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

service. Apparently, cone valves

are not

used

in

sludge

systems

and one

manufacturer's representative stated

that

the

valve

is not

intended

for

such service

[34].

Gate

Valves

Gate valves

are

seldom used today

in

sludge service

because

of

their tendency

to

collect debris

and

solids

as

compared

to the

preferable plug valve

[27].

Pump Seal Systems

Most sludge pumps

are

specified with seal water

sys-

tems

rather than mechanical seals

due to the

nature

of

pump

service.

The

seal water

flow

requirement should

be

reviewed with

the

pump manufacturer. Seal water

is

taken

from

a

water system

but

never

from

the

pumped

fluid

discharge. Potable water must

be

sepa-

rated completely

from

seal water

by an air gap in the

seal water tank

or an

acceptable

backflow

preventer

system.

Seal water lines should have

a

pressure-reducing

valve

on the

seal water header supplying seal water

to

a

series

of

pumps.

The

valve prevents

the

seal water

from

entering

the

pump casing

at an

excessive pres-

sure

that could dislodge

the

packing

and

accelerate

packing

and

shaft

sleeve wear. Seal water leaving

the

pump

should

be

conveyed

by

piping

to the floor

drain

system

and

never

be

allowed

to

overflow

the

pump

base

or

pump room

floor.

Pump bases should

be

curbed with drain outlets.

A

typical water seal system

schematic

is

shown

in

Figure

19-13

and a

suggested

pump

drain system

is

shown

in

Figure

19-14.

Flowmeters

The

nature

of

sludge makes

it

necessary

to use flow-

meters that

do not

obstruct

the flow.

Sludge metering

systems

are

expensive,

often

inaccurate,

and

always

require preventive

and

corrective maintenance. Unless

positive-displacement pumps

are

used, meters

are

nevertheless necessary

to

measure

the flow of

sludges.

It

is

also desirable

to

measure solids content continu-

ously

so

that

the

operators

can

determine

the

mass

of

(dry)

solids transferred

to

track process control.

If

some means

of

sampling solids concentrations

is not

included,

the flow

data will only track volumes

of flu-

ids and not

mass

of

(dry)

solids.

Venturi,

magnetic, ultrasonic,

or

in-line density

flowmeters

can

be

used.

The first

three

are

typically

preferable

to the

fourth

due to

pipeline obstruction

considerations.

One

advantage

of a

positive-displace-

ment

pump

is

that

the

pump itself

can be

used

as a

meter

if the

number

of

revolutions

or

strokes over

a

specific

time period

is

recorded.

A

concern about meters

in

sludge pipelines

is

pipe-

line cleaning. Flowmeters should

be

installed

in the

piping system with

a

bypass

for

routine maintenance,

Figure

19-13.

A

typical

water

seal

connection.

Courtesy

of

Stearns

&

Wheler

Engineers

and

Scientists

and J.

Kenneth

Fraser

&

Associates.

calibration,

and (if

needed)

a

water purging system

(see Figure

20-12).

A

pipe

spool

piece

should

be

available

to

replace

the

meter when

it is

removed

for

servicing

or

when

the

line must

be

pigged.

The

relative accuracy

of

each type

of

sludge meter

is

debatable. Many engineers believe

the

Venturi

sys-

tem

is the

best overall choice when accuracy, mainte-

nance,

and

long-term reliability

are

considered.

Others prefer magnetic

or

ultrasonic meters.

Grinders

or

macerators

are

sometimes provided

in

sludge piping systems

for

shredding solids, rags,

and

other debris

to

reduce wear

on

downstream pumps

and

sludge handling

or

processing equipment.

When

an

upstream sludge grinder

is

used

to

protect

the

pump,

add an

interlock between

the

sludge grinder

and

the

pump

to

keep

the

grinder operating whenever

the

pump

is

running. However,

a

manual bypass

should

be

installed around

the

grinder

so it can be

repaired.

Pipe

Flushing

and

Draining

Flushing

sludge piping systems

is a

common mainte-

nance

chore

in

water

and

wastewater treatment plants.

Flushing

requires

an

adequate velocity

and

pressure

of

water

to

dislodge solids, move obstructions,

and

gener-

ally

clean

the

pipe. Rushing connections should

be

lib-

erally

provided

to

allow convenient operations. Flushing

water

should

be

available

to the

piping system

on

both

the

suction

and

discharge sides

of

sludge pumps.

Pipe

Cleaning

Stations

A

well-designed system

for

pipe cleaning allows

the

piping system

to be

maintained

in

mint condition

and

reduces

the

pumping power requirement.

