How to Size a Dosing Pump: Flow and Back-Pressure Calculation Step by Step
How to determine dosing pump flow (L/h), back-pressure (bar), chemical compatibility and control type, with a worked example and a selection checklist that avoids the most common sizing mistakes.
1) Start with Process Data: Flow, Pressure and Chemical
Dosing pump selection starts with process data, not with the catalogue. You need three basic values: the hourly quantity of chemical to be dosed (L/h or mL/min), the back-pressure at the injection point (bar) and the chemical itself (concentration, density, viscosity).
The most common flow formula is: required dosing flow (L/h) = main line flow (m³/h) × target dose (ppm) ÷ chemical concentration (%) ÷ 10. For example, dosing 2 ppm of active chlorine into a 50 m³/h water line with 12% sodium hypochlorite requires 50 × 2 ÷ 12 ÷ 10 = 0.83 L/h.
This value should be the operating point, not the pump's maximum capacity. Selecting the pump so that it runs between 30% and 80% of nominal capacity improves both accuracy and diaphragm life. For a 0.83 L/h requirement, a pump in the 1 to 2 L/h class is the right choice; the SEKO Tekna and Invikta solenoid ranges start at 0.4 L/h for exactly this kind of duty.
2) Get the Back-Pressure Right: The Most Common Mistake
Back-pressure is not just line pressure: it is the sum of line pressure at the injection point, the injection valve spring pressure (typically 0.5 to 1.5 bar), friction loss in the discharge line and the static head difference.
The flow value on the pump label is valid at a specific pressure. A solenoid diaphragm pump rated 10 L/h at 1 bar can fall to 6 or 7 L/h at 7 bar back-pressure. Always select from the curve at the actual operating pressure, and keep in mind that solenoid pumps top out around 20 bar, while motor-driven diaphragm pumps are the answer beyond that.
The suction side is critical too: the line from tank to pump inlet should be as short as possible, and for gassing chemicals such as hypochlorite a flooded suction or a degassing head should be preferred. Otherwise the pump gas-locks and dosing stops.
- • Main line flow and target dose (ppm) tabulated
- • Injection point pressure plus injection valve spring pressure added up
- • Pump selected to run in the 30 to 80% capacity range
- • Suction line and gas venting checked
3) Chemical Compatibility: Head and Diaphragm Materials
It is the wetted parts (head, diaphragm, valve balls, seals), not the pump body, that are selected for the chemical. The PVDF head plus PTFE diaphragm combination is the safe standard for hypochlorite, acids and most aggressive chemicals; a PP head is the economical alternative for mild chemicals.
For high-density fluids such as sulphuric acid, the valve ball material (glass, ceramic, stainless steel) and O-ring selection (FKM, EPDM) must be verified separately. For abrasive fluids such as polymers and slurries, peristaltic technology should be evaluated.
For chemicals you are unsure about, consult the manufacturer's compatibility table or ask Vanera's engineering team with the chemical name, concentration and temperature; the wrong diaphragm choice ends in perforation within weeks.
- • Chemical name, concentration and temperature documented
- • Head and diaphragm material confirmed against the compatibility table
- • Valve ball and O-ring materials verified for density and chemistry
- • Peristaltic option considered for abrasive or gassing fluids
4) Control Type: Manual, Pulse or 4-20 mA
For simple processes with constant flow, a manually adjusted pump is sufficient. But if the main line flow varies, proportional dosing driven by pulses from a water meter delivers both chemical savings and dose stability.
Where measurement-based control (chlorine, pH, conductivity) is required, models with 4-20 mA input or integrated control are used. In the SEKO Tekna series, pulse and 4-20 mA inputs are standard, which makes a later move to automation easy.
If you have automation plans, define relay outputs (level alarm, fault) and remote monitoring needs from the start. Replacing a pump later is always more expensive than choosing the right model in the first place.
5) Worked Example, Cost Drivers and Next Step
Scenario: 30 ppm of corrosion inhibitor is to be dosed into a cooling tower with 120 m³/h circulation, the product is used as a concentrate and the injection point is at 3 bar. Required flow: 120 × 30 ÷ 100 ÷ 10 = 3.6 L/h.
Back-pressure: 3 bar line + 1 bar injection valve + 0.5 bar line loss ≈ 4.5 bar, so a 5 bar class is selected. Result: a solenoid dosing pump in the 5 to 7 L/h at 7 bar class with PVDF/PTFE head and pulse input gives the ideal operating point.
What drives the price of a dosing pump is not the flow figure alone: the pressure class, the wetted materials (PVDF/PTFE against PP), the control features (manual, pulse, 4-20 mA, integrated measurement), accessories such as injection valve, foot valve and level switch, and whether a motor-driven diaphragm pump (5 to 1000 L/h) is needed instead of a solenoid unit. A quotation is only accurate when these are defined.
If you would like to run the same calculation for your own process, send your flow, pressure and chemical details to Vanera's engineering team through the Quick Quote form on our dosing pump page. As an authorised SEKO dealer we return a model recommendation verified on the pump curve, with the accessories listed.