Selection Principles of Valve Bