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

This chapter

is

confined

to

those design consider-

ations

that facilitate operations

and

maintenance

by

improving

labor

efficiency

and

thereby reducing down

time

and the

cost

of

replacing parts. Operations

and

maintenance procedures themselves

are not

consid-

ered.

See

also Chapters

12, 25, and 26,

which contain

much

advice relative

to

designing

for

ease

in

operations.

Sections 24-9

and

24-10

are

especially valuable.

Section 24-9 contains

the

opinions

of

many opera-

tors with

an

aggregate

of two

centuries

of

experi-

ence

with wastewater pumping stations. Most

of

their comments also apply

to

water pumping sta-

tions. Section 24-10

is a

distillation

of

knowledge

gained

from

telephone interviews with

15

public

sanitation agencies that operate more than

2700

pumping

stations

and

site visits

to

more than

50 of

these stations.

Chapter

24

Designing

for

Easy

Operation

and

Maintenance

BAYARD

E.

BOSSERMAN

Il

GEORGEJORGENSEN

GARY

ISAAC

ROBERT

SANKS

CONTRIBUTORS

Nicholas

J.

Arhontes

Roger

J.

Cronin

Ronald

P.

Kettle

Earle

C.

Smith

Dale

Stiller

John

A.

Redner

24-1.

Site

Selection

When selecting

a

site

for a

pumping station, give care-

ful

consideration

to

access,

not

only

for

operating per-

sonnel

but

also

for

equipment. Equipment

can

vary

from

a

pickup truck

for

small stations

to a

crane

for

large stations. Weather conditions

may

influence

access requirements.

For

example, road gradients

may

be

limited

in

areas

of

heavy snowfall.

Route

the

force main

to

minimize

the

possibility

of

column

separation, even

if it

means relocating

the

pumping station.

24-2.

Landscaping

the

pumping station

to fit or to

blend into

the

surroundings

is

desirable,

but

design

the

landscaping

for

low

maintenance

by

using ornamental woody peren-

nials

and

ground cover that require little attention.

Install

automatic sprinkler systems activated

by

mois-

ture

probes

and a

timer.

24-3.

Hydraulics

The

hydraulics

of

pumping systems

are

covered

in

several

other

chapters,

but it is

worth repeating that

if the

analyses

are

improper, inadequate,

or do not

include

all of the

operating conditions (including start-up, shut-down,

and

power

failure),

excessive maintenance

may

result.

Head-Capacity

Curves

A

careful

study

of the

full

range

of

conditions under

which

pumping will occur

is

essential

so

that

the

proper pump, impeller,

and

motor

can be

chosen.

Such

a

study

is

particularly needed where

a

battery

of

pumps

discharges into

a

manifold.

As

additional

pumps

come

on

line,

the

total

flow and

head increase,

but

as the

head increases,

the flow

from

each individ-

ual

pump decreases.

In a

smaller pump

(or one

with

a

lower

shut-off

head than

the

others),

the

discharge

may

drop

to

zero, which

can

cause excessive radial

loads.

If the

analysis

is

incomplete,

the

pumping con-

ditions

may be

outside

the

limits

of the

pump curve

for

a

particular pump (refer

to

Section 10-6).

Pressure

Gauges

Taps

(in

bronze pipe saddles secured

to the

pipe with

double

bronze

or

stainless-steel straps)

for

screw con-

nections

of

pressure gauges

are

useful

on

both

the

intake

and

discharge pipelines. Permanently mounted pressure

gauges

can be

used,

but

portable gauges, which

can be

used

when needed,

are

usually better because they

can

be

easily recalibrated

and can

serve

for

many locations

(see Chapter 20). Fixed gauges

are

subject

to

equipment

vibration and, unless periodically recalibrated, will

not

remain

reliable.

The

maintenance schedule should

require pressure readings

at

regular intervals (e.g.,

monthly)

along with simultaneous

flowmeter

readings.

These records

can be

used

to

detect abnormalities such

as

clogging

of the

force

main

or

impeller wear.

Flow

Measurement

To

check

the

efficiency

of the

pumping system,

a

flowmeter

in

conjunction with

the

pressure gauges

facilitates

checking

the

power consumed against

the

water output.

It is

also

a

good idea

to

make

it

easy

to

calibrate

the flowmeter in

place;

for

water systems,

a

means

of

inserting

a

pitot tube

is a

good

way to do so.

Time-volume measurements using

the wet

well,

a

storage tank,

or a

clear well

are

reliable

and can

often

be

used.

The use of

tracers (dye dilution)

is

accurate,

almost universally applicable,

and

easy

to use if

taps

(one

to

introduce

the

tracer

and

another

at a

point

far

downstream

for

withdrawing samples)

are

installed

(see Section 3-9).

The

lengths

of

straight inlet

and

outlet pipe recom-

mended

by the

manufacturer must

be

observed

for all

types

of

meters; otherwise

the

meter readings will

probably

be

erroneous.

The

errors increase exponen-

tially

as the

inlet

and

outlet pipes

are

shortened.

Surge

Control

Water hammer need

not

instantly rupture

a

pipe

to be

damaging. Surges that continue

for

many years gradu-

ally

destroy

a

pipe's

integrity through material

fatigue,

so

surge control

is

required

in

most pumping stations

to

prevent damage

to the

piping system. Several meth-

ods of

surge control

can be

employed:

•

Pump-control valves (which

are

popular)

for

start-

up

•

Motor starters that ramp

up and

down when pumps

are

started

or

stopped

•

Flywheels

to

control

the

rates

of

acceleration

or

decay

of

liquid system velocities.