Two

systems

used

for

cleaning pipe

are (1)

rodding

out

each

straight section

of

piping

and (2)

cleaning

the

entire

pipe with pipe-cleaning pigs.

Rodding

Rodding requires specifying cleanouts

on all

elbows,

tees,

and

crosses within

the

pumping station. Require

pipe-sized cleanouts

for

diameters

up to 100 mm (4

in.)

and

cleanouts one-half

the

pipe

size

on

larger pipe

at

bosses

on all

bends

or

other angled

fittings.

Addi-

tionally

(or

alternatively), bends

can be

replaced

by

crosses with removable blind

flanges for

cleanout

or

rodding.

All

cleanout plugs should

be fitted

with

25-

mm

(1-in.)

or

larger plug valves with hose adapters.

The

sludge piping

can

then

first be

drained

or

cleared

by

high-pressure water

in

preparation

for

rodding.

Cleanouts

for

rodding must

be

located

no

farther than

15

m (50 ft)

apart unless special rodding

tools

are

available.

Pigging

The

main advantage

of

cleaning pipe with pigs

is

that

the

pipe length

is

practically unlimited. Cleaning pigs

range

from

soft

swabs

to flexible,

abrasive pigs

to

rigid

steel

scraper pigs. Block

ice or

bagged

ice

cubes

can

also

be

used; with this method, retrieval

is

unnec-

essary

and

permanent jamming cannot occur.

Flexible

pipe-cleaning pigs

can

clean several pipe sizes,

but are

Figure

19-14.

Pump

drain

system.

This

arrangement

is

also

applicable

to

horizontal

pumps.

Courtesy

of

Stearns

&

Wheler

Engineers

and

Scientists

and J.

Kenneth

Fraser

&

Associates.

limited

in

turning around pipe

crosses.

Transmitter-

cleaning

pigs enable

the

maintenance crew

to

locate

underground

pipe

failures

accurately.

Cleaning

the

piping with pigs requires provisions

for

the

insertion

and

retrieval

of the

pigs.

The

pigs

are

inserted

in an

isolated length

of

pipe

one

size larger

than

the

piping

to be

cleaned

and

located

as

close

to

the

primary pumps

as

possible.

The pig

launcher

is

fit-

ted

with

a

pressure gauge,

a

drain,

a

high-pressure

water

fitting, and an

easily removed blind

flange

or

mechanical coupling.

The

pump discharge

and the pig

launcher pressure gauges should

be

easily read

from

one

point. Ideally, this control point

is

protected

from

a

pipe rupture

and

includes other controls

as

neces-

sary.

The

pressure gauges last longer

if

they

are filled

with

glycerin

and

protected

by an

isolation diaphragm

and

a

snubber

(glycerine-filled capillary tubes)

to

keep sludge

out of the

gauge

and

attenuate sharp pres-

sure spikes.

The

details

of

cleaning pipe with pigs

are

discussed

by

Play

ford

in the

Proceedings

[14,

pp.

878-879].

The

ease

of

cleaning pipes with pigs

and the

increasing cost

of

power warrants

the

installation

of

pipe-cleaning, launching,

and

receiving stations

on

most

pipe systems

and

networks.

Safety

Factors

for

Sizing

Sludge piping systems should

be as

short

as

possible

with

a

minimum

number

of

bends. Bends (unless

replaced

by

tees

or

crosses) should

be the

long radius

type

to

minimize

the

headless.

In

addition,

the

head-

loss contributed

by

check valves,

flowmeters, and

other piping

accessories

should

be

considered criti-

cally

in the

analysis

of the

system.

If at all

possible,

omit

check valves

and fittings

that would cause plug-

ging

of

solids

or add

unnecessary

headless.

In

each sludge pumping system,

at

least

two

pumps

sized

at

full

capacity should

be

provided

so

that

at

least

one

full-capacity standby pump

is

available.

It

may

well

be

appropriate

to use a

positive-displace-

ment

standby pump because

of its

reliability

and

abil-

ity

to

move heavy sludge.

19-5. Long-Distance

Pumping

wastewater sludges through pipelines

1.6

km

(1

mi)

long

(or

longer) requires greater attention

to

issues that

are not as

critical

in

pipelines

of

shorter

length. Small variations

in the

unit dynamic headloss

are

magnified

by the

length

of

piping

in a

long pipe-

line. Dynamic headlosses

are

significantly

affected

by

•

Concentration

of the

sludge

•

Type

of

sludge (raw, secondary,

or

digested)

•

Variability

of

blends

if a

mixture

of

sludge

is

pumped

•

Additives used

in the

wastewater

or

sludge treat-

ment

process (polymers

or

metal salts).

