Dosing Pump Calibration and Maintenance: Small Pumps, Big Consequences

Dosing pumps are among the smallest machines in a water treatment plant, often fitted on a simple stand next to a chemical tank. Yet they control some of the most critical processes: antiscalant to protect RO membranes, sodium metabisulphite to remove chlorine, coagulants for clarification, hypochlorite for disinfection and acid or alkali for pH correction. When a dosing pump drifts out of calibration, the consequences can be expensive and sometimes hidden until damage is done.

Why Calibration Drifts

A dosing pump’s actual output can differ from its setting for many reasons:

• Wear of diaphragms, valves and seals
• Air locking in the suction or discharge line
• Changes in back pressure in the injection line
• Changes in chemical viscosity with temperature or concentration
• Partially blocked foot valves or strainers

A pump set at a given percentage of its stroke and speed may deliver noticeably less than expected, and the operator may never know without checking.

How to Calibrate a Dosing Pump

The most practical method uses a calibration column, a graduated transparent cylinder installed on the suction side of the pump.

Step by Step

• Fill the calibration column from the chemical tank
• Isolate the tank so the pump draws only from the column
• Run the pump at its normal setting for a measured time, such as one minute
• Read the volume drawn from the column
• Calculate the actual dosing rate in litres per hour
• Compare it with the required rate and adjust stroke length or frequency
• Repeat until the actual output matches the target

If no calibration column is installed, the bucket-and-stopwatch method can be used, but it is less convenient and less safe with hazardous chemicals. Installing calibration columns on every critical dosing pump is a worthwhile investment.

Calculating the Required Dose

Before calibrating, you must know the target. The required chemical feed rate depends on:

• The desired dose in mg/l at the injection point
• The water flow rate
• The concentration of the chemical solution in the tank

For example, if a plant treats a known flow and needs a specific antiscalant dose, the operator can calculate how many litres per hour of diluted solution are required. When dilution ratios or flow rates change, the pump setting must be recalculated.

Routine Maintenance Checklist

Daily

• Confirm the pump is running and the chemical level in the tank is adequate
• Look for leaks at fittings and the pump head
• Check that the injection point is not blocked

Weekly

• Verify output using the calibration column
• Inspect suction strainers and foot valves
• Check for air bubbles in suction lines

Periodic

• Replace diaphragms, valve balls and seals according to manufacturer recommendations
• Clean injection valves, which can scale or clog
• Inspect tubing for hardening or cracking, especially with oxidising chemicals

Safety During Maintenance

Dosing pumps handle hazardous chemicals. Before maintenance, isolate the pump, relieve pressure and flush the head with water where appropriate. Operators should wear goggles, gloves and protective clothing. Chemical tanks should be clearly labelled.

Linking Dosing to Process Control

Where flow varies, a fixed-rate dosing pump overdoses at low flow and underdoses at high flow. Flow-paced dosing, using a signal from a flow meter, keeps the dose proportional. For pH or chlorine, closed-loop control with an online analyser adjusts dosing automatically.

Keep Records

Record calibration results, chemical consumption and maintenance activities. Comparing chemical consumption with water treated is an excellent cross-check. If chemical use falls without any process change, the pump may be failing.

Conclusion

Dosing pumps are small, but they have a large influence on plant performance. Regular calibration, simple maintenance and good records ensure each chemical is dosed exactly as intended, protecting equipment, saving chemicals and keeping water quality consistent.

Need help with your water or wastewater project? Aguapuro Equipments designs, supplies and services water treatment, effluent treatment and bottling solutions. Send us an enquiry or contact our team to discuss your requirements.