Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones
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Figure
11-34.
Multistage
centrifugal
pump
for
high-
pressure
discharge.
Courtesy
of
Grundfos
Pumps
Cor-
poration.
with
those
of
other types
of
pumps, consider
the
cost
of
the
total system, including
all
piping,
wet or dry
wells,
screens,
fittings,
valves,
variable-
speed
con-
trols,
and
other accessories
as
well
as
operating
and
maintenance
costs. Note that operators like screw
pumps
because
the
good ones, when properly
installed,
are so
trouble
free.
Open
Screw
Pump
The
open screw pump consists
of a
torque tube
to
which
spiral
flights
are
attached,
a
lower submerged
bearing,
an
upper radial
and
thrust bearing,
a
gear
reducer
(typically driven
by an
electric motor),
and a
trough
in
which
the
screw rotates
at a
constant speed.
The
limiting speed ranges
from
20
rev/min
in
large
pumps
to 75
rev/min
in
small ones
and
cannot
be
exceeded
without
water spilling over
the top of the
screw.
Pockets formed between
the flights,
torque
tube,
and
trough trap
the
liquid
and
move
it up the
incline
in a
continuous
flow. The
steel screw
can be
protectively
coated. Steel troughs
can be
used
for
smaller
pumps,
but
concrete troughs
are
most com-
mon.
Fabricated
deflectors
are
usually
installed
in the
pump
trough along
the
uptake side
of the
spiral. Their
concave
surfaces extend
the
circular
arc of the
wall
of
the
trough
and
improve pump
efficiency.
The
pump drive consists
of a
motor,
vee
belts [except
for
motors
of
more than
100
kW
(150
hp)],
and
reducing
gears.
A
backstop
is
usually
furnished
to
prevent reverse
rotation
of the
spiral when
the
unit
is
stopped.
If
the
headloss
at the
upper collector
or
chute point,
which
is
about
0.3 m (1 ft)
above
the
discharge pool,
is
ignored,
the
maximum overall system
efficiency
of
the
pump
at
design
flow may
reach 80%.
At
approxi-
mately
30% of
design capacity,
the
efficiency
drops
to
about
60% due to
friction
and
"slippage"
(backflow)
of
fluid
between
the flights and the
trough.
Installation
of the
pump
is
relatively simple
in
con-
cept, although there
is
some
difficulty
in
placing con-
crete
on
such steep slopes. Once
the
pump
is
aligned,
the
radius
of a
concrete trough
is
grouted
in
place with
a
screed attached
to the
edge
of the
screw.
Enclosed
Screw
Pumps
Except
for the
following differences,
the
enclosed
screw
pump
is
similar
to the
open type:
•
Because
the flights are
welded
to the
outer cylinder,
there
is no
slippage (backflow).
If the
pump
is
stopped,
the
water
is
retained between
the flights.
(In
cold
climates, provide
for
reverse operation
after
shut-down
to
prevent freezing.)
• The
lower bearing,
a
self-aligning
set of
rollers
mounted above
the
high-water level,
is
easily acces-
sible.
The
rollers
carry most
of the
radial load.
•
Because there
is no
slippage,
the
efficiency
of the
pump
is
very high
and
stays high even
at low
dis-
charge.
• A
massive concrete structure
is not
needed.
Air
Lift
Pumps
Air
lift
pumps
are
pneumatic devices that
do not fit the
categories
of
Figure
11-1.
The
pump consists
of a
sim-
ple
tube immersed
in the
sump
or wet
well with
a
high
volume
of
low-pressure compressed
air
admitted
at
the
bottom
of the
tube.
The
reduced-density
air-liquid
mixture
in the
tube
is
raised
by the
static head
in the
wet
well,
and it
overflows
into
the
open discharge
channel (Figure
11-9).
The
pump
is
extremely simple
and can be
constructed
in the field. It is
suitable
for
raw
sewage, sludge,
and
sandy
or
dirty water.
The
hydraulic
efficiency
is
very
low and
rarely
reaches 35%. This does
not
include
the
compressor
efficiency,
which reduces
the
overall
efficiency
even
more.
