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

engine room ventilation

air is filtered,

specify

dry-

type

filter

elements. Larger engines require

an air

intake system ducted outside

of the

building

to

avoid

upsetting

the

building ventilation system,

and the

pumping

station designer should

be

responsible

for

such

an air

intake system.

If

turbocharged engines

are

installed,

a

separate silencer

is

needed

to

control

intake noise.

Figure

14-1.

Radiator

cooling.

If the

radiator

is

remote from

the

engine,

the fan is

motor

driven;

if the

radiator

is at

the

engine,

the fan is

engine

driven.

The

thermostat controls

the

temperature

of the

water leaving

the

engine.

The

water circulating

pump

is

engine driven. After Brown

and

Caldwell

Consultants.

Figure

14-2. Heat-exchanger

cooling.

With

the

proper heat exchanger,

the

cooling

water

can be the

flow

from

the

pumping

station

and

need

not go to

waste.

The

thermostat maintains

the

engine water outlet temperature.

The

exhaust

silencer

can be a

heat exchanger

or a

boiler

for

heat reclamation.

After

Brown

and

Caldwell

Consultants.

14-13.

Exhaust

Silencing

Exhaust silencers must

be

selected specifically

for the

application because

of

engine back-pressure limitations

and

should, therefore,

be

furnished

by the

engine manu-

facturer.

Chambered, ported types appear

to be the

best.

There

are

four

different

grades

of

effectiveness:

•

Industrial

—

minimal

sound attenuation

•

Commercial

—

suitable

for

nonresidential

areas

•

Residential

—

applications

where

a

high degree

of

silencing

is

required

•

Critical

—

maximum

silencing

for

locations near

hospitals

and

very quiet residential areas.

Do not

rely solely

on the

silencer manufacturer's

rating

for the

equipment. Instead, specify noise reduc-

tion

by

octave band.

In

critical

applications, specify

a

result, that

is, the

noise

level

on the

decibel-

A

scale

(dbA)

at a

specific distance

from

the

exhaust.

An

acoustical engineer should

be

consulted

in

such

instances (see also Chapter 22).

Because

of

their design, exhaust heat boilers provide

a

high degree

of

silencing

and

approach residential

quality. Specify inspection ports

and

drain plugs

for all

silencers

and for the gas

side

of all

exhaust heat boilers.

(Davited inspection doors

are

preferred

for

boilers.)

Stainless steel (type 347)

may be

advisable

for

silencers

serving engines that operate only intermittently.

14-14.

Pollution

Control

Federal regulations

and

many local

air

pollution control

codes require some

form

of

exhaust emission control.

Figure 14-3. Ebullient

cooling

system.

In the

steam

condenser,

steam

is

condensed

and

returned

to the

separator (the

conversion

is 950

Btu/lb

of

steam).

In the

steam separator, heated water flashes

to

steam (the conversion releases

950

Btu/

Ib of

water).

The

asterisks

(*)

indicate

the

components

that

are

necessary

to

reclaim engine waste heat.

If

heat

reclamation

is

not

desired,

the

steam

condenser,

the

steam separator,

and the

exhaust

boiler

can be

eliminated,

and the

system

will

operate

by

boiling

off

water,

but

provide

for

blowdown

of

precipitated

solids.

After Brown

and

Caldwell

Consultants.

Pollutants

of

interest include

particulates,

and

oxides

of

sulfur

(SO

x

)

and

nitrogen

(NO

x

).

Many control technol-

ogies have

been

attempted

—

with

mixed

results.

Generally speaking, exhaust converters using

noble metal catalysts have

not

worked well even when

using

a

clean-burning

fuel

such

as

natural gas. Turbo-

charged engines, using

a

very lean mixture

in the

combustion chamber, have achieved better (but

not

reliable) performance.

