

that carries

the

pump impeller. Radial loads

for

multivaned impel-

lers pumping clear water

are

relatively small,

but

radial loads

for

impellers pumping

raw

sewage

are

large,

they

fluctuate

(twice

per

revolution

for a

two-

vane

impeller),

and

with large solids present,

the fluc-

tuations

are

irregular.

Shafts

or

bearings

and

seals

in a

close-coupled unit pumping sewage

may be

short

lived, particularly

at

high discharge heads, unless

the

unit

is

specially designed

for

severe service.

Motor Specifications

Motors located indoors

and

above grade

can

have

open dripproof enclosures. These

are the

least expen-

sive,

and

they allow motors

to run

cooler

than other

enclosures. Some designers suggest splashproof

enclosures

for

some environments

so

that

the

entire

area

can be

washed

with

a

hose.

To

provide protection

from

flooding,

specify

installations below grade

to be

either

(1)

open, dripproof enclosures with encapsu-

lated motor windings

or (2)

totally enclosed, fan-

cooled motors. Outdoor installations should

be

speci-

fied

as

Weather-Protected

II

enclosures.

For

Class

1,

Division

1

hazardous

locations,

specify that

"motors

shall

be

rated Class

1,

Division

1,"

which

is a

totally

enclosed, explosionproof motor.

For

Class

1,

Division

2

locations,

the

open, dripproof

enclosure

is

accept-

able

for

squirrel-cage motors

if

there

are no

switches

or

other arc-producing components

in the

motor.

Other specifications

for

motors should include:

•

Class

B

temperature

rise

•

Class

F

insulation

•

480-

V,

three-phase power

for

motors

from

2 to

about

60 kW (2 to

about

75 hp)

•

2300-V three-phase power

for

motors

of 75 kW

(lOOhp)

or

more

• A

service

factor

of

1

.

1

5

•

High energy

efficiency

and a

high power

factor

(because these terms have

no

accurate

definitions,

state

the

actual values

in the

specifications)

•

Blower, fan,

and

centrifugal pump motor torque

characteristics that

fit

NEMA Standards Design

B

•

Antifriction ball bearings with

a

life

of

100,000

for

motors

up to 300 kW

(400 hp); ball

or

roller bear-

ings

can be

used

•

NEMA code letters

F or G for

motors

of 150 kW

(200

hp) or

less

(see Chapter 13).

Variable

Speeds

Use V/S

drives only

if

necessary

and do not use

com-

plex control systems (such

as

AFD)

for

remotely located

pumping

stations except with

the

client's

full

under-

standing

of the

problems stated

in the

subsection enti-

tled "Adjustable-Frequency Drives"

in

Section

15-11.

Specify

unit responsibility (see Chapter

16)

for the

drive

and

pump, but,

if AFD is to be

used, either

(1)

specify

also

that guaranteed service

and

critical parts will

be

available within

a

reasonable

distance (say,

160 km or

100

mi) or (2)

make sure that

the

client will have trained

electronic technicians

on the

staff

or

that there

is a

reli-

able,

qualified

service organization nearby.

If V/S is

nec-

essary

and AFD

cannot

be

justified

and if

slightly higher

power losses

can be

tolerated, consider

the

simpler slip

drives such

as the

eddy-current coupling (see Sections

15-1

and

15-11).

One way to

"improve"

the

efficiency

of

slip

drives

is to

design

so

that

a

pump rarely operates

at

less than about

85% of

full

speed. Also consider

the

almost

100%-reliable

combined squirrel-cage motor/

diesel

engine drive where

the

engine with

a

larger rating

is

used both

for

peak

flow and for

standby power.

If V/S

is

used

for one

pump, there must

be a V/S

standby.

A

combination

of a V/S

pump

and a C/S

pump

is

satisfac-

tory

only

if the

discharge

capacity

of the V/S

pump

exceeds

the

capacity

of

the

CIS

pump

by at

least

50%.

