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

D

is the

throat diameter

of the

intake

(in

either meters

or

feet),

so for a

throat

of

1.0

m

(3.28 ft),

for

example,

the

discharge

is

limited

to 6.2

m

3

/s

(220

ft

3

/s).

There

are two

designs

of the FSL The one

with

the

lowest

profile

(and hence

the

unit

of

least cost)

is

shown

in

Figure 12-30 (page 356). Flow

may

approach

the

intake

at any

angle

up to

90°.

In

studies

of

a

model with

a

throat diameter

of 200 mm (8

in.),

forebay

velocities

of

1.2

m/s

(4

ft/s),

or

even twice

as

much,

caused

no

problems, according

to

Fletcher

[15],

and

velocities

in the

throat

met all

requirements

for

uniformity

and

lack

of

swirling

or

vortices. Corre-

sponding prototype

velocities

can be

found using

the

model similitude relationships presented

in

Section

3-

12.

Caution with respect

to

high forebay velocities

is,

however,

advised.

Not

every design with

FSI

intakes

works

equally well, however,

and the

configuration

of

the

forebay does

affect

swirling

and

vortices,

as

dem-

onstrated

in

model tests

by

Fletcher

[16].

The

dividing

wall

in the

entrance

is not a

problem, because trash

racks

are

assumed

to be

placed

in the

forebay

to

elim-

inate debris. Without trash racks, expect stringy mate-

rial

to be

deposited

on the

nose

of the

dividing wall.

A

schematic drawing

of a

wastewater pumping sta-

tion

wet

well containing pumps

originally

designed

to

discharge

a

total

flow of

6300

m

3

/h

(40

Mgal/d)

is

shown

in

Figure 12-31 (page 357).

The

pumps were

very

noisy

and

impeller noses were quickly eroded,

so

impellers

had to be

replaced

frequently. When nearly

double

the

capacity

was

required,

a

model study

was

made.

The

model proved that even

a

very short cas-

cade (less than

0.3 m or 1 ft in the

prototype) into

the

Figure

12-28.

A

sump recommended

in the

Hydraulics Institute Standards

[2].

(a)

Plan;

(b)

Section A-A. Improve-

ment

is

shown

by

dashed lines.

wet

well caused great masses

of air

bubbles

to be

driven

to the floor

where

the

pump intake currents

captured them, thereby creating noise

and

erosion.

The

poor characteristics were overcome

by

installing

the

baffle

channel, shown

by the

dashed lines

in the

figure.

Water

from

the

cascade

falls

into

the

channel

and

must enter

the

sump horizontally. Hence,

the

bub-

bles

are not

driven

to the floor of the

sump

and

thus

can float to the

surface.

No

bubbles entered

any

pump

intake

after

the

baffle

was

installed even

at the

high

discharge

of

4700

m

3

/h

(30

Mgal/d)

per

pump,

and the

pumps

ran

smoothly even

at a

station discharge

of

7500

m

3

/h

(118

Mgal/d).

The wet

well still traps

sludge

and

scum, however,

and

remains unsatisfactory

for

that reason.

Air

bubbles entrained

by a

free

fall

can

persist

for a

surprisingly long time

and are

readily drawn into

pump intakes with devastating

effects

on

capacity,

head,

and

efficiency.

One % of air

can,

for

example,

reduce

the

efficiency

and

throughput

of

some pumps

by

as

much

as

15%.

Pumps with

a low

specific speed

(n

s

)

are

more sensitive

to air

than those with

a

high

specific

speed. With

a

sufficient

residence time

in the

sump

and

enough distance between

the

inlet

to the

sump

and the

pump intake,

the air

bubbles

can be

eliminated. Information given

by

Falvey [17]

is

useful

for

estimating this distance.

Solids-Bearing

Waters

Many

waters (raw water,

stormwater,

and

wastewater)

contain solids that settle rapidly

in

traditional

wet

well

designs

as the

velocity

at the

exit

of the

inlet pipe

falls

to

very

low

values

in the

sump.

As a

result,

all of the

above

designs

are

generally unsuitable

for

such

waters.