These

and

other methods

are

discussed

at

length

in

Chapter

7.

Neither

of the first two

methods, however,

provides

any

surge control

for

power

failure.

As

explained

in

Chapter

7, if the flowrate

exceeds about

30 L/s

(500

gal/min)

and the

total dynamic head

exceeds

12 or 15 m (40 or 50

ft),

a

surge control anal-

ysis

is

needed

and

other surge control devices

or

sys-

tems

may be

required.

If

surge

can

occur

in the

intake pipe, consideration

must

be

given

to

surge control both

in the

discharge

and

intake piping.

Air-Vacuum

Release

Valves

Try

to

avoid

the

need

for

air-vacuum

release valves

in

water

service (and make every

effort

to

avoid them

in

wastewater

service) because they become unreliable

if

not

properly maintained. Because these valves seldom

receive

the

maintenance required

to

ensure 100% reli-

ability

and

because

no

designer

can

guarantee proper

maintenance

in the

future,

relying

on

air-vacuum

release valves

to

positively prevent water hammer

is

unwarranted.

If a

water hammer control device

can

fail,

it

will

do so

sometime

and is,

therefore, inade-

quate

protection.

Use

other defensive strategies, such

as

alternate pumping locations and/or

a

suitable force

main

profile

obtained

by

relocation,

deep

trenching,

or

even tunneling.

If

necessary,

increase

the

rotational

inertia

of the

pump

and

motor

by

adding

a

flywheel.

If

alternatives

are

completely impractical, conduct

a

careful

analysis

of the

system

for the

proper size

of

air-

vacuum

release

valve openings. Always install such

valves

in

pairs, particularly

in

sewage systems,

to

pro-

vide

a

backup

if one

fails.

Use the

tall type

and

stain-

less-steel trim

for

sewage service.

If

iron valve bodies

are

used, require

an

epoxy coating

to

prevent corrosion.

Provide

flushing

facilities

for all air and

vacuum

release

valves

and

specify

weekly maintenance

in the O&M

manual.

Note that only

an air or a

vacuum release valve

might

be

needed instead

of the

combination.

Intake

Sumps

Follow

the

recommendations given

in

Sections 12-6

and

12-7

in

designing clear water

and

(especially)

wastewater

wet

wells. These designs provide nearly

ideal hydraulic environments

for

pump intakes,

and

with

only

the

added cost

of an

ogee spillway

in

storm

water stations plus (for wastewater stations)

a

small

(1.5

to 4.5 L/s or 25 to 70

gal/min)

wash

water hose,

they

can be

easily

and

quickly cleaned.

For

clean waters, some engineers

may

prefer

to

follow

the

designs

in

Hydraulic Institute Standards

of

1983

[1] or in the

later

ANSI/HI

Standards [2,3],

but

beware! Those designs

are

unsuitable

for

water con-

taining

either settleable

or floating

debris, because,

without

modification, they cannot easily

be

cleaned.

(Mixing

the

contents during pumping

is one

modifica-

tion

that

is

sometimes

effective,

but

designers should

contact manufacturers that make equipment

specifi-

cally

for

that purpose.)

If

there

are

unusual

conditions

or if the flow is

very

large, model studies should

be

used

to

optimize

the

geometry

and the

need

for

baffles

and

suppression

of

swirling

and

vortices.

If

practical, provide enough storage

in the wet

well

and

influent

sewer

so

that short-term (e.g.,

1 h)

power

outages

can,

at

least

for

dry-

weather

flows,

occur

without

damage.

Heat

generated

by

inrush current during start-ups

can

damage motors.

The

sump should

be of

adequate

size

to

limit

the

starts

per

hour

to the

manufacturer's

recommendation.

24-4. Mechanical Considerations

One of the

most costly elements over

the

life

of the

facility

is

maintenance labor. Hence, developing

a

concept

or

method

for the

disassembly

of

piping,

pumps,

check valves, gate valves, traps,

flowmeters,

and

so on for

maintenance

or

removal

and

replace-

ment

is

important, even

if

three-dimensional

drawings

or

spatial models

are

necessary. Include such plans

in

the O&M

manual.

The

following guidelines make

the

maintenance

or

replacement

of

pipe,

fittings, and

pumps

easy:

•

Locate these elements

at a

convenient height.

•

Orient them

so

that removing

the

covers does

not

drench

the

workers.

•

Include

a

drain

and a

vacuum cock

for

emptying

the

pipe.

•

Install strong,

safe,

lifting

eyes

in the

ceiling

for a

chain hoist

in

small stations, monorails

and

electric

hoists

for

small

to

medium stations,

and

traveling

bridge cranes

for

large stations. Even

in

small sta-

tions, consider

a

trolley rail

for

moving

the

heaviest

load

to a

hatch accessible

to the

upper-level hoist.

Hoists should travel over

all

heavy equipment

and

terminate

at a

doorway. Alternatively, especially

for

vertical turbine pumps, install roof hatches

for

access

by

a

mobile crane parked outside.

The

hatches

and

floor

space must

be

adequate

for

removing

the

largest

equipment.

It is bad

practice

to

build

the

upper part

of

the

station around equipment already

in

place.

•

Provide plenty

of

clearance [0.8

m (30

in.) mini-

mum,

1 m (36

in.) recommended] between

the

pip-

ing and

pumps

or

other equipment

for

workers

and

their wrenches

and

other tools. Make sure that nuts

and

bolts

are

accessible. Increase

the

clearance

for

larger pumps. There must

be

adequate space under

the

crane

or

hoist

to lay

removed parts

on the floor.