The

reason

is

that many

of the

systems utilize sensors that

are

located

in

hostile envi-

ronments such

as

combustion chambers

and

exhaust

gas

streams,

and

they depend

on

sensitive instrumen-

tation

systems. Maintenance costs

are

high because

of

the

labor required

to

keep

the

equipment properly

tuned

for

optimum emission performance.

Emerging technologies using parameters easily

measured outside

the

combustion chamber

and

exhaust

gas

stream, such

as

fuel

charge temperature

and

pres-

sure, engine torque,

and so on,

promise improved per-

formance

and

reduced maintenance costs. Some

guidance regarding current (January 1996) emission

control technology used

by

various manufacturers

of

turbocharged

engines

is

given

in

Table

14-7.

14-15.

Vibration

Isolation

Depending

on the

application,

the

subsoil conditions,

and

the

nature

of the

building housing

the

engines,

the

following

alternatives

can be

used

to

isolate engine

Table

14-6.

Recommended

Governor

Types

for

Engines

Application

Governor type

Generation

Standby

generator (single Droop, hydraulic

unit)

Standby

generator Isochronous, with

(two,

in

parallel) synchronizing

feature

electronic/hydraulic

Continuous

duty

multiple Isochronous, with

units

or

cogeneration synchronizing

feature

with

utility

electronic/hydraulic

Direct drive

Constant

speed Droop, hydraulic

Variable

speed Electrohydraulic

or

pneumatic/hydraulic

vibrations

from

the

building

(in

descending

order

of

isolation

efficiency).

See

also Chapter

22.

•

Separate foundation, with vibration-absorbing

filler

between

the

building

and the

engine foundations

•

Heavy

inertia

block,

mounted

on

energy-absorbing

material

or

spring-type

isolators

•

Spring-type energy-absorbing isolators mounted

on

a

heavy engine subbase (the subbase

may be

filled

with

concrete

to

lower

the

center

of

gravity)

•

Cork

and

elastomer type energy-absorbing sand-

wich

material under

the

engine subbase.

Controls

Start-up

logic

Cranking

routine

Actuation

of

associated remote

devices

Status

Shut-down

logic

Actuation

of

engine-mounted

and

remote

devices

Status

Emergency

shut-down logic

Actuation

of

engine-mounted

and

remote devices

Status

Manual

start-up/shut-down

Timer

and

logic

for

lube

oil

priming

pump

Timer

and

logic

for

remote devices

such

as

cooling

fans

and

pumps

a

Initiates

emergency

shut-down.

Monitors

Oil

temperature

and

pressure

Jacket

water temperature

Engine speed

Intake

manifold

vacuum

(gas engines)

Cylinder

exhaust temperature

(larger,

continuous-duty engines)

Main

bearing temperature

(larger, continuous-duty engines)

Vibration

(larger engines)

Alarms

Low

oil

pressure

3

Jacket

water temperature (radiator

and

heat

exchanger cooling

only)

a

Low

water

level

8

High

oil

temperature

Cylinder exhaust temperature

(large,

continuous-duty engines)

Vibration

(larger

engines)

a

Overspeed

a

Table 14-5.

Recommended

Engine

Controls,

Monitors,

and

Alarms

Table

14-7.

Fuel

and

Exhaust

Emission

Control

Systems

System/types

Manufacturer

Fuel

Exhaust

emission

Caterpillar

CI/OC

CSfES

Cooper

CI ES

Duete

MWM DT FS

Jennbacher

DT

FS/T

Waukesha

CI/DT

ES

CI

=

Combustion chamber injection

DT =

Draw-through

OC =

Open

chamber/carburetion

ES =

Exhaust sensors

CS

=

Combustion chamber sensors

FS =

Fuel

charge

sensors

T =

Output

torque

Before

choosing which vibration isolation system

to

use,

carefully

evaluate

the

natural frequency

of the

structure

and the

effect

transmitted vibration

may

have

on

sensitive electrical

and

electronic equipment.

