312 CHAPTER TWELVE
WATER HAMMER
Water hammer is a change in pressure, either above or below the normal pres-
sure, caused by a variation of the flow rate in a pipe.
The equation for the velocity of a wave in a pipe is
(12.63)
whereUvelocity of pressure wave along pipe, ft /s (m/s)
Emodulus of elasticity of water, 43.2 106 lb/ft^2 (2.07 106 kPa)
density of water, 1.94 lbs/ft^4 (specific weight divided by accelera-
tion due to gravity)
Ddiameter of pipe, ft (m)
Epmodulus of elasticity of pipe material, lb/ft^2 (kg/m^2 )
tthickness of pipe wall, ft (m)
PIPE STRESSES PERPENDICULAR
TO THE LONGITUDINAL AXIS
The stresses acting perpendicular to the longitudinal axis of a pipe are caused
by either internal or external pressures on the pipe walls.
Internal pressure creates a stress commonly called hoop tension. It may be
calculated by taking a free-body diagram of a 1-in (25.4-mm)-long strip of
pipe cut by a vertical plane through the longitudinal axis (Fig. 12.11). The
forces in the vertical direction cancel out. The sum of the forces in the hori-
zontal direction is
pD 2 F (12.64)U
B
E
B
1
1 ED/EptFF
D
F F
FIGURE 12.11 Internal pipe pressure produces
hoop tension.