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

risks. Consider omitting powered ventilation

for wet

wells

by

ensuring that there

is no

need

for

personnel

to

enter routinely (but note that

a

vent

is

required).

Wet

well

fans. Power

air

into

the wet

well

at the top

with

high-energy

diffusers,

and

power

the

exhaust

air

out

at a low

elevation using slightly more exhaust vol-

ume

than supply volume

to

create

a

small negative

pressure (see Section

23-2).

(Opening

or

closing

doors defeats

the

purpose

of

ventilation

by a

single

fan

whether

the air is

powered

in or

out.) Design

for

Class

1,

Division

2

(NEC Code),

and use

corrosion-

resistant materials (such

as

fiberglass)

and

explosion-

proof construction

in the

exhaust system.

Wet

well

exhaust

is

very damp

and

corrosive. Protect

screens

from

freezing.

Inadequate

ventilation

for dry

well. Ventilation

in

dry

wells

of

either water

or

sewage pumping stations

is

required

to

limit

the

humidity

and

accumulation

of

toxic

or

explosive gases. (Explosions have occurred

in

water

pumping stations.)

Six air

changes

per

hour

for

continuous

ventilation

or 30

complete

air

changes/h

for

intermittent ventilation

is a

practical minimum.

Intermittent

ventilation

may be

hazardous because

it

might

not

prevent

the

accumulation

of

explosive

or

toxic

gases.

Other

Considerations

Inadequate

headroom. Design

for

2.1

m

(7 ft)

clear-

ance under

all

pipe

flanges,

hand wheels,

and

other

protuberances.

But if

such

a

clearance cannot

be

met,

pad the

protuberance

and

hang

a row of

light chains

around

it as a

warning. However, locate

oil or

water

traps

low

enough

so

they

can be

serviced easily.

Obstructions

in

walkways.

Lay out dry

well

and

other

floors

to

provide easy access

to all

maintenance points

(such

as

packing glands,

oil and

grease points,

and

pres-

sure

gauges)

so

that workers need

not

climb over piping

or

other obstructions.

A

good rule

is to

require

a

1.07

m

(42

in.) clearance (horizontally) between pumps, motors,

valves,

etc.,

and any

obstruction.

Ladders

and

circular stairs. Avoid ladders

if

possi-

ble, because

(1)

emergency egress

is

impeded,

(2)

per-

sonnel

can

carry tools only with

difficulty,

and (3)

disabled workers cannot

be

carried

at

all. Observe

OSHA rules

for

landings (required

at

maximum inter-

vals

of 3 m or 10

ft), ladder cages, nonskid treads,

and

toe

plates. Make ladders easy

and

safe

to

enter.

Circular stairs

are

only marginally better. Straight

stairways

with landings conforming

to

local building

codes

are

justifiable,

considering

safety,

convenience,

and

daily

use for 20

years

or

more.

If a

compromise

must

be

made, consider ship stairs

for

minimum

run

length

(1

horizontal

to 2

vertical). Check local build-

ing

codes

for

double-access requirements

and for

stairs

to

augment elevators.

Guards.

Install guards around dangerous rotating

machinery, belt drives,

and

other moving machinery.

Rescue. Provide

a

means

for

moving

a

helpless

worker

to the

building exit. Consider

the

need

for gas

masks

and

self-contained breathing apparatus (always

in

pairs)

at

sewage pumping stations.

Codes.

Become familiar with

the

safety

regulations

of

such codes

as

OSHA, NFPA,

and

NEC.

27-5.

Hydraulics

Sumps

and wet

wells inadequate

or

mismatched with

pumps. Review Section 12-6. Unless

the

design

is

similar

in

geometry, size,

and flowrates to an

existing

successful

design, consider model tests. Sumps

or wet

wells

for

systems larger than about

3

m

3

/s

(50,000

gal/

min)

or

those with unusual approach conditions

should

always

be

designed

from

model tests because

the

rules published

in the

various standards

and

design

guides

can

(and

often

do)

lead

to

poor hydraulic con-

ditions.

For

small-

to

medium-sized inlets, refer

to

Prosser

[2] and to the

text

by

Dicmas

[3].

Pipe

roughness overestimated.

If the

actual friction

headloss

is

less than

the

estimate,

the

actual discharge

will

be

greater than predicted.

The

greater discharge

increases

the

horsepower requirement and, thus, over-

loads

the

pump

and

motor. Design

for the

code-speci-

fied

roughness

—

e.g.,

in the

Ten-State Standards [1],

C=

120 for

lined

and

plastic pipe

or C = 100 for

unlined

pipe

—

or

design

for the

roughness expected

after,

say,

20 yr of

service. Then redraw

the

station

curves

for new

pipe (e.g.,

C =

145),

find the new

oper-

ating

point,

and

specify

a

pumping unit that

can

handle

all of the

pumping conditions between

the two

extremes.

For final

calculations,

use the

actual, true

inside pipe diameter

for

headloss

and flow

calculations.

Prerotation

in wet

wells.

The

vortexing

that results

from

prerotation entrains air, reduces

flow, and

leads

to

vibration

and

noise

in

pumps.

See

Sections 12-6

and

12-7

for

advice about

wet

wells.

Suction

inlets

too

small

for

rated

flows.

Cavitation

and

vibration will occur

and the

pump will

not

dis-

charge

the

designed capacity.

The

"average"

velocity

at

the

bell mouth

of the

inlet should

not

exceed

1

.7

m/s

(5.5

ft/s),

and the

velocity

in the

inlet pipe should

not

exceed

2.4 m/s (8

ft/s),

but see

also Table 12-1 (and

nearby

text)

and

Section 17-3.

Pump

volutes inadequately submerged. Always

place

the low

water

cutoff

above

the

pump volute

or,

more conservatively, above

the

stuffing

box.

Suction

inlets

inadequately

submerged.

The

required

submergence

is

based

on

empiricism,

and

opinions

vary.

Probably

the

best rule

is

given

by

Equation

12-1.

Gratings

and

other devices above

the

inlet

can

reduce

the

required

submergence,

but

they collect debris

in

wastewater

wet

wells.

Be

careful

and

conservative.

For

authoritative

and

extensive discussions,

refer

to

Prosser

[2] and

Dicmas

[3].

See

also

the

Hydraulic Institute's standards

[4].

Vertical

pipes

connected

to

invert

of

manifolds.

The

solids

in raw

water

and

wastewater

can

collect

in the riser

and,

given time, plug

it.

Connect

the riser to the

side

of

the

manifold with

a

short length

of

horizontal pipe.

Float

switches

unt

ether

ed

in

turbulent

wet

wells.

Untethered

float

switches tangle,

so

attach them

to a

weighted chain (which permits easy removal)

or to a

vertical pipe. Also consider other sensors

described

in

Section 20-3.

Pipe

profile

incompatible

with

hydraulic

profile.

Plot

both

profiles

on the

same drawing

for

comparison.

