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

95.

Carbon towers work well,

but

they still allow

the

escape

of a

slight odor that

is not

always accept-

able

to the

neighbors.

On

heavy

odor

days, how-

ever,

carbon

is

reasonably satisfactory.

The

carbon needs

to be

changed about twice

a

year,

and

it is

dirty

to

handle.

96.

Hypochlorite

generating towers require lots

of

servicing

time

and the

hypochlorite

generators

are

too

inconsistent. They need

to be

recharged

often,

and

on

heavy odor days

the

odor control

is

erratic.

Because

of the

salt, these units corrode rapidly.

97.

Potassium permanganate/alumina pellets require

too

much maintenance. They

are

expensive

and do

not

last. Jones: North

Mercer

Island

pumping

sta-

tion

was

originally

designed

with

''filters"

for

these

pellets,

but the

system

did not

work

well

(an

experience

we and

others

have

had)

and was

replaced

with

a

hypochlorite

wet

scrubbing

system.

98.

Masking agents

are

ineffective.

99.

Proper maintenance equipment must

be

included.

100. Provide

a

proper storage area

for

replacement

chemicals.

Overflow

Structures

101.

Structures should

be

kept

as

inconspicuous

as

possible

but

easily

accessible.

102.

Use flap

valves rather than hydraulic

or

electri-

cally actuated valves.

103.

A

measuring weir with recorder would

be

useful

together with

an

alarm

to

signal when

overflow

is

occurring.

104.

A

catch basin

to

collect solids would

be

desir-

able

to

reduce clean-up problems.

Potable

Water

Supply

105.

No

significant problems have been identified

with

potable water systems.

106.

All

outlets should

be

above

flood

level with

no

possibility

of

cross-connections.

107. Non-potable outlets should

be

clearly signed.

Raw

Wastewater

Pumps

108. Provide easy access

to

pumps

and a

clear work-

ing

space around them.

109. Pumps

are

much more trouble-free when speed

and

head

are

low.

110.

Impellers should

be

nickel iron

or

stainless steel

for

better wear.

Jones:

Some

types

of

stainless

steel

have excellent wearing properties

but

come

at an

exorbitant cost premium.

We

prefer

cast

iron with

2 to 3%

nickel.

111.

Provide guards

for all

drive

shafts.

112.

We do not

like mechanical seals

and are

replac-

ing the few we do

have with packing glands.

Jones: Mechanical seals generally

do not

func-

tion

well where

the

shaft

may

deflect

because

of

high

radial thrust conditions.

113.

Plumb seal water pressure lines

and

drain lines

to

keep them

out of the way of the

operators. Provide

seal water drains

at the

bottom

of

pump casings.

114.

Valves should

be

located

to

make operation

easy. Make sure manual actuators have enough

mechanical advantage

to

make valves easy

to

open

and

close.

115.

Bleed-off

s

are

required. They should

be

large

and

plumbed into

a

drain.

116.

Provide covers

for

volutes during repairs.

117.

Provide spare

parts

.

Security

118.

Provide vandal-proof

doors

,

vandal-proof

lights

,

and

unbreakable windows.

119.

Fencing

may be

required

in

some areas.

120. Provide moderate outside lighting switched

on

and off by a

timer.

121.

For

stations

in

remote locations,

it is

wise

to put

special security locks

on

station doors.

122. Avoid long driveways

to

facilities.

123. Isaac:

We

have "locked covers"

for

manholes

that

make

it

difficult

for

amateurs

to

stuff

debris

into

the

sewer.

Sanks:

For

discouraging vandal-

ism and

midnight dumpers, some manholes

in

Europe

have

a

special,

heavy

(140-kg

or

300-lb)

cast

iron hatch (below

the

manhole cover) that

can be

lifted

only

with

a

hoist

.

Stairwells

124. Stairwells located

on the

outer perimeter

of the

sta-

tion leading

to the

various doors

are

preferred.

125.

Fiberglass treads

are

preferred.

126. Aluminum handrails

are

preferred over galva-

nized

iron.

127.

Provide adequate headroom over stairs.

128. Circular stairs

are

better

than

ladders

or

ship-

type companion

way

s, but

straight stairs with

intermediate landings

are

best. Sanks: Design

stairs

so

that

it is

easy

to

carry

an

injured

worker

or a box

of

tools.

129.

We

prefer non-skid spiral stairwells

for

smaller

stations

but

like wide straight steps

if

there

is

enough room.

Sump

Pumps

130. Sump pumps should

be

industrial-grade units

of

relatively

large size capable both

of

passing

wastewater solids

and of

dewatering

an

entire

flooded

dry

well.

131.

There should

be at

least

two

pumps.

132. Check valves should have

an

external

arm

with

a

spring

or

counterweight

to

prevent slam

and to

indicate whether

the

pump

is

pumping.

133.

Ease

of

access

is

critical.

The

pumps should

be

conveniently located

in the dry

well

for

ease

of

cleaning

and

pump removal.

134. Gratings over sumps should

be

light

in

weight

and

readily removable. Pumps should

be

easy

to

flush

out.

Grease

collection

on the

surface

of

sumps

is a

problem that takes either manual

removal

or a lot of

hosing down.

135.

Built-in

floor

drains

are

preferable

to

open chan-

nels

along

one

wall.

The floor

drains should

be

of

relatively large size.

136. Lead/follow switches should

be

provided.

137.

Add an

alarm

so

that operators

can

tell when

the

follow

pump

is

activated.

138.

Either

air

bubblers

or float

controls

are

satisfactory.

139.