The

deposition

of

solids

can

change

the

hydraulic characteristics

of the

basin appreciably, and,

if

any

organic material

is

present,

its

decomposition

results

in

toxic, malodorous,

and

corrosive gases.

Usually such solids

can be

removed

from

these sumps

only

with

difficulty

and at

considerable

expense.

Designs

for

solids-bearing waters

are

specifically

avoided

in the

Hydraulic Institute Standards

[2]

with

the

statement "Figures

apply

to

sumps

for

clear liq-

uids.

For

fluid-

solids

mixtures

refer

to the

pump man-

ufacturer"

In a

later

edition [3],

the

wording

is

"This

standard

applies

to

handling clear

liquids"

and

these

warnings preclude,

for

example,

the use of

such

sumps

as

illustrated

in

Figures 12-26

to

12-29

for

solids-bearing waters. When,

despite

the

warning, tra-

ditional designs

are

used

for

water containing solids,

sludge accumulates,

often

quickly. Sometimes sumps

are

divided

(as in

Figure

12-31)

to

allow

one

side

to

remain

in

service while

the

other

is

isolated

and

man-

ually

cleaned.

But the

manual

cleaning

of

sumps

is a

Figure

12-29.

A

sump

with

right-angle

change

of

flow.

Not

recommended.

labor-intensive,

disagreeable

chore

in a

hazardous

space. Cleaning

by

pumper truck

is

expensive.

It is

unlikely

that

such

wet

wells would

be

frequently

(say,

weekly)

cleaned,

so if the fluid is

wastewater, odors

are

likely

to be

severe. Clearly,

a

design that permits

quick,

easy, inexpensive cleaning

is

needed.

Until

1997,

the

Hydraulic Institute Standards con-

tained

no

advice about

the

design

of

pump intakes

for

solids-bearing waters beyond

the

statement that:

(1)

flat floor

areas should

be

minimized,

(2)

approach cur-

rents

should

be

kept high (above

1 m/s or 3

ft/s)

to

minimize

deposition,

and (3)

previous successful

designs should

be

followed.

No

means

for

accom-

plishing

these goals were depicted.

But

from

1994

to

1997,

the

Intake

Design

Committee

of the

Hydraulic

Institute addressed

(1)

improved designs

for

clear

waters

and (2) for the first

time,

specific

and

detailed

design recommendations

and

geometries

for

pump

intake basins

for use

with solids-bearing waters.

The

of the

Hydraulic Institute Standards will

include

the

trench-type

wet

well

and

endorse

the

pre-

sentation

in

Section

12-7.

Pump

Intake

Basin

Design

To

be

acceptable,

a

generic

intake basin design must

provide

a

good hydraulic environment

for the

pump

Figure

12-30.

Formed

suction

intake

(FSI).

(a)

Elevation;

(b)

plan;

(c)

isometric

view.

Courtesy

of

U.S.

Army

Corps

of

Engineers.

intakes.

If the fluid

contains solids,

the

design must

allow

ease

in

cleaning. Three kinds

of

sumps that

can

easily

be

kept clean are:

• The

trench-type sump

in

which extreme drawdown

creates

an

irresistible current that quickly sweeps

all

solids into

the

last pump suction.

This

design

is

suitable

for

medium

to

large nonclog column

pumps,

dry

-pit

pumps,

and

submersible

pumps.

•

Hopper-bottom sumps with steep sides

and

floors

so

small that settleable solids

do not

accumulate

and

in

which

occasional

drawdown allows scum

to

be

removed

by the

pumps. They

are

suitable

for two

or

even three small submersible

or

self-priming

pumps

in a

small, round

wet

well.

•

Vigorous mixing

of the

contents

during

a

short

period

of the

normal pump operation cycle. Mixing

can

be

accomplished

by:

(1)

using

a

propeller (usu-

ally

coupled

to a

submersible motor),

(2)

bypassing

part

of the

pump discharge into

the

sump,

or (3)

bypassing some

of the flow in the

force main back

into

the

sump just above

the

pump intakes.

