What causes water hammer?

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

Answer: (A)

Water hammer is a pressure surge that happens when a flowing column of water is suddenly stopped or its speed is rapidly reduced. The driving factor is momentum: moving water carries momentum, and when a valve closes abruptly or flow is interrupted, that momentum has to be absorbed somehow, creating a rapid, high-pressure wave that travels through the pipe.

The bigger the flow velocity, the greater the momentum, so high-velocity water (over a certain speed) produces a much larger pressure spike. That’s why the statement about water moving at velocities over 5 feet per second best explains water hammer—the risk and severity rise with faster flow.

If the water is moving slowly, a sudden stop still creates a pressure rise, but it’s much smaller and often negligible. Describing water simply as coming from the main or through valves doesn’t pin down the trigger or magnitude of the surge—the crucial idea is the abrupt change in velocity of high-velocity flow. To reduce risk, use slow-closing valves and consider surge protection devices like air chambers or surge tanks.