Boiler Feed Pumps
Boiler feed pumps transfer treated feedwater from a feed tank or deaerator into a steam boiler. The pump must raise the water pressure above the boiler operating pressure while also overcoming losses through pipework, valves, economisers and other system equipment.
AxFlow supplies industrial boiler feed pumps for small process boilers through to demanding high-pressure steam and power-generation systems. Our range includes horizontal and vertical multistage centrifugal pumps, low-NPSH side-channel pumps and specialist canned motor pumps.
What Is a Boiler Feed Pump?
A boiler feed pump is a high-pressure pump that delivers feedwater into a steam boiler at the flow and pressure required to maintain the correct boiler water level.
Feedwater usually consists of returned condensate combined with treated make-up water. The boiler feed pump typically takes this water from the feedwater tank or deaerator and increases its pressure sufficiently for it to enter the boiler.
Reliable boiler feedwater pumping is critical because insufficient flow can cause low boiler water levels, interrupt steam production and place the boiler at risk.
How Does a Boiler Feed Pump Work?
Most industrial boiler feed pumps use a centrifugal pumping principle. Water enters the first-stage impeller, where rotational energy increases its velocity and pressure. In a multistage pump, the water then passes through several impellers and diffusers arranged in series.
Each stage adds pressure while the flow remains broadly constant. This allows a multistage boiler feed pump to generate high discharge pressures without requiring an excessively large single impeller.
The pump may operate continuously with a modulating feedwater valve or variable-speed drive, or intermittently in response to a boiler level controller. The correct arrangement depends on boiler design, steam demand and the required level-control philosophy.
Why Are Multistage Pumps Used for Boiler Feedwater?
Multistage centrifugal pumps are commonly selected for boiler feed applications because they combine high-head capability with smooth, continuous flow.
Key advantages include:
- High discharge pressure from a compact pump
- Efficient transfer of clean, low-viscosity feedwater
- Stable continuous flow
- Horizontal and vertical configurations
- Materials suitable for hot treated water
- Compatibility with variable-speed control
- Multiple seal, bearing and balance-device options
Multistage pumps are also available with low-NPSH first-stage designs for installations where suction conditions are restricted.
Boiler Feed Pump Applications
Boiler feedwater pumps are used wherever steam is generated for heating, power or industrial processing.
Typical applications include:
Industrial Steam Boilers
Reliable feedwater supply for steam boilers used in manufacturing, food production, chemical processing, hospitals, laundries and other industrial facilities.
Power Generation
High-pressure boiler feed pumps for conventional power stations, combined-cycle plants, biomass facilities, waste-to-energy plants and industrial cogeneration systems.
Process Steam Systems
Feedwater transfer for boilers supplying steam to reactors, heat exchangers, evaporators, dryers, sterilisers and other process equipment.
Heat-Recovery Steam Generators
Boiler feed pumps used to supply water to heat-recovery steam generators and waste-heat boiler systems.
Condensate and Feedwater Systems
Pumps for moving hot condensate to a deaerator or feedwater tank and delivering the combined feedwater stream into the boiler.
A condensate return pump and a boiler feed pump perform different duties. The condensate pump returns recovered condensate to the feed system, while the boiler feed pump raises the final feedwater pressure above boiler pressure.
How to Choose a Boiler Feed Pump
Selecting the correct boiler feedwater pump requires a review of the complete steam and feedwater system—not simply the boiler pressure.
Required Feedwater Flow
The pump must provide the boiler manufacturer’s required feedwater rate at maximum steam output. The calculation should also account for boiler blowdown, make-up water requirements, operating margin and any expected future demand.
Avoid applying excessive flow margin, as an oversized pump may operate inefficiently and spend more time below its recommended operating range.
Total Discharge Head
The required pump head should include:
- Boiler operating pressure
- Static elevation
- Pipe and fitting losses
- Economiser pressure drop
- Feed-control valve pressure drop
- Check-valve and equipment losses
- A suitable operating margin
Boiler pressure alone is not the complete pump duty.
Feedwater Temperature
Hotter feedwater has a higher vapour pressure, reducing the pressure margin available at the pump inlet. Feedwater temperature must therefore be considered when calculating NPSH available and selecting seals, bearings and materials.
NPSH and Cavitation
The system’s NPSH available (NPSHa) must exceed the pump’s NPSH required (NPSHr) by an appropriate margin throughout the full operating range.