Geometry

is the

favored

method

for

facilitating clean-

ing,

so the first two

types

are

preferred

for new

construc-

tion.

The

third

is

useful

for

retrofitting

existing sumps.

The

concept

of the

trench-type pump sump

was

developed

by

Caldwell

in the

late

1950s.

The

design

has

been used

at

many locations.

In

particular,

it was

used

in 27

pumping stations

in

Metropolitan Seattle

by

Metropolitan Engineers,

a

consortium

of

four

consult-

ing

engineering

firms

managed

by

Brown

and

Caldwell

Consultants.

Six

examples

are

described

in

Chapter

17.

Note that trench-type sumps

differ

from

traditional

designs

by

having pump intakes

in a

deep,

narrow

trench and,

in

defiance

of

conventional wisdom,

in

tan-

dem

with

the

inlet

so

that water must theoretically pass

one

inlet

to

reach another.

In

spite

of

this violation

of

recommendations

by the

BHRA

[1],

Hydraulic Institute

Standards [2],

and

ANSI/HI

Standards

[3, 4],

these

wet

wells have been eminently

successful

as

demonstrated

in the field for

four

decades

with pump

sizes

ranging

from

63 L/s

(230

m

3

/h

or

1000

gal/min)

to 4.7

m

3

/s

(17,000

m

3

/h

or

75,000

gal/min). None

of

these sumps

Figure

12-31.

A

divided

sump,

(a)

Plan;

(b)

Section

A-A.

Not

recommended

(see

text).

has

ever required retrofitting

for the

design

flowrates,

and

no

pump installed

in one has

failed

to

perform sat-

isfactorily.

Trench-type

pump sumps possess

four

distinct

advantages

over almost

all

other kinds

of wet

wells:

•

Pump inlets

are

located near

the

bottom

of a

deep

trench

and far

below

the

invert

of the

upstream con-

duit

or

channel. Consequently, currents near pump

intakes

are

low

—

almost

stagnant

—

and

water tends

to

enter suction bells uniformly around their periph-

eries.

•

Incoming

flow is

coaxial with

the

pump intakes,

so

Phenomenon

2 has no

chance

to

develop.

•

Cleaning

is

quick, easy,

and

essentially devoid

of

manual

labor. Cleaning every week

or

more

often

is

practical.

• The

footprint

(or

plan area) required

is

minimal.

Figure

12-32.

Trench-type sump

for V/S or C/S

pumps,

(a)

Plan

view

for

solids-bearing

water;

(b)

Section

A-A

for

sol-

ids-bearing

water,

for

clean

water,

omit

ogee

ramp,

shorten

sump,

and use

cones

under

all

intakes;

(c)

Section

B-B

for

solids-bearing water

and dry pit

pumps;

(d)

Section

B-B for

solids-bearing water

and

submersible

pumps;

(e)

Sec-

tion

B-B for

clean water

and

column

pumps;

(f)

Floor

flow

splitter

and

fillets

for

suppressing

floor

and

wall

(subsur-

face)

vortices

at

upstream pumps. (See Figure

12-33

for the end

pump.)

Research with models constructed

to a

linear scale

of

approximately

1:4

and

tested between 1991

and

1995

at

Montana State University

and

(under contract

to the

university)

at

ENSR Consulting

and

Engineer-

ing

laboratories

in

Redmond, Washington,

by

Sanks,

Jones,

and

Sweeney

[7]

plus assistance

from

Beaty

[18]

in

testing

a

full-scale model

at the

Fairbanks

Morse Pump Corp. plant

has

resulted

in

minor design

improvements over

the

Metropolitan Seattle stations

for

normal operation

and in

major improvements

for

cleaning.

An

improved

design,

in

which

recom-

mended

dimensions

are

shown,

is

depicted

for wet

well-dry

well pumps

in

Figure 12-32. Note that both

plan

and

cross-section views

are

symmetrical below

Figure

12-32.

(Continued).

Recommended

Dimensions

of

Trench-Type

Sumps

Item

Dimension

All

Waters

A

>

2.5 D.

Usually about

4.5 D

unless suction

pipes

splayed

to

make room

for

pumps

and

motors.

