What are the selection requirements for boiler feed water pumps?
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Operating conditions of the boiler feed pump: The boiler feed pump is used to deliver high-temperature, high-pressure water to the boiler drum in the boiler feed system, in order to maintain the proper operation of the boiler. Continuous water supply is crucial for steam boilers, as it not only prevents overheating but also avoids further damage to the boiler. Its specific operating conditions are as follows: 1) High pressure 2) High temperature 3) High speed (acceleration and/or speed variation) 4) Variable conditions (with peaks and valleys). Types of boiler feed pumps: The common boiler feed pumps are mainly of horizontal design, supported at both ends by bearings, and are multi-stage centrifugal pumps. There are mainly three types: 1) Radially split multi-stage segmented centrifugal pump (BB4 type), 2) Double-shell multi-stage centrifugal pump (BB5 type), 3) Axially split multi-stage centrifugal pump (BB3 type)
It should be noted that API 610, version 11, specifies that radial split-case pumps must be used if the operating conditions fall under any of the following situations: a) if the temperature of the medium transported by the pump reaches 200 °C or higher (a lower temperature limit should be considered if the pump may be subject to thermal shock)； b) Liquids with a relative density of less than 0.7 at the specified pumping temperature ； c) The rated outlet pressure of the pump exceeds 10 MPa·G. Interpretation: In other words, the BB3 type of pump shall not be used when the temperature of the pumped medium reaches 200 °C or higher, or when the rated outlet pressure of the pump exceeds 10 MPa·G. However, in practical engineering applications, despite there possibly being successful experiences in overcoming these two limitations, the BB3 type pump needs to be chosen carefully for safety reasons.
For specific boiler feedwater applications, there are no fixed rules for selecting pump types. This depends to a large extent on the manufacturer’s application experience and product availability, as well as the user’s experience in terms of maintainability and reliability. If applied in conditions of variable load and/or thermal shock, special consideration must be given to using a sufficiently robust and proven pump design. Comparison of the three types of boiler feedwater pumps: 1) Both the BB4 segmented pump and the conventional BB5 double-housing pump have a radial split structure with radial sealing surfaces; their geometric shape results in sealing surfaces that are smaller and simpler compared to those of the BB3 axial split structure. 2) The geometric design of the BB4 segmented pump and the BB5 double-housing pump is suitable for higher pressure conditions. Dual-shell pumps offer the highest safety and reliability when the pressure exceeds 25 MPa·G. 3) Compared with the BB4 type segmented pump, the BB5 type double-housing pump has fewer external sealing mating surfaces. 4) The BB3 type axially split and BB5 type double-shell pumps allow the entire assembled rotor to be removed and balanced as a single unit. These rotors may have impellers arranged back to back in order to reduce the hydrodynamic thrust borne by the thrust bearings. Meanwhile, the inner casing of the BB5 pump may also adopt an axially split structure (see Figure 4).
Material selection: The internal components of the boiler feedwater pumps used in power plants are typically made of 400 series stainless steel (martensitic stainless steel). When the pumping temperature exceeds 120 °C, BB3 and BB4 pumps use 400 series stainless steel housings. The outer cylinder of the BB5 type pump can be made of cast steel or forged steel, and is usually designed in accordance with ASME standards. When forged carbon steel materials are used for the outer cylinder, a layer of stainless steel is typically welded on the inner surface areas exposed to high flow rates (areas prone to erosion) to improve corrosion resistance. Shaft seal: The standard shaft seal type for boiler feed water pumps is a single-face, balanced mechanical seal ； Standard flushing protocol: Plan 23. Drive method: Boiler feed pumps generally have two drive methods: turbine-driven and motor-driven. Turbine-driven pumps are usually used as main pumps during normal operation ； Motor-driven pumps are usually used for starting up and in emergency situations (as a backup). Pumps that are operating normally can also be driven by electric motors. For boiler feed pumps used in larger or massive power plants, since the pressure provided by the deaerator is usually not sufficient to meet the suction requirements of the feed pump, a pre-pressurization pump is often necessary.
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