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

speeds.

Calculate

the

actual pump

shaft

input power

(bhp)

based

on

pump

efficiencies

•

Motor

efficiencies

reduced

for (1) low

speeds,

(2)

low

torques,

and (3)

non-sine-wave input

(-3%)

•

Converter

and

transformer

efficiencies

for the

actual loads

•

Overall drive

efficiency

at

each

of

several speeds

obtained

as the

product

of

pump, transformer, con-

verter,

and

predicted

motor

efficiencies

•

Wire power (input power

to the

motor) obtained

by

dividing

the bhp by the

overall drive

efficiency

•

Power needs calculated

from

hours

per

year

of

operation

at

various discharge

and

pump speeds

(see Example

29-1).

Compare

various

types

of

drives

for

each

specific

application

on the

basis

of the

present worth

of

cost,

life

and

replacement,

maintenance,

and

power (per

Example

29-1).

The

results

cannot

be

predicted

reli-

ably

if

any

of

these factors

is

ignored.

Motors

Standard

motors cannot

be

used with

any AF

drive.

All

drive types impress

a

non-sinusoidal wave

form

on

the

motor.

As

motor speed decreases, motor

cool-

ing

provided

by

shaft-driven

fans

decreases,

but

cool-

ing

is not the

only consideration when applying

adjustable-frequency

drives. Rapid change

in

voltage

versus

time caused

by

some types

of

drives

has

been

the

source

of

rapid insulation

failure

in

some motors.

Furthermore, stray currents, apparently induced

by

some variable-frequency drives, have resulted

in

elec-

trolytic corrosion

of

bearings

and

other components.

To

break

the

circuit

of

these stray currents,

a

thin,

dielectric

material

can be

specified

for

electrically iso-

lating

the

shaft

from

the

motor

frame.

Not all

prob-

lems associated with this technology have been

solved,

but

consensus standards

for

motors

for

appli-

cation

to AF

drives

are

evolving.

The

claim frequently made

by

drive manufacturers

that

standard motors

can be

used

is not

true.

As it

hap-

pens,

AF

converter application

to

existing motors

is

frequently

successful, because motors

are

often

over-

sized

for the

application,

and

older motors were fre-

quently

designed more conservatively than

are

mod-

ern

motors.

Motors

for use

with

AF

drives must

be

specially

designed

for

this service.

For

example, such motors

are

constructed with stator wires double-dipped

in

var-

nish (wires

in

standard motors

are

single-dipped); thin,

paperlike insulation

is

added

in the

slots;

and

Class

H

insulation

is

used

for

Class

A

temperature

rise.

Also,

as

stated above,

it is

wise

to

insulate

the

bearings

from

the

motor

frame

to

interrupt stray currents

and

thus

prevent electrolytic corrosion.

Be

very

conservative

in

specifying

motors

for use

with

AF

drives.

Step-output converters

and PWM

converters that

do not use

IGBT converters

are

generally satisfactory

with

existing

1.15

service

factor

motors driving cen-

trifugal

pumps

if the

motor

is not

loaded above

the

nameplate rating (service

factor

not

used). However,

both

the

foregoing

AF

converters

are

less

efficient

and

more expensive than modern converters.

IGBT converters switch their output

at a

very high

frequency

(Figure

15-24b).

They impress

a

high rate

of

voltage change (dV/dt),

and the

switching action

also overshoots, thereby impressing high peak volt-

ages

on the

motor. Both characteristics

put

greater

stress

on the

motor insulation than does

a

sine wave.

The

motor insulation system must either

be

designed

for

this stress,

or an

output reactor must

be

inserted

between

the AF

converter

and the

motor. Specially

designed

"power

converter duty" motors

are

more

desirable

because

a

reactor

reduces

the

efficiency

of

the

drive system

by

about

1%.

Also, always locate

the

converter within

45 m

(150

ft) of the

motor, because

long motor leads

magnify

the

peak voltages produced

by

the

converter.

If the

drive cannot

be

located

close

to

the

motor, always

use a

reactor between

the

converter

and

the

motor.

Power converter duty motors have insulation sys-

tems designed

for the

stresses imposed

by

AFDs.

In

addition

to the

usual motor parameters that must

be

specified

when

ordering

a

converter-duty motor,

the

speed

range

and

load speed/torque

characteristic

must

be

specified.

The

motor manufacturer will then design

the

motor cooling

for the

application. Motor

manufac-

turers catalog motors

for

speed ranges

as

great

as

10:1

when

driving constant torque

loads

—

a

much more

severe service than driving

a

centrifugal pump over

a

typical

2:1

speed range.