For

submersible pumps,

see

Mfr.

a

>2D

S (1 +

2.3F)D

where

F=

v[gD)-°-

5

.

W

As

small

as

possible

but

keep

v

avQ

above trench

<

0.3

m/s

(1

ft/s)

for all

flows

and

water levels.

Wastewater

C 0.5 D.

Last pump intake

>

0.25

D

below upstream

intakes

and

D/4

above

floor.

Lower

the

floor

if

desired.

H

1

2.33

/?

where

h

is

head

on

sluice gate.

R

2

0.67

H

1

a

>

45° for

plastic lining;

a

>

60° for

concrete

surfaces.

Clean water

C

0.25

D

<

C

<

0.5 D.

Cone always preferred

but

is

required

for C < 0.5 D.

oc

>

0° but 45°

preferred

by

some experts.

the

water line,

so

asymmetrical

flow to the

intakes

is

avoided. During normal operation,

the

water

jet

from

the

inlet spreads slightly

and the

velocity

is

dimin-

ished somewhat before

the jet

strikes

the

rear wall

and

returns,

thereby setting

up a

recirculation

pattern

above

the

trench

from

which water

drifts

slowly

downward

to the

pump intakes.

For

wastewater applications,

the

sidewalk shown

in

Figure

12-11

a

and c is

strongly recommended

for

access when workers

are

hosing grease

off the

walls.

Try

to

avoid

any

obstruction (beams

or

struts) across

the wet

well, because they interfere with water lances.

Use the

sidewalk

as a

beam

to

resist earth loads.

Although

the

cross-section

is

massive,

the

actual vol-

ume

of

concrete

is

small when compared with

the

"typical"

wet

well. Furthermore, many pumping sta-

tions

are

relatively deep

and

well below

the

maximum

groundwater

level,

so

massive concrete volumes

are

required

to

prevent

flotation.

The

preferred position

of a

submersible pump,

shown

in

Figure 12-32d, requires

a

suction nozzle

appended

to the

pump.

The

disadvantage

of the

nozzle

is

its

cost (because

it may

have

to be

custom

fit, and the

pump

volute might have

to be

modified),

the

extra head

space required

to

pull

the

pump

out of the wet

well,

and

the

requirement

of

some sort

of

cradle

or

dunnage

to

support

it on the floor or on a

truck bed. (Without

a

nozzle,

a

submersible pump

is

stable

on a floor.)

When

the

pump volute

is

small enough,

the

pump

can be

low-

ered into

the

trench

(by

deepening

the

recess

for the

discharge elbow)

so

that

no

nozzle

is

required.

After

installing

the

discharge

fitting, the

recess

can

then

be

filled

with

lean concrete troweled into place with only

enough

room

for the

pump

to be

pulled out.

The

trench-type

wet

well

is

readily adaptable

for

pumping

clear water

by

omitting

the

ogee

ramp,

shortening

the

sump,

and

using

a

cross-section like

that

in

Figure 12-32e.

As

there

is no

need

for

super-

critical velocities

for

cleaning,

the

suction bells

can

be

lowered

to

D

/4

with

a

cone beneath

the

bell.

The

advantage

of the

trench-type sump

is its

small size,

low

cost,

and the

benign hydraulic environment

for

the

pumps.

Caveat

Because

of the

success

of the

trench-type sump,

it is

to

be

preferred over

all

others wherever applicable.

But

unless

the

designer

has had

experience with

installations

of

larger sizes,

the

trench-type sump

should

not be

universally applied

to

pump sizes larger

than

about 1900

L/s

(6800

m

3

/h

or

30,000

gal/min)

without

design-specific model testing.

Valves

for

suction

or

discharge pipes larger than

400 mm

(16

in.)

are

large, heavy,

and

costly. Consider

the use of

draft

tube inlets

(as in

Figures 17-9

and

17-12) with sluice gates

for

isolation

for

larger pump

intakes.

See

subsection

"Sumps

for

Large

Pumps"

near

the end of

Section

12-7.

Sumps

for

large pumps that

run

continuously (e.g.,

circulating water pumps

in

power plants) should

be

practically perfect

as

proven

by

model studies.

