Reliable chemical dosing is fundamental to water treatment plant performance. This guide covers the recurring failure modes for diaphragm, peristaltic, and progressive cavity pumps with practical corrective actions.
Symptoms: Pump running, counter incrementing, but no chemical flow confirmed by lack of tank level change; possibly audible cavitation noise; pressure gauge on discharge shows near-zero.
Root cause: Air locked in suction line or pump head; foot valve failed or missing; suction lift excessive (>3 m for most diaphragm pumps); chemical too viscous for self-priming; check valve stuck open.
Vent the pump head and suction line manually — open vent screw if fitted, or disconnect suction briefly to allow air to escape. Verify foot valve is present and operational (for suction-lift installations). Reduce suction lift by repositioning pump closer to chemical tank. If chemical is high-viscosity (polymer solution, lime slurry), increase suction pipe diameter. Check discharge check valve — if stuck open, prime is lost on every stroke.
Symptoms: Chemical leaking from pressure relief vent or pump head joint; loss of dose rate; pump head filling with water from behind diaphragm; diaphragm visible as torn or perforated.
Root cause: Chemical attack by incompatible reagent (e.g., strong oxidant attacking PTFE-composite diaphragm); mechanical fatigue from over-pressurisation; abrasive particles in chemical (lime, ferric) scoring diaphragm surface; diaphragm incorrectly seated at last maintenance.
Replace diaphragm immediately — continued operation risks chemical release. Identify diaphragm material compatibility with your chemical: PTFE for most acids and oxidants; EPDM for caustic; Viton for solvents. If fatigue failure, check back-pressure — must not exceed pump max discharge pressure; install back-pressure valve if system pressure is variable. For lime or ferric dosing, use heavy-duty diaphragms rated for abrasive service; replace at 6-month intervals as preventive measure.
Symptoms: Dose rate erratic — sometimes over-dosing, sometimes under-dosing despite constant stroke setting; calibration check shows variable volume per stroke; audible ‘chattering’ from valve.
Root cause: Particle deposits (calcium carbonate, ferric hydroxide, polymer gel) preventing check valves from seating fully; worn ball or seat allowing reverse flow; valve installed in wrong orientation.
Remove and inspect both suction and discharge check valves. Soak in 5% citric acid (lime, carbonate deposits) or 10% HCl (iron deposits) for 30 minutes. Inspect ball and seat for pitting or scoring — replace both if worn. Ensure ball is free-floating and seats correctly. For polymer dosing, use oversize valves (DN10 minimum) with ball-check design — flap valves block with polymer gel. Flush check valves with clean water after any extended shutdown.
Symptoms: Chemical spill from peristaltic pump housing; audible change in pump sound (slapping or irregular); dose rate drops suddenly; tube inspection reveals cracking, flattening, or pinhole.
Root cause: Incorrect tube material for chemical (e.g., natural rubber with strong oxidants); roller pressure too high causing mechanical fatigue; running dry — no chemical in tube causing excessive heat; chemical temperature above tube rating.
Replace tube immediately with correct material: Marprene or Viton for strong acids and oxidants; natural rubber for mild chemicals only. Verify tube pressure rating exceeds system back-pressure. Adjust roller clearance per manufacturer specification — excessive squeeze dramatically reduces tube life. Never run peristaltic pumps dry — install low-level float switch on chemical tank to stop pump before emptying. Track tube change intervals — standard Marprene life: 500–2,000 hours depending on chemical.
Symptoms: Flow rate below setpoint despite constant speed; increased power draw; audible high-pitched squeal; pump hot to touch.
Root cause: Stator (elastomer) worn by abrasive solids in chemical (lime slurry, polymer with grit); running dry — single most damaging condition for PC pumps; incorrect stator hardness for chemical temperature; suction inlet blocked.
Stop pump immediately if running dry — even 30 seconds dry running can destroy a stator. Install dry-run protection sensor on suction (pressure or flow). For lime slurry dosing, specify hardened stator (NBR 70 Shore A minimum) and replace at 6-month intervals. Ensure suction piping is generously sized (minimum DN50 for lime slurry). For hot chemicals, verify stator elastomer is rated for your maximum temperature.
Symptoms: Process performance changing despite unchanged dose setpoint; chemical tank depleting faster or slower than expected; ratio controller showing unexpected chemical:flow ratio.
Root cause: Diaphragm fatigue reducing effective stroke volume; check valve wear allowing backflow; pump speed drift on variable-speed units; chemical density change affecting flow meter reading.
Perform manual calibration check: collect output into graduated cylinder for 60 strokes — compare to expected volume per stroke × 60. For peristaltic: measure actual rpm with tachometer. For all types: verify chemical concentration has not changed (affects mass dose even if volume is correct). Recalibrate at minimum monthly intervals for critical chemical dosing (coagulant, disinfectant). Install totalising flow meter on discharge line for continuous calibration verification.
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