FLOW • HEAD • POWER • NPSH

Boiler Feed Pump Calculator

Build a transparent preliminary pump duty from steam production, feedwater conditions, system pressure and piping losses.

Q×H→P

Preliminary engineering calculationDo not purchase, install or operate equipment from this result alone. Final selection requires qualified engineering review and boiler/pump manufacturer approval.

Enter boiler and system data

Feedwater flow

Start with the boiler’s maximum continuous steam production.

Maximum steam outputkg/h Continuous blowdown% Blowdown percentage basisMatch the convention used by the blowdown calculation. Flow margin% Pump duty cycle%Use 100% for continuous/modulating duty; lower values increase required pump capacity. Feedwater temperature°C

Required differential head

Use gauge pressure for the boiler and feedtank entries.

Boiler operating pressurebar(g) Feedtank surface pressurebar(g) Control/check valve allowancebar Static elevation risemPositive when the boiler entry is above the feedtank liquid surface. Pipe and fitting lossm Head margin%

Efficiency assumptions

Use efficiencies from the selected pump and motor at the intended duty point when available.

Pump efficiency% Motor efficiency%

Calculate NPSH available optional

Enter liquid level and suction loss together. Feedtank pressure and temperature entered above are reused.

Local atmospheric pressurebar(a)Use site absolute atmospheric pressure, especially at altitude. Liquid level above pump centrelinemA pump above the liquid level has a negative value. Suction line lossm Pump NPSHr from curvemOptional; use the value at the calculated flow and selected speed.

Plant data stays in the browser and is not submitted or stored.

STEP-BY-STEP METHOD

Boiler feed pump flow calculation

The pump must replace steam leaving the boiler plus continuous blowdown. The calculator then applies the entered design margin and adjusts for the percentage of time the pump is available to run.

Blowdown based on steam

Base feed = steam × (1 + blowdown %)

Blowdown based on feedwater

Base feed = steam ÷ (1 − blowdown %)

Design mass flow

Base feed × flow margin ÷ duty cycle

Design volume flow

Mass flow ÷ water density

A continuously modulating pump can use a 100% duty cycle. An intermittent on/off pump may need a substantially greater capacity because the required hourly water must be delivered during a shorter running period.

Boiler feed pump head calculation

Pressure differenceBoiler gauge pressure + valve allowance − feedtank gauge pressure

Pressure difference is converted to metres of the entered-temperature water using:

Pressure head = ΔP ÷ (ρ × g)

Static elevation rise and pipe/fitting loss are added to pressure head. The selected head margin is applied last:

Design head = (pressure head + static lift + friction head) × (1 + head margin)

The correct pump is selected at the intersection of the required design flow and head—not from shut-off pressure alone.

Pump and motor power methodology

Hydraulic power

P_h = ρ × g × Q × H

Pump shaft power

P_shaft = P_h ÷ η_pump

Motor input

P_input = P_shaft ÷ η_motor

The result is not a nameplate recommendation. Final motor sizing must use the complete pump power curve and account for service factor, voltage, starting method, ambient temperature, enclosure, applicable codes and any operating case that can overload the driver.

NPSH available and hot feedwater

Hot water has a higher vapor pressure, reducing the pressure margin before flashing. The optional calculation uses:

NPSHa = (surface absolute pressure − vapor pressure) ÷ (ρg) + liquid level − suction loss

NPSHa belongs to the system. NPSHr belongs to a specific pump and changes with flow and speed. Obtain NPSHr from the manufacturer’s curve, then apply the additional margin required by the project specification or governing standard. A simple NPSHa greater than NPSHr comparison is not, by itself, a complete cavitation assessment.

Water density and saturation vapor pressure are estimated by interpolation from temperature property data aligned with IAPWS water/steam formulations. Confirm properties separately for final design, especially for pressurised deaerators and very hot feedwater.

Worked boiler feed pump example

Steam output10,000 kg/h

Blowdown and flow margin3% of steam + 15%

Feedwater temperature90°C

Pressures10 bar(g) boiler, open tank, 1 bar allowance

Additional head8 m elevation + 12 m friction + 10% margin

Efficiencies70% pump, 90% motor

Approximate result12.27 m³/h at 149.9 m head; about 7.67 kW motor input

The result uses an interpolated water density of 965.1 kg/m³ at 90°C.

Use the “Load example” button to reproduce the calculation and review each intermediate value.

Engineering checks before pump selection

Operating envelope

Check minimum, normal and maximum boiler loads against the pump curve and minimum continuous stable flow.

Suction design

Verify NPSH margin, tank pressure and level, suction-pipe velocity, strainers and hot-water flashing risk.

Mechanical design

Confirm materials, seals, bearings, temperature/pressure ratings, balance devices and required redundancy.

Controls and transients

Review control-valve authority, recirculation, start-up, standby switchover, trips and boiler level-control response.

Technical references

Reference material: U.S. Department of Energy pump hydraulic-power definition, Spirax Sarco feedtank and cavitation guidance, Xylem NPSH guidance, and the IAPWS industrial water and steam property formulation.

Boiler feed pump calculator FAQs

What does this boiler feed pump calculator calculate?

It estimates design feedwater mass and volume flow, differential pressure, total dynamic head, hydraulic power, pump shaft power, motor input power and optional net positive suction head available.

How is boiler feed pump flow calculated?

Maximum steam production is increased for blowdown, the selected flow margin and any reduction in available pumping duty cycle. Water density at the entered feedwater temperature converts mass flow to volume flow.

Should blowdown percentage be based on steam or feedwater?

Both conventions are used. Select the basis matching the source data. If blowdown is a percentage of steam, feedwater equals steam multiplied by one plus the percentage. If it is a percentage of feedwater, feedwater equals steam divided by one minus the percentage.

How is boiler feed pump head calculated?

Pressure head is calculated from boiler pressure plus valve allowance minus feedtank pressure. Static elevation rise, piping loss and the selected head margin are then included.

Why is feedwater temperature required?

Temperature changes water density, volume flow, pressure-to-head conversion and vapor pressure. Hot feedwater also reduces the suction margin available before flashing and cavitation.

What is NPSH available?

NPSHa is a property of the suction system. This calculator uses feedtank surface absolute pressure, water vapor pressure, liquid level above the pump and suction-line losses.

Where does NPSH required come from?

NPSHr is a characteristic of a specific pump at a specific flow and speed. Obtain it from the manufacturer’s pump curve and apply the margin required by the project specification or applicable standard.

Is calculated motor input the motor nameplate size?

No. It is the estimated electrical input at the calculated duty and entered efficiencies. Final motor selection must consider the full pump curve, service factor, starting method, voltage, ambient conditions and applicable codes.

Can this calculator select a boiler feed pump model?

No. It defines a preliminary duty point. A qualified engineer and the boiler and pump manufacturers must verify curves, materials, minimum flow, seals, temperature limits, NPSH margin, redundancy and transient conditions.

Are entered system values stored?

No. The calculations run locally in the browser, and the plugin does not submit or store entered plant data.