Trench-Type

Sumps

for

Solids-Bearing

Waters

The

characteristics

of a

good sump

for

solids-bearing

waters,

as

illustrated

in

Figure

12-32, are:

a.

Water enters

the

sump horizontally with

no

free

fall.

LWL

during normal operation

is far

enough

above

the

pipe invert

to

limit

the

entrance velocity

to

about

1 m/s

(3.5

ft/s).

For V/S

pumping,

HWL is

at

the

soffit

of

inlet pipe,

but for C/S

pumping,

HWL can be

anywhere above

the

inlet

pipe,

b.

A

motorized sluice gate allows isolating

the

sump

from

the

inlet pipe.

The

mechanism must allow

setting

the

gate opening accurately (perhaps

by

means

of a

limit switch) even under static pressure

from

upstream

storage,

c. The

trench

is 2 D

wide

by

about

2.5 D

deep.

But

actual

depth

is

governed

by: (1)

required submer-

gence

of

pump intakes (see Equation 12-1),

(2)

pump

intake

floor

clearance (usually

D/2),

and (3)

the

required projected

net

cross-sectional area

above

the

trench (see item

e

below).

The top of the

trench should, however,

be no

less than

1.9 D

above

any

intake unless model studies indicate that

less

is

allowable,

d. The

invert

of the

influent

pipe

or

channel

is no

less

than

2.0 D

above

the

highest bell mouth

to

keep

incoming currents well above

the

pump intakes

unless model studies indicate that less

is

allowable,

e. The

projected

net

cross-sectional area (gross area

minus

area

of

column

or

submersible pumps)

of

the

water above

the

trench

is

sufficient

at any

depth

(and

consequent

flowrate) in the

inlet

pipe

to

main-

tain

an

average velocity (i.e.,

a

nominal plug

flow

velocity) that never exceeds

0.3 m/s (1

ft/s).

f

.

The

ogee ramp (used only during cleaning) extends

from

the

invert

of the

inlet

to the floor of the

trench.

The

minimum radius

of the

upper ramp curve

is

given below

by

Equation 12-2.

The

radius

of the

reverse curve should

be at

least half

as

great.

A

short tangent between

the two

curves

is

desirable.

g.

A

water guide (shown

in

Figure 12-32) keeps

water

in a

rectangular section

from

the

inlet

to the

trench. Without

the

guide, water climbs

up the

sloping sides,

falls

back into

the

trench,

and

inter-

feres

with

the flow of

water along

the

trench during

cleaning,

h.

Above

the

trench,

the

sides

are

sloped

at

least

45°

if

lined with,

for

example,

PVC (or

another plastic

equally slick)

or 60° if the

surface

is

smooth con-

crete (steeper

is

better)

to

assist solids

to

slide into

the

trench.

PVC

makes

it

easier

to

hose grease

and

debris

off the

walls.

For

wastewater,

the

entire

sump

above

LWL

should

be

lined with

a

plastic

such

as

Linabond®

[19]

to

prevent

corrosion,

i.

Intakes

may be

spaced

as

close

as 2.5 D

center

to

center. Closer spacing

may be

used subject

to

model

study.

For

intakes

so

close, splay

the

pump

suction

pipes apart

so

that pumps

and

valves have

a

clear space

on

three sides

of no

less than

l.lm

(42

in.),

j.

Upstream intakes

are no

less than

D

12

above

the

floor to

avoid interference with

the

supercritical

flow

of

water during cleaning. Interference pat-

terns generated

at the

sides

of the

trench during

cleaning

may

cause shock waves

(or

"rooster

tails")

that

can

reach

a

height

of

D/2.

Their

impingement

on a

suction bell

can

interfere with

the flow of

water

underneath,

k. For flat floors as in

Figures 12-32c

to

12-32e,

the

entrance velocity

at the

mouth

of the

pump suction

bell (based

on the OD of the

bell)

should

be

1

.

1

m/s

(3.5 ft/s)

or

less

for V/S

pumping

and 0.9 m/s

(3.0 ft/s)

or

less

for C/S

pumping.

