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

Admittance

Probes

The

admittance system

is a

modern improved version

of

the

older capacitance probe.

It

consists

of a

slender,

usually

Teflon®-coated

rod

that extends into

the

mon-

itored liquid.

The rod is

excited with

a

low-power,

high-frequency

voltage.

An

electrical

current

flows

between

the rod and a

ground reference, which

may

be

either

the fluid

itself

(if it is a

conductor)

or a

ground plane

on the

wall

of the

tank

(if the fluid is

essentially

a

nonconductor).

The

current

flow

varies

as

the fluid

level changes

and

provides

a

measurement

of

the

tank level.

The

system

is

totally solid state, which

produces neither motion

nor

mechanical

force.

Solid-

state systems

are

most

useful

where

a

transmitted sig-

nal is

required. They

do not

produce

a

direct mechani-

cal

readout,

but

they

can be fitted

with

a

local

electronic

indicator

to

display

the

level.

Ultrasonic

Measurement

The

ultrasonic level measurement system consists

of a

transducer (suspended above

the fluid) and

associated

Table

20-1.

Continued

Station

type

Parameter

Clean

water

Wastewater

Reasons

Sulfur

dioxide

gas

leak

X

Personnel protection

Maintenance dispatch

Public protection

Hydrogen

sulfide

gas X

Personnel protection

Ventilation

equipment status

X X

Equipment protection

Fire

X X

Equipment protection

Maintenance dispatch

Fire brigade dispatch

Unauthorized intrusion

X X

Equipment protection

Public protection

Air

pressure

X X

Maintenance dispatch

Fuel level

X X

Inventory

Control power voltage

X X

Maintenance dispatch

Battery

chargers operating

X X

Maintenance dispatch

Sample pumps running

X

Maintenance dispatch

Engine systems

Engine

running

X X

System operation

Engine available

X X

System operation

Engine

malfunction

X X

Maintenance dispatch

Table

20-2.

Level

Measuring

Elements

3

Clean

Element

water

Wastewater

Discrete

Analog

Comments

Air

bubbler

EE

High

cost,

general application

Float

EP

EF

Low

cost

Ultrasonic

EE

High

cost,

general application when

instrument

air is not

available

Immersible

head transmitter

E F NA E

Replaces

the

diaphragm

box

Differential

pressure

E G NA E

High cost, limited application

Conductivity

EF

E NA

Low

cost

Admittance probe

GG

EG

Moderate cost

Displacer

EF

EG

Limited range

Diaphragm

box

GF

GG

High cost,

obsolete

Microwave

EE

High cost,

use

instead

of

ultrasonic

for

difficult

conditions

(see

text)

a

E,

Excellent;

G.

good;

F,

fair;

P.

poor,

NA, not

applicable.

electronics.

The

transducer periodically emits

a

pulse

of

sonic energy toward

the

liquid,

and the

pulse

is

reflected

by the

liquid

surface

back

to the

transducer.

The

transit time

of

this pulse

is

approximately propor-

tional

to the

distance between

the

transducer

and the

liquid

surface.

The

speed

of the

sonic pulse

is

affected

by

the

density

of the gas

above

the

liquid and,

hence,

by

both barometric pressure

and

temperature. Good-qual-

ity

ultrasonic level systems

are

fully

compensated

and

provide very accurate level measurement.

But

many

systems

are

either uncompensated

or

only temperature

compensated

and

cannot measure with accuracy. Read-

ings

may

also

be

affected

by

foam,

which causes either

a

false

reflection

or a

loss

of

reflection. Discontinuities

in

the

vessel walls

may

also cause

false

reflections.

The

better quality ultrasonic level systems have

microprocessors that reject spurious reflections. Never-

theless, when applying ultrasonic level measuring

devices, check

the

angle

of the

emitted sonic pulse

to

ensure that

the

space within

the

transmission cone

is

clear

of

obstructions. Manufacturers rate ultrasonic

level transducers

for

operation

from

- 40 to

+9O

0

C

(-40

to

+20O

0

F),

but field

installations have

not

been satis-

factory

below

freezing

or in

windy

locations.

The

ultra-

sonic system

is

totally solid state

and

does

not

provide

a

direct mechanical indication

of

level.

Modern microprocessor-based ultrasonic level sys-

tems

are

accurate

and

reliable.

They have supplanted

bubbler systems

in

many facilities where maintenance

of

instrument

air

supply equipment

has

become

uneconomical

or

onerous.

