Selecting the right boiler feed pump
How do I determine which type of boiler feed pump to select?
Boiler feed pumps can range from serving relatively small commercial boilers to large power plants. Small boilers can use a variety of pump types for this purpose. However, large power plants will use multi-stage rotodynamic pumps between bearings.
For pressures up to approximately 1800 pounds per square inch (psi) (125 bar), typical boiler feed pump types include multi-stage horizontal ring section designs (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 types of boiler feed pumps used are generally BB4 or BB5 types, with the BB5 types meeting the highest requirements in terms of pressure, flow rate and power.
There are no fixed rules for selecting the pump type for a specific boiler feed application. Much depends on the combination of manufacturers’ experience with applications and product offerings, as well as users’ experience with maintainability and reliability. If the plant is cycled instead of base loaded, special consideration should be given to using a sufficiently robust and proven pump design.
Some users choose to pay particular attention to the main construction features of the pump:
- Both ring section type BB4 and double casing type BB5 pump types have radial seals, which are geometrically simpler than the axial pump seal BB3.
- The BB4 ring section and BB5 double casing pump geometries inherently lend themselves to higher pressure containment. The double sump pump type has the highest containment
- for pressures above 3600 psi (250 bar).
- The BB5 double casing pump has fewer seals than the BB4 annular section pump.
- Type BB3 split case axial pumps and BB5 double case pump allow the entire assembled rotor to be removed and balanced as a unit. These rotors may have opposed wheel arrangements which reduce the hydraulic thrust carried by the thrust bearing.
Boiler feed pumps used by electric power utilities are typically constructed with 400 series stainless steel internals. Types BB3 and BB4 use 400 series stainless steel housings when the temperature pumping exceeds 120 C (250 F). The outer casing of the Type BB5 pump can be cast or forged steel, designed using the American Society of Mechanical Engineers (ASME) Section VII as a guide for pressure limiting and bolting. When a forged carbon steel material is used for the outer barrel, the internal surfaces exposed to high flow velocity may be welded metal coated with stainless steel to improve erosion resistance.
For more information on boiler feed pumps, see “Power Plant Pumps: Guidelines for Application and Operation” at www.pompes.org.
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