We

have numerous submersible sump pumps

as

well

as

submerged extended-shaft pumps with

encapsulated

motors located above

the

pumps.

Although

both types

are

acceptable,

we

prefer

the

latter.

Our

experience with submersibles

has

been

less satisfactory, possibly because

the

pumps have

not

been rated

for the

intended service. Isaac:

Many

submersible pumps

are not

designed

for

passing

wastewater

solids

—

many

are too

light

for

this

duty,

and

seals

tend

to

fail

and

cause motor

shorts.

Jones: Once

the

shaft

seal

fails,

the

motor

windings

are

sure

to go

that

is the first

indication

to the

operating

staff

that

there

is a

prob-

lem.

After

much

trouble

with submersibles,

we

switched

to wet pit

pumps with

the

motors above

grade

where

they

are

less

vulnerable

and the

oper-

ator

can

service them. According

to

NFPA

820,

submersible

pumps

in

sumps

receiving

wastewater-

contaminated

drainage

must have

explosionproof

enclosures.

Sanks:

Submersible pumps

can be

satisfactory

if

they

are

carefully

selected

for the

service

conditions,

are of

rigid

design,

and

have

moisture

probes

to

warn

of

seal

failures.

Support

Systems

140.

We are firmly

convinced that support features

of

a

pumping

facility

are of

paramount importance.

Some support systems that

can

make

a

major

difference

in

ease

of

maintenance (but

are

some-

times given

too

little attention) include:

(1)

sump

pumps;

(2)

cranes, hoists,

lifting

eyes,

and

access hatches;

(3)

alarms;

(4)

doorways;

and

(5)

landscaping. Sanks: These topics

are

dis-

cussed

herein under separate subheadings.

Switches

141. MANUAL/OFF/AUTO switches

are too

easily

left

on

MANUAL with

the

possibility

of

damag-

ing the

equipment.

142. Either

use

PUSH-TO-TEST/

OFF/AUTO

or add a

timer

to the

MANUAL position that automatically

turns

the

switch

to

AUTO

after

a

preset period.

Telephones

143.

For

safety

and

convenience, every station should

have

a

telephone

—

particularly

in

deep stations.

Jones:

Portable phones

(often

carried

in the

main-

tenance

vehicles

anyway)

are

good substitutes.

Toilets

144. Every station should have

a

toilet

and

washing

facilities

for the

convenience

and

safety

of

oper-

ators

and

maintenance personnel.

Valves

145. Locate

for

easy isolation

of

pumps

and

force

mains.

146.

It

should

be

possible

to

close

all

valves manu-

ally

if

necessary,

and

overhead valves should

be

activated with

a

chain.

Large valves should have

a

setup

for

operation with

an air

wrench.

147.

Do not use

knife valves

on

discharge lines,

because they tend

to

leak. Isaac: Some

manufac-

turers

make quality

knife

valves that

are

ade-

quate

for

this service,

and

specifications

can be

written

to

ensure this quality.

148.

Use

gate valves

for pig

launchers.

149.

Force

mains discharging

into

channels should

have

flap

gates instead

of

valves. [See Figures

12-19

and

12-21.]

150. Valves over walkways

or

located

in

walking

areas should have

a

clearance

of

2.1

m (7

ft).

151.

Avoid cheap gate valves

on

discharge lines.

Isaac: Avoid cheap valves everywhere.

152.

We

have

no

problems with hydraulic systems

to

operate valves. Jones: Hydraulic systems

unquestionably

require more maintenance

and

are

messier than electric

actuators,

but

hydrau-

lie

actuators

are

better

for

protection against

hydraulic

transient conditions

and flooding.

Variable-Speed

(V/S)

versus

Constant

Speed

(C/S)

Pumps

153.

Variable-

speed

pumps

are

preferred

and

seem

to

do a

better

job of

keeping

the wet

well

clean.

154.

Our

types

of V/S

drives include liquid rheo-

stats,

eddy-current couplings,

and

adjustable-

frequency

drives (AFDs).

All

have been satis-

factory,

but

each

has

shortcomings.

155. Liquid rheostats

are

relatively slow

in

response

to wet

well level changes. They have more main-

tenance problems. They

can

become fouled

by

electrolyte leaks. Changing electrolyte should

be

done with care

and

only

by

trained technicians.

156. Eddy-current couplings

are the

most mainte-

nance-free

system

in

use. They

run

well with

very

few

problems

and

have proven

to be

very

reliable.

But we

have

had

difficulty

recently

in

obtaining parts

from

certain manufacturers

who

no

longer build them

in the

smaller sizes,

56 to

112kW(75tol50hp).

157. AFDs

are

quick

in

response,

but

they

are

subject

to

nuisance shutdown

from

small changes

in

incoming line voltage.

All of

ours

are

sensitive

to

voltage dips. Overheating

is a

problem.

In

general, AFDs give more trouble than other

types

of

variable-

speed

drives.

Sanks:

AFDs

have

recently

become more versatile

and

robust.

Automatic restart with power line synchroniza-

tion

is

obtainable

and

should always

be

speci-

fied.

Some drives

now

produce

a

nearly

perfect

sine

wave without

the

spikes that cause heating.

The

overheating problem

at low

speed

can be

solved

by

adding

a fan

driven

by a

small motor

directly

connected

to 60 Hz, ac

power.

158.

The

control system must

be

tailored

for the

application

and flowrate.

Ventilation

159. Good ventilation

is

very important

for

both

equipment

and

operators.

160.