Because unbalanced forces

in

engines vary widely

between types

and

manufacturers,

the

vibration isola-

tion

system should

be

made

a

part

of the

engine manu-

facturer's

responsibility.

Specify

the

maximum

transmissibility

and the

maximum amplitude

for the

engine during operation

and on

start-up.

14-16.

Lubrication

Oil

Storage

and

Supply

Oil

should

not be

stored

in

large volumes inside build-

ings

because

of fire

codes

and

insurance costs.

Automatic

lubrication oil-makeup units

are

designed

to

sustain only modest

pressures,

so

direct pumping

is

not

possible.

The

solution

is to use a

small

day

tank

similar

to the

30-gal

day

tank

for

fuel,

as

illustrated

in

Figure 14-4.

The

system operates

on a fill-and-draw

basis. Waste

oil is

pumped

to a

storage tank

by

means

of

a

portable, air-operated pump.

For

large

engines,

use

a fixed

pump

and

permanently connected

oil

drain

piping.

Do not use

galvanized (zinc-coated) piping

or

appurtenances

in any

portion

of the

lubrication

oil

storage

or

supply system, because

the

zinc

may

con-

taminate

the

oil.

The

building design should incorpo-

rate provisions

to

contain

any

spills caused

by day

tank

overflow

or

rupture.

14-17.

Fuel

Oil

Storage

and

Supply

Diesel

fuel

injection systems

are not

designed

for

pumped supply

but

must, instead, draw

fuel

by

gravity

from

a day

tank.

The

system shown

in

Figure 14-4

ensures

the

constant supply pressure required

for

proper injector operation.

A

small quantity

of

excess

fuel

must

be

drained back

to the

fuel

storage tank

from

the

engine

fuel

oil

system when

the

engine

is in

operation. Never

use

galvanized (zinc-coated) piping

or

appurtenances

in any

portion

of the

fuel

oil

storage

or

supply system.

14-18.

Gaseous

Fuel

Storage

and

Supply

The

design

of

storage

and

supply systems

for

gaseous

fuels

is

governed

by

NFPA

37 and

NFPA

58. In

gen-

eral,

the

local utility's storage

and

transmission sys-

tem

provides adequate service

for

natural

gas and no

separate storage system should

be

necessary.

If a

pres-

sure-charged engine

is

allowed, however,

a

separate

pressure-boosting system

may be

required.

Be

particularly

careful

when designing gaseous

fuel

systems,

because

the

effectiveness

of

carburetor sys-

tems depends

on

stable

fuel

supply pressures.

If

possi-

ble, design biogas systems that

do not use

compression

and

storage systems

for the

following reasons:

• The

power required

for

compression

and the

initial

cost

of

compressors

and

storage vessels reduces

the

economic advantages

of

biogas.

•

Because biogas

is

contaminated with liquids,

sulfur

dioxide,

and

particulate

matter,

the

condensate

causes corrosion

and

other maintenance problems

in

compression equipment.

The

gaseous

fuel

supply

to

naturally aspirated

engines

may be

troublesome because

of the

relief

valve required

in

paragraph 4-21

of

NFPA

37. The

relief

valve

is

extremely

sensitive

and may

leak,

which

results

in the

loss

of

fuel.

Be

aware

of

this par-

ticular provision

and be

prepared

to

deal with

it. One

obvious solution

is to

employ

a

fuel

transport pressure

of

less than

3.5

kPa

(0.5

lb/in.

2

)

gauge within

the

engine room.

1

4-1

9.

Service

Piping

Care must

be

exercised

in the

design

of the

service

piping that connects

the

engine

to

various piping sys-

tems. Examples

of

service piping include exhaust,

fuel,

lubrication oil, starting air,

and

cooling water.

The

engine

is a

source

of

vibration,

so flexible

pip-

ing

sections

(or flexible

metal hose) should

be

pro-

vided

at

each engine connection.

The

piping

on

both

sides

of the flexible

section should

be firmly

anchored

to

prevent

the

transmission

of

motion that could cause

the

joints

or flexible

hoses

to

fail.