Avoid

high spots

in

pipe

profile

—

especially

in

waste-

water

service where

the

height

of the

pipe should never

be

above

the

mirror image

of the

hydraulic

profile

(see

Figure 7-1).

In

water service,

the

pipe

profile

should

never

be

above

the

hydraulic profile

by

more than

7 m

(23

ft.),

and

then control equipment (e.g., air-vacuum

valves)

must

be

installed

at

high points.

Use the

services

of

experienced

personnel

or

consultants

for air

release

needs

and

detailed surge analysis. Prevent column sepa-

ration

in the

force main

at all

costs.

A

complete tran-

sient

analysis should always

be

made

for all

systems

exceeding, say,

TDH = 12 m (40

ft),

Q = 30 L/s

(500

gal/min),

and v = 1.5 m/s (5

ft/s).

Beware

of

water

hammer

with

an

expulsion

of air at the top of

wells.

Make

certain that thrust blocks

and

anchors

at

bends

and

elbows

can

resist both static head

and

surges.

Sewage systems

are

especially vulnerable

to

water

hammer because

of

buildup

of

grease

and

foam

in

control

devices

such

as air

valves.

Pilot-operated

surge-control

valves

for

sewage

are

unreliable

due to

the

potential clogging

of

pilot lines,

so

avoid them

in

wastewater

pumping stations. Avoid water hammer

in

sewage force mains

by

controlling

the

pipeline

profile

and/or

by

adding

flywheels to

drivers.

Air

chambers close

to

swing check valves. Never

put

swing check valves close

to air

chambers, because

the

short water column

can

accelerate very quickly

and

cause valve slam with exceedingly high pressures

that

can

explode pipe

or

break valve casings.

Backflow

Prevention

Cross

connection installed.

Use

reduced-pressure-

principle

backflow

preventers

or,

better, completely

separate potable water

from

the

station (seal water,

washdown) water supply with

a

positive

air

gap.

Applicable

plumbing

code

not

checked. Under

some

codes,

a

reduced-pressure

backflow

preventer

is

not an

acceptable substitute

for an air

gap.

Backflow

preventer

not

equipped

with drain. Under

some conditions,

the

backflow

preventer

can

release

a

large stream

of

water, which requires

a

high-capacity

drain that

can

inundate

the dry

well,

so

exhaust

the

vent pipe outside

the

building above

flood

level.

Backflow

preventer installed

at too low an

eleva-

tion. Always install

backflow

preventers above

any

possible

flood

level.

Potable water. Water downstream

from

a

backflow

preventer

is

subject

to

contamination

and is not

pota-

ble. Provide only potable water

at

sinks.

Potable water faucets with hose threads. Hoses

are

possible sources

of

cross connection

and

contamina-

tion.

Do not

install hose valves (bibbs)

on

potable

water

pipelines.

Hose

valves

(bibbs)

without warning signs.

Place

warning

signs indicating that

the

water must

not be

drunk.

27-6.

Pumps

Improper

selection

of the

type

of

pumps

and

pumping

station.

A

design concept that

may be

entirely suitable

for

one

agency

or

application

may be

entirely unsuit-

able

for

another, even

if the

stations happen

to

serve

the

same hydraulic conditions.

For

example,

the use of

a

submersible sewage pump design

may be

appropri-

ate for an

agency that already

has a

significant

number

of

such facilities

and has the

attitude, experience, per-

sonnel,

and

equipment

to

maintain them. Such

a

pumping station

may be

quite unsuitable

for an

agency that

has no

experience

with such

a

facility

and

lacks

the

equipment

and

personnel

to

service

and

maintain

it. A dry

well

may be the

best option

for the

second agency

in

spite

of the

additional

first-time

con-

struction

cost.

In

summary:

all

aspects

of the

facil-

ity

—

initial

construction cost, power costs, ease

of

operation

and

maintenance, availability

of

equipment

and

skilled maintenance personnel,

and so

on

—

must

be

considered

in

evaluating

the

various types

of

sta-

tions

and

selecting

the

most appropriate.

Large

submersible pumps. Large [more than

75

kW

(100 hp)] pumps

may

have very high operation

and

maintenance (O&M) costs

if the

pumping station

is

not

tailored

for

ease

in

handling

big

pumps. Ease

in

handling

large submersible pumps

is

particularly

important

for wet

well installations

due to the

long,

heavy

power cables that (because they cannot

be

han-

died

manually) make large submersible pumps

entirely

different

from

smaller ones

in

their require-

ments

for

trouble-free removal.

There

are

many horror

stories

of

power cables

cut on

sharp edges

or

flexed

beyond their limit, pumps jammed

on

guide rails that

are

thereby bent,

the

need

for

massive mobile cranes

to

pull pumps located

an

excessive distance

from

a

parking

area, long discharge pipes that vibrate exces-

sively,

pumps dropped

by

inexperienced crane crews,

and

even

of a

discharge elbow pulled

out

with

the

pump.

In

addition

to

these scenarios

of

woes, large

pumps

(especially those

of

moderate quality) tend

to

break down more

often

than

do

smaller pumps. Many

utilities seem

to

agree that large submersibles should

be

confined

to dry

pits where they

can be

serviced

in

place.

In at

least

one

large utility, however,

wet

well

installations

of

submersible pumps

are

favored

regard-

less

of

size (see

the

subsection

"Submersible

Pumps"

in

Section 25-6).

Cable

attachment details omitted. Lack

of

fore-

sight

in

planning

for the

removal

of

submersible

wet

pit

pumps (especially those over

75 kW or 100

hp)

can

make removal

difficult

and

expensive

and may

even

require entry

in

confined

hazardous spaces (with

all

the

associated personnel

and

safety

requirements).

Discuss

the

removal

of the

pump with

the

manufac-

turer

and the

utility, decide what method

is to be

used,

and

tailor

the

facilities

to

make

the job as

easy

as

pos-

sible. Detail

the

method

in the O&M

manual,

and

describe

the

cable attachments

in the

specifications

and

show them

on the

plans.

See

subsection "Sub-

mersible Pumps"

in

Section 25-6.

It is

attention

to

details

such

as

these that makes

the

difference

between easy

and

difficult

maintenance

and

between

good

and bad

engineering.

Unreasonably

conservative estimates

offlowrates

and

pipe roughness.

Fear

of

insufficient

capacity

for

future

flows

often

leads

to the

most

frequent

and

worst

blunder

of

all

—

choosing

pumps

and

pipes

too

large

for

the

actual

flows to be

encountered.

The

results

are

(1)

pumps that operate

too far to the

right

of the

bep,

exceed their capacity, vibrate,

and

cavitate;

and (2)

motors that

are

overloaded

and

therfore overheat.

Pumps

and

motors have short lives,

and

parts must

frequently

be

replaced.

Avoid

this blunder

by

choosing

the

pumps

and

impellers

for

stable operation

at low (as

well

as

high)

flows,

for

smooth pipe

as

well

as

aged pipe,

and for

flooded

wet

wells (with lowered static

lift)

as

well

as

for

low

water levels.

