Operation of boiler feed pumps

The operation of a boiler feed pump consists of several stages.

1. Suction

The feedwater is usually supplied from a deaerator or condensate tank to the suction side of the pump.

Deaerators remove oxygen and gases from the water to reduce corrosion.

Important factors include:

• Sufficient NPSH available

• Low resistance in the piping system

• Stable water supply

Insufficient NPSH can lead to cavitation.

2. Impeller Acceleration

The rotating impeller accelerates the water centrifugally outward, increasing the velocity of the water.

This is the first energy transfer stage of the pump.

3. Pressure Boost in Multiple Stages

After each impeller, the water flows through a diffuser or guide vane before entering the next stage.

Each stage further increases the pressure.

This is why boiler feed pumps are usually designed as multistage pumps. A single impeller would often be unable to generate sufficient pressure.

4. Diffuser / Casing Conversion

Inside the diffuser, velocity is converted into pressure energy.

This improves pump efficiency and ensures stable pressure build-up.

5. Discharge to the Boiler

After the final stage, the water leaves the pump through the discharge connection and flows toward the boiler drum or economizer.

There, the water becomes part of the steam generation process again.

6. Recirculation Protection

When the flow rate becomes too low, overheating can occur.

For this reason, many installations are equipped with a minimum flow recirculation line. This returns a minimum amount of water back to the tank.

This protects the pump against:

• Overheating

• Unstable operation

• Internal damage

A boiler feed pump is usually a multistage centrifugal pump designed for high pressure and high reliability.

1. Casing

The casing forms the pressure-resistant housing of the pump. It must withstand temperature fluctuations and high internal pressure.

Common designs include:

Barrel Casing

In a barrel casing design, the hydraulic assembly is enclosed within a heavy pressure shell. This design is often used for very high-pressure applications.

Advantages:

• High safety

• Strong construction

• Suitable for severe operating conditions

Ring Section Pump

In a ring-section design, the pump consists of multiple segments assembled together.

Advantages:

• Modular design

• Easier maintenance

• Suitable for medium-high pressure applications

2. Rotor and Impellers

Multiple impellers are mounted on the shaft. Each impeller increases the pressure step by step.

This means:

• Stage 1 increases the pressure

• Stage 2 increases it further

• Stage 3 increases it further

• And so on

This ultimately creates the high discharge pressure required for the boiler.

3. Bearings and Balancing

Due to the high pressure, radial forces and axial thrust are generated on the rotor.

For this reason, boiler feed pumps are equipped with:

• Heavy-duty bearings

• A balancing drum or balancing disc

• Precise alignment

• Low-vibration construction

This prevents damage to the shaft, bearings, and seals.

4. Shaft Seal

To prevent leakage, the following sealing solutions are commonly used:

• Mechanical seals

• Seal systems with flushing

• In some cases, gland packing

At high temperatures and pressures, reliable sealing is essential.