Six things to check when selecting a dosing pump
Flow and pressure alone are not enough. Chemical compatibility, suction conditions, control mode and accessories decide whether the pump will work in the field.
Choosing a dosing pump is often reduced to a single question: “how many litres per hour?” Yet what comes back as a field problem is rarely the flow itself — it is the conditions around it.
1. Flow and back pressure are read together
The catalogue flow is valid at a given back pressure; as pressure rises, flow falls. Always select on the flow at your operating pressure. In the MEMDOS SMART LB series, for example, the LB 5 delivers 5.3 l/h up to 15 bar while the LB 20 delivers 19.2 l/h at 4 bar.
2. Calculate the dosing flow you need
With line flow, target dose and solution strength known:
Q(l/h) = (line flow m³/h × dose mg/l) / (10 × solution %)
Example: 120 m³/h of drinking water at 1.5 mg/l with 12% sodium hypochlorite needs about 1.5 l/h. Add a safety margin — a pump running mid-range is both more accurate and longer-lived than one running at its limit. You can also use the dosing calculator on this site.
3. Materials in contact with the chemical
Dosing head, valves, diaphragm and seals must suit the chemical. Common options are PVC, PP, PVDF and stainless steel; sodium hypochlorite, sulphuric acid, caustic and polymer each call for different choices. Hoses and the injection valve must belong to the same material family — a leak starts at the weakest link.
4. Suction conditions and gassing media
Gassing chemicals such as sodium hypochlorite can lock the pump head with gas. The answer: mount above the tank, use a vented dosing head and keep the suction line short. Catalogue suction lift figures (e.g. 3 mWC) apply to non-gassing media.
5. Control mode
- Manual: fixed dosing, small plants.
- External pulse: dosing proportional to a flow meter — the dose stays constant as the line flow changes.
- 0/4–20 mA: closed-loop control from an instrument (pH, ORP, residual chlorine).
6. The accessories that turn a pump into a system
- Pressure relief valve: protects pump and line against a closed valve.
- Pulsation dampener: smooths the pulsating flow and reduces losses in long pipelines.
- Injection valve: introduces the chemical at the right point and prevents backflow.
- Calibration column: the only correct way to measure real flow in litres.
Always calibrate at commissioning: catalogue curves are for water at 20 °C, and the density and viscosity of the real medium change the result.
See the MEMDOS SMART LB page for an example, or write to us and we will make the selection together.
