Sizing a boiler feed pump starts with getting the duty point right. Get the flow or head wrong and you end up with cavitation, wasted energy, or a pump that cannot keep up with the boiler at full load.
For a quick preliminary duty point, run a free boiler feed pump calculation and then refine the result against the manufacturer’s pump curve before you commit to a model.
1. Start with the right feedwater flow
The pump must deliver more than the boiler’s rated steam output. Start with maximum steam generation, then add continuous blowdown and any minimum-flow recirculation the system needs. Apply a sensible design flow margin on top, and keep each allowance visible so the total can be reviewed later.
Convert the final mass flow to volumetric flow using the feedwater density at the actual operating temperature. Hot feedwater is less dense than cold water, so skipping this step understates the flow the pump curve will see.
2. Do not underestimate total dynamic head
Total dynamic head (TDH) is the energy the pump must add to move water from the feed tank into the boiler. It combines pressure head from the boiler operating pressure, static elevation between the suction source and the injection point, and every system loss: pipe friction, fittings, valves, the economizer, and the feed control valve.
A common mistake is sizing from boiler pressure alone. The control valve needs pressure drop to regulate, the economizer adds resistance, and long pipe runs add friction. Leave these out and the calculated head comes out far too low.
3. Check your NPSH margin before you select a pump
Net positive suction head available (NPSHa) describes the suction energy your system provides. It must exceed the pump’s required NPSH (NPSHr) at the actual operating flow, with a comfortable margin.
Hot feedwater makes this critical. As feedwater temperature rises, vapour pressure rises and NPSHa falls, increasing cavitation risk. Tank elevation, suction pipe diameter, strainers, and deaerator pressure all influence the result, so check the margin at the hottest expected condition, not just the normal one.
4. Size the motor for the real operating range
Hydraulic power is the useful energy delivered to the water. Shaft power is higher because the pump is not perfectly efficient, and the motor must cover shaft power across the expected operating range, not just at one rounded calculation point.
Enter realistic pump and motor efficiencies, add a motor sizing margin, and then round up to the next standard motor rating. Confirm service factor, starting method, and voltage with the manufacturer.
Common boiler feed pump sizing mistakes to avoid
- Using steam flow with no allowance for blowdown or recirculation
- Ignoring the feed control valve pressure drop
- Treating hot feedwater as cold water
- Mixing gauge and absolute pressure in NPSH checks
- Choosing a motor with no margin above the calculated shaft power
- Skipping the pump curve check entirely
From estimate to pump selection
An online estimate gives you a defensible preliminary duty point: design flow, TDH, shaft power, and an NPSH check. The next step is placing that duty point on a manufacturer’s curve and confirming efficiency, absorbed power, and NPSHr at the exact flow.
If you are comparing quotations or replacing an existing unit, run the numbers first with the free boiler feed pump calculation tool linked above, then take the duty point to your supplier for a final selection.