Question 57 AEL01 - Assistant Engineer - Limited

The Correct Answer is D **Explanation for Option D (Excessive feedwater temperature) being correct:** Excessive feedwater temperature is a primary cause for a centrifugal feed pump to lose suction. This is because high temperatures lower the Net Positive Suction Head Available ($NPSH_A$). When water is heated, its vapor pressure increases significantly. If the feedwater temperature rises high enough—particularly if it approaches or exceeds the saturation temperature for the pressure at the pump eye—the water will flash into steam (vaporize) inside the low-pressure zones of the pump impeller. This phenomenon is known as cavitation. Steam bubbles entering the pump interrupt the smooth flow of liquid, cause noise and vibration, and critically, displace the liquid, leading to a catastrophic loss of pumping capacity (the pump "loses suction"). **Explanation of why the other options are incorrect:** * **A) Increased suction head pressure:** Increased suction head pressure ($NPSH_A$) improves the pump's ability to maintain suction and prevents cavitation. This condition would make the pump *less* likely to lose suction. * **B) Decreased feedwater temperature:** Decreased temperature lowers the water's vapor pressure. This increases the Net Positive Suction Head Available ($NPSH_A$), making it easier for the pump to handle the flow without flashing. This condition would make the pump *less* likely to lose suction. * **C) Pump recirculating line being open too much:** An open recirculating (minimum flow) line helps cool the pump and ensures sufficient flow through the pump when the boiler demand is low (preventing overheating and internal damage). While excessive recirculation can reduce system efficiency, it generally helps maintain stable suction conditions and does not directly cause a loss of suction. (Note: A pump running against a completely closed discharge valve without a minimum flow line *would* overheat and eventually lose suction due to vaporization, but opening the minimum flow line mitigates this risk.)