Microwave

The

microwave level measurement system

is

similar

to the

ultrasonic, except that

a

microwave frequency

pulse

is

emitted toward

the

liquid surface. Microwave

pulses penetrate

foam,

and the

speed

of

microwaves

is

affected

very

little

by the

density

of the gas

above

the

liquid

or by

very

low or

high temperatures. Thus,

although

considerably more expensive than

the

ultra-

sonic system,

the

microwave system operates reliably

under

much more

difficult

conditions.

lmmersible

Head

Transmitter

Immersible electronic head transmitters

are

designed

to be

completely immersed

in the

liquid being moni-

tored.

A

cable containing both

the

signal leads

and a

pressure reference tube connects

the

transmitter

to a

junction

box

above

the

vessel

or

channel.

The

pressure

element

is

typically

the

strain gauge type

described

under pressure transmitter.

Immersible

head transmit-

ters were originally designed

for oil field

borehole

use. They

are

economical, rugged,

and

reliable.

They

are

cylindrical

in

shape,

and

some users drop them

into

the

liquid through

a

pipe such that

the

sensing

diaphragm, which

forms

the

bottom

end to the

trans-

mitter,

is flush

with

the

bottom

of the

pipe.

This con-

venient installation

protects

the

transmitter

from

gross

solids present

in

wastewater applications. These units

have

been used

as a

complete replacement

for air

bub-

Figure

20-4.

Displacement-type

liquid

level

control,

(a)

Mercury

switch

closed;

(b)

mercury

switch

open.

biers in clean water applications. However, the typical

size

(25 mm or 1

in.)

is not

suitable

for

narrow spans

(less than

700 mm or 2.3

ft). With 75-mm (3-in.) dia-

phragms, they

can be

used

for

controlling

V/S

pumps

where

narrow spans

are

required. With reasonable fre-

quency

of

cleaning, they

are

suitable

for

wastewater

applications

if the

enclosing

pipe

is

extended below

LWL

to

protect

the

transmitter

from

scum.

Conductance

Probes

Conductance probes utilize

the

ability

of

virtually

all

liquids

to

conduct electricity. When

the

probe

is

immersed,

a

small electric current

flows

between

probes

or

between

a

probe

and the

tank ground,

and

this current

is

detected

by a

sensitive relay. Conduc-

tance probes

are

suitable only

for

discrete

measure-

ments. They

are

used primarily

in

clean water

applications.

In

wastewater applications, grease

deposits

and the

necessity

of

using

low

electrical

energy levels

to

prevent ignition

of

hazardous atmo-

spheres generally rule

out

these devices.

Air

Bubblers

The

bubbler

is

probably

the

most common liquid level

measurement device used

in

wastewater applications.

It

consists

of a dip

tube into which

a

small amount

of

air

is

metered.

The

back pressure produced

is a

mea-

sure

of the

liquid level above

the

tube outlet.

Any

pres-

sure-measuring

device

can

then

be

used

to

produce

either

an

analog

or a

discrete signal,

and a

pressure

gauge

can be

used

for a

direct readout.

The

bubbler

works

well with

any

liquid including sludge

and

scum.

Its

advantages also include

safety

in

explosive atmo-

spheres

and

longevity

in

corrosive environments.

The

perceived disadvantages

are the

complexities, contin-

ual

maintenance, problems with leaks,

and

high

cost.

But

if the

advice

in

this section

is

followed,

air

bubbler

systems

are

easily maintained, leaks

are

easily

fixed,

and

the

system

is the

most

reliable

of

all. Some bub-

bler systems have been replaced

by

tethered

floats

(sometimes

not the

best

of

choices),

by

ultrasonic

measurement systems,

or by

immersible head trans-

mitters. Consider

the

latter

two if the

bubbler

is the

only device requiring instrument air.

A

complete

air

bubbler assembly

for

level measure-

ment

is

shown

in

Figure 20-5. Instrument

air is the

most

vulnerable

of all

systems,

so do not use a

cheap installa-

tion. Oil-free

air

(which cannot

be

obtained with oil-

pumped

and filtered

air) must

be

supplied

by

oil-less

compressors.

Air is

most commonly supplied

by

dual-

redundant,

piston-type compressors derated

from 550 to

690

kPa

(80 to 100

lb/in.

2

)

to 400

kPa

(60

lb/in.

2

)

dis-

charging

to an

oversized receiver

so

that

the

compressor

does

not

cycle

frequently. Use

refrigerated

air

dryers

and

specify

a hot gas

bypass

to

prevent freezing. Provide

for

air

purging

of the dip

tube

at no

less than

200 kPa (30

Ib/

in.

2

).

Neither compressor should have

to

operate more

than

5

min/h

to

produce

28

L/h(l

ft

3

/h) of

air.

The

pressure regulating valve, shown

in

Figure 20-5,

reduces supply pressure

to a

level

low

enough

to

prevent

possible

damage

to the

pressure element

if the dip

tube

plugs

but

high enough

to

ensure purge

flow at

maximum

liquid

level.