In

lift

stations with multiple

pumps,

for

example, characteristically only

one

pump

operates most

of the

time

—

and

it

does

so at

substan-

tially

reduced total head.

So the

pump should operate

near

its bep for

that condition. Also select pumps

and

motors

to

operate safely over

the

full

range

of

condi-

tions

to be

encountered.

Low flows

are,

if

anything,

more

critical

than high ones.

See

Figures 10-27

and

10-30.

If

future

flows

will

be

considerably larger than

present ones,

try the

following

in the

order listed:

(1)

trim

impellers

now and

replace them with larger ones

when needed,

(2)

leave space

and

hook-ups

for

another,

future

pump,

or (3)

plan

on

replacing present

pumps

and

motors with larger ones when needed. Pip-

ing

must

be

satisfactory

for

both

the

present (with

velocities high enough

to

transport solids)

and the

future

(with friction headloss within bounds).

Hazen-

Williams

C

values

for

lined steel, ductile iron,

and

plastic

pipe should

be in the

range

of 130 to 150 for

normal operation (see Sections

3-2 and

3-3). Future

flows

might

require

a

second, parallel force main.

During start-up, always test both suction

and

dis-

charge pressure

at

each pump

for the

widest range

of

conditions encountered.

If

necessary, change

or

trim

impellers

to

suit

the

true system requirements

and to

reduce noise

and

vibration

(an

embarrassment

to the

designer). Test motors

for

current draw

to be

sure they

are not

overloaded

at any

condition. Make sure

the

ventilation system

can

adequately cool

the

motors.

Never allow variable speed units

to

operate

at

speeds lower than recommended

by the

pump manu-

facturer.

If flows are too

low,

add a

smaller unit

or

pro-

gram

the

controller

to

operate

on a

start-and-stop,

fill-

and-draw

basis while making sure

the

electrical equip-

ment

can

withstand

the

frequency

of

pump starts.

Net

positive suction head

(NPSH).

Always make

sure

NPSH

A

is at

least 1.35 times

NPSH

R

,

and

also

never less than

1.5 m (5 ft)

more than

NPSH

R

(see

Section 10-4

and

Figure

10-13).

Pump

efficiency

poor. Reliability

and

ease

of

main-

tenance

are of

prime importance,

but

search diligently

for

the

best pump because

an

increase

of

only

2 or 3%

in

pump

efficiency

returns

a

startlingly

high present-

worth

cost saving.

Pump

and

impeller selection poor. Avoid

the

larg-

est

impeller

in a

family

of

impellers because

of

vibra-

tion

and

reduced bearing life.

Try to use a

diameter

that

is no

more than

90% of the

largest diameter.

Select

a

pump

to

provide good

efficiency

above

the

expected head range

so

that

the

heads

"bracket"

the

highest

efficiency

of the

pump.

For V/S

units, specify

(or

select) pumps

so

that

the

operating point

lies

to the right of the

best

efficiency

point (bep). Thus,

the

pump selected will

be the

small-

est

applicable

and

will operate through

the

zone

of

best

efficiency

at the

various normal (reduced) speeds.

But

specify

(or

select) pumps with

the

operating point

at

or

near

the bep for C/S

pumps.

If a

greater capacity

is

anticipated,

specify

the

future

operating conditions.

Relationship

between

impeller,

speed,

flow,

head,

and

power

incorrect.

When calculating

flows

(which

vary

as

revolutions

per

minute)

for

reduced speeds,

also calculate

the

head (which varies

as

revolutions

per

minute squared)

to find a new

point

on the

pump

H-Q

curve. Draw

the new

pump

H-Q

curve through several

such

points

to an

intersection with

the

station

H-Q

curve

to find the

true

flow and

head

at the

reduced

speed.

To

avoid mistakes

in

applying

the

affinity

laws,

calculate

the

power

from

head,

flow, and

pump

effi-

ciency

—

not

from

the

relationship between power

and

revolutions

per

minute cubed. Note that manufacturers

of

V/S

pump drives generally

assume

—

erroneously

—

that

power varies

as

revolutions

per

minute cubed,

and,

hence, their curves

of

driver

efficiency

are

inaccu-

rate (see Section 10-3, Example 10-3, and, especially,

Figure 15-4

and

Section 15-3).

Excessive

radial loads. Avoid applications where

pumps operate

for

extended periods near

the

shut-off

head. Radial forces developed

in the

casing

and

acting

on

the

impeller

at low

speeds (see Figure

10-18)

cause

rapid bearing wear

and

damage

the

wearing rings,

shaft

sleeves,

and

packing.

If

such operation

is

neces-

sary,

specify

custom-made pumps with heavy-duty

shafts

and

bearings.

Machinery

vibration. Test pumps

for

vibration dur-

ing

factory

pump testing,

but be

aware that foundation

or

supports

in the field are

different

and

affect

vibra-

tion. Support vibrating machinery (such

as

engines)

on

spring isolators.

(It is a

fallacy

that massive

foun-

dations

can

control vibration.) Isolate piping

from

vibrating

machines with vibration isolators (see Chap-

ter 22 for a

discussion).

Inadequate

or

unclear pump

performance

specifica-

tions.

Manufacturers

differ

in

their presentation

of

pub-

lished pump performance data. Hence,

it is

important

to

define

the

conditions

of

performance clearly

so

that

competing products

can be

compared

on the

same

basis.

One

basis

for

comparison

is to

specify

energy,

head-capacity,

and

efficiency

criteria

for the

complete

pump,

for

example

(1) flange to flange for dry

well

pumps,

(2)

pool

to

discharge

flange for wet

well pumps,

or

(3)

pool

to

discharge elbow outlet

flange for

sub-

mersible pumps.

Single-vane impellers.

A

single-

vane

impeller

is

asymmetrical,

and no

amount

of

machining

can

bal-

ance

it

dynamically

in

water. Some manufacturers

can

and

do

trim these impellers; others

do

not.

As

alterna-

tives

to

trimming,

(1)

consider

V/S

drives unless

an

unaltered impeller

can be

found

to

match

the flow

conditions

or (2) use a

symmetrical impeller, which

can

be

trimmed

and

balanced more easily.

High

pump speeds. Avoid high speeds (especially

for

sewage pumps) because wear

is a

function

of the

second

or

third power

of

speed.

Use the

slowest prac-

tical speed (such

as 705 to

1150

rev/min)

to

reduce

maintenance costs.

Impellers

improperly

chosen

for

deep

well turbine

pumps. Because

shaft

stretch becomes important

for

deep pump settings greater than about

30 m

(100 ft),

specify

closed impellers

and

consult

the

pump manu-

facturer

or a

specialist.

Oil-lubricated

deep

well pumps.

In

general,

oil

lubrication should

not be

used

in

pumps

for

potable

water.

Prelubrication

system omitted

in

lineshaft

deep

well

turbine pumps.

A

system

for

supplying water

to

the

bearings during starts

is

required. Consult

the

pump

manufacturer

for

assistance

in the

design.