Also observe

the

following

considerations:

•

Install

the flexible

hose

as

close

as

possible

to the

exhaust

outlet.

•

Avoid

fittings or

lengths

of

pipe between

the

engine

exhaust

connection

and the flexible

hose;

the

weight

could damage

the

exhaust manifold.

• Try to

make

flexible

piping connections

as

close

as

possible

to the

engine crankshaft.

•

Install

flexible

sections parallel

to the

engine crank-

shaft

to

increase their service

life.

•

Require

flexible

piping sections

to be

designed

for a

specific

number

of

full

displacement cycles

(10,000,000

is

suggested).

•

Install isolating valves

and

automatically controlled

valves,

such

as

fuel

and air

valves,

in the

rigid

gas

piping upstream

from

any

pressurized

flexible

sec-

tion

to

avoid

the

escape

of

fuel

to the

engine room

if

the flexible

section

fails.

• Use

braided stainless-steel hose rated

for a

working

pressure

at

least twice

as

great

as the

system's max-

imum

pressure.

•

Provide

flexible

duct connectors between engine-

mounted radiators

and

built-in exhaust louvers.

14-20.

Building

Envelope

When designing structures

for

engine installations,

consider these caveats:

•

Avoid

the

transmission

of

engine vibration

by

con-

ducting

a

thorough analysis

of

each building ele-

ment

and

eliminating harmonic natural frequencies.

•

Provide

a

small room

for

engine maintenance tools

adjacent

to the

engine installation.

• If

permitted

by the

project budget, conduct

a

sound

spectrum

analysis

and

provide noise-control features.

In

critical applications where building noise emis-

sions

may be

objectionable, employ

an

acoustical

consultant.

Coordinate

the

acoustical control

efforts

with

the

ventilation system

and

architectural design.

•

Avoid crowding engines.

A

clear passage

of 2 to 2.4

m

(6 to 8 ft)

between adjacent units

is

suggested.

•

Provide some type

of

hoisting means (e.g.,

a

bridge

crane

or a

rolling gantry with chain hoist)

to aid in

engine maintenance.

• Be

aware

of

special engine maintenance clearance

requirements.

One

example

is the

vertical distance

above

the

heads that

is

necessary

to

pull

the

pistons

on

some engine designs.

Figure

14-4.

Schematic

for a

lubricating

oil

supply

system.

The day

tank

is

less

than

30 gal and is

located

high

in the

superstructure.

The

float

switch

controls

the

pump

on a

fill

and

draw

basis.

After Brown

and

Caldwell

Consultants.

•

Design

for

explosion venting (see NFPA 37).

•

Except

for gas

compressors, explosionproof

electri-

cal

devices

for

engine rooms

are not

required

by

any

code.

Engine rooms, however, should

be

designed

for

explosion venting when

gas

engines

are

used (see NFPA

37 and

NFPA 68).

•

Provide

a

means

for

draining

the

crankcase.

A

por-

table air-operated pump

to

remove

the oil

from

the

engine sump

is

preferred.

If

supplied

from

a fixed-

service

air

system,

the air can

also

be

used

for

other

purposes

(e.g.,

torque wrenches

and

other air-driven

tools).

14-21.

Ventilation

systems

for

engine rooms should

be

designed

to

provide

the

following:

•

Balanced room pressure

•

Powered intake

and

exhaust

for

continuous-duty

applications

• A

maximum

room temperature

of

43

0

C

(1

1O

0

F)

•

Positive

air flow

along each side

of

each engine

(which

is

necessary

for

personnel comfort)

• Air flow

from

driven equipment (pump, gear, gener-

ator)

directed toward

the

engine

• 10% of the

total

air flow

exhausted through

the

roof

to

clear

out

smoke

and

fumes;

the

rest should

be

exhausted

at the

wall

at the

rear

of the

engine

•

Adequate engine combustion

air if

engine-mounted

air

intake systems

are

provided

—

a

variable that

depends

on the

engine load

and the

number

of

engines

in

operation

•

Filtered

air for

continuous-duty applications, tem-

pered

air

only

to

13

0

C

(55

0

F)

in the

winter; cooling

is

necessary only

in hot

climates.