Operational Problems

Any

short interruption

of

power

(sufficient

to

make

a

light

flicker) can

trip

an

older AFD,

after

which

the

motor must

be

allowed

to

come

to

rest before restart-

ing

—

a

requirement annoying

in

thunderstorms

and

windstorms,

which could cause several interruptions

per

day. Some modern AFDs, however,

are

designed

to

operate

through such momentary (less than

0.5 s)

power

losses,

and

AFDs

can

even

be

designed

to

syn-

chronize with

a

spinning

motor

—

but

these

features

must

be

specified

if

they

are

wanted,

and

they increase

the

cost

of the

AFD.

Designing and/or specifying

the

interfacing

between

the

motor

and AFD is not a

simple task.

One

way

to

promote compatibility

is to

specify

"unit

responsibility,"

in

which

the

pump, motor, converter,

and

isolating transformer

(if

any)

are all

furnished

by

one

supplier (the pump manufacturer)

who has

full

responsibility

for the

proper operation

of the

entire

pumping

unit (see Chapter 16).

The

pump manufac-

turer

is in no

position

to

warrant

the

compatibility

of

the

motor

and the AF

converter,

so

specify that

the

entire

power package

be

supplied

to the

pump manu-

facturer

by the AFD

manufacturer,

who

must ensure

the

compatibility

of the

motor

and the AF

converter.

Because there

are

torsional pulsations

in the

output

of

a

motor powered

by an AF

converter,

it is

strongly

recommended that

a

torsional analysis

be

done

for any

drive

of

more than

300 kW

(400 hp). Vertical motors

supplied with variable-speed systems

are

subject

to

"reed

frequency" vibration.

The

motor,

the

pump,

and

the

control manufacturers must coordinate

to

elimi-

nate this potential problem (see "Unit Responsibility"

in

Appendix

C).

Certain speeds

may

have

to be

blocked

out of the

variable-speed range. These speeds

must,

of

course,

be

passed through during drive accel-

eration

and

deceleration,

but the

drive must

not be

allowed

to

operate there.

A

second problem that

can

occur

if

there

is an

anti-reversing ratchet

in the

drive

shaft

is

that dwelling

at

certain speeds

can

cause

an

oscillation about

the

vertical axis, hammer

the

ratchet,

and

damage

or

destroy

it.

Adjustable-Frequency

Drive

Specifications

A

summary

of

specifications

and

requirements

for a

reliable, trouble-free

AFD

system includes:

• A

power system analysis.

A

source voltage range

of

+10%,

-5%

from

nominal (usually

460

volts)

is

specified

in

NEMA standards

for AF

drives. Most

electrical equipment

is

designed

for

+10%

from

nominal.

If the

plant electrical system cannot meet

the

tighter tolerance required

by AF

drives,

specify

the

voltage range that

the

drive must accept.

•

Voltage

disturbances. Modern drives have micro-

processor controllers. These controllers

are

capable

of

maintaining drive operation through short power

supply

voltage sags

and

immediately

and

automati-

cally

restarting

the

drive

if the sag

exceeds

the

hold-

up

interval.

Specify

"flying

restart."

•

Grounding. Protect

the

microprocessors

by

specify-

ing

a

ground grid

in

which each electrode, when

isolated,

has a

maximum ground resistance

of 2

ohms

—

a

protection

significantly

better than that

specified

in the

NEC.

•

Motor insulation

and AFD

compatibility.

Static

power converters impose unusual stress

on

winding

insulation. Early insulation

failure

will occur

if the

motor

is not

designed

for

this duty.

For

six-step

inverters,

specify

conformance

to

NEMA

MG

1,

Part

30

(1000 volt peak with

12

jus

rise

time).

For PWM

inverters, specify conformance

to

NEMA

MG

1,

Part

31

(1600 volt peak with

0.1

JLIS

rise

time).

•

Drive tuning.

To

ensure optimum performance,

the

AFD

must

be

tuned

to the

motor

and

load character-

istics.

Some modern drives have

a

"self-tuning"

function

that permits

the

drive

to

adjust

some

parameters

to

motor characteristics,

but no

drive

can

analyze load requirements. Specify that

the

drive

be set up by a

factory-authorized technician.

Do not

rely

on

contractor electricians

to set up AF

controllers.

•

Voltage-vector

control.

For

best performance, spec-

ify

drives with voltage-vector control

to

regulate both

voltage

and

frequency

to

produce

a

sine wave output

for

optimum motor magnetization

and

performance.