Light-duty

pumps. Many pumps with attractive

prices

are too

light

for

constant

or

severe

service.

Require rigid construction.

For

example,

shaft

deflec-

tion

at the

packing gland should

not

exceed 0.05

mm

(0.002

in.),

nor

should deflection

at

wearing rings

(outboard side

of

impeller) exceed about 0.20

mm

(0.008

in.).

A

custom-designed pump

may be

needed

to

meet these requirements. Even sump pumps should

not

be

light-duty machines (see Example

12-1).

Mechanical

seals

on

pump

shafts

with

a

long

over-

hang

from bearing

to

impeller.

If

pumps

are

variable

speed

or

operate frequently

at low flow (or

near

the

shut-off

head), high radial loads, which bend

the

shafts

and

cause excessive wear

on

mechanical seals,

are

produced. Most seals cannot withstand such

shaft

distortions. Pumps must

be

engineered

for

such appli-

cations. Specifying

a

maximum

shaft

deflection

of

0.20

mm

(0.008

in.)

at the

impeller

and a

B-IO

bear-

ing

life

of

50,000

to

100,000

h at the

worst combina-

tion

of

operating conditions promotes longer

life,

low

maintenance,

and

life-cycle economy.

Bearings

and

seals

or

packing

difficult

to

replace.

Packing

and

bearings

are the

most frequently replaced

items

on

pumps. Make seal replacement easy

by

ensuring easy access

to the

seal (such

as

avoiding

close-coupled pumps

and

motors) and,

for

bearing

replacement, provide

a

convenient means

for

lifting

and

storing motors

and

pump parts.

Bearing

life

not

specified.

Specify bearing

life

and

insist that

the

manufacturer

furnish

certified

bearing

life

calculations

for the

design point, pump speed,

and

impeller selected

for the

worst combination

of

speci-

fied

continuous

operation conditions.

The

worst com-

bination

of

operating conditions

is

particularly critical

for

V/S

pumps.

Be

especially

careful

with high radial

load conditions when operating near

shut-off,

espe-

cially

if the

head exceeds about

15m

(50

ft).

Positive-displacement

pumps

unprotected.

A

clogged line

can

break

a

positive-displacement pump

unless

it is

protected

by (1) a

bypass line with

a

pres-

sure-relief valve such

as a

simple air-loaded pinch valve

or

(2) a

pressure-actuated switch

to

stop

the

motor.

Poor

or

nonexistent pressure gauge installations.

Every

station should either have gauges

or

outlets

for

gauges

on the

pressure side (and also

on the

suction

side,

if

practical)

of a

pump

or

pipe (see Figure

20-6).

By

using quick-connects with double

shutoffs,

one

gauge

can

serve several locations. Ease

of

frequent

calibration

and

inconsequential wear

are two

advan-

tages

of

portable gauges.

Sewage

pumps inadequate

for

passing solids.

Specify

that

the

pump must

be

certified

by the

manu-

facturer

to

pass

a

particular size

of

sphere. Further-

more, test

the

pump

at the

site before installation.

The

Ten-State Standards

[1]

requires

at

least

75 mm (3

in.).

Many

100-mm

(4-in.)

or

even

150-mm

(6-in.)

pumps

cannot pass

a

75-mm

(3-in.) sphere.

Cleanouts

not

specified.

Some pumps

are

built

without

suction elbows

or

suction piping containing

hand

holes

for

removing rags

or

other debris

from

non-clog pumps,

and

such pumps cannot

be

cleaned

without

dismantling. Always

specify

hand holes

for

unclogging sewage pumps

and

consider

how to

drain

the

pump

and its

piping conveniently (such

as a

drain-

pipe

to the

sump).

Use

of

main pumps

for

scum control.

The

buildup

of

scum

in wet

wells

can be

controlled with main

pumps

by

manually bypassing

the

low-level pump

control

and by

operating

one or

more pumps

at

full

speed until

the

pumps break suction prime.

The

short-

term

vortexing

sweeps

the

scum into

the

pumps.

Equip

the

manual bypass feature with

a

spring

return-

to-automatic

switch

to

restore

the

water level

to the

standard

LWL and to

avoid

the

hazard

of

operator

error (see also

Section

27-16).

Variable

-speed

drives

in low

friction

head

systems.

Variable-speed drives should

not be

used

if the

system

can

operate

at

constant speed. Furthermore,

V/S

should

not be

used

for

centrifugal pumps

in

systems

where

(1) the

slope

of the

head-capacity curve

of the

pump

is

nearly

flat or (2)

simple, four-position

wet

well level controls

are

used.

Variable-speed

is

appropriate

for (1)

positive-dis-

placement pumps;

(2)

centrifugal

pumps

in

systems

where

an

instantaneously variable demand will occur,

such

as a

tankless water system;

(3)

situations where

relatively

precise

water level regulation

is

required;

or

(4)

situations where sudden changes

of

discharge

due

to

start-up cannot

be

tolerated.

But be

careful,

because

V/S

design

in low

friction

head systems requires

a

skillful

design

of

pumps

and

controls because

a

very

small change

in

speed

may

produce

a

very large

change

in

discharge.

Use

of

pressure gauges

at

start-up.

Impellers

may

not

be

compatible with

the

system hydaulics because

of

errors

or

even blunders

in

determining TDH.

To

avoid

this

one

source

of

vibration

and

noise, always

use

calibrated pressure gauges

on

both suction

pipe

and

discharge pipe

for

testing

the

pump. Compare

with

manufacturer's curve

and

trim

or

change impel-

lers

if

necessary

to

keep

the

pump operating point

within

-15%

to

+10%

of the

bep.

27-7.

Valves

Check

Valves

Improperly

Selected

Be

careful

to

match

the

valve

in

type

and

size

to the

system

and to the flow.

Swing

checks. Oversized valves operate improp-

erly,

but

reducers

and

smaller valves

can be

used

for

low

flows to

obtain

sufficient

velocity

to

open

the flap-

per

fully.

An

external

arm or a

position indicator

is

desirable

to

show

the

position

of the flapper.

Slam. Check valves

may

slam

and

cause disastrous

water hammer. Avoid slam

by

using valves that close

either very quickly

(so

that

no

reverse

flow can

occur)

or

very slowly

(so

that

the

water column comes gradu-

ally

to

rest). Quick-closing valves

are

preferable.

Slow-closing valves tend

to

promote water hammer

unless

the

closing mechanism (such

as an

external

dashpot)

is

adjusted

carefully

and

matched

to the

sys-

tem.

A

water hammer analysis

and an

expert

to

adjust

the

closing mechanism

is

required (review

the

subsec-

tion

entitled

"Slam"

in

Section 5-4).

Quick

closure.

The

quickest closure

is

provided

by

center-guided

lift

(or

"silent")

check valves followed

by

double-disc

(or

"double-leaf

or

"double-door")

check valves,

but

these types

are for use

only

in

clear

water service. Closing characteristics

of

swing check

valves

with counterweighted versus spring-loaded

lever arms

is

controversial (see

"Slam"

in

Section 5-4).

