Question 59 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is A. ### Why Option A is Correct Option A, "close off the discharge valve, and watch for a rise in pressure," describes the simplest and most common field method for determining the basic health and maximum head capability of a centrifugal pump. This process is known as determining the **Shutoff Head** (or Dead Head). * **Procedure:** By closing the discharge valve completely, the flow is stopped (zero GPM). The pressure gauge on the pump discharge should then display the maximum pressure the impeller can generate at the current speed. * **Verification:** If the measured shutoff head matches the pressure specified on the pump's performance curve (or significantly exceeds the normal operating pressure), it confirms that the pump's impeller is intact, the speed is correct, and the pump is capable of achieving its design head (pressure) rating. A low shutoff head strongly suggests internal issues like a worn or damaged impeller, incorrect speed, or severe internal recirculation/wear ring failure. ### Why the Other Options Are Incorrect **B) momentarily close off the suction valve, and watch for a rise in pressure:** This procedure is highly risky and inaccurate for checking performance. Momentarily closing the suction valve risks severe damage due to **cavitation** (since flow is suddenly restricted while the impeller is turning) and **starvation**. While it might show a momentary drop or instability in pressure, it does not provide a definitive, standardized shutoff head value required for performance verification, unlike closing the discharge valve. **C) use a clamp-on ammeter and compare the readings to past records:** Monitoring motor current (amperage) is an excellent method for continuous monitoring and detecting gradual wear or impending failure (Predictive Maintenance). However, measuring shutoff head (Option A) is a more direct, simpler, and universally accepted method for explicitly determining the pump's current hydraulic capability (head) compared to its design specification. Amperage primarily reflects the load (power consumed), which is related to flow and efficiency, but not necessarily the maximum pressure-generating ability of the pump. **D) closely observe the pump discharge temperature:** Discharge temperature is a lagging indicator. While excessive temperature rise can indicate very low or zero flow operation (recirculation) or inefficient operation, it is not the standard or simplest primary metric used to confirm the pump's ability to achieve its design head specification. A pump can be operating poorly (e.g., worn impeller producing low head) without exhibiting a significant immediate temperature change, especially in large systems.