No

less than

10 air

changes

per

hour

(ac/h)

for dry

wells

and 20

ac/h

for wet

wells

is

required

to

pre-

vent

corrosion

and

odors. Jones:

We

recommend

10

ac/h

for

pump

rooms

and a

minimum

of

20

ac/h

for wet

wells

and

screen rooms

(40

ac/h

in

south-

ern

states

where

the

wastewater

is

warmer

and the

need

for

hydrogen

sulfide

production

is

greater).

161.

The

general design principle

of

using both inlet

and

exhaust

fans

to

introduce

air at the

ceiling

and

discharge

air

into ducts

at the floor has

been

effective

in

providing

a

pleasant work environ-

ment

and

protection

of the

equipment.

For

example, there

has

never been condensation

on

any

equipment

—

even

on

tanks. Sanks: Proper

ducting

is

just

as

important

as the

number

of

air

changes

per

hour.

162.

If the dry

well

is

vented

to the wet

well,

set up

the

duct work

so

that

a

high

wet-

well

level can-

not flood the dry

well.

A few

problems

in

early

designs were corrected

in

subsequent designs.

The dry

well

in the

Bellevue Pumping Station

was

flooded

shortly

after

being placed

in

ser-

vice. Power failure caused

a

high

wet

well level

and

allowed wastewater

to flow

through

a

venti-

lation duct between

the wet and dry

wells.

Jones:

Any

ducting whatsoever between

wet and

dry

wells

is now

prohibited

by

NFPA

820.

163. Sail switch alarms

in the

ventilation ductwork

systems were provided

in

many

of the

early sta-

tions

and

were subsequently connected

to the

main treatment plant control centers over leased

telemetry lines. These caused numerous nui-

sance calls

to

check stations that could have

been serviced during

the

next routine inspection.

164.

The

high

(10

to 20

ac/h)

air

exchange rates spec-

ified

in

early stations

has

created some

difficul-

ties when trying

to

retrofit

facilities with odor

control equipment. Replacing

fans

or

adjusting

sheave

sizes

has

largely

alleviated

this problem.

165.

If

exhaust gases must

be

scrubbed

for

odor con-

trol, high-volume ventilation becomes very

expensive. Jones:

This

consideration puts

the

engineer

at a

decided disadvantage.

Ventilation

at

lesser rates [than above] creates,

in

effect,

confined

spaces that require three-person

entry

teams

and

safety

equipment.

It

also exposes

the

engineer

to the

potential

for

liability claims

if

an

accident

occurs. Given

the

value placed

on

human

life,

the

added

expense

of an

odor con-

trol

system

for the

greater ventilation rates

seems

a

justifiable,

needed expense.

Wet

Wells

166.

The

long, narrow self-cleaning [trench-type]

pump intake basin works well

in

practice.

All of

our

future

pumping stations will

be of

this type.

167. Seattle

Metro's

wet

wells

are

pumped

and

hosed

down

at

least weekly

and

even more frequently

in

troublesome locations.

168.

The

older

"traditional"

wet

wells

for

constant-

speed (C/S) pumps were dangerously odiferous

and

huge piles

of

grit accumulated rapidly.

169.

We

prefer suction bells that terminate near

the

floor

over other types

of

intakes.

170. Provide high-pressure

"hose-down"

facilities.

Sanks:

Supply

at

least

1

.6

LIs (25

gallmin)

at a

pressure

of

no

less

than

550

kPa

(80

lblin?)

at the

nozzle.

111.

Smaller

wet

wells, where storage

can be

pro-

vided

in the

sewer lines (and

not in the wet

well)

have

fewer problems

in

general.

172. Minimize control instruments

in wet

wells.

173. Eliminate heavy gratings that must

be

lifted

to

gain access

to the wet

well

for

hose-down and/

or

maintenance. [See Gratings]

174.

In

other cities,

wet

wells

are

often

dark, dismal,

and

poorly

ventilated

—

a

scourge

for

operations.

Yet

operators must enter them frequently

to

con-

trol odors

and

remove

grease

and

accumulated

solids. Some

of the

features that make frequent

wet

well entry less objectionable are:

(a)

Easy-opening, counterbalanced access

hatches,

(b)

Ventilation systems that provide enough

air

exchanges

(15-20

ac/h)

to

prevent conden-

sation

and

remove potentially

harmful

gases,

(c)

Wash-down stations with

local

explosion-

proof START/STOP switches

to

provide

large volumes

of

water

at

high

pressure,

(d)

Rood-

and

explosionproof lighting that pro-

vides good visibility

for any

operation

chores,

(e)

Wet

wells with

no bar

screens.

Our

experience

has

been that with

V/S

pumping,

flow and

debris

come into

the

station constantly

and

uni-

formly,

so

they move through

the

system with

minimal

clogging.

Our

experience

with

inlet

bar

screens

has not

been very satisfactory.

When operators rake

the

screens,

a

considerable

amount

of

debris reaches

the

pumps

in

bunches

and

tends

to

clog them. Most stations

are not

equipped with facilities

to

handle removed

screenings except

to

hand-carry them

in

buck-

ets

—

a

sure

way to

make operators unhappy.

Bosserman:

Provide

screenings

removal

equip-

ment

at

stations

where

bar

screens

are

deemed

necessary.

Jones:

If

screenings

removal

equip-

ment

is

provided,

then

screening

treatment

equipment

(washing,

dewatering,

and

pressing)

equipment

and

facilities

for

storage

and

removal

are a

must

to

prevent

odors

and

mini-

mize

labor-intensive

operation.

24-10.

Survey

of Two

Thousand

Wastewater

Pumping

Stations

During 1996,

one of the

editors

[9]

investigated

the

per-

formance

of a

great many

raw

wastewater pumping sta-

tions

in

several large sanitation agencies

in the

United

States

[10,11].

