Boiler Feed Pump Calculator
Start from boiler steaming capacity and this tool sizes the feed pump: required flow with evaporation margin, total dynamic head from drum pressure plus static lift and friction, shaft power at your assumed efficiency, and the next standard motor rating. SI first, with imperial output alongside.
Pump duty
| Flow (L/s) | |
| Flow (US gpm) | |
| Pressure head component | |
| Total head | |
| Shaft power | |
| Shaft power + 10% margin |
Motor selected from the IEC standard series (0.75 … 132 kW). This sizes the duty point only — confirm available NPSH against the pump curve before selection.
Feeding a vessel that is already at pressure
A boiler feed pump does something no ordinary transfer pump does: it pushes water into a vessel that is actively resisting, at full working pressure, continuously. That single fact reshapes the whole calculation. Static lift and pipe friction — the terms that dominate most pumping problems — become minor corrections next to the pressure head, which at 10 bar is already more than a hundred metres of hot water before a single metre of pipe is considered.
The second peculiarity is the fluid. Feedwater arrives hot from a deaerator or hotwell, both less dense than cold water and dangerously close to flashing into steam at the pump suction. Density shifts the volumetric duty a few percent; the proximity to saturation governs whether the pump survives at all. Feed pumps that fail in service overwhelmingly fail from cavitation on the suction side, not from a discharge calculation that came up short — which is why sizing the duty point, as this tool does, is the beginning of the selection rather than the end of it.
How to use the boiler feed pump calculator
- Enter the steam output. Use the boiler's rated evaporation in kilograms per hour. Feedwater demand equals steam production in steady state — every kilogram that leaves as steam must be replaced.
- Add a design margin. Boilers surge, blowdown consumes water, and controls modulate. Feed pumps are conventionally sized 15–25% above rated evaporation; 20% is the common default.
- Enter the boiler pressure. Working pressure in bar gauge. This is the dominant term in the head calculation — at 10 bar it alone accounts for over 100 metres.
- Add static lift and friction. Static lift is the geometric height from feed tank level to the boiler inlet; friction covers pipework, valves, the feed regulator and the non-return.
- Set the pump efficiency. Small multistage feed pumps land near 55–65%; larger units reach 70%+. When a duty curve exists, read the efficiency at the actual duty point rather than the best point.
- Read shaft power then motor size. Shaft power is the hydraulic demand; the motor is chosen from the IEC standard series at least 10% above it, so the drive never runs at its ceiling.
Key formulas
- Flow: Q (m³/h) = steam (kg/h) × (1 + margin) ÷ ρ
- Pressure head: Hp = P × 100,000 ÷ (ρ × 9.80665)
- Total head: H = Hp + static lift + friction losses
- Shaft power: Pshaft (kW) = Q × H × ρ × 9.80665 ÷ (3.6×10⁶ × η)
- Motor: next IEC standard rating ≥ Pshaft × 1.1
Worked example
A 5,000 kg/h boiler at 10 bar, fed from a 105 °C deaerator (ρ = 955 kg/m³) with 3 m static lift, 10 m of friction, 20% margin and 60% pump efficiency: Q = 5,000 × 1.2 ÷ 955 = 6.28 m³/h; Hp = 10 × 100,000 ÷ (955 × 9.80665) = 106.78 m, so H = 119.8 m. Shaft power = 6.2827 × 119.777 × 955 × 9.80665 ÷ (3.6×10⁶ × 0.60) = 3.263 kW; with the 10% margin that is 3.589 kW, so the selection is a 4 kW IEC motor.
Feedwater density reference
| Temperature | Density (kg/m³) | Typical source |
|---|---|---|
| 20 °C | 998 | Cold make-up |
| 80 °C | 972 | Warm hotwell |
| 90 °C | 965 | Hotwell / feed tank |
| 105 °C | 955 | Pressurised deaerator |
Saturated-water property values. Above 100 °C the feed tank must be pressurised, which is precisely why deaerator NPSH deserves checking.
Things to keep in mind
- Check NPSH available before anything else. Hot feedwater leaves little margin; the suction line, tank height and temperature together decide whether a correctly sized pump will actually work.
- Provide minimum-flow protection. A feed pump throttled near shut-off heats its own contents; a recirculation line or orifice back to the tank prevents it destroying itself during low-demand periods.
- Duty and standby, always. Loss of feedwater is a boiler safety event, not an inconvenience — alternate two full-duty pumps.
- Blowdown is real consumption. Continuous and intermittent blowdown leave the boiler as water, not steam; the design margin is partly there to cover it.
- Efficiency at the duty point, not the best point. Reading the curve's peak efficiency flatters small feed pumps and undersizes the motor.
Frequently asked questions
How is boiler feed pump capacity determined?
Start from the boiler's rated steam output in kg/h — in steady operation, feedwater mass must equal steam mass — then convert to volume using feedwater density at its actual hot temperature, and add a margin of roughly 20% for blowdown, load swings and control modulation. Sizing to bare evaporation leaves nothing for the surges that occur every time a large steam user opens.
Why does feedwater temperature matter for pump sizing?
Two reasons. Density: hot feedwater at 105 °C is about 955 kg/m³ versus 998 cold, so the same mass flow needs a few percent more volume. Far more critically, NPSH: hot water sits close to its boiling point, and a deaerator at 105 °C provides very little margin against flashing. Feed pumps cavitate from insufficient suction head far more often than they fail from undersized discharge capacity.
What head does a boiler feed pump need?
Total head is boiler pressure converted to metres, plus static lift from the feed tank, plus friction through the feed line, regulator and check valve. Pressure dominates: 10 bar alone is roughly 107 m of hot water. Add a working allowance so the pump can actually force water in against a boiler already at pressure — feeding a vessel is fundamentally different from filling a tank.
Why choose the motor above the calculated shaft power?
Shaft power is the hydraulic duty at one design point; real operation wanders. Running-out along the curve at low system resistance, cold-start viscosity and voltage dips all push demand above the calculation. Standard practice adds at least 10% and then rounds up to the next IEC frame, which is what this tool does — motors are only sold in that discrete series anyway.
Should I use one pump or two?
Steam plant convention is duty plus standby: two pumps each capable of the full duty, alternated in service. A boiler that loses feedwater goes to low-water lockout — and, without protection, to serious damage — so the standby is regarded as essential rather than optional. Many jurisdictions and insurers make a second means of feeding a condition of operation.
Does this calculator size a modulating or on-off feed system?
It sizes the pump duty point, which both arrangements need. On-off systems run the pump at that point intermittently against a level switch. Modulating systems hold it running against a control valve or variable-speed drive, in which case check the minimum-flow requirement too — a feed pump throttled near shut-off heats its own water and can be destroyed by it, which is why recirculation lines exist.
Last updated: 24 July 2026