SHORT ANSWER
Size for the complete controllable range, not only the maximum dose.
Calculate minimum, normal and maximum required flow, then define discharge pressure, suction conditions, chemical concentration, temperature, viscosity, required accuracy and control method. Select diaphragm or plunger construction and wetted materials accordingly. Leave reasonable adjustment margin without choosing a pump so large that normal operation sits at the bottom of its range.
Convert the process requirement into a dose range
For water treatment or proportional dosing, chemical flow may be calculated from process flow, target dose and solution strength. Check units carefully and account for active concentration, density and preparation dilution. For batch addition, divide the required volume by the available dosing time and include mixing or reaction constraints.
State all three points: minimum, normal and maximum. A pump selected only for the peak may not provide stable accuracy at the normal point. Also define whether the stated capacity is expected delivered flow or nominal nameplate capacity.
Discharge pressure must exceed the real system requirement
Include downstream vessel pressure, static elevation, injector loss, piping friction, check valves, filters and any control valve. Provide minimum and maximum pressure because a changing system affects delivered volume and check-valve behavior. Confirm the pump's permissible pressure at the selected capacity and stroke or speed.
Suction conditions also matter. Keep suction lift low, pipework short and airtight, and line size adequate for viscosity. Gas-forming chemicals can cause loss of prime; a flooded suction, degassing head or suitable liquid-end arrangement may be needed.
Match the complete liquid end to the chemical
Provide chemical name, concentration, normal and maximum temperature, density, viscosity, vapor behavior and any solids. Review head, diaphragm or plunger, check balls, seats, seals and tubing. A compatible pump head with an incompatible elastomer is still an incorrect selection.
| Duty factor | What to specify |
|---|---|
| Corrosive chemical | Exact composition, concentration, temperature and cleaning fluid. |
| Viscous additive | Viscosity at operating temperature, suction length and tank level. |
| Off-gassing liquid | Vapor tendency, storage temperature and preference for flooded suction or degassing head. |
| Slurry or crystals | Particle size, concentration, settling and flushing method; standard check valves may not suit. |
Define manual, pulse or analog control before ordering
A basic mechanical pump may use manual stroke-length and speed adjustment. Automated dosing may need 4–20 mA, pulse input, frequency control, remote start/stop or fieldbus integration. State fail-safe behavior, required turndown, signal isolation and whether local manual override is needed.
Accuracy claims require context. Repeatability depends on stable suction, discharge pressure, correct priming, valve cleanliness and calibration. Install a calibration column or use a measured drawdown method to verify delivered flow under site conditions.
Accessories are part of the metering system
- Back-pressure valve: stabilizes low or varying discharge pressure and helps prevent siphoning.
- Pressure-relief valve: protects the pump and line from a blocked discharge.
- Pulsation dampener: reduces pressure ripple that can disturb instruments and injection quality.
- Foot valve and strainer: support priming and keep large debris out of the check valves.
- Injection valve: provides a defined injection point and helps prevent reverse flow.
- Calibration column: enables direct verification of delivered capacity.
Frequently asked questions
How much sizing margin is appropriate?
Enough to cover credible peak demand and adjustment, but not so much that normal flow falls below the pump's stable controllable range.
Why use a back-pressure valve?
It can create stable pressure, prevent siphoning and improve check-valve performance when system pressure is too low or variable.
Should the pump be calibrated?
Yes. Calibrate with the actual installation and operating pressure, then recheck after maintenance or chemical changes.
Send us the metering-pump specification.
Include dose range, discharge pressure, chemical, concentration, temperature, viscosity, suction layout, control signal, site power and accessory needs.
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