For all
except very
low
heads,
the
pump
requires large submergence.
11-11.
Summary
of
Typical
Pump Applications
The
principal uses
of the
pumps shown
in
Figure
11-1
are
given
in
Table
11-1.
The
listed discharge capabili-
ties
of
pumps
are
intended
to
convey only
a
general
Figure
11-35.
An
enclosed screw
pump.
After
CPC
Engineering
Corp.
Table
11-1.
Application
of
Pumps
Capacity
Type
of
pump (see
Figure
11-1)
m
3
/h
gal/min
Service
Overhung
impeller
Clear
liquid
to
2300
to
100,000
Water
Nonclog
9-2300
40-100,000
Wastewater
Wet
pit
volute
50-9000
250-40,000
Water
VTSH®
450-5000
2000-22,000
Wastewater
Self-priming
23-910
100-4000
Wastewater, water
Vortex
10-230
50-1000
Sludge, wastewater
Cutter
11-230
50-1000
Wastewater
Grinder
<23
<100
Wastewater
Submersible nonclog
9-11,000
40-50,000
Wastewater
Submersible vortex
11-60
50-250
Sludge, wastewater
Submersible grinder
<23
<100
Wastewater
idea
of the
available pump sizes. Some manufacturers
may
have standard designs smaller
or
larger than
those listed. Custom-made pumps (which
can be of
any
size)
and
European pumps (some
of
which
are
very
large indeed)
are
excluded.
No
distinction
is
made
in
Table
11-1
between clean
water
and
dirty water (with gritty material such
as
sand
in
suspension), because
a
clean water pump
can
be
made satisfactory
for
dirty water
if it is
constructed
of
abrasion-resistant materials
and if it is
equipped
with,
for
example,
the
appropriate kinds
of
seals.
Wastewater
is
assumed
to be
unscreened
and to
contain both organic
and
inorganic solids (including
stringy
materials) that require
a
nonclog design.
Wastewater
from
a
treatment plant
can be
pumped
with
water pumps. Unless
a
sludge pump
is
specifi-
cally marked
as
suitable
for
pumping wastewater,
it
would
not be
used
for
such service even though such
use
is
possible.
11-12.
References
1
.
Hydraulic Institute Standards
for
Centrifugal,
Rotary,
&
Reciprocating
Pumps,
14th ed.,
Hydraulic Institute
Parsippany,
NJ
(1983).
2.
ANSI/HI
9.1-9.5-1994,
Pumps—
General
Guidelines.
Hydraulic Institute, Parsippany,
NJ
(1994).
11-13.
Supplementary
Reading
1.
Pump manufacturers' catalogs, such
as
Centrifugal
Pumps
and
Vertical
Pumps, Fairbanks Morse Pump
Corp., Kansas City,
KS
(n.d.).
2.
Anderson,
H.
H.,
Centrifugal
Pumps,
3rd ed.
Trade
&
Technical Press, Morden, Surrey, United Kingdom
(1980).
3.
Pollak,
F.
Pump
Users'
Handbook,
2nd
ed.,
Trade
&
Technical Press, Morden, Surrey, United Kingdom
(1980).
4.
Karrasik,
1.
J.,
W. C.
Krutzsch,
W. H.
Fraser,
and J. P.
Messina, Pump Handbook,
2nd
ed.,
McGraw-Hill,
New
York (1985).
5.
SWPA, Submersible Sewage Pumping Systems
Handbook,
Lewis Publishers, Chelsea,
MI
(1986).
6.
Hicks,
T.
J.,
Pump Selection
and
Application, McGraw-
Hill,
New
York (1957).
7.
Stepanoff,
R.
J.,
Centrifugal
and
Axial Flow Pumps,
Wiley,
New
York (1967).
8.
Sarvanne,
H.,
and H.
Borg, Submersible Pumps
Handbook,
Oy E.
Sarlin,
AB,
Helsinki, Finland (n.d.)
9.
PSI
—
Pump
Selector
for
Industry,
Worthington
Pump
Division, Dresser Industries, Harrison,
NJ
(n.d.).
