As the impeller wears on a pump, the pump efficiency will

• A. decrease
• B. improve
• C. increase
• D. not change

Answer: (A)

Worn impellers reduce the pump’s ability to transfer energy to the fluid, so efficiency falls. As the impeller wears, its effective diameter gets smaller and blade surfaces lose sharpness, increasing leakage around the blades and through the clearance between the impeller and the housing. This leakage and disturbed flow lower the head and the amount of fluid energy the impeller can impart for a given flow. Since pump efficiency is the ratio of hydraulic power out to shaft power in, the energy transferred to the fluid drops while input power may stay the same or rise due to added losses, causing overall efficiency to decrease. So, as the impeller wears, the pump becomes less efficient.