How to Use This Boiler Feed Pump Calculator
This boiler feed pump calculator turns a handful of boiler and piping parameters into the numbers a pump datasheet actually needs: design flow rate, total dynamic head (TDH), NPSHa, and hydraulic, shaft and motor power. It works in either metric or US units and computes water properties (density and vapour pressure) from the feedwater temperature you enter, rather than assuming cold-water conditions like a plain flow/head calculator would.
- Choose your unit system — Metric (bar, °C, m, kg/hr, kW) or US (psi, °F, ft, lb/hr or GPM, hp).
- Enter feedwater flow — either from the boiler's steam generation rate plus blowdown, or as a direct feedwater flow if you already know it.
- Enter boiler pressure and feedwater temperature — these drive both the pressure-head and the NPSHa calculation.
- Enter elevation and friction losses for the suction and discharge piping, plus any feedwater control valve drop.
- Enter pump/motor efficiency and electricity rate to get shaft power, motor power and an annual energy cost estimate, then click Calculate.
Boiler Feed Pump Sizing Formulas Explained
Every result on this page comes from the same four calculation steps a pump engineer would work through by hand. They are shown here so you can audit the numbers or reproduce them in a spreadsheet.
1. Feedwater flow rate
Volumetric flow (m³/hr) = Feed mass flow (kg/hr) ÷ ρ(T)
Design flow = Volumetric flow × (1 + Flow margin%)
ρ(T) is the density of water at feedwater temperature, taken from saturated-steam-table data rather than a fixed cold-water value — this matters because feedwater near 90–130°C is 3–7% less dense than water at room temperature.
2. Total Dynamic Head (TDH)
TDH = Pressure head + (Discharge elevation − Suction elevation) + Friction losses + Valve drop
Design TDH = TDH × (1 + Head margin%)
A widely used shortcut is "1 bar ≈ 10.2 m of head," but that constant assumes cold water. This calculator derives the head directly from the actual feedwater density at your operating temperature, which is more accurate for hot condensate and deaerator feed systems.
3. Hydraulic, shaft and motor power
Shaft power (kW) = Hydraulic power ÷ Pump efficiency
Motor input power (kW) = Shaft power ÷ Motor efficiency
In US units the equivalent hydraulic horsepower formula is BHP = (Flow[gpm] × Head[ft] × SG) ÷ 3960. This calculator always computes internally in SI units and converts for display, so metric and US results stay mathematically consistent with each other.
4. NPSHa (Net Positive Suction Head available)
The vapour pressure term is what makes boiler feed pump suction calculations different from an ordinary water pump: hot feedwater is close to its own boiling point, so even a small suction friction loss or a poorly elevated deaerator can push NPSHa below the pump's required NPSHr and cause cavitation.
Worked Example: Sizing a Boiler Feed Pump
Scenario: A 10,000 kg/hr steam boiler operates at 10 bar g. Feedwater comes from a deaerator at 90°C, mounted 3 m above the pump centreline, delivering feed to an injection point 4 m above the pump. Blowdown is 3%, suction friction is 0.5 m, discharge friction plus valve drop is 6.5 m of head equivalent, pump efficiency is 70%, motor efficiency 93%, and a 15% flow margin and 10% head margin are applied.
Flow: 10,000 × 1.03 = 10,300 kg/hr → at ρ ≈ 965.3 kg/m³, that's ≈ 10.67 m³/hr → with 15% margin ≈ 12.28 m³/hr design flow.
Head: 10 bar → ≈ 103.6 m of head at 90°C density, plus net static (4 − 3 = 1 m), plus 7.0 m friction/valve loss ≈ 111.6 m → with 10% margin ≈ 122.8 m design TDH.
Power: Hydraulic power ≈ (12.28 × 122.8 × 965.3 × 9.81) ÷ 3,600,000 ≈ 3.96 kW. Shaft power ≈ 3.96 ÷ 0.70 ≈ 5.66 kW. Motor power ≈ 5.66 ÷ 0.93 ≈ 6.09 kW → round up to a standard 7.5 kW motor.