10.
Matley,
J.,
and the
staff
of
Chemical Engineers
(Eds.).
Fluid
Movers: Pumps, Compressors, Fans,
and
Blowers, McGraw-Hill,
New
York (1979).
Table
11-1.
Continued
Capacity
Type
of
pump
(see
Figure
11-1)
m
3
/h
gal/min
Service
Impeller-between-bearings
Axial
split,
single-stage
23-23,000
100-100,000
Water
Axial
split,
multi-stage
23^60
100-2000
Water
Vertical
(turbine)
lineshaft
Deep
well
9-2300
40-10,000
Water
Short
setting
9-23,000
40-100,000
Water
Barrel
pumps
9-9100
40-40,000
Water
Vertical
(turbine)
submersible
Deep
well
23-160
100-2000
Water
Short
setting
23-1000
100-8000
Water
Horizontally
mounted
Axial
flow
9-110,000
40-500,000
Storm
water
Positive
displacement
Plunger
0-120
0-540
Sludge
Lobe
<450 <2000
Sludge
Progressive
cavity
<100
<480 Sludge
Screw
(open)
2300-14,000
10,000-60,000
Wastewater,
storm
water
Screw
(enclosed)
110-8000
500-35,000
Wastewater,
storm
water
Ejector
0.5-140
2-600
Wastewater
Other
(omitted
in
Figure
11-1)
Air
lift
11-570
50-2500
Water,
activated sludge
The
purpose
of
this chapter
is to
describe:
(1) how
pumps
are
selected,
(2)
some important aspects
of
pump
installation,
and (3)
superior pump sump designs.
The
process
of
selection
is
difficult
to
explain.
For
seasoned engineers,
it
probably "just
happens"
because
the
knowledge gained
from
years
of
experi-
ence guides them through
a
winnowing process
in
which
several possible selections
are
eliminated until
only
a few
candidates remain.
Up to
this point
the
selection
can be
based
on a set of
guidelines (Section
12-4).
But
then
it
becomes
a
matter
of
relying
on
experience
to
select
the
best alternative
from
those
that
have survived this initial screening
process.
Illustrative
problems taken
from
practice
in
which
the
design
was
developed
in
U.S. Customary Units
are
presented
in
those units. Problems with
no
history
are
Chapter
1
2
Pumps:
Selection,
Installation,
and
Intakes
GARRM.
JONES
ROBERT
L.
SANKS
CONTRIBUTORS
Stefan
M.
Abel
in
Virgil
J.
Beaty
Roger
Cronin
John
L.
Dicmas
Rick
A.
Donaldson
Erik
B.
Fiske
Paul
R.
GaIIo
Richard
O.
Garbus
James
G.
Gibbs,
Jr.
Mayo
Gottliebson
Alan
W.
O'Brien
Ned
W.
Paschke
Otto
Stein
Charles
E.
Sweeney
William
Wheeler
presented
in SI
units.
But the use of
both units simul-
taneously
is
avoided
in
this chapter.
12-1.
Initial
Screening
Before
the
process
of
selecting pumping equipment
can
begin, many factors relating
to the
application
must
be
established.
As
these factors
are
defined,
one
inevitably
begins
to
identify
the
type
and
size
of
equipment that will
be
most suitable. Initial selection
factors,
listed generally
in
order
of
precedence,
include:
•
Quality
of the fluid to be
pumped (clean
or
sandy
water, wastewater)
•
Required design capacity (maximum
flowrate)
•
Operating conditions (head, maximum
and
mini-
mum
flowrates,
submergence, and/or NPSH).
Once these
factors
have been evaluated
and the
ini-
tial decisions have been made,
the
following factors
must
be
considered:
•
Mode
of
operation (such
as
in-line pumping, pump-
ing
from
a
well,
or
pumping
from
a
clear well
to a
reservoir)
•
Type
of
driver (motor
or
engine, constant
or
vari-
able speed)
•
Station location, configuration,
and
constraints
(such
as the
number
of
pumps, horizontal
or
vertical
pumps, units
in
parallel
or
series,
wet
well
or dry
well
pumps,
and
submersible pumps).
