Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones
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can be
estimated.
Row
through centrifugal pumps
after
power failure
is a
function
of
many variables, including
•
inertia
and
speed
of the
pump, driver,
and
water
within
the
casing;
•
length
and
profile
of the
pipe;
•
steady-
state
hydraulic gradeline;
•
velocity
of flow; and
•
suction conditions
in the wet
well.
The
true shape
of the
hydraulic gradelines requires
solution
by a
computer.
6-8.
Criteria
for
Conducting
Transient
Analysis
Every
pump
and
pipeline system
is
subject
to
transient
pressures,
but in
practice,
it is
impractical
to
spend
the
time
and
expense necessary
to
analyze
all of
them.
The
following empirical guidelines, which seem
to be
satisfactory
in
most (though certainly
not
all) situa-
tions,
can be
used
to
decide whether
a
complete tran-
sient analysis
is
required.
Do Not
Analyze
•
Pumping systems with
flow
less than
23
m
3
/h
(100
gal/min).
Discharge piping
is
usually such that
velocity
is low and
transient pressures
are
low. Even
if
transient pressures
are
high, small diameter
(100-
mm
or
4-in.) piping
has a
high pressure rating
and
can
usually withstand
the
pressures.
•
Pipelines
in
which
the
velocity
is
less than
0.6 m/s
(2ft/s).
•
Distribution systems
or
pipe networks
(as in
com-
munity
potable water systems).
The
many junctions
significantly
dissipate
the
pressure waves.
•
Reciprocating pumps, because virtually every
reciprocating pump should have
a
pulsation damp-
ener
on the
discharge (see
Ekstrum
[7] for
methods
of
sizing such dampeners).
•
Pumping systems with
a
static
differential
pressure
between suction
and
discharge
of
less than about
9
m (30
ft).
Warning:
it is
possible that
a
very
low
static head
coupled with
a
relatively high dynamic head could
result
in a
column separation problem.
Do
Analyze
•
Pumping systems with
a
total dynamic head greater
than
14 m (50 ft) if the flow is
greater than about
1
15
m
3
/h
(500 gal/min).
•
High-lift pumping systems with
a
check valve,
because high surge pressures
may
result
if the
check valve slams shut upon
flow
reversal.
Figure 6-8. Successive
hydraulic
gradelines
following
power
failure.
Adapted
from
Walters
[4, p.
271].
• Any
system
in
which column separation
can
occur:
(1)
systems with
"knees"
(high points),
(2) a
force
main
that needs automatic
air
venting
or air
vacuum
valves,
or (3) a
pipeline with
a
long (more than
100
m
[300
ft]),
steep gradient followed
by a
long, shal-
low
gradient.
•
Some consultants analyze
any
force main larger than
200 mm (8
in.)
when longer than
300 m
(1000
ft).
Checklist
Some additional insight
can be
gained
from
the
fol-
lowing conditions, which tend
to
indicate
the
serious-
ness
of
surge
in
systems with motor-driven centrifugal
pumps.
A
serious surge
may
well occur
if any one of
these conditions exists.
If two or
more conditions
exist,
a
surge will probably occur with
a
severity
pro-
portional
to the
number
of
conditions
met
[8-1
1].
•
There
are
high spots
in
pipe
profile
•
There
is a
steep gradient: length
of
force main
is
less than
20 TDH
•
Flow velocity
is in
excess
of
1
.2
m/s (4
ft/s)
•
Factor
of
safety
(based
on
ultimate strength)
of
pipe
(and
valve
and
pump casings)
is
less than
3.5 for
normal operating pressure
•
There
can be
slowdown
and
reversal
of flow in
less
than
t
c
•
There
is
check valve closure
in
less than
t
c
•
There
is any
valve closure
(or
opening)
in
less than
10s
•
There
can be
damage
to
pump
and
motor
if
allowed
to run
backward
at
full
speed
•
Pump
can
stop
or
speed
can be
reduced
to the
point
where
the
shut-off
head
is
less than static head
before
the
discharge valve
is
fully
closed
•
Pump
can be
started with discharge valve open
•
There
are
booster stations that depend
on
operation
of
main pumping station
•
There
are
quick-closing automatic valves that
become inoperative
if
power
fails
or
pumping
sys-
tem
pressure
fails.