•

Design

the

piping with enough sleeve and/or

grooved-end couplings

to

permit

the

easy removal

of

pumps, valves, etc. Blocking,

tie

downs,

or

bri-

dles

are

required

for

sleeve couplings.

•

Provide valves

to

isolate pumps

so

that

any

pump

can be

removed without draining

the

header

and

force

main.

•

Specify

spacer

shafts

between pumps

and

motors

to

allow

for the

quick

and

easy removal

of

backheads

and

rotors

of end

suction pumps

to

make replace-

ment

of

seals,

shaft

sleeves,

and

wearing

rings

easy

without

moving

the

motor

or

pump.

•

Provide

a

spool

of the

same length

as the flowmeter

and

shut-off

valves

on the

header

to

permit

the

removal

of the flowmeter. An

alternative arrange-

ment

is

depicted

in

Figure

20-12.

•

Provide adequate lighting

for

safe

working condi-

tions.

•

Install GFCI protection

on all

service receptacles.

Locate

the

following items

in

places that

are

easily

accessible

for all

routine maintenance:

•

Traps that must

be

periodically cleaned

•

Pump vent piping

•

Grease

fittings

(especially intermediate bearings

on

long

shafts)

•

Intermediate bearings

on

long

shafts

(safely

acces-

sible)

• Oil

reservoirs

•

Dry-break, quick-connect couplings

on

hydraulic

hoses

•

Dehydrating canisters

•

Mechanical

or

electrical devices that need periodic

adjustment.

If

necessary, provide permanent steps and/or plat-

forms

for

access.

Miscellaneous

considerations include

the

following:

•

Keep walking areas clear

of

overhead obstructions

to a

height

of at

least

2.2 m (7

ft).

If

obstructions

cannot

be

eliminated, cover them with padding

and

hang

warning devices (heavy strings

or

light chains,

for

example)

on

both sides.

•

Install convenient hose bibbs.

A

proper

backflow

preventer

must

be

installed

on the

potable water

connection

to the

station.

•

Install

a

lavatory

or at

least

a

wash basin.

•

Provide access

that

complies

with

OSHA require-

ments

to

ladders

and

manhole steps.

•

Provide adequate storage space

for

spare lubricants,

parts,

and

special tools.

Nine

power plants were surveyed

by the

Electric

Power Institute

to

identify

problems that

affect

pro-

ductivity

and the

safety

of

maintenance personnel

[4].

The

suggestions made

are

also

useful

for

pumping sta-

tion

designers. They include

the

following:

•

Provide easy

access

to

adjustment points,

test

points,

and

filling

and

draining locations

on all

equipment.

•

Arrange equipment

so

that access

to the

malfunc-

tioning

unit

does

not

require

the

disassembly

of

adjacent

units.

•

Provide

sufficient

clearance

to

allow

for

effective

personnel interaction when

a

team

effort

is

required.

•

Ensure that equipment clearances

in

hazardous

areas allow access

by

personnel encumbered

by

protective

garments

and

associated gear.

•

Provide

a

clearance

of at

least

l.lm

(42

in.)

at the

front

or

back

of

electrical panels.

•

Code

or key

interchangeable units that

are

function-

ally

dissimilar

to

prevent their insertion

in the

wrong

unit.

•

Provide places

to put and

support components

being removed

or

installed.

• Use

hinged doors rather than cover plates (which

are

time consuming

to

remove) where

frequent

access

is

needed.

•

Place controls

and

associated instrumentation

within

easy visual

and

manual reach

of

normal

work

positions.

A

universal complaint

was the

difficulty

of

accessing

equipment that required attention. About

30% of the

maintenance time could have been saved

had

ideal

or

unrestricted access been possible.

Remember that some types

of

equipment (e.g.,

submersible well pumps

and

submersible

wet pit

sew-

age

pumps)

are

maintenance intensive

and

must

be

removed

and

reconditioned

at

regular intervals,

and

this

may

cost about

one

third

as

much

as a new

pump.

Parts

for

some kinds

of

equipment (e.g., variable-fre-

quency

drives

and

some foreign-made pumps)

are

very

expensive

or,

worse yet,

may be

available only

after

long delays. Investigate this aspect before using

such equipment

and

conduct lifetime present-worth

analyses

of the

maintenance labor

and

parts

as

well

as

of

the

costs

for

energy. Avoid considering only

the

first

cost.

24-5.

Electrical

Considerations

According

to the

NEC, electrical control panels must

have

a

minimum clearance

in

front

of at

least

• 0.9

Im

(3 ft) for O to

15OV

•

1.07

m (3 ft, 6

in.)

for 151 to 600 V

•

1

.22

m (4 ft) for 601 to

2500

V

•

1

.52

m (5 ft) for

2501

to

9000

V.

The

same clearance

is

required

at the

back

if the

panel

is

a

free-standing unit. Free-standing panels

are

rec-

ommended over wall-mounted

panels.

A

lock-out stop should

be

installed

at

each motor

(or

at any

rotating equipment) that cannot

be

seen

from

the

control panel

to

prevent accidental energiza-

tion

while

the

equipment

is

being serviced.

A

time-

running meter

for

each pump

can

furnish

useful

infor-

mation

for

scheduled maintenance. Include heaters

or

desiccant containers

(to

keep electrical equipment

dry)

and put

control panels

in a

location

free

from

dust,

fumes,

and

H

2

S.

For

large pumping stations, consideration should

be

given

to

computerization

for

monitoring

and

con-

trolling

the

system.

Make certain that

the

electrical engineer provides

a

liberal number

of

convenience outlets

for

tools

and

maintenance equipment, such

as

lights

and

fans

(120

V,

single-phase),

and for

welders (480

V,

three-phase).