14-22.

Maintenance

The

manufacturer's

instructions

for

maintenance

should,

of

course,

be

followed

exactly,

but a

designer

contemplating

an

engine drive should recognize

the

owner's involvement.

The

maintenance

of

engines

depends

on

whether

(1) the

engine

is

used

as the

prime mover

and is,

thus,

in

regular operation

or (2)

the

engine

is on

standby

and

(including exercising)

operates,

on

average, only about

50

h

per

year.

Prime Movers

The

life-cycle costs

for a

continuous duty diesel

engine

are

compared

with

those

for a gas

engine

in

Tables 14-8

and

14-9.

The

total costs

are

based

on an

operating

period

of 20

years.

Standby

Engines

Standby engines operate

so

seldom that maintenance

depends

on

time

as

well

as

wear. Maintenance tasks

for

a

125-kW

diesel

engine-generator

set are

shown

in

Table

14-10.

On a

yearly contract,

the

total cost

of

maintenance labor

and

parts would

be

about

$700/yr

if

an

allowance

of 2 h

driving time

is

assumed

per

yearly

visit.

There

is a

considerable

difference between

the

maintenance

of

light, standby units

and

continuous-

duty

prime movers.

For

example,

the

service interval

for

valve overhaul

in a

heavy-duty engine

is 20

times

as

long

as the

interval given

for the

light-duty engine

in

Table

14-

10.

Exercising Engines

Instructions vary,

but the

following recommendations

are

good practice

for

modern engines.

Continuous-Duty

Engines

At

least once every

6

months,

all

engines should

be

operated

at the

rated load

for at

least

5 to 10 min

after

reaching operating temperatures

to

ensure that

the

engine

is

capable

of

developing

the

required rated

power.

Standby

Service

Every

6

months

to a

year,

all

engines should

be

full-

load tested

as

follows:

Operate

at 1 or 2 min at no

load,

5 min at 25%

load,

5 min at 50%

load,

5 min at

75%

load,

and 5 min at

rated load.

Engines should

be

no-load exercised

at

least once

each month.

The

engine should

be run

just long

enough

to

ascertain that

the

starting

and

control sys-

tems

are

operating properly

and

that proper

oil

pres-

sure

is

obtained. Excessive no-load operation causes

the

exhaust system

to

"slobber"

—

a

problem that

is

mostly cosmetic (messy). Slobber,

an

oily discharge

from

the

manifold stemming

from

fuel

that

is

partly

unburned,

is

especially noticeable

in

modern direct-

injected

diesel engines.

The

exercising

can be

done

by

an

automatic timing device installed

in the

trans-

fer

switch. These devices

are

usually

1-week

cycle

timers.

There

are

2-

week

cycle timers,

but

they

may

not

be

available

for

most switches. Many engineers

dislike automatic

exercisers,

because

(1) an

operator

should

be

listening

to an

engine while

it is

exercised,

(2)

the

maintenance crew

may

depend

too

much

on

automation

and

neglect

the

trouble-forestalling

inspection

of the

running engine,

and (3)

very

few

automated exercising systems

can

adequately exer-

cise

the

engine under load.

The

owner should

be

con-

sulted

about automatic versus manual control

of

exercising.

Manufacturers'

Recommendations

Although

the

above exercising instructions comprise

one

manufacturer's

recommendation

for

modern engines,

there

are

many other makes

and

models

and

many older

engines. Owners should

be

advised

to

follow

the

maker's

recommendation

for the

particular engine model

and the

particular circumstances

of

service.

Table

14-8.

