Which option best represents the official cause of water hammer when hot water enters a low pressure line?

Which option best represents the official cause of water hammer when hot water enters a low pressure line?

• 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)

The key idea is that water hammer in this scenario comes from a vapor or void in the liquid column and the rapid rejoining of that column, creating a high-pressure surge. When hot water enters a low-pressure line, it can flash to steam in the cooler, under-pressurized pipe, forming a vapor pocket. This vapor void disrupts the moving liquid column, and as conditions change (pressure rises and the vapor collapses), the liquid column is forced to fill the void abruptly. The result is a sudden, high-pressure transient—the classic water hammer. So the option that best represents the official cause is the filling of a voided line (column rejoining) because it directly describes the mechanism by which the pressure surge develops after a vapor pocket forms and then collapses. The other possibilities don’t pin down this mechanism: rapid valve actuation explains hammer from a valve event rather than vapor-induced rejoining; the triggering event (hot water entering a low-pressure line) describes the condition but not the mechanism; and a vague “cause of water hammer” is too general to capture the specific process at play here.

The key idea is that water hammer in this scenario comes from a vapor or void in the liquid column and the rapid rejoining of that column, creating a high-pressure surge. When hot water enters a low-pressure line, it can flash to steam in the cooler, under-pressurized pipe, forming a vapor pocket. This vapor void disrupts the moving liquid column, and as conditions change (pressure rises and the vapor collapses), the liquid column is forced to fill the void abruptly. The result is a sudden, high-pressure transient—the classic water hammer.

So the option that best represents the official cause is the filling of a voided line (column rejoining) because it directly describes the mechanism by which the pressure surge develops after a vapor pocket forms and then collapses. The other possibilities don’t pin down this mechanism: rapid valve actuation explains hammer from a valve event rather than vapor-induced rejoining; the triggering event (hot water entering a low-pressure line) describes the condition but not the mechanism; and a vague “cause of water hammer” is too general to capture the specific process at play here.