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

tions because clean water does

not

contain

reliable

reflective

surfaces.

Doppler

flowmeters do

have

the

outstanding

advantage

that there

is

nothing

to

foul

or

corrode.

Users

consider

them

to be

maintenance-free

devices.

They

are

frequently

used

on

sludge pipelines where

accuracy

is not

particularly important

and the flow is

very

difficult

to

measure

at all

with other devices.

The

maximum

velocity

is

limited only

by

what

can be

practically pumped through

a

pipeline,

and the

mini-

mum

velocity

is

limited only

by the

resolution

of the

frequency

measurement

circuitry—about

0.3 m/s (1

ft/s)

for

most units.

In

sludge service, however,

the

minimum

velocity must

be

enough

to

prevent

a

buildup

on the

pipeline walls, which would change

the

effective

pipe

diameter.

The

Doppler

flowmeter is

also available

as a

moder-

ately

priced,

nonintrusive

flow

switch. Accuracy

is not

critical

in

this application.

The

Doppler

flow

switch

is

particularly

useful

as a

pump discharge

flow

detector,

where

limit switches cannot

be

used

on a

discharge

check valve.

If the

Doppler

flow

switch

is

located

immediately downstream

of a

pump, reliable signals

are

produced even with clean water, because

the

bub-

bles

from

the

pump impeller

reflect

the

sonic pulse.

Transit-Time

Ultrasonic

Flowmeters

The

transit-time ultrasonic

flowmeter

(see Figure

20-14)

consists

of at

least

one

pair

of

piezoelectric

transducers mounted

on

opposite

sides

of the

pipeline

and

offset

from

each other

by 30 to

45°. Additional

transducer pairs

are

often

used

to

provide

the

ability

to

average

the flow

profile.

An

ultrasonic pressure

pulse

is

injected

by one

transducer

and the

period

of

time

for it to

reach

the

second transducer

is

measured.

The

process

is

then reversed.

The

difference

in

time

required

for the

pulse

to

propagate upstream

and

downstream through

the fluid is

proportional

to the

average

fluid

velocity between

the

transducers. This

metering system

is

capable

of

very high accuracy,

and

when

multiple

transducer pairs

are

used,

it can

aver-

age out

irregularities

in the flow

profile.

The

process

of

reversing

the

signal direction cancels errors that

would

be

caused

by

variations

in the

speed

of

sound

through

the fluid.

This meter

does

not

perform well

in

dirty water

or

wastewater applications.

The

same sol-

ids,

gas

bubbles,

and

other

fluid

discontinuities that

make

the

Doppler meter work

diffract

the

pressure

pulse

so

that

it

arrives

at the

receiving transducer

as a

burst

of

noise,

not as a

single pulse. High-quality

Figure

20-14.

Transit-time

ultrasonic

flowmeter.

Figure

20-13.

Doppler

flowmeter.

electronics

can

deal with

the

noise

to a

certain extent,

but

they increase

the

cost

of the

meter substantially.

The

transit-time ultrasonic

flowmeter is

especially

useful

on

very large pipes where cost becomes less

of

a

factor.

The

maximum velocity

is

limited only

by

what

can be

pumped through

a

given size pipe,

and

the

minimum velocity

is

limited

by the

resolution

of

the

timing

circuits—

0.3

m/s

(1

ft/s).

Propeller

Meters

As

its

name implies,

a

propeller meter contains

a

pro-

peller installed coaxially

in the

center

of the

pipe,

as

shown

in

Figure

20-15.

Flow turns

the

propeller

at a

speed proportional

to the

liquid velocity.

It is a

low-

cost device used primarily

in

clean water applications

where

it

gives excellent service. Depending

on the

manufacturer

and the

meter size,

the

mechanical

nature

of the

equipment limits

the

usable

flow

range

to

about

0.15

to 3.0 m/s

(0.5

to 10

ft/s).

Its

only disad-

vantage

(for clean water service)

is the

need

to

replace

bearings

from

time

to

time.

Turbine

Meters

Turbine

flowmeters

have many blades

on a

rotor whose

speed

is a

linear

function

of fluid flow

velocity within

an

error

of

about ±0.5% over

a

very wide

flow

range

of

10

or

even 20:1.

The

blades

are

vulnerable

to

particu-

late

matter,

so the

meter

is

used only

for

clean water.

Vortex

Flowmeters

In

vortex

flowmeters, a

transverse,

flat bar

(called

a

"bluff

body") across

the

pipe splits

the flow

into

two

paths

and

causes vortices

to

shed alternately

from

the

sides

of the bar at a

frequency that

is (1)

linearly pro-

portional

to

velocity

and (2)

independent

of fluid

den-

sity

over

flow

ranges

up to

30:1.

The

meters

can be

used

for

liquids, gases,

and

cryogenic

fluids.

There

are

no

moving parts,

and the

construction allows sand

in

dirty

water

to

pass without obstruction.

The

shedding

rate

for

vortices

is

linear above

a

Reynolds number

of

10,000.