The

initial part

of the

project (February

and

March) consisted

of

telephone interviews with

operation

and

maintenance supervisors

and

chief engi-

neers

at 15

large public sanitation agencies.

These

15

agencies operated

a

total

of

more than

2700

raw

waste-

water pumping stations. Based

on the

interviews,

four

agencies (City

of

Houston, Texas; City

of

Baton Rouge,

Louisiana; City

of

Nashville, Tennessee;

and

King

County

Department

of

Metropolitan Services, Seattle,

Washington) were visited

in

April.

These

agencies

operate

a

total

of

more than 1000 pumping stations.

Interviews were conducted with

O&M

personnel

to

determine what problems they frequently encountered

and

to

establish their preferences

for

improving

O&M

characteristics

of the

pumping stations. Finally, more

than

50

pumping stations were visited. Operations were

observed,

and the

actual station

O&M

personnel were

interviewed.

The

following

is a

short summary

of

some

of

the key

issues that were identified.

1.

How Not to

Design

a Wet

Well

The

design engineers

for

several pumping stations built

within

the

past

five

years

in one

agency insisted that

baffles

per the

Hydraulics Institute Standards

[1,2,3]

needed

to be

designed into

the wet

wells

to

provide qui-

escent conditions

for the

pumps

and to

provide greatest

pump

efficiency.

The

resulting

baffle

system

did

just

that.

It

provided

very

quiescent conditions.

As a

result,

the

grit

and

solids, settling

to the

bottom

of the wet

wells, caused severe odor problems

and

sometimes

clogged

the

intake

pipes

to the

pumps.

It

takes

an

eight-

person crew

to

remove

the

grit

from

the wet

wells,

because

the wet

wells

are

considered

to be

confined

spaces.

Willing

to

accept

a

little

reduction

in

pump

effi-

ciency

in

exchange

for

eliminating

the

cost

of

removing

the

grit that accumulates

in wet

wells with

baffles,

the

agency

no

longer installs

baffles

and has

begun

to

demolish

the

existing

baffles.

Sanks:

The

designers

did

not

heed

the

warning

in the

Hydraulic

Institute Stan-

dards:

all HJ.

Standard

wet

wells,

(up to and

including

the

1994 edition)

are for

clear water

only.

If

they

are

nevertheless

to be

used

for

solids-bearing waters, pro-

visions

for

cleaning accumulated grit

and

scum must

be

made.

Deposition occurs wherever currents decrease

much

below about

0.6

mis

(2ft/s).

Demolishing

the

baf-

fle

would

make vigorous mixing

(as a

means

of

solids

suspension

and

removal)

more

effective.

See

Sections

12-6

and

12-7

for

viable alternative designs.

Bosserman:

If a

designer uses conventional

wet

well

design, consideration must

be

accorded

to the

removal

of

scum from

the

water

surface

and

grit

and

other settleable solids from

the

bottom

of the

sump.

Unfortunately,

only

recently

(with

some exceptions)

have

designers

and

published design

standards

or

guidelines giving this factor

the

serious consideration

that

it

deserves. Access must

be

convenient

for

bring-

ing

in

cleaning equipment such

as

hoses

to

"wash

grease

off

the

sump

walls

and

vacuum hoses

or

other

means

to

remove

the

accumulated sediments.

The wet

well

walls

and

ceiling should

be

lined with smooth

PVC

to

prevent

the

attack

on

concrete

by

sulfuric

acid

(derived

from hydrogen

sulfide)

above

the

water line

and to

make

it

easy

to

wash grease

off

the

walls. Grat-

ings, sidewalks,

or

landings that

do not

interfere

with

hoses

should

be

provided

for the

convenience

of the

cleaning

crews.

2.

More

Comments

on Wet

Well

Design

The

vast majority

of the dry

well pumping stations

in

these agencies have sealed

wet

wells without ventila-

tion. They contain

a

"breather"

vent pipe, typically

150 mm (6

in.)

in

diameter, coming

out of the wet

well roof. When

O&M

personnel have

to

enter

the wet

wells (which

is not

very often), they

use

portable

blowers

and

ducting

to

supply

the

necessary ventila-

tion.

In the

latest designs,

the wet

wells

are

lined with

PVC.

One

agency (King County Department

of

Met-

ropolitan Services) that does desire routine

and

conve-

nient personnel access

to the wet

well used

the

following

design

in

their most

recent

pumping station

(Interurban),

started

up in

March,

1996:

(a)

A

catwalk

of FRP

grating over

the

entire length

of

the wet

well about

0.6 m (2 ft)

above

the

high-water

level),

so

that

personnel

can

walk

over

the

entire

wet

well.

The

clearance

between

the

catwalk

and the

walls

is

about

0.3

m (1 ft) to

permit unobstructed wash-down

of

the

walls,

(b)

Variable-speed pumping,

so

that

the wet

well water level does

not fluctuate

very

much.

(c)

Explosionproof (NEMA

7)

lighting fix-

tures,

accessible

from

the

grating

for

replacement,

(d)

Walls

and

ceiling

are

lined with PVC.

(e)

No

screens

or

grinders,

(f)

The

ventilation system

is so

effective

(the

editor entered

the wet

well) that there

was

hardly

any

"sewage"

odor,

in

spite

of the

fact

that

raw

wastewater

was

just

2 ft

below

the

grating. There

are

several ventilation

ducts with

diffusers

on

each,

to

distribute

the air

into

the wet

well.

3.

