How to Size a Dosing Pump: Step-by-Step with Worked Examples

An undersized dosing pump cannot keep up when the dose goes up. An oversized one runs at a tiny stroke setting, where accuracy suffers. Sizing takes five numbers and one formula. This guide walks through it with three worked examples from water and effluent treatment.

The five numbers you need

1. Flow to be treated in m³/h: the water or process flow the chemical goes into.
2. Dose in ppm (mg/L). Your chemical supplier, consultant or lab test gives you this. Note both the normal and the maximum dose.
3. Chemical strength in g/L, as it will sit in the dosing tank (after any dilution).
4. Discharge pressure at the injection point, in bar or kg/cm².
5. The chemical itself, its temperature and concentration, which decide the liquid end material.

Step 1: Convert chemical strength to g/L

Suppliers quote strength in different ways, and mixing them up is the most common sizing error.

• % w/v (weight per volume): g/L = % × 10. A 10% w/v solution holds 100 g/L.
• % w/w (weight per weight): g/L = % × 10 × specific gravity. A 30% w/w solution with SG 1.15 holds 30 × 10 × 1.15 = 345 g/L.

If you dilute the chemical in a day tank, use the diluted strength. For example, 1 part of 345 g/L chemical in 4 parts water gives 345 ÷ 5 = 69 g/L.

Step 2: Calculate the chemical flow

Since 1 ppm = 1 mg/L = 1 g/m³:

Chemical needed (g/h) = flow (m³/h) × dose (ppm)
Pump flow (L/h) = chemical needed (g/h) ÷ strength (g/L)

Work this out for both the normal and the maximum dose.

Step 3: Choose the pump capacity

Metering pumps are adjusted by stroke length, and most hold their best accuracy across the upper part of their range. A good rule is to choose a pump so that:

• your normal dose sits at about 50–80% of the pump’s rated capacity, and
• your maximum dose is still within its rated capacity.

If the gap between normal and maximum dose is very wide, ask about variable speed (VFD) control or two pumps instead of one oversized pump.

Step 4: Check pressure

The pump must deliver against the line pressure at the injection point, plus losses in the discharge pipe and any check or back pressure valve. Add a safety margin, and fit a pressure relief valve on the discharge, set above the normal operating pressure but below the pump’s rated pressure.

Worked examples

For the WTP case, a 1000 LPH diaphragm metering pump with a PP liquid end would run at about 45% stroke at normal dose and still cover a higher dose in monsoon, when raw water turbidity rises.

Step 5: Choose the pump type and material

• Plunger pump for higher pressure and non-hazardous chemicals. See plunger vs diaphragm metering pumps.
• Diaphragm pump where leakage is not acceptable, such as hypochlorite or strong acids.
• Liquid end material to suit the chemical: SS316, Alloy 20, PP, PVDF or PTFE are common. Your industry page lists typical choices, for example water and wastewater treatment.

Common sizing mistakes

• Confusing % w/w with % w/v, or forgetting specific gravity.
• Sizing for the maximum dose only, so the pump runs at 10–15% stroke most of the time.
• Ignoring the dilution in the day tank.
• Choosing the pump on flow alone, without checking discharge pressure.
• Forgetting future plant expansion. If capacity will grow within two to three years, size for it now or plan a second pump.

Frequently asked questions

What does ppm mean in chemical dosing?

Parts per million. In water, 1 ppm is 1 mg of chemical per litre of water, which is the same as 1 g per m³.

Can one pump dose the same chemical at two points?

Yes, with a duplex or multi-head pump. Each head has its own stroke adjustment, so each point is set separately.

What should I send to get a pump recommendation?

The flow, normal and maximum dose, chemical name and strength, temperature, discharge pressure, and the power supply at site. We will confirm the pump model and material.

About Jagdish Engineering Works

JEW has manufactured plunger and diaphragm metering pumps and pressure relief valves in Goregaon (East), Mumbai since 1969. We also repair pumps, supply spares and offer annual maintenance contracts.