•

Isolation.

A

contactor

to

disconnect

a

drive physi-

cally

from

the

motor

is not

necessary,

but

consider

specifying

contactors

to

isolate shunt

filters

when

the

drive

is not

running.

If the

drive

is

equipped

with

a

phase-

shifting

transformer,

specify

a

line

contactor

to

eliminate

the

transformer losses when

the

drive

is not

running.

•

Harmonic currents. Power factor correction

is

usu-

ally

not

required

for AF

drives.

But if

power factor

correction

is

required because other loads

on a

power system have

a

lower power factor, traps must

be

provided

to

keep

harmonic currents

out of the

capacitors.

A

power system analyst

can

design suit-

able trapped capacitor schemes

for

power systems

with

AF

drives.

• A

temperature-controlled environment.

A

maximum

ambient temperature

of

4O

0

C

(104

0

F)

is

specified

in

NEMA standards. Unless

the

space

is air

condi-

tioned,

the

temperature requirement

may not be

met.

AF

drives generate substantial heat. Specify

a

realistic ambient temperature requirement.

If the

space

is to be air

conditioned (mandatory

in

many

geographic areas), obtain realistic heat dissipation

(including

heat

from

transformers) values

for the

drive

from

the

manufacturer,

specify

these

as

maxi-

mums,

and

provide

the

value

to the air

conditioning

system

designer.

•

Standby generators.

If the

drive must

be

powered

by

a

standby generator, harmonic currents drawn

by

the

drive must

be

reduced

to a

minimum. Heavy

harmonic

filtration

with shunt

filters may

result

in a

leading power

factor.

Use

six-phase rectification,

series

reactors, and/or active

filters in

preference

to

shunt

filters to

reduce harmonics.

Be

sure

the

gener-

ator

is

capable

of

handling

the

harmonic current

load

and any

leading power factor resulting

from

any

filters. Be

aware that

the

generator must

be

oversized

to

meet these requirements.

•

Corrosive

gases. Hydrogen

sulfide

in the

atmosphere

must

be

limited

to 3 ppb or

less. Place

the air

intake

on

the

upwind side

of the

building,

and

pass

the air

through

a filter of, for

example, activated carbon.

•

Noise.

An

annoying

hum

(sometimes

an

ear-pierc-

ing

high-frequency

screech)

is

generated

by AF

controllers,

and

motors

are

also noisy,

so

consider

sound-absorbing

walls

and

ceilings

to

reduce

the

reverberation. IGBT controllers, however,

are

quiet

and

emit only

a

slightly noticeable hum.

In

conclusion, power system design

for AF

drives pre-

sents

problems that have been largely ignored

in the

past

and

have resulted

in

many unsatisfactory installa-

tions. Modern

AF

drives

and

proper power system

design should result

in a

problem-free

variable-

speed

pumping

system with

a

life-cycle cost lower than that

of

any

other

V/S

drive.

Emergency

Generators

The

harmonics generated

by the

converter

are fed

back

to the

standby generator and, unless

the

genera-

tor

voltage regulators

are

specifically

designed

for

compatibility with

the

AFDs,

the

generator

may not

operate

the

drive.

The

generator

manufacturer

must

match

the

generator

to the

specific

AFD to be

used.

Maintenance

The

problems

of

repairing AFDs

are

controversial.

Some

manufacturers

claim that diagnostics

and

plug-

in

circuit boards make troubleshooting

and

most

repairs relatively easy. Most circuit boards, however,

are

expensive,

and

untrained, nonelectrical personnel

should

not

touch them. Moreover, replacing

a

thyris-

tor

requires

a

very skilled technician

and the

proper

tools. Nevertheless,

a

competent maintenance electri-

cian

can

service AFDs

if

•

Training

in

power electronics

is

provided

at

start-up

and

on a

periodic basis.

• The

proper tools

are

available.

• A

spare

of

every type

of

printed circuit board

and

power electronic component

is

kept

on

hand. Many

boards, even

for

different

sizes

of

AFDs,

are the

same,

so the

inventory

may not

have

to be

large.

(Circuit

boards

usually

cost between

$750

and

$2500

each;

the

cost

of

repair

is

about half

as

much.)

Power electronic components, though,

are

different

for

different

sizes

of

drives. Another rea-

son

for

keeping spare circuit boards

is

that some

manufacturing

firms

continue

to

drop

out of the

solid-state business,

and

orphan electronics boards

may

be

very costly

to

replace

and

slow

in

delivery.

Routine maintenance

is low and

consists only

of

keeping

the

equipment clean

and all

connections tight.