Example

of

Unwarranted

Optimism

in the

Design

Engineer

A

consulting engineering

firm

decided

to use

close-

coupled vertical wastewater pumps, because

the wet

well

was of a

design that could

not

possibly

be

flooded.

There

was a

solid, unpierced concrete wall

between

the wet

well

and the dry

well

from

the

bot-

tom

slab

of the

structure

all the way up to the

roof slab

of

the dry

well containing

the

pumps.

The

following

sequence

of

events occurred

one

day:

(a)

There

was a

storm, during which

a

power

fail-

ure

occurred,

(b)

In

accordance with Murphy's

Law

("If any-

thing

can go

wrong,

it

will"),

the

standby

engine generator failed

to

start.

(c)

There

was a

rest room with

a

toilet

on the

roof slab over

the dry

well.

The

toilet

was

connected

via a

pipe

to the wet

well. There

was

no

check valve

in the

pipe.

The

waste-

water backed

up in the wet

well,

flowed up

the

pipe, over

the

toilet, across

the floor,

and

down

the

stairway into

the dry

well that

couldn't

be flooded.

4.

Maintaining

Dry Pit

Pumps

versus

Submersible

Pumps

All

the O&M

personnel

at all the

agencies

visited

felt

that

it is

much easier

to

maintain

a dry pit

pump,

unless

the

submersible pump

was

manufactured

by

one

specific manufacturer. This

one

manufacturer

(a

large corporation)

was

almost unanimously acclaimed

as

having

the

most

reliable

pump

on the

market, with

O&M

requirements that were comparable

to

those

for

conventional

dry pit

pumps.

Three

other manufactur-

ers

were frequently cited

as

providing good pumps,

although

not

quite

as

good

as

Manufacturer

No.

1.

(a)

Their main reason

for

stating that

it is

much

easier

to

maintain

a dry pit

pump

is

that with

a dry pit

pump,

the O&M

staff

can

inspect

it

and

tell when something

is

amiss

and

repair

it

before

the

problem becomes

too

serious.

Such inspection

is

impossible with sub-

mersible pumps, because they cannot

be

observed routinely

and

must

be

hauled

out

of

the wet

well

for

repairs. Consequently,

there

is a

greater tendency

to let the

sub-

mersible pump

run

until

it

simply

fails.

Then

the

cost

of

repair

is

usually very

high,

(b)

Submersible pumps

are

quite variable

in

fre-

quency

of

maintenance required. Some

agencies have

had to

pull submersible pumps

as

frequently

as

every

6

months. Others have

gone

as

long

as 6 or 7 yr

before having

to

pull

the

pumps

for

maintenance.

The

typical inter-

val

seems

to be

about

2 yr.

Seal

and

bearing

failures

are the

most common problems with

submersible pumps. Damage

to

power cables

and

motor windings occur less frequently.

(c)

It can

take

a

crew

of 6 to 10

workers

to

remove

a

submersible pump

from

a wet

well.

Depending

on the

size

of the

pump,

the

crew

typically

consists

of the

following workers:

(i)

1 or 2

crane operators

(ii)

2 or 3

electricians

(iii)

3 to 5

other

persons

actually

to

remove

the

pump.

(d)

Most agencies have discovered that

O&M

costs

for

submersible pumps larger than

75

to

150 kW

(100

to 200 hp)

become

extremely high. Generally speaking,

the

agencies have discovered that:

(i)

Submersible pumps

and

motors larger

than

75

kW

(100

hp)

break down more

often

than smaller

ones,

(ii)

Removal

of

large submersible pumps

takes

a lot

more

effort

than they

had

originally thought.

In

general,

it

requires more personnel

to

remove sub-

mersible pumps than conventional

dry

pit

pumps. With large pumps (greater

than

75 kW or 100 hp)

workers

in at

least

one

large public utility usually

must

enter

the wet

well

to

disconnect

the

electrical power cable

from

the

pump,

unseat

or

disconnect

the

pump

from

the

connecting piping,

and

help

guide

the

pump

out of the wet

well.

When

the

pump

is

reinstalled

after

being repaired, workers must again

enter

the wet

well

to

reconnect

the

power cable

and

reconnect

the

pump

to

its

discharge piping.

As the wet

well

is

classified

as a

confined space, this

effort

must

be

made

by

workers wear-

ing

rubber suits

and air

masks

and

accompanied

by

observers

and

supervi-

sors above.

Sanks:

Contrast that sce-

nario

with

the

experience

of a

large

utility

that manages

425

pumping sta-

tions

of

which

170

have submersible

pumps including many

large

ones.

No

worker

ever goes down into

a

submers-

ible

pump

wet

well

except

to

work

on

guide

rails

or

brackets.

The

pumps

can

be

pulled

up or

replaced

in

half

an

hour

by

a

crew

of

three. Minor service

(such

as

changing

oil or

checking

shafts

for

deflection)

is

completed

on the

pump

while

it

rests

horizontally

in a

cradle

beside

the wet

well hatch.

The

cable

is

disconnected

at the

motor starter

and

accompanies

the

pump

and

motor

to

the

shop

for

major

repairs

such

as the

replacement

of

seals

and

bearings.

The

power cable

is not

disconnected from

the

motor.

The

difference

between

the

two

kinds

of

operations lies

in the

training

of

the

workers, attitude

(do the

best

you can

with what

you

have),

and

designs

that make these operations

easy

(see Section 25-6).

(iii)

The

frequency

of

repair

of

submers-

ible pumps

is

greater than that

for

con-

ventional

dry pit

pumps.