All

outlets should

be GFI

protected.

Use

hermetically sealed switches

and

relays

in

wastewater pumping stations.

Use

NEMA

7

equip-

ment

for

hose-down

spaces.

Post

a

single-line diagram

of the

power

system

—

especially

if

there

is

auxiliary power.

24-6. Architectural Considerations

The

entire layout, including housing, drainage, stor-

age for

equipment,

and

safety,

is

very important

to the

maintenance operation. Give proper consideration

to

the

following items.

For the

housing:

•

Access

for the

installation

and

removal

of

equip-

ment (for example,

use

double doors

or

roll-up

doors

to

remove large

pieces

of

equipment such

as

pumps,

motors,

or

engines)

•

Heating

and air

conditioning, which

may be

neces-

sary

depending

on

weather conditions

•

Convenience

for the

cleaning

and

painting

of the

facility.

For

drainage:

• The

drainage system

and

sump

for

wash-down water

•

Drainage

from

the

sump

of

water pumping stations

piped

to a

storm drain

or

sewer

(if

this

is not

possi-

ble and a

French drain

is

used, consider

the

ground-

water table

for all

seasons

of the

year;

if a

submersible sump pump

is

used

in a

wastewater

pumping

station

and the

discharge

is

piped

to an

adjacent

downstream manhole,

it may be

possible

to

dewater

a

small

flooded

station)

• A

drip line

for

water-lubricated packing

and a

drain

system

to the

sump

•

Sloping

the floor to floor-drains to

eliminate puddles.

For

storage:

•

Cabinets

to

store records, spare parts,

and

tools

•

Record

drawings (plans) should

be

stored

at

each

station.

For

safety

and

security:

•

Sewage pumping stations must

be

equipped with

an

adequate forced-air ventilating system

•

Adequate lighting

to

supply

at

least

200 to 300

luxes

(20 to 30 ft • cd) at the floor or,

alternatively,

about

100

luxes

(10

ft • cd)

with auxiliary

portable

lights

and GFI

receptacles

•

Carefully

consider security

and the risks of

vandalism

•

Intrusion protection with

a

remote alarm.

Miscellaneous. Include

the

following:

•

There

should

be a

means

of

checking

the

water

level

in

water wells because

it

helps

in

determining

the

proper pump head, drawdown,

or

changes

in the

water level

in the

aquifer.

• It may be

desirable

to

have

a

color-code

system

for

the

piping, particularly

if a

water station pumps into

more than

one

pressure zone.

24-7. Standby

Facilities

Consider using standby pumping facilities

for

impor-

tant

pumping stations. Most states require

a

second

source

of

power

for

sewage pumping.

Possible

sources include

•

Another utility source (feeding

from

two

separate

substations

is

almost always adequate)

• A

nearby major industrial plant with

a

large electri-

cal

generating capability

• A fixed,

dedicated engine-generator

set

• A

dual engine/motor drive with

an

appropriate,

automatic clutch that disengages

the

engine when

the

motor

is

energized

and

engages

the

pump

shaft,

thus

allowing

the

motor

to

spin

freely

when

the

engine

is

running

• A

portable

engine-generator

set

that

is

readily

accessible

and

well maintained.

Standby

diesel

or

natural

gas

engines should

be

exer-

cised

for at

least

20 to 30

min

after

reaching operating

temperature once each month

at

nearly

full

load (see also

Section 14-22). Low-load operation results

in

carbon

deposits

in the

exhaust

of

diesel engines,

and

inade-

quately

exercised engines require periodic dismantling

for

cleaning.

An

engine sized

for

pumping peak

flows or

operating under

the

worst conditions means that

the

engine

may

operate

at

one-third

or

less

of its

capacity

when

exercised.

One

solution

is to

design

the

electrical

system

so

that

a

portable load bank

can be

used

to

exer-

cise engines under

full-load

conditions. Reliability

can

be

improved

by

electrically heating

the

engine,

by

trickle

charging

the

batteries,

and by

vigilant

maintenance

—

all

of

which should

be

included

in the O&M

manual.

24-8. Specifications

Include provisions

in the

specifications requiring

the

supplier

of

pumps

and

other equipment

to

furnish

pump

curves, maintenance data, parts lists,

and the

manufacturer's

recommendations

for

service

and

maintenance.

The

names

and

locations

of

parts suppli-

ers

should

be a

part

of the

records.

Designers should recognize that many old, poorly

designed pumps

are

still

being sold (some

by

well-

known

manufacturers). Specifications obtained

from

the

manufacturers

(or any

organization representing

them) coupled with

low

bids

may

lead

to the use of

these shoddy pumps.

For

normal service, specify

a

limit

for

shaft

deflection

of no

more than

0.15

or

0.18

mm

(0.006

or

0.007

in.)

at the

wearing ring

and

0.05

mm

(0.002

in.)

at a

mechanical seal.

The

allowable

deflection

at

stuffing

boxes

is

greater, but, because

operators

may

want

to

replace

a

stuffing

box

with

a

mechanical seal,

the

more restrictive deflection

is

preferable.

Interviews with manufacturers

at

pump

exhibits

and

with pump maintenance crews

or

super-

visors

of

several utilities provide valuable instruction

in

maintenance economy,

and a

present-

worth

analy-

sis of the

expected

future

costs

of

maintenance, labor,

parts,

and

power

is

revealing.

24-9.

Operators'

Preferences

The

opinions

of

operators

are

invaluable. This section

contains

a

potpourri

of

selected opinions

on a

variety

of

subjects. Although these comments come

from

operators

of

wastewater pumping systems, most

of

them apply

to

fresh

water systems

as

well. Comments

by

non-operators

are

given

in

italics,

and the

commen-

tators

are

named.

