Question 68 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C **Explanation of Correct Option (C):** The primary purpose of a lead-lag pump arrangement in a potable water system is to optimize efficiency, manage demand fluctuations, and ensure redundancy. * **Lead Pump Function:** The designated "lead" pump handles the normal, day-to-day fluctuations in water demand (e.g., standard usage, small spikes). During these periods of relatively low to moderate demand, the lead pump cycles on and off as needed to maintain the required system pressure. * **Lag Pump Function:** The "lag" pump serves as a backup or supplementary pump. It remains idle (lagging) until the system pressure drops significantly, indicating that the lead pump cannot meet the current high demand (e.g., peak usage periods, fire system activation). The lag pump then automatically starts to assist the lead pump, ensuring adequate pressure and flow. Once the high demand subsides, the lag pump shuts off, and the lead pump resumes sole operation. This configuration minimizes unnecessary wear on the second pump and saves energy. **Why Other Options Are Incorrect:** * **A) Enabling the lead pump to pump against a shut-off head during periods of relatively low demand and the lag pump to recirculate when the demand is high.** * This is incorrect. Pumps should be prevented from operating against a "shut-off head" (zero flow), as this causes overheating and damage. Recirculation (pumping water back to the source) is sometimes used for cooling or minimum flow requirements, but it is not the primary purpose of the lead-lag arrangement, nor is it typically done by the lag pump when demand is high. * **B) Enabling both potable water pumps to cycle on and off together in response to system demand changes.** * This is incorrect. If both pumps cycled together, it would defeat the purpose of staged operation, resulting in excessive energy consumption during low demand periods and equal wear on both pumps, reducing the overall lifespan efficiency benefit of the lead-lag system. * **D) Enabling the lag pump to cycle on and off during periods of relatively low demand and the lead pump to assist the lag pump only when the demand is high.** * This is incorrect because it reverses the roles. By definition, the **lead** pump handles the normal, low-demand cycling, and the **lag** pump only engages during high-demand or emergency situations.