If

the two

pumps have equal capacities,

the V/S

pump will

"starve"

on too

little

flow and

wear

out

quickly,

for the

reasons given

in

Sections 10-6

and

15-5.

Flat

H-Q

curves

for

both

the

pump

and the

system

must

be

avoided because, with

an

acute angle

of

inter-

section,

small changes

in

head (due

to air in the

force

main,

changes

in the wet

well level,

or a

change

in

speed

due to fluctuating

voltage) causes highly unsta-

ble

operation.

The

best operational approach

is to

start

a

second

V/S

pump when

the

lead

V/S

pump capacity

is

slightly exceeded

and to run the two

pumps

at the

same speed

in a

"load-

sharing"

operation, which

is

more

efficient

than operating

the two

pumps

at

differ-

ent

speeds

in

"staggered

operation"

(see Figure 15-8).

Miscellaneous

but

Important

Considerations

The

following miscellaneous considerations

are

also

important:

• For flood

protection

in dry

wells, consider

(1)

sub-

mersible motors,

(2)

vacuum

and

pressure epoxy

encapsulation

of the

motor windings with

a flood

level

float to

disconnect

the

motor automatically before

inundation,

or (3)

vertical pumps with extended

shafts.

• For

high-service duty, select

the

slowest speed that

can

perform satisfactorily. Slow speed motors

and

pumps

cost more,

but

wear

is a

function

of

speed

to

approximately

the

third power.

•

Consider slip

or

high slip

(8-13%)

motors with pos-

itive-displacement pumps

for

high static heads

or

for

long force mains.

•

Keep

the

motor control panels

in a

clean,

dry

envi-

ronment

above grade

and

above

the flood

level,

but

locate them

as

close

to the

motor

as

possible

to

keep

the

leads short.

•

Calculate

the

number

of

motor starts

per

year

and

obtain motors that will last

for

many years

by

opti-

mizing

life-cycle costs. Specify

the

minimum cycle

times

and the

minimum

life

required

for

motors

and

obtain

the

manufacturer's guarantee

in

writing

(never verbally).

The

best economy

is

often

found

in

a

special, more expensive motor.

•

Limit

the

permissible deflection

of the

impeller

shaft

at the

wearing rings under

the

worst specified

continuous operating condition (see Sections 10-6,

11-3,

and

Appendix

C).

•

Specify

the

bearing grade. Some designers require

certified

calculations

of

pump bearing

life

at the

design operating condition.

Engines

Internal combustion engines

are

used sparingly

as

prime

movers. Nevertheless, they

are

more reliable than electric

motors,

often

more economical (especially

if

operating

on

biogas),

offer

improved protection against surge dam-

Type

Electric motor (direct drive)

Electric motor (belt drive)

Electric motor (gear drive)

Engine

(direct drive)

Engine

(gear drive)

Engine/motor

(with

clutch

or

declutching)

Engine-generator

standby

duty

Power

limits

No

practical limit

600 hp

No

practical limit

No

practical limit

No

practical limit

Generally

less than

200 hp

No

practical limit

Reliability

Good,

depending

on

utility

Good

to

poor

depending

upon

frequency

of

belt

and

pulley replacement

Good (depending upon

utility),

but has

additional

maintenance

cost

Excellent, especially with

on-site

fuel

storage

or

production

Same

as

direct-drive engines

Superior,

especially with

on-

site

fuel

storage

or

production

Superior

Capital

cost

Low

Moderate

to

high

High

Very

high

Table

25-11.

Guide

for

Selecting

Drivers

Application

criteria

age

(because

of the

increased inertia

of

moving parts),

and

are

easily controlled

in V/S

operation.

But

they

are

noisy,

complex,

and

require skilled mechanics

for

main-

tenance

and

repairs.

If

engines

are

seriously considered,

consult

the

application engineering departments

of

equipment

manufacturers

—

certainly

before

proceeding

with

the

design.

Never

install engines below grade.