Other design

and

operating considerations

for

long

sludge pipelines include

•

Periodic venting

of

gases

released

from

solution

or

generated

by the

digestion process

•

Correct sizing

of the

pipeline

to

allow

for

large

flowrate

variation over

the

system lifetime

•

Providing sludge storage

or a

backup pipeline

to

enhance reliability

and

allow

for

pipeline mainte-

nance

•

Selection

of

pumps that

can

perform satisfactorily

over

a

wide range

of

discharge pressures

as

concen-

trations

and

sludge blends change.

The

designer

of a

long sludge pipeline must give

serious consideration

to

measuring

the

characteristics

of

the

range

of

sludge blends, solids concentrations,

chemical additives,

and

sludge conditions (including

temperature) likely

to be

encountered.

The

need

for

site-specific sludge testing

is

highlighted

by the

vari-

ability

of the

published data

[13,

18]

on

sludge char-

acteristics

—

particularly

when

major

investments

are

to be

made

in

pumping transport

and

storage systems.

Extensive testing

is

even more necessary when undi-

gested secondary sludges

are to be

pumped. These

complex problems

are

covered

in

more detail

by

Carthew

e/

a/.

[18].

The use of

specially designed diaphragm pumps

capable

of

large

flowrates at

pressures

up to

2400

kPa

(350

lb/in.

2

)

has

been successful.

The

application

of

such

pumps

is

unusual,

but

several advantages include

(1)

low

maintenance,

(2)

high reliability,

(3)

excellent

protection against

overpressuring

the

pipeline,

(4)

clean, quiet operation,

and (5)

separation

of

sludge

from

most

of the

pump. Diaphragm pumps

not

specif-

ically

manufactured

for

sludge

or

slurry service

do not

have

these advantages.

19-6. References

1.

Metcalf,

L.,

and H. P.

Eddy, American Sewage

Practices.

Volume

HI:

Disposal

of

Sewage,

3rd

ed.,

McGraw-Hill,

New

York

(1935).

2. EPA

Process Design Manual

for

Sludge

Treatment

and

Disposal,

EPA

625/1-79-011,

pp.

14-4-14-17,

14-29-14-

37,

U.S.

Environmental

Protection

Agency,

Municipal

Environmental

Research

Laboratory,

Office

of

Research

and

Development,

Cincinnati,

OH

(September

1979).

3.

Babbitt,

H.

E.,

and D. H.

Caldwell,

"Laminar

flow of

sludges

in

pipes with special reference

to

sewage

sludge."

University

of

Illinois Engineering Experiment

Station,

Bulletin

Series,

No. 319

(1939).

4.

Caldwell,

D.

H.,

and H. E.

Babbitt,

"The

flow of

muds,

sludges,

and

suspensions

in

circular

pipe,"

Trans,

of

the

American

Institute

of

Chemical Engineers,

37, 237

(April

1941).

5.

Wolfs,

J.

R.,

"Factors

affecting sludge force

mains,"

Sewage

and

Industrial

Wastes,

22, 1

(January

1950).

6.

Babbitt,

H.

E.,

and D. H.

Caldwell, "Turbulent

flow of

sludges

in

pipes,"

University

of

Illinois Engineering

Experiment

Station, Bulletin Series,

No. 323

(1940).

7.

Brisbin,

S.

G.,

"Flow

of

concentrated

raw

sewage

sludges

in

pipes,"

Journal

of

the

Sanitary Engineering

Division

American Society

of

Civil Engineers,

83(SA3),

1274

(

June

1957).

8.

Chou,

T.

L.,

"Flow

of

concentrated

raw

sewage sludges

in

pipes,"

Journal

of

the

Sanitary Engineering Division

American

Society

of

Civil Engineers,

84(SAl),

1557

(February 1958).

9.

Span,

A.

E.,

"Pumping sludge long

distances,"

Journal

of

the

Water

Pollution Control Federation,

43,

1702

(August

1971).

10.

Dick,

R.

L.,

and B. B.

Ewing,

"The rheology

of

activated sludge," Journal

of the

Water

Pollution

Control

Federation,

39,

543

(1967).

11.

Bourke,

J.

D.,

"Sludge handling characteristics

in

piped

systems,"

Proceedings

of the

Northern Regional

Conference

of the

California

Water

Pollution

Control

Association,

Monterey, California (October

19-20,

1973).

12.

Vesilind,

P.

A.,

Treatment

and

Disposal

of

Wastewater

Sludges,

2nd

ed., Chapter

4, Ann

Arbor

Science

Publ.,

Ann

Arbor,

MI

(1979).

13.

Mulbarger,

M.

