How do I determine what type of boiler feed pump to select?
Boiler feed pumps can range from serving relatively small commercial boilers to large electric generating power plants. Smaller boilers can use a variety of pump types for this purpose. However, larger generating plants will use multistage between bearing rotodynamic pumps.
For pressures up to approximately 1,800 pounds per square inch (psi) (125 bar), the typical boiler feed pump types include horizontal multistage ring-section (type BB4, Image 1) or axial split case designs (type BB3), although in some applications there may be a preference for a double case pump (type BB5, Image 2). For higher pressures, the boiler feed pump types used are usually BB4 or BB5 types, with BB5 types serving the largest pressure, flow and power requirements.
There are no fixed rules for selecting the pump type for a specific boiler feed application. A lot depends on a combination of manufacturers’ application experience and product offerings as well as user experience in maintainability and reliability. If the plant will be cycled instead of base loaded, then special consideration must be given to the use of a sufficiently robust and proven pump design.
Some users choose to give particular consideration to the main pump construction features:
- Type BB4 ring-section and type BB5 double case pump types both have radial sealing joints, which are geometrically simpler than the BB3 axial pump sealing joint.
- The BB4 ring-section and BB5 double case pump geometries inherently lend themselves to higher pressure containment. The double case pump type has the highest containment
- for pressures exceeding 3,600 psi (250 bar).
- The BB5 double case pump has fewer sealing joints than the BB4 ring-section pump.
- Type BB3 axial split case pumps and the BB5 double case pump allow for the entire assembled rotor to be removed and balanced as a unit. These rotors may have opposed impeller arrangements that reduce the hydraulic thrust carried by the thrust bearing.
The boiler feed pumps used by power generating utilities are typically built with 400-series, stainless steel internals. Types BB3 and BB4 use 400-series, stainless steel casings when the pumping temperature exceeds 120 C (250 F). The outer barrel of pump type BB5 may be cast or forged steel, designed using American Society of Mechanical Engineers (ASME) Section VII as a guide for pressure boundary and bolting. When forged carbon steel material is used for the outer barrel, the internal surface areas exposed to high flow velocity may be weld-metal overlaid with stainless steel to improve erosion resistance.
For more information on boiler feed pumps, refer to “Power Plant Pumps: Guidelines for Application and Operation” at www.pumps.org.