If

high accuracy

is

required,

a

constant-flow

regulator

is

needed

to

prevent variations

in

purge

flow

from

introducing

dynamic headloss variations.

The

regu-

lator

is set to

maintain

a

constant

differential

pressure,

usually

20 kPa (3

lb/in.

2

),

across

the

needle valve

so

that

the

valve becomes

a

constant-flow

regulator.

The flowrate

Figure

20-5.

Air

bubbler

assembly.

is

measured

by the

rotameter.

Usually,

the

differential

pressure

regulator, needle valve,

and flow

indicator

are

furnished

as a

factory-assembled unit.

Ak flow is

typi-

cally

adjusted

to

about

0.025

m

3

/h

(1.0

ft

3

/h), but the

flowrates

are

sometimes

a

third

as

much.

Dip

tubes should always

be

installed

so

that they

can

be

easily rodded

out if

they become plugged.

The

bottom

of the dip

tube

is

notched

to

keep bubble size

as

small

as

possible

to

maintain

the

highest accuracy.

Some engineers (and owners) prefer straight

dip

tubes

(for

easy rodding) made

of

13-mm

(

l

/

2

-in.)

Schedule

40 PVC or

stainless-steel (ss) pipe attached

to the

wet-

well wall with pillow blocks. Others prefer

to

install

soft

ss

tubing

in

50-mm (2-in.) conduit

of

harder

ss or

galvanized

steel encased

in the

concrete.

The

conduit

must

be

bent

so

that

the dip

tube

can

enter

the

pool,

but

the

bends must have

a

long radius

to

make

it

easy

to

insert

the dip

tube. Rodding must

be

done with

a

flexible rod

(preferably

furnished

by the

contractor).

In

either installation, make

all

joints,

especially

the

rodding port, easily accessible

for

finding

and

fixing

leaks. Joints should never

be

encased

in

concrete.

Pressure instruments used with bubbler systems

should

be

installed adjacent

to the

bubble pipe (dip

tube)

to

keep tubing lengths

as

short

as

possible

because long tubing runs

(e.g.,

30 m or 100 ft) can add

significant

time delays

to the

measurement, increase

the

possibility

of

leaks that disable

a

bubbler system,

and,

in

closed-loop control systems, cause instability.

One

way to

eliminate these

problems

is to

mount

the

purge

panel

and

pressure instruments

on a

wall

or

hand

rail

adjacent

to the

bubble pipe

to

keep tubing

runs

to

less than about

3 m (10

ft).

High

accuracy

is not

usually

required

for

pump con-

trol—especially

if the

range

of wet

well levels

is 1 m

(3 ft) or

more

—

so the

high cost

can be

somewhat

reduced

by

simplifying

the

system.

The

differential

pressure regulator

is

often

omitted even though this

causes

the

purge

flow to

vary

with liquid level,

and

small

errors result

from

changes

in

system dynamic

pressure

losses

and

changes

in

bubble size.

If an

analog

signal

is

required,

a

bubbler with

an

electronic

change-

of-pressure transmitter

is

satisfactory. Providing

a

dif-

ferent

(or

even

an

additional) system

for

backup

is

good

practice,

so use a float

switch

for the

high-level alarm.

Diaphragm

Boxes

A

diaphragm

box

consists

of a

closed

box

with

an

elastomer diaphragm

forming

one

wall.

The

system

is

obsolete

and has

been supplanted

by

immersible

elec-

tronic pressure elements, which

may be

installed

in

the

same

fashion

and are

much more accurate.

Differential

Pressure Elements

Differential

pressure units

may be flange

mounted

to

a

tank

to

expose

the

pressure element directly

to the

fluid.This

approach

is

practical only when

(1)

freez-

ing

does

not

occur

or can be

prevented

and (2)

access

to the

side

of the

tank

is

readily available.

If the

tank

is not at

atmospheric pressure,

a

second connection

must

be

made above

the

maximum

fluid

level.

The

pressure exerted

on the

pressure element

is

then

a

direct indication

of the fluid

head above

the

connec-

tion point. This system, which

is

simple

and

very

accurate,

is the

favored

level measuring system

in

industrial applications,

but it

does

not

work

if the

spe-

cific

gravity

of the fluid in the

tank

is

unpredictable.

An

isolating valve

can be

placed between

the

pressure

unit

and the

tank

to

facilitate instrument maintenance

without emptying

the

tank,

but

this modification

entails

a

length

of

piping between

the

pressure ele-

ment

and the

tank that

is

subject

to

clogging

in

dirty

water applications.

