Which phenomenon results from rapid valve closure that creates a pressure surge in piping?

• A. Cavitation
• B. Water hammer
• C. Thermal expansion
• D. Surge tank

Answer: (B)

When a valve closes rapidly, the moving water wants to keep flowing, so the flow cannot stop abruptly and a sudden high pressure builds up upstream. This rapid pressure rise is called water hammer. It can cause loud banging and potential damage to pipes and fittings, especially if valves are slammed shut or the system lacks dampening.

Cavitation isn’t the right description here because it involves pressure dropping below the liquid’s vapor pressure leading to vapor bubble formation, not a surge of pressure from stopping flow. Thermal expansion refers to volume increase from heating, not the effect of quickly stopping flow. A surge tank is a device used to dampen or absorb these surges, not the phenomenon itself.