Pump types, curves, cavitation, short cycling, clogging, overheating, and troubleshooting sequence

Pump Types and Where They Fit

Municipal wastewater lift stations most often use submersible centrifugal pumps: the motor and pump sit in the wet well, the pump slides down guide rails, and the discharge connection seals automatically against a rail base elbow. Dry-pit (dry-well) centrifugal pumps are used at larger or older stations; they keep the motor out of the sewage but require a separate dry well and closer attention to suction conditions and flooding.

Grinder pumps shred solids and feed low-pressure (pressure-sewer) systems, usually serving single homes or small clusters; they are not the normal choice for a large gravity-fed municipal station. Self-priming pumps mount above the wet well and re-prime after each cycle, which makes them convenient to service but vulnerable to losing prime if the priming chamber leaks.

For solids handling, the impeller style matters. A non-clog (semi-open or enclosed) impeller passes a rated sphere — often 3 inches in municipal service — so wipes and rags travel through rather than wrap the vanes. A vortex (recessed) impeller sits back in the volute and spins the liquid so most solids never touch the vanes; it clogs less but is less efficient. Chopper/grinder impellers cut solids before discharge. When a pump that has run clean for years suddenly clogs daily, suspect a change in the influent (a new restaurant adding FOG, ragging from wipes) before condemning the pump itself.

Curves, Head, and the Operating Point

A centrifugal pump does not deliver one fixed flow. It operates where the pump curve (head it can produce at each flow) intersects the system curve (head the piping demands at each flow). Memorize the head components:

A partially closed discharge valve, grease buildup, air pocket, or blocked force main raises the system curve and reduces flow (the operating point slides up and left). A worn impeller or wrong rotation lowers the pump curve. A variable frequency drive (VFD) reduces speed and, by the affinity laws, drops flow proportionally, head with the square, and power with the cube of speed — the reason VFDs save energy at part-load.

Cavitation, Clogging, and Overheating

Cavitation occurs when the absolute pressure at the impeller eye falls below the vapor pressure, so vapor bubbles form and then collapse violently. It happens when net positive suction head available (NPSHa) drops below the pump's required value (NPSHr). Operators hear a rattling, gravel-in-the-pump sound, see vibration, and measure reduced flow, while the impeller pits over time. Causes include low wet well level, vortexing, clogged or restricted suction, excessive suction lift, and operation far off BEP.

Clogging is the everyday wastewater failure. Rags, wipes, grease, grit, and debris jam the impeller, volute, check valve, or suction opening. Telltales: high amperage, low flow, poor drawdown, vibration, repeated overload trips, or a pump that sounds loaded but does not move the level.

Overheating can come from rapid cycling, clogged cooling passages, low submergence (a submersible relies on sewage for cooling), blocked flow, bad bearings, failed seals, voltage imbalance, or running against a closed valve. A motor that overheats after repeated short cycles may have no hydraulic fault at all — the control range is simply too narrow.

Troubleshooting Sequence

Work the steps in order so you do not jump to the wrong (often most expensive) repair:

1. Make the site safe — traffic control, electrical hazards, confined-space boundaries, lockout/tagout.
2. Verify the actual wet well level and whether overflow is imminent.
3. Check power — phase loss, breaker, motor starter, overloads, hand/off/auto (HOA) position, control power.
4. Confirm level devices and alarms — floats free, transducer plausible, alarm investigated not merely reset.
5. Check valves and hydraulics — suction path, discharge isolation valve, check valve, discharge pressure, air-release points, force main restriction.
6. Compare pump data — amperage, run time, starts per hour, vibration, temperature, drawdown rate, historical trend.
7. Pull or isolate equipment only after the station is protected by the second pump, standby power, storage, vacuum truck, or bypass.
8. Document cause, action, runtime changes, alarm times, and follow-up maintenance.

Exam trap: do not pick the costliest mechanical repair before confirming control status, valve position, actual level, and power. Most pump station failures are control, clogging, or valve problems long before they are motor failures.

Reading Electrical and Hydraulic Clues Together

Amperage is the cheapest diagnostic gauge in the station. High amps with low flow usually means a clog, a partially closed discharge valve, or a worn bearing dragging the shaft. Low amps with low flow points the other way — an air-bound pump, a broken impeller, or a pump running on a worn-out (flat) curve. Amps swinging up and down suggest cavitation, vortexing at low submergence, or surging from a trapped air pocket in the force main. Compare the reading against the nameplate full-load amps (FLA); sustained operation above FLA trips the overload, and chronic overload trips are a symptom, not the disease.

Match electrical clues to drawdown: if the well drops at its normal rate, the pump is moving design flow regardless of the noise; if it barely moves, trust the level, not the running light. This pairing — amperage plus measured drawdown — separates a true hydraulic restriction from a mere control or sensor fault and is exactly the reasoning the exam rewards.