One

agency

thinks (although exact records

are not

available) that submersible pumps

must

be

repaired

20 to 40%

more

often

than

dry pit

pumps.

(e)

One

agency reported that

it

takes

a

six-per-

son

crew

up to

half

a day to

remove

a

sub-

mersible pump.

If

maintenance then

has to

be

performed, additional time

and

labor

is

needed

to

transport

the

submersible pump

to

the

maintenance shop,

repair

it,

transport

it

back

to the

pumping station,

and

reinstall

the

pump.

A

conventional

dry

well pump

can be

repaired with

a

fraction

of

this time

and

effort.

However, this agency also

felt

that

good-quality submersible pumps

(of

which

there

are not

very many manufacturers,

in

their opinion)

are

more reliable than

the

con-

ventional

dry

well pumps

in the

sense that

they

do not

fail

as

often

as dry

well

pumps,

(f)

A

majority

of the

agencies send

all

their

submersible pumps

to

private shops

for

repair.

The

most common repairs

are

replacing bearings, replacing seals,

and

rewinding motors. Typically,

it

cost

$4000

to

$8000

to

repair

a

submersible pump

and

sometimes much more.

For

example,

it

could cost

as

much

as

$27,000

to

replace

the

rotor

and

stator

on a 160 kW

(215

hp)

submersible pump.

Use an

ENRCCI

of

5600

for

these

costs

(see Chapter 29).

(g)

It can

cost

$600

to

purchase

a new

upper

bearing assembly

for a

submersible pump.

A

lower

seal

assembly

for a

submersible

pump

can

cost

$2400.

By

comparison,

the

seals

(conventional packing)

and

bearings

for

a dry pit

pump

can be

purchased

for

only

$200.

(h) In the

opinion

of the O&M

staffs

at

most

of

the

agencies,

the

degree

of

effort

required

to

maintain various types

of

wastewater

pumping

stations

(from

easiest

to

most dif-

ficult)

is

as

follows:

(i)

Submersible pumps

in a dry

well,

(ii) Conventional

dry

well pumps

in a dry

well

(iii) Submersible pumps

in a wet

well.

(i)

The

sales representatives

for

submersible

pump

manufacturers tend

to be

very opti-

mistic regarding

the

effort

that

it

will take

to

maintain

a

submersible pump.

A

careful

and

realistic life-cycle cost analysis should

be

performed

before deciding

to use

submers-

ible pumps, especially

if the

contemplated

pumping station

is

fairly

large

—

greater

than

440 or 660 L/s (10 or 15

Mgal/d)

capacity.

5.

Depth

of Wet

Wells

for

Submersible Pumps

It

was

observed

and

reported that

in

deep

wet

wells,

the

heavy electrical power cables

to the

pump motors

can

sometimes stretch

and

crack.

The

turbulence

in

deep

wet

wells

can

also cause

the

long cables

to

whip

and

bend, thereby damaging

the

cover,

the

power

wires,

or

especially

the

connection

at the

motor cap.

When

a

crack develops

in the

cable cover, hydrogen

sulfide

can

enter

the

cable

and

corrode

the

power

wires.

It is

expensive

to

purchase

a

replacement power

cable

and it

sometimes takes

four

to six

weeks

to

obtain

a new

cable

from

the

manufacturers, most

of

which

are

overseas.

Sanks:

Stretching

can be

con-

trolled

by

fastening

the

power cable

to a

stainless-

steel

wire

rope

designed

to

carry

the

tension. Some

manufacturers

regard cable replacement

as

emer-

gency

repair

and

keep

cable

on

hand

so

that replace-

ment

may

require

as

few

as

three work days.

Another factor

in wet

well depth, especially impor-

tant

for

removing large pumps,

is

that

the

long, thick

power cables cannot

be

bent

to a

short radius without

damaging

the

cable. Consequently,

it

requires much

care

and

effort

to

lift

both

the

pump

and the

cable

together without overstressing

the

latter.

It is for

this

reason that some agencies disconnect

the

cable

from

the

pump

before

lifting

the

pump

out of the wet

well. Agen-

cies with experience

in

removing large submersible

pumps

prefer

to

limit

the

maximum allowable

wet

well

depth

to

about

12m

(40

ft). Sanks: Excessive

flexure

near

the

motor connection

can be

controlled

by a

cable

saddle

or by the use of

semi-flexible

stainless-steel

cable

sheathing that controls

the

radius

of

bending.

It

may

be

preferable

for the

cable

to

exit

downward

from

the

motor

caps

of

large

pumps

to

prevent excessive

bending

when

the

pump

(with

cable

attached)

is

lifted.

Cable

sheathing

or a

saddle

is

still

required

because

the

cable

must

be

tight during normal pumping operation

to

keep

it

from swinging with

the

currents

and

thus

wearing

out the

cable connection

at the

motor cap.

6.

Discharge Elbows

for

Submersible Pumps

A

major problem that

has

sometimes been encoun-

tered (fortunately,

not

very often) with submersible

pumps

is the

discharge elbow

in the wet

well tearing

loose

from

its

anchor

bolts.

When that happens,

it

requires

a

major

effort

to

repair

it. The wet

wells

are

classified

as

confined spaces

per

OSHA,

so it

takes

an

eight-person crew

to do the

work

in

compliance with

all the

confined-

space

observation

and

supervision

requirements.

It is

believed that

the

cause

of

this prob-

lem is as

follows:

(a)

A

pump ingests

a

large rock

or

some

solid,

(b)

The

pump then vibrates

as it

tries

to

expel

the

solid.