The
steps taken
to
complete
the
initial screening pro-
cess
are
described
in the
following subsections.
By
necessity, this
process
is
presented
in a
stepwise pro-
gression.
In
fact,
it is an
iterative procedure, with
trade-offs
from
the
ideal
until
the
apparent optimum
selection
is
found.
Fluid
to Be
Pumped
The
characteristics
of the fluid to be
pumped have
obvious
effects
on the
pump selection process.
The
selection
may not be so
immediately obvious
as one
might
think, however. Some
raw
water supplies
may
contain
significant
quantities
of
sand, grit,
and float-
ing
or
suspended material.
For
such waters, solids-
passing
capability
and
wear resistance (features
commonly
found
in
wastewater pumping equip-
ment)
may be
appropriate. Pumps intended
to
func-
tion
on
unscreened wastewater should
be of the
type
with
appropriate
eye
inlet velocities, rounded
and
blunt
impeller leading edges,
and a
configuration
for
nonclog service with rounded cutwater
and
adequate
clearances
to
pass
a
desired minimum (specified)
solids size.
On the
other hand, pumps intended
for
use
with
screened wastewater
or
treatment plant
effluents
of
various qualities
need
not be
selected
to
pass large solids. However, reconstituting
of
solids
in
the wet
well, stringy materials,
and
abrasion
due
to
sand
and
grit must
be
considered. Note that abra-
sive
wear appears
to
vary with
the
third
or
fourth
power
of
speed,
and
that unbalance (due,
for
exam-
ple,
to
fouling
by
rags)
is
more serious
at
higher
speeds. Consequently,
the
slower speeds
are
prefera-
ble for all
applications
but
particularly
for
wastewa-
ter
pumps.
Required
Design
Capacity
The
required design capacity (both initially
and at a
future
date), including
the
maximum, normal,
and
min-
imum
flows to be
pumped, must
be
considered when
selecting
the
type
and
size
of
pumping equipment.
Unless
the
station
is
intended
to
accommodate
a
wide
range
of flows
(caused, perhaps,
by
substantial
further
growth
in the
service area,
by
daily
or
seasonal
changes,
or by
substantial storm
inflow
in a
sewerage
system),
the
following advice, derived
from
experi-
ence,
is
useful.
• Try to
accommodate
the
peak demand with
two or
three duty pumps.
• Try to
accommodate
the
normal demand with
one
duty
pump.
• Try to
limit
the
number
of
pump sizes.
To
reduce
the
inventory
of
spare parts,
one
size
is
best.
Two
sizes
are
acceptable.
These considerations, which keep
the
station size
to a
minimum, must
be
balanced against initial
requirements.
In
some instances,
the
best solution
may
be to
install small pumps initially that
are to be
replaced
at
some
future
date with larger equipment
instead
of
more pumps. Note that
the
largest capital
cost item
is the
structure itself (e.g., excavation, con-
crete, ventilation).
In
most instances, minimizing
the
number
of
pumps minimizes
the
capital cost
of the
station.
If
smaller pumps
are
used initially,
the
suction
and
discharge connections should
be
sized properly
for
the
future
units;
use
reducers
as
necessary
for the
smaller original units. Full-size drivers
and
starting
equipment
may be
preferable
for the
initial pumping
equipment.
In any
event, remember
to
allow space
for
future
equipment
and
plan
how it
will
be
installed.
Once
the
preliminary pump sizes have been selected,
the
next step
is to
examine
the
complete range
of
operat-
ing
conditions
to be
imposed
on the
equipment.
Operating
Conditions
The
full
range
of
operating conditions should
be
examined
to
understand
the
application completely
(see Section 10-8). Operating conditions (minimum
flow,
minimum/maximum
discharge heads, NPSH
and/or submergence limitations,
and
other require-
ments)
may
dictate pump selection. Some examples
(drawn
from
actual experience) include
the
following:
• In a
wastewater pumping station
on a
small site
where over
70% of the
tributary
flow was
delivered
by
off-site
constant-speed (C/S) pumps
at
several
pumping
stations, large-capacity variable-speed
(V
/S)
pumps were required.