Other manufacturers state that

all but

simple prob-

lems require

a

factory-authorized technician,

who is

sometimes accompanied

by an

electrical

engineer

at a

charge

of

about

$700

per

person-day portal

to

portal

plus travel time

and

expenses. Repairs

often

take

2 or

3

days.

A

worst-case scenario might

be a day for

travel

and

diagnosis,

a day

waiting

for

parts,

and a day

for

installation

and

start-up, which could total

$4000

or

$5000.

Troubles might occur yearly, more

often,

or

never. Typically,

one

might expect

no

trouble

at all for

2 or 3 yr

after

start-up with only occasional problems

later. Note, incidentally, that

one

lightning strike

can

zap all of the

AFDs,

so

make

sure

that

the

station

is

protected

from

lightning (see Section 8-5).

Some expert consultants with considerable

and

lengthy

experience have

had

nerve-racking problems

with

AFDs,

and

they approach decisions regarding

their

use

with reservations

and

caution

—

especially

for

municipal projects. They

point

to the

problem

of

com-

petition with industry

in

hiring

and

keeping trained

technicians

and the

difficulty

in

procuring spare parts

for

public utilities,

and

they urge that spares

be

included

in the

original

bid

package. Make sure that

no

critical

items

are

omitted.

The

industry

is now

more mature than

it was a

decade ago,

the

equipment continues

to

improve,

reli-

ability

is

better than

it was in the

past,

and new

com-

ponents continue

to

reduce

the

price

of

AFDs.

The

improvements, however, tend

to

make electronic

equipment

obsolete

and

parts

may

become unavail-

able.

It

seems wise

to

plan

on

replacing electronic

equipment every

10 to 15 yr, and

this cost should

be

included

in the

cost analyses

of

alternatives.

The

cost

of

repairs

is

quite uncertain,

but for

economic analy-

ses

of

alternatives,

the

assigned annual cost should

probably

lie

between

5 and 10% of the fob

price

of the

equipment.

There

are

records

of

annual costs well

above

10% and

other records

of

almost zero.

If

C/S

drives

are to be

considered

as an

alternative

to

V/S

drives, include

a

comparison

of (1) the

construc-

tion

costs

of the wet and dry

wells,

(2)

energy usage,

and

(3)

maintenance needs

as

shown

in

Example

29-1.

Summary

When

evaluating

V/S

drives, avoid

the

entrapment

of

com-

paring

only

the

efficiencies.

Instead, compare power losses

in

kilowatts

at

several

of the

loads

and

speeds

to be

imposed.

An

alternative

is to

compute

the

annual energy

requirement

for

various increments

of

discharge

(as

shown

in

Example

29-1),

in

which

the

proper power rate schedule

is

used. Then

add

maintenance, replacement,

and

capital

expenditure

and

reduce

all of the

costs

to

present

worth.

Include isolation

transformers

in the

capital cost.

Finally, consider

the

intangibles

of

simplicity

and

reliability together with

the

owner's situation, such

as

the

number

and

size

of

pumps

and

pumping stations,

the

size

of the

public utility,

the

location (especially

with

respect

to

repair

facilities),

the

cost

of

power,

the

abilities

of the

operators,

the

effects

on

downstream

operations,

and the

qualifications

of the

supervisors.

The

energy savings

in one or two

small drives would

not

justify

the

training

and

spare parts inventory

needed

for

AFDs.

If, on the

other hand, there

are

half

a

dozen large drives, justification

for

using AFDs

(or

any

other

V/S

drive)

is

likely.

The key is to find the

overall economics

for

each

situation.

Eddy-Current

Couplings

The

eddy-current coupling (Figure

15-26)

is a

unit

of

approximately

the

same size

as the

motor

and is

placed

between

the

motor

and the

pump.

The

input

shaft

is

cou-

pled

to the

motor, which

is

usually

a

squirrel-cage

induction

motor.

The

output

shaft,

coaxial with

the

input

shaft,

is

coupled

to the

pump.

A

small control panel

is

electrically connected

to the

eddy-current coupling.

An

eddy-current coupling consists

of two

basic

elements:

(1)

an

iron

ring (or

drum) keyed

to the

input

shaft

and

rotating with

it at C/S and (2) a

multipole

electromagnet keyed

to the

output

shaft

(see Figure

15-27).

The

electromagnet pole

faces

are

very close

to

the

inner surface

of the

iron ring,

and the

magnets

are

energized

by

low-power

dc

current

from

the

control

panel through brushes mounted

on the

frame

and

slip

rings mounted

on the

shaft.