The

meters

are

available

in

sizes

from

38 to

400 mm

(1.5

to 16

in.),

and

their

use in

chemical

and

industrial

fields is

increasing because

of

their accu-

racy (which

is

slightly greater than

the

accuracy

of a

Venturi

meter), range,

and

insensitivity

to fluid

prop-

erties. These excellent characteristics

are no

less

applicable

to the

measurement

of

water.

Summary

For

clean water applications,

the

propeller meter

is a

good choice

for all

sizes

of

pipes.

For

sewage,

the

Venturi

meter

is

widely used even though keeping

the

pressure ports clean

is

troublesome. Venturi meters

for

pipes

larger than

600 mm (24

in.) become compara-

tively

costly

and

require straight-approach pipes that

may

be

inconveniently long. Install

a

freshwater purge

system

and a

pipe configuration that allows

the

Ven-

turi

meter

to be

removed

for

cleaning.

The

magnetic

meter

is

much preferred

by

many experts because

of

its

superior accuracy

and

reliability, although mainte-

nance

is

still

a

problem. Both Venturi

and

magnetic

meters

are

heavy

in

sizes larger than

300 mm

(12

in.),

so

hoisting beams should

be

installed overhead. Con-

sider

the

Doppler meter

for

measuring

the flow of

sludge.

The

accuracy

of

Doppler meters

is not

good

(but

usually adequate), maintenance

is

negligible,

and

operators like them. Except

for raw

wastewater ser-

vice, consider

the

vortex meter.

20-6. Open

Channel

Flow

Measurement

Weirs

and flumes are

used

for

open channel

flow

mea-

surement. Weirs

are

applicable only

to

clean water

and

Figure

20-15.

Propeller

meter.

treated wastewater because they cannot pass heavy

solids.

Flumes

may be

used

in

both clean

and

waste-

water

applications.

Weirs

A

weir

is

simply

an

obstruction across

a

channel,

and

weirs

are

built

in a

number

of

configurations,

including

rectangular,

vee-notch,

and

trapezoidal. Vee-notch weirs

typically

have

an

included angle

of

30°, 60°,

or

90°.

Weirs

are

fabricated

in the field,

usually

of

concrete

or

steel,

and the

care used

in

their construction

is a

major

limitation

on

their accuracy. Because

the flow

down-

stream

from

a

weir must

fall

freely

away

from

the

crest,

weirs introduce substantial headloss into

the

channel.

Depending

on the

weir configuration,

the flow

var-

ies as a

power

(1.5-2.5)

of the

upstream level,

so the

change

in

level

is not

great with respect

to the flow,

and

level instruments must

be

extraordinarily accu-

rate.

The

usable range

is

typically

20:1,

but the

accu-

racy

is

limited because errors

often

exceed

5%.

Flumes

are

built

in a

number

of

configurations

including

Parshall

(Figure 20-16), Palmer-Bowlus (Figure 20-17),

parabolic,

and

cutthroat. Except

for the

Palmer-Bowlus

flume

(see Section 3-7), they

are

purely empirical devices

and

their accuracy

is

limited

by the

degree

to

which

the

field

installation

resembles

the

laboratory model.

Approach

conditions that produce irregular

flow

profiles

and/or

flows

greater

or

less than

that

specified

for the

size

give

poor

and

unpredictable results. Properly sized

and

installed,

flumes

provide reliable measurement

of

clear

water

and

sewage

—

even

with gross

solids.

The

head

change developed

by a flume

with

a

rectangular throat

is

proportional

to the flow to

approximately

the

0.65 power.

Flumes have

a

wide usable

range

—

typically

20:1.

20-7.

Chlorine

Residual

Measurement

Water

is

often

chlorinated

in

water pumping stations.

Standard analyzers

in

"package"

units

are

available

to

Figure

20-16.

Parshall

flume,

(a)

Plan;

(b)

longitudinal

section.

Figure

20-17.

Palmer-Bowlus

flume,

(a)

Cross-section;

(b)

longitudinal

section;

(c)

plan.

control

the

dose

or to

measure

the

residual

chlorine.

Refer

to the

treatise

by

White

[8] for a

complete dis-

cussion

of

chlorination.

20-8. Utility

and

Environmental

Measurements

Utility

and

environmental measurements include

a

large

variety

of

miscellaneous measurements that pro-

vide information about

and

protection

of the

pumping

station

itself. These measurements

are not

unique

to

pumping

stations. They might

be

made

in any

facility

containing

valuable equipment.

Electrical

Parameters

Electrical parameters such

as

volts, amperes, watts,

and

power

factor

are

easily measured either with

direct reading instruments

or

with transducers that

convert

these readings

to

standard transmission sig-

nals

for

activating alarms

or

telemetering

to a

distant

location. Most electric power-monitoring instruments

operate

on

rather simple electromagnetic principles

and

are

very accurate, reliable,

and

virtually mainte-

nance

free.

Temperature

There

are a

great

many

ways

to

measure temperature.

Mercury

thermometers should

be

confined

to the

labo-

ratory

because

of the

extreme hazard

of a

mercury

spill.