(c)

This vibration creates forces

and

movements

in

the

pump

and its

discharge elbow that

are

much

greater than those

the

elbow anchor

bolts

and

associated concrete anchoring sys-

tem

were designed

to

withstand,

(d)

Each time such

an

event occurs,

it

weakens

the

concrete

and

anchoring system

a

little

more,

and

eventually,

the

bolts tear

out of

the

concrete

or the

concrete breaks,

and the

agency

has a

major repair problem. Sanks:

Beware

of

light anchor bolts.

A

bolt smaller

than

22 mm

(

7

I

8

in.)

is a

toy. Heavier bolts

cost

very

little more than smaller ones. Bolts

should

be

(1)

buried

deep

enough

to

develop

their

ultimate tensile strength

and (2)

sur-

rounded

by a

strong cage

of

reinforcement

to

prevent cracking

of

the

concrete.

7.

Variable-Speed

Pumping

Many agencies

use

variable-speed pumping exten-

sively. Those with significant experience feel

it is a

very effective

and

efficient

means

of

operating

raw

wastewater pumping stations. Most

new

variable-

speed units

are

adjustable-frequency drives (AFD).

One AFD

manufacturer

in

particular

was

cited

as

providing

especially

reliable

equipment

and

excellent

service

and

training support.

The

drives feature insu-

lated gate bipolar transistor (IGBT) technology, which

results

in an

extremely quiet design compared with

that

of

other older units. There

is

only

a

slight

60-

cycle

background hum, compared with

the

ear-pierc-

ing

noise sometimes produced

by

older designs.

In

addition,

the

IGBT design runs

cooler

and

does

not

emit nearly

as

much heat, thereby reducing

the

air-

conditioning requirement

for the

control room.

8.

Standby

Engine-Generators

Although

standby power

is

usually thought

to be

essential

for raw

wastewater pumping stations, there

are

sometimes exceptions

to

this rule.

One

agency

(City

of

Houston)

has

none

in any of

their

330

pump-

ing

stations. Another agency (City

of

Baton Rouge)

has

none

in

more than

400

pumping stations. Both

have

concluded that their power supply

is

very reli-

able,

and

there

is no

need

for

standby generators.

Some

of

their older stations have connections

for use

with

portable generators,

but the

newer stations

do

not. These agencies have determined that

the

hydrau-

lics

of

their systems allows them

to

withstand power

outages

of 3 to 5

hours duration before

any

wastewa-

ter

spills occur,

and the

power failures have never

lasted that long. Both have portable generators

(up to

400 kW)

that they

can use

when

needed,

but

they

are

typically

only used

10 to 12

times

per

year.

Not

hav-

ing

to

provide permanently installed standby genera-

tors

or

dual power feeds

to

these several hundred

pumping

stations

has

saved considerable construction

cost.

Sanks:

Each

of the

above agencies

may

have

both

a

fortunate

and a

unique situation. Each facility

should

be

evaluated

for

standby power need

on the

basis

of

size

and the

consequences

and

probability

of

power

failure.

See

Section

13-14

for

developing

a

more

balanced viewpoint

and for

examples

of

power

outages

that have caused disasters.

9.

Odor Control

Most

O&M

staff

stated that they prefer packed carbon

towers, because they

are

easy

to

maintain. Both

the

activated

carbon type

and the

impregnated carbon

type

have been used. Jones:

The

impregnated carbon

type

has a

well-established poor

safety

record

(fires,

hazardous

chemicals)

and is not

recommended.

The

carbon systems last

a

long time.

At

some pumping sta-

tions,

the

carbon

has not had to be

changed

in

over

3

yr.

Wet

scrubbers (such

as

caustic soda

and

sodium

hypochlorite)

are

high-maintenance

systems.

Sodium

hypochlorite corrodes everything

it

touches when

it

spills

and is a

hard system

to

maintain.

10.

Flow

Metering

Four

agencies

(City

of

Houston, Texas;

Ann

Arundel

County,

Maryland; Orlando, Florida;

and

Frederick

County,

Department

of

Public Works, Maryland, with

a

combined total

of

more than

900 raw

wastewater

pumping

stations) have

no flowmeters at any of

their

pumping

stations. They just read

the

elapsed time

meters (ETMs)

for the

pumps,

and

knowing

the

approximate capacity

of

each pump, they

can

then

approximate

the flowrate.

They

see no

need

for

greater

accuracy. Flowmeters

are

installed

at the

treatment

plants where

the flow

data

are

needed. Typically,

flow

measurement devices

of

some sort

are

needed only

at

pumping stations

to

indicate

flow

build-up over time

to

allow

the O&M

staff

and

engineering

staff

to

deter-

mine when they need

to add

additional pumps

or to

expand

a

pumping station.

In the

opinion

of

these

agencies, time meters

furnish

adequate data.

11.

Pump

Speeds

Most agencies prefer

no

more than 1200

rev/min

speed

for any

pump (dry

pit or

submersible)

or

per-

haps even

no

more than

900

rev/min maximum speed.

Some

of the

pumps

run at

speeds

up to

1800

rev/min.

These higher-speed units wear

out

much more quickly

than

pumps operated

at 600 to

1200 rev/min. Packing

has

to be

replaced more

often,

and the

bearings

and

shaft

sleeves wear

out

faster

and

must

be

replaced

more

often.

In

general, pumps with operating speeds

above 1200 rev/min have significantly higher

O&M

costs than slower speed units.

12.

Cranes

and

Monorails

Almost

all

agencies have permanently installed bridge

cranes

or

monorails

in

their conventional

dry

well/wet

well pumping stations.