C.,

et

al,

"Pipeline

friction losses

for

wastewater sludges," Journal

of the

Water

Pollution

Control

Federation,

53, 8,

1303-1313

(August

1981).

14.

Sanks,

R.

L.,

C. W.

Reh,

and A.

Amirtharajah

(eds.),

Conference

Proceedings, Pumping Station Design

for

the

Practicing Engineer,

pp.

854-899.

Montana State

University, Bozeman,

MT

(1981).

Available through

interlibrary

loan.

15.

Govier,

G.

W.,

and K.

Aziz,

The

Flow

of

Complex

Mixtures

in

Pipes,

Van

Nostrand

Reinhold,

New

York

(1972).

16.

Cain,

C.

B.,

et

al.,

"Design

of

90-mgd wastewater

reclamation plant," Journal

of the

Environmental

Engineering

Division American Society

of

Civil

Engineers,

107(EEI),

29

(1981).

17.

Rimkus,

R.

R.,

and R. W.

Heil,

"The rheology

of

plastic sewage sludge,"

in L. K.

Cecil,

ed.,

Second

National

Conference

on

Complete

Water

Reuse,

American Institute

of

Chemical

Engineers,

Chicago,

DL

(May

4-8,

1975).

18.

Carthew,

G.

A.,

C. A.

Goehring,

and J. E. van

Teylingen,

"Development

of

dynamic headloss criteria

for raw

sludge pumping," Journal

of the

Water

Pollution

Control

Federation,

55,

472-483

(May

1983).

19.

Hanks,

R.

W.,

and B. H.

Dadia,

"Theoretical

analysis

of

the

turbulent

flow of

non-Newtonian slurries

in

pipes,"

Journal

of the

American Institute

of

Chemical

Engineers,

17, 554

(May 1971).

20.

Hanks,

R.

W.,

"Principles

of

slurry

pipeline

hydraulics,"

in N. P.

Cheremisinoff

(ed.),

Encyclopedia

of

Fluid Mechanics, Vol.

5,

Slurry Flow

Technology,

Gulf

Publishing, Houston

(1986).

21.

Darby,

R.,

"Laminar

and

turbulent pipe

flows of

non-

Newtonian

fluids," in N. P.

Cheremisinoff

(Ed.),

Encyclopedia

of

Fluid

Mechanics, Vol.

7,

Rheology

and

Non-Newtonian

Flows, Gulf Publishing, Houston (1988).

22.

Pipeline Design

for

Water

and

Wastewater,

American

Society

of

Civil Engineers, Report

of the

Task

Committee

on

Engineering Practice

in the

Design

of

Pipelines,

New

York

(1975).

23.

Karassik,

I.

J.,

W. C.

Krutzsch,

W. H.

Fraser,

and J. P.

Messina, Pump Handbook

2nd

ed.,

Chapters

2 and 9,

McGraw-Hill,

New

York

(1986).

24.

Henshaw,

T.

L.,

Reciprocating Pumps, Chapter

6, Van

Nostrand Reinhold,

New

York (1987).

25.

Baumeister,

T.

(ed.),

Standard Handbook

for

Mechanical

Engineers,

8th

ed.,

Chapter

14,

McGraw-

Hill (1978).

26.

Wylie,

E.

B.,

and V. L.

Streeter,

Fluid Transients,

Chapter

14,

McGraw-Hill,

New

York (1978).

27.

Water

Pollution

Control

Federation

and the

American

Society

of

Civil Engineers,

Wastewater

Treatment

Plant Design, WPCF Manual

of

Practice

No. 8.

Lancaster Press, Lancaster,

PA

(1977).

28.

Burrage,

K.,

President,

Penn

Valley Pump

Co.

Private

communication (September 1997).

29.

Jones,

G.

M.,

Vice President, Brown

and

Caldwell

Consultants. Private communication (February

1989).

30.

Svaras,

J.

C.,

Application

Engineer,

Alfa Laval Pumps,

Inc. Private communication (September 1997).

31.

Caballero,

R.

C.,

County Sanitation District

of Los

Angeles. Private communication (undated; tested

March

1983).

32.

Cunningham,

J.,

Chief Operator, Clinton Wastewater

Treatment Plant. Private communication (January

1989).

33.

O'Keefe,

W,

"Valves

for

Corrosives

and

Slurries,"

Power,

Special

Report,

pp.

51-516

(May

1986).

34.

Allis-Chalmers

Valve Division. Private communication

(January

1989).

The

purpose

of

this chapter

is to

introduce

the

basic

instrumentation

and

control devices that

are

needed

and

used

in

pumping stations.

