Question 69 QMED05 - Machinist-Pump Technician

The Correct Answer is D **Explanation for Option D (Correct Answer):** Centrifugal pumps operate on the principle of converting rotational kinetic energy into hydrostatic pressure and flow. When the discharge valve is closed, the pump is operating against a "shut-off head." While the motor is still expending energy to spin the impeller, the fluid (bilge water) is not allowed to leave the casing. This continuous churning of the same body of water within the volute generates significant friction and turbulence. Virtually all the mechanical energy input from the motor that is not converted into pressure is converted into heat, rapidly raising the temperature of the trapped fluid and, consequently, the metallic components of the **pump** casing and impeller, leading to overheating. **Why Options A, B, and C are Incorrect:** * **A) motor overload will open:** While operating a pump against a closed discharge causes the motor to draw less current than at the design point (Best Efficiency Point), it still draws a substantial "shut-off power." If the pump were running for an extremely extended period, the motor *might* eventually overheat (Option B), but the immediate and characteristic consequence of running a centrifugal pump dead-headed is not typically tripping the thermal overload. Centrifugal pumps generally draw the least power at zero flow. * **B) motor will overheat:** Although the motor is working and will generate heat, the power draw at shut-off conditions is usually lower than the maximum power draw (which typically occurs slightly past the design point or at higher flows). Therefore, the immediate and most critical failure mode is the overheating of the *pump* components (impeller, casing, mechanical seals) due to fluid friction, not the instantaneous thermal overload or overheating of the motor. The pump overheats significantly faster and more dangerously. * **C) relief valve will open:** Centrifugal pumps, unlike positive displacement pumps (such as gear or piston pumps), do not require a relief valve installed between the pump and a closed isolation valve. This is because they are inherently non-positive displacement; they simply recirculate the fluid internally when the maximum head is reached. The pressure will rise to the shut-off head, but it will not rise indefinitely to a point that requires a relief valve to protect the pump against casing overpressure.