Install

a

water connection

for

convenient

flushing

of

any

pipe

used between

the

vessel

and the

dia-

phragm. Flushing

can

done

be at

regular intervals

or a

continuous small

flow can be

used

to

isolate

the

pres-

sure cell

from

contaminated

fluid.

20-4. Pressure Measurements

Pressure

is a

basic measurement that

is

made

not

only

for

itself

but

also

for

converting many other measure-

ments into readable parameters.

For

example,

many

level

and flow

measurement systems produce

a

pres-

sure that

can

infer

a

value

for

another parameter.

Pres-

sure

is, in

fact,

almost always measured

as a

differential

pressure. Atmospheric pressure

is

nor-

mally

the

reference,

but

virtually

all

pressure-measur-

ing

elements

can be

arranged

for

connection

of an

independent reference pressure.

The

various types

of

pressure elements

are

summarized

in

Table 20-3.

Pressure gauges should

be

installed

on

both

the

suction

and

discharge

spools

(short

pipe

section)

of

every pump (see Figure

20-6).

Instead

of fixed

gauges,

however, pressure taps with short,

12-mm

(

l

/

2

-in.)

pipes terminating

in

shut-off

cocks

and

quick-con-

nects allow more accurate portable gauges

to be

used.

Portable gauges

are

advantageous because only

two

are

required

and

they

can be

kept

in

calibrated condi-

tion. Suction

and

discharge pressures

are

valuable

checks

on the

condition

of the

station because they

can

be

used

to

diagnose such

troubles

as

blockages,

deposits

in the

pipe,

and

worn impellers.

In

conjunc-

tion with

the

manufacturer's pump curve, pressure

measurements

can be

used

to

measure

flow

with

an

error

of no

more than about

7%

(refer

to

Section 3-9).

An

alternative

to the

installation

of a

pressure gauge

on

the

discharge

of

every pump

is a

single pressure

gauge

installed

on the

manifold; this

is

advantageous

because only

one

gauge

is

needed

and the flow and

pressure

are

likely

to be

more stable than

at a

gauge

near

the

pump.

The

disadvantages

are the

need

to

shut

off

all of the

pumps except

the one

under observation

and

the

loss

of

head between pump

and

gauge.

Bourdon Tubes

The

Bourdon tube

is the

most common

of

pressure-

measuring elements.

It is

used

in

virtually every pres-

sure gauge

and in

many pressure

switches.

It

consists

of

a

semicircular length

of flattened

tubing. When

pressure

is

applied

to the

inside

of the

tube

it

tends

to

straighten,

and the

mechanical motion

can be

coupled

to

indicators, switch mechanisms,

and

transmitter

mechanisms.

It is a

motion-balance device.

The

tube

can be

fabricated

from

copper-based

and

stainless-

steel alloys, which

offer

moderate corrosion resis-

tance. Sewage

and

dirty water must

be

excluded

from

the

Bourdon tube

by a

diaphragm seal,

as

shown

in

Figure 20-6.

All of the

tubing (including

the

Bourdon

tube)

above

the

diaphragm seal

is filled

with glycerin.

The

case

is

also

filled

with glycerin, thereby immers-

ing

the

rack

and

pinion gears both

to

prevent corrosion

and

to

dampen rapid pressure

fluctuations. The

snub-

ber

(a

micropore

filter) can be

omitted, particularly

if

the

shut-off

cock

is

normally closed. Bourdon tube

pressure elements

are

suitable

for

moderate

to

high

pressures

—

generally

more than

100

kPa

(15

lb/in.

2

)

although low-pressure gauges

are

available.

For

hard-to-gauge liquids such

as

sludge,

one

form

of

Bourdon gauge installation consists

of an

annular,

Figure

20-6.

Proper

Bourdon

pressure

gauge

assembly.

Table

20-3.

Pressure-Measuring

Elements

3

Element

Clean

water Wastewater

Discrete

Analog

Comments

Bourdon

E

GEG

Low

cost,

general

application

Diaphragm

E

GEG

Low

cost,

low

pressure

Bellows

G

PEG

Special

applications

Force

balance

E E NA E

Becoming

obsolete

Strain

gauge

E E NA E

Moderate

cost,

general

application

Capacitance

E E NA E

Moderate

cost,

general

application

Manometers

E NA G E

High

cost,

special

application

a

E,

Excellent;

G,

good;

F,

fair;

P,

Poor;

NA, not

applicable.

flexible,

impermeable tube

or

liner that

is

recessed

so

that

its

inner wall

is flush

with

the

pipe.

The

tube

is

connected

to the

Bourdon gauge,

and

both

are

filled

with

a

sensing

fluid.

Various

fluids and

tube materials

are

available.