Data

for

Diesel

and

Natural

Gas

Engines

3

'

6

Item

Diesel engine

Gas

engine

Size

of

engine,

hp

1150 1150

Engine output,

kW 820 820

Engine speed, rev/min

1800 1200

Total engine price,

$

158,000.00

268,000.00

Down

payment,

$

58,000.00

68,000.00

Loan interest rate,

%

10.00 10.00

Inflation

rate,

%

4.00 4.00

Months

payment

due

120

Taxes,

$

9,875.00

16,570.00

Insurance,

$

0.00 0.00

Operation, h/yr 8000

8000

Fuel

No. 2

diesel

Dry

natural

gas

Fuel

price

1.00

$/gal

0.35

ft

3

Fuel consumption

at

average load

59.06

gal/h

9500

ft

3

/h

Oil

10W30

SAE 30

Oil

consumption

at

average load

0.14

gal/h

0.05

gal/h

Oil

change interval

500.

h

1000.

h

Sump

size,

gal

81.1 111.7

Overhaul interval

Overhaul

1

(top end)

6530.

h

16,006.

h

Overhaul

2

(major)

13,061.

h

48,017.

h

Overhaul

3

80,002.

h

a

Courtesy

of

Caterpillar,

Inc.

b

Prices

effective

June

1997.

ENRCCI

=

5700.

Table

14-9.

Financial Summary

for

Twenty

Years

of

Operations^

Diesel

cost

Natural

gas

cost

Item

$

$7h

tf/kW-h

$

$7h

tf/kW-h

Selling price

158,000

0.99 0.12

268,000

1.68 0.20

Interest 58,581

0.37 0.04

117,162

0.73 0.09

Taxes

9,875

0.06 0.01

16,570

0.10 0.01

Insurance

(omitted)

— — — — — —

Fuel

9,450,000

59.06

7.20

5,320,000

33.25

4.05

Oil

288,500

1.80 0.22

151,400

0.95 0.12

Preventive

maintenance 241,500

1.51 0.18

153,900

0.96 0.12

Components

567,500

0.36 0.04

107,600

0.67 0.08

Overhauls

937,900

5.86 0.72

295,800

1.85

0.23

Total cost

11,202,000

70.01 8.54

6,430,000

40.19 4.90

With

inflation

16,584,000

103.65

12.64

9,393,000

58.71 7.16

a

Courtesy

of

Caterpillar,

Inc.

b

Prices

effective

June

1997.

ENRCCI

=

5700.

Table

14-10.

Typical

Maintenance

Schedule

for a

125-kW

Standby Diesel

Engine-Generator

3

Frequency

Interval

Maintenance

task Operating time Calendar time

Inspect

fuel,

oil

level, coolant

8 h 1

mo

b

Inspect

air

cleaner, battery

5Oh

1 yr

Clean governor linkage,

breather,

air

cleaner

100

h 1 yr

Clean

fuel

filter,

replace

oil

filter,

change

crankcase

oil, check switchgear

200 h 1 yr

Clean commutator, collector

rings, relays, cooling

system; inspect brushes,

valve

clearances, starting

and

stopping systems,

water

pump

500 h 1 yr

Check injectors, grind valves

(if

required), remove

carbon, clean

oil

passages,

replace secondary

fuel

filter,

grease bearings,

clean generator,

100Oh

—

a

Courtesy

Onan

Corporation.

b

mo,

Month.

Batteries

Starting

batteries must

be

reliable

and

capable

of five

10-s

start attempts without

a

loss

of

cranking power.

Maintenance-free

batteries

are

recommended with

either

a

two-stage trickle charger

or,

better,

a float

charger that

is

used

to

sense

the

charge

on the

battery

and

to

limit

the

charging current accordingly.

Heaters

Moisture

is the

enemy

of

electrical equipment, includ-

ing

generators. Install space heaters inside

the

genera-

tors

to

keep them

as dry as

possible. Install heaters

on

the

engine cooling water

to

keep

the

engine warm.