Carefully

select

check valves

for the

specific

system.

Slow

closure. Reverse

flow

will occur

and the

pump

will

run

backward unless restrained. Impellers

must

be

keyed,

not

screwed

to the

shaft.

The wet

well

must

be

able

to

accept

the

reverse

flow.

Rags

in

sew-

age flowing

backward

may jam

swing checks,

so use

ball, lubricated plug,

or

cone

valves

for

slow closure

in

wastewater service.

One

requirement

is a

stored-

energy closure system that operates

the

valve quickly

through

the first

stage

of

closure, then slowly

to

shut-

off

so as to

minimize water hammer. Swing check

valves

are

satisfactory

for

water service

if

equipped

with

dashpots that

can be

regulated

to

meet system

requirements.

Swing

check valves without cover

plates.

In

waste-

water

service,

the

lack

of a

cover plate means that

the

entire valve must

be

removed

to

extract debris,

so

require removable cover plates.

Other Aspects

of

Valves

Improper

location. Location

is

often

critical

for any

type

of

valve. Never allow swing checks

or

gate valves

in

a

vertical pipe carrying

raw

sewage

or

grit-laden

raw

water.

When

the flow

stops, grit settles

and

clogs

the

valve

(or the

valve bonnet

of a

gate valve),

and the

valve

will eventually

fail.

If

valves must

be

placed

on a

vertical pipe carrying dirty water,

use

only eccentric

plug,

ball, cone,

or

pinch valves. Never install

butterfly

valves

near (within

2

pipe diameters

of)

elbows

or

pumps

because

the

curved streamlines

may

cause

so

much

torque

on the

vane that

the

valve actuator will

not

function.

Never install valves

in a wet

well.

Air-

and

vacuum-release valves

in

sewage service.

Go

to any

lengths

to

avoid using

air-release

and

vac-

uum-relief valves

for

sewage

service.

Relocate

the

force

main

or the

pumping station.

Dig

deep trenches

or

tunnel.

If the use of

such valves

is

unavoidable,

realize that they

are

high-maintenance items, that they

frequently

clog,

and

that they

are

unreliable

in

waste-

water service.

Specify

them

in

pairs

to

provide

a

backup,

and

specify

sewage types with stainless-steel

trim

and flushing

connections. Require

not

less than

monthly

cleaning

in the O&M

manual. Removal

of all

internal parts

and

shop cleaning

is

more

effective

than

field

flushing.

Throttling.

Except

for

slow starts

and

stops

to

con-

trol water hammer,

seek

methods other than throttling

to

control

flow

(such

as

variable speed,

spillage,

a

bypass,

or a

smaller impeller) because throttling wastes

energy.

Gate valves should

not be

throttled

for

extended

periods because they

are

quickly ruined

by

vibration.

Low

velocity through

a

throttling valve results

in

poor

control,

so if

throttling

is

unavoidable, consider using

a

reducer,

a

smaller valve,

and an

expander.

Nearly

all

butterfly

valves

are

unsuitable

for

severe

throttling service

(as in

pump-control valves).

AWWA

C504

specifications alone

are

insufficient

to

ensure

the

adequacy

of

valve seats. Some (but

not

all)

replaceable

body seats

are

satisfactory. Some manu-

facturers

of

pump-control systems

who

either

use

top-

of-the-line

makes

or buy

customized valves

of

special

design

may be

able

to

offer

advice.

Air-release

valves

in

transmission

and

force

mains.

Air-release valves

can

cause water hammer (see Sec-

tion 7-4) unless appropriately sized.

Vacuum-relief

valve

danger.

Be

extremely wary

of

using

vacuum-relief valves

in a

pipeline

if the

pump

valve

is of the

slow-closing type.

If the

water column

reverses

its

direction

and the

velocity increases while

the

check valve

is

still mostly open (and unable

to

pre-

vent high

velocities),

an

explosive force

can be

pro-

duced that

is

sufficient

to

throw slabs

of

concrete

into

the

air. Hydraulic analyses

amd

consultation with

the

vacuum-relief valve manufacturers

are

necessary

to

determine whether such

a

problem

can

occur.

Air in

piping. Preferably, arrange piping

so

that

air

is

automatically expelled

as the wet

well

or

sump

fills.

Air

entrapped between

the

pump inlet

and a

check

valve

can air

bind

the

pump,

so

install manual

or

auto-

matic

(or

both

in

sewage service)

air-release

valves

at

all

possible

points

of air

entrapment, such

as the

volutes

of

horizontal pumps, pressure gauges, suction

piping,

and

both upstream

and

downstream

from

the

check valve.

A

manual

air

release

should have, say,

a

12-

or

19-mm

(1/2-

or

3/4-in.) gate valve (for water)

or a

ball valve (for water

or

sewage) followed

by an

air

break

and

drainpipe

to the floor

sump.

Natural

rubber seats. Because natural rubber

in

wastewater valves

is

attacked

by

petroleum products,

specify

a

resistant synthetic elastomer.

Thin

elastomer seats. Elastomers

may be cut by

grit

in

slurry service,

so

thick layers should

be

speci-

fied.

Urethane

is

preferable

to

either natural

or

syn-

thetic rubber.

Zinc

and

aluminum bronze. Avoid zinc

and

untem-

pered

aluminum bronze

in

valves

for

waters that cause

dezincification

(see

C508-93,

2.2.2.4

and

C504,

Fore-

word

1 1 1

.7

and

main text

2.2.

1 1

.2).

Valve

handles. Chain wheels

in

sewage

wet

wells

are an

explosion hazard. Hand wheels should

be

placed within easy reach

and

away

from

walls

or

other

knuckle-cracking obstructions.

Use

either hand

wheels

or

chain wheels with worm

gears

instead

of

levers

on

plug valves

150

mm (6

in.)

or

larger.

Valve

stems protruding into walkways. Protruding

valve

stems constitute

a

safety

hazard

in

walkways,

so

relocate

them.

Power-actuated

valves without manual actuators.

The

valve cannot

be

moved

if the

power actuator does

not

work. Always

specify

an

auxiliary manual closure.

Power actuators

not

designed

for

continuous

throt-

tling

service. Valves designed

for

throttling should

have

actuators that

are

specified accordingly; other-

wise, maintenance

is

excessive.

Hand

wheel with nonstandard direction. Valves

should either open when

the

hand wheel

is

turned

counterclockwise

or the

hand wheel should

be

perma-

nently marked with

the

direction

for

opening.

Valve

permits cavitation. Cavitation causes exces-

sive

noise, wear,

or

premature failure

of

valves.

Select

and

install valves

so

that cavitation does

not

occur.

Valves

must operate within

the

limits recommended

by

the

manufacturer.

Valve

fails

in the

wrong manner. Design

the

valve

and

appurtenant piping

so

that

the

valve acts

in a

safe

manner when power fails

(as it

will).

Valve

cannot

be

exercised without

raw

wastewater

discharge.

