Guide to Material Selection for Valve Body & Trims

In modern industrial production, valves play an indispensable role as the key components for controlling fluid flow. Their performance and reliability directly affect the safety and efficiency of the entire system. Whether in the fields of petroleum, chemical engineering, pharmaceuticals, food processing, or energy, valves act like precise "switches," accurately regulating the direction, flow rate, and pressure of fluids. The selection of materials for the valve body and valve trim is undoubtedly one of the important factors determining the performance of the valve. The quality of the materials not only affects the service life of the valve but can also directly impact the safety of the production process. Therefore, the scientific and rational selection of materials for the valve body and valve trim is crucial for ensuring the smooth operation of industrial production. This article will delve into the key points of selecting materials for valve bodies and various types of valve trim, combining practical examples and industry standards to analyze the performance characteristics and applicable working conditions of different materials.

Selection of Valve Body Materials

In the selection and design of industrial valves, the choice of valve body material is the first and most critical step. The valve body, as the main structure of the valve, determines whether the valve can operate safely and stably under specific working conditions.

1. Classification of Materials and Standard References

In ASME B16.34 Table 1, steel and nickel alloy materials for valve bodies and bonnets are clearly classified into three groups. Group 1 consists of ferrous metals, which typically have high strength and toughness, making them suitable for applications that require high pressure and temperature resistance, such as carbon steel, widely used in general industrial fluid transportation systems. Group 2 includes austenitic stainless steel materials, known for their excellent corrosion resistance, especially in the presence of corrosive media like acids and alkalis, making them ideal for industries with high corrosion resistance requirements, such as chemical and food processing. Group 3 comprises nickel alloys associated with special pressure-temperature rating tables. Nickel alloys, with their superior high-temperature and corrosion resistance, are widely used in high-end fields such as aerospace and nuclear industries.

In addition to these, other materials are specified in ASME B16.1 (cast iron), ASME B16.24 (copper alloys), and ASME B16.42 (ductile iron). Cast iron is cost-effective and easy to machine, making it suitable for low-pressure, ambient-temperature applications involving water and steam. Copper alloys, with their good thermal conductivity and corrosion resistance, are often used in instrument valves and applications requiring high cleanliness. Ductile iron, with its superior strength and toughness compared to gray iron, can partially replace carbon steel in certain applications.

2. Importance of Temperature-Pressure Tables

Special attention should be given to the notes on temperature and heat treatment ranges in these tables. By consulting the temperature-pressure tables, one can determine the maximum allowable working pressure of different materials at various temperatures, providing a basis for selecting materials that meet mechanical strength requirements. For example, a certain carbon steel material may be able to withstand high pressure at ambient temperature, but its strength will gradually decrease as the temperature rises. If this material is still chosen for high-temperature applications without considering the effect of temperature on strength, it could lead to dangerous situations such as valve leakage or rupture during use. Therefore, when selecting valve body materials, it is essential to determine the appropriate material based on the actual working temperature and pressure, in conjunction with the temperature-pressure tables, to ensure the safe and reliable operation of the valve under the designed conditions.

3. Consideration of Material Degradation Mechanisms

In addition to mechanical strength, materials may also experience performance degradation due to various factors during actual use. For instance, corrosion can occur when the acid, alkali, or salt components in the fluid medium chemically corrode the valve body material, reducing its thickness and strength, and shortening the valve's service life. Stress cracking is another issue that cannot be overlooked. Under alternating stress, micro cracks may form within the material and gradually propagate, eventually leading to valve failure. Moreover, low- and high-temperature conditions can also affect material performance. Low temperatures can make materials brittle, while high temperatures may cause creep and grain growth, altering their mechanical properties. In hydrogen-containing environments, hydrogen atoms may penetrate the metal material, leading to hydrogen embrittlement and a significant reduction in material toughness. When selecting valve body materials, it is necessary to fully consider these factors that can lead to performance degradation and choose materials with corresponding corrosion resistance, stress cracking resistance, and high- and low-temperature tolerance based on the actual working conditions to ensure the long-term stable operation of the valve under complex conditions.

Selection of Valve Trim Materials

After determining the valve body material, the next focus should be on the selection of valve trim materials. The valve trim, which comes into direct contact with the fluid, directly affects the valve's sealing, durability, and reliability.

1. Gate Valve Trim Material Selection

The valve trim of a gate valve mainly includes the closure member, seat sealing surface, stem, and upper sealing seat. According to API standards, the corrosion resistance of the valve trim must be at least equal to that of the valve body to prevent the trim from corroding and failing before the valve body, which would affect the valve's sealing performance. To avoid seat wear, a key factor is to provide a different hardness for the pair of sealing surfaces that mate with each other, unless both sealing surfaces are hard. For example, when the seat is made of a softer material, the closure member can be made of a harder material. During the valve closing process, the compression of the closure member against the seat can improve the tightness of the seat surface, enhancing sealing effectiveness while reducing wear between the closure member and the seat. In practical applications, some rational valve trims may have wear-resistant hard surface on one or both surfaces. By hard surface with an alloy material that has high hardness, wear resistance, and good corrosion resistance, such as hard metal, the sealing surfaces can maintain long-term wear resistance, corrosion resistance, and durability, thereby extending the service life of the valve.

