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

C/S and V/S

drives.

Such

a mix can be

successful only

if

the V/S

pumps

are

larger than

the C/S

pumps.

The

reasons

are

given

in the

following subsections.

If,

however,

all of the

pumps

are to be of the

same size,

a

mix

will neither save money

nor be

successful,

as

explained

by the

following reasoning.

Many,

perhaps most, pumping requirements

can be

satisfied

with

two

duty pumps

and a

standby.

If any of

the

pumps

has a V/S

drive,

the

standby must have

a V/S

drive also,

so

only

one

pump

can be a C/S

unit. Assume

each pump

has a

capacity

of 100

units. Below

100

units

only

a V/S

pump

can

operate. Suppose

the flow is

slightly

more than

100

(e.g., 102),

and the C/S

pump

is

started.

The C/S

unit

can

discharge nothing

but 100

units,

so the V/S

pump must

either

slow

to 2

units

or

cycle

on and off at its

minimum desirable

speed.

Cen-

trifugal

pumps cannot tolerate less than

30 or 35% of

their maximum capacity without excessive vibration,

cavitation,

and

damage

to

bearings

and

shafts.

As the

standby

cannot

be

involved (consider

it out of

service),

the

only solution (for discharge

to

equal

influent)

is to

add a

fourth pump

and

only

a V/S

will

do. Now we

have

three

V/S

drives

and one C/S

drive plus

a

larger

dry

well with more piping.

No

savings

there!

If the V/S

pump

is

allowed

to

cycle

on and

off,

the

discharge

surges

and the wet

well must

be

made large enough

for

adequate pump cycle

periods.

In

a

second example, assume

the

peak

influent

rate

is 300

units

and

that most

of the

time

the

influent

rate

ranges between one-ninth

and

one-fifth

of the

peak

flow

—

a

situation that could easily

be met

with

four

identical

V/S

pumps (one

a

standby).

As the flow

increases

from

33

units, only

one

pump would

be

used

until

the flow

reaches

100,

and

then

a

second pump

would

come

on at 50

units

and the first

pump would

drop

to 50

units

also.

If the flow

were

to

drop,

the two

pumps would continue running until

the flow

dropped

to

95 (or

perhaps

to 90) to

avoid switching pumps

on

and off too

frequently. When

the flow rises to

200,

three pumps

are

activated

at 67

units each

and

they

remain

on

until

the flow

drops

to 190 (or

perhaps

to

180). Except

for

very

low flows, a

pump

is

never

Figure

15-9.

Variable-speed pumps

for

steep system

H-Q

curves.

required

to

operate below

45% of its

capacity.

The

regime

is

shown

in

Table

15-1

in

columns

1

through

3

and

rows

2 and 3.

Now

suppose

one of the V/S

drives (see columns

2

and

3) is

replaced with

a C/S

drive (see columns

4 and

5)

to

make

a

mixed system.

The

mixed system would

work,

but it is not

optimum, because when

the C/S

pump

is

running,

the V/S

pumps

often

operate

at

min-

imum

discharge rates where their energy

efficiency

is

lowest.

Note, too, that

it is

simpler (and easier)

to

pro-

gram

for

V/S

-only

drives.

The C/S

pump would rarely

be

used,

and it

might deteriorate through idleness.

If

the

C/S

drive were changed

to

V/S,

the

life

of the

pumps

would

be

increased

by

25%,

and the

operating

staff

would have much more

flexibility

in

managing

the

station equipment.

Three Pumps

Because

the

standby must

be a V/S

pump,

a

three-

pump

system requires

two V/S

pumps

and one C/S

pump.

The V/S

pumps should each

be

capable

of

delivering about

60% of the

peak

influent

rate (PIR),

and

the C/S

should deliver

no

more than about

40% of

the

PIR.

One of the V/S

pumps operates

as the

lead

unit.

