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.
Pump piping and valves
At low- or no-flow conditions, a multistage high-head BFW pump can easily generate excessive pressure and heat, which can lead to pump failure. To prevent this, a 'minimum recycle system' should be provided to ensure that a minimum flow is maintained through the pump. The pump vendor generally specifies the minimum flow requirements for its pumps. Some pumps may require a minimum flow of 30% (or more) of the rate pump flow. This requirement can be met in a number of ways but in general consists of having a recycle line back to the deaerator with orifices and valves to control the amount of flow.
The simplest system consists of a recycle line with orifice(s); this is used in small BFW pumps. The orifices are sized for the minimum flow rate as specified by the vendor and to let down the pressure from the pump discharge. For some very small and simple pumping systems, this 'minimum recycle system' maintains a continuous recycle flow even during normal operation. The wastefulness inherent in this system, however, is undesirable, and the system generally isn't used in units above 20 kW.
An alternative system calls for use a special valve sometimes known as a recirculation valve. With this setup, as the flow decreases, the valve automatically opens the recycle line, maintaining the minimum required flow at all times. As flow to the boiler increases, the recirculation valve automatically closes the recycle. The recirculation valve provides some pressure reduction in the recycle flow, but the system should be checked to determine whether additional orifices are required for pressure reduction.
An additional alternative pressure-control method uses instrumentation to detect flow and open a control valve in the recycle line. A flow meter in the main feed line measures the water flow to the boiler. When the flow drops below the minimum flow specified by the pump vendor, a controller begins opening the recycle control valve in the recycle line. When the flow meter measures zero flow, the control valve will be fully open. Orifices should again be installed in the recycle line if additional pressure reduction is required.
For large BFW pumps, a sophisticated control arrangement typically manages the minimum recycle system..Usually, one or two standby BFW pumps are specified as backup to the normally operating BFW pump. The standby pump should start automatically if the pressure drops in the feed water line or if the operating BFW pump fails.
At least one of the pumps should be capable of operating during an electric power failure. This is accomplished by having at least one pump driven by a steam turbine. In most cases, even if there is a site electric power failure, there would be enough steam remaining in the steam drum and headers to operate a steam-turbine-driven BFW pump until the steam system could be safely shut down. An electric-motor-driven pump on emergency power could also be used. In many plants, a steam-turbine-driven BFW pump is employed as the main operating pump, whereas an electric-motor-driven pump is used as standby. Other plants, however use a steam-turbine-driven BFW pump as the standby pump. For steam-turbine-driven BFW pumps on standby, the steam turbine should be kept warm by operating the unit on a slow roll. In this way, a small amount of steam is bled to the steam turbine to keep the casing and impeller warm and slowly turn the turbine and pump.
Suction and NPSH
Suction piping should be as short and direct as possible with the minimum number of bends. Pump vendors have suggested that the suction piping be at least two sizes larger than the pump suction nozzle.
During commissioning, the suction piping is flushed to remove pipe scale and construction debris. A temporary strainer will be installed in the suction line during startup and left installed until it is ensured that the line is clean. The free area of the strainer's mesh should be at least four times the area of the suction pipe. Proper differential pressure must be provided to monitor the temporary strainer's pressure drop.
Other purposes for a deaerator are to provide the required net-positive suction head required (NPSHR) for the BFW pump and to serve as a storage tank to ensure a continuous supply of BFW during rapid changes in BFW demand. The available net-positive suction head provided to a BFW pump can drop enough during a pressure excursion to cause cavitations and damage to the pump’s internal parts. A careful analysis of various operating profiles and scenarios can ensure that the BFW pump operates safely during the pressure fluctuations that occur after a steam turbine trip or large load change.
A deaerator is installed at some elevation above the BFW pump to provide the NPSH required by the pump. By definition, the NPSHR is the total suction head over and above the vapor pressure of the liquid pumped. The deaerator elevation minus the dynamic losses and other losses in the BFW suction piping between the deaerator and the BFW pump indicates the NPSH available (NPSHA) to the BFW pump.
The difference between the value of the NPSHA and that required (NPSHR) by the BFW pump gives the NPSH margin. The NPSH margin or the NPSH margin ratio (NPSHA/NPSHR) is an important factor in ensuring adequate service life of the BFW pump and minimizing noise, vibration, cavitation, and seal damage. The NPSH margin requirement increases as the suction energy level of the pump increases. For example, high suction-specific speed or high peripheral velocity of impeller will result in a higher NPSH margin requirement.
As an indication, for a BFW pump, this ratio could be in the range of 1.6 to 2.4. Traditionally, NPSH margin ratios around or above 2 were encouraged. Indeed, for small and medium-size steam generation systems, it is possible to achieve NPSH margin ratios of around 2. For very large steam generation plants, particularly those using high-speed special BFW pumps with relatively high NPSHR, a NPSH margin ratio around 2 can result in an extraordinary high elevation of deaerator, which might not practically be possible. In those cases, all possible scenarios should be carefully studied, and a proper NPSH margin ratio should be selected with respect to all aspects of design. For those cases, a NPSH margin ratio between 1.6 and 1.8 could be used after studying all possible operating cases and abnormal or transient scenarios.
Generally, a high NPSH margin improves the ability of the BFW pump to handle a deaerator pressure transient. Once a design is determined to have an adequate NPSH margin, it should also be determined if the NPSH margin is adequate during all possible cases of pressure transients. It’s not uncommon to find that the BFW pump was originally specified based on steady-state conditions and did not consider the deaerator pressure transients that occur during a steam system trip (with the boiler remaining in service) or a sudden steam turbine load reduction. If the NPSH available to the BFW pump during the pressure transient drops below that required by the pump for only a short period of time, cavitation and damage to the pump internals often result.
Boiler feed water pumps are critical components of any steam generation system; as such, their reliability is extremely important. NPSH margin, pump piping, all steady-state and transient operating cases and details of upstream and downstream of the BFW pump need attention.