V/S
equipment
had to
be
capable
of
reacting quickly because
little
or no
storage could
be
provided
in the wet
well.
• At a
water booster station, C/S, vertical-turbine bar-
rel
pumps
(described
in
Chapter
11)
were
selected
because
of the
limited NPSH available
at the
pump-
ing
station site.
The
selection minimized
the
cost
of
the
station
by
eliminating
the
need
for a
forebay
and
substructure.
•
High static
and
total head conditions
and the
desire
to use
only
one
pumping stage
at a
wastewater
pumping
station limited
the
number
of
pump
designs available
for
consideration.
•
Widely
fluctuating
inlet head conditions
at a
water
booster pumping station required pumps with
a
high
shut-off
head
and a
steep head-capacity curve.
• A
nearly
flat
system curve
at a V/S
interstage pump-
ing
station
in a
treatment plant ruled
out
high spe-
cific
speed pumps because
a dip in the
head-capacity
curve
(see Figure
10-16)
would have caused unstable
operation.
• A
requirement
for
start-up
and
shut-down against
a
closed valve eliminated propeller pumps
from
consid-
eration
at a
water pumping station because
of the
high
power
requirement while
the
valve
is
closed. Opera-
tion
against
a
closed valve would require larger
motors,
switchgear,
conductors, etc. (see Figure
10-
16d).
An
alternative, however, would have been
to
bypass
to the wet
well during start-up. This alternative
was
not
attractive because
of the
additional mechani-
cal
equipment, valve wear,
and
noise considerations.
•
Plotting
the
system characteristics
for all
antici-
pated operating conditions
is
most
helpful
and is
considered mandatory regardless
of
station size.
Station
Location
and
Configuration
Various
aspects
of the
site selected
for a
pumping sta-
tion
may
force
one
pump option
to be
favored over
all
others. Site considerations
may
include
the
following:
•
Size:
A
small site
may
require
the use of
vertical
pumps
(such
as
vertical turbines
in
barrels)
to
reduce
the
size
of the
station
floor
plan.
•
Hydraulic
profile:
The
required NPSH
(NPSH
R
)
at
the
pump's (suction) inlet
may
result
in a
deep station,
thereby
discouraging
the use of
horizontal pumps
in a
dry
well because
of the
cost associated
with
a
large
floor
plan. Conversely,
if a
station
can be
shallow,
horizontal pumps
may be
preferred
because
of the
prospect
for
reduced:
(1)
vibration,
(2)
headroom
required
for a
crane,
and (3)
height
of
superstructure.
•
Environment: Weather conditions
may
rule
out
equipment that could otherwise
be
mounted out-
doors.
Concern over noise
emissions
may
eliminate
drivers
or
gear boxes mounted
at or
above grade,
and
submerged motors (which
are
quiet) might
be
favored.
Pumping
stations
may, however,
be
sound-
proofed
(see Chapter 22).
Mode
of
Operation
In
most situations, pumps suitable
for V/S
applications
are
those with continuously sloping head-capacity (H-Q)
curves, such
as
those shown
in
Figure
10-16b.
The
range
of
type numbers
can be
approximately
20 to
1250
in SI
Units
(specific speeds
of
1000
to
7000
in
U.S. Custom-
ary
Units). Pumps with
flat H-Q
curves
are
usually
unsuitable
for V/S
operation
if two or
more pumps oper-
ate in
parallel
or if a
portion
of the H-Q
curve parallels
the
station system curve. Pumps with
a dip in the H-Q
curve
are
also unsuitable
for V/S
(Figure
10-16d)
unless
the
pump
H-Q
curve intersects
the
system
H-Q
curve
cleanly
and
well beyond
the dip at all
possible
operating
conditions. Pumps with
a
steep
H-Q
curve
or a
power
curve
that
rises
to the
left
(Figure
10-16c
or d) are not an
optimum
choice
if the
pump must
be
started
and
stopped
against
a
closed valve. Selection
of
pumps
with
such
operating characteristics
may
require larger motors,
starters, electrical conductors,
and
conduits
to
sustain
the
added load
on
start-up
and
shut-down.