Criteria
for
determining whether
to use
simple hand
calculations
or a
more detailed computer program
are
also given
in
Pipeline Design
for
Water
and
Wastewa-
ter
[8, p.
65].
Shut-downs
will occur,
so
plan
for
them. They
can
result
in low
pressures
and
column separation
at
knees
in
steep pipelines.
Air
venting valves that close
too
rapidly while
an
empty pipe
is
being
filled
also cause
destructive hydraulic transients. Even
on
low-lift
pumping stations, depending
on the
pipe profile,
col-
umn
separation
can
occur
in the
vicinity
of the
dis-
charge header
or
farther downstream.
Computers
There
is no
simple, easy
way to
perform reliable tran-
sient analyses. Computer modeling
is the
most
effec-
tive means available,
but
there
are
practical constraints
on
time
and
cost. Both computer time
and the
labor
needed
to
analyze
and
review
are
expensive,
so the
extent
of the
analysis should
be
related
to the
size
and
cost
of the
project.
For
example, spending
$1000
to
$5000
to
analyze transients
for a
$1,000,000
project
is
probably worthwhile even
if no
problem
is
found.
6-9. References
1.
Parmakian,
L,
Waterhammer
Analysis,
Dover,
New
York
(1963).
2.
Rich,
G.
R.,
Hydraulic Transients,
Dover,
New
York
(1963).
3.
Wylie,
E.
B.,
and V. L.
Streeter,
Fluid Transients,
Feb
Press,
Ann
Arbor,
MI
(1982,
corrected
copy
1983).
4.
Walters,
G.
Z.,
Analysis
and
Control
of
Unsteady Flow
in
Pipelines,
2nd
ed.,
Butterworths,
Stoneham,
MA
(1984).
5.
Chaudhry,
M.
H.,
Applied Hydraulic Transients,
Van
Nostrand
Reinhold,
New
York
(1979).
6.
Wood,
D.
J.,
and S. E.
Jones,
"Water
hammer
charts
for
various
types
of
valves,"
Journal
of the
Hydraulics
Division,
Proceedings
of
the
American
Society
of
Civil
Engineers,
167-178
(January
1973).
7.
Ekstrum,
J.
D.,
"Sizing
pulsation
dampeners
for
reciprocating
pumps,"
Chemical Engineering (January
12,
1981).
8.
Pipeline
Design
for
Water
and
Wastewater,
American
Society
of
Civil
Engineers,
New
York
(1975).
9.
AWWA
Mil,
Steel
Pipe—
A
Guide
for
Design
and
Installation,
p. 54,
American
Water Works
Association,
Denver,
CO
(1985).
10.
Kerr,
S.
L.,
"Minimizing
service
interruptions
due to
transmission
line
failures:
Discussion,"
Journal
of the
American
Water
Works
Association,
41, 634
(July
1949).
11.
Kerr,
S.
L.,
"Water
hammer
control,"
Journal
of the
American
Water
Works
Association,
43,
985-999
(December
1951).
The
advantages, disadvantages,
and
typical uses
of
various
kinds
of
control devices (such
as
tanks
and
valves)
and
control strategies (such
as
timing
of
valve
operations, slow
filling
of
empty pipelines,
and
ade-
quate
maintenance) used
to
limit transients
are
dis-
cussed
in
this chapter. Coping
with
transients
is
site
specific,
so the
discussion
is
limited
to
examples that
illustrate
only
a few of the
many
different
problems
designers
may
encounter. Chapter
6 is
prerequisite
to
this
chapter.
The
approach
to
design
is the
same
for all
strate-
gies.
If
surges
are
expected
to be
severe (see
Section
6-8),
the
piping system
is
modeled
for a
computer
solution
and the
magnitudes
of
surges
for the
critical
conditions
are
determined.
If
they
are
excessive,
a
promising
control scheme
is
selected
and the
system
is
then analyzed again
by
computer.
The
process
of
selecting
an
appropriate candidate solution
and
ana-
lyzing
the
results
is
continued until satisfactory con-
trol
and
reliability
are
achieved
for
minimum
(or at
least
supportable)
cost
—
a
decision that usually
requires
considerable judgment.
