SPECIFYING AND MAINTAINING BOILER FEEDWATER CENTRIFUGAL PUMPS

Boiler feedwater (BFW) pumps are a special class of high-pressure centrifugal pumps used to pump purified BFW at high pressures to boilers to produce steam. They are usually high-speed, high-power units with specific requirements for each service, material selection and pump design.

Steam turbine-driven BFW pumps are often used in steam generation systems. Properly designed steam turbine drivers are typically more reliable than electric motors for high-speed BFW pump applications. As they consume steam generated in the boiler, they are usually more efficient, and easier to control.

However, electric motor-driven BFW pumps are still needed for start-up, back-up and standby. Electrical motor drivers can be started faster, making them a better option for standby and spare pump trains.

The same model of BFW pump is generally used in both electric-driven and steam-driven BFW pumps to encourage commonality, and ease of operation and maintenance.

In a steam generation system, a steam-turbine-driven BFW pump is provided for normal operation. 2 x 100% electric motor-driven pumps are provided for start-up, backup and standby. Overall, 300% of the installed capacity is provided. As the required availability is 100%, such an expensive solution is needed.

In a critical, large steam generation system, 2 × 50% steam-turbine-driven BFW pumps (total 100%) are provided for normal operation and 3 × 50% electric-motor-driven pumps (overall 150%) are furnished for backup and standby.

A 30% electric-motor-driven BFW pump is also provided for start-up of the large unit. In this case, installed BFW pumps amount to 280% of the required capacity, and provide 180% of spare and standby capacity. In addition, a 10% to 15% flow margin should also be considered for each BFW pump.

Components and sub-systems

Casings for BFW pumps should be designed to withstand their maximum working pressures. This should be assessed during extreme conditions such as: the shutoff point at maximum speed (when the discharge valve is closed); failure of the discharge non-return valve (check valve); obstruction of flow in the suction piping; or a combination of these, with hot water, cold water and other possible thermal shocks.

The pump casing should enable the removal of the internal assembly without disturbing the piping. Therefore, casing design, fabrication and assembly need great care as many considerations are involved.

Many pumps usually use flange connection at both suction and discharge for connection to the piping. However, for many BFW pumps, discharge flanges have been a source of leakage and trouble.

In small or medium BFW pumps, flanges of one or two pressure ratings above the calculated rating should be used to provide a margin and prevent leakage. However, this is expensive.

For high-pressure and large BFW pumps, flange connections at the discharge may not be feasible, particularly if pressure and temperature are high. The connection between the pump casing and BFW at the discharge nozzle (and sometimes both the suction and discharge) is usually welded.

Corrosion and erosion, major issues for BFW pumps, require careful material selection. All areas subject to possible erosion should be protected by linings or alternative methods. BFW pump shafts should be machined from high-quality forgings of a carefully selected grade of stainless or alloy steels. They should be heat treated. Additionally, the rotor assembly must be properly designed, fabricated, tested and balanced.

Impellers should be of the shrouded type and fitted to the rotor shaft in such a manner as to be readily removable, and to ensure freedom from thermal distortion, due to high operating speeds and loadings. High speed, speed variations and high head place them under considerable stress, requiring careful stress analysis and fatigue calculations.

All stage impellers should have a useful lifetime of more than 140,000 hours. Each impeller and each rotating part should be individually balanced. In addition, the rotor assembly should be dynamically balanced and the assembled rotor checked for concentricity.

Tilting-pad journal bearings are often used for BFW pumps. However, small BFW pumps might use rolling-element bearings. Provision should be made to permit vertical and horizontal adjustment of each bearing by shim manipulation. The return oil flow from each bearing should be visible and the oil should remain at local temperature.

In addition, sensors are needed for flow, temperature and pressure of the lubrication oil system (both supply and return), as well as measurement of vibration and temperature. Duplex thermocouples should be installed in the white metal of each journal bearing and within each pad of thrust bearings. Bearings should be arranged so they are readily accessible for examination and replacement without removal of the rotor assembly from the pump.