Why should you avoid operating a pump at shut-off (zero flow) and what happens to the head?

• A. Head is near its maximum; efficiency is very low, torque and heat increase risk of motor overload; cavitation may occur.
• B. Head is near zero and efficiency is at maximum.
• C. Head is independent of flow, and torque remains unchanged.
• D. Head increases with flow while efficiency remains high.

Answer: (A)

When a centrifugal pump runs with no flow, the impeller is still imparting energy to the fluid, but there’s nowhere for the fluid to move, so the pressure builds up inside the pump. On the typical performance curve, as flow approaches zero, the head approaches its maximum value—the shut-off head—because the energy goes into pressurizing the fluid rather than moving it downstream. Since almost no fluid is being delivered, the useful hydraulic power is almost zero (P_h = ρ g Q H with Q near zero), while the motor still has to supply power to spin the pump and overcome internal losses. That means the torque required is high and the shaft power goes into heat in the fluid, bearings, and windage, raising the risk of overheating or motor overload. Cavitation can occur under this condition because the suction-side pressure can drop toward the vapor pressure when flow is very small, allowing vapor bubbles to form and potentially causing damage when they collapse.