Use a

diesel

fuel

that

is

compatible

with

the

ambient

temperature

at the

storage location.

In

this chapter,

the

uses

of

variable-speed (V/S) pump

operation

in

water

and

wastewater pumping stations

are

presented

and

discussed.

The

principal adjustable-

and

variable-speed drives

are

reviewed

and

compared.

The

primary goal

of V/S

wastewater pumping

is to

keep

the

station discharge rate equal

to the

influent

rate

at

all

times.

The

station acts

as

though

it

were

a

sewer

discharging

by

gravity.

A

pumping station that meets

this

goal requires very little

(if

any) storage

in the wet

well,

does

not

rapidly cycle

any of the

pumps,

and

does

not

cause abrupt, large changes

in the

discharge rate.

The

primary goal

of V/S

booster pumping

is to

maintain

a

nearly constant pressure

at all

times

regardless

of the

demand.

The

discussion given here

for

V/S

booster pumping also applies

to

high-

service

(high

head) pumping

for

reducing water hammer

in

very

long transmission mains. Otherwise,

V/S is

rarely

used

for

water pumping.

Chapter

1

5

Variable-Speed Pumping

MAYO

GOTTLIEBSON

ROBERT

L.

SANKS

CONTRIBUTORS

Stefan

M.

Abel

in

Roberts.

Benfell

Harrison

C.

Bicknell

Ronald

W.

Duncan

Stanley

S.

Hong

Casey Jones

Paul

C.

Leach

Timothy

Nance

Michael

R.

Olson

E.

O.

Potthoff

Arnold

R.

Sdano

Richard

N.

Skeehan

Earle

C.

Smith

Patricia

A.

Trager

In

common usage,

the

term

"variable

speed"

indi-

cates that

the

speed

of the

pump

is

regulated

to

pro-

duce

the

desired discharge.

Electrical

engineers,

however,

consider "variable

speed"

to

mean that

the

motor speed varies with

the

load, while

the

term

"adjustable

speed"

means that

the

speed

is

nearly

independent

of

load.

An

example

of an

adjustable-

speed drive

is an

adjustable-frequency drive (AFD),

sometimes mistakenly called

a

"variable-frequency"

drive.

An

example

of a

variable-speed drive

is a

liquid

rheostat

in

which, although speed varies with both

load

and

control,

the

proper speed

can be

attained

by

the

external control. Thus, pump speed

can be

varied

at

will

by

either

an

adjustable-

or a

variable-speed

drive.

In

this chapter, both kinds

of

drives

are

called

"variable

speed"

or

"V/S"

for

convenience,

but

when

communicating

with electrical engineers,

confusion

can

be

avoided

by

strict usage

of

correct terminology.

Applications

to

both water

and

wastewater pump-

ing

are

described

in

Sections 15-1, 15-3,

and

15-11.

Sections

15-2

and

15-4 through 15-6

are

generally

devoted

to

wastewater pumping,

and

Sections 15-7

through

15-10

are

confined

to

water booster pumping.

15-1.

Variable

Speed versus Constant Speed

The

difference

between

V/S and

constant-

speed

(C/S)

pumping

is

profound

and has

far-reaching conse-

quences,

so the

choice should

not be

lightly made.

Some reasons

for the

choice

of V/S or C/S for a

partic-

ular

pumping station

may be

compelling while

others,

which

may be

significant

for a

different

situation,

may

be

trivial. Nevertheless,

all of the

reasons should

be

considered thoughtfully.

Advantages

of

Variable-Speed

Pumping

The

major

advantages

of V/S

pumping, stemming

from

the

objective that discharge always equals

influ-

ent

flowrate,

are:

• No

storage

is

required,

so the wet

well

can be

smaller, shallower,

and

less expensive than

for C/S

pumping,

and

size might compensate

for the

high

cost

of the V/S

equipment.

