TM 5-814-2/AFM 88-11, Vol. 2
to the point of change. The time required for this
to control and limit these surge phenomena, the
roundtrip is given by:
following practices will be followed.
Tc = 2L/a
(1) Gravity check valves. For simple cases
where
involving small to medium sized pump stations with
Tc =
critical time in seconds
gradually rising force mains (no intermediate high
L
= length of force main between
points) of less than 1000 feet in length, and with static
point of flow change and point of
discharge heads of less than 50 feet, a gravity operated
discharge in feet, and
check valve will usually be sufficient. Gravity type
a
= velocity of pressure wave in feet
check valves may be either swing checks utilizing
per second
outside lever and weight (or spring) set to assist closure,
When flow is completely stopped (Q = O) in a time
or then may be ball checks. Swing check valves are
interval greater than Tc, the maximum theoretical
usually installed horizontally, while ball check valves
pressure increase is not fully developed. However,
may be either vertical or horizontal. For additional
when flow is stopped in a time interval less than or equal
protection, a pressure relief valve may be installed in
to Tc, the change is said to be instantaneous, and the
conjunction with check valves to allow reversing flow to
maximum pressure increase is developed as given
reenter the wet wall. Pressure relief valves must be
below.
specially designed for sewage applications. As an
c. Maximum pressure increase. The maximum
alternative to relief valves, a hydro-pneumatic tank may
be utilized.
theoretical pressure increase or surge caused by water
(2) Automatic control valves. In situations
hammer is calculated from the following:
where long force mains are required, pipe profiles must
hw = aV/g
conform to existing ground elevations for economic
where
reasons. This normally will result in high points in the
hw =
pressure increase in feet
force main, with the possibility of water column
V = velocity of fluid in the pipeline
separation at the high points in the force main, with the
prior to flow change in feet per
possibility of water column separation at the high points
second
during pump shutdown or power failure. The pressures
g
= acceleration due to gravity, or
generated when these separated columns come to rest
32.2 ft/sec2 at sea level, and
against closed valves or against stagnant columns may
a
= velocity of pressure wave in feet
be large, and are again determined by paragraph c
per second
above. In general, where force mains are greater than
Some typical values of a are given in table 5-3 below.
1000 feet in length or contain intermediate high points,
Table 5-3. Water hammer wave velocities.
and where pumping stations are large in capacity, or
Pipe Material
a, ft/sec
static discharge heads are greater than 50 feet, control
Asbestos-cement
2700--400
valves will be automatically operated (1) cone, (2) plug,
Ductile iron
3100--4200
(3) ball, or (4) butterfly valves. Normal operation of
Steel
2700--3900
these valves upon pump shutdown, is to slowly close the
Concrete
3300--3800
valve while the pump continues to run. When the valve
Plastic
1100--1500
is closed, a limit switch then stops the pump motor. On
1200--1600
power failure, an emergency hydraulic or other type
d. Methods of control. Whenever a pump is shut
operator closes the valve slowly. The time of valve
down, or power to the station fails, the pump motor is
closure is of utmost importance. Valves should be half
suddenly cut off. Pump speed along with flow and
closed when the velocity in the force main has dropped
velocity in the force main are quickly decelerated by
to zero. The time required to reach zero velocity can be
pressure waves, which travel up the pipeline and back in
calculated with the following formula:
accordance with Newton's second law of motion. When
t = LV/g Hav
the velocity is reduced to zero, reverse flow through the
where
pump would occur if a gravity operated check valve or
t
= time in seconds
an automatic control valve were not installed on the
L
= length of force main in feet
pump discharge line, and did not close properly.
V = velocity of fluid in pipeline in feet
Reverse flow fully accelerated through the pump could
per second, and
cause transient flows and pressures well above
Hav = average
decelerating
head
maximum design conditions. A swing check valve
including pipe friction in feet
which stuck open temporarily, and then slammed shut
under these conditions, would re5-8 suit in a large
pressure surge as given by paragraph c above. In order
5-8
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