Wet-Well Level Alarm During a Holiday Weekend
A duplex submersible lift station serves a residential basin of about 1,200 homes. Each pump is rated 350 gpm at the design head; the wet well is 8 feet in diameter with a 10-foot operating depth. The station discharges through 2,400 feet of 8-inch force main to a gravity manhole. It is Friday afternoon before a three-day holiday. The SCADA high-level alarm has just annunciated. The on-call operator arrives and finds: both pumps showing "run" on the controller, wet well level rising slowly, discharge pressure gauge reading lower than normal, and no visible overflow yet. The station has a standby generator that is not running; utility power appears normal. The operator's phone shows the alarm history: three high-level alarms in the past 10 days, each clearing on its own.
1. What is the operator's immediate priority order, and why?
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Priority one is preventing an overflow and protecting people, in that order of urgency. The operator should first confirm the actual wet well level against the SCADA reading (float or manual measurement) because a false high-level alarm changes the response. If the level is genuinely high and rising with both pumps running, the operator should attempt to increase throughput and reduce inflow: check that both pumps are actually moving water (valve positions, check valves, air binding), and consider whether a portable pump or vacuum truck can be staged before the well tops out. Simultaneously, the operator should notify the supervisor and begin the spill-contingency notification chain, because a holiday weekend means response resources are thin and the window to act is short. The key judgment is that "both pumps running" plus "low discharge pressure" plus "rising level" points to a downstream restriction or pump wear, not a power problem - so chasing the generator would waste the critical minutes.
2. Interpret the combination of symptoms: both pumps running, low discharge pressure, slowly rising level, and repeated self-clearing high-level alarms. What is the most likely cause and how would you confirm it?
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Both pumps running with low discharge pressure and a still-rising well suggests the pumps are moving some water but not achieving normal head - consistent with worn impellers, a partially blocked or failing check valve, air binding, or a restriction/break in the force main. The repeated self-clearing high-level alarms over 10 days suggest an intermittent condition that worsens with flow, such as a partially obstructed force main or a check valve that hangs. Confirmation steps: read discharge pressure at each pump individually (run one at a time if the wet well allows), compare to the pump curve and the normal operating point; check valve position and stroke; listen and feel for cavitation or air; verify the force main by checking the receiving manhole for reduced or surging flow. If one pump alone holds pressure and the other does not, the fault is in that pump or its check valve. If both are weak, suspect the common force main. The operator should not assume the pumps are fine just because the controller says "run."
3. Given the holiday timing, what supervisory and contingency actions are warranted even if the level stabilizes?
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Even if the level stabilizes, the operator should not leave the station unattended on a three-day weekend with a 10-day pattern of alarms. Appropriate actions: notify the supervisor and document the alarm history and findings; arrange for a portable standby pump or vacuum truck to be available; schedule a force-main and check-valve inspection as soon as practical; and consider setting up more frequent monitoring or a temporary level trend watch. The operator should also verify the standby generator is functional, since a power loss during the holiday with degraded pumping would be far worse. The judgment point is that a self-clearing alarm is not a resolved problem - it is a warning, and the responsible move is to reduce exposure before the long unattended period rather than wait for a failure.