Filling of a voided line (column rejoining) is identified as a cause of water hammer. Which option represents the official cause?

• 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 when moving fluid is forced to change velocity suddenly, creating a pressure wave that travels through the pipe. The official way this is categorized in the question is the broad statement “Cause of Water Hammer,” since the phenomenon is defined by the momentum change and the resulting pressure surge, not by any single triggering event. Specific triggers like rapid valve actuation, filling of a voided line, or hot water entering a low‑pressure area can all produce that surge, but they’re different ways the same underlying effect can occur. So the best representation of the official cause is the general label for the cause of water hammer.