Redesign

the

system

so

that valves

can be

exercised with

no

resulting problems.

Improper

support. Improperly supported valves

or

pipe

can be

distorted, which causes binding.

Use an

adequate number

of

supports.

Water

valves

in

wastewater

service.

Use

only valves

designed

for

wastewater

in

wastewater service. Rags

can

wrap around

shafts

or

catch

on

protuberances.

Freezing.

Valves must

be

protected

from

freezing.

If

the fluid is

warm enough, insulating

the

valve hous-

ing may be

sufficient

unless there

is a

possibility

of

zero

flow.

Plug

valve installed backward.

Eccentric

plug

valves

seat

in

only

one

direction. Analyze

the

require-

ments

carefully

and

indicate

the

seating direction.

In

sewage

service, install eccentric plug valves with

the

shaft

horizontal

so

that

the

plug

is in the

upper portion

of

the

valve

in its

open position.

Cleaning

tools will

not

pass.

Select valves that

allow

passage

of a

cleaning

"pig."

Otherwise,

the

pip-

ing may

have

to be

disassembled

for

cleaning

—

a

seri-

ous

blunder.

Isolating

valves

added

to

pressure

relief

or

other

control

valves.

It

must

be

possible

to

repair

a

valve

by

isolating

it.

Always design

the

system

so

that safety

is

not

compromised

by the

forgetfulness

of a

careless

worker

(in

reopening

a

valve,

for

example).

Control

piping

of

incorrect diameter.

It is

impor-

tant

to use the

correct diameter with respect

to the

size

and

speed requirements

of

valve actuators.

27-8.

Mechanical

Cranes

and

hoists. Cranes

and

hoists must have

access

to the

equipment with vertical

falls

to

meet

OSHA requirements. Piping

and

other obstructions

above pumps

or

heavy valves must

be

located

so as

not to

interfere. Design

the

facilities

to

permit

lifting

all

heavy equipment straight

up and

depositing

it

directly

on a

truck bed.

Lifting

eyes.

Lifting

eyes

are an

abomination,

but if

(for

some reason) they must

be

used,

at

least

design

them

so

that

stresses

(in

steel)

do not

exceed

69,000

kPa

(10,000

lb/in.

2

)

at the

smallest cross section.

Never

use

cast iron

or

malleable iron

for any fittings to

hold

lifting

eyes (see Figure 25-2).

Obstruction

in

walkways. Avoid them.

For

exam-

ple, seal water

can be

carried

in ss

piping buried

in

floors.

Overhead piping

and

other objects

are not

obstructions

if the

headroom

is at

least

2.1

m (7

ft.)

Unharnessed

sleeve couplings. Sleeve

(Dresser®)

couplings,

flanged

adapters, bell

and

spigot joints,

and

mechanical joints

can

blow apart

at any

time unless

they

are

longitudinally restrained.

The

restraint

can be

provided

by

burial

in

soil, confinement between

rig-

idly restrained piping

or

equipment,

or

harnessing

with

enough bolts

to

resist

the

longitudinal forces

due

to

water pressure (plus water hammer)

in the

joint.

Long

shafts.

Either avoid

the use of

long

shafts

or

else

plan

for

them carefully. Provide

sufficient

head-

room

for

disassembly. Always balance long

shafts

dynamically,

and

always investigate torsional vibra-

tion, which

is not

apparent

to the

senses

and is

there-

fore

more dangerous. Provide access

for

lubrication

and

consider framing

the

concrete structure

for

both

the

support

of and

access

to any

intermediate steady

bearings. Also confer with

the

shafting manufacturer

when

considering

the use of

tubing large enough

to

allow

for the

omission

of

intermediate bearings.

Pipe

flanges

of

different

pressure class than pump,

valve,

or fitting flanges.

Coordinate

the

plans

and

specifications

to

ensure

flanges

will

fit.

Piping

system

inflexibility.

Flexibility

must

be

suf-

ficient

for

the

replacement

of

valves

and fittings.

Con-

sider

flange

adapters

or

grooved-end

(Victaulic(D)or

sleeve

(Dresser®)

couplings

at

strategic locations.

Sludge

lines clogging. Sludge

or

concentrated

raw

sewage tends

to

clog when

at

rest

or

when pumped

at

velocities that

are too

low. Design

for

velocities above

the

critical velocity, which

is

about

1 m/s (3

ft/s)

for

2%

sludge,

1.5 m/s (5

ft/s)

for 4%

sludge,

and 2.4 m/s

(8

ft/s)

for 10%

sludge. Refer

to

Chapter

19 for the

calculation

of

headloss. Design sludge systems

for

easy

cleaning

—

for

example,

by

using crosses instead

of

elbows

to

make rodding

easier.

Water

meter inaccurate

or

readout

and

capacity

improper.

Water meters

are

inaccurate unless

the

entrance pipe

is

straight

for 15 to 20

pipe diameters

for

"head"

meters

and 2 to 5

pipe

diameters

for

magnetic

meters. Downstream

flow

disturbances also

affect

meter accuracy.

Do not

"fudge"

on the

manufacturer's

recommendations

for the

required straight inlet

and

outlet distances. Choose capacity

and

readout units

for

present (not

future)

flow

variations. Calibrate

in

situ.

Inadequate

well

head design. Many problems

can

occur, such

as

improper well casing ventilation, depth

gauge,

tap for raw

water samples, clearance

of top

casing above

the

floor,

and

pressure gauge

for

pump

testing. Review sanitary codes

and

engage

a

well

pump

specialist.

Flywheels

used

for

water hammer control. Fly-

wheels impose very severe service

for

bearings.

Use

oversize bearings

(100,000

hour,

L-IO

life)

and flexi-

ble

couplings

to the flywheel

shaft.

27-9.

Electrical

Control

system

too

complex

for

contractor

or

opera-

tors.

Assess

and do not

exceed

the

level

of

sophistica-

tion

of

local contractors

and the

client's personnel.

Keep

it

simple.

Corrosion

in

control panels. Locate control panels

in

a

clean,

dry

environment.

If

they must

be in a

damp

environment,

use

NEMA

4 or 12

panels

and

purge

them

with

dry

air. NEMA

4

panels require

thermo-

static

heaters

in

most

of the

United States because

of

nighttime

moisture condensation

in the

cabinet.

Use

moisture-absorbing cartridges

for

corrosion control

in

all

panels.

Excessive

starts

per

hour.

Starting motors

and

motor starters

too

frequently reduces their

life.

Con-

tact

the

manufacturers

—

not

the

sales

representative

—

of

both

the

motor

and the

starter regarding

the

number

of

allowable starts

per

hour

and

obtain guarantees

in

writing.

Consider

(1)

"soft-starting"

with reduced-

voltage,

solid-state starters

for

unlimited starts

per

hour

or (2) V/S

pumping.

Big

motors.

Big

motors

may not

start

on the first

try.

Then

the

operators must wait

for

hours

for a

sec-

ond

try.

To

avoid

the

delay,

specify

enough thermal

capacity

for at

least

two

starts.

