Classification,Structure and selection considerations for boiler feed pumps
I. Operating Conditions of Boiler Feed Pumps
Boiler feed water pumps are used to deliver high-temperature, high-pressure water to the boiler drum in the system to maintain the operation of the boiler. Continuous water supply is crucial for steam boilers, not only to prevent overheating but also to keep the boiler safe. Its specific operating conditions are as follows:
1) High voltage
2) High temperature
3) High speed
4) Variable operating conditions
1. Common boiler feed water pumps are mainly horizontal structures, supported at both ends of the bearings, and are multi-stage centrifugal pumps. There are three main types:
(BB3 type)
centrifugal pump
(BB4 type)
Radial split multistage segmental centrifugal pump
(BB5 type)
Dual-casing multistage centrifugal pump
2. Comparison of three types of boiler feed water pumps:
1) Both the BB4 segmental pump and the conventional BB5 double-casing pump are radially split , while the inner casing of the special BB5 pump may adopt an axially split structure. The BB3 is an axially split type.
2) The geometry of the BB4 segmental pump and the BB5 double-casing pump is suitable for higher pressure conditions. The BB5 double-casing pump has the highest safety and reliability when the pressure exceeds 25 MPa·G.
3) Compared with the BB4 type segmental pump, the BB5 type double-casing pump has fewer external sealing surfaces .
4) The BB3 type axial split and BB5 type double-casing pumps allow the entire assembled rotor to be disassembled and balanced as a unit.
II . Precautions for Water Pump Selection
① Mechanical seals should be used for drive shaft sealing whenever possible, especially for feed water pumps in steam boiler rooms. Although packing seals are simple to maintain, they have a high failure rate and maintenance rate, and require a large workload. Therefore, mechanical seals should be selected, with a service life of more than 8000 hours.
②If a variable frequency water supply system is selected, the pump head should be slightly higher than the design value. Since the pump head will decrease by 20-30m after 3-5 years of operation due to impeller wear and increased clearance, the selected pump head should be about 30% higher than the design value to ensure normal water supply after many years and to maintain the energy-saving effect of the frequency converter, thereby extending the pump's service life.
③When the water supply temperature is normal, no special specification is required when selecting a pump. If the water supply temperature is >100℃, the water supply temperature should be emphasized when selecting a pump. This is because high-temperature pumps and low-temperature pumps differ in materials, structure, and anti-cavitation functions.
④ When the highest point of the water pump inlet is lower than the water level in the tank (or pool), no special instructions are needed when selecting a pump. When the highest point of the water pump inlet is higher than the water level in the tank (or pool), a pump with strong suction should be selected.
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