In field

applications where only

a

discrete

signal

is

required,

the

most common device

is a

closed

chamber

in

which

a fixed

amount

of gas or

liquid pro-

duces

a

variation

in

pressure with changes

in

tempera-

ture.

A

bimetal element consists

of two

metal strips with

different

thermal expansion

coefficients

formed into

a

spiral.

As the

temperature changes,

the

spiral tends

to

wind

tighter

or

unwind. Bimetal elements

are

com-

monly

used

in

residential applications both

to

indicate

the

temperature

and to

control

on-off

devices such

as

furnaces

and air

conditioners. These devices

are low

in

cost

but do not

provide

the

long-term accuracy

of

liquid-

or

gas-filled

systems. Where electronic trans-

mission signals

are

required, coils

of

platinum, cop-

per,

or

nickel alloys (which change

resistance

as the

temperature

changes)

are

usually used

for

temperature

transmitters.

Nickel

alloy

is

Jow

Ju

cost

and

com-

monly used

for

heating, ventilating,

and air

condition-

ing,

but it has a

nonlinear temperature-resistance

coefficient.

Platinum

has a

linear

coefficient,

is

extremely

stable,

and is

commonly used where accu-

rate, long-term measurement

is

required.

Environmental

Safety

Instruments

Methane, hydrogen

sulfide,

and

oxygen depletion

monitoring

are

required

in

wastewater pumping sta-

tions. Instruments

for

measuring these parameters

are

costly

and

require substantial

maintenance

—

par-

ticularly when installed

in wet

wells. Only

fixed

hydrocarbon monitors

are

required

in wet

wells

by

the

NFPA. Hydrogen

sulfide

detectors

are too

unsta-

ble for

permanent installation

and

these monitors

should

be

carried

by the

personnel. Oxygen deple-

tion

is not

generally considered

to be a

problem

in

wet

wells.

The

calibration

of any of

these instruments should

be

checked

at

least monthly.

For

small facilities, por-

table instruments carried

by

maintenance personnel

may

be a

better

solution

if

acceptable

to the fire

mar-

shal. Portable instruments should

be

carried

by

main-

tenance personnel anyway, because they

frequently

must

enter collections system manholes where perma-

nently installed equipment

is

never present.

Explosive

Atmosphere

Methane

(an

explosive gas) occurs

in

underground

water

sources

and

forms

naturally

in

wastewater facil-

ities,

and

fuel

vapors sometimes occur

due to

acciden-

tal

leakage

or

intentional dumping into sewers.

The

most

common

explosive-gas-monitoring

device

is the

catalytic detector, which consists

of two

heated ele-

ments.

One is

exposed

to the

ambient

gas and the

other

is

isolated.

If the

ambient

gas is

combustible,

it

burns

on the

exposed element, which raises

its

tem-

perature above that

of the

isolated element.

The

differ-

ence

in

temperature measures

the

combustible

gas

present. Because catalytic elements

are

easily poi-

soned

by the

hydrogen

sulfide

frequently

present

in

wastewater

facilities, detector

life

is

limited. Some

manufacturers

produce catalytic detectors with

improved resistance

to

hydrogen

sulfide

poisoning

and

typically guarantee

the

detector

for one

year.

In

extreme conditions

infrared

detectors, which

are

totally

immune

to

poisoning,

are

available,

but in

1997 these cost about

$2500

per

point

—

f

our

times

as

much

as the

improved catalytic

type.

Chlorine

Gas

Leak Detectors

Chlorine

gas

leak monitoring

is

required

in

either

water

or

wastewater pumping stations

if

chlorination

equipment

is

present. Permanently installed monitor-

ing

equipment

is

necessary

at any

chlorination loca-

tion, because chlorine leaks

can

present

a

hazard

to

the

public outside

of the

station. Where permanently

installed monitoring systems

are

used, audible

and

visual

warning devices must

be

installed

at all

entrances

to the

monitored space.

Hydrogen

SuI

fide

Gas

Detectors

Hydrogen

sulfide

is a

ubiquitous, dangerous

gas as

deadly

as

hydrogen cyanide. Because

it is so

common

around

sewage works (who

has not

smelled

a

faint

odor

of

rotten

eggs?),

it is

often

ignored

and

many

people have been killed

or

have

suffered

permanent

brain

damage

from

this insidious gas. Below ground,

sample

for

hydrogen

sulfide

continuously.

The

effects

of

various concentration

are

given

in

Table 20-5.

Detectors

are

seldom installed permanently

in

pump-

ing

stations because they should

be

carried

at all

times

by

personnel

in

wastewater facilities when working

in

any

confined

space where sewage

is

present.

Oxygen Depletion

Oxygen

depletion monitors

are

available

from

the

same

companies that

manufacture

explosive

gas and

hydrogen

sulfide

detectors.

A

common type

of

unit

has

a

galvanic cell containing

two

dissimilar elec-

trodes

in a

basic electrolyte. Oxygen

diffusing

through

the

sensor cell

face

initiates

a

redox reaction that gen-

erates

a

minute current proportional

to the

oxygen

partial pressure.