When

the

influent

rate imperceptibly exceeds

60% of the

PIR,

the

level

in the wet

well

rises,

which

causes

the C/S

pump

to

start.

The C/S

pump delivers

40% of the

PIR, while

the

speed

of the V/S

unit

drops

to

pump

20% of the

PIR. Thus,

the

lead

V/S

pump

and

the

C/S

pump operate together

as one

large

V/S

unit

that

discharges

a

combined total exactly equal

to the

influent

rate.

Once started,

the C/S

unit operates continuously

until

the

influent

rate

falls

to

less

than

55% of the

PIR,

at

which point

the V/S

unit

is

still operating

at

approx-

imately

30% of its

bee

(the exact percent depends

on

the

pump).

After

stopping,

the C/S

pump remains

off

until

the

influent

rate again exceeds

60% of the

PIR,

which

may be

hours, days,

or

even weeks. This

method

provides

the

best possible automatic operation

of

the C/S

pump.

The V/S

pump does

not

operate

at

discharge rates below

the

recommended minimum,

and

a V/S

pump

is

always available

as a

standby

to

replace

the

lead

V/S

pump

or the C/S

pump

if

either

fails.

Four

Pumps

A

four-pump combination

can be

configured

either

as

(1)

two V/S and two C/S

pumps

or (2)

three

V/S

pumps

and one C/S

pump.

Table

15-1.

Minimum,

Minimum

Optimum,

and

Maximum

Relative

Pumping

Rates

for a Mix of

Drivers

Column

1234

5

Pumps

1 V/S 2 V/S 3 V/S 1 V/S + 1 C/S 2 V/S + 1 C/S

Q

min

33 66 99 133 190

O

Q*

IQO

^optimum

min

~

J

L

^

u

Qmax

100 200 300 200 300

Two V/S and Two C/S

Pumps

Each

V/S

pump should

be

capable

of

discharging

45%

of

the

PIR,

and

each

C/S

pump should discharge

not

more than 27.5%

of the

PIR.

Either

of the V/S

units operates

as the

lead pump.

When

the

influent

rate exceeds

45% of the

PIR,

a C/S

pump

is

started

and the V/S

pump discharge drops

to

17.5%

of the

PIR. These

two

pumps

act

together

as

one

large

V/S

pump

to

discharge

a

total exactly equal

to the

influent

rate.

The

second

C/S

unit

is

started when

the

influent

rate exceeds 72.5%

of the

PIR,

at

which point

the V/S

pump

again discharges 17.5%

of the

PIR.

The

second

C/S

unit operates continuously until

the

influent

rate

falls

to

about

70% of the

PIR,

and the first C/S

pump

operates continuously until

the

influent

rate

falls

to

about

40% of the

PIR.

The

minimum discharge rate

of

the V/S

pump

occurs

just before

either

C/S

pump

is

stopped,

and

this rate

is at

least 12.5%

of the

PIR.

The C/S

pumps

are

usually automatically alternated

at

the end of

each cycle

of

either

or

both.

The V/S

pumps

are

usually alternated every

24 h. The

second

V/S

unit acts

as a

standby

for the

lead

V/S

pump

as

well

as for

either

of the C/S

pumps.

Three

V/S

and One C/S

Pumps

In

this combination,

all

pumps

are of the

same size,

each with

a

capacity

of

33.3%

of the

PIR.

One V/S

pump

is the

lead unit. When

the

influent

rate exceeds 33.3%

of the

PIR,

a

second

V/S

pump

comes

on

line

and the two

operate

in a

load-sharing

mode.

The C/S

unit

is

started when

the

influent

rate

exceeds

66.7%

of the

PIR.

The V/S

pumps discharge

all of the

influent

above

the

capacity

of the C/S

unit.

The C/S

pump continues

to

operate until

the

influent

rate

falls

to

about

60% of the

PIR,

and the lag V/S

pump

is

stopped when

the

influent

rate

falls

to

about

30%

of the

PIR.

