Select the right metering pump based on 4 key parameters: required dosing flow rate (l/h), back pressure at the injection point (bar), required accuracy (±%), and the chemical being dosed (determines pump head material). A wrong selection leads to under/over-dosing, incorrect dosage, or premature pump head failure.
Step 1 — Calculate the Dosing Flow Rate (l/h)
Dosing flow rate = (process flow rate × target dose) ÷ chemical solution concentration. Example: treating 50 m³/h of water, dosing PAC at 20 mg/L (ppm) using a 10% PAC solution (≈100 g/L):
- PAC mass required: 50,000 L/h × 20 mg/L = 1,000,000 mg/h = 1 kg/h
- Solution flow rate: 1,000 g/h ÷ 100 g/L = 10 l/h
Select a pump with a flow range covering this value, with margin for adjustment. See general flow ranges on the metering pump page.
Step 2 — Determine the Back Pressure at the Injection Point
Back pressure = pressure in the pipe/tank at the injection point + losses through the check valve and injection tube. The pump must generate pressure greater than the back pressure for the chemical to enter. Solenoid and mechanical diaphragm pumps typically handle ~10–20 bar; higher pressures (up to ~250 bar) require piston/plunger pumps.
Step 3 — Select Accuracy & Control Method
Dosing accuracy (typically up to ±1% for pumps meeting API 675) is determined by the technology and control method: manual stroke adjustment, 4–20 mA signal, or pulse from a flow meter. Processes requiring flow-proportional dosing should use 4–20 mA or pulse control.
Step 4 — Select the Right Technology
Solenoid (electromagnetic) for small flow rates, moderate pressure, low cost; mechanical/hydraulic diaphragm for medium flow rates and stability; piston/plunger for high pressure and large flow rates. See a detailed comparison in the article diaphragm, piston, or solenoid metering pump.
Step 5 — Select Pump Head Material Based on Chemical
Pump head, diaphragm, and valve materials must be compatible with the chemical: PVC/PP for common chemicals; PVDF for strong acids and oxidizers; PTFE for a wide range of chemicals; SS316L for high pressure and solvents. Specify the chemical name and concentration for correct material selection.
Pre-Order Checklist
- Chemical + concentration + temperature
- Dosing flow rate (l/h) and adjustment range
- Back pressure at the injection point (bar)
- Accuracy + control method (manual / 4–20 mA / pulse)
- Standard requirements (API 675 / ATEX) + CO-CQ
Thái Khương Pumps is an authorized distributor of Doseuro (Italy) metering pumps — covering the full range from compact solenoid to API 675-compliant piston pumps. Send your specifications for model and material selection consultation.
Operating Range by Technology — Doseuro Authorized Data
The table below summarizes the actual operating ranges of metering pump lines from Doseuro (Italy) distributed by Thái Khương Pumps — for quick cross-referencing of system requirements against suitable technology.
How to read the table: take the dosing flow rate calculated in Step 1 and the back pressure from Step 2, then select the line that covers both values with adequate safety margin. Avoid selecting a pump running near the top of its range — positive displacement pumps operate more reliably in the mid-range, and having margin allows for dose increases when inlet water conditions change.
Conversely, do not oversize: a large pump running at a few percent stroke will dose unevenly and be difficult to calibrate — proper sizing matters more than “going bigger to be safe”.
| Technology (Series) | Flow Rate | Selection Notes |
|---|---|---|
| Solenoid — electromagnetic diaphragm (SDP type S) | up to ~22 l/h | Small-dose injection, IP65 enclosure, low capital cost — small-scale water treatment |
| Compact mechanical diaphragm (SR type FM) | 14 – 47 l/h · up to 10 bar | Economy option for standard chemical dosing, compact construction |
| Mechanical diaphragm / piston (SR type A·B) | 49 – 1,400 l/h (diaphragm) · piston up to 1,458 l/h, AP version up to 230 bar | Industrial workhorse: wastewater treatment, power generation, F&B — continuous duty |
| Hydraulic diaphragm API 675 (PDP) | up to 4,000 l/h · accuracy ±1% (10–100% capacity) | Oil & gas, demanding chemical service — API 675 standard, 4–20 mA control |
Pump Head Material Selection by Common Chemical
Incorrect pump head material is the most common cause of premature failure in metering pumps. The pump head, diaphragm, and valve assembly come into direct contact with the chemical thousands of cycles per hour — incompatible materials will swell, become brittle and crack, or corrode within just a few weeks of operation.
Note that material compatibility varies with both concentration and temperature: the same chemical at low concentration may be compatible with PP, but at high concentration may require PVDF or PTFE. When changing the dosed chemical or switching to a different chemical supplier, cross-check the pump manufacturer’s compatibility chart before continuing to use the existing pump head.
The combinations below are verified configurations for common chemicals in water treatment:
| Dosed Chemical | Recommended Material Combination | Notes |
|---|---|---|
| PAC, polymer (coagulants, flocculants) | PP or PVDF pump head | High-viscosity polymer: select larger valve size, lower stroke speed |
| NaOCl — Bleach (disinfection) | PVDF + PTFE diaphragm + ceramic valve balls | Avoid stainless steel — NaOCl causes pitting corrosion on stainless steel |
| Acid H₂SO₄, HCl (pH adjustment) | PVDF / PTFE | Specify concentration when ordering — different concentrations require different materials |
| NaOH (alkali) | PP / PVC (depending on concentration and temperature) | Check solution temperature — concentrated alkali generates heat when diluted |
Frequently Asked Questions
What parameters should I use to select a metering pump?
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Dosing flow rate (l/h), back pressure at the injection point, required accuracy, and the chemical (which determines pump head material). Pumps meeting API 675 deliver accuracy up to ±1% with stroke control.
Solenoid, mechanical diaphragm, or piston — which should I choose?
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Solenoid for small flow rates, moderate pressure, low cost; mechanical/hydraulic diaphragm for medium flow rates requiring stability; piston/plunger for high pressures up to several hundred bar. Continuous industrial-scale operation should prioritize hydraulic diaphragm or piston.
Why does a metering pump dose incorrectly despite being properly calibrated?
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Three common causes: air entering the pump head (loss of prime), check valve stuck/worn, and back pressure changes from calibration conditions. Installing a back-pressure valve and degassing unit helps maintain consistent dosing.
Spare parts for metering pumps: see the catalog of in-stock items by model.
Send the chemical, dosing flow rate, and injection point pressure — Thái Khương Pumps engineers will advise on model + material and provide a quotation with CO-CQ within 24 hours. Request a Quotation →