When

the

driving motor

is

started

and the

iron ring

revolves,

the

electromagnet

is

energized through

the

brushes

and

slip

rings.

Electrical currents (eddy cur-

rents)

in the ring are

induced

by the field of the

electro-

magnet,

and

they create magnetic

fields. For

brevity,

it

can

be

considered that

the

electromagnet

is

driven

by the

magnetic

fields in the ring. The

difference

between

the

rotational speed

of the ring and

that

of the

electromagnet

is

called

"slip."

The

amount

of

slip depends

on the

mag-

nitude

of

electrical current through

the

electromagnet.

Efficiency

The

efficiency

of the

drive

is

determined

by

three factors:

•

Motor

and

coupling

friction

and

windage losses

•

Motor core, winding

and

stray losses

•

Slip

loss,

which

is by far the

major

loss

•

Direct current supplied

for

coupling's

field

excitation.

The

slip

loss

is the

power

lost

in

speed

reduction

due to

slippage

of the

electromagnet with

respect

to

the ring.

Because

of low

friction

and

windage

in the

coupling,

the

output torque

is

only slightly less than

the

input torque.

If

friction

and

windage

losses

are

neglected,

the

input

and

output torques

are

equal.

Hence,

the

slip

efficiency

(power out/power

in) is

e

s

=

f

^

1

,

=

"°

(15-4)

2Tt^r

H

1

where

n

0

is

output

shaft

(pump) speed

and

n

{

is

input

shaft

(motor rated)

speed.

Thus,

the

slip

efficiency

depends only

on the

ratio

of

input

and

output

speeds.

It

is

linear

and

independent

of

load.

The

loss

of

power

due to

slip

is

/

3

S

=

PP(J-

-l)

(15-5)

where

P

p

is the

power output

to the

pump.

So the

power

lost

due to

slip depends

on the

ratio

of

speeds

and the

magnitude

of the

load

at the

lower speed. Furthermore,

the

slip loss

is the

same

for any

type

of

slip drive,

including

wound-rotor/liquid-rheostat

slip drives.

The

load

for

centrifugal

pumps

may

vary between

the

third

and

fourth

power

of the

speed

over

the

useful

speed ranges,

so the

power lost

may be

relatively

small even though

the

efficiency

may

seem

to

indicate

otherwise.

For

example,

at 75%

speed

for a

load that

varies

as the

fourth

power

of the

speed,

the

power lost

would

be

PS

=

(0

'

15)4

(&75~

1

}

=

ai

°

5

or

10.5%

of the

power required

at 75%

maximum

speed.

Although these losses

are not

negligible, they

should

be

considered objectively

in a

life-cycle engi-

neering cost study that includes

not

only power costs

but

also

the

maintenance, reliability,

and

expected

life

of

the

equipment

in

comparison

to

other drives.

Typical power

losses

for an

eddy-current coupling

driving

a

pump

are

shown

(as

"slip

drives")

in

Figure

15-25.

Stationary

Field

Eddy-Current Couplings

By

altering

the

design

of the

unit,

the

rotating

field of

an

ordinary eddy-current coupling

can be

converted

to

a

stationary

field,

which eliminates

the

brushes

and

slip

rings. Two

U.S. companies

offer

this design.

The

efficiencies

of the new and old

designs

are

almost

the

Figure

15-26. Air-cooled eddy-current pump

drives,

(a)

Vertical

mount,

salient

pole,

rotating field (with

brushes);

(b)

horizontal

mount,

stationary

field (without

brushes).

Courtesy

of

Dynamatic Corp.

same

and the

prices

are

comparable,

but

eliminating

the

slip rings

and

brushes reduces

the

maintenance

to

an

amount comparable with

a

squirrel-cage induction

motor.

Liquid

Rheostats

A

complete drive consists

of a

liquid rheostat with

short electric power cables connected

to the

brushes

of

a

wound-rotor motor.

Operation

The

slope

of the

speed—

torque

curve

of an

induction

motor varies with changes

in the

resistance

of the

rotor circuit.

The

lower portions

of the

speed-torque

curves

of any

induction motor

are

shown

in

Figure

15-28

for

many values

of

external resistance

in the

rotor circuit,

and a

portion

of the

speed-torque

curve

of a

typical centrifugal pump

in a

typical

sys-

tem

is

superimposed

on the

motor

speed-torque

curves.

At

any

given speed,

the

motor

and

pump must

operate

at the

intersection

of the

pump

and

motor

speed-torque

curves.