The

third

V/S

pump

is a

standby

to

replace either

the

lead

or lag V/S

pump

or the C/S

unit.

Combination

of

Five

or

More

Pumps

The

capacity

of

each

C/S

pump should

be

limited

by

***H™U

(15

'

2)

where

Q

c/s

is the

capacity

of a C/S

pump

as a

percent-

age of the PIR and

N

v/s

is the

number

of V/S

pumps.

The

capacity

of

each

V/S

pump should

be

l-SGr/s

Q

^~Nl

(15

'

3)

/v

v/s~

L

where

Q

v/s

is the

capacity

of a V/S

pump.

Consider

a

station with three

C/S and

three

V/S

pumps. From Equations

15-1

and

15-2,

the

capacity

requirements

are

100

2c/s

-3TT5-

22%PIR

Q

c/s

>

l

-j^>

16.5%

PIR

and

the

station would operate with

one V/S

pump

as a

standby.

None

of the C/S

pumps

is

cycled

at any

influ-

ent

rate,

so wet

well storage

is not

required.

In

actual practice,

the

system curve

is not at a

con-

stant

head,

and the

pumps

are

selected

to

deliver their

nominal

ratings

at the TDH of the

maximum station

discharge (PIR).

15-6.

Special Design Considerations

Maximum

Speed

Limit

A

pump must never

be

operated

at a flowrate

greater

than

that shown

by the

manufacturer's plotted curve.

In

other words, never extrapolate

a

pump

H-Q

curve.

Hence,

it may be

necessary

to

limit

the

speed

of a

pump

to

ensure that

the

pump curve

not

only inter-

sects

the

system curve

but

also overlaps

it

somewhat.

Consider

the

range

of

possible system curves,

and

also

note that pipe

friction

can

(particularly when

the

pipe

is

new)

be

lower than anticipated, which

may

further

limit

the

speed range

of the

pump.

A

discharge pressure-control override

can

also

be

used

when several stations discharge into

a

common

force

main where

the

head

at any

station

can be

very

low at

times.

Control

of

Pumping

Units

A

pump-control

system

should

be

designed

so

that

the

failure

of

any

component

or the

trip

of

any

single pro-

tective

device

(downstream

of the

main

breaker)

does

not

disable

more

than

one

pump.

A

complete

and

sepa-

rate

V/S

drive

for

each

V/S

pump

is

strongly recom-

mended

to

ensure that

the

facility

can

operate

in a

normal manner with

any

single

V/S

drive

out of

service.

Pump

Failure

Detection

Pump failure detection

is a

highly desirable feature

that

can

usually

be

included

in the

pump-control

sys-

tem

at

little cost.

If the

pumps

are

equipped with out-

side-lever check valves,

the

valves

can be

fitted

with

limit switches

to

detect

flow (or the

lack

of flow). The

limit switch

is

actuated

by the

lever when

the

valve

is

fully

closed;

it is

deactivated when

the

pump begins

to

deliver

a

small

flowrate.

Regardless

of the

cause,

the

check valve limit switch

can

detect

the

failure

of the

pump

to

discharge.

The

electrical circuitry

is

designed

to

deenergize

the

motor

of any

pump whose check valve remains

closed

for a

predetermined period

of

time (usually less

than

3 min for V/S

pumps) when

the

pump

is

required

to

operate.

An

alarm

is

signaled,

and the

standby

pump

replaces

the

unit that failed.

Check valve limit switches

can

also prevent energi-

zation

of the

motor

of any

pumping unit whose

dis-

charge check valve

is not

fully

closed. Some drives

can be

severely damaged

or

destroyed

and

shafts

can

break

if the

motor

is

energized while

the

pump

is

backspinning.

If

the

check valves

do not

have outside levers,

a

pressure switch located between

the

pump discharge

nozzle

and the

check valve

is

sometimes used

as a

flow

indicator. Actually,

the

check valve only indi-

cates pressure,

not flow, and is

misleading

if flow is

blocked

by a

downstream obstruction.

