Corrosive compounds. Too-high heat. Sudden plummets in pressure. All can contribute to failure in boiler feed water (BFW) pumps, leading a plant's operations to grind to a costly halt. Avoiding BFW pump problems starts with understanding the key differences among different types of modern BFW pumps and the insidious actors most likely to take them out of commission.
BFW pumps—typically, high-pressure units that take the condensate that results when boiler-produced steam condenses and feed it back into the boiler—range in size up to 10 MW, though sometimes they can be larger. Both electric-motor-driven pumps and steam-turbine-driven pumps are used. The driver is usually connected to the pump body by some form of properly selected mechanical coupling.
In some steam-generation system designs, large industrial condensate pumps may also serve as the BFW pump. In either case, to force the water into the boiler, the pump should generate sufficient pressure to overcome the steam pressure developed by the boiler. This is usually accomplished through the use of a multi-impeller, high-speed centrifugal pump. Speed variation using a variable-speed electric motor driver or a variable-speed steam turbine driver is the most common capacity control method for BFW pumps.
Another form of BFW pump capacity control runs constantly and is provided with a minimum flow device to stop overpressuring the pump on low flows. The minimum flow usually returns to the tank or deaerator. This is only used for relatively small BFW pumps.
Boiler feed water
Boiler feed water is used to supply ("feed") a boiler to generate steam. The BFW is usually stored, preheated and conditioned in a feed water tank and supplied to the boiler by a boiler feed-water pump system.
Corrosive compounds, especially oxygen and carbon dioxide, need to be removed, usually via a deaerator. Residual amounts can be removed chemically by use of oxygen scavengers. Additionally, feed water is typically alkalized to a pH of higher than 7 to reduce oxidation and to support the formation of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkaline agents such as sodium hydroxide (caustic soda) or ammonia into the feed water. Corrosion in boilers results from the presence of dissolved oxygen, dissolved carbon dioxide, dissolved salts, and other materials.
Deposits can reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any nonvolatile salts and minerals that will remain when the feed water is evaporated should be removed, because they will become concentrated in the liquid phase and require excessive "blowdown" (draining) to prevent the formation of solid precipitates. Even worse are minerals that form scale. Therefore, the makeup water added to replace any losses of feed water should be demineralized and deionized.
In a typical steam generation system, the BFW pump takes suction from the deaerator and discharges high-pressure BFW to the boiler. BFW pump design and fabrication—especially when it comes to material selection and manufacturing methods (such as welding methods)—should take into account all characteristics of the feed water that will be pumped.
Pump selection and sizing
The BFW pump capacity is established by adding to the maximum boiler flow a margin to cover boiler operating swings and eventual capacity reduction in capacity caused by wear. This margin likely will be between 20% and 25% for small and medium plants and around 15% for very large plants. As an indication, a margin of 20% is generally specified for commonly used BFW pumping systems.
The BFW pump discharge piping should contain an isolation valve and check valve. The check valve should be installed between the pump discharge and the isolation valve. The purpose of the check valve is to protect the pump from excessive pressures and prevent reverse flow through the pump. The pump discharge should also be equipped with a minimum recycle system.
The first types of BFW pumps, single- or two-stage between-bearing pumps, aren't seen frequently and aren't suitable for many large or high-pressure BFW pumping systems. The second BFW pump type is a heavy-duty axially-split case horizontal pump usually with opposing impellers. They are axially split multistage between-bearings pumps sometimes known as BB3 per the American Petroleum Institute's API-610 standard. These units are specifically designed for heavy-duty, medium and high-pressure applications—up to, for example, 270 Barg and 200°C operating temperature (values are noted just as rough indications). Again as a rough indication, they can be used up to 3,500 m3/h capacity. They are traditional BFW pump designs and they are very popular with experienced, traditional operators. The speed of these BFW pumps could be up to 6,500 rpm.
The third BFW pump type is a double-casing radially split multistage between-bearings pump (barrel pump), also known as BB5 as per API-610. These units are specifically designed for heavy-duty, high- and very-high-pressure applications—up to 450 Barg. Different designs, such as multistage diffuser or double-case volute types, are available for these pumps.
The full BB5 BFW pump ranges and designs are known as modern-generation BFW pumps; they have been designed to produce advanced BFW pumps with high speeds and a minimum number of impellers with reduced lifetime costs. All the pump internals can be withdrawn quickly without disturbing pump alignment or pipeworks. This reduces maintenance time and costs. As a very rough indication, they can be used up to 3,000 m3/h capacity. They are modern BFW pumps and are very popular with operators and designers today. The speed of these BFW pumps could be as high as 8,000 rpm or more.