Which of the following is a recommended method to prevent water hammer when starting large-capacity pumps?

• A. If possible, reduce system flow prior to tripping large-capacity pumps.
• B. Start with discharge valve fully open.
• C. Venting should be ignored.
• D. Initiate flow with full throttling.

Answer: (A)

Water hammer is a pressure surge that occurs when fluid momentum changes rapidly, such as when a large pump starts and suddenly accelerates water through a long piping system. The bigger the flow and the faster the acceleration, the larger the pressure spike that can develop and potentially damage pipes and valves.

Reducing system flow before starting large-capacity pumps lowers the velocity and inertia of the fluid that the pump must accelerate. With less flow to overcome at start-up, the acceleration head—and thus the transient pressure rise—is smaller, making the incident less severe and less likely to cause damage. This proactive step directly mitigates the primary cause of water hammer during pump re-start.

Discharging fully open would allow a large, rapid fill into the system, increasing the surge rather than reducing it. Ignoring venting can leave air pockets that cause separate hammer effects, and starting with full throttling, while helpful in some cases, is not as reliable or direct a control as pre-start flow reduction.