Although
the
details
of
computer analyses
are
beyond
the
scope
of
this book, some
of the
factors
involved
in
choosing
a
solution
are
included.
Standards
and
specifications given
by a
coded
number
(e.g.,
AWWA
C401)
are
referenced
in
Appen-
Chapter
7
Control
of
Hydraulic
Transients
BAYARD
E.
BOSSERMAN
Il
CONTRIBUTORS
Robert
C.
Glover
William
A.
Hunt
Joseph
R.
Kroon
M.
Steve
Merrill
Gary
Z.
Watters
dix
E.
Addresses
of the
publishers
are
given
in
Appen-
dix
E
7-1
.
Overview
of
Hydraulic
Transient
Control
Strategies
Every piping system
for
water
and
wastewater should
be
evaluated
for
water hammer. From
a
review
of the
plan
and
profile
of the
pumping station
and
pipeline
system
as
well
as the
operating scheme,
it is
fre-
quently possible
to
determine
where potential
hydraulic transient problems
may
exist
and
what
means might
be
taken
to
control them.
For a
quick
check
to
determine
the
potential
for
column separa-
tion,
draw
a
mirror image
of the
hydraulic
profile,
as
in
Figure
7-1.
If the
pipe lies well above
the
mirror
image, column separation might occur
or
even
be
likely.
If
column separation
can
occur,
or if the
crite-
ria
for
required analysis
in
Section
6-8 are
met,
use
(or
contract for)
a
trustworthy computer analysis
of
the
system
[I].
Make every
effort
to
eliminate
(or,
at
least,
to
miti-
gate) transients
by
avoiding knees, high spots, steep
gradients near
the
pump,
and air
(or, much worse, vac-
uum)
in
pipes.
When
any of
these
conditions cannot
be
avoided,
use a
combination
of
pipe strength
and
control strategies
to
provide adequate protection
at
reasonable cost.
Recommended Options
for
Wastewater
Systems
Because sewage contains
(1)
microorganisms that
form
hydrogen
sulfide
and
sulfuric
acid;
(2)
grease
(sometimes
in
large quantities) that
floats,
tends
to
stick
to
walls,
and can
eventually develop into
a
hard
blanket;
(3)
stringy material (rags, paper) that tend
to
wrap around
any
protrusion;
and (4)
grit that sinks
into
any
depression, only
a few
strategies
can be
used
to
control water hammer
in
wastewater pumping.
See
Section
7-7 for
additioinal details.
Briefly,
surge con-
trol strategies
are
limited
to the
following, more
or
less
in
order
of
preference:
•
Rerouting
the
force main
to
avoid knees
or
negative
gradients
so
that
air
traps
are
eliminated and, poten-
tially,
the
need
for
special surge protection
can be
avoided.
•
Using
an
alternative source
of
power, such
as an
engine
on
some pumps,
so
that
the
entire system
cannot
fail
at
once. Properly designed, engine
drives
offer
superior protection against power
fail-
ure. Engines rarely
fail
or
shut down suddenly even
when
starved
for
fuel,
and
—
unlike
motors
in
con-
stant
speed (C/S)
operation
—
in
normal operation,
speed
can be
increased
or
decreased gradually.
•
Using pump control valves that open
and
close
gradually,
or (on
very long pipelines) using variable
speed (V/S) drives
for
ramping speed
up and
down
slowly.
Either prevents
the
continual pounding
of
the
system.
The
latter, however,
offers
no
protection
when
power failure occurs.
•
Increasing
the
rotating moment
of
inertia
of the
pump
and
driver
by
adding
a flywheel.
•
Using high-strength
pipe.
This option
is
limited
to
small systems,
but if the
force main
is 200 mm (8
in.)
in
diameter
or
smaller,
it may be the
cheapest
solution.
But be
aware that
the
continual pounding
of
surges
may
cause leaks
and
eventual failure.
•
Providing
air
inlet (vacuum relief) valves
at
critical
points
to
prevent
the
high surge that follows column
separation.
Use
only swing check valves
as in
Figure
7-2, because other types
of
valves
may
clog with
grease
or
particulate
material. Install such valves only
where
the
pipe slopes upward
to its
discharge
(or at
worst
is
level),
so
that
the air
admitted
can be
swept
downstream
to a
free
surface.