The

typical range

is O to 25%

oxygen

concentration.

Fire

Severe

fire

hazards

do not

normally exist

in

water

and

wastewater

facilities because they

are

usually con-

structed

of

noncombustible materials

and

very little

of

the

contents

can

burn.

In

very large facilities,

the

con-

trol

equipment

may

present

a

significant

fire

hazard,

and

prompt detection

of a fire may

reduce

the

damage

and

facilitate repair. Because these electrical

fires

pro-

duce

little visible smoke

or

heat,

"products-of-com-

bustion"

detectors

are the

most suitable type.

In

some

Table

20-5.

Effects

of

Hydrogen

Sulfide

Concentration

in air

Effect

3 ppb Max

concentration

for

electronic

systems

per ISA

10

ppb Max

concentration

for

electrical

equipment

per

NEMA

100 ppb

Threshold concentration

for

smell

1 ppm

Offensive

odor

of

rotten eggs

5 ppm

Deadens olfactory senses

10

ppm Max

24-h exposure limit

per

OSHA

15

ppm Max 8-h

exposure limit

per

OSHA

50 ppm Max

30-min/d

exposure

per

OSHA

300 ppm

Instant death

for

sensitive people

500 ppm

Instant death

for

everyone

46,000

ppm

Lower explosive limit (LEL)

facilities,

the

local

fire

marshal

may

require

a fire

alarm

system conforming

to

applicable building codes

to

protect personnel.

The

design

of

such systems

is

usually

outside

of the

competence

of

instrumentation

engineers,

and

specialists should

be

retained.

Unauthorized

Intrusion

Intrusion alarm systems

are

frequently needed

in

urban

areas

not

only

to

protect

the

pumping station,

but

also

to

protect

the

intruders

from

injury.

Most

pumping

stations have

a

limited number

of

possible

entry

points,

and

simple door switches

are

adequate.

If

there

are

windows, passive infrared detectors

may

also

be

needed.

To be

effective,

intrusion alarms must

be

transmitted

to a

continuously

staffed

facility, such

as a

police

or fire

station.

Air

Pressure

Where instrument

air

and/or starting

air

systems

form

an

essential part

of a

pumping station, pressure alarm

systems

are

required. Simple pressure switches

are

adequate

for

this purpose.

Fuel

Level

Where

fuel

is

stored on-site

for

engine-driven pump-

ing

units

or

emergency generators,

the

fuel

supply

must

be

monitored.

A

simple

staff

gauge

is

fre-

quently

used.

If

remote monitoring

is

required, some

type

of

level measurement system

is

used. Automatic

monitoring

of

diesel tanks

can be

accomplished with

bubblers,

floats,

radio-frequency

probes,

or

ultra-

sonic units. Buried liquid

fuel

tanks must

be

fur-

nished with leak detection systems. Package tank

monitoring units, which include

the

leak detection

and

level monitoring,

are

available

and

recom-

mended.

Propane tanks

are

more

difficult.

Pressure mea-

surement,

which

is

often

used

for

monitoring propane

tanks,

does

a

better

job of

measuring tank temperature

than

liquid level until

the

tank

is

almost empty.

A

dif-

ferential

pressure element connected

to the

liquid

withdrawal

nozzle

on the

bottom

of the

tank with

a

compensating connection

to the gas

withdrawal noz-

zle on the top of the

tank provides

a

simple, accurate

measure

of

tank level. Such transmitters

are

readily

available with

the

static pressure rating

of

4140

kPa

(600

lb/in.

2

)

required

for

this application. Alterna-

tively,

the

tank

can be

mounted

on

platform scales

to

give

a

direct measure

of

propane quantity.

Control Power Availability

Where possible, control circuitry should

be

designed

so

that

it is not

dependent

on a

single control power

supply.

If a

single control power supply

is

used,

it

should

be

monitored

by an

undervoltage

relay

to

pro-

vide

an

alarm

if

failure

occurs.

Battery

Charger Operation

Chargers designed

for

constant-voltage-float charging

of

stationary batteries

are

available with built-in alarm

circuitry

that provides

a

warning when

the

charger

fails.

Sample Pumps Running

Sample pumps

are

used

to

transfer sample

flows to

chlorine residual analyzers and/or containers

for

sub-

sequent laboratory analysis.

In

noncritical applica-

tions,

a

motor-starter auxiliary contact

or

control relay

connected across

the

motor gives

a

reasonable indica-

tion

of

pump operation.

A

running pump, however,

is

not

a

sure indication

of flow.

Because

a

valve might

be

closed

or the

pipe might

be

blocked,

a flow

switch

is

needed

in

critical applications.

Vane

switches installed

in

a

pipeline

can be

used

in

clean water applications.

In

wastewater applications,

a

thermal-dispersion

flow

switch

(which

is a

heated temperature-sensitive ele-

ment encased

in a

smooth cylinder that

is

inserted into

the

pipeline)

is

better. When

flow is

present,

the

ele-

ment

temperature stays near

the fluid

temperature,

but

when

flow

stops,

the

element temperature

rises

because

the

heat

is not

dissipated

effectively.

