During rainfall, the wet well inflow increases and the system shows rapid cycling; lag pump motor shaft turns backwards when off. What is the most likely valve problem?

During rainfall, the wet well inflow increases and the system shows rapid cycling; lag pump motor shaft turns backwards when off. What is the most likely valve problem?

• A. Check valve on lag pump is clogged
• B. Lag pump check valve stuck closed
• C. Lead pump check valve stuck closed
• D. Check valve on lead pump clogged

Answer: (A)

This question hinges on how backflow through a discharge check valve can drive a pump when it should be idle. In a two-pump lift station, check valves on each pump prevent water from flowing back into the pump when it’s off. If the lag pump’s discharge check valve is clogged, it may not seal properly. Water in the discharge line can push backward through the lag pump and into the impeller while the pump is supposed to be stopped. That backflow can cause the lag pump shaft to turn backward when the motor is off, which matches the observation. The result is unstable operation and rapid cycling as the system keeps trying to balance the rising wet-well level with ineffective discharge. If the lag pump’s check valve were simply stuck closed, backflow would be prevented and you wouldn’t expect reverse rotation of the lag pump. Issues with the lead pump valve would affect the lead pump’s operation, not explain the lag pump turning backward. So the most likely problem is the lag pump’s discharge check valve being clogged, allowing backflow and backward rotation when off.

This question hinges on how backflow through a discharge check valve can drive a pump when it should be idle. In a two-pump lift station, check valves on each pump prevent water from flowing back into the pump when it’s off. If the lag pump’s discharge check valve is clogged, it may not seal properly. Water in the discharge line can push backward through the lag pump and into the impeller while the pump is supposed to be stopped. That backflow can cause the lag pump shaft to turn backward when the motor is off, which matches the observation. The result is unstable operation and rapid cycling as the system keeps trying to balance the rising wet-well level with ineffective discharge.

If the lag pump’s check valve were simply stuck closed, backflow would be prevented and you wouldn’t expect reverse rotation of the lag pump. Issues with the lead pump valve would affect the lead pump’s operation, not explain the lag pump turning backward. So the most likely problem is the lag pump’s discharge check valve being clogged, allowing backflow and backward rotation when off.