Fuel

Consider

diesel, natural gas, biogas,

and LPG

(pro-

pane)

fuels.

For

engines over

560

kW

(750 hp), con-

sider dual

fuel.

Gasoline

is

unsatisfactory because

its

storage properties

are

very poor,

and it has a low flash

point, which increases

the

danger

of

explosion

and fire.

Diesel

is

much less hazardous,

and it can be

stored

for

approximately

a

year,

but

double storage tanks

are

required

to

guard against leakage.

An

advantage

of

LPG is its

unlimited storage

life,

but it is

heavier than

air, very explosive,

and the

building envelope must pos-

itively

prevent leakage

from

accumulating

in low

areas.

Duty

The

term

"duty"

means

the

time-based utilization

of

the

driver

and the

specifics

of the

application such

as

direct drive

or

electrical generation

and

emergency

or

standby

duty.

•

Direct drive

is the

most

efficient.

Gear reducers per-

mit any

pump speed

at the

best engine revolutions

per

minute.

If

gears

are

required, they should have

a

nonreverse mechanism

to

prevent reverse rotation

of

the

engine

and the

interruption

of

lubrication.

•

Continuous duty engines should

be

designed with

conservative rating factors

to

meet

the

objectives

of

reliability,

low

operating costs,

and

long service

life.

•

Standby

duty

engines should

be

selected

for

rapid

starting

and an

ability

to

develop

full

load quickly.

Specify

starting aids, such

as

two-

stage

trickle

chargers

for

batteries, jacket water

and

lube

oil

heaters,

and

high torque versus speed.

•

Electrical generation requires

a

careful

analysis

of

the

loads (see Example

9-1)

and

thoughtful

applica-

tion

to the

generator (see Example 9-10).

The

man-

ufacturer

should

confirm

the

analysis

and

make

the

final

equipment selection.

Standby

may be

supplied

in

four

ways:

•

Dual electrical power systems. Dual power does

not

guarantee that

a

general

failure

of

interlocked sys-

tems (which

has

occurred over very wide areas)

or

Table

25-1

!.Continued

Operating

cost

Moderate

to

high

Moderate

to

high below 1720 rev/min

Moderate

to

high

Moderate

to

low, depending upon

fuel

cost.

Can be

very

low if

engine waste heat

can

be

recovered

for

heating

or

cooling

Same

as

direct drive engine

Moderate

to

low, depending upon

fuel

cost

Moderately

low in

standby duty. Must

exercise frequently

Application

criteria

Speed

limits

Essentially limited

to

1800,

1200,

900,

or 720

rev/min

for 60 Hz

current;

variable

speed

possible

No

practical

limit

to

mechanically

adjustable speed

No

practical

limit

either

up or

down

Variable speed (through motor speed

adjustment)

Limited

to

available engine

speeds;

variable speed easily achieved

No

practical

limit

either

up or

down

Variable

speed

easily achieved

Practical

limit

is

1200

rev/min; variable

speed

is

easily achieved with engine

Same

as

electric

motor

Complexity

Varies depending upon size

and

system

configuration; synchronous drives

can

be

complex

Moderate,

but if

speed

changes

constantly, belts wear quickly.

If

speed

is

seldom adjusted, grooves

worn

into

cone

pulleys make

adjustment

impossible,

so

specify

chromium plated pulleys

Moderate

to

very complex

Moderate

to

very

complex

Moderate

to

very complex

Very

complex

that

a

failure

in the

main switchyard

(as by a

light-

ning

strike) will

not

occur.

•

Standby engines with either clutches

and

direct

drives

or an

engine-generator.

• Use of

only engines

as

prime movers with

one

engine

and

pump standby. Diesel

fuel

provides

the

utmost

in

reliability. Natural

gas is

much more reli-

able than electrical power.

For

example,

the

only

interruption

in

natural

gas to an

area

in

Southern

California

occurred

in the

1932

Long Beach earth-

quake.

•

Storage tanks

or

reservoirs

for the

time anticipated

to

restore power.