This

temperature change

can be

detected

by

suitable

elec-

tronic circuitry.

The

thermal-dispersion

flow

switch

is

much

more expensive than vane switches,

but the

sys-

tem is

fairly

resistant

to

fouling.

20-9. Pumping

Unit

Monitors

Most pumps

are

driven

by

constant-speed electric

motors

of

moderate

(less

than

1

10

kW or 150 hp)

size.

Such

units require only

the

standard protective

devices provided with

any

electric motor, such

as the

replica-type thermal overload protection normally fur-

nished

in

standard motor controllers.

"Replica"

means

that

the

thermal overload device, which

is

located

in

the

starter where

it is

heated

by

motor current, dupli-

cates motor heating.

It is

most common

in

motors

up

to 150 kW

(200 hp).

In

larger motors,

it is

better

prac-

tice

to add

temperature monitors

in the

windings.

Temperature monitors should also

be

used

on

most

adjustable-frequency

drives. Always equip engine

drives with cooling water temperature monitors

and

lubrication

oil

pressure monitors.

Bearing Temperature

Bearing housings

of

larger (greater than

150 kW or

200 hp)

units

may be

provided with temperature mon-

itors. Frequently, these

are filled-system

temperature

switches

and may be

specified

to be

furnished with

the

pumping

unit. Alternatively, resistance temperature

detectors (RTDs)

or

thermocouples (TCs)

may be

installed

in

bearing housings

and

connected

to

exter-

nal

electronic monitoring equipment. Both

the

RTDs

and

the TCs are

more accurate

and

easier

to

adapt

to

remote monitoring.

Motor Winding Temperature

Motor winding temperature monitors provide much

better

overtemperature

protection than replica-type

thermal overloads. Three types

are

commonly used:

(1) end

turn

snap switches

(Klixons™),

(2)

positive

temperature

coefficient

(PTC) thermistors installed

in

the

winding slots,

and (3)

resistance temperature

detectors (RTDs) installed

in the

winding slots. Klix-

ons™

are not

adjustable,

may

produce

a

winding

hot

spot where attached,

and are not

recommended.

The

PTC

thermistors

are

recommended

for

motors

up to

about

750 kW

(1000 hp),

and

RTDs

are

recommended

for

larger motors.

Pump Speed

In

many applications,

a

motor-speed meter

is an

ade-

quate

measure

of

pump speed

and can be

obtained

as

an

accessory with adjustable-speed drive equipment.

Where

an

actual measure

of

pump

shaft

speed

is

required (either

for

positive interlocking purposes

or

because

the

adjustable-speed drive equipment cannot

provide

a

speed signal), either

a

belt-driven tachome-

ter or a

reluctance pickup device

may be

used. Belt-

driven

tachometers

are

mechanical devices with bear-

ings

and

brushes that require

significant

maintenance.

The

belt cannot

be

replaced without opening

the

cou-

pling between

the

driver

and the

pump.

A

reluctance

pickup

consists

of a

split gear clamped

to the

pump

shaft

and a

magnetic pickup coil assembly mounted

close

to

(but

not in

contact with)

the

gear.

As the

gear

teeth

pass

the

pickup coil,

a

small alternating current

voltage

is

generated that

can be

detected

by

suitable

electronics. These devices

are

essentially maintenance

free

and are

very accurate.

Vibration

monitoring

is a

specialized

field, and

most

engineers

defer

to the

recommendations

of the

machinery

builder. There

are

three parameters that

may

be

monitored:

(1)

acceleration,

(2)

velocity,

and

(3)

shaft

displacement.

The

choice depends

on

machinery

characteristic dynamics such

as

speed,

mass,

and

bearing type.

In

general, low-cost acceler-

ometers utilizing

a

spring-suspended mass attached

to

a

machine housing give

a

little protection. Elaborate

electronic systems costing thousands

of

dollars

may

be

able

to

provide

a

degree

of

warning

of

incipient

failure,

but

nothing substitutes

for

periodic inspection

by

a

competent operator.

Engine

Driver Systems

Engine

drivers

of all

sizes require extensive instrumen-

tation

(see Chapter 14).

The

instrumentation

is

usually

furnished

as

part

of the

engine-control equipment.

Electric

or

electronic sensors should

be

used

on the

engine itself with signals wired

to a

separate, vibra-

tion-free

panel. Only sensors designed specifically

for

engine service should

be

mounted

on the

engine.

20-10.

Control

Equipment

Control equipment used

in

pumping stations

has

advanced

from

crude, motor-driven

cam

stacks

to the

compact, electronic,

stored-program

microprocessor

controllers used

in the new

designs.

The

older systems

were readily understood

by

personnel accustomed

to

maintaining mechanical devices,

but

many operators

now

can

deal with modern control equipment. When

there

is a

failure,

either

the

device must

be

returned

to

the

vendor

or the

vendor must come

to the

site

to

make

repairs. This situation

is

unacceptable

for

equip-

ment

that must

be

kept operating.