If the

resistance

is

decreased,

the

slope

of the

motor torque curve

is

increased,

which

causes

the

motor

to

accelerate

on its

curve

(while

the

pump

accelerates

on its

curve) until

the

motor

output torque exactly equals

the

torque required

by

the

pump.

A

liquid rheostat

is

basically

a

large, three-phase

resistor. Electrical current

flows

between copper

or

bronze electrodes through

an

electrolyte

(a

very weak

solution

of

distilled water

and

sodium carbonate) that

makes

a

good

and

stable resistor.

The

electrical resis-

tance between electrodes varies inversely with

the

submerged area

of the

electrodes

and

also varies

directly

with

the

distance between

the

electrodes.

The

electrolyte

is the

resistor.

The

resistance

of the

elec-

trodes themselves

is

insignificant.

Pneumatic

Control

The

model most commonly used

for

sewage pumping

is

illustrated

in

Figure

15-29.

One

rheostat

electrode

is

connected

to

each rotor brush

of its

associated motor.

The

principle

of

operation

is

that

of a

manometer.

The

air

pressure,

jP

a

,

in the

sealed reservoir equals

the air

pressure,

P

a

,

at the

bottom

of the

bubbler. This

air

pressure raises

the

level

of the

electrolyte

on the

elec-

trodes

by an

amount,

D. The

depth,

D, in the

rheostat

equals

D in the wet

well

and

also equals

P

a

(expressed

in

height

of

water column).

The air

compressor

is

rela-

tively

small

and

(without storage) supplies

air

directly

to the

piping

as

shown.

The

three-way solenoid valve

vents

the

sealed compartment

to

atmosphere when

the

drive

is

shut down.

The

bubbler requires about

1.7

m

3

/h

(1

ft

3

/min)

of

air at

very

low

pressure,

and an air

compressor pow-

ered

by a

small

(1/4-kW

or

1/4-hp)

motor

is

adequate

for

drivers

up to 600 kW

(800 hp).

Each liquid rheo-

stat

includes

an

internal

air

compressor.

Mechanical

Controls

Another

type

of

liquid rheostat uses movable

and

sta-

tionary

electrodes.

Each

of a set of

upper, movable

electrodes

is

connected

to one of the

motor rotor

brushes

and

stationary, lower electrodes

are

connected

together

to

form

a

wye-connected, three-phase resis-

tor.

The

electrodes

are

fully

submerged

at all

times,

and

resistance between

the

movable

and

stationary

electrodes

is

varied

by

changing

the

level

of the

upper

electrodes. Usually

a

small, single-speed motor

and

sprocket chains

are

used

to

raise

and

lower

the

mov-

able plate assembly.

The

motor receives "up-hold-

down" signals

from

external equipment.

Figure

15-27.

The

principle

of an

eddy-current coupling.

Semal

feSances

ed

"

t0rqUe

^*

**

'

^'

"^

0

'

08

^

8

'

Centn

'

fU8a

'

PUmp

and

a

wou

nd-rotor

motor

with

Figure

15-29.

Pneumatic control

system

for a

liquid

rheostat.

Cooling

Because

the

electrolyte must

be

cooled,

a

heat

exchanger

is

required.

Up to

ratings

of

about

19 kW

(25

hp),

a

water-

to-air

heat

exchanger

can be

incorpo-

rated

into

the

same enclosure with

the

liquid rheostat

if

ambient temperatures permit. Larger units require

an

external water-to-water heat

exchanger,

and the

most

common

is a

baffled

steel sleeve around

the

pump

discharge piping.

The

annular clearance

between sleeve

and

pipe

is

about

13 mm

(1/2 in.)

and

the

sleeve length

is

determined

by the

maximum heat

losses that

can

occur based

on the

maximum slip loss.

A

small circulation pump (rarely exceeding

1/4 kW or

1/4 hp) is

adequate.

Efficiency

The

slip

efficiency

is

_

n

p

_

motor

shaft

speed

H5

6")

s

n

m

motor rated

speed

in

which

n

p

is the

speed

of the

pump

shaft

and

n

m

is

the

full-speed rating

of the

motor.

The

slip

efficiency

is

identical

to

that

for an

eddy-current coupling

or any

other type

of

clutch.

The

total drive

efficiency,

determined

by

adding

slip losses

and

motor losses,

is

about

the

same

as

that

of

an

eddy-current coupling.

The

friction

or

windage

losses

(1 to 2%) in the

coupling

are

absent,

but in

rat-

ings

below about

300 kW

(400 hp), wound-rotor

motor

efficiencies

at

loads above

a

rating

of

approxi-

mately

25% are

typically

1 to 2%

less than those

of

squirrel-cage machines.

