Which sequence correctly describes priming a diesel fire pump that is not yet primed?

• A. Start engine, engage priming system, open priming valve, observe suction gauge rise and prime completion, close priming valve, then open discharge lines gradually.
• B. Start engine, open discharge lines, observe gauge rise, then engage priming system.
• C. Start engine, stop if gauge doesn't rise, wait, then restart.
• D. Start engine, never engage priming valve; rely on atmospheric.

Answer: (A)

Priming a diesel fire pump that isn’t primed requires filling the suction side with water so the pump can develop suction instead of pulling air. The priming system pushes water into the suction line; as air is displaced, the suction gauge rises. When that gauge shows a stable rise indicating the suction line and pump are filled with water, you close the priming valve to hold the prime. With the prime established, you then open the discharge lines gradually to bring the system online smoothly and build pressure without a sudden surge.

Opening discharge lines before priming would not allow the suction to fill properly and the pump could cavitate or stall. If the suction gauge doesn’t rise, you wouldn’t proceed by assuming it’ll fix itself—you’d troubleshoot the priming path. Relying on atmospheric pressure to prime is not effective for a diesel pump, since it won’t reliably fill the suction and establish suction.