8.6 Lift Station Electrical Devices, Level Detection, Telemetry, Gas Detection & Capacity Adjustment
Key Takeaways
- Single phasing keeps a three-phase motor running while current spikes on the remaining legs, so phase-loss relays and three-leg amperage checks are standard protection.
- A robust lift station uses a continuous level device for control plus independent float switches for high-level alarm and backup, so one fouled sensor cannot cause an overflow.
- A communication-failure alarm at the SCADA end is essential, because a silent station looks identical to a station with nothing to report.
- Cycle time equals fill time plus draw-down time, where fill time is usable volume divided by inflow and draw-down time is usable volume divided by pump rate minus inflow.
- Slowing a pump with a VFD below the speed that maintains roughly 2 feet per second in the force main allows grit and grease to settle out and restrict the main.
The 18 percent content area
Lift Station Operation and Maintenance is 18 percent of the WPI Wastewater Collection Operator Class I exam, and the outline splits it into three device families plus one control task: ensuring electrical devices (fuses, motors, relays, starters) function; ensuring electronic devices (alarms, controllers, gas detection, level detection, telemetry RTUs, SCADA, PLCs, control devices) function; ensuring mechanical devices (piping, pressure relief valves, compressors, water heaters, chemical addition, pumps, valves, wet wells, vacuum valves, force mains, air relief, seals, air exchangers and exhaust fans, bar screens) function; and adjusting equipment such as SCADA, VFD, PLC, and control panel to increase or decrease pumping capacity for proper flow.
The electrical chain
Power flows through a predictable chain, and troubleshooting follows it in order:
Utility service → main disconnect → transfer switch (if standby power) → motor control center bus → circuit breaker or fused disconnect → starter → overload relay → motor
| Device | Function | Failure signature |
|---|---|---|
| Fuses | One-time overcurrent protection | A single blown fuse on a three-phase circuit causes single phasing |
| Circuit breaker | Resettable overcurrent and short-circuit protection | Trips on overload; repeated trips mean a real fault, not a nuisance |
| Motor starter (contactor) | Switches motor power on command | Pitted or welded contacts; chattering from low control voltage |
| Overload relay | Thermal or electronic protection against sustained overcurrent | Trips on a bound pump, high head, or single phasing |
| Control transformer | Steps voltage down for the control circuit | Failure kills the control circuit while power remains on the load side |
| Relays and contactors | Logic and switching | Sticking, coil failure |
| Motor | Converts electrical to mechanical energy | Insulation breakdown, bearing failure, winding short |
Single phasing deserves emphasis. If one of three phases is lost — a blown fuse, a broken utility conductor, a loose lug — a running three-phase motor keeps turning but draws greatly increased current on the remaining phases and overheats rapidly. It will not restart if stopped. Phase monitors and phase-loss relays are installed to shut the motor down, and reading amperage on all three legs is standard practice for exactly this reason.
Variable frequency drives vary motor speed by varying frequency and voltage. In a lift station they allow the pump to match inflow rather than cycling on and off, which reduces starts, smooths flow to the treatment plant, and can keep velocity in the force main above the scouring minimum. VFD-specific issues include harmonics, the need for inverter-duty motors, shaft currents requiring grounding rings, and heat in the enclosure — VFD cabinets need working cooling.
Level detection
Level control determines when pumps start and stop, and level failure is the leading cause of both dry running and overflows.
| Device | Principle | Notes |
|---|---|---|
| Float switches | Tilt or mercury switch on a tethered float | Simple, cheap, reliable; fouled by grease and rags |
| Submersible pressure transducer | Hydrostatic pressure at the sensor | Continuous level signal; needs a vented cable; fouls |
| Bubbler | Air pressure required to bubble from a fixed tube | Tolerant of debris; needs a compressor |
| Ultrasonic | Time of flight to the liquid surface | Non-contact, but confused by foam, grease buildup on the transducer face, and turbulence |
| Radar | Microwave time of flight | Tolerant of foam and vapor; more expensive |
| Conductance probes | Electrode contact with liquid | Coats with grease |
A robust station uses a continuous level device for control plus independent float switches for high-level alarm and pump backup. That redundancy is what prevents a single fouled sensor from causing an overflow. A high-high float wired directly to the alarm dialer, bypassing the PLC, is cheap insurance.
Alarms, telemetry, and gas detection
- Telemetry by radio, cellular, or fiber links the station's RTU or PLC to SCADA. Alarm points that matter: high wet well level, pump failure or fail-to-start, power failure, phase loss, intrusion, and communication failure. A communication-failure alarm at the SCADA end is essential, because a station that goes silent looks identical to a station with nothing to report.
- Autodialers provide a backup path for critical alarms independent of SCADA.
- Gas detection. Lift station wet wells generate hydrogen sulfide and, where industrial or illicit discharges occur, methane and flammable vapors. Fixed detection in dry wells and enclosed structures, plus personal four-gas monitors for anyone approaching an opening, are standard. Detection must alarm both locally and remotely.
- Ventilation. Wet wells and dry wells have exhaust fans and air exchangers, and the WPI outline lists them explicitly. Continuous ventilation of a dry well and forced ventilation before and during any wet well entry are basic.
Adjusting pumping capacity
The listed task is "adjust equipment (SCADA, VFD, PLC, control panel) to increase or decrease pumping capacity for proper flow." In practice:
- Raise or lower start and stop setpoints to change the working volume and therefore the cycle time.
- Change lead, lag, and standby assignments and enable alternation so pumps share runtime.
- Adjust VFD speed setpoints or the level-to-speed control curve so the station matches inflow.
- Verify minimum velocity in the force main. Slowing a pump too far drops force main velocity below the roughly 2 ft/s needed to keep solids moving, and the main begins to accumulate grit and grease. This is the most common unintended consequence of adding a VFD to an existing station.
Worked example of cycle time. A wet well has a usable volume between start and stop setpoints of 900 gallons. Inflow is 60 gpm and the pump delivers 220 gpm.
- Fill time = 900 ÷ 60 = 15.0 minutes
- Draw-down time = 900 ÷ (220 − 60) = 900 ÷ 160 = 5.6 minutes
- Cycle time = 15.0 + 5.6 = 20.6 minutes, giving about 2.9 starts per hour
That is comfortably within typical motor limits. If the usable volume were only 300 gallons, cycle time would fall to 6.9 minutes and starts would rise to nearly 9 per hour, which shortens motor life.
A three-phase lift station pump motor continues running but trips its overload relay after a few minutes, and a check finds greatly elevated current on two legs and none on the third. What has occurred?
A lift station wet well has a usable volume of 1,200 gallons between the start and stop setpoints. Inflow is 80 gpm and the pump delivers 260 gpm. What is the cycle time?
What is the most common unintended consequence of slowing a lift station pump too far with a variable frequency drive?