Feed tank pressure, water temperature, pump elevation, suction-pipe losses and minimum tank level all affect NPSHa. Low-NPSH first-stage impellers, increased static head or a revised suction arrangement may be required where hot feedwater is close to its saturation conditions. Boiler feed pump manufacturers use axial inlets and specialist suction impellers to reduce NPSH requirements on demanding services.
Operating Range and Control
Confirm the minimum, normal and maximum boiler feedwater demand. The pump should operate within its permitted hydraulic range and, wherever practical, close to its best-efficiency region.
Variable-speed control can reduce unnecessary throttling and match pump output to changing steam demand. However, the selected pump’s minimum continuous flow must still be maintained.
Minimum-Flow Protection
A centrifugal boiler feed pump should not operate indefinitely against a closed discharge or below its permitted minimum flow.
A minimum-flow bypass, automatic recirculation valve or suitable control arrangement may be required to prevent internal recirculation, overheating and hydraulic instability.
Materials and Water Chemistry
Pump casing, impellers, shaft, wear components, seals and elastomers must be suitable for:
- Feedwater temperature
- Operating pressure
- Oxygen content
- pH and treatment chemicals
- Chloride concentration
- Start-up and shutdown conditions
Material selection should reflect the actual boiler-water treatment programme rather than assuming that all feedwater is chemically identical.
Reliability and Redundancy
Steam-critical installations commonly use duty and standby pumps so that boiler operation can continue during maintenance or an unexpected pump failure.
Duplex arrangements can also alternate the operating pump, helping balance run hours and preserve standby availability.
Boiler Feed Pump Maintenance
Preventive maintenance helps protect boiler availability and pump efficiency.
Good practice includes:
- Monitoring suction and discharge pressure
- Checking flow and motor current
- Trending vibration and bearing temperature
- Inspecting seals for leakage
- Maintaining coupling alignment
- Checking minimum-flow protection
- Monitoring feedwater temperature and tank level
- Inspecting impellers, diffusers and wear components
- Investigating noise, cavitation or unstable pressure promptly
Changes in vibration, noise, flow or power consumption can indicate cavitation, bearing wear, internal wear, misalignment or operation away from the intended duty point.
When a Standard Boiler Feed Pump May Not Be Suitable
A conventional horizontal multistage pump may not be the best option where:
- NPSH available is extremely limited
- The liquid contains significant vapour or entrained gas
- Feedwater contains solids or heavy contamination
- Installation space requires a vertical arrangement
- Flow is very low relative to the required head
- A sealless or canned motor design is mandatory
- Operating pressure exceeds the selected range
In these cases, a low-NPSH side-channel pump, vertical multistage pump, specialist high-pressure pump or alternative feed-system arrangement may be required.
Frequently Asked Questions
What is a boiler feed pump used for?
A boiler feed pump delivers treated water from a feedwater tank or deaerator into a steam boiler. It raises the water pressure above the boiler pressure and overcomes losses through the feedwater system.
Which pump is best for boiler feedwater?
A multistage centrifugal pump is commonly the best choice for industrial boiler feedwater because it can produce high pressure efficiently while delivering continuous flow. Low-flow, high-head or low-NPSH duties may be better suited to a side-channel pump.
How is a boiler feed pump sized?
A boiler feed pump is sized using the required feedwater flow, total discharge head, feedwater temperature, NPSH available, operating range and control method. The calculation must include system losses and control-valve pressure drop—not boiler pressure alone.
Why are multistage pumps used as boiler feed pumps?
Each impeller stage adds pressure to the feedwater. Connecting several stages in series allows the pump to generate the high head required to overcome boiler pressure efficiently.
What causes cavitation in a boiler feed pump?
Cavitation can occur when pressure at the pump inlet falls too close to the feedwater vapour pressure. High water temperature, low feed-tank level, restrictive suction pipework or insufficient static head can all reduce NPSH available.
What is the difference between a boiler feed pump and a condensate pump?
A condensate pump returns condensed steam to the feedwater tank or deaerator. A boiler feed pump then raises the treated feedwater to a pressure high enough to enter the boiler.
Can a boiler feed pump use variable-speed control?
Yes. Variable-speed drives can regulate boiler feedwater flow and reduce throttling losses. The control system must still maintain the pump’s permitted operating range and minimum continuous flow.