The

advantages

are a

protected

gauge,

responsive pressure readings,

and

protection

from

clogging.

The

disadvantages

are

cost (perhaps

five

times

the

cost

of a

Bourdon gauge),

a

small amount

of

hysteresis,

and

some loss

of

accuracy.

Diaphragms

Diaphragm pressure elements

are

used

for

lower pres-

sure

spans than those

of

Bourdon tubes.

A

variety

of

materials

can be

used

to

provide corrosion resistance,

and

mechanical

configurations

are

available that per-

mit

mounting

the

diaphragm

flush

with

a

vessel wall

to

permit

use

with

solids-bearing

fluids.

Housings

can

be

designed

to

protect

the

diaphragm

from

high over-

pressures. Large sizes

can be

made

to

provide

suffi-

cient sensitivity

for

measuring very

low

pressure

spans.

The

limited amount

of

mechanical motion

is

sufficient

for

direct actuation

of

precision switches

and

transmission mechanisms. Diaphragms

are

also

coupled

to

Bourdon tubes

by

liquid-filled

capillary

tubes

to

produce

a

common type

of

chemical

seal.

Bellows

A

bellows

is a

stack

of

diaphragms. Bellows elements

can

measure very

low

pressure spans and,

at the

same

time, produce large amounts

of

mechanical motion

for

operating indicators

and

switch

and

transmission ele-

ments. Bellows elements

are not

available

in

corrosion-

resistant

materials, their

configuration

is

unsuitable

for

solids-bearing

fluids, and

they cannot

be

protected

against

overpressure.

Force-Balance Transmitters

The

force-balance mechanism

is

frequently

coupled

to

a

diaphragm

to

produce

a

very rugged pressure trans-

mitter

(see Figure

20-2).

Mechanical power available

from

the

transmitter pneumatics

or

electronics

is

used

in

a

null-balance circuit

to

oppose

movement

of the

diaphragm.

As a

result, mechanical motion and,

hence, wear

is

limited

to a

very small amount. Force-

balance transmitters

can be

built

in

very

low

pressure

spans

and can

withstand high surges

and

overpres-

sures

without

damage

or

alteration

of the

calibration.

The

pneumatic force-balance mechanism

is the

stan-

dard industrial pressure transmitter,

but the

electronic

versions

have become

obsolete.

Strain Gauge

and

Capacitance Transmitters

These transmitters have replaced

the

electronic force-

balance transmitter. Modern

electronic

circuitry permits

the

mechanical deformation

of a

diaphragm element

to

be

limited

to a

very small amount

and

still provide suf-

ficient

movement

to

produce

a

usable output. These

transmitters

are

extremely rugged and,

in

addition, pro-

vide error values

of

less than 0.25%

of

span.

Manometers

A

manometer

is

simply

a

U-tube

(see Figure 3-4),

which

contains

an

indicator

fluid

(such

as

mercury,

air,

and

carbon

tetrachloride)

of

known density.

If a

pressure

difference

exists between

the two

ends

of the

U-tube,

the

indicator

fluid

elevation

in the two

legs

is

different,

and

this

difference

is a

measure

of the

pres-

sure.

If any but the

lowest pressures

are to be

mea-

sured, either

the

U-tube must

be

very tall

or the

density

of the

indicator

fluid

must

be

great. Mercury

has

been

used frequently,

but it is a

very poisonous

substance that must never

be

used

for

potable water

and

should

be

avoided

for any

application.

Manometers

are

useful

for

direct readout

of

very

low

pressures. Mechanisms

for the

conversion

of

indi-

cator

fluid

position

to

useful

motion

for

switch

and

transmitter

operation

are

complex

and

expensive.

Manometers

are

very accurate, however,

and are

sometimes used

in

conjunction with

air

bubblers

to

monitor reservoir levels where very small changes

in

level represent large quantities

of

water.

20-5. Flow

Measurements

in

Pipes

Flow

is an

easy measurement

to

make

in

clean water

applications,

but

very

difficult

to

make

in

wastewater

applications. Most

flow

elements convert

flow to

some

other variable that

is

more readily measured, such

as

pressure, level,

or an

electrical signal.

The

accuracy

of

flow

measurement

is

frequently

of

considerable con-

cern,

and

sustained accuracy

is

difficult

to

achieve

without

giving great care

to the

installation

and the

continued maintenance

of the

instruments (see ASME

MFC-3M

[3]).

Instrument types frequently used

for

flow

measurement

are

summarized

in

Table 20-4

and

discussed

by

Miller

[4].

The

relative costs

for

some

of

the

more common types

of flowmeters,

including

the

4- to

20-mA

output

but

excluding installation,

are

shown

in

Figure 20-7.