The

information pre-

sented should

be of

value

to (1)

designers,

who

must

plan

for and

specify

equipment,

(2)

project engineers,

who

must

decide

how a

pumping station will operate

and

what kind

of

equipment will

be

needed,

and (3)

owners,

who

must maintain

the

equipment selected.

The

types

of

instruments

and

their principles

of

opera-

tion

are

discussed

in

detail

by

Liptak

[I].

Specific

control devices

and

their advantages

can be

found

in

manufacturers'

literature. However, what

is

needed

(and

presented

in

this chapter)

is a

careful

analysis

of

the

relative

merits

of

instruments specifically

for use

in

pumping stations.

20-1.

Reliability

Instruments

for

pumping stations should

be

selected

to

provide long

life,

low

maintenance,

and

high reliabil-

ity

in

damp, corrosive environments.

These

require-

Chapter

20

Instrumentation

and

Control Devices

ROBERTS.

BENFELL

CONTRIBUTORS

Russell

H.

Babcock

Mead

Bradner

Harold

D.

Oilman

Mayo

Gottliebson

Paul

C.

Leach

John

Leak

Earle

C.

Smith

James

Taube

Morton

Wasserman

ments

are not

necessarily unique

to

pumping stations,

because many industrial applications have similar

requirements.

But

industrial users

are

frequently less

concerned with extended

life,

and

most such users

maintain highly skilled, full-time maintenance

staffs

to

service

sophisticated

equipment. Pumping station

operators

often

have limited sophisticated mainte-

nance skills

and

usually rely

on

outside maintenance

services. Furthermore, most pumping stations

are

designed

to

operate

for 20 yr or

more without major

reconstruction. Accordingly,

the

simplest instrument

that

will perform

the

required

function

should

be

selected

for

pumping stations,

and

premium materials,

such

as

stainless

steel,

should

be

used wherever

possi-

ble to

provide maximum resistance

to

environmental

conditions.

Because many pumping stations

are

operated unat-

tended

and the

potential property damage

is

signifi-

cant

if a

serious malfunction occurs, instruments

should

be

selected

for

their inherent reliability. Back-

up

systems should

be

provided,

and the

consequences

of

component failure must

be

carefully considered.

Instrument

design

is a

highly specialized

field

that

requires knowledge

of

both instrumentation equip-

ment

and fluid

mechanics.

Any

engineer

who

designs

instrumentation

and

control systems must carefully

consider

the

application limitations

of the

systems

to

be

specified

and

should seek

the

services

of

qualified

specialists

if

unusual

and/or complex systems

are

required.

20-2.

Instrument Selection

There

are

many instruments

and

instrument systems

on the

market that were designed specifically

for

water

and

wastewater facilities.

But

only some

of

this

equipment

was

designed

with

serious consideration

given

to the

requirements

of

such facilities.

Unfortu-

nately, much

of it was

designed with

low

cost

as the

primary consideration

to

compete

in a

market

in

which

much equipment

(1) is

bought

from

the

"low

bidder"

and (2) is

probably inferior

to

equipment that

was

designed primarily

for the

industrial marketplace.

Every

instrument

or

functional instrument system

must

be

evaluated solely

on the

basis

of its

suitability

for

the

application. Instruments

and

instrument

sys-

tems designed

for the

water

and

wastewater market-

place

are not

inherently better

or

worse than

instruments

and

instrument systems designed

for

industrial

applications

for

service

in

pumping stations.

Environmental

Conditions

The

environmental conditions

to

which

the

various

instruments

in a

pumping station

are

subjected vary

considerably

from

station

to

station

and

from

location

to

location within

a

pumping station. Most pumping

stations have areas

of

high humidity, many have areas

of

extreme temperature,

and

most wastewater pump-

ing

stations have areas that

are

corrosive

or

classified

as

hazardous

by the NEC

[2].

Instruments that

are

resistant

to

these conditions

are

generally available.

However, instrument life,

reliability,

and/or

maintain-

ability

are

adversely

affected

by

service under such

conditions.

Effects

of

corrosive gasses

and

high

humidity

can be

reduced

or

eliminated

by

adequate

ventilation

or by

removing

as

much

of the

instrumen-

tation equipment

as

possible

to

more hospitable areas

of

the

station. Electrical instruments

in

hazardous

atmospheres

can be put

into explosionproof cases,

but

pneumatic

instruments

and

intrinsically

safe

elec-

tronic instruments

are

easier

to

maintain.

Reliability

and

Maintainability

In

many instruments,

the

"motion-balance"

principle

is

used

for

operation. These instruments have springs,

racks

and

pinions,

and

bearings that

are all

subject

to

wear.