2. Check Valve Trim Material Selection

The valve trim of a check valve is defined as the closure member and seat sealing surface. Similarly, the reference standards provide a trim table, and the selection of trim materials should follow principles similar to those for gate valve trim, that is, the corrosion resistance should be at least equal to that of the valve body. To enhance the service life and reliability of check valve trim, some rational trims may also have wear-resistant hard surface on one or both surfaces. This is because, during the operation of a check valve, the closure member frequently contacts and separates from the seat sealing surface. Under long-term conditions of wear, corrosion, and erosive media, without sufficient wear and corrosion resistance, the closure member and seat sealing surface can easily be damaged, leading to valve failure and an inability to properly prevent fluid backflow. Hard surface with wear-resistant material can effectively enhance the wear and corrosion resistance of the closure member and seat sealing surface, ensuring that the check valve can reliably prevent fluid backflow over the long term and maintain the normal operation of the system.

3. Ball Valve Trim Material Selection

The closure member of a ball valve is defined as the internal metal parts of the valve, such as the ball, stem, and metal seat or seat support ring. These components can have the same nominal chemical composition as the valve body, as well as similar mechanical and corrosion resistance properties. This means that when selecting materials for ball valve trim, materials that match the valve body are usually preferred to ensure the overall consistency of mechanical and corrosion resistance properties of the valve. For example, when the valve body is made of stainless steel, the ball, stem, and metal seat of the trim are also typically made of stainless steel to avoid compatibility issues or corrosion problems due to differences in material properties. Additionally, since the ball of a ball valve comes into tight contact with and rotates against the seat sealing surface during valve opening and closing, there are certain requirements for the surface roughness and hardness of the ball and seat sealing surface to ensure good sealing performance and reduce wear.

4. Butterfly Valve Trim Material Selection

All materials of a butterfly valve that come into contact with the process fluid should comply with the manufacturer's standards, unless otherwise specified. The valve trim of a butterfly valve is defined as the valve body seat sealing surface, butterfly plate, the connection between the butterfly plate and the shaft (e.g., keys, pins, screws, etc.), and all internal fasteners that come into contact with the process fluid. The materials of the shaft and bushing should have similar corrosion resistance to the trim materials to prevent damage due to electrochemical corrosion between different materials. When selecting materials for butterfly valve trim in practice, it is necessary to take into account the nature of the fluid medium, working temperature, pressure, and other factors. For instance, when conveying highly corrosive acidic media, the trim materials of the butterfly valve should be selected from acid-resistant materials, such as non-metallic materials like fluoroplastics or metallic materials like stainless steel. For high-temperature, high-pressure conditions, the trim materials need to have sufficient strength and high-temperature resistance to ensure the normal operation of the butterfly valve under harsh conditions and effective control of fluid flow.

5. Plug Valve Trim Material Selection

For plug valves, the materials of the plug and stem must have corrosion resistance at least equal to that of the valve body. This is because the working principle of a plug valve involves controlling the flow of fluid by rotating the plug, and the plug and stem come into frequent contact with the fluid medium during valve operation. If their corrosion resistance is insufficient, they can easily corrode and fail, affecting the normal operation and sealing performance of the valve. Therefore, when selecting materials for plug valve trim, it is essential to ensure that the plug and stem materials meet the requirements for corrosion resistance, while also considering the strength and wear resistance of the materials to ensure the reliability and stability of the valve over the long term.

6. Globe Valve Trim Material Selection

The valve trim of a globe valve is defined in API 602. The trim of a globe valve mainly includes the disk, seat, stem, and other components. The selection of materials for these components also needs to follow API standards to ensure that their corrosion resistance matches that of the valve body and that they meet the strength requirements of the valve under working pressure and temperature conditions. In practical applications, the selection of materials for globe valve trim also needs to consider the operating frequency of the valve and the nature of the fluid medium. For example, for globe valves that require frequent operation, the stem material should have good fatigue resistance to prevent stem breakage due to frequent opening and closing. For globe valves conveying high-temperature, high-pressure steam, the disk and seat materials need to have good high-temperature resistance and sealing performance to ensure reliable fluid shutoff under high-temperature, high-pressure conditions.

Conclusion

The selection of materials for valve bodies and valve trim is a complex and critical process that involves the comprehensive consideration of numerous factors. From the classification and standard references of valve body materials to the specific requirements of valve trim materials for different types of valves, and the performance characteristics of special sealing components, every aspect is vital. In practical applications, it is essential to strictly adhere to relevant standards and specifications while conducting detailed analysis and evaluation based on specific working conditions. This includes not only the consideration of material mechanical properties, corrosion resistance, and temperature tolerance but also the dynamic changes in actual working conditions, such as temperature fluctuations, pressure variations, and fluid corrosivity. Only through comprehensive and meticulous analysis can the selected materials meet the design requirements in actual use, avoiding valve failures or malfunctions caused by improper material selection.