MIL-HDBK-1022
Tc
=
2L/a = 22,000/3,692 = 1.08 seconds
From the table of values for "a", the surge pressure
wave velocity (a) is 3,692 fps (1125.3 m/s). The maximum
pressure in any pipeline occurs when the total discharge is
stopped in a period of time equal to or less than the critical
time. Since the valve will theoretically close prior to this,
Equation (2) should be used to determine the pressure rise.
In this case, the final velocity (Vo) will be assumed to be
zero because the critical time is greater than the valve
closure time.
P1 - P = (V1wa)/(144g)
= (6.8151.53,692)/(14432.2) = 279 psig (1925
kPa)
P1 = P+273 = 60+279 = 339 psi (2337 kPa)
Initial velocity (V1) was found by dividing the given
flow rate of 600 gpm (38 L/S) by the cross sectional area of
the 6-inch (150 mm) diameter, Schedule 40 pipe.
Considerations will have to be made for this system to deal
with the maximum predicted pressure.
(4) When the valve closure time is longer than
the critical time, the surge will be less than predicted by
Equation (2). The equation used to calculate surge pressure
rise for this situation is:
EQUATION: P1 - P = [2Lw (V1 - VC)]/(CgT1.3)
(3)
where:
P1 = maximum pressure (psig or Pa)
P = pump shutoff pressure (psig or Pa) (equal to
system static pressure)
L = length of pipe (ft or m)
V1 = initial velocity (fps or m/s)
VC = velocity at TC (fps or m/s)
w = specific weight of the fluid (lbm/ft3 or kg/m3)
g = gravitational constant (32.2 ft/s2 or 9.81 m/s2)
C = unit constant (144 in2/ft2, 0.101 (kg/m2)/Pa
a = surge pressure wave velocity (fps or m/s)
T = valve closure time (sec)
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