What causes water hammer in a piping system?

• A. Water coming through the valves
• B. Water moving at velocities of over 5 feet per second
• C. Water coming directly from the main to a valve
• D. Water moving at velocities of 5 feet per second or less

Answer: (B)

Water hammer is caused by the inertia of moving water. When water is flowing and a valve or pump is suddenly closed, the moving water can’t stop instantly. That momentum is converted into a rapid pressure increase that travels through the pipe as a surge. The faster the water is moving, the bigger that surge can be, so velocities over about five feet per second are enough to cause a noticeable water-hammer effect. This is why rapid valve closures or pump trips at high flow rates lead to banging noises, pressure spikes, and potential damage in piping systems.

So the best way to understand it is that the abrupt change in velocity of a high-speed water column creates a pressure wave—not just water simply passing through valves or coming from the main. When the flow is slower (five feet per second or less), the resulting surge is much less significant.