Each valve should
be
sized
to
match
the
volumetric
flow of
water with
the
volumetric
flow of air at a
pressure drop
of
about
14
kPa
(2
lb/in.
2
)
or
less. Fleming
[2] has
described
a
successful
installation
of air
inlet valves
for a
7100-m
3
/h
(45-Mgal/d)
pumping station.
•
Providing
air
scouring velocities (per Table
B
-9)
at
knees (together with vacuum relief valves
as per
Figure 7-2)
or in any
pipe laid
flat or at
downward
gradients.
If
required
by the
owner
or a
regulatory
agency, adding
air
release
valves could serve
a
use-
ful
purpose
in
preventing small accumulations
of air
and
eliminating
an
environment conducive
to
corro-
sion. (Such valves should
be of the
tall body
form
and
made
of
stainless steel
to
resist corrosion
caused
by
hydrogen
sulfide.)
If,
however,
the air
release valves
are
clogged,
the
high velocity will
scour
out the air (or
gas) anyway
and
thus prevent
a
catastrophe.
To
minimize friction losses, only
the
pipe
on a flat or
downward gradient need
be
made
small
to
obtain
the
high
velocities
required. (Note
that
the
custom
of
never decreasing
the
diameter
of
Figure
7-1.
Construction
for
indicating
the
probability
of
water
column
separation
in
pipelines.
a
sewer line does
not
apply
to
force mains.) Air-
scouring velocities must
be
reached
frequently
—
at
least once
per
day.
In
the
device
of
Figure 7-2,
the
saddle
is on the
side
of the
pipe
to
inhibit
the
entrance
of
scum
and
grit. Install
the
check valves
in
pairs
so
there
is
always
protection while
one is
being serviced.
The
clapper
is
held
closed
by a
light counterweight
or
spring
until
a
partial vacuum
in the
force main over-
comes
the
counterweight
or
spring (usually
at 7 to
14
kPa or 1 to 2
lb/in.
2
)
and
opens
the
valve
to
admit
large (equal
in
volumetric
flow to the flow of
water)
quantities
of air and
thus prevent
the
formation
of
water
vapor.
The
three-way valve must
be of a
type
that
can
never close
off
both check valves
at
once.
The
velocity
of flow in
wastewater force mains
should
reach
1.2
m/s (4
ft/s) once every
day or so to
ensure self-scouring.
It is
wise
to
include pigging
facilities
because pigging removes
the
bacterial slimes
that
produce hydrogen
sulfide.
Options
Undesirable
for
Wastewater Systems
Protection strategies that
are
undesirable
or
impracti-
cal in
varying degrees
for
wastewater systems include:
• Air
chambers.
The use of air
chambers
is
highly
controversial,
but in any
situation,
it is
always
a
means
of
last resort. Opponents state that
grease
will
enter
the
tank with each surge event,
and
even-
tually
the
grease will harden
and can
clog piping,
thus eliminating
the
supposed
protection.
Some
operators have stated that they immediately close
valves
to
remove surge tanks
from
the
system
because
of
grief with them
in the
past. Other opera-
tors have
not
experienced
the
above
difficulties
and
have
not
been upset over surge tanks
in
their pump-
ing
stations.
A few
have been enthusiastic, because
the
tanks
effectively
controlled surges.
On the
other
hand,
many have complained about
the
mainte-
nance required
for the
compressors, level controls,
pressure
relief valves,
and
grease
accumulations
on
sight
gauges. Proponents also claim that running
a
stream
of
fresh
water (with,
of
course,
proper
back-
flow
prevention) into
the
tank keeps
out the
waste-
water,
although opponents reiterate that,
as
grease
floats
and
surges introduce wastewater anyway,
grease will accumulate
and may
eventually prevent
proper functioning
and
expose
the
designer
to
law-
suits.
In a
water-short situation,
a
steady
flow of
fresh
water into
a
surge tank
at a
rate needed
to
purge
the
tank
effectively
may be
unacceptable.
•
Air-
vacuum
valves.
The
protection
these
valves
afford
is
most uncertain,
and
their life-cycle cost
is
exorbitant.
