Boiler Feed Pump Calculator
Calculate the required feedwater flow, pump head, hydraulic power, shaft power, motor power, and NPSH margin for preliminary boiler feed pump sizing.
Calculation Results
What Is a Boiler Feed Pump?
A boiler feed pump transfers feedwater from a deaerator, feed tank, condensate system, or other source into a steam boiler. The pump must create enough pressure to overcome the boiler-side pressure while also supplying the required feedwater flow. Its duty is affected by the vertical elevation between source and boiler, friction in the discharge piping, valves and fittings, and losses through equipment in the feedwater path.
Boiler feed pumps are common in industrial boilers, steam power plants, process steam systems, manufacturing facilities, and utility systems, but the exact arrangement can vary substantially. Some systems use atmospheric feed tanks, while others use pressurized deaerators or multiple pump stages. The selected pump therefore needs to match the actual operating envelope rather than a generic boiler rating alone. Preliminary boiler feed pump sizing establishes a design flow and head so engineers can compare candidate pumps, estimate power, and identify suction-side risks before moving to detailed hydraulic design and manufacturer selection.
How Boiler Feed Pump Sizing Works
Preliminary boiler feed pump calculation can be organized into four connected steps. First, determine the feedwater mass flow from the design steam generation rate and blowdown, then convert it to volumetric flow using water density and apply a flow margin. Second, determine the pressure head required to move water from the source pressure to the boiler pressure and add static elevation and estimated system losses. Third, calculate hydraulic power, pump shaft power and estimated motor input using the specified efficiencies. Finally, check suction conditions by comparing NPSHa with the manufacturer's NPSHr plus the chosen safety margin.
- Determine required feedwater flow.
- Determine required pump head.
- Calculate hydraulic, shaft, and motor input power.
- Check NPSH availability against pump requirements.
Boiler Feedwater Flow Calculation
For a preliminary estimate, feedwater demand can be related to the boiler's design steam generation and continuous blowdown. If blowdown is entered as a percentage of steam generation, the required feedwater mass flow can be approximated as:
The mass flow is converted to volumetric flow using the estimated or manually entered water density:
A project-specific flow design margin is then applied. This relationship is intended for preliminary sizing; actual plant feedwater demand can also be affected by operating cases, control strategy, intermittent blowdown, recirculation, start-up conditions, redundancy philosophy, and other system requirements.
Boiler Feed Pump Head Calculation
Pump total dynamic head must account for the pressure difference between the source and the boiler as well as elevation and hydraulic losses. Depending on the system, equipment and control-valve losses may need to be included in the piping-loss input.
Pressure basis must be consistent. The calculator assumes gauge pressure for the boiler and source pressure inputs used in the head calculation, so their difference can be converted directly into pressure head. If your engineering basis uses different reference conditions, convert the pressures consistently before using the result.
Boiler Feed Pump Power Calculation
Hydraulic power is the rate at which the pump adds hydraulic energy to the water. Pump shaft power is higher because the pump is not 100% efficient, and estimated motor electrical input is higher again because the motor also has losses.
Lower assumed efficiency increases the estimated shaft and motor input power. The calculated motor power is an estimated input requirement, not a final motor nameplate selection. Use manufacturer recommendations, service factor, starting method, operating envelope, ambient conditions, and project electrical criteria for final selection.
Why Feedwater Temperature Matters
Boiler feedwater is often hot, especially when a deaerator or heat-recovery system is used. Temperature changes water density, so a given mass flow corresponds to a different volumetric flow as the feedwater heats up. Temperature also changes water vapor pressure, which directly affects the suction pressure margin available before cavitation can occur.
For that reason, a boiler feedwater pump calculation should not always assume a density of 1,000 kg/m³. This calculator uses a lightweight temperature-based property estimate for preliminary work. Final design should use verified thermodynamic properties for the actual feedwater pressure and temperature, particularly for hot or pressurized systems.
NPSH and Cavitation
Net positive suction head available (NPSHa) describes the suction-side head available above the liquid's vapor-pressure head. It is determined by the system, including absolute pressure above the liquid surface, static suction head, feedwater vapor pressure, and suction piping losses. Net positive suction head required (NPSHr) is different: it is a characteristic supplied by the pump manufacturer and varies with pump flow, speed, and design.
NPSHr should come from the manufacturer's pump curve at the relevant operating point. NPSHa should then be compared with NPSHr using an appropriate project safety margin. This calculator classifies the result as SAFE when the user-entered NPSH safety margin is fully met, CAUTION when NPSHa remains above NPSHr but does not meet that margin, and INSUFFICIENT when NPSHa is at or below NPSHr.
