Material Selection for Boiler Feed Pumps
There are vast varieties of applications wherein centrifugal pumps are deployed. The scope extends from household applications to critical services such as primary coolant pumps for nuclear reactors. The ultimate aim for an end user is that the pump should perform the required service with excellent reliability and that it proves out its intended service life. In this instance, service life indirectly means the pump materials’ measure of resistance to corrosion, erosion, and wear of the parts.
As different applications demand different materials of construction for the equipment, proper material selection is a paramount to the success of the pump. The question of reliability of the equipment is more pronounced for industrial and power generation applications.
By Abhijeet Keer, Design Engineer
Power and Steam Generation Processes
One of the most important applications in power generation is the boiler feed pump. The power generation cycle is a closed loop known as ‘Rankine Cycle’. In this cycle, electricity is produced through the rotation of a steam turbine, which is in turn driven with the help of high pressure steam. This steam is produced by feeding hot water to boiler through the boiler feed Pump (BFP). The BFP takes the suction from the deaerator and delivers it to the steam drum. The BFP has to handle high pressures and temperatures, thus the selection of materials of construction plays a vital role.
Due consideration has to be given to feed water composition, operating pressure, as well as temperature. Usually, multistage centrifugal pumps are employed for this service.
In the case of steam generation, purity of water matters a lot. In this regard, it is important to have water that is free from any minerals for boilers and other power generation equipment, such as turbines. There are two basic issues that need to be dealt with: the first is scale formation and second is corrosion.
Dissolved minerals cause hardness in the water and are capable of depositing scales on the heat transferring surfaces. The minerals and scales are removed by treating water with processes such as Ion exchange and reverse osmosis. However, removing these minerals will not suffice. Once the water is demineralized, it becomes hungry, due to the ion imbalance, and will dissolve gases from the surrounding air like O2 and CO2. Although using the process of demineralization can ensure the removal of dissolved solids, there is another process called Deaeration, which ensures the removal of dissolved gases; especially O2 to 7 ppb. Complete removal of CO2 is desirable as the formation of Carbonic acid causes the pH of the water to fall in the region of 5 to 6.
The final quality of the water has a significant influence over the selection of materials for the pump equipment. Material selection is the balance between three factors: corrosion resistance, strength, and cost.
Commonly Used Materials
As depicted by Iron-Iron Carbide diagram (see Figure 2) there are two partitions which divide the most commonly used engineering materials, namely, steel and cast iron. For boiler feed pumps, cast iron, carbon steel, and martensitic stainless steel are commonly used. Although the pump is made up of number of parts, and each undergoes separate considerations for the selection of materials; regarding pressure retaining components, the above crude classification can be accepted.
Cast irons contain carbon in the range of 1.7% to 4% by weight. Lower tensile strength and brittleness limits the use of cast irons for boiler feed applications. The only justification, therefore, for using cast iron for pressure retaining components could be lower initial costs.
Iron surfaces when corroded, form metal oxides. When iron reacts with water in an oxygen-free environment a protective layer of iron oxide, known as Magnetite, is formed. This provides a protective barrier against further corrosion. In the case of high purity water that has a high temperature and low conductivity, however, cast iron is unable to produce the surface-passive film of Magnetite. This inability is caused by a number of factors such as: pH, conductivity, dissolved O2 and temperature. If there is oxygen present inside the water, it will continue to cause an erosion corrosion reaction, until it completely ‘eats’ out the material. Moreover, typical limits of pressure and temperature could be below 85 bar and 150°C. The values are generic and careful evaluation of application is required. It is therefore important to be aware of the maintenance of proper operating conditions. Hence, use of cast iron is limited.
Another commonly used material is carbon steel which contains up to 1.7% of carbon by weight. Like cast iron, high purity water at high temperature is detrimental to carbon steel. Unlike cast iron, carbon steel is unable to form metal oxide layer, which is as tenacious as that of cast iron, making it even more susceptible to corrosion. The justification for using carbon steel could therefore be its high strength. Ideally, carbon steel is used when the application media are non-corrosive and have temperatures up to 400˚C.