The

power required

by air

compressors

and

other accessories

is

negligible.

Fluid

Couplings

(Hydrokinetic)

In

fluid

couplings,

the

drive consists

of a

squirrel-cage

induction

or

synchronous motor

and

coupling.

The

coupling,

enclosed

in an

attractive housing, contains

these

major

components:

• An

impeller, which

is

mounted

on and

keyed

to the

input

shaft

• A

runner, which

is

mounted

on and

keyed

to the

output

shaft

• A

scoop tube

• A

casing, which

is

mounted

on and

keyed

to the

input

shaft.

A

very small electrically driven pump

is

mounted

on

the

outside

of the

housing.

The

pump delivers

oil

from

a

sump

at the

inside bottom

of the

housing

to the

casing.

Oil is

free

to flow

from

the

casing into

the

impeller

and

runner.

The

impeller

and

runner have internal blades simi-

lar

to

those

in a

recessed

impeller

of a

pump.

Rotation

of

the

impeller imparts energy

to the oil in

which

it is

partially submerged.

The oil is

"pumped"

to the

run-

ner and

imparts energy

to the

runner

blades,

thereby

causing

the

runner

and the

output

shaft

to

rotate.

The

depth

to

which

the

impeller

and

runner blades

are

submerged

in oil

directly determines

the

torque that

is

transmitted

to the

runner

and the

output

shaft.

The

depth

of oil in the

casing,

and

thus

the

submergence

of

the

impeller

and

runner blades,

is

adjusted

by a

"scoop

tube" that removes

oil

from

the

casing

and

returns

it to

the

sump.

The

scoop tube

can be

positioned

from

the

exterior

of the

housing through

a

linkage,

and its

posi-

tion

determines

the oil

level

in the

casing.

Slip-Recovery

Drives

A

slip-recovery drive consists

of a

wound-rotor induc-

tion motor

and a

solid-state

controller.

The

controller

is

electrically connected

to the

rotor

brushes

of the

motor,

and the

effective

resistance

in

the

controller

is

electronically

adjustable (see

liquid

rheostats). Rotor circuit current does

not

actually pass

through

resistors,

but is

rectified,

inverted

to 60 Hz,

and

returned

to the

motor

stator

terminals.

The

power

losses that occur

in

resistor-type secondary controllers

do not

exist

in the

slip-recovery drive, which

is

slightly more

efficient

than

the

AFD.

The

controller

should

include

all of the

necessary capacitors

for

power factor correction

and all of the

circuit breakers

required

for

line disconnect.

Variable-Ratio

Belt

Drives

The

variable-ratio belt drive consists

of a

squirrel-

cage induction motor

and two

variable pitch sheaves

with

a

belt,

shafting,

bearings,

a

sheave-pitch-chang-

ing

mechanism,

and a

large spring,

all of

which

are

enclosed

in a

housing.

One

sheave (input)

is

either

on a

shaft

directly cou-

pled

to the

motor

shaft

or

mounted

on an

extended

motor

shaft.

The

second sheave

is

mounted

on the

out-

put

shaft.

Power

is

transmitted

from

the

input

to

out-

put

sheaves

by a

heavy-duty

vee-belt.

Output

shaft

speed changes

are

accomplished

by

changing

the

effective

diameters

of the

sheave hubs

by

changing

the

spacing between

the

sheave discs.

The

discs

are

inside tapered

so

moving

one

disc

(along

the

shaft)

closer

to the

other disc

of the

sheave

forces

the

belt

to

engage

the

disc

at a

greater distance

from

the

hub.

The

position

of one

disc

on the

input

sheave

is

changed

by an

external linkage.

The

posi-

tion

of the

disc

of the

output sheave

is

changed

by an

internal spring

to

keep belt tension relatively constant

regardless

of

changes

in

output speed.

The

discs

should

be

chrome plated

to

reduce wear.

Hydrostatic

Drives

A

hydrostatic drive consists

of the

following:

• A

squirrel-cage induction motor

• An

adjustable displacement, multicylinder hydrau-

lic

pump

• A fixed

displacement, multicylinder hydraulic motor

• An oil

reservoir.

The

hydraulic pump

is flexibly

coupled

to the ac

motor,

and its

shaft

turns

at

C/S.

The

cylinder block

(which

is

cylindrical

in

shape)

is

keyed

to the

shaft

and

contains

the

pistons.