The

accuracy

of flowmeters is

(for most types)

sig-

nificantly

affected

by

piping configurations

in the

vicinity

of the flow

element. Because designing

the

approach conditions

to

match factory

or

laboratory

flow

test

facilities

is

ordinarily impossible,

the

manu-

facturers'

claims

of

accuracy

are

rarely realized

in

practice.

The

effects

of

nonstandard approach condi-

tions

on

orifice

and

Venturi meters have been thor-

oughly

explored

and

illustrated graphically

by

Starrett

et

al.

[6].

Investigations

of field

installations show that

errors greater than

10% are the

rule

in

most installa-

tions

and

errors

from

50 to

200%

are not

uncommon

[7].

In

situ

verification

of

calibration

is

essential

for

confidence

and

accuracy. Calibrations with

the

"meter

provers"

(often

used

in the

process industries)

is

impractical

for the flows

typically encountered

in

pumping

stations. Adequately sized tankage associ-

ated with

a

pumping station

can

often

be

isolated

and

used

for an

approximate volumetric calibration,

but

much

more accurate

and

reliable results

can be

obtained

with tracer techniques

(see

Section

3-9).

Pipeline traverses with either

an

inserted magnetic

probe

or a

pitot tube should

be

considered only

as a

last resort. Accurate traverses

are

difficult

to

accom-

plish

in the

presence

of

swirling and/or pulsating

flow

conditions caused

by flow

profile

disturbances.

Orifice

Plates

An

orifice

plate

is a

simple,

flat

restriction

(in a

pipe-

line) that produces

a

differential pressure proportional

to the

square

of the flow

velocity

in

accordance with

Bernoulli's principle

(see

Figure 20-8). Small reduc-

tions

in flow

cause

a

great reduction

in the

differential

pressure,

so

orifice

plates should

not be

used

for flows

that

vary over

a

range greater than

8:1.

Because

the

abrupt

restriction does

not

pass solids, this instrument

is

unsuitable

for

sewage,

and

head recovery down-

stream

is

poor

so

there

is a

high permanent headloss.

Orifice

plates require

a

fully

developed

flow

profile

to

produce accurate measurements,

and,

depending

on

pipe

and

orifice size,

a

straight pipe

6 to 45

diameters

long

is

required upstream.

The

straight pipe require-

ment

downstream

is 5

diameters.

The

orifice plate sys-

tem

becomes unpredictable

for

Reynolds numbers

below

10,000.

Venturi Tube

The

Venturi tube (shown

in

Figure

20-9)

also

pro-

duces

a

differential

pressure proportional

to the

square

of the

velocity. With

its

smooth approach

and

recovery cones,

the

Venturi tube works well

in

clean

water

applications

and

reasonably well

in

wastewater

Table

20-4. Flow-Measuring

Elements

3

Clean

Primary

element

3

wastewater

b

Wastewater

b

Secondary

element

3

Comments

Orifice

plate

G X

Pressure

High

headless,

range

8:1,

error

±1-2%

C

Venturi

meter*

1

G F

Pressure

High

cost,

range

8:

!,error

±1-2%

Flow

tube

G F

Pressure

Medium

cost,

range

5:1,

error

±1-2%

Elbow

meter

F P

Pressure

Cheap,

range

4:

!,error

±2%

e

Magnetic

E E

Electronic

High

cost,

accurate,

range

10:

!,error

±0.5-1%

Doppler

P G

Electronic

Low

cost,

no

intrusion

into

pipe

range

10:1,

error

±2-20%

Transit-time

E F

Electronic

Special

applications,

error

±1%.

ultrasonic

Propeller

E X

Mechanical

Low

cost,

accurate,

range

10:

!,error

±2%

Turbine

E X

Mechanical

Accurate,

range

100:1,

error

±1%

Weir

E X

Level

High

headloss,

standard

open

channel

device,

fair

accuracy,

range

20:1,

error

±2%

Flume

G G

Level

High

cost,

fair

accuracy,

range

20:

!,error

±4%

Vortex

G X

Pressure

change

Range

20:1,

insensitive

to fluid

properties,

frequency

error

±

1

%

a

See

Chapter

2 for

definition.

5

E,

Excellent;

G,

good;

F,

fair;

P,

poor;

X, not to be

used.

c

Typical

error

to

expect

under

"best"

field

conditions with premium quality instruments

and

frequent, careful

recalibration.

Expect

double

the

stated error

if

recalibration

is

infrequent.

d

Venturi

meters

are

considered

to be the

standard

flow

measuring device

for

custody transfer.

6

OnIy

if

accurately calibrated

in

situ.

Figure

20-8.

Orifice

meter.

applications. Head recovery

is

excellent.