Los

Angeles

County

Sanitation

Districts

The

following comments

are

paraphrased

from

con-

versations with Kettle

[5].

Wet

Wells

The

Lancaster Reclamation Plant

wet

well

in

Figure

11-12

is

very poor, because

it is so

large,

and

because

the floor is so flat.

Large quantities

of

grit

and

scum

accumulate.

Serious consideration

was

given

to

rebuilding

it

with

floor

slopes

of 45° and

pumps

or

pump

inlets

set

into

a

small pocket

so

that scum could

be

readily sucked out,

but the

cost

of

rebuilding

was

prohibitive.

Valves

Trouble

was

experienced with high-quality eccentric

plug

valves

in

digester

lines.

They were

replaced

with

solid wedge gate valves exercised weekly,

and

there

have

been

no

problems

with them.

In

fairness, how-

ever,

the

plug valves

had not

been exercised. Sanks:

Others

have used eccentric plug valves with complete

success.

Lescovich

[6]

warns that excessive tightening

of

the flange

bolts

on

eccentric plug valves squeezes

the

rubber gasket

out of the flanges and

binds

the

plug.

Bolts should

be

only

tight enough

to

prevent

leakage.

Bubblers

Air

tubing

was

plagued

by

plugging with

grease

and

with

leaks that could

not be fixed

because

the

tubing

was

buried

in

concrete. These ills, plus

the

mainte-

nance

of

compressors

and

associated mechanical

equipment, makes

the air

bubbler system irritating.

Pressure cells were substituted

at all

pumping sta-

tions

by

attaching

a

150-mm

(6-in.)

flange to the dry

pit

wall below LWL, installing

a

150-mm gate valve

and

drilling through

the

wall into

the wet

well.

A

short spool

is

attached

to

contain

the

pressure

cell

and

a

fresh

water line

is fitted to flush the

cavity

continuously with 0.06

L/s (1

gal/min)

of

fresh

water.

An

electronic control

box

completes

the

installation. Sanks: Leaks

can be fixed

when

air

tub-

ing

is

installed

in the

open

or in

conduit

and all

joints

are

accessible.

Suitably derated

air

compres-

sors

are

trouble-free.

Packing

Glands

versus

Mechanical

Seals

According

to

Redner [7],

new

pumps

are

always

required

to

have mechanical seals,

and the

seals that

come with

the

pump

are

immediately replaced with

the

owner's standard tungsten carbide-faced seals.

To

fit

mechanical seals

to

their older pumps that were

equipped with packing glands required machining

the

pumps

for

installing larger

shafts

and

bearings.

The

maximum

allowable deflection

at the end of the

shaft

is

(and was)

0.025

mm (3

mils). More deflection

would

quickly destroy

a

mechanical seal.

Packing glands leak,

of

course,

so

housekeeping

is

a

problem, they must

be

adjusted weekly

(a

continual

source

of

maintenance),

and

shaft

sleeves eventually

become scored

and

must

be

replaced

at

considerable

expense. Mechanical seals

are

preferred, because they

require

no

maintenance, last

for

many years

(10 or

more),

and

eventually

pay for

themselves, although

their

first

cost

is

many times greater than

the

modest

cost

for

packing glands.

Sanks:

The

costly

mechanical

seal

can be

reconditioned

to be

like

new for

about

a

third

of

the

cost

for a new

one.

A

mechanical seal

can

be

reconditioned several times.

County

Sanitation

Districts

of

Orange

County,

CA

Fresh water

for

seal water systems

is

avoided

in

most

pumping

stations.

By

using graphite-teflon packing,

the

wastewater itself, without filtration, becomes

the

seal water. This scheme

has

worked well

for a

decade,

and,

according

to

Arhontes [8],

the

wastewater does

not

cause

excessive

wear. This system

is

economical

and

more reliable because there

is no

dependence

on a

city

water supply that might

be

interrupted

by an

earthquake. When pumps

are

dismantled

to

replace

worn

impellers,

new

shaft

sleeves

are

also installed.

In

contrast

to the

antipathy

to air

bubbler systems

in

the Los

Angeles County Sanitation Districts (see

above),

air

bubbler systems

are

preferred,

and the

Orange County engineers have designed

a

standard

package

for the

entire system.

The air

tubing

is

installed

in

stainless-steel conduit

to

allow

the

bubbler

system

to be

easily removed

for fixing

leaks.

Seattle

Metro

The

operational friendliness

of 29

Seattle

Metro

—

now

King County (Washington) Department

of

Met-

ropolitan

Services

—

pumping

stations with

V/S

drives

and two

older

"traditional"

designs with

C/S

pumps

were carefully studied

after

two

decades

of

operation

by 10

senior operators with

an

aggregate

total

of 200

years

of

operating

experience.

Most

of

the

29

pumping stations have trench-type sumps

of

the

type illustrated

in

Figures

17-14

to

17-20.

In

addi-

tion,

there

are two

"traditional"

designs with large

flat

floors. The

following conclusions were reached

by

the

operators

in a

day-long seminar held

in

1989

and

attended

by

Garr

Jones (one

of the

original principal

designers

of

these stations)

and

Gary Isaac (Superin-

tendent

of

Operations).

/Access

1.

We

recommend catwalks

of fiberglass or

alumi-

num

gratings with rails (for safety)

to all

parts

of

the wet

well

and for all

out-of-reach locations

where routine maintenance

is

required.

Alarms

2.