But
because their
first
cost
is the
lowest
of
all, they
may
sometimes
be
demanded
by the
owner.
If
they
are to be
used
despite
these
objec-
tions,
the
owner should
be
warned
in the
prelimi-
nary
design report
and in the O&M
manual that
even
the
best
of
these valves will require
a
high
level
of
maintenance
—
a
minimum
of
servicing
Figure
7-2. Vacuum relief (air
inlet)
valve scheme
for
sewage service
(a)
Plan;
(b)
Section A-A.
monthly
—
wherein
the
best maintenance
is
replacement
of the
valve's interior mechanism
and
cleaning
the
replaced parts
in the
shop. Some
engineers insist
on
placing such valves
in
pairs
so
that
if one
fails,
the
other might still protect
the
pipeline.
Recommended
Options
for
Water
Systems
All of the
recommended strategies
for
wastewater also
apply
to
water
systems.
Additional options
for
water
systems
include:
• The use of air
chambers
or
standpipes (see Table
7-1).
• The use of one or
more
air
release
and
vacuum
relief
valves
—
an
inexpensive
first-cost
solution
but
one in
which inspection
and
cleaning
is
required
twice
per
year
as a
minimum
One
major
pipeline
failed
because
in 20
years
of
service,
the air
release
and
vacuum relief valves
had
never been main-
tained. Recognize that
the
failure
of any
means
for
protecting
a
pipe
can
lead
to
utter disaster.
Means
for
coping with transients
are
discussed
in
detail
in
Section 7-6.
Power
Failure
Power
failure
will occur several
to
perhaps many
times,
so
plan carefully
for it.
Power
failure
at a
pumping
station results
in an
initial rapid downsurge
in
the
discharge header
and
piping close
to the
pump-
ing
station.
The
change
in
head that occurs
in the
pump
discharge piping
after
power failure
(or
pump
shut-off)
is
shown
in
Figure 6-8.
The
following meth-
ods can be
used
to
reduce
the
magnitude
of a
down-
surge.
•
Increase
the
inertia
of
pump
and
motor
by
adding
a
flywheel.
Engines have higher moments
of
inertia
than
do
electric motors,
and it is
easy
to
increase
the
size
of the
flywheel.
Flywheels control downsurge
and
can
prevent column separation (which
is
espe-
cially
important
in
sewage systems), whereas con-
trol valves (which control upsurge) cannot.
•
Install
an air
chamber (see Figure 7-3) near
the
pumping
station and, perhaps,
at
other points
as
well.
Check valves near
air
chambers must
be
fast
acting
but
slamproof because
the
short water col-
umn
between
the air
chamber
and the
check valve
can
quickly accelerate.
Column
Separation
The
most serious consequence
of
downsurge
is
col-
umn
separation, which must always
be
avoided.
It is
more likely
to
occur
if
there
are
knees
in the
pipeline
(as
in
Figure 6-8)
and is
almost certain
to
occur
at a
high point. Column separation
can
also occur
at
other
places along
the
pipe.
The
location
and
extent depends
on
the
relative relationship between
the
dynamic head,
the
total head,
and the
profile
of the
pipeline (review
Section 6-7).
A
quick
way to
determine whether col-
umn
separation
is
likely
is
shown
in
Figure
7-1.
If the
mirror image
of the
hydraulic gradient intersects
the
pipeline profile, column separation will occur. Col-
umn
separation
may
also occur
as
depicted
in
Figure
6-8. Various means
for
preventing column separation
in
raw
sewage force mains include (but
are not
limited
to) the
following:
•
Relocate
the
pumping station
and the
associated
pipeline
to
avoid knees
or
high points.
•
Reroute
the
pipeline
or
bury
the
pipe deeper
to
achieve
a flat
gradient near
the
pump
so
that succes-
sive
hydraulic grade lines
(as in
Figure 6-8)
do not
intersect
the
pipe.
•
Install one-way
air
valves
or,
preferably,
a
check
valve
to
admit
air
into
the
pipeline
on the
down-
surge
and to
trap
the air on the
upsurge.
• Add a flywheel
sized
to
prevent
the
column separa-
tion.
