Which option represents the official cause of water hammer when a voided line column rejoining occurs?

Which option represents the official cause of water hammer when a voided line column rejoining occurs?

• 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 comes from a rapid change in the velocity of the liquid in a pipe, which sets up a pressure wave because the fluid has inertia and resists sudden motion changes. In a voided line that rejoins, the collapsing air or vapor pocket causes the water column to abruptly change its motion, producing a sharp pressure spike. The option that best represents the fundamental idea is the general statement describing the cause of water hammer itself, since it points to the underlying mechanism—the rapid momentum change of the liquid—rather than a specific triggering event. The other choices describe particular situations that can trigger or accompany a surge, but they do not capture the core cause.

Water hammer comes from a rapid change in the velocity of the liquid in a pipe, which sets up a pressure wave because the fluid has inertia and resists sudden motion changes. In a voided line that rejoins, the collapsing air or vapor pocket causes the water column to abruptly change its motion, producing a sharp pressure spike. The option that best represents the fundamental idea is the general statement describing the cause of water hammer itself, since it points to the underlying mechanism—the rapid momentum change of the liquid—rather than a specific triggering event. The other choices describe particular situations that can trigger or accompany a surge, but they do not capture the core cause.