EI 11C201
CEMP-E
1 March 1997
where:
Tc
= critical time in seconds
L
= length of force main between point of flow change and point of discharge
a
= velocity of pressure wave.
When flow is completely stopped (Q = O) in a time interval greater than T c, the maximum
theoretical pressure increase is not fully developed. However, when flow is stopped in a time
interval less than or equal to Tc, the change is said to be instantaneous, and the maximum
pressure increase is develop as
given below.
c. Maximum pressure increase. The maximum theoretical pressure increase or surge caused
by water hammer is calculated from the following:
hw
=
aV/g
where:
hw
= pressure increase in meters (feet)
V
= velocity of fluid in the pipeline prior to flow change in meters (feet) per second
g
a
= velocity of pressure wave in meters (feet) per second
Some typical values of a are given in table 7-3 below.
Table 7-3. Water Hammer Wave Velocities.
Pipe Material
a, m/sec (ft/sec)
*
Asbestos-cement
820-1040 (2700-3400)
*
Ductile iron
940-1280 (3100-4200)
*
Steel
820-1190 (2700-3900)
*
Concrete
1010-1160 (3300-3800)
*
Plastic
340-460
(1100-1500)
*
370-490
(1200-1600)
*
d. Methods of control. Whenever a pump is shut down, or power to the station fails, the
pump motor is suddenly cut off. Pump speed along with flow and velocity in the force main are
quickly decelerated by pressure waves, which travel up the pipeline and back in accordance with
Newton's second law of motion. When the velocity is reduced to zero, reverse flow through the
pump would occur if a gravity operated check valve or an automatic control valve were not
7-10
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