Warning systems (whether internal

or

transmitted

elsewhere

for

monitoring) should

be

limited

to

those critical

for

station operation.

Too

many

alarms

can

cause

operator

complacency

and

fail-

ure

to

respond when response

is

really

needed.

We

have eliminated station-occupied

and

ventila-

tion-system failure alarms

in an

effort

to

reduce

troublesome calls.

3.

Having early warning

of

high water followed

by

flood

warning usually gives operators enough

time

to

respond.

4. It

should

be

impossible

to

clear alarms

at the

treatment plant when

the

acknowledge button

is

pressed.

Alarms should

be

cleared

only

at the

remote station,

but

when

two

stations

are

inter-

connected,

it

should

be

possible

to

clear alarms

at

either one.

5.

There should

be

alarm lights

for all

significant

water

levels

in the wet

well.

6.

Explosive

gas

detectors

are

desirable

if

they

are

maintained properly. Jones:

NFPA

820

mandates

the use

of

explosive

-gas

detectors.

Auxiliary Power

7. All

stations should have back-up power, either

engine-generators

or

dual power feeds.

8.

Dual power feeds

are

preferred over engine-gen-

erator sets

and

seem

to be

more reliable. High

winds

can

sometimes disrupt both feeds,

so at

least

one

should

be

underground.

Sanks:

The

reli-

ability

of

dual feeds

is

suspect because

they

offer

no

protection

in the

event

of

an

area-wide

power

failure.

9. We

prefer natural

gas for

engine-generator sets.

Jones:

The use

of

gas is

site-specific.

In the

South,

where

it is

common practice

to

shut down

gas

producing

and

distributing installations

for

days

at a

time when

a

hurricane

threatens,

diesel

is the

fuel

of

choice.

In the

Pacific

Northwest,

gas may

be

the

best choice because buried utilities

are

secure

against windstorm

and

most

floods.

How-

ever,

leaks

are

very

hazardous.

As the

engine

is

presumably

in an

unattended station,

a

hydrocar-

bon gas

detector

is a

better choice

for

shutting

off

fuel

than

a

fuel pressure detector.

The

latter gives

rise

to

spurious shut-downs. Cronin:

Gas may be

unavailable

if

power

failure

is due to

earthquake.

Also,

gas

engines

are

larger

than diesel engines.

10.

Portable engine-generator sets

are of no

value

in

snow

or for

remote

stations.

11.

Stationary engine-generator sets

may not

start

automatically. Jones:

For

reliability, exercise

them

with regularity

and

long enough

at or

near

full

load operating temperatures

to

cook

off

mois-

ture

in the

crankcase

and

elsewhere.

See

also

the

advice

in

Section 14-22.

12. We

think

engine-generators

should shut down

automatically

when commercial power

is

restored. Isaac:

I

disagree. Operators should

be

dispatched

to

check

all

systems. Jones: There

is

great

potential

for

damage

if

control circuits

are

not

reset

and

full

power

is not

restored

to

divided

busses

as a

consequence

of

any

power

failure,

no

matter

how

brief.

13.

Adequate noise control

is

needed

for

engines.

Sanks:

See

Chapter

22 for

measures

to

reduce

noise.

14.

Storage tanks

for

diesel

fuel

should

be

large

enough

for at

least

one day of

operation. Jones:

Typically,

power failures

rarely

last longer than

one

hour, although storms, earthquakes,

and

other

disasters have interrupted power

in

every

section

of the

country

for

several days, thus

prompting

engineers

to

design

for

excessive stor-

age. Diesel fuel deteriorates,

so the

exercise pro-

gram should

be

able

to

recycle

the

fuel inventory

every

6 to 12

months.

15.

There

should

be

load banks

or

some

other

means

to

exercise engines under

no

less than about

90

percent

of

full

load.

16.

The

transfer switch

and the

station's main breaker

should

be

sized

to

take

all of the

station's

load.

17.

PLCs (programmable logic

controllers)

should

be

programmed

to

start pumps

one at a

time.

Auxiliary Water Systems

18.

Solenoid valves

to

main pumps should

all be on

essential power.

19.

In

addition

to a

backflow

preventer,

we [in

Seat-

tle]

must also have

a

water storage tank with

an

air

gap,

and we use a

regenerative turbine pump

to

pressurize

the

system. Sanks: Multistage cen-

trifugal

pumps

are

also excellent

for

either seal

water

or

wash water (see Figure

11-34).

20.

Make sure that water pressure

is

available

in the

station

while

the

tank

is filling.

Controls

21.

Keep controls

as

simple

as

possible.

22. We

prefer

the

control room

to be

isolated

from

the

pumps. Sanks:

All

controls should

be

above

the

flood

plain.

Jones: Modern electronic con-

trols

should

be

located

in

air-conditioned rooms

with

air

treated

to

remove hydrogen

sulfide

and

particulates.

23.

PLCs work well

but

need

a

manual backup

for

those times when

the

system goes down. Cronin:

All

stations should have duplex PLCs. Sanks:

Some

operators would like

a

pump-down selector

switch with

a

timer

to

return

the

switch

to

auto-

matic

operation

after

a

preset time. Jones: Timers

are

not

recommended. Cleaning operations should

be

supervised

and the

station returned

to

normal

operation

with operating

personnel

present.

24.

Duty switches need

to be set so

operators

can

select

any

combination

of

lead/follow pumps.

Sanks:

Light switches

and

other switches

in dry

wells

should

be

waterproof

so

that

the

entire

dry

well

can be

hosed down.