In the

opinion

of the O&M

staffs

at

several

of the

agencies, design engineers need

to do a

better

job of

considering

the

crane layout

and

how

the

cranes will

be

used.

The O&M

personnel also

stated that they need more access space around equip-

ment

to be

able

to use the

crane systems

effectively.

In

some

of the

pumping stations,

the

crane

layout

was so

poor that rework

was

necessary

to

make them

function

properly.

For

example:

(a)

Some cranes were designed

so

that that

the

crane blocks

had to be

operated

at an

angle

to

remove

pieces

of

equipment

—

a

viola-

tion

of

OSHA crane

safety

regulations,

(b)

In

another pumping station,

a

bridge crane

was

supposedly designed

so the

trolley

could

be

centered over

the

motors

and

pumps

for

removing them. Unfortunately,

the

designers placed

a 90°

elbow

and

dis-

charge riser directly

on the

pump discharge

nozzle. When

the

trolley

was

moved

toward

the

motor,

it

struck

the

riser

pipe

and

could

not be

centered over

the

motor

or

pump.

(c) In

several pumping stations

at

four

agen-

cies,

the

overhead bridge cranes could

not

be

easily used because

of

interferences

from

structural beams. Better coordination

is

required between

the

mechanical engi-

neers

who lay out the

pumps, piping,

and

crane systems,

and the

structural engineers

who

design

the

structural beams

and

sup-

porting columns.

Sanks:

Project

leaders,

take

note!

13.

Accessibility

of

Equipment

A

nearly universal complaint

of the O&M

staffs

was

that more space

or

clearance needs

to be

provided

around

equipment

for

maintenance access.

In

some

of

the

pumping stations visited,

the

access

was so

cramped that maintenance

personnel

could

not

even

remove

the

bolts

to the

pump

flanges

without

a

major

team

effort.

In

others, access

to the

stuffing

boxes

was

restricted

so

that changing

the

packing

was

difficult.

Designers need

to

give much better thought

to

impor-

tant

issues such

as:

(a)

How

will packing

or

mechanical seals

be

changed?

(b) How

will pump

shafts

be

removed

so

that

the

shaft

sleeves

can be

replaced?

(c)

How

will impellers

be

removed

and

replaced?

(d)

How

will

flanges be

disassembled

and

reas-

sembled?

(e)

How

will

the top

covers

on

check valves

be

removed?

Designers need

to

keep

in

mind that

all

these main-

tenance activities require

a

team

effort

of two to

four

people

to be

carried out,

and

there must

be

sufficient

space around

the

equipment

so

they

can

work together

effectively.

14.

Screens

and

Grinders

Generally speaking, most agencies have

no

screens

or

grinders

in any of

their pumping stations. Typically,

O&M

staffs

see no

need

for

them

and

consider them

to

be

high-maintenance items.

At

some

older

stations,

the

existing screens have been taken

out of

service,

because

they served

no

real

purpose

and

required

maintenance.

Of the 15

agencies investigated, only

one

(City

of

Nashville, Tennessee) required screens

or

grinders

at all of

their stations. That agency reported

they

consistently

get

large quantities

of

construction

debris

(pieces

of

brick

and

masonry, lumber, drill bits,

bolts, nails, etc.)

in

their sewage

and

they need

to

have

screens

or

grinders ahead

of the

pumps

to

avoid exces-

sive wear

of the

pumps. This problem does not, how-

ever, appear

to be a

typical

or

common occurrence

in

municipal sewage systems.

15.

Check

Valves

Most agencies normally

use

hydraulically

buffered

swing check valves

or at

least require springs

or

weights

on the

lever arms

of the

check valves.

In

pumping

stations built within

the

past

5 to 10 yr, the

trend

toward hydraulically

buffered

controls

on the

swing

check valves

was

especially evident. Sanks:

See

also

Section

5-4.

With

a

heavy

spring

or

counter-

weight,

the

liquid

in the

valve

is a

hydraulic

buffer

as

effective

as a

dash

pot

(although

the

spring

increases

the

headloss).

16.

Mechanical

Seals

Another

definite

trend

was the use of

mechanical seals

in

the

pumps

for new

pumping stations. Agencies that

have

investigated mechanical seals most extensively

prefer

the

split-seal type with tungsten carbide (and

not

ceramic) faces. Split seals

are

easy

to

install

and

disas-

semble. Leakage

is

much less with

a

mechanical seal

than

with packing,

and the

mechanical seal lasts much

longer than packing.

One

agency

reported

that

in

their

life-cycle

cost analysis, they concluded that

a

mechan-

ical seal pays

for

itself

in 3 yr

because

of the

reduced

maintenance requirements.

The

mechanical seal lasts

10

yr

—

an

excellent bargain,

in

their opinion.

17.

Packing

Another trend observed, especially

in

those agencies

that have

not

switched

to

mechanical seals (Orange

County Sanitation District

and

City

of

Baton Rouge),

was

the use of

packing materials that

do not

require

an

external

clear water

flush. At

least

two

agencies have

acquired extensive experience with such materials.

One

agency (Orange County Sanitation District)

has

been using

one of

them

[12]

for

more than

10

yr, and

they

report excellent success.

The

packing lasts

two to

three times

as

long

as

conventional packing.

The

seal-

ant

material

is so fine

that

it

screens

out

wastewater

solids that leak

up the

shaft,

so

that

the

solids cannot

come

in

contact with

and

abrade

the

shaft

sleeve.

These

agencies

no

longer

even have

the flushing

water

systems

installed

in

their

new

wastewater pumping

stations.

18.

Level

Sensors

in the Wet

Wells

Bubbler

systems

are by far the

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

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