With these significant limitations, many individuals could question why these material are commonly used. Boiler feed applications need high strength at elevated temperatures in order to contain high pressures effectively. So, in cases where cast iron is inadequate due to poor mechanical properties at elevated temperatures, the ideal choice could be carbon steel. The main alloying elements of carbon steel are Cr, Ni, Mo, and Cu. Although Cr is typically added to steel in order to increase corrosion resistance, here, Cr enhances the mechanical strength of the component. Cr is a strong carbide former and is able to form it at low percentage by weight; in this case, about 0.5% maximum. The formation of chromium carbide ultimately improves hardenability of the material and improves tensile strength. This often helps the parts resist deformations at elevated temperatures. This means, the material will not lose its strength at elevated temperatures. Cr is often added with element such as Ni. Ni, when present in small amounts, improves toughness.
Commonly used material for high pressures and temperatures, beyond the limitations of carbon steel and cast irons, are the martensitic stainless steels. The carbon percentage in martensitic stainless steel varies from 0.06% to 0.15% by weight. Unlike the other two, it gives good balance between corrosion resistance as well as mechanical strength. Additionally, it also offers good cavitation resistance as the presence of chromium in these material grades is from 11.5% to 14%. The martensitic stainless steel containing high carbon content are strengthened because of precipitation of iron carbide at low temperature tempering. This feature, specifically, is of no use for high temperature services because of lower creep resistance. Hence the steels containing lower carbon content are strengthened with the carbide formation at high temperature tempering. The addition of chromium will provide moderate corrosion resistance.
Martensite is a phase which is strong and hard, which is why it has improved strength when compared to carbon steel. However, if left untreated or without tempering, it will be too brittle to be used for the intended purpose. Hence, after quenching to form Martensite, it is tempered to improve its toughness at the expense of some loss in strength. Other important alloying elements in the steel, such as Ni and Mo, increase toughness and high temperature strength, respectively. The presence of Ni and Mo improves corrosion resistance, while Ni also helps to make the oxide layer more tenacious. This feature helps the material to be more corrosion resistant in case of high fluid velocities.
Material Grades and Examples
There are various material grades available in the industry, as per various international standards. Depending upon the different geometry of the components, suitable grades from the various classes, for cast iron, carbon steel, alloy steel or chrome/martensitic stainless steel, will be chosen. The choice to use cast or wrought grade will be based on the components under consideration. Depending on the manufacturing processes or assembly processes, changes may have to be made in terms of mechanical properties, such as hardness, or thermal properties, such as coefficient of expansion.
There are a couple examples worth mentioning. If considering fitment of impeller on the shaft of the usual boiler feed multistage pump, it could be noted that, if chromium steel was chosen, then wrought grade of the material such as F6NM, or rolled bar of 410 materials, can be used. However, attention has to be given to surface treatments, such as hard chrome plating, which will avoid galling tendency between the impeller material and shaft material. This will also call for an accurate manufacturing process for the shaft, as after chrome plating has been applied to the shaft, it can only have grinding allowance. Conventional machining is not possible with such a hard material.
Another example is using a casing as well as impeller wear rings by maintaining a hardness difference of at least 50 BHN. It is common to use the same cast grade of chrome steel material for both the components, impeller and casing, to avoid galling; one must therefore consider using wear rings.
In the end, it must be remembered that the selection of materials determines the useful life of the equipment, which is the measure of resistance to environmental as well as liquid conditions. Material is going to degrade eventually, hence compromise and trade-offs have to be made thoughtfully so that intended service life will be achieved.
About the Author
Abhijeet Keer is a Design Engineer, who has been working in the field with centrifugal pumps for over six years. With strengths in mechanical construction and materials, he has gained valuable knowledge working in design with major players in pump industry, such as KSB limited and Kirloskar Brothers Limited. He completed his Bachelor’s degree in Mechanical Engineering from University of Mumbai, India. His professional experience covers new product design and developments, material selection and application engineering and complete mechanical constructions.