25. It

should

be

possible

to

control

the

pump either

at

the

control panel

or at the

pump unit. Jones:

This

seems

to me to be a bad

idea, because

it

causes

an

unnecessary

complication

in

controls.

Worse,

it can

lead

to

damage

by

making

it

possible

for an

opera-

tor to

lock

out

equipment

or

leave

it in the

manual

mode

when leaving

the

station. Furthermore, with

hand-held,

low-power radios

so

readily

available,

one

operator

can be at the

machine

and

another

can be

manipulating

the

controls. Cronin:

If

the

cli-

ent

insists

on a

control

at the

pump when

in

manual

mode,

install

an

operation light

to

remind

operator

to

reset

the

controls

to

"Auto."

Sanks:

Do so

regardless

of

the

location

of

the

manual control.

26. It is

nice

to

have

an

indicator

of the

water level

in

the

pump room

for

stations with deep

dry

wells.

Jones:

The

operator should also have

the wet

well

level

instruments available when operating

the

pump

in

manual mode.

27.

Bubbler systems

are

still

our

standard,

but

sonar

has

improved

in

reliability recently. Back-up

floats for

high-

water

alarms

are

also needed. Otherwise

we do

not

recommend

float

switches except

for

constant

speed pumps. Sanks:

See Air

Bubblers, Section

20-

3, for

proper

design. Jones: Bubblers

are

immune

to

power

failure

whereas

electronic

instruments

are

not.

Float switches

are

mechanical devices

and

sub-

ject

to

wear.

Bubblers

and

pressure

cells

(with

virtu-

ally

no

moving parts)

are

inherenelty

more reliable.

Cranes,

Hoists,

Lifting

Eyes,

and

Access

Hatches

28. All

equipment weighing more than

45 kg

(100

Ib)

should

be

accessible

by

crane.

29.

Bridge cranes, properly located

to

allow direct

vertical

access

to

major station equipment,

are

absolutely necessary.

30. The

crane hoist should

be

electrically operated.

Removing

a

heavy object

from

far

below grade

with

a

manually operated hoist

is a

waste

of a

journey-level worker's valuable time.

3

1

.

Include

the

"inching"

mode

of

operation

—

a

fea-

ture (available

from

most manufacturers) that

allows slow

and

precise positioning

of

large items

such

as

motors.

32.

Extend crane rail girders outside

the

building

if

pos-

sible. Jones:

A

better

design

allows

trucks

to

move

into

the

building

and

under

the

crane

for

loading

heavy

equipment

directly

onto

the

truck

bed.

33.

Lifting

eyes should

be an

integral part

of

station

design

and

need

to be

strategically located over

critical equipment.

34.

Hatches

or floor

covers

with

plenty

of

clearance

for

pump

removal should

be

located over each pump.

They should

be

light

in

weight (preferably alumi-

num)

and

either spring-loaded

or

easily removable.

35.

Openings around hatches need proper barricades

built

into

the

hatch design

to

ensure

a

safe

work-

ing

area.

36.

When

lifting

eyes

are

needed, locate them prop-

erly

and

design them with

an

adequate

reserve

of

strength. [See Figure

25-6.]

Custom-Engineered

Pumps

37. We are firmly

committed

to

high-quality equip-

ment

and

custom-engineered pumps with larger

shafts

and

bearings.

38. The

best metallurgy

for

impellers

and

pump cas-

ings

(particularly

for raw

wastewater)

is

cost-

effective

in

prolonging

useful

life.

For

example,

our

pumps have generally lasted

20 yr or

more.

Pumps

in

four

stations

finally

wore out,

and our

engineers,

unfamiliar

with

the

thought that went

into

the

original designs, replaced them with stan-

dard

pumps. Troubles developed

at the

outset,

and

the

standard pumps lasted only

4 yr or

less.

39. We now

replace custom-engineered pumps with

units

of the

same quality. Where grit

is a

problem,

pump

casings should

be

hard cast iron.

Design Implications

40. Use

common sense

in

laying

out

piping

and

valves

so

stations

can be

operated

safely

and

with

easy access

to all

equipment.

41.

Good hydraulics

is

important.

42.

Design

for the

lowest head

in

pumping.

Do not

allow

a

free

fall

of

wastewater into

a

pool below.

43.

Involve

the

operators

in the

design

of the

station.

44.

Work with

the

neighbors

so

that they

can

claim

ownership

in the final

station with regard

to

col-

ors, landscaping,

and

noise

and

odor control.

45.

Success requires good design,

a

good contractor,

and

good inspection.

Doorways

46.

Doors should allow heavy equipment

to be

loaded

easily onto maintenance vehicles

for

transport

to

the

shop.

47.

Measure

the

utility's trucks

to be

certain doors

are

wide

enough.

At two of our

stations,

the

entrance

door

had to be

modified

to

allow adequate clear-

ance

for

maintenance vehicles.

48.

Roll-up doors

are

preferred over double doors.

Drains

49. All

areas should have adequate drains

and all

floors

should slope about

1%

to the

drain

to

avoid

standing water.

50. In the

immediate vicinity

of the

drain,

the

slope

should

be

increased

to 2%.

51.

Drains should

be

large

and

must

not

have

check

valves.

52.

Pipe

drains

are

preferred

to an

open rectangular

drain

along

one

wall.

Floor

Finishes

and

Coatings

53. Use

non-skid coatings where water

is

prevalent.

54. Use finishes

that

are

easy

to

keep clean.

Urethane

finishes are

satisfactory

and

control concrete dust.

55. Do not

paint

floors;

paint peels

in a

matter

of

months.

56. The

newer

sealers

have

not

been

holding

up

well.

Force

Main

Draining

and

Cleaning

57.