The

time cannot

be

known,

so

consider

a

suitable

safety

margin.

If

standby electrical generation

is

required

for a

pumping

station with

electric

motors

and V/S

drives,

using

engines

and

direct drives (with microprocessors

to

trace

the

system head curve)

is

more economical

in

both

capital

and

operating costs (assuming

a

56-kW

(75-hp)

station

and

energy

at

60/kW

• h for

electricity

and

360/therm

for

gas).

Maintenance

for

engines

is

much

greater than

for

motors,

but the

savings

in

capi-

tal and

operations

is

more than adequate

for the

main-

tenance

and

ultimate replacement

of

engines

(see

Section

8-4 for

engine-generator

requirements, Exam-

ple

9-10

for

engine-generator

sizing,

and

Chapter

14

for

engine requirements).

Number

of

Pumping

Units

versus

Capacity

of the

Station

Optimize

the

size

of the wet

well

and the

number

of

pumping

units

so

that

the

permissible number

of

starts

per

hour

is

never exceeded. Consider these factors:

•

Present average

and

peak

flows

•

Ultimate design average

and

peak

flows

•

Time period until

the

ultimate design

flows are

obtained

•

Desirability

of

adding pumping units

in the

future

•

Size

and

number

of

pumping units required

•

Size

of

standby power unit(s) needed

to

operate

the

station.

In

general,

the

minimum number

of

pumping units

is as

follows:

• One

pump

and one

standby

for

small stations,

e.g.,

<160

m

3

/h

(<700

gal/min)

• Two or

three pumps plus

a

standby

for

medium

sta-

tions,

e.g.,

160-450

m

3

/h

(700-2000

gal/min)

•

Three

to five (or

more) pumps plus standby

for

large stations. Consider

V/S or a

combination

of

V/S

and C/S

pumps

(see

Chapter

15).

25-8. Station

Auxiliaries

Just

as

much care

is

required

in the

engineering

and

equipment

selection

for

station auxiliary systems

as for

the

main pumping equipment.

In

retrospect, station reli-

ability,

as

determined

by the

ability

of the

main pumping

equipment

to

perform their intended

function,

is

greatly

dependent

on

these systems.

A

host

of

benefits

flow

from

properly engineered station auxiliary systems:

• A

wholesome environment,

free

from

condensation

and

dangerous gasses,

and

affording

protection

for

personnel, equipment,

and

structure alike (ventila-

tion system, Section 25-3).

•

Reliable protection against

flooding

(sump pump-

ing

system).

•

Convenient removal

and

replacement

of

heavy

equipment components (hoisting equipment).

•

Reliable supply

of air (if

needed)

for

instruments

(compressed

air

systems).

•

Reliable supply

of

housekeeping

and

shaft

seal

water

for

cleanliness

and

protection

of

equipment

(utility

water supply).

•

Warning

of

probable system trouble before

it

becomes

a

crisis (system controls

and

alarms).

Some insights into

the

considerations that enter

into design

and

equipment selection

for

these systems

to

provide

the

best investment

are

considered

in the

following.

Ventilation

Guidance

for the

design

of

ventilation systems

for

wastewater

pumping stations

is

provided

by

Chapter

24 and by

NFPA

820.

Hence, this chapter contains

no

further

discussion

of the

subject.

Sump

Pumping

System

Often

given short

shrift

in the

design

of

many

installa-

tions, sump pumping stations provide

a

vital service:

they

keep

the floor

dry.

To

perform this

function

satisfac-

torily, they must

be

able

to

accomplish

the

following:

• The

sump pumping station force main must

dis-

charge with

an air

break above

the

highest possible

water

surface

at the

point

of

disposal

for

station

drainage.

The

best place

is at

some location where

isolation

from

the

station

is

possible

—

for

example,

upstream

from

the

influent

manhole, where

a

sluice

gate

has

been provided,

or

into

the

main pumping

station discharge manhole.

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

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