Which option best represents the official cause of water hammer in voided line scenarios?

Which option best represents the official cause of water hammer in voided line scenarios?

• A. Cause of Water Hammer
• B. Rapid valve actuation (valve slam)
• C. Filling of a voided line (column rejoining)
• D. Hot water entering a low pressure line (low pressure discharge)

Answer: (A)

Water hammer happens because the liquid’s velocity is changed very quickly, and the moving water resists that abrupt change due to its inertia. That resistance forces a pressure rise in the pipe, launching a pressure wave through the system. In a voided line, the presence of an air or vapor pocket means the moving water has to push on or compress that gas as the flow changes, which can amplify the resulting pressure surge. So the fundamental premise is the abrupt change in fluid velocity that creates a propagating pressure wave—the essence of water hammer. The other options describe specific triggers or scenarios (such as valve slam, rejoining of a water column, or hot water flashing) but they are instances that lead to the same pressure surge, not the general mechanism itself.

Water hammer happens because the liquid’s velocity is changed very quickly, and the moving water resists that abrupt change due to its inertia. That resistance forces a pressure rise in the pipe, launching a pressure wave through the system. In a voided line, the presence of an air or vapor pocket means the moving water has to push on or compress that gas as the flow changes, which can amplify the resulting pressure surge. So the fundamental premise is the abrupt change in fluid velocity that creates a propagating pressure wave—the essence of water hammer. The other options describe specific triggers or scenarios (such as valve slam, rejoining of a water column, or hot water flashing) but they are instances that lead to the same pressure surge, not the general mechanism itself.