The

overall reliabil-

ity

of

modern solid-state controllers, however,

is

much

better than that

of the

older mechanical devices,

so

modern

devices

are now

frequently

applied

to

pump-

ing

stations, even

in

locations

far

from

skilled

mechanics.

If

solid-state controllers

are to be

used,

it

is

essential

that emergency manual control systems

be

provided, that spare control devices

be

stocked

by the

operator,

and

that operators have

or

gain adequate

skill levels

to

make repairs

by

replacing defective

modules.

Motor Controllers

A

motor controller

is

required

for

every motor

in a

pumping

station. Article

430 of the

National Electrical

Code

[2]

establishes

the

minimum requirements

for

(1)

motor controllers,

(2)

motor

and

branch-circuit

overload protection,

(3)

motor branch-circuit, short-

circuit,

and

ground-fault protection,

(4)

control circuit

characteristics,

and (5)

disconnecting methods.

Beyond

these code requirements,

a

great number

of

variations

in

motor controllers

are

established

by the

characteristics

of the

power source

and the

require-

ments

of the

driven equipment.

Motor Branch-Circuit,

Short-Circuit,

and

Ground-Fault

Protection

Either

a

fused

switch

or an

automatic circuit breaker

is

used

for

branch-circuit, short-circuit,

and

ground-fault

protection. Fuses provide higher short-circuit inter-

rupting

capacity than circuit breakers,

but

they must

be

replaced

after

a

circuit interruption.

A

single

fuse

may

blow, which causes damaging single-phase

power

to be fed to a

three-phase motor. Circuit break-

ers

always open

all

conductors feeding

a

motor circuit

and

may be

reset

following

a

circuit interruption.

Where

the

available short-circuit current exceeds

the

interrupting

capacity

of

circuit breakers,

fuses

may be

used

in

conjunction with

the

circuit breaker

to

provide

the

advantages

of

both.

But

this combination system

is

objectionable

in

terms

of

cost (30% added

to the

cost

of

the

motor

and

starter), space requirements,

and

convenience. Although standard

fuses

are

readily

available,

the

special

fuses

used

in

conjunction with

a

circuit breaker

may be

difficult

to

obtain

in an

emer-

gency.

The

type

of

short-circuit

and

ground-fault

pro-

tection

to be

used

is not a

clear-cut decision,

and the

selection should

be

made

in

conjunction with

the

pref-

erences

of the

facility

operator.

Motor Starters

Motor starters typically provide motor branch-circuit

overload protection

as

well

as a

means

of

starting

the

motor. Both magnetic

and

solid-state motor starters

are

available, but,

at

this time, solid-state units

are

still

very

expensive (they

add

about

50% to the

cost

of the

motor

and

starter)

and are

seldom used except

in

spe-

cial applications.

Full-Voltage,

Nonreversing

(FVNR)

Magnetic

Starters

The

FVNR starter

is the

most commonly used type

of

magnetic starter.

It

connects

and

disconnects

an

elec-

tric motor with

the

power source

in

response

to a

rela-

tively

low

energy control signal.

Reduced-Voltage,

Nonreversing

(RVNR)

Starters

The

RVNR

starter must

be

used where

the

capacity

of

the

electrical

power source

is

limited

in

comparison

with

the

size

of

motor. This

is a

common limitation

in

pumping

stations located

far

from

other plant facilities

and

in

residential districts. Electric power utilities fre-

quently

stipulate that reduced-voltage starters

be

used

under these conditions.

Reduced-

voltage

starters

are

also

helpful

in

getting

a

motor started

on an

emer-

gency generator.

The

various types

of

RVNR

starters,

along with their advantages

and

disadvantages,

are

listed

in

Table 20-6 (see also Section 8-3).

Multispeed

Motor

Starters

Multispeed motors

are

sometimes used

to

provide

two

or

more pumping capacities

from

a

single pumping

unit. Multispeed motors

may be of the

two-speed/one-

winding, two-speed/two-winding,

or

four-speed/two-

winding types, depending

on the

required speed

ratios.

The

type

of

multispeed

starter used depends

on

the

type

of

motor

to be

started.

The

design

of the

con-

trol circuitry used with multispeed starters must

ensure that

the

motor

is

deenergized

for a

sufficient

time

to

allow

the

motor magnetic

flux to

decay during

speed changes.

If the

time

for

decay

is too

short,

extremely high transient torques

may be

generated

when

the

motor

is

reenergized. Magnetic

flux

decay

time

varies

from

0.5 s for

small motors

to

over

1 s for

large motors.

For

adjustable-speed motor drives, refer

to

Chapter

15.

20-11.

Control

Logic

Control

logic

is

required when equipment operates

automatically. Although control logic

is

trivial

in

many

small pumping stations,

it can

become quite

complex

in

larger pumping stations with multiple

pumping

units and/or numerous auxiliary devices.