An

operator,

who

should

be

standing

by,

should stop

the

motor

if it

fails

to

reach

full

speed

in 10

seconds. Correct

the

cause

of

the

malfunction before attempting

a

restart.

Junction

boxes

in wet

wells. Avoid placing junction

boxes

in wet

wells

or at any

location where water could

enter

the

box. Boxes

in

moisture-laden atmospheres

accumulate

water that

can

travel along conductors

and

cause

serious

faults.

In a wet

well,

a

worker must enter

a

hazardous,

confined

space

to

disconnect,

for

example,

a

submersible pump.

One

means

to

keep junction boxes

out

of wet

wells

is

shown

in

Figure

17-21.

If

there

is no

alternative,

specify

watertight motor

and

device termi-

nations.

Keep moisture

from

entering

by filling the

junction

box

with potting compound and,

for

submers-

ible motors, also

specify

non-wicking cable.

Inadequate

lighting

of

work areas. Review lighting

needs with

the

operating personnel

and the

electrical

engineer. Operating panels should

be

lighted with

a

minimum

of 500 lux (50 ft

-

cd),

but 200 lux (20 ft

-

cd) is

adequate

for the

pump room.

Portable

work

lights

are

cheaper than

fixed

lights

and are

actually

better. Check

the

hazard rating. Install guards

on

lamps

and

ensure easy access

for

replacement.

Add a

few

incandescent lamps

to

provide instantaneous light

to

augment lighting that requires

a

long warm-up time

before

reaching

full

brilliance.

Inadequate

or

dangerous outlet receptacles.

Always

specify

GFCI

receptacles

and

always test

them. Plan enough

receptacles

for

power tools, elec-

tric hoists,

and

lights.

Phase

not

protected.

Use

breakers,

not

fuses,

on

motor-control

and

other three-phase circuits. Specify

a

three-phase monitoring instrument

on the

incoming

power line

to

protect against

undervoltage

or a

loss

of

phase.

Improperly

sized circuit breakers. Specify properly

sized circuit breakers

to

protect against

(1)

short cir-

cuits,

(2)

overloading over

a

short time interval,

and

(3)

nuisance tripping.

All

electrical

equipment must

have

adequate withstand ratings.

Improper

grounding.

All

electrical (and mechani-

cal) equipment must

be

properly grounded (see

"Grounding"

in

Section 8-5).

Power failures. Power failures will occur,

so

design

the

system

to

prevent column separation

and

other

disasters.

Add

backup

in the

form

of

stored

energy

to

operate

slow-closing

valves.

Power

supply

inadequate. Check with

the

power

company

before beginning

the

design.

Positive-displacement

pumps with high static

heads

or

long

force mains. These sometimes require high-

slip

(8 to

13%)

"D"

motors

to

accelerate properly

the

tons

of

liquid

in a

force main. Check

the

pump

and

operating characteristics with

the

motor manufacturer.

Transfer

switches without time

delay

in

neutral.

The field

voltages

on

motors driving loads with

a

large

WR

2

do not

collapse

rapidly.

If the

motor

is

trans-

ferred

quickly

from

one

power source

to

another (say,

to

an

emergency generator

or

vice versa)

and if the

two

sources

are out of

sync, mechanical forces that

are

large enough

to

twist

shafts,

break couplings,

and

tear

windings

out of

motors

are

produced.

Specify

a

trans-

fer

switch with

an

adjustable time delay

in

neutral.

Usually

20 to 30

cycles

is a

sufficient

delay.

Wet

well electrical equipment

not

explosionproof.

Always

classify

sewage

wet

wells

as

Class

I,

Group

D,

Division

1 or 2 in

accordance with

the

NEC. Elec-

trical

equipment should

be

explosionproof.

Use

non-

sparking

tools

and

safe

maintenance

practices

when

working

in or

around

wet

wells.

27-10.

Structural-Architectural

Anchor bolts

incorrectly

located

or

with inadequate

projection.

Anchor bolts

out of

place

is an

all-too-fre-

quent

problem that

can be

solved

by (1)

using sleeves

to

permit minor adjustments

of

bolts

and (2)

fastening

bolts

to a

plywood template before casting

the

con-

crete

to

ensure exact placement. Admonish

the field

inspector

to

check

and

recheck

to

ensure correct

placement.

An

alternative

is to

place anchors

in

accu-

rately located

drilled

holes

with epoxy

(or a

similar

quick-setting,

two-component glue)

in

breakable cap-

sules,

but

some experienced engineers will

not use

them,

because

the

strength

of the

anchor depends

so

much

on the

quality

of the field

work.

In

corrosive

environments,

use

extra-heavy

bolts,

or use

stainless

steel

or

nonferrous

material.

Inadequate

detailing. Show details

of

such ele-

ments

as

water stops, roof

flashing and

drainage, lou-

vers,

weather stripping, clearance around equipment,

and

provisions

to

guard against freezing. Detail roof

and

wall penetrations

for

adequate

clearance

between

engine exhaust pipes

and

combustible materials.

Ensure

that

the

work

of

structural, mechanical, electri-

cal, instrumentation,

and

architectural

disciplines

is

closely coordinated.

Differential

settlement between structure

and

exte-

rior

piping.

Be

sure that

the

exterior pipeline

has

enough

flexibility in its

joints adjacent

to the

wall

or

support

both

the

structure

and the

pipe

on

piles.

Always

use

wall sleeves. Because

of the

high proba-

bility

of

differential

settlement

or

frost

heave, inter-

rupt

plastic electrical conduit with metal conduit

through

exterior walls.

Foundation

problems. Obtain

the

services

of a

qualified

geotechnical engineer

and

adequately inves-

tigate

the

site. Include

field

borings

or

test pits (see

"Subsurface

Investigations"

in

Section 25-2

and

"Geotechnical

Considerations"

in

Section 25-10).

Inadequate

hatches. Make hatches large enough

(at

least

700 mm x 700 mm or 28 in. x 28

in.)

for a

worker

with

a

belt

of

tools

to

enter.

In

sewage

wet

wells,

allow

at

least

700 mm x 900 mm (28 in. x 36

in.)

for a

self-contained breathing apparatus

in a

back-

pack. Consider aluminum

checkerplate

with stainless-

steel hardware, positive latch openers,

and

inside grab

rails

for

easy access

to the

ladder

or

stairs.

Check

OSHA standards

for

hatches

and

their locations.

Level

floors

and, consequently, puddles. Slope

floors

at

least

1%

to a floor

drain

(2% is

better). Con-

crete

floors can be

cast level

if a

wearing slab

of

vari-

able thickness

is

applied later,

but

there

is

considerable

difficulty

in

obtaining

a

good bond.

Inadequate

monorails

or

cranes. Preferably,

arrange

the

station

to

provide monorails (for small sta-

tions)

or

cranes (for medium-sized

to

large stations)

above

all

heavy equipment,

and

make

it

easy

to set

heavy

items

on a

truck

in a

doorway. Consider motor-

izing

all

motions

for

cranes

and

hoists with capacities

greater than

680 kg

(

3

/

4

tons).