When to Use This Calculator
- Preliminary pump sizing
- Engineering estimates
- Conceptual plant design
- Equipment comparison
- Checking pump duty requirements
- Educational calculations
- Reviewing existing pump operating conditions
It is not a substitute for final pump selection, certified pump curves, detailed hydraulic analysis, or pressure-system engineering review.
Engineering References
2026 Update. The calculator uses established engineering relationships and recognized water/steam property references. Thermodynamic property calculations should be based on the applicable IAPWS formulation or verified steam-table data.
IAPWS-IF97 is an industrial formulation for thermodynamic properties of water and steam and is intended particularly for industrial applications including the steam power industry.
iapws.org/documents/release/IF97-RevNIST provides thermophysical property data including density, enthalpy, entropy, viscosity and other fluid properties.
webbook.nist.gov/chemistry/fluid/Water species reference within the NIST Chemistry WebBook.
webbook.nist.gov/cgi/cbook.cgi?ID=C7732185Standard gravitational acceleration used in calculations: g = 9.80665 m/s².
Frequently Asked Questions
1. What is a boiler feed pump used for?
A boiler feed pump moves feedwater from a feed tank, deaerator, condensate system, or other source into a boiler. It must provide enough flow while overcoming the pressure difference between the source and boiler, elevation change, piping and equipment losses, and any required design allowance. The final pump must be checked against the manufacturer's performance curve.
2. What information do I need to size a boiler feed pump?
For a preliminary estimate, gather the design steam generation rate, blowdown rate, feedwater temperature, boiler and source pressure, elevation difference, discharge-system losses, flow and head margins, pump and motor efficiency, plus suction-side pressure, suction head, suction losses and pump NPSHr. Good input data matters more than adding unnecessary precision to the equations.
3. Should boiler pressure be entered as gauge or absolute pressure?
For the pump-head portion of this calculator, gauge pressure is assumed because the pressure difference is what drives the required pressure head. Use a consistent pressure basis for both boiler and source pressure. NPSH is different: the calculation converts vessel gauge pressure to absolute pressure before comparing it with water vapor pressure.
4. What is a typical pump efficiency for preliminary calculations?
Pump efficiency varies significantly with pump type, size, speed and operating point. A preliminary assumption such as 70–80% may be reasonable for some centrifugal pump estimates, but it is not a universal design value. Use the efficiency shown on the manufacturer's pump curve at the intended duty point whenever reliable selection data is available.
5. How much flow margin should I use for a boiler feed pump?
There is no single margin that fits every project. A preliminary allowance is often added for uncertainty, operating range, future conditions or control requirements. The appropriate margin depends on the boiler system, owner criteria, redundancy philosophy and manufacturer guidance. This tool lets you enter a project-specific flow margin instead of applying a fixed rule.
6. What is the difference between NPSHa and NPSHr?
NPSHa is determined by the suction system: pressure above the liquid, elevation, vapor pressure and suction losses. NPSHr is a pump characteristic supplied by the manufacturer for a specific flow and speed. A pump selection should provide enough NPSHa above NPSHr, including the safety margin required by the project or applicable guidance.
7. Can I use this calculator for a power plant boiler?
It can support early-stage estimates for power-plant duties, but high-energy boiler feed systems usually need more detailed hydraulic, thermodynamic and pump-selection work. Final selection should consider certified curves, operating cases, minimum-flow protection, recirculation, transients, pressure class, materials, redundancy and plant-specific requirements beyond this preliminary calculator.
8. Can I use this calculator for an industrial steam boiler?
Yes, it is intended to be useful for preliminary industrial boiler feedwater estimates when the required inputs are known. It can help define a candidate duty point for flow and head and estimate power. The result should still be verified against the actual piping system, controls, boiler connection conditions and the selected pump manufacturer's data.
9. Why does feedwater temperature affect NPSH?
As feedwater temperature rises, water vapor pressure increases. That reduces the pressure margin available before local boiling or cavitation can occur at the pump suction. Temperature also changes water density. Hot deaerated feedwater therefore deserves a careful suction-side check using the actual vessel pressure, elevation, suction losses and manufacturer NPSHr.
10. Does this calculator select a specific pump model?
No. The calculator produces a preliminary duty estimate, not a vendor model selection. After calculating design flow and head, review manufacturer curves for a pump that operates near an appropriate efficiency region and then check NPSHr, minimum flow, power, materials, pressure rating, speed, controls, and the manufacturer's recommended motor rating.
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Engineering Disclaimer
This calculator provides preliminary engineering estimates and is not a substitute for detailed hydraulic analysis, certified pump curves, manufacturer specifications, applicable codes, or qualified engineering review. Boiler and pressure systems can involve high pressure and temperature. Verify all operating conditions and equipment selections before design, procurement, installation, or operation.