A

better

flow

indicator

is a

vane switch

in

clean water applications

and,

for

sewage,

a

thermal-dispersion switch

or a

Doppler

flow

switch

(see

Section 20-5). When used

for

sewage

or

dirty water,

any

pressure device (such

as

a

bourdon tube) should

be filled

with

a

clear liquid

(glycerin,

for

example), which

is

separated

from

the

dirty

water

by a flexible

diaphragm.

Power-Operated

Check

Valves

If

the

pump discharge check valves

are

power oper-

ated

(hydraulic

or

electric),

it may be

necessary

to

include

a

relatively sophisticated control system

to

protect

the V/S

pump drives against backspinning

while

the

drives

are

energized. When

a

pumping unit

is

required

to

operate

and the

pump drive

is

activated,

the

check valve should

not

begin

to

open until

the

pressure

on the

upstream side

of the

valve exceeds

the

pressure

on the

downstream side. (The pressure

can be

sensed,

as

described previously.) When

the

pump

is no

longer needed,

the

check valve should close

first, and

the

pump should

not be

stopped until

the

valve

is at

least

95%

closed.

The

pumping

unit

should

be

deenergized when

• The

pump discharge pressure does

not

exceed

the

pressure

in the

force

main within

a

short interval

(say,

60 s)

after

the

pump

is

started

• The

check valve does

not

open

fully

within

a few

minutes

after

the

pump discharge pressure exceeds

the

pressure downstream

from

the

check valve

• The

check valve does

not

close

fully

within

a

short

interval (say,

3

min)

after

the

pumping unit

is

sig-

naled

to

shut down.

The

control system should also prevent energiza-

tion

of the

motor

of any

drive whose check valve

is

not

fully

closed.

15-7.

Analysis

of

Variable-Speed Booster

Pumping

The

purpose

of a V/S

booster pump

is to

maintain

a

nearly constant discharge pressure while delivering

the

variable demand

of a

closed distribution network.

Pump speed variation controls

the

pump discharge

pressure,

and the

discharge rate

is

determined

by the

system demand.

To

analyze

the

operation

of a

pump that discharges

into

a

hydraulic network, both

the V/S

pump charac-

teristic curves

and the

curve

of

required pump

differ-

ential head versus demand must

be

considered.

The

pump

must always operate

at the

intersection

of the

impeller's characteristic curve

(at the

speed

of

rota-

tion) with

the

required

differential

head curve.

Pump

Characteristic

Curves

The

curves

for a

typical horizontal split-case pump,

with

its

impeller

trimmed

to

deliver

681

m

3

/h

(3000

gal/min)

at

33.5

m

(110

ft) at

maximum speed,

are

shown

in

Figure

15-10.

Note

the flatness of

these

curves.

Figure

15-10.

Characteristic curves

for a

booster

pump.

Required

Differential

Head

Curves

The

significant

contrast between wastewater

and

booster pumping

is

that

the

system

H-Q

curves

for the

two are

derived

differently

and

usually have

different

shapes.

The

system

H-Q

curve

for a

booster

pumping

facility

is

actually

a

"required

differential

head,"

which

represents

the

differential

head that must

be

added

to

the

suction head

to

produce

the

desired

constant

dis-

charge pressure

at all

demands.

There

are

four

basic types

of

suction pressure vari-

ation.

• The

suction pressure decreases with

an

increase

of

the

pump discharge rate

due to

headlosses

in the

suction

piping system.

• The

suction pressure remains constant regardless

of

demand.

An

example

is a

pump that takes suction

directly

from

a

tank

in

which

the

water level

is

auto-

matically

maintained relatively constant

by a float

valve

on the

supply inlet.

• The

suction pressure varies independently

of

demand.

An

example

is a

relatively small pump that

takes suction

from

a

large city main

in

which

the

pressure varies

but is

essentially

unaffected

by flow

through

the

pump. Thus, there

are an

infinite

num-

ber of

suction head curves.