Because
of
transient control measures
or
because
of
the
details
of the
discharge piping,
the
pump
may
be
required
to run
backward during
the
period shortly
after
shut-down. Operation
of the
pump
at
shut-off
or
at
reduced capacity
may
increase vibration
and
signif-
icantly
reduce bearing
life.
Type
of
Driver
The
type
of
driver
influences
pump selection.
For
example, horizontal pumps
may be
more appropriate
than
vertical units
if
engines
are to be
used, although
note that right-angle gears
can be
used,
as
shown
in
Figure
12-1.
Available motor speeds
may
also
influ-
ence
the
pump selection process.
Miscellaneous
Considerations
Other considerations that
can
influence
the
selection
of
pumping equipment include
•
Client preference
•
Local custom (e.g.,
the use of
vertical turbine
pumps
in the
West
vs.
horizontal, double-suction,
split-case pumps
in the
East)
•
Budget
and
initial cost versus operation
and
mainte-
nance cost
•
Delivery time
•
Experience (i.e.,
an
engineer's previous experience
with
a
particular manufacturer
or
pump type)
•
Funding agency
or
statutory restriction (e.g.,
a
pro-
hibition
against foreign manufacturers, proprietary
specificiations,
etc.)
•
Local availability
of
parts
and
service.
12-2.
Final
Selection
Final selection involves
a
review
in
more critical detail
of
the
considerations used
in the
initial screening pro-
cess
as
well
as
detailed discussions with
the
selected
manufacturers
and the
owner.
Specific
actions
to
pre-
cede
a final
selection include
the
following:
•
Perform
a
detailed evaluation
of
system hydraulics
and
consider
the
performance
of
candidate pumps
at
all
possible operating conditions.
•
Furnish each candidate manufacturer with
the
details
of the
hydraulic requirements (including
the
system
curve),
the
preliminary station layout,
and
preliminary specifications.
Ask for
written com-
ments
and a
proposal
specific
to the
project with
budget equipment prices.
•
Show
the
owner's representatives
the
station layout
with
a
rough construction cost estimate
and the
pre-
liminary equipment selections, preferably
in a
pre-
liminary
design report. Request their comments.
Once comments have been received
from
the
owner
and
pump manufacturers,
the final
selection
of
equipment, layout
of the
pumping station,
and
writing
of
detailed pumping unit specifications
can
begin.
Figure
12-1.
Large engines
driving
pumps
through
right-angle
gears. West Point Sewage Treatment
Plant,
Seattle
Metro.
Designed
by
Brown
and
Caldwell
Consultants. Photo
by
George Tchobanoglous.
12-3.
Illustrative Examples
The
examples
in
this section
are
presented
to
illustrate
the
pump selection process. Bear
in
mind that
the
process
is
one
that cannot
fully
be
described because
a
great deal
of
the
selection
effort
is
simplified
through experience.
All too
often,
sump pumps (see Figure 12-2)
are
given
too
little attention
by
designers.
In
reality, these
are
pumping stations
in
their
own
right.
The
equip-
ment should
be
selected
and the
installation should
be
designed with
the
same care
as
that
afforded
to the
main pumping equipment.
Figure 12-2. Sump
pumps
in
Henderson
Street
Pumping
Station, Seattle
Metro.
Designed
by
Brown
and
Caldwell
Consultants. Photo
by
George Tchobanoglous.
Example
12-1
Sump
Pumping
System
A
pumping station
is
designed
to
pump unscreened wastewater
at a
rated capacity
of 20
Mgal/d
at
a
total head
of 67 ft.
Each
of the
three main pumps
is to be
driven
by a
175-hp
(with nominal
200-hp
motors) variable-speed system.
