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Boiler Feed Pump Calculator

Size a boiler feed water pump from the boiler's steam output and pressure: required flow, discharge pressure, total head, NPSH available, pump power and the recommended motor size.

100% Private Temperature-corrected density Metric & US units

Boiler & System Data

Capacity

kg/h
%
%

Discharge side

bar(g)
The pump must deliver 3% above this (ASME Section I, PG-61).
bar
m

Suction side

bar(g)
°C
m
bar

Efficiency

%
%
Pump duty point
—
—
Flow rate
—
—
Total head
—
—
Discharge pressure
—
—
Pump shaft power
—
—
Recommended motor
—
—
NPSH available
—
—

Calculation steps

How to size a boiler feed pump

A boiler feed pump has to deliver enough water to replace all the steam leaving the boiler plus blowdown, at a pressure high enough to force water into the drum even when the safety valves are lifting. Sizing comes down to two numbers — flow and head — which then give the power.

1. Required flow

Feed water (kg/h) = Steam output × (1 + blowdown %) × (1 + capacity margin %)
Flow (m³/h) = Feed water (kg/h) ÷ water density at feed temperature (kg/m³)

A 10–25% capacity margin is common practice to cover pump wear, load swings and control. Density matters: water at 105 °C is about 4.5% lighter than cold water, so the same mass needs more volume.

2. Discharge pressure and head

Discharge = 1.03 × safety valve set pressure + valve/economizer/pipe losses + static lift
Suction   = tank pressure + static height − suction losses
Head (m) = (Discharge − Suction) ÷ (ρ × g)

ASME Section I (PG-61) requires the feed supply to deliver water at a pressure at least 3% above the highest safety valve setting.

3. Power and motor size

Hydraulic power (kW) = ΔP (kPa) × Flow (m³/s)
Shaft power = Hydraulic power ÷ pump efficiency
Motor = next standard size ≥ shaft power × 1.15

Frequently Asked Questions

At least the boiler’s maximum continuous steam output plus blowdown, with a design margin on top — typically 10–25%. Many plants also install a second, identical standby pump.

The pump must overcome the safety valve set pressure plus 3% (ASME PG-61), the pressure drop across the feed control valve, check valves, economizer and piping, and the height difference between the pump and the drum water level.

NPSH available is how far the suction pressure is above the water’s vapour pressure, expressed as head. Feed water is hot and close to boiling, so a low NPSHa makes the pump cavitate. NPSHa must exceed the pump’s NPSH required with a safety margin, which is why deaerators are mounted high above the feed pumps.

Small multistage feed pumps are typically 50–70% efficient and large ones 75–85%. Use the value from the pump curve at the duty point once you have a selection.

Hotter water is less dense, so the volumetric flow and the head for a given pressure rise both increase, and its vapour pressure is higher, which reduces NPSH available.

This calculator is for preliminary sizing. Final pump selection must be checked against the manufacturer’s curves, the applicable boiler code and a qualified engineer’s review.

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