A

simplified motion-balance Bourdon-tube

mechanism used

for

pressure measurement

is

shown

in

Figure

20-1.

An

increase

in

pressure within

the

Bourdon tube straightens

it

slightly, which

lifts

the

linear voltage differential transformer (LVDT)

core.

The

primary

coil

is

excited

with

a

high-frequency

alternating current,

and the

secondary

coils

are

con-

nected

in

series

opposition

so

that

the two

voltages

produced

are of

opposite

phase.

As the

core

moves

up,

the

magnetic coupling between

the

lower secondary

coil

and the

primary

coil

decreases

and the

magnetic

Figure

20-1.

Motion-balance

mechanism.

coupling between

the

upper secondary coil

and the

primary coil increases. Suitable phase-sensitive

elec-

tronics

are

used

to

produce

a

direct current output

pro-

portional

to

core position.

"Force-balance"

instruments reduce

the

amount

of

movement

and,

therefore, reduce wear.

A

simplified

force-balance mechanism used

for

pressure measure-

ment

is

shown

in

Figure

20-2.

An

increase

in

pressure

within

the

housing displaces

the

diaphragm

to the

left,

which

rotates

the

force beam clockwise about

the

ful-

crum

and

unbalances

the

null detector.

The

amplifier

detects this unbalance

and

increases

its

output, which

tends

to

rotate

the

force beam counterclockwise until

balance

is

again established.

The

current passed

through

the

solenoid

force

motor

is

also

the

output

current, which

is

typically

4 to 20 mA. By

designing

the

solenoid

to

provide

a

linear response

and

using

a

high

gain null detector

and

amplifier,

the

motion

required

to

provide

a

signal

is

reduced

to a

very small

value.

The

only parts actually strained

are the

dia-

phragm

and the

seal

at the

fulcrum.

The

stiffness

of

these parts

is

made

insignificant,

which results

in a

very

stable pressure-measuring device.

Many

modern instruments utilize

solid-state

cir-

cuitry

with

no

moving parts. Ordinarily, force-bal-

ance

and

solid-state instruments

are

more

reliable

and

require less maintenance than motion-balance

instruments. However, motion-balance instrument

mechanisms

are

more easily understood

by

mainte-

nance personnel familiar with

mechanical

equip-

ment. Consider

who is

going

to

maintain

the

instruments before selecting exotic modern equip-

ment that

may

require highly skilled specialists

for

routine servicing.

Utilities

Many

instruments require either electrical power

or

instrument

air for

their operation,

and

such facilities

constitute

a

major

cost

of

instrumentation systems.

Electric power must

be (1)

conditioned

to

remove

surges, which

may

interfere with proper instrument

operation

or

even damage sensitive electronic

cir-

cuitry,

(2)

converted

to the

required voltage levels,

and

(3)

regulated

to

ensure stable operation.

In

many

pumping

stations, provision must

be

made

to

ensure

that electric power

for

instruments

is

maintained

dur-

ing

service power outages.

Pneumatic instruments require clean,

dry air

except

for

bubbler systems connected

to

pressure

switches.

If the

instrument

air

supply

is not of

ade-

quate quality, instrument reliability

suffers

and

mainte-

nance requirements increase very significantly. Because

the

cost

of

purchasing

and

maintaining adequate instru-

ment

air

equipment

is

significant,

electronic instrument

systems

are

most

often

selected

for

modern systems.

However, pneumatic instruments have

significant

advantages

in

high temperature, corrosive,

and

haz-

ardous locations. Also, level measurements

are

more

Figure

20-2. Force-balance mechanism.

reliably

made with pneumatic bubbler

systems

than

with

electronic systems.

In

many pumping stations,

particularly large waste water stations, pneumatic

instruments

can be

used advantageously

if the

mainte-

nance workers

are

familiar

with

this type

of

equip-

ment.

It

is

true that explosionproof cases

are

available

for

field

instruments,

but

there

is no way to

service

or

calibrate them without opening

the

case,

which

makes them

"nonexplosion"

proof. Intrinsically

safe

circuitry sounds easy

but is

difficult

to

accomplish.

In

warm

climates, pumping stations

are

often

outdoors

and

sunlight

can

heat

the

cases

to

temperatures that

electronic devices cannot withstand. Chlorine

and

hydrogen

sulfide

can

quickly destroy

electronic

cir-

cuitry.

The

choice

of

electrical

or

pneumatic equipment

may

depend

on the

skills

of the

service

personnel.

A

skilled technician

can

disassemble, clean,

and

reas-

semble pneumatic equipment quickly

(often

in

much

less

than

an

hour)

in the field.

