the velocity of the wave, which is the speed of sound). In a 1,000-ft pipe, the wave can make a complete round trip in less than one half second.
The pressure created by this shock wave is directly proportional to both the wave velocity and the velocity of the water flowing in the pipe. Although the equation below does not take into account the effect of pipe length, diameter and elasticity, it will provide some insight as to the additional pressure created by a water hammer pressure wave.
P(additional) = aV / 2.31g
P is the additional pressure the shock wave creates, a is wave velocity, V is the velocity of the flowing water in the pipe in feet per second, g is the universal gravitational constant @ 32-ft/sec2 and 2.31 is the pressure conversion constant. At a pipeline velocity of 5-ft/sec², the additional pressure created by the shock wave is approximately 328-psi. Increasing that velocity to 10-ft/sec increases the additional pressure to about 657-psi. Obviously, systems that are not designed to accommodate such an increased pressure are often damaged or even destroyed.
Next month we will explore the three major causes of water hammer and the factors that contribute to the magnitude of the shock wave it generates. We will also see why water hammer can be more damaging in low pressure systems.
