01

Start with active-chemical demand

For water-like streams, a useful starting relationship is active chemical mass flow in kg/h = process flow in m³/h × target dose in mg/L ÷ 1,000. This works because 1 mg/L across 1 m³ is 1 gram. Confirm whether the stated dose means active chemical, neat product or prepared solution.

  • Process flow: use the maximum dosing case
  • Dose: confirm active basis
  • Check whether the process flow varies
  • Separate normal and maximum demand

02

Convert mass demand to solution volume

Solution flow in L/h = active chemical demand in kg/h ÷ (active mass fraction × solution density in kg/L). Use the supplier's concentration and density at the relevant temperature. Do not assume that volume percentage and mass percentage are interchangeable.

  • Mass fraction: 10% = 0.10
  • Use actual solution density
  • Correct for on-site dilution
  • Keep units visible through every step

03

Worked example

A 500 m³/h water stream requires 8 mg/L active chemical. Active demand is 500 × 8 ÷ 1,000 = 4 kg/h. If the dosing solution is 20% by mass with density 1.10 kg/L, required solution flow is 4 ÷ (0.20 × 1.10) = 18.2 L/h. The pump is then selected for the required operating range and system pressure—not simply rounded to the largest available model.

  • Normal required flow: 18.2 L/h in this example
  • Check maximum process flow and dose
  • Add only justified margin
  • Verify minimum controllable flow

04

Finish with hydraulic checks

The calculated flow is only one input. Confirm discharge pressure, suction lift or flooded suction, viscosity, vapour pressure, acceleration losses and accessory pressure drops. The selected pump must meet capacity at the actual differential pressure and provide useful adjustment across the operating range.

  • Maximum back pressure
  • Line and fitting losses
  • Relief-valve setting
  • Calibration method
  • Duty/standby philosophy

Frequently asked questions

How much sizing margin should be added?

Use a project-specific margin that covers credible process variation and uncertainty. Excessive oversizing can force normal operation into the bottom of the pump's adjustment range.

Can pump stroke setting be used as proof of actual flow?

No. Verify delivered flow by calibration or an appropriate flow-measurement method under representative system conditions.