In
addition
to
these
four
methods, column separa-
tion
in
water transmission
pipelines
can be
prevented
by
the
following means:
•
Install open surge tanks (standpipes)
at
knees
or
high
points
if the
required height
is not
excessive.
The
stored water
can
control both high
and low
pressures (see Figure
7-4 and
refer
to
Chapter
10 in
Walters
[3]).
• If the
hydraulic gradeline (HGL)
is too
high
for an
open surge tank,
a
one-way (Figure 7-5)
or a
two-
way
(Figure 7-6) surge tank
can be
used
to
keep
the
HGL
above
the
pipe.
A
check valve allows
flow
from
the
tank
to the
pipe
but not
from
the
pipe
to
the
tank.
•
Install
air
valves, more specifically called "vacuum
relief
and air
release valves"
or
"combination
air
valves" (Figure 5-16), that admit
air
into
the
pipe
and
thereby prevent
a
vacuum. Upon
the
following
upsurge,
the air
must
be
exhausted slowly enough
to
prevent overpressure when
the two
water col-
umns
meet.
•
Install
an air
chamber (pneumatic surge tank). More
than
one
might
be
required
for
very long
pipelines.
Except
for
pipeline rerouting
and the
flywheel,
all
of
these methods require routine maintenance, which
cannot
be
guaranteed
and is not
always provided.
Start-Up
Pump start-up
can
cause
a
rapid increase
in fluid
velocity that
may
result
in an
undesirable surge,
but
usually
it is not a
problem unless
the
type number
of
the
pump exceeds about
135
(specific speed exceeds
approximately
7000).
(Refer
to
Figure
10-16
for
type
number
and
specific
speed.) Methods
for
controlling
such
surges,
in
order
of
increasing cost,
are as
follows:
• If
there
are
several pumps, start them
one at a
time
at
intervals
from
four
to ten
times
the
critical period
(t
c
=
2L/d).
•
Program
a
pump-control valve
to
open slowly
(again
from
four
to ten
times
t
c
)
after
the
motor
starts. Pump motors typically reach
full
speed
within
1 to 2 s
after energization
(see
Figure
7-7).
• Use a
variable-speed drive
for
each pump ramped
up to
full
speed slowly enough
(from
four
to ten
times
t
c
)
to
avoid high surges.
• Add an air
chamber (hydropneumatic tank),
but
only
if
quick-acting, slam-resistant check valves
are
used.
Figure
7-3. Horizontal
air
chamber
for
clean water
service,
(a) End
elevation;
(b)
Side
elevation.
Figure
7-4.
Open-end
surge tank
or
standpipe.
Figure
7-5.
One-way
surge tank.
Shut-Down
Normal pump shut-down
may
also cause surges. They
can
be
controlled
to
remain within acceptable bounds
by
the
following methods:
•
Turn pumps
off one at a
time
at
intervals
from
four
to
ten
times
t
c
.
•
Program
a
pump-control valve
to
close
slowly
(four
to
ten
times
t
c
)
before
the
motor
is
stopped.
• If
pumps
are
equipped
with
variable-speed drives,
ramp down slowly.
•
Increase
the
inertia
of the
motor
and
pump unit
so
that
it
coasts
to a
stop over
a
longer interval.
• Add an air
chamber.
The air
chamber should
not be
needed
for
normal pump shut-down,
but if one is
installed
for
other reasons,
it
would
be an
effective
control method.
Figure
7-6.
Two-way
surge
tank.
Check
Valve
Slam
If
the
pumping station
is
small,
or if
there
are
several
duty
pumps,
the
sequencing
of the
pump shut-down
is
usually
adequate
to
prevent valve slam. However, when
the
last pump shuts down,
or
when power
fails,
the
liq-
uid
comes
to
rest
and
then reverses.
An
unrestrained
check valve disc, seating
after
the fluid flow
reverses,
closes
with
a
resounding
and
disconcerting slam that
may
shake
the
building.
The
slam
may or may not be
accompanied
by a
significant
hydraulic surge.
If
slam
does occur, there
are
several remedies that may,
depending
on
circumstances, mitigate
the
problem.