The
pumps
are to be
located
in a
room with
a
finished
floor
elevation
of
95.33
ft. The
motors
are to be
located
at an
elevation
of
110.00
ft and the
nom-
inal
finished
grade outside
the
station
is at an
elevation
of
128.75
ft. The
maximum water sur-
face
elevation
in the
influent
sewer system
is to be
limited
by the
elevation
of
overflows
set to
function
at
121.00
ft. A
typical station
cross-section
is
shown
in
Figure 12-3.
A
hydraulically
powered sluice gate isolates
the
pumping station upon power
failure,
abnormally high levels
in
the wet
well,
or
other system malfunctions.
The
configuration shown
has
several advantages:
(1)
no
intermediate
shaft
bearings (which increase cost
and
maintenance
and
often
give trouble)
are
needed,
(2) the
station
is
quieter,
and (3) the
upper
floor can be
used
for
other purposes such
as
for
the
motor control center.
As for the
possibility
of flooding,
modern motors with encapsulated
windings
can
withstand inundation (although
not
while running) without damage. Note that
because
the
pump cannot
be
operated under water without damage, sump pumps
or
portable
pumps
must
be
used
to
remove water
from
a flooded
station.
Problem: Select sump pumps
and
design
a
sump pump installation
to
protect
the dry
well,
the
motor room,
and the
pump room.
The
owner wants submersible pumps.
Solution:
The
purpose
of the
sump pump installation
is (1) to
accept liquids
from
incidental
leakage
from
pumps, drainage
from
station maintenance
and
housekeeping operations,
and
Figure
12-3.
Cross-section
of a
typical
pumping
station. After Brown
and
Caldwell
Consultants.
excessive
inflow
caused
by
broken
or
leaking couplings
in the
wastewater piping
and (2) to
dis-
charge these liquids
to a
suitable location
for
disposal. Because
the
installation serves
a
waste-
water
pumping station
and
wastewater solids
may be
encountered
from
spillage during
maintenance operations
and at
other times,
the
pumps must
be
selected
to
pass solids
at
least
IV
2
in. in
diameter.
The
pumps must discharge
to the
influent
sewer upstream
from
the
sluice
gate
(to
provide
a
positive means
for
dewatering
the
station)
at an
elevation that protects
the
sta-
tion
against back siphonage.
The
required discharge cannot
be
computed,
but it is
considered that
a flowrate of
150
gal/min
will protect
the
station. Under worst-case conditions (substantial
inflow,
perhaps caused
by a
par-
tially
separated pipe
or
pump casing joint),
the
maximum discharge required might
be on the
order
of 300
gal/min. Such
an
unlikely event
can be met by
operating
the
standby pump
as
well.
Head-capacity
curve.
The
influent
manhole (upstream
from
the
pumping station)
is
located
approximately
25 ft
away
from
the
structure
and the
sump
is to be at the
opposite
end of the
structure, which
is
about
45 ft in
length.
The
sump pump force main
is to
terminate
in the
influ-
ent
manhole
at a
centerline elevation
of
125.00
ft. The
estimated length
of
piping between
the
sump
and the
point
of
discharge
is
135
ft. The
drain discharging
to the
sump
is to
have
an
invert
elevation
of
94.55
ft. The
pump-control elevations
are to be
selected
to
maintain
free
drainage
conditions
in the
drain system
at all
times during normal operation.
The
minimum static
lift,
therefore,
is 125 -
94.55
=
30.45
ft
when
the
sump water level
is
94.55
ft
(see Figure 12-4).
The
maximum
static
lift
is 125
-
91.5
=
33.5
ft. and the
average
lift
is
(30.45
+
33.5)/2
= 32 ft.
The
strategy that prompted
the use of six
ON-OFF
switches
in
Figure
12-4
is the
best possi-
ble for
achieving
the
following.
The
follow pump will
not
start until
after
the
high-water alarm
is
activated. Either excess
flow
into
the
sump
or
failure
of the
lead pump will initiate
an
alarm.
The
operator must
go to the
station
to
investigate
the
alarm condition
and
switch
the
follow
Figure 12-4.
A
sump
pumping
system. After Brown
and
Caldwell
Consultants.
See
Figure 21-4
for a
different strat-
egy
for
ON-OFF
switches.