At a pumping station equipped with centrifugal pumps, what caused the discharge pressure to suddenly increase and the discharge quantity to suddenly decrease?

At a pumping station equipped with centrifugal pumps, what caused the discharge pressure to suddenly increase and the discharge quantity to suddenly decrease?

• A. A discharge valve was closed
• B. A suction valve was closed
• C. The pump amperage was decreased
• D. The voltage suddenly increased

Answer: (A)

Throttling the discharge line changes the head against which a centrifugal pump must operate, which in turn alters the flow it can deliver. When the discharge valve is closed, back pressure in the discharge system rises. The pump has to push against this higher head, so its discharge pressure increases while the flow falls, since higher head reduces the amount of fluid the pump can move at a given speed. This combination—higher discharge pressure with lower discharge flow—is the typical response to closing a discharge valve. Closing a suction valve would starve the pump and generally reduce both flow and discharge pressure. Decreasing the amperage reduces the motor’s available torque, usually lowering both flow and pressure instead of increasing them. Increasing voltage could alter motor speed and potentially affect flow, but it doesn’t produce the simultaneous rise in discharge pressure and drop in discharge quantity described here.

Throttling the discharge line changes the head against which a centrifugal pump must operate, which in turn alters the flow it can deliver. When the discharge valve is closed, back pressure in the discharge system rises. The pump has to push against this higher head, so its discharge pressure increases while the flow falls, since higher head reduces the amount of fluid the pump can move at a given speed. This combination—higher discharge pressure with lower discharge flow—is the typical response to closing a discharge valve.

Closing a suction valve would starve the pump and generally reduce both flow and discharge pressure. Decreasing the amperage reduces the motor’s available torque, usually lowering both flow and pressure instead of increasing them. Increasing voltage could alter motor speed and potentially affect flow, but it doesn’t produce the simultaneous rise in discharge pressure and drop in discharge quantity described here.