•
Ensure quick closure
of the
valve
before
the flow
can
reverse
to
cause
a
slam
•
Ensure slow, gentle seating
of the
disc (especially
during
the
last
5% or so of
closure)
by
adding
an
oil-filled
dashpot
to an
outside lever attached
to the
disc axle
•
Substitute
a
pump-control valve that cannot slam.
It
must
be
equipped
with
a
stored energy closure
mechanism
to
ensure operation when power
fails
•
Equip
the
check valve with
an
oil-filled dashpot
to
allow
the
valve
to
close
quickly
but to
seat gently
• Use
rubber-cushioned
flapper
seats
to
prevent
metal-to-metal
contact
and to
cushion
the
closure
•
Install
a
pressure-actuated relief valve (see Figure 7-8).
For a
more extensive discussion, refer
to
"Slam"
in
Section 5-4.
Choosing
Check
Valves
Choosing
the
right check valve
is
vital.
Often
there
are
profound
differences
in the
same kind
and
style
of
valve
offered
by
different
manufacturers. Headlosses
Figure
7-8.
Pressure-actuated
surge
relief
valve.
in
some makes
of
swing check valves
are
twice
as
great
as in
others,
and the
massiveness
of
stressed
parts varies widely.
Swing check valves should always
be
equipped with
external levers, which
are
useful
in
several ways.
The
position
of the
lever indicates whether
flow is
occur-
ring,
and the
lever
can be
equipped with
an
inexpensive
switch that shuts
off
power
to the
motor
(after
a
timed
delay)
if flow
does
not
occur.
The
lever
can be
equipped
with
either springs
or
counterweights, which
can be
adjusted
to
mitigate slam
or
disc
flutter.
Dashpots
can
also
be
added
to
control slam.
But
whether counter-
weights, springs, dashpots,
or
pump-control valves
are
used,
it is
mandatory that
the
valves
be
properly
adjusted
in the
start-up procedure
and
equally impor-
tant that operators understand
the
valve operation
and
practice
the
necessary preventive maintenance.
Because many contrary opinions prevail, designers
should
be
extremely careful
in
specifying check
valves. Investigate them thoroughly
by
obtaining
and
analyzing
advice
from
several sources.
Filling
Empty Pipelines
Small
air
bubbles, which
collect
at
summits
of
pipe-
lines,
can be
bled
off
with
air
release valves without
creating hydraulic transients. However,
the
initial
fill-
ing
of
pipelines must
be
done cautiously
with
veloci-
ties kept below
0.3 m/s
(1
ft/s)
and
with
air
release
valves
open
to
exhaust
the air
slowly. Avoid
full-
capacity start-ups until
all of the
large
air
bubbles
are
exhausted.
Provide
properly
sized
air and
vacuum
valves
or
slow-acting pump-control valves
for
pumps
with
long discharge columns. These measures com-
plete
air
evacuation without sudden slamming
of
valves, which creates high transient pressures. Always
include
the
start-up procedure
for
empty pipelines
in
the
O&M
manual.
Pipelines that
are
nearly
flat may
require
air
release
valves spaced
at
intervals
of
approximately
400 to
Figure 7-7.
Pump-control-valve
system.
To
ensure
the
highest
available
operating
pressure,
take
the
control water
supply
from
both
sides
of the
check valve. Provide separate
speed-control
needle
valves
on
each side
of the
double-
sided
diaphragm
to
control
the
water flowrate
out of the
diaphragm actuator.
800
m
(V
4
to
V
2
mi) to
vent
air
properly during
filling.
Knees
and
high points require both
air and
vacuum
valves.
At
best,
air
pockets
in
pipelines increase
flow
resistance
by as
much
as 10% or
even more.
At
worst,
air
pockets
can
generate pressures
as
high
as ten
times
normal operating pressures,
and air
trapped
at
high
points reduces
the
water cross-sectional area and,
thus, acts
like
a
restriction
in the
pipe.
Air
release cocks
or
valves must
be
installed
in
pump
casings
(or at
high points
of
pump manifolds)
to
prevent
air
binding.
If the HGL can
fall
much below
any
part
of the
system
on a
downsurge,
an air and
vac-
uum
valve
is
needed
to
prevent vapor cavities. Limit
the
vacuum
to
about half
of an
atmosphere,
and
exhaust
the air
slowly
so
that
it
acts
as a
cushion.