For

example, some pumping stations have several pumps

of

different

sizes

and

require control logic

to

sequence

them

in a way

that matches

the

pumping demand

as

closely

as

possible. Pumping stations discharging into

long force mains

frequently

require power-operated

check valves that

are

opened only

after

an

oncoming

pumping unit

has

been started (and

are

closed before

the

pump

is

stopped)

to

reduce surges that

can

occur

when

a

pump suddenly starts

or

stops.

Table

20-6.

Reduced-Voltage,

Nonreversing

(RVNR)

Motor

Starters

Starter

type

Characteristics

3

Autotransformer

Moderate

cost,

good

starting

current

reduction,

uses

standard

motors

Primary

resistor

Moderate

cost,

poor

starting

current

reduction,

uses

standard

motors,

not

applicable

to

high-voltage

(over

600 V)

motors

Primary

reactor

Same

characteristics

as

primary-resistor

starter,

but may be

used

with

high-voltage

motors

Part

winding

Low

cost,

poor

starting

current

reduction,

requires

a

special

motor

Wye-delta

High

cost,

moderate

starting

current

reduction,

requires

a

special

motor

Solid

state

High

cost,

excellent

starting

current

reduction,

presently

not

applicable

to

high-voltage

motors

a

Moderate cost

is

100%,

low

cost

is

95%,

and

high cost

is

140%.

Control logic

can be

provided either

by

electrome-

chanical

devices, such

as

relays

and

drum program-

mers,

or by

solid-state devices, such

as

programmable

logic controllers

and

microcomputers. Mechanical

devices

are

usually more expensive

for all but the

sim-

plest control systems,

and the

reliability

of

large num-

bers

of

mechanical devices

in

complex control

systems

is

poor because dirt

on a

single relay contact

can

cause

a

malfunction.

Solid-state systems have

become very reliable

if

properly designed

and

installed,

but

they

are

more

difficult

to

repair when

a

malfunction

occurs.

Relays

Two

types

of

relays

are

available: machine tool

and

miniature

plug

in.

Machine-tool relays

are

compara-

tively

large

and are

capable

of

operating large motor

starters. Because

a

large armature

and

contact mass

must

be

moved quickly when

a

machine-tool relay

operates, these relays

are

subject

to

mechanical

fail-

ure in

high duty-cycle applications. Miniature plug-

in

relays have

a

limited contact rating

and are

prone

to

early failure

if

applied

to

switching heavy loads.

Miniature

relays, however,

may be

quickly

and

eas-

ily

replaced. Therefore, miniature plug-in relays

should

be

used

for

logic level circuits

and

machine-

tool relays should

be

used

for

switching large motor

starters.

Timers

Most machine-tool timing relays provide time delays

by

forcing

air

through

an

adjustable

orifice.

Time

delays

of up to

approximately

1 h can be

reliably

obtained

in

this

fashion,

but the

accuracy

is not

partic-

ularly

good. Where longer time delays

or

precise tim-

ing

is

required,

synchronous-motor-driven

timers

are

used. Solid-state timers

are

also available

in

either

large-frame

or

miniature plug-in configurations.

Solid-state timers

can

provide high-accuracy timing

with

delays

of

many hours.

Drum

Programmers

Complex

sequential logic requires large numbers

of

relays. Drum programmers provide

a

much simpler

method

of

accomplishing such logic. Drum program-

mers

can be

actuated

by

either solenoids

or

motors.

Either type

is

suitable

for low

duty-cycle applications,

but

the

motor-actuated type

is

more reliable

in

high

duty-cycle

applications.

The cam

stacks that

are

used

in

many existing pumping stations

are a

form

of

motor-driven drum programmer.

Programmable-Logic Controllers

Programmable-logic controllers (PLCs) were origi-

nally

developed

for the

automotive

and

machine tool

industries. These units

are

industrial-grade computers

that

have been designed

to

emulate conventional relay

logic.

Reliability

and

maintainability have been devel-

oped

to a

very high level. Programming

can be

accomplished

by any

competent control

electrician,

and

repairs

are

facilitated

by

extensive diagnostic sys-

tems

and

extensive

use of

plug-in components. PLCs

are

less expensive

and

more reliable than discrete

components

for all but the

simplest

of

logic systems,

and

they

are

greatly preferred

for

pumping station

control systems

if

control logic

is

needed.

Microprocessor Controllers

Microprocessor controllers

are

similar

to

programma-

ble-logic controllers,

but (1)

they require computer

programming skills

to

alter their logic

and (2)

they

ordinarily lack diagnostic facilities

and

plug-in com-

ponents. These controllers frequently appear

in

pre-

programmed vendor packages.

This type

of

equipment

is

useful

in

applications

such

as

constant-speed,

multipump

wastewater

lift

stations

and

multipump water system booster stations,

but

it

should

not be

used

for

overall control

of a

pump-

ing

system unless

the

facility

operator has,

or is

will-

ing

to

develop,

the

requisite maintenance

skills.

When

compared with programmable-logic

controllers,

the

simpler design

of a

microprocessor controller does

provide high performance

at low

cost.

20-12.

