Steam generation systems are critical units in many industrial and power plants, and the boiler feedwater (BFW) pump plays a key role in the operation of these systems. The BFW pump is a special kind of pump that requires careful design and operation.
BFW pumps are used for feeding water to a boiler, where heat energy will be supplied and feedwater will be changed into steam under pressure. Nowadays, BFW pumps commonly operated at temperatures of 120–260 C (or more), operating pressures of 50–250 Barg (or above), and power ranges of 1–9 MW. High-speed pumps of 3,000 RPM or more are normally employed in BFW services. Due to the extreme conditions in which BFW pumps operate, they are prone to failure when improperly designed or operated and are a major cause of steam system unavailability. Specifying, purchasing, installation, commissioning, operation, and reliability improvements on BFW pumps require a deep knowledge and experience of their hydraulic, process, thermal, mechanical, and dynamic behaviors.
|An installation of a boiler feedwater pump.|
BFW pumps for steam boilers should be capable of withstanding severe thermal shocks in order to protect the boiler—since a boiler failure would result in an unacceptable and costly plant shutdown due to steam unavailability or damage.
For safe and reliable operation, the boiler should have an uninterrupted supply of feedwater, which is within close temperature limits of the rated temperature to avoid thermal shock and possible damage or even catastrophic failure. It is therefore essential that a standby BFW pump be available at all times. The standby BFW pump should be capable of accepting within a few seconds the full flow of full-temperature water irrespective of its standing temperature, which may be 100-150 C lower than that of the running pump. The major factor in the mechanical design of a BFW pump is the thermal shock to which the pump may be subjected. In addition to the stresses imposed by pressure and by operation, consideration should therefore be given to the stresses due to differential thermal expansion during rapid changes of temperature.
The shell of the barrel of a BFW pump is usually exposed to severe temperature fluctuations on its inner wall, while its outer wall, usually exposed to atmosphere, would tend to lag in temperature behind the inner walls during temperature changes. This makes BFW pumps prone to temperature stress within the metal due to the differential expansion between the inner and outer walls.
The thermal shock can cause risk of internal and external misalignments if the elements of the pump are allowed to expand unequally with respect to the axis of the shaft. Internal misalignments could be because of misalignment of bearings and internal parts of pumps. External misalignments are the misalignment of pump, gear unit (if used), and driver. Stresses can generally cause risk of misalignment if they are unsymmetrical around the shaft axis.
The symmetric design is the key for reliability and safety of BFW pumps. Generally, in modern turbo-machine designs particular for extreme temperatures (high or low temperatures) and high pressure applications, symmetry about the shaft axis is an important consideration. The symmetry of construction, of flow and of stresses in order to withstand rapid temperature changes and high pressures without distortion or misalignment issues, should always be respected for BFW pump design and selection. It is essential that all thermal flow during transient conditions, all water flow, all pressure containing sections, and all stresses should be as symmetrical as possible about the shaft axis.
BFW pumps are most often under rapid heating and relatively slow cooling. The heating rate could be around 400–550 C per minute and the cooling rate would be around 40–70 C per minute. The thickness of the high-pressure BFW pump casings could be 20–100 mm, which is usually the thickest in pumps. In most severe thermal shock situations, there is a possibility that stresses at 5–15 percent of the casing thickness pass the allowable stress limit, particularly for large and high-pressure pumps at high heating-up rates. There is often no danger in this case, since the frequency of such shock is relatively small with respect to the fatigue range of the material, and since the reversal shock of cooling is much less severe. Based on thermal and stress studies for BFW pump casings, a high tensile steel should be used. The resulting reductions of the casing thickness had the advantage of a very much lower temperature differential between walls, and since the material had a higher yield point and consequently higher allowable stresses. In these designs, maximum experienced stresses would not pass the limits. An improvement of corrosion and erosion resistance could also be obtained with the higher tensile steels.
The bolts holding the casing elements together to form a pressure containing system are partially exposed to the air, and consequently will change temperature less rapidly than the casing wall, which is in contact with the BFW. When the pump is heated suddenly, the bolt stress is increased by the differential expansion between the hot casing and the cool bolts. During this heating period, the stresses in the bolts should not exceed the allowable stresses (allowable stresses should be defined based on yield stresses of the materials with sufficient safety factors). Conversely, when the pump casing is cooled to a temperature below that of the bolts, the resulting differential expansion will cause a reduction of bolt tension. It is essential that at their minimum stress conditions, the bolts contain a sufficient margin of tension to hold the main joints against risk of leakage.
Corrosion & erosion
The BFW should be specially treated to avoid problems in the boiler and downstream systems. Untreated boiler feedwater can cause corrosion and fouling. Corrosive compounds, especially O2 and CO2, should be removed, usually by the use of a deaerator.
Deposits reduce the heat transfer in the boiler, reduce the flowrate, and eventually block boiler tubes. Any non-volatile salts and minerals that remain when the BFW is evaporated should be removed, as they can 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.
The treatment of feedwater to give minimum corrosion and scale formation of the boiler may result in a liquid that is quite erosive at the flow speeds associated with a high-pressure (high-speed) pump.
In other words, the BFW treatment is usually designed to give minimum corrosion at the boiler. Such treatment may, however, result in a liquid which is strongly corrosive and erosive at the high flow speeds and pick-up speeds associated with high-pressure pumping. The generation of high heads per stage involves correspondingly high flow velocities in the BFW system. The BFW system can be highly corrosive and erosive at high velocities, since the protective film of the salt or oxide of the metal, normally found in static corrosion, is eroded away by the high velocity of the system. This corrosion and erosion occurs despite the fact that there may be no abrasive particles in BFW. The proper type of stainless steel or alloy steel should be used in BFW pumps to prevent corrosion and erosion.