Enter these same numbers into the calculator above to see the full breakdown, including NPSHa. Your own system will differ, but the calculation sequence is identical.
Common Boiler Feed Pump Calculation Mistakes to Avoid
Using cold-water density
Sizing hot feedwater with room-temperature water properties understates required head and overstates NPSHa, hiding a cavitation risk that only appears once the plant is running.
Ignoring blowdown
Skipping the 2–5% blowdown allowance quietly undersizes the pump — a boiler that blows down 3% needs 3% more feedwater than its steam rating implies.
Stacking safety margins
Applying a large margin to flow, then another to head, then oversizing the motor "to be safe" compounds into a pump that runs inefficiently at part load and cycles excessively.
Forgetting suction friction in NPSHa
Even a small strainer or check valve pressure drop on the suction side can erase most of the NPSH margin on a hot feedwater system — it should never be assumed to be zero.
Frequently Asked Questions
How do you calculate a boiler feed pump?
Boiler feed pump calculation has three parts: first find the required feedwater flow rate from the boiler's steam output plus blowdown and a design margin; second, add up the total dynamic head (TDH) the pump must overcome — boiler pressure converted to head, static elevation, pipe friction and any control valve drop; third, combine flow and head with pump and motor efficiency to get hydraulic, shaft and motor power. This calculator performs all three steps automatically.
What is the formula for boiler feed pump power?
Hydraulic power (kW) = flow (m³/hr) × head (m) × water density × 9.81 ÷ 3,600,000. Shaft power = hydraulic power ÷ pump efficiency, and motor input power = shaft power ÷ motor efficiency. In US units, hydraulic horsepower = flow (GPM) × head (ft) × SG ÷ 3960.
How is NPSHa calculated for a boiler feed pump?
NPSHa equals atmospheric head plus static suction head, minus the vapour pressure head of the feedwater at its own temperature, minus suction line friction losses. Because feedwater often sits close to saturation temperature, the vapour pressure term is significant, and NPSHa needs a healthy margin above the pump's required NPSHr.
What flow rate should a boiler feed pump be sized for?
Size for the boiler's maximum continuous steam rating, plus blowdown (typically 2–5%), plus a design margin (typically 10–25%) for peak demand and future growth. Many plants land somewhere between 1.1 and 2.5 times the nominal steaming rate depending on how quickly the boiler swings with load.
How much head does a boiler feed pump need?
Total head is boiler operating pressure converted to metres/feet of feedwater, plus static lift from pump to injection point, plus discharge and suction friction losses, plus any feedwater control valve drop. A rough rule of thumb is 1 bar ≈ 10.2 m of head, adjusted for actual feedwater density.
Why does feedwater temperature matter in the calculation?
Hot feedwater is less dense, which changes the volumetric flow for a given mass flow and increases the head needed to produce a given pressure. It also has much higher vapour pressure, which lowers NPSHa and raises cavitation risk if the suction system doesn't have enough margin.
What NPSH margin is considered safe for a boiler feed pump?
Most manufacturers and the Hydraulic Institute recommend at least 1–1.5 m (3–5 ft) of margin between NPSHa and NPSHr, with more margin for high-energy pumps. A small or negative margin risks cavitation, noise, vibration and premature impeller wear.
How do I convert boiler horsepower to feedwater flow?
One boiler horsepower corresponds to about 15.65 kg/hr (34.5 lb/hr) of steam evaporated from and at 212°F. Multiply boiler horsepower by this factor, add blowdown and a design margin, then convert the resulting mass flow to a volumetric flow rate.
Is this boiler feed pump calculator accurate enough for pump selection?
It uses standard Hydraulic Institute and steam-table relationships and is accurate enough for preliminary sizing, budgeting and checking vendor numbers. For final selection, always confirm the operating point against the manufacturer's actual pump curve and site conditions.
What causes a boiler feed pump to be undersized or oversized?
Undersizing usually comes from ignoring blowdown, using too small a design margin, or underestimating friction losses — it shows up as boiler water level dropping under peak load. Oversizing usually comes from stacking multiple safety margins — it shows up as pump cycling, recirculation losses and poor part-load efficiency.