Altitude

Valves

Altitude valves (see Section 5-5)

are

sometimes used

with

water system storage

to

control pumps indirectly.

The

pumps

are

typically started

by a

time

clock.

When

the

storage tank

is

full

the

altitude valve

closes,

and a

discharge pressure switch located

at the

pump detects

an

increase

in

system pressure

and

shuts

the

pump off.

Because

no

electrical

connections

are

required

between

the

reservoir

and the

pumping station, this

pump

control

is

simple

and

economical.

The

operation

of

pressure-controlled valves

is

explained

in

manufacturers'

literature.

20-1

3.

Monitoring

and

Data

Acquisition

Providing information about

the

operation

of

pumping

station equipment

is

particularly

useful

for

large, unat-

tended pumping stations.

The

simplest systems con-

sist

of

only

an

event recorder

(to

record pump

operation)

or an

annunciator with memory

(to

give

visiting operators

an

indication

of

malfunctions).

Telemetry

systems

may be

added

to

transmit station

status

to an

attended location

for

immediate indication

of

failure.

Full supervisory control

and

data acquisi-

tion (SCADA) systems

are

capable

of

monitoring

pumping

station operation

and

controlling station

operation

from

a

distant location.

Annunciators

An

annunciator monitors

one or

more discrete con-

tacts

and

activates

a

warning light

or

alarm

on

mal-

function.

The

addition

of

memory causes

the

annunciator

to

retain transitory alarms

to

inform

a

vis-

iting operator

of

something gone wrong

in the

worker's absence. Recording annunciators provide

a

printed record

of

each alarm occurrence

as

well

as the

time,

data, acknowledgment,

and

return

to

normal

condition.

Recorders

Recorders produce

a

permanent

record

of

analog

or

discrete variables

on

charts, which

are

frequently

retained

for

many

years. Flows and/or water levels

are

recorded

in

virtually

all but the

smallest pumping

stations.

The

major

disadvantages

of

recorders

are as

follows:

(1)

using information

from

recorder charts

is

tedious;

(2) it

takes considerable

effort

to

keep pens

working

and

charts

replaced;

and (3) the

mainte-

nance

of

recorders

is

particularly poor

in

many

pumping

stations.

Recorders,

therefore, should

be

sparingly used.

Telemetry

Transmitting

data

to an

attended receiving station

makes

them more (and immediately)

useful

as

com-

pared with data stored

in a

remote pumping station.

Telemetered alarms provide immediate warning

of

malfunctions.

Centralized recorders

are

easier

to

maintain,

and

trends

can be

monitored continuously.

Communication

channels

are a

major

part

of any

telemetry system. They

may be

leased

from

common

carriers

(such

as

local

telephone

companies),

or the

pumping

station utility

may

construct

its

own. Histori-

cally,

the

performance

of

leased facilities

has

been

less than satisfactory. Data channels require high

reli-

ability

—

particularly

during storms when leased

facili-

ties

are

least reliable. Leased channels

are

primarily

designed

for

audio (voice) transmissions, which

are

not

seriously impaired

by

intermittent

noise.

But

noise

is

a

severe problem

for

data transmissions. Conse-

quently,

dedicated communication facilities

are

fre-

quently

constructed even though they

are

almost

always more expensive than leasing.

The

various types

of

communication channels used

for

telemetry

and

SCADA systems

are

listed

in

Table

20-7.

The

type

of

telemetry equipment required

depends

on the

type

and

amount

of

data

to be

trans-

mitted

and

whether remote control capability

is

desired.

In the

simplest system,

the

data

are

simply

displayed

on

annunciators, indicators,

and

recorders.

In

larger systems, video display terminals

and

data-

logging typers

are

frequently

used.

Tone

Equipment

Tone equipment

is a

system

for

modulating discrete

contact signals into

the

voice-frequency range

for

transmission over voice-grade communication chan-

nels.

It

permits multiple signals

to be

transmitted over

a

single communication channel

by

using

different

carrier

or

center

frequencies

for

different

signals.

By

utilizing multipoint communication channels, this

system

is

suitable

for

multiple signals

from

a

single

station

and for

multiple signals

from

different

sta-

tions.

For

best performance,

the

number

of

different

car-

rier

frequencies

used

on one

channel should

be

limited

to

about ten.

Two

types

of

tone equipment

are

avail-

able: amplitude modulation (AM)

and

frequency-

shift-keyed

(FSK) modulation. Amplitude modulation

has

very

poor

resistance

to

communication channel

interference

and

should never

be

used.

Frequency-

shift

modulation

has

some resistance

to

communica-

tion channel interference,

but is

still generally unsatis-

factory.

Tone systems

are

ordinarily capable only

of

transmitting data

in one

direction

—

from

the

remote

station(s)

to the

central station.

Digitally

Encoded

Tone

Equipment

Tone equipment manufacturers have generally

recog-

nized

the

interference problem,

and

most

now

offer

digital

encoding

systems that

permit

the

tone

equip-

ment

to

reject errors. Digital encoding also permits

several

pieces

of

information

to be

transmitted using

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

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