Use

slow speeds,

because pumping station operators

are

rarely

skilled

in

the

operation

of

hoisting equipment. Make access

hatches light (for easy

lifting)

and

large enough

to

remove equipment without dismantling

it. At the

very

least, small stations should have

lifting

eyes

for

porta-

ble

hoists (see Figure 25-2).

Inadequate

access

and

clearances. Allow enough

room

for the

removal (and replacement)

of

every

piece

of

equipment without disassembly

or

with mini-

mum

disassembly. Avoid cramped working areas

by

allowing

at

least

1.1 m (42

in.) around

all

equipment

(including

appurtenant piping)

for

heads, hands,

feet,

rumps,

wrenches,

and

lights

for

maintenance repairs

and

operations.

Carefully

draw plans

for

complicated

equipment such

as

engines

or

generators together with

all

attached auxiliary

or

appurtenant equipment

to

ensure that there

is

about

3 m (10 ft) of

clearance

for

ease

in

dismantling

for

repair.

Inadequate

strength,

stiffness,

and

watertightness.

Avoid

using

the

minimum design requirements

in the

ACI

318

Code. Heavier construction

is

required

so use

ACI

350

code.

Inadequate

ventilation

for

motor cooling. Provide

for

close coordination between mechanical

and

elec-

trical disciplines. Consider

air

conditioning, espe-

cially

if

noise must

be

suppressed.

Vibration.

Avoid resonance frequencies

by

sup-

porting pipes

at

frequent

intervals

and

using deep

beams

and

thick

floor

slabs. Isolate

diesel

engines

by

setting them

on

spring supports (see Chapter 22).

Wet

pit

atmosphere

seeping

into pump room. Com-

pletely sequester

wet

pits

from

the

pump room

so

that

neither

gas nor

sewage (even

if the wet

well over-

flows)

can

enter

the

pump room. Seal wires within

interconnecting conduits placed

at the

highest practi-

cal

elevation. Sump pumps should always discharge

above

the

highest

possible

water level

to a

sewer out-

side

the

pumping station.

27-11.

Specifications

Equipment

specifications

not to

code. Equipment

specified

by a

single named standard

or by

sole-

source

procurement must meet federal

and

state require-

ments.

Pump

testing. Require witnessed factory tests

of all

large

and

important pumps,

but use

judgment before

requiring witnessed tests

for

small pumps. Require

the

field

operational tests described

in

Section 16-6.

Incomplete

or

conflicting

specifications.

The

project

engineer must ensure that specifications

for all

materi-

als,

workmanship,

and

submittals

are

complete

and

must

coordinate them with

the

plans (see Chapter

17

and

Appendix

G).

Specifications

not

incorporating

unit

responsibility.

Specifications

should require that

a

factory-certified

representative

of the

manufacturer

be

responsible

for

the

complete coordination

of

multiple-component

equipment packages (such

as

pumps,

V/S

driving

equipment, drivers,

and

associated controls) when fur-

nished

by a

single manufacturer. Otherwise,

you

must

be

prepared

to

coordinate

all the

parts

and

assume

responsibility

for

such diverse aspects

as

motor size,

motor

heating characteristics, control system character-

istics,

pump torque requirements versus motor torque

output

at

variable speed, coupling sizes, coupling bore

sizes,

shaft

length, torsional analysis,

shaft

guard

design,

and a

host

of

other concerns over

which

—

not

being

the

manufacturer

—

you

have little

or no

control.

27-12.

Economics

Failure

to

select

an

appropriate

pump.

Thoroughly

investigate

a

pump's

efficiency,

ease

of

maintenance,

energy

costs,

and

compatibility with other pumps

in

the

system.

Reliance

on first

cost

for bid

selection.

A

penalty/

reward

specification

can be

added

in a bid

clause

to

include

the

present worth

of

anticipated energy

use

(see Chapter 29). Pump specifications

can be

written

for

low

maintenance (see Chapters

11,

12, and

16).

Obsession

with

economy.

How

well

the

station

works

and how

easily

and

conveniently

it can be

main-

tained

is

vital.

Two or

three points

of

efficiency

or a 4

to 5%

savings

in

construction cost

is

trivial

in

compar-

ison

with

a

design that ensures

a

maximum

of

conve-

nience

and a

minimum

of

maintenance

and

repairs.

27-13.

The

Future

and

Remodeling

Future

expansion

not

considered. Design

the

station

to

allow

for an

increased discharge

by (1)

specifying

pumps that

can

accommodate larger impellers,

(2)

adding another intake with

a

blind

flange, or (3)

choosing oversized piping

(especially

on the

suction

side)

so

that pumps

and

motors

can be

replaced with

larger ones.

Fasten

a

lifting

eye on

large blind

flanges

because they

are

otherwise

difficult

to

handle.

Develop

a

strategy

for

adding

or

changing pumps

and

piping

so

that

the

station need

not be

shut down

for the

change.

A

valve with

a

blind

flange is one

way,

but

valves

not

exercised will

freeze

and, thus,

be

worse than useless.

The

strategy must

be

safe

and

sure

and

should

be

written

into both

the

specifications

and the O&M

manual.

Reliance

on

record

drawings

and

specifications.

Never trust

record

drawings. Always make

an

accurate

field

survey

(or

engage

an

expert

to do it) to

determine

the

actual static head, TDH,

flowrate,

pipe roughness

coefficient,

possibility

of

obstructions

in the

force

main, wire horsepower, exact

locations

of

pipe

and

flanges, and any

other measurements that

may

affect

the new

plans, before beginning

the

design.

27-14.

Illustrative

Examples

Schematic diagrams

of two

pumping stations, each

containing

at

least

10

blunders,

are

shown

in

Figures

27-1

and

27-2. Many

of

these blunders

are

discussed

in

this chapter,

but

others

are

found

elsewhere.

Sharpen your wits

by finding

them before turning

to

the

answers

in

Appendix

D.

27-15.

Design Reviews

A

design review strategy should

be

imposed prior

to

beginning

the

design. Ideally, review teams should

be

made

up of the

personnel

listed

in

Table

27-1

.

For a

typical large project, reviews might well take

place

on the

following schedule:

•

Predesign

review

•

Intermediate reviews

(at 10, 50, and 90%

comple-

tion plus

a field

check)

•

Regulatory agency review

•

Postconstruction

review.

For

smaller projects,

the

reviews might

be

made

less

often

(or at

different

times),

but

project leaders should

realize that reviews

by

those

not

directly associated

with

the

detailed

design

are

vital

if

blunders,

errors,

and

problems

of

inadequate design

are to be

minimized.

The

cost

of

such reviews

is

small compared with

the

cost

of field

correction

of

mistakes,

the

value

of the

firm's

reputation,

and

malpractice insurance premiums.

Appendix

G

contains checklists

for the

following

disciplines:

•

Civil design

•

Structural/architectural design

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

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