Given the lift station data below, what is the most likely cause of the problem? Wet well inlet is normal for dry weather flow. Wet well is alternating between excessively high & low levels. Lead pump starts at right level, level continues to rise, pump stops at right level. Lead pump check valve arm remains stationary in lowered position when pump starts and stops. Lag pump check valve arm rises when lag pump starts and lowers when it stops. Force main pressure remains the same when lead pump runs, but increases when lag pump runs. Level drops when lag pump runs. Low-level pressure switch normal. High-level pressure switch normal. Electrical controls are all in automatic.

Given the lift station data below, what is the most likely cause of the problem? Wet well inlet is normal for dry weather flow. Wet well is alternating between excessively high & low levels. Lead pump starts at right level, level continues to rise, pump stops at right level. Lead pump check valve arm remains stationary in lowered position when pump starts and stops. Lag pump check valve arm rises when lag pump starts and lowers when it stops. Force main pressure remains the same when lead pump runs, but increases when lag pump runs. Level drops when lag pump runs. Low-level pressure switch normal. High-level pressure switch normal. Electrical controls are all in automatic.

• A. lead pump clogged
• B. force main pressure too high
• C. lag pump is air-bound
• D. both pumps are clogged

Answer: (A)

Start with how two pumps should behave in a lift station: when a pump is delivering water into the force main, the discharge check valve opens and the force main pressure responds, while the wet well level falls as water is pumped out. In this scenario the lag pump is clearly delivering: its check valve arm rises when it starts and lowers when it stops, the force main pressure increases when it runs, and the wet well level drops while it operates. That shows the lag pump is effectively moving water to the force main. The lead pump, however, tells a different story. The wet well level continues to rise while the lead pump runs, and its check valve arm stays in the lowered position, not moving as the pump starts or stops. A check valve that remains stationary in the lowered position indicates the discharge path is not opening, so water isn’t entering the force main despite the pump running. This pattern points to a blockage or restriction in the lead pump’s discharge path—most likely the pump itself being clogged (impeller or discharge line). Other possibilities don’t fit as well: if the lag pump were air-bound, it wouldn’t move water effectively and the force main pressure wouldn’t rise with its operation; if both pumps were clogged, the lag pump wouldn’t be able to raise pressure either; the observed pressure increase with the lag pump running confirms it is delivering, not blocked.

Start with how two pumps should behave in a lift station: when a pump is delivering water into the force main, the discharge check valve opens and the force main pressure responds, while the wet well level falls as water is pumped out.

In this scenario the lag pump is clearly delivering: its check valve arm rises when it starts and lowers when it stops, the force main pressure increases when it runs, and the wet well level drops while it operates. That shows the lag pump is effectively moving water to the force main.

The lead pump, however, tells a different story. The wet well level continues to rise while the lead pump runs, and its check valve arm stays in the lowered position, not moving as the pump starts or stops. A check valve that remains stationary in the lowered position indicates the discharge path is not opening, so water isn’t entering the force main despite the pump running. This pattern points to a blockage or restriction in the lead pump’s discharge path—most likely the pump itself being clogged (impeller or discharge line).

Other possibilities don’t fit as well: if the lag pump were air-bound, it wouldn’t move water effectively and the force main pressure wouldn’t rise with its operation; if both pumps were clogged, the lag pump wouldn’t be able to raise pressure either; the observed pressure increase with the lag pump running confirms it is delivering, not blocked.