Major Selection Considerations
Previous sections have shown how the construction materials influence the pressure, pressure drop, and temperature ratings of a particular valve type. Material selection is also guided by other requirements, including the material properties and characteristics that are required to ensure compliance with good valve design standards, and the material properties and characteristics that are required to ensure valve compatibility with a specific control valve application. Some of the material selection relate directly to overall valve design include:
• Material strength; i.e., the ability to retain pressure and to withstand considerable pressure drops.
• The thermal expansion coefficients of all valve components (which must be matched in order to prevent dimensional distortions that could result in gasket leaks and excessive stresses on valve components).
• Wear properties; e.g., resistance to wear, such as sliding wear and oxidative wear.
• Wear-couple compatibility; i.e., resistance to galling. Galling is a unique form of wear that results when two incompatible materials that are in sliding contact with each other become welded together, and are then torn apart.
The above grouping of parameters is generally addressed by valve manufacturers who establish ‘standard’ options for materials and material combinations that will provide satisfactory valve performance in a wide range of applications. Because material options are pre-engineered, valve specifiers can focus on specific application requirements and select material options that are compatible in terms of:
• Corrosion resistance
• Erosion resistance
• Temperature ratings