The

approach cone tends

to

correct aberrations

in flow

profile

caused

by

poor approach conditions. Some

error

is

introduced

by

irregular

flow

profiles,

but the

error

is

much less than

with

orifice

plates. Standard

Venturi

tubes

are

acceptable

as

custody

transfer

flow-

metering devices

and are

frequently

used

in

clean

water applications

as

revenue meters.

The

principal disadvantage

of a

standard

Venturi

meter

is the

high cost

for

large sizes

and

long laying

length (see

Starrett

et

al.

[6]).

A

number

of

proprietary

designs (see,

for

example, Figure 20-10) have been

developed

to

shorten

the

length

and

reduce

the

cost.

The

performance

of

these proprietary designs

is

gen-

erally excellent,

but the

volume

of

laboratory data

available

to

substantiate their performance

may not be

as

great,

and

their

use as

custody

transfer

meters

may

not be

fully

acceptable.

When

Venturi

tubes

are

used

in

dirty water

or

wastewater

applications,

solids

must

be

excluded

from

the

differential

pressure measurement connections.

The

most common method

is to

install

a

water purge

system,

but

even this

is

sometimes troublesome with

raw

sewage. Diaphragms,

flush-mounted in the

throat

wall,

may

also

be

used,

but

some

difficulty

is

encoun-

tered

in

maintaining measurement accuracy

with

such

arrangements.

Elbow Meter

A

90°

bend

can be

converted

to an

elbow meter

by

installing pressure taps

on the

inside

and

outside

of

the

bend

at

either 22.5°

or 45° [4] and

measuring

the

differential

pressure. Pressures

at

taps installed

at

Figure 20-7. Relative

1993

costs (ENRCCI

=

5050)

for

flowmeters

with

4- to

20-mA

outputs

but no

processor

or

installation

costs. Courtesy

of

WaterWorld Review

[5].

Figure

20-10.

A

typical

proprietary

flow

tube.

angles greater than

45°

are

erratic. Because

the

differ-

ential pressure

is

very

low,

an air

manometer (Figure

3-4)

is,

probably,

the

best type

of

measuring device.

The low

cost

and

ease

of

installation

(or the use of an

existing elbow) make

it a

useful

flowmeter for field

testing

in

stations

not

otherwise equipped with

a flow-

meter.

Magnetic

Flowmeters

The

magnetic

flow

tube (shown

in

Figure

20-1

1)

is an

open cylinder that produces practically

no

headloss

and

readily passes solids.

It

functions

with

any

con-

Figure

20-11.

Magnetic

flowmeter.

ductive

liquid, such

as

water

or

sewage.

It

consists

of

a

flow

tube that develops

a

magnetic

field

across

the

flow

profile.

When

a

conductor passes through

a

mag-

netic

field, an

electrical

potential

proportional

to the

conductor's velocity

(in

accordance with Faraday's

law)

is

produced. With proper design

and

modern

electronics,

very accurate

flow

measurements (which

are

virtually immune

to

distorted

flow

profiles)

can be

obtained.

The

principal

difficulty

is

with wastewater contain-

ing

grease, which

can

coat

the

electrodes

and

interfere

with

the

potential measurement. This problem

has

been overcome somewhat

by

high-quality electronic

circuitry that

can

measure

the

potential

across very

poor conductors. Nevertheless, magnetic

flowmeters

for

wastewater applications should

be

installed

to

facilitate

cleaning.

A

piping configuration that allows

the

electrodes

to be

cleaned

without removing

the

flow

tube

from

the

pipe

is

shown

in

Figure 20-12.

Alternatively,

the

meter

can be

installed

in the

straight

pipe with

a

Dresser coupling

on one

side

and a

Victau-

lic®

coupling

on the

other side

(or

with

Victaulic®

couplings

on

both sides)

to

allow

the

meter

to be

removed while

the flow

goes around

the

installation

in

a

bypass pipe. Another cleaning method sometimes

used consists

of pig

launching

and

removal

stations.

Use

only

soft

pigs

for

magnetic

flow

tubes. Always

arrange

the

magnetic

flow

tube

so

that

(1) no

stress

is

placed

on the flow

tube

and (2)

mating

flanges can be

separated

to

permit easy removal

of the

tube.

The

magnetic

flow

tube liner must

be

both noncon-

ductive

and

nonmagnetic.

Polyurethane,

Teflon®,

and

ceramic

are the

most commonly used

liners.

Polyure-

thane

provides good abrasion resistance

and is the

stan-

dard

liner material.

Teflon®

is

used

for

meters

in

sewage sludge service

for its

superior resistance

to

grease,

but it has

poor abrasion resistance

and

must

never

be

used

if the

sludge contains much grit. Ceramic

liners

are

available

for

meters

up to

about

150 mm

(6

in.)

and are

resistant

to

both

grease

buildup

and

abrasion.