•

Fewer

—

although

larger

—

pumps

are

needed

in

medium-

to

large-sized stations with savings

in

space

for the

pump room

and

less equipment

to

maintain.

•

Downstream

flowrates fluctuate

slowly

and do not

upset

treatment processes caused

by

surges

nor do

the

sudden surges caused

by C/S

pumping pound

downstream

piping.

• The

almost constant

wet

well surface

and

short

resi-

dence time tend

to

reduce

the

production

of

odors

and

corrosive gasses.

•

Energy savings

are

likely

due to the

higher average

wet

well surface elevation

and the

usually lower

pumping

rates

and

lower pipe

friction

losses.

• If

adequate cooling

is

provided, motor

life

is

pro-

longed because

of the

infrequent

starts.

• The

high inrush current that characterizes across-

the-line

starts

of C/S

drivers

is

greatly reduced

in

adjustable-frequency

drives (AFDs)

and

liquid

rheostats (although

not in

eddy-current couplings).

• The

soft

and

reduced number

of

starts

do not

cause

annoying

power dips

in

nearby buildings. Power

dips caused

by

across-the-line

starts

for C/S

pumps

can be

avoided with

soft

start equipment,

but the

cost

of

such equipment makes adjustable-frequency

drives

more attractive economically.

Throttling

the

engine

of a

direct engine drive

is a

simple, inexpensive,

and

reliable means

of

achieving

V/S,

but

read Sections 14-2

and

14-22

and

consult

the

engine manufacturers regarding

the

problems

of

oper-

ating

diesel

or gas

engines

at

less than

full

load.

Wastewater

The

reasons

for

using

V/S

pumping

for

wastewater

include

the

following:

•

Continuous pumping

and the

very short liquid resi-

dence time

in V/S

operation eliminates

or

reduces

(1) the

deposition

of

organic

solids

—

a

problem

in

C/S

pumping because

of the

difficulty

of

resuspen-

sion;

(2)

putrefaction

—

so

the

sewage

is

easier

and

less costly

to

treat;

(3) the

production

of

odors

and

corrosive, poisonous gases;

and (4)

cyclic rise

and

fall

of

the

water

surface

with

the

resulting pumping

of

sewer gases into

the

atmosphere.

• V/S

pumping eliminates

the

long,

free

fall

of

sew-

age and the

entrainment

of air

bubbles that,

if

sucked into

the

pump, greatly reduce pump

effi-

ciency, although this advantage disappears

in a

comparison with

C/S

pumping

if a

sloping

approach

pipe

is

used (see Section 12-7).

• The

average static head

in C/S

pumping

can be

reduced

by 0.6 m (2 ft) or

more

in V/S

pumping

by

keeping

the wet

well level

the

same

as the

normal

level

in the

influent

sewer (see Section 12-6

and

Figure 12-35). Turbulence

is

thereby virtually elim-

inated,

and the

release

of

odorous gases

is

substan-

tially reduced.

The

energy cost savings

due to the

consistently high

wet

well level

may be

significant.

• One

important reason

for

using

V/S

pumping

is to

produce gradual changes

in flow

that

do not

upset

(and

thereby reduce

the

efficiency

of)

downstream

processes

such

as

sedimentation. Sudden

flow

changes caused

by C/S

pumps, even

at

significant

distances

from

a

treatment plant,

can

upset

the

entire treatment

process.

At one

utility,

it was

found

that

such

flow

changes propagated through

the

entire primary/secondary treatment

process

and

were upsetting

the

dechlorination

system. Forcing

the

operators

to use the V/S

drives (that

had

been

installed but,

due to

indifference,

not

used) solved

the

problem.

Water

is

customarily pumped

for

long periods

(of

sev-

eral hours)

at

uniform rates

(often

to fill

reservoirs

or

tanks),

so

there

are

fewer

reasons

for

choosing

V/S

for

pumping water than

for

pumping wastewater,

although

they

may be

just

as

compelling.

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

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