Keep force mains small

to

maintain adequate

velocity. They must

be

large enough

to

accommo-

date only

the

sewage volumes

to be

removed.

58.

Sump pumps

and

drains should

be

plenty large

enough

to

take care

of

accidental spills. [See Sec-

tion

12-3.]

59. Low

turbulence discharge structures reduce

hydrogen

sulfide

damage.

60. Pig

launching facilities should

be

provided

for all

force

mains. Pigs should

not be

larger than

600 mm

(24

in.).

Use

dual

force

mains

if

necessary

to

avoid

larger

pipe.

Use

dual headers

for

dual

force

mains.

61.

Design

the

force mains

for

ease

in

accessibility

in

the

station, along

the

alignment,

and at the

dis-

charge structure.

Gear

Boxes

62. We

dislike gear boxes because

of the

heat,

noise,

and

oil

leakage. Sanks:

They

also tend

to

induce

vibrations.

Gratings

63.

Removable gratings over

the wet

well (e.g., Kirk-

land

Pumping Station, Figure 17-14)

are a

nuisance.

To

facilitate hosing

the

side walls, design

a

catwalk

along

the

full

length

of the wet

well.

64. If

gratings cannot

be

avoided,

use

non-skid

fiber-

glass easily

lifted.

Even aluminum

is too

heavy.

Horizontal versus Vertical Pumping Units

65.

Pumps

and

motors

in a

horizontal configuration

are

excellent, because alignment

is

better than

in

vertical units,

but

they require

a

larger footprint

area

and are

more susceptible

to flooding.

66.

Avoid

the use of

intermediate

shafts

if

possible.

Vertical

units require

shafts,

and

vibration prob-

lems tend

to

develop.

67.

Isaac:

We

have some units with

flywheels.

They

have

not

been

a

problem,

but

nevertheless

they

should

be

avoided unless necessary

to

solve

a

hydraulic

problem. Jones:

To the

best

of my

knowledge,

they

have worked well.

Instrument

Air

Systems

68. Air

tanks (receivers) should have hand

bleed-off

s.

Automatic

bleeds

are not

necessary.

69.

Dual compressors should

be

valved

so

that

one can

be

removed while keeping

the

second

in

service.

70.

Both compressors should

not be on the

same

switch gear.

71.

Purge valves should

be

spring-loaded

to

return

to

normally

closed

position.

72. Air

dryers

are

recommended

for all

instrument

air.

73.

Design

for

high

flowrates at low

pressure:

240 to

350

kPa

(35 to 50

lb/in.

2

)

is

adequate.

74.

Lead/follow compressor selection

is

better than

automatic

alternating compressor operation.

Jones:

We

have

not

advocated

automatically

alter-

nating

lead

sequencers.

We

think

it is

just

one

more

thing

to go

wrong. Instead,

we

furnish

the

operating

staff

with hour meters

on the MCC

cubi-

cle

doors

and a

lead!

follow

selector switch.

This

approach

leaves

it up to the

operator

to

decide

when

to

select

the

other unit

to

function

as the

lead

unit

in a

lead!

follow

automatic starting sequence.

75.

Some

new

level control systems eliminate

the

need

for

instrument air: sonar

and

pressure cells

are two

examples.

Landscaping,

Irrigation,

and

Shrubs

76.

Many

of our

stations

are

located

in

areas

of

heavy

public

use and

high visibility,

so

good landscap-

ing

is

necessary. Landscaping should blend with

surrounding

area

and be

kept simple with ease

of

maintenance

in

mind. Low, slow-growing, dis-

ease-resistant plants should

be

specified. Avoid

ivy

and

thorny bushes.

77.

Lawns should

be

kept

to a

minimum. Consider

grasses that need

a

minimum

of

weeding

and

mowing.

78.

Automatic sprinkler systems with timers should

be

installed.

Sanks:

Moisture probes

are

also use-

ful

for

irrigating when necessary.

Lighting

79.

Good lighting

is

very important. Interior lights

should come

on

instantly.

80.

Where gases

can

exist, lighting should

be

explo-

sionproof.

81.

Pumping stations should

be

well

lit

outside

for

protection

from

vandalism.

82.

Include emergency back-up lights

by

battery

for

each pumping station

floor.

83.

Some lights take

too

long

to

light (high-pressure

sodium)

and

take

a

very long time

to

turn

on

right

after

they have been turned off,

so

choose other

types. Sanks: Sodium vapor lights

are all

right

for

outside

lights that turn

on

automatically.

84.

Locate

fixtures so

that access

by

ladder

for

chang-

ing

bulbs

is

easy.

Noise

Control

85.

Noise control

is an

important consideration

for

employee health

and

public relations.

86.

Engine-generator sets

may

require noise control.

87.

Variable frequency drives

are

also noisy

and may

require special treatment. Sanks:

AF

converters

are

sensitive

to

dust, hydrogen

sulfide,

and

tem-

peratures higher than

32

0

C

(90

0

F),

so an

air-con-

ditioned

enclosure should

be

considered.

88.

Consider

the use of

noise absorption walls

and

ceiling.

89.

Odor control systems will probably require noise

abatement.

90.

Rubber-mounted equipment helps

to

absorb

vibration

and

reduce noise.

Odor

Control Facilities

91.

Facilities should

be

built

to

suit

the

environment.

92. The

discharge

of air

from

the wet

well should

be

diffused.

Consider mixing

wet-

well

air

with

fresh

air

from

a

booster fan.

93.

Consider noise problems

from

odor control

or

ventilation

fans.

94.

Packed

wet

chemical scrubbers work well.

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

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