• The

suction pressure decreases with

an

increase

in

the

pump discharge rate

and

also varies indepen-

dently

of

demand. Examples

are (1) a

booster pump

in

a

city water main

in

which pressure varies

due

both

to the

pump discharge

and to

other demands

and

(2) an

in-line transmission booster pump.

Suction

Pressure

Varies

with

Demand

The

curve

of

required differential head that

the

pump

must

develop

is

shown

by the

solid lines

in

Figure

15-11

to be the

desired discharge head minus

the

curve

of

suction

head.

But in

actual

practice,

the

booster pumps commonly used

do not

maintain

a

con-

stant

discharge pressure. Because

of the

requirement

of

the

control system

for a

change

in

signal

to the

drive,

the

pump discharge pressure usually

has a

droop

of

about

10%

from

zero demand

to

maximum

demand.

The

droop

is not

usually linear,

but its

actual

curve

is not

important;

a

linear droop

is

used here

for

simplicity.

The

required

differential

head curve incor-

porating

the

droop

is

shown

by the

dashed line

in

Fig-

ure

15-

11

c, and it is

also superimposed

on the

pump

characteristic curves

in

Figure

15-10,

where

it is

labeled

"required

head."

Droop

can be

eliminated with

a

reset feature

in the

control system. Elimination

of

the

droop, however,

is

complex, expensive,

and not

usually

necessary because

a 10%

droop

is not

objec-

tionable.

The

curve

of

pump

speed

versus demand shown

in

Figure

15-12

can be

obtained

as

explained

for

Figure

15-3.

The

pump

can,

of

course, operate

at any

speed

between

the

limits shown

by the

solid line

and at

speeds less than

1150

rev/min,

as

indicated

by

the

dashed

line.

However, operation

at

zero

flow, or

even

at

low

discharge

rates,

should

be

avoided

as

explained

in

Section 15-3. Again,

the

speed-demand curve

is the

most

significant

depiction

of V/S

booster pump opera-

tion.

Figure

15-11.

Differential head

for a

suction head varying

with

demand,

(a)

Discharge head;

(b)

suction head;

(c)

required differential head.

Note:

(a) - (b)

=

(c).

Suction

Pressure

Constant

The

development

of the

required differential head

curve

is

shown

in

Figure

15-13.

The

superposition

of

this

curve

on the

pump curves

of

Figure 15-10

is

shown

in

Figure 15-14.

The

zero

Q

speed

is

found

at

the

intersection

of the

1450-rev/min

pump curve with

the

required head curve,

but if it

were not,

it

could

be

found

as

explained

in

Section 15-3.

If

a

pump with

a

very

flat

curve

is

used

as

shown

in

Figure

15-15,

the

zero

Q

speed

can be

calculated

to be

1760

rev/min

and the

pump would always operate

Figure

15-13.

Differential

head

for a

constant

suction

head,

(a)

Discharge

head;

(b)

suction

head;

(c)

required

differ-

ential

head.

Note:

(a)

- (b) =

(c).

Figure

15-12.

Speed versus

demand

for a

booster

pump.

within

1% of

maximum speed regardless

of

demand.

(All

of the

curves shown here

are

published curves

for

actual

pumps

offered

by

well-known manufacturers.)

Hence

there

is

nothing

to be

gained

by

VIS

operation

of

a flat-curve

pump

with

a

constant suction head.

The

flat-curve

pump

provides good pressure regulation

where

the

demand

fluctuates

rapidly.

The use of

steep-

curve pumps

is

sometimes desirable because they per-

mit

some adjustment

of the

discharge pressure with-

out

changing

the

pump

impellers.

The

discharge

pressure

of a

steep-curve

V/S

pump

fluctuates

with

abrupt

changes

in

demand because

the

inertia

of the

rotating elements prevents instantaneous changes

of

pump

speed.

