Selection Principles of Valve Internal Parts’ Materials

API600 Standard gives the definition of a gate valve's internal parts, which refers to the valve stem, the sealing surface of the valve seat, the sealing surface of the valve plate, the upper sealing surface and other contact media. In order to facilitate serialization and standardization, API600 gives a common combination of valve internal parts' materials, and gives them the internal part number. Other valves can also refer to the definitions of these internal parts and the names of the materials.

One of the advantages of API Standard is that the engineering designers do not need to rigidly adhere to the internal part numbers and the internal parts' materials they represent given by the standard, and can specify the appropriate materials of the main internal parts according to the actual working conditions. When the internal part numbers and the internal parts' materials they represent given by the standard are suitable for the working conditions, we can select and indicate them in the valve material table/valve data sheet.

It should be said that the standard combination of materials listed in API Standard can meet more than 80% of the engineering needs. If the internal part numbers and the internal parts materials they represent given by the standard can not meet the requirements of the actual working conditions, the engineering designers can choose the appropriate combination of the internal parts' materials and indicate them in the valve materials table/valve data sheet respectively. Otherwise, the manufacturers will determine them by customs. Here are some of the basic principles of the selection of internal parts' materials.

Under the working temperature, the stem materials should have metallurgical stability. For example, when 13Cr material is used as the stem material, we should pay attention to its temperature limit conditions.

In the application of media conditions, stem materials should have chemical stability. For example, in corrosive media environments, the corrosion resistance of valve stems' materials can not be lower than that of the main materials of valves.

In the application of media conditions, the stem materials should have the required mechanical properties and wear resistance (hardness). For example, under normal circumstances, the strength and hardness indicators of valve stems should not be lower than those of the valve bodies' materials and if necessary, have surface treatment to increase the surface hardness of the valve stems.

In the application of medium conditions, the valve clacks' or valve seat sealing surfaces' materials should have chemical stability. For example, corrosion resistance of valve clacks' or valve seat sealing surfaces' materials can not be lower than that of the valve bodies' materials.

In the application of medium conditions, especially where cavitation or erosion may occur, the valve clacks' or valve seat sealing surfaces' materials shall have the desired cavitation or abrasion resistance. If necessary, overlaying appropriate cemented carbide on single side (give priority to valve seats) or double side of the surfaces .
 
In the application of media conditions, the selected valve internal parts' materials should not have a negative impact on the main material. For example, when an electrochemical corrosion environment is foreseen, the engineering designers should evaluate the possible accelerated corrosion due to the potential difference between the internal parts' materials and the valve bodies' materials.

Valve clacks' or valve seat sealing surfaces' materials should have weldability.

In the application of media conditions, consider other restrictions on the materials of the valve stems, valve clacks or valve seat sealing surfaces. For example, consider the restrictions on materials in the sulfide stress corrosion environment and low temperature environment.

Take into account the economy principles. For example, the overlaying cemented carbide of the valve seats and the sealing surfaces of the valve clacks will increase the valve manufacturing costs, especially of carbon steel valves of small diameters, single or double-sided cemented carbide costs accounting a large proportion for the entire valves' costs.