Dosing Pump Maintenance and Troubleshooting: Diaphragms, Vapour Lock, Calibration

A large share of service calls logged as "the pump is not dosing" have nothing to do with the pump body. Air leaks on the suction side, a blocked foot valve strainer, crystallised valve balls and lost back pressure are the four dominant root causes. A dosing pump can be electrically perfect and still deliver zero flow because of any one of them.

Diagnosing in the right order saves hours. Start with what you can see: chemical level, hose stiffness, staining under the head and visible bubbles in the suction line. Then measure. If the stroke counter is running but the delivered volume is low, the problem sits on the hydraulic side. Board or solenoid faults account for a small fraction of service calls and belong at the end of the list, not the start.

Build the maintenance plan in three layers: a weekly visual check, a quarterly calibration verification and an annual wear kit. All three belong in one logbook. Without recording the chemical, ambient temperature and daily running hours, there is no way to see which part is wearing out early and why.

A diaphragm is a planned wear part, not a failure item. For PTFE-faced diaphragms, the interval accepted in practice is 12 months or 4,000-8,000 running hours, with hot ambients, concentrated acids and continuous full-load duty pushing that towards the lower end. Output usually drops by 10 to 20 percent before a diaphragm actually ruptures, and that drop is the natural trigger for planned replacement.

Valve cartridges are the second critical item. Ceramic or glass balls and their seats should be changed together, typically every 12 to 24 months. Replacing the ball and reusing the seat is a short-lived repair, since both surfaces wear as a pair. Head screws must be tightened in a cross pattern to the manufacturer's torque and re-torqued after the first 24 hours, because PTFE gaskets cold-flow and loosen slightly on day one.

Pump technology reshapes the whole wear map. Among solenoid, peristaltic and motor-driven diaphragm dosing pumps, the peristaltic type has a single wear part, the hose, with a life typically limited to 500-1,500 running hours, but it never suffers valve ball or priming problems. Spare part budget belongs in the technology decision, not after it.

Vapour lock happens when a gas bubble trapped in the dosing head absorbs the stroke. Liquid is incompressible, so every diaphragm movement displaces chemical; with gas in the head, the stroke merely compresses and expands the bubble and nothing leaves the line. Sodium hypochlorite, hydrogen peroxide and some polymer solutions are the classic offenders.

Hypochlorite degrades faster as temperature and concentration rise, and a 12-15 percent solution gases noticeably above 25 °C. When you commission a sodium hypochlorite dosing system for drinking water, keeping tank temperature below 25 °C, shading the tank and turning stock over within 30 days attacks the problem at source rather than at the pump.

The durable fix is hydraulic layout. Place the pump head below the tank liquid level for flooded suction, keep the suction hose under 2 metres on a continuously rising run and remove the U-bends that trap gas. For gassing chemicals, specify a dosing head with automatic air bleed. When turning the output down, leave stroke length at 100 percent and reduce frequency instead; below about 30 percent stroke length the pump struggles to sweep gas out of the head and repeatability suffers.

The suction side is the most neglected part of a dosing installation. On solenoid metering pumps, suction lift is practically limited to 1.5-2 metres, and with gassing chemicals that figure should be brought to zero, meaning flooded suction. Hose bore must never be smaller than the pump connection, and should be stepped up one size as the run gets longer.

The foot valve at the end of the line is effectively a check valve: it holds liquid in the suction line when the pump stops so that prime does not have to be re-established at every start. It must hang vertically, 50 to 100 mm above the tank floor. A foot valve resting on the bottom draws sediment and blocks its strainer within weeks.

Air leaks are invisible but expensive. One loose union on the suction side fills the line with an air-liquid mixture, and dosed volume then swings unpredictably. Do not over-tighten unions beyond hand-tight plus the specified turn, and renew the conical seals at every diaphragm change. Using transparent suction tubing lets an operator see bubbles directly and cuts diagnosis time sharply.

A diaphragm metering pump is a positive displacement device, but it stops behaving like one when discharge pressure falls below suction pressure. You need at least 1 bar of differential between the two. Dosing into an open channel, a vented tank or the suction of another pump creates a siphon, and chemical keeps flowing even while the metering pump is stopped.

The remedy has two parts. Fit a back-pressure valve on the discharge set 1 to 2 bar above line pressure, and a spring-loaded injection valve at the injection point. Injection valve springs usually crack somewhere between 0.5 and 1.5 bar depending on the model, so a low-spring type cannot stop siphoning on its own and is no substitute for a back-pressure valve. The two components work together, not instead of each other.

The opposite error is just as common. If the pump is rated at 10 bar and line pressure climbs to 12 bar, output collapses, the relief valve opens and chemical returns to the tank. To fix the deviation permanently, redo the flow and back-pressure calculation for the pump and select a pressure class that leaves at least 20 percent margin over the actual line pressure.

Calibration measures what actually leaves the line, not what the pump display claims. The method is simple: move the suction hose from the chemical tank into a graduated calibration cylinder, run the pump at real operating pressure and its real setting, and divide the level drop by elapsed time. Each run should last at least 60 seconds or 100 strokes, repeated three times and averaged.

Set a clear acceptance criterion. If measured output is within 5 percent of the setpoint, the system is healthy. A deviation between 5 and 10 percent can be trimmed out by adjustment. Anything beyond 10 percent is a fault rather than a setting issue, and should be investigated in order: vapour lock, worn valve balls, tired diaphragm, lost back pressure.

Repeat calibration quarterly, and additionally after every change of chemical or concentration, every wear part replacement and every modification that alters line pressure. Temperature and viscosity affect the result too, so the same setting can deliver differently at 5 °C in winter and 35 °C in summer. Wear chemical-appropriate gloves, goggles and a face shield, and never break the cylinder connection before depressurising the line.

Wetted parts come in four groups: dosing head, diaphragm, seals and valve balls. Head material is usually PP, PVDF, PVC or stainless steel; the diaphragm face is PTFE; seals are FKM or EPDM; balls are ceramic, glass or PTFE. If any one of the four is wrong for the chemical, the maintenance interval loses all meaning and parts fail well before half their planned life.

The warning signs are easy to recognise. Swollen, softened seals point to the wrong elastomer; a discoloured or dull head surface indicates chemical attack on the plastic; a white crust in the ball seat means crystallisation. With chemicals that precipitate calcium, such as sodium hypochlorite, flushing the suction line with clean water once a week noticeably delays that build-up.

The table below is a starting point only. The final decision comes from the manufacturer's compatibility chart read against concentration and temperature together, and above 40 °C those values deserve one grade of extra conservatism. A 10 percent and a 50 percent solution of the same chemical can call for different materials. PVDF handles most acids well but is not recommended for concentrated or hot caustic, where PP is the safer head material.

Spare planning is a comparison between downtime cost and stock cost. On lines that cannot stop, such as chlorination or pH control, holding one wear kit per pump is the right call. Plants running 3 or 4 identical pumps with the same head material can usually share a single kit, while non-critical auxiliary duties are covered by a planned annual order.

A standard wear kit contains the diaphragm, suction and discharge valve assemblies with balls and seats, O-rings, head gaskets and the injection valve spring. Add a metre of spare suction and discharge tubing, a foot valve strainer and correctly sized union seals. Store kits labelled with the pump model and head material code from the nameplate, keeping different head materials in separate bins.

Vanera matches head material to the chemical for SEKO dosing pumps, lists wear kit contents by pump model and supports field calibration and commissioning. Share the chemical, concentration, flow rate, line pressure and daily running hours and you will get a kit and interval recommendation back.