Electronic

equipment usually

has to be

removed

and

returned

to the

factory.

Accuracy

requirements

for

instrument systems

are

generally

not

severe.

The

exception

to

this rule

is

"custody

transfer"

flow

measurements (situations

in

which

money passes

from

one

entity

to

another based

on flow

quantities) that occur

in

water

or

wastewater

applications. Instrument systems usually consist

of a

number

of

interdependent elements, each

of

which

contributes

to the

total measurement

error—

errors

that

are

always greater than

the

ones published

in

manufacturers'

literature.

A

major

source

of

error

is

the

piping

configuration

at the

point

of

measurement.

It

seldom resembles

the

piping

in the

hydraulics labo-

ratory

that

was

originally used

to

establish instrument

performance.

The

error

of

many

instruments

is

stated

as a

per-

centage

of

span. Most instruments ordinarily

operate

at

a

point considerably below

the

maximum cali-

brated range.

If

high accuracy (such

as in

custody

transfer)

is

required, extreme care must

be

taken

to

be

sure that

the

instruments

are

properly applied

and

installed

and

that unnecessary error

is not

introduced

by

less than optimum configurations

of

multiple

instruments

in a

system. Where instrument systems

are

used

for

custody transfer,

it

must

be

recognized

that

a

certain amount

of

error

is

inevitable,

and

con-

tracts must

be

written

to

recognize

and

accept

the

error.

Package

Systems

Package systems

are

available

to

perform many

instrumentation

functions

commonly used

in

pumping

stations. Common examples include pump sequence

control

and

chlorination

control.

The

obvious advan-

tage

of a

package

is

that

it can be

applied

by a

designer

who is not

skilled

in

instrumentation system

design.

The

following

are

disadvantages

of

package sys-

tems:

• It may be

impossible

to find one

that

does

exactly

what

is

desired,

which

may

lead

to

compromises

in

control strategy.

•

Packages

may be

difficult

or

even

impossible

to

modify

if the

control strategy needs

to be

changed.

•

Nonstandard signals between elements

are

fre-

quently

used,

so all

replacement parts must

be

obtained

from

the

original vendor,

who may

discon-

tinue

the

model

or

even

go out of

business.

•

Packages tend

to be

designed

so

that only

the

origi-

nal

vendor

can

service them.

•

Many

are

designed without adequate consideration

of

the

environment

in a

pumping station

or the

need

for

reliability

and

long

life.

This

is not to say

that packages should never

be

used.

They frequently provide cost-effective solutions, par-

ticularly

in

relatively simple applications.

But

take

the

same care

in

selecting

packages

as in

selecting

com-

ponents

for

custom-designed instrument systems.

Measurement

Types

There

are two

basic types

of

measurement: analog

and

discrete.

Analog measurements produce

a

signal

or

readout proportional

to the

value

of a

certain parame-

ter, such

as flow or

level. Discrete measurements pro-

duce

a

true/false indication, such

as

valve

"open/

closed"

or

pump

"running/stopped."

Signal Transmission

Many

instruments provide only

a

local indication

of a

measurement.

A

pressure gauge

is a

common exam-

ple. But,

frequently,

signal transmission

to

another

location

is

required. Wherever

possible,

use

standard

signal transmission systems, such

as 4 to 20

mA

for

electrical systems

and 20 to 100

kPa

(3 to 15

lb/in.

2

)

for

pneumatic systems.

These

systems allow unre-

stricted interconnection

of

instruments

of

different

manufacturers.

Many nonstandard systems have been

used

over

the

years

in

water

and

wastewater facilities.

Some

are

well established

and

conversion devices

are

readily available

to

incorporate such signals into stan-

dard systems,

but

such conversion devices

(1) add

more equipment

to

maintain,

(2)

reduce reliability,

and

(3)

increase system error.

Variables

Measured

Variables

or

parameters frequently measured

or

sensed

are

listed

in

Table 20-1.

Not all of

them

would

be

used

in any but

very large

stations.

The

purpose

of a

pumping station

is to

pump

—

not

to

col-

lect data.

If

there

is not a

well-defined

use and

need

for

data,

do not

collect

them.

If a

device

is not

essen-

tial, omit

it.

20-3.

Level

Measurements

Level

is one of the

most common measurements made

in

pumping stations. Instrument types

frequently

used

for

this measurement

are

summarized

in

Table 20-2

more

or

less

in

order

of

popularity.

Floats

A

float is a

buoyant body that

rides on the

surface

of a

liquid

and

includes

a

method

of

monitoring

its float

elevation.