The

entire assembly rotates

with

the

shaft.

The

piston rods bear

and

slide

on a

"swashplate"

that does

not

rotate.

The

swashplate

can

be

tilted

by an

external device (through

a

linkage)

to

change

the

effective

displacement

of the

pump.

The

hydraulic motor

is

connected

to the

hydraulic

pump

by

means

of two

hoses.

The

output

of the

pump

is

circulated through

the

motor

and

back

to the

pump

intake.

The

shaft

of the

hydraulic motor

is flexibly

coupled

to the

load

shaft.

The

motor construction

is

similar

to the

pump, except that

its

swashplate tilt

is

fixed. The

motor output

is

connected

to the

shaft

of

the

sewage pump. Changing

the

swashplate angle

changes

the

discharge rate

of the

hydraulic pump thus

controlling

the

speed

at

which

the

motor turns.

Refer

to

Table

15-3

for

comparative advantages

of

hydrostatic drives.

Other

Drives

Other drives

are

available

but are not

described here

because they

are not

widely used

for

pumping water

or

wastewater. Obviously, this situation

may

change,

so be

alert

to new

products. Many types

of

rotating

machinery that

are

quite satisfactory

for

some

specific

use, however,

are

unsuitable

for

municipal pumping.

Be

cautious

in

applying

a new

product

by

making

a

thorough investigation

of

installations

and

their main-

tenance records.

15-12.

References

1.

Karassik,

I.

J.,

W. C.

Krutzsch,

W. H.

Eraser,

and J. P.

Messina,

Pump

Handbook,

2nd

ed.,

McGraw-Hill,

New

York

(1985).

2.

Davis,

C.

V.,

and K. E.

Sorensen,

Handbook

of

Applied

Hydraulics,

pp.

40-16,

McGraw-Hill,

New

York

(1969).

3.

Fair,

G.

M.,

J. C.

Geyer,

and D. A.

Okun,

Water

and

Wastewater

Engineering,

Vol.

1,

Water Supply

and

Wastewater

Removal,

John

Wiley,

New

York

(1966).

Two

approaches

to

writing specifications

for

pumps

and

drivers

are (1)

investing "unit responsibility"

for

the

complete pumping unit (driver, shafting, pump)

with

a

single manufacturer

and (2)

separating driver,

shafting,

and

pump

so

that each

can be

furnished

by a

different

supplier

and

assembled

at the

jobsite

by the

general contractor. These

two

approaches

are

dis-

cussed

and

compared

in

Section

16-1.

The

remainder

of

the

chapter

is

confined

to the first

approach (unit

responsibility)

and an

explanation

of how to

apply

it

to

custom-engineered pumps

and

drivers

for a

high-

lift

sewage pumping station. "Custom

engineered"

means that most manufacturers must

modify

their

"off-the-shelf

pumps with larger shafts, more rugged

bearings,

or

some special materials

to

comply with

the

specifications.

Other manufacturers

may

produce

high-quality pumps that need

no

improvements.

This chapter

is

keyed

to

(and

is an

explanation

of)

Appendix

C,

which contains

the

specifications

as

written

for two

high-lift

pumping stations. Each sta-

tion

has

four

pumps discharging into

two

force

mains. Paragraph 1.02B

of

Appendix

C

shows

how

to

vest

the

manufacturer with complete

responsibil-

ity

for

properly coordinating sizes, system analyses,

Chapter

1

6

Pump-Driver

Specifications

DAVID

L.

EISENHAUER

GARR

M.

JONES

CONTRIBUTORS

Richard

O.

Garbus

Philip

A.

Huff

Melvin

P.

Landis

Ralph

E.

Marquiss

Earle

C.

Smith

Morton

Wasserman

and

procurement

of the

complete pump

and

driver

system.

Specifications should

be so

clear

that they cannot

be

misunderstood (see

"Specification

Language"

in

Section

28-1).

1

6-1

.

Comparison

of

Two

Approaches

to

Writing Specifications

Many

experts believe that

the

unit-responsibility

approach

is

superior

for

public works projects.

The

benefits

of

this approach include

the

following:

•

Fixing complete responsibility

for the

equipment

with

a

single manufacturer

•

Eliminating problems

of

misfit

or

mismatch

in

either size

or

performance (such

as

torsional

reso-

nance, reed frequency,

and

critical speed)

•

Providing

the

owner with

a

single source

for

spare

parts, troubleshooting,

and

future

modifications

•

Ensuring adequate design

in

subjects

for

which

the

consultant's personnel

may be

lacking

in

training

or

experience.

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

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