For

meters larger than

150

mm (6

in.),

used

with

liquids that contain

significant

amounts

of

grit,

Figure

20-9. Venturi meter.

use

polyurethane

and

limit

the

velocity

to 3

m/s

(10

ft/s).

The

grit content

of raw

sewage

is

usually

low

enough

to

allow

a

peak

flow

velocity

of 4.5

m/s

(15

ft/s)

and the

grease content

is

usually

low

enough

to

allow

a

mini-

mum

velocity

of 0.3

m/s

(1

ft/s),

so the

usable range

is

15:1.

With

sludge,

the flow

velocity range

is

about

1.0

to 3.0

m/s

(3 to 10

ft/s)

unless grit

is

effectively

removed

before entering

the

meter. Always provide

a

means

for

easily cleaning

the

electrodes. Failure

to

observe these limitations

has

frequently

resulted

in

unsatisfactory

service

in

wastewater applications.

Nonetheless,

the

magnetic

flowmeter

remains

the

most

satisfactory

wastewater

flowmeter. In

clean water

flow

applications,

the

magnetic

flowmeter can

function

from

0.03

to 6.0

m/s

(0.1

to 20

ft/s),

but the

electronics

gen-

erally

limit

the

usable range

to

about

20:1.

Probable errors

in

magnetic

flowmeters are

typi-

cally

stated

as

0.5%

of flow for

liquid velocities

greater than

1.5

m/s (5

ft/s)

and

0.0075

m/s

(0.025

ft/s)

for

velocities less than

1.5

m/s. Most vendors

can

fur-

nish

meters with half

of

these errors,

but

typically

there

is

extra cost

for

custom calibration.

The

mag-

netic

flowmeter is one of the

most accurate meter

types

available.

The

initial cost

may

seem high,

but the

simplicity,

reliability,

and low

maintenance makes

its

life-cycle

cost competitive with other meter types

up

to

about

400 mm (16

in.)

in

diameter.

For

applications

other than sludge

and

larger than

400 mm,

consider

the

transit-time ultrasonic meter.

Sonic

Meters

There

are two

types

of

sonic

flowmeters: (1)

Doppler

meters

and (2)

transit-time meters. Other than their

common

use of

ultrasonic pressure waves,

the

operat-

ing

principles

of the two are

completely

different,

and

their applications

are

different.

The

Doppler meter

is

useful

for

measuring

flows in fluids

containing

at

least

100

ppm

of

suspended

solids

or air

bubbles

for

which

the

transit-time meter

is

useless.

Unlike

the

Doppler

meter,

the

transit-time meter

is

very accurate.

Doppler

Meter

The

Doppler meter (see Figure

20-13)

consists

of a

pair

of

piezoelectric transducers, which

are

usually

contained

in a

common case

and

mounted

on the

out-

side

of the

pipeline.

One

transducer injects

an

ultra-

sonic pressure wave

at

frequency/!

into

the

pipeline,

and

this pressure wave

is

reflected

by

solids,

gas

bubbles,

or

other discontinuities

in the

liquid back

to

the

second transducer.

The

reflected frequency,

/

2

,

varies

by an

amount proportional

to the

reflecting

surface

velocity. According

to the

principle

of

Chris-

tian Doppler,

/a

=

/,(;£)

(20-1)

where

s is the

velocity component axial

to the

normal

pressure wave propagation

and v is the

velocity

of

sound

in the fluid. The

practical

execution

of

this prin-

ciple

is

fraught

with

difficulties.

The

sound pressure

waves

must

be

precisely controlled,

and the

reflecting

particles must

be

moving coaxially with

the

pipe

at a

speed representative

of the flow

(which varies

from

wall

to

center).

There

are a

large number

of

error sources

in

Dop-

pler

flowmeters:

(1)

the

reflecting

surfaces

may not be

moving

at the

same speed

as the

average velocity

of

the

liquid;

(2)

they

may not be

moving parallel

to the

main

liquid

flow; (3) the

frequency

shift

is

affected

by

the

speed

of

sound

in the

liquid, which

is a

function

of

temperature, pressure,

and

density

and

cannot

be

fully

compensated;

and (4) the

angle

of

injection

of the

pressure wave

is

difficult

to

control accurately.

In

summary,

the

Doppler meter

is

inaccurate. Errors

as

great

as 20%

occur regularly

in

practical installations.

Doppler meters

are

used only

in

dirty water applica-

Figure

20-12.

Piping

plan

to

allow

cleaning

a

magnetic

meter

in

situ.

With

permission

of The

Foxboro

Company.

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

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