A float may be

suspended

from

a rod or a

cable that mechanically links

it to an

elevation moni-

toring device,

or the float may

contain

a

magnet

and

simply

slide

up and

down

on a rod

containing reed

switches that

are

magnetically actuated. Another type

of

float,

sometimes called

a

"tethered

float,"

consists

of

a

spheroidal, hollow vessel with

a

mercury switch

inside. When

lifted

(or

buoyed),

the float

changes

position

and

actuates

the

switch. Tethered

floats

(Fig-

ure

20-3)

are

frequently

used

in

wastewater applica-

tions with some degree

of

success. Floats have these

disadvantages:

•

They

are

difficult

to

adjust.

•

Access

to the wet

well

is

required

for

servicing.

• The

electric cable

flexes and

wears out,

so the float

must

be

replaced periodically

(2 to 3

yr).

Float devices, which

are

available

in a

wide variety

of

materials,

are

simple

and

inexpensive. They

are not

well

suited

to

analog transmission applications

because

of the

complications involved

in

converting

the

motion produced

to a

transmission signal,

but

they

are

good

for

discrete applications

and

local

indication

applications. Because

the

presence

of floating

debris

in

wastewater

may

foul

the floats, use

only large

floats

and

call

for

periodic

washdown

in the O&M

manual

in

such service.

Although

popular, tethered

floats

have sometimes

given

trouble

and

they

do not

carry

a UL

label

for use

in

Class

I,

Division

1

hazardous areas such

as

sewage

wet

wells.

The

hazard

can be

circumvented

by

using

very

low

energy, intrinsically

safe

control circuitry,

but,

because this adds another level

of

design compli-

cation, some engineers avoid them.

Displacers

differ

from

floats in

that

the

buoyant body

has

a

density greater than that

of the

liquid being mon-

itored.

The

displacer,

shown

in

Figure 20-4,

is a

cylin-

drical

body whose height

is at

least equal

to the

maximum

level span

to be

monitored.

As the

displacer

is

immersed,

it

loses apparent weight

by the

weight

of

the

liquid displaced

(in

accordance with

Archimedes'

principle),

and

this apparent weight loss stretches

the

spring slightly

and

actuates

the

mercury switch used

to

indicate liquid level

and

start

or

stop pumps. Built-

in

limit stops prevent overstretching

of the

spring.

Displacers

do not

produce

any

significant

mechanical

motion,

but

rather work

on the

force-balance princi-

ple. They

are

very suitable

for

both analog

and

dis-

crete measurements,

but do not

produce

a

direct

readout

of

level.

The

useful

measurement range

is

lim-

ited

by the

practical cylinder

length

—

less

than

10 ft.

Displacers

are

better than

floats in

wastewater applica-

tions because

the

liquid

rises and

falls

on a

smooth

cylinder producing

a

washing action, although

fouling

is

still

possible.

Figure

20-3.

Tethered

floats.

Table

20-1.

Parameters

Frequently

Monitored

Station

type

Parameter Clean water Wastewater

Liquid level

Wet

well

X

Suction storage

X

Distribution storage

X

Dry

well

X

Pressures

Distribution system

X

Pump

suction

X X

Pump

discharge

X X

Flow

Influent

or

effluent

X X

Pump

status

Running

X X

Speed

X

Available

X X

Maintenance dispatch

Primed

X X

Dry

suction

X

Bearing temperature

X X

Motor temperature

X X

Operation time

X X

Time

start/stop

X X

Start/stop

sequence

fail

X X

Vibration

X X

Water

in

motor

X X

Valve

and

gate positions

X X

Chemical feed systems

Storage weight

or

level

X X

Feed rate

X X

Chlorine

residual

X X

Feed pumps running

X X

Utilities

and

environment

Voltage

X X

Amperage

X X

Wattage

X X

Power factor

X X

Temperature

X X

Explosive atmosphere

X

Chlorine

gas

leak

X X

Reasons

Pump

control

Maintenance dispatch

Pump

control

Maintenance dispatch

Pump

control

Maintenance dispatch

Pump

control

Troubleshooting

Pump

control

Troubleshooting

Billing

Chemical feed control

System

operation

System operation

System

operation

Pump

control

Equipment

protection

Equipment protection

Equipment

protection

Maintenance

dispatch

Troubleshooting

Litigation defense

Maintenance dispatch

Equipment

protection

Maintenance dispatch

System operation

Inventory

Maintenance dispatch

System

operation

Feed rate control

Maintenance dispatch

Alternate source control

Troubleshooting

Expansion planning

Power cost control

Equipment

protection

Maintenance dispatch

Personnel protection

Equipment protection

Personnel protection

Maintenance dispatch

Public protection

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

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