To

ensure

effec-

tive disposal

of the

largest grit particles during

cleaning, however,

the

entrance velocity

at

full

dis-

charge capacity should

not be

less than about

0.6 m/

s

(2.0

ft/s).

Velocity

in the

suction pipe should

not

exceed

2.5 m/s (8

ft/s).

If

these restrictions

are

incompatible

with standard

bells,

the

bells

can be

machined

to a

smaller size.

1.

The

last pump intake clears

the end

wall

by,

prefer-

ably,

no

more than

D

/4

to

inhibit

surface

vortices.

The

intake must

be no

more than

D/4

above

the

upstream

trench

floor. The

intake

can be

lowered

by

sloping

the floor

downward

from

the

adjacent

intake

to a

lower elevation

and

retaining

a floor

clearance

of

D/4.

The

lowered suction intake cre-

ates

a

more vigorous

jump,

m. An

anti-rotation

baffle,

as

shown

in

Figure

12-33,

must

be

installed

at the

last pump inlet, because

otherwise

the

hydraulic jump, necessary

for

quick

cleaning,

may not

reach

the

last pump.

The

cone

is

also necessary,

and the

attached vane

is

desirable,

n.

Floor

vanes

as

shown

in

Figure 12-34

are

installed

under

all

upstream suction bells

to

inhibit swirling

caused

by floor

vortices,

o.

Valve

and

connection

is

installed

for a 3

8

-mm

(l

l

/

2

-in.)

hose

for

washing walls with about

3 to

4.5 L/s (50 to 70

gal/min)

of

water

at

about

600

kPa

(90

lb/in.

2

)

of

pressure

at the

nozzle.

The

trench-type sump

of

Figures 12-32a

to

12-32e

was

developed

from

model studies made between 1992

and

1995

in

which

the

maximum suction

bell

velocity

was

1.07

m/s

(3.5

ft/s).

The

adequacy

of

this design

has

been demonstrated

by

those model tests backed

by the

Figure

12-33.

Details

at

last

intake

to

ensure supercritical

velocity

along

full

length

of

trench,

(a)

Section B-B;

(b)

Section A-A.

success

of

two-score

wet

wells

of the

type shown

in

Figures

17-13

through

17-17,

which have operated

at

V/S

without problems

in the

Seattle area

for 10 to 30 yr.

They have

flat floors

ranging

from

1.88

to 2.6 D

wide,

suction-bell

floor

clearances

of

D/2,

and no floor

vanes

or

cones,

but the

intake velocity

was

nominally limited

to

1

.07

m/s.

The

addition

of

vanes

and

cones

as in

Fig-

ures

12-33

and

12-34

provides

an

added factor

of

safety

against swirling

and

vortices.

At

higher intake veloci-

ties, however,

the

submerged

floor and

wall vortices

are

expected

to

worsen. Model tests reported

by

ENSR

in

June

of

1997 [22]

and

discussed

by

Williams

et

al

[23]

revealed that both

subsurface

vortices

and

excessive

swirling

at an

intake velocity

of

1.5

m/s

(4.9 ft/s) could

be

controlled

with

a flow

splitter

and fillets as

shown

in

Figure 12-32f.

The flow

splitter should extend

from

the

ogee ramp

to a

point between

the

last

two

pumps.

A

cone

per

Figure 12-33 should

be

installed under

the

last

pump.

The fillets can

extend

to the end

wall.

The flow

splitter eliminates

floor

vortices,

and the fillets

reduce

wall

vortices,

but

swirling

is not

controlled. Swirling

can

be

reduced with

two or

more vanes inside

the

suc-

tion

bell.

The

vanes should have smooth rounded edges

at

angles

of no

more than

15 or 20° to the

streamlines

to

prevent

the

attachment

of

stringly

material.

The

design

shown

in

Figure 12-32f

has not yet (in

1998) been

tested

in the field.

The

acceptable velocity ranges

for

pump intakes

proposed

by the

Intake Design Committee (consisting

of

users, engineering contractors,

and

producers con-

vened

and

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

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