The erosion-corrosion and the corrosion-fatigue have been reported for BFW pumps. The erosion-corrosion is the acceleration or the increase in rate of deterioration or attack on a metal because of relative movements between a corrosive fluid and metal surfaces. Cavitation damage is usually considered a special form of the erosion-corrosion, which is caused by the formation and collapse of vapor bubbles in the liquid near a metal surface. The corrosion-fatigue is defined as the reduction of fatigue resistance due to the presence of a corrosive medium. The corrosion-fatigue is also influenced by the corrosive to which the metal is exposed.
The oxygen content, temperature, pH, and solution composition can influence corrosion-fatigue. The corrosion-fatigue resistance might be improved by using proper coatings. However, a coating is usually discouraged in BFW pump applications. A proper design with a correct material selection is nearly always selected. Too often, the corrosion-fatigue process could result in cracks in pump components. The high-speed of BFW pump rotating parts favored the growth of cracks, and finally the component could be broken. There have been some unplanned shutdowns of steam generation systems because of the corrosion-fatigue in BFW pumps.
The material selection is an important consideration for BFW pumps. The use of suitable grades of stainless steel in BFW pumps have resulted in better reliability, safety, and the long lasting of various components and parts.
Reliability & Availability
High availability, usually above 99 percent, has been required for a BFW pumping system. High availability is required in BFW applications in order to keep the steam generation unit running at its own maximum availability. Operators don’t want to have to shut down the plant (whether an industrial plant or a power generation unit) for failure of an auxiliary pump system. A standby BFW pump is necessary for nearly any BFW pump system.
A considerable amount of attention is being given to suction piping and suction system performance. An important reason is the possibility of cavitation. The size of steam generation units has been increased constantly in the last couple of decades. The capacity of steam generation units has been increased steadily in the last 50 years, which resulted in the demand for higher capacity BFW pumps, higher speeds, and more NPSHR. However, the height at which a deaerator is installed (which is related to the NPSHA) has not been increased with the same rate. In modern large steam generation unit designs, the height at which the deaerator is installed and the BFW pump suction piping require special attention.
Multistage BFW pumps are often designed and built in two different configurations:
- The “In-Line” configuration (also known as the “Equidirectional” configuration).
- The “Back-to-Back” configuration (also known as the “Opposite-Impeller” configuration).
Advantages and disadvantages of the two designs are analyzed and described in this section. In the selection, different factors such as hydraulic, structural, dynamic, and operational considerations should be respected. Particular attention is required for the axial load balance and the lateral dynamic behavior, with new and worn clearance conditions.
The in-line configuration is simpler, more compact, and in many cases more efficient. In this configuration, the flow leaving the impeller outlet is conveyed into the diffuser and then to the eye of the subsequent impeller.
The “back-to-back” configuration consists of two groups of impellers, with one group installed opposite the other group. The number of impellers in the first group is half of the total number of impellers if the number is even. If the total number of impellers is odd, the number of impellers in the first group of impellers is usually half of the total minus one. After the first group of impellers, the BFW flow is conveyed via two crossover channels to the second group of impellers, which are situated opposite the pump. During this crossing, the flow is subjected to a pressure drop. The hydraulic efficiency of the pump—and as a consequence the overall efficiency—is affected by this pressure drop.
On the other hand, the in-line configuration brings a very high axial load, due to the sum of the axial thrusts of every impeller. A balancing drum is necessary to balance the thrust and to reduce the load acting on the thrust bearing.
Back-to-back pumps are always well balanced (especially when the number of stages is even), and the balancing drum is less critical. This is an important issue, particularly when all clearances begin to increase. Assuming a uniform wear of all seals and rings, when clearances are increased with respect to design ones, a back-to-back pump is still well balanced, while for an in-line pump the axial load increases to a high value, which could be 5-10 times the rated axial load of an axial bearing.
Different flow leakages are reported on balancing drums for both pump configurations. An in-line pump balancing drum is usually subjected to a total differential pressure of all stages. On a back-to-back pump, the total differential pressure is usually subdivided in two balancing drums. The diameter and clearances of an in-line pump balancing drum are greater because of a higher axial load. Therefore, the balancing drum total leakage is greater for an in-line pump compared to a back-to-back pump. Leakages in an in-line pump drum could be 30–65 percent more than ones in a back-to-back pump. This could affect the volumetric efficiency and the overall performance of a BFW pump.
It is difficult to give a general instruction on which configuration is the best for BFW services. Both configurations are used today for different BFW pumps. Generally, the best selection is dependent on the application. However, there is a preference for large, high-pressure multistage BFW pumps.
The back-to-back configuration has small hydraulic and technological disadvantages for the crossover channel required to convey the flow from the first group to the second group of impellers. On the other hand, it seems more advantageous for the balancing of axial load and for volumetric efficiency, mostly in worn clearance conditions. Different dynamic studies indicated that the back-to-back configuration can lead to a rotor behavior, which is less sensitive to the increase of clearances. The damping factors of this configuration are usually high. In most applications, particularly large, high-pressure multistage BFW pumps, the back-to-back configuration can increase the reliability of the pumps.
Amin Almasi is a senior rotating machine consultant in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical), IMechE (CEng MIMechE), holds bachelor’s and master’s degrees in mechanical engineering, and is a registered professional engineer in Queensland. He specializes in rotating machines, including centrifugal, screw, and reciprocating compressors, gas turbines, steam turbines, engines, pumps, subsea, offshore rotating machines, LNG units, condition monitoring, and reliability. Mr. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, subsea, and reliability. He can be reached at email@example.com.