Alternatively,
especially
for raw
sewage, reroute
the
pipeline
to
produce
a
uniform
or,
better,
an
increasing
gradient with
no
knees.
7-2. Control
of
Pumps
None
of the
methods
for
controlling water hammer
is
universally
applicable. Some methods might control
one
cause
of
water hammer
but
leave
the
system
unprotected
from
other causes. Some methods
may be
unacceptable
for a
variety
of
reasons, such
as
exces-
sive
maintenance
or
unreliability. Because
a
single
device
is
often
inadequate, several must
be
used
to
provide
full
protection. Several schemes
for
control-
ling pumps
can be
used
to
limit surges during start-up
and
shut-down. Some methods
offer
limited control
of
surges
due to
power failure.
Pump
Sequencing
By
controlling
the
sequence
of
pump start-up
and
shut-down
so
that
the
starting
and
stopping
of
several
pumps
are
staggered,
the
magnitude
of
change
in flow
at
any one
time
is
reduced
—
often
to
acceptable
levels
for
normal operation. Sequencing
is
automatic,
reli-
able,
and
inexpensive.
Pump-Control
Valves
By
interlocking
the
pump with control valves
in its
discharge piping, transient problems caused
by
pump
start-up
and
shut-down
can be
greatly reduced.
The
control valves
are set to
open
and
close
slowly
(4 to
10
times
t
c
).
Upon start-up,
the
pump
operates
against
a
closed valve.
As the
valve opens,
the flow
into
the
pipeline gradually increases
to the
full
pump capacity.
Upon shut-down,
the
control valve slowly
closes
to
decelerate
the flow,
after
which power
to the
pump
is
shut
off
(but
not
until
the
valve
is
fully
closed).
To
circumvent power
failure,
the
valves should
be
operated
by a
stored-energy auxiliary power source
such
as
trickle-charged batteries
for
electric
systems
or a
compressed
air
tank (with enough capacity
to
operate every valve through
two
cycles)
and
either
a
water
or an oil
valve actuator. Ball valves
are
excellent
for
either water
or
sewage service.
For
water service,
butterfly
valves
can be
used,
but
they have poor throt-
tling
characteristics when slow closing
is
required
to
control head rise following
a
power failure.
Diaphragm-actuated
globe
valves operating
off
pipe-
line pressure with
a
check valve feature
can
also
be
used.
For
sewage service,
eccentric
plug valves
can be
used. Pump-control valves, however, cannot prevent
downsurge
on
power failure.
Increasing
the
Rotational
Inertia
The
moment
of
inertia
of the
pumping system
has an
important
effect
upon hydraulic transients
in the
pip-
ing
downstream
from
a
pumping
station.
Upon pump
power failure,
the
pump speed
and
head rapidly
decrease
and a
negative pressure wave,
or
downsurge,
is
propagated down
the
pipeline.
The
greater
the
moment
of
inertia,
the
slower this
decrease
in
head
and
pump
speed
and the
lower
the
magnitude
of the
resulting downsurge becomes. Thus, adding
a fly-
wheel
increases
the
moment
of
inertia
of the
system
and
slows
the
decrease
in
pump speed
and
head.
A fly-
wheel
can be
added
to an
electric
motor
by
extending
the
housing
or
frame
(usually toward
the
pump)
to
enclose
the flywheel and to
support bearings above
and
below
the flywheel.
When
the
conditions
are
suit-
able, high inertia
is the
most reliable method
of all for
control
of
surge
in
both normal operations
and
power
failure,
and, except
for
maintaining
the
bearings,
it is
maintenance
free.
An
alternative
is to use an
engine.
It
is
easy
to
enlarge
the flywheel, and
engines inherently
take longer
to
come
to a
stop (even when starved
for
fuel)
than
do
motors.
Variable-Speed
Drives
A
variable-speed drive that
can be
ramped
up or
down
slowly
has
several
advantages
for
controlling tran-
sients during normal operation:
(1)
enhancement
of
motor
life
due to
infrequent starts and,
for
adjustable
frequency
drives,
low
inrush current;
(2) flicker of
lights (annoying
to
nearby residents) caused
by
inrush