Engineering tool · metric units
Boiler Feed Pump Calculation: Power, Head & Motor Sizing
Enter feedwater flow, total differential head, fluid density and efficiencies to get hydraulic power, shaft (brake) power and the recommended motor rating in kW and HP. Built for quick sizing checks during design, commissioning, and troubleshooting.
For preliminary sizing. Confirm final selection against the manufacturer pump curve and NPSH data.
What a boiler feed pump calculation actually solves
A boiler feed pump (BFP) pushes treated water into a boiler against the drum pressure. Sizing it correctly means answering three linked questions: how much water the boiler needs per hour, how much head the pump must develop to overcome boiler pressure and system losses, and how much power the driver needs to deliver that duty continuously. Get any one wrong and you either starve the boiler or oversize the motor and waste energy for the life of the plant.
The calculator above works in metric units and ties these together. You give it flow, head, density and efficiencies; it returns the water (hydraulic) power, the shaft power the pump shaft must absorb, and a motor rating that already includes your design margin. From there you round up to the nearest standard motor frame.
The core formulas
Hydraulic power
P_hyd (kW) = (ρ · g · Q · H) ÷ 3.6×10⁶
Shaft (brake) power
P_shaft = P_hyd ÷ η_pump
Motor rating
P_motor = P_shaft × (1 + margin) ÷ η_motor
Q in m³/h, H in m, ρ in kg/m³, g = 9.81 m/s². The constant 3.6×10⁶ converts the result to kilowatts.
Hydraulic power is the useful work done on the water. Because no pump is perfect, the shaft has to supply more than that — divide by pump efficiency, typically 60–80% for a multistage feed pump. The motor then has to cover the shaft demand plus a margin for fouling, wear and process swings, divided by its own efficiency. The result is the continuous rating you select against.
Worked example
A small package boiler needs 25 m³/h of feedwater against 1,200 m of head, using hot water at 960 kg/m³, a 70% efficient pump and a 93% efficient motor with a 15% margin.
| Step | Working | Result |
|---|---|---|
| Hydraulic power | (960 × 9.81 × 25 × 1200) / 3.6e6 | 78.5 kW |
| Shaft power | 78.5 / 0.70 | 112.1 kW |
| Motor (with margin) | 112.1 × 1.15 / 0.93 | 138.6 kW |
| Standard frame | round up | 160 kW |
The duty needs about 139 kW at the motor terminals, so you would select the next standard size up — 160 kW — leaving headroom without grossly oversizing. The calculator runs exactly this chain whenever you change an input.
Getting the head right
Total differential head is where most sizing errors creep in. It is not simply the boiler pressure expressed in metres. Add every component the pump has to overcome.
Boiler pressure
The dominant term. Convert drum design pressure to metres of head using H = (p × 10⁵) / (ρ × g) for pressure in bar.
Static lift
The vertical distance from the deaerator or feed tank water level up to the boiler inlet connection.
Friction losses
Pipe, valve, economiser and feed-control-valve drops at maximum flow. These rise sharply with flow rate.
Control margin
Extra head so the feed control valve keeps authority. Reserve a portion of head for the valve to throttle against.
Flow rate: more than just steam output
Feedwater demand is not equal to steam output. You must add continuous blowdown and a safety margin for peak firing. A common rule is to size feed flow at the boiler’s maximum continuous rating plus blowdown, then add roughly 10–25% so the pump can recover drum level quickly after a load swing.
Feed flow estimate
Q_feed = Steam rate × (1 + blowdown%) × margin
For example, a boiler raising 20,000 kg/h of steam with 3% blowdown and a 15% margin needs about 20,000 × 1.03 × 1.15 ≈ 23,690 kg/h, which at roughly 960 kg/m³ is near 24.7 m³/h. That is the value to enter as flow.
Don’t forget NPSH
Frequently asked questions
Why divide hydraulic power by pump efficiency?
Hydraulic power is the energy delivered to the water. Internal losses — friction, recirculation, mechanical drag — mean the shaft must supply more. Dividing by pump efficiency (typically 0.60–0.80 for multistage feed pumps) gives the actual shaft, or brake, power.
What density should I use for feedwater?
Use the density at the actual suction temperature, not 1,000 kg/m³. Hot deaerated feedwater near 100–150°C sits around 920–960 kg/m³, which lowers both head-in-metres and power slightly versus cold water.
How big a design margin is reasonable?
10–25% is common on the motor rating to cover wear, fouling, process swings and standard frame steps. Continuous-duty or critical boilers lean toward the higher end; tightly metered packages can use less.
How do I convert boiler pressure to head?
Head in metres = (pressure in bar × 100,000) ÷ (density × 9.81). For 10 bar and 960 kg/m³ that is about 1,062 m, before adding static lift and friction losses.
Does this give the motor or the pump power?
Both. The result panel shows hydraulic power, shaft (brake) power, and the recommended motor rating after margin and motor efficiency. Select the next standard motor size above the motor figure.
Can I use this for multistage high-pressure pumps?
Yes. The formulas are independent of stage count — they depend only on flow, total head, density and efficiencies. Multistage pumps simply achieve high head across several impellers, but the power calculation is the same.
Method based on standard centrifugal pump power relationships (hydraulic power, efficiency chain) as used in pump and boiler sizing practice. Verify final selection against manufacturer curves and applicable codes.