Figure

15-15.

A

flat

booster

pump

curve.

Figure

15-14.

Booster

pump

and

required

differential

head

curve

for a

constant

suction

head.

Suction

Pressure

Varies

Independently

of

Demand

The

development

of the

required

differential

head

curves

is

shown

in

Figure 15-16.

The

minimum suc-

tion head requires

the

maximum required

differential

head

and

vice versa.

The

pump

and

differential

head

curves

are

similar

to

those shown

in

Figure

15-14

except that there

are an

infinite

number

of

required

head curves

as the

suction head changes between

the

minimum

and

maximum

of

Figure

15-16b.

The

maxi-

mum

speed pump curve

and the

required head curve

for

maximum suction head must

intersect

(which

may

require

a

steep-curve pump).

If

the

pump curve

is

very

flat, the

pump speed changes

are

almost entirely

to

compensate

for

suction pressure

variations.

But

regardless

of the

manner

in

which

the

suc-

tion

head varies,

the

drive automatically regulates

the

pump

speed

to

deliver

the

desired discharge pressure

at

any

demand

within

the

capability

of the

pump.

Suction

Pressure

Varies

both

Independently

and

Dependently

of

Demand

The

development

of the

required differential head

curves

is

shown

in

Figure 15-17

for a

minimum suc-

tion

head

of

12.2

m (40 ft) and a

maximum suction

head

of

21.3

m (70

ft).

The

differential head

is not a

parabola with

its

apex

at

zero discharge because

the

droop causes

it to be

skewed.

The

required

differential

head curves

for the

extremes

of

suction head

in

Figure

15-17c

are

super-

imposed

on the

pump curves

of

Figure 15-10

and

shown

in

Figure

15-18.

The

pump operates

on an

infi-

nite number

of

system curves between those

for

mini-

mum

and

maximum suction head.

15-8.

Minimum

Discharge

Rate

A

minimum speed feature

in a V/S

drive cannot,

by

itself,

prevent

the

operation

of the

pump

at low

dis-

charge rates. Without

a

bypass,

the

pump cannot

deliver more water than

the

amount that

is

being drawn

from

the

distribution system.

If the

drive prevents

operation

at

speeds lower than

a

preselected minimum,

the

pump must operate

on its

curve

for

that speed when

the

demand

is

less than

the flowrate at

which

the

mini-

mum

speed curve intersects

the

required head curve.

If

the

pump with

the

characteristic curves

of

Figure

15-18 were prevented

from

operating

at

speeds less

than

1300 rev/min,

the

pump would operate

on the

solid line ABCD shown

in

Figure 15-19.

For

example,

at

a

demand (and discharge)

of 114

m

3

/h

(500 gal/

min),

the

pump would develop

a

head

of 29 m (96

ft).

If

the

minimum demand

is

less than

the

minimum rec-

ommended pump discharge rate,

the

pumps should

be

protected

by one of the

three methods discussed below.

Suction

Pressure

Varies

with Demand

Flowmeter

and

Bypass

Include

a flowmeter and a

bypass with

a

modulating

valve,

as

shown schematically

in

Figure

15-2Oa.

The

Figure

15-16.

Differential

head

for a

suction head

that

varies

independently

of

demand,

(a)

Discharge

head;

(b)

suc-

tion

head;

(c)

required

differential

head.

Note:

(a) - (b) =

(c).

Figure

15-18.

Suction varying both dependently

and

independently

of

demand combined

with

pump

curves.

Figure

15-17.

Differential head

for

suction

pressure

varying both dependently

and

independently

of

demand,

(a)

Discharge

head;

(b)

suction head;

(c)

required differential head. Note:

(a) - (b) =

(c).

Figure

15-19.

Booster

pump operation

at

minimum suction head.

Figure

15-20. Flowmeter

and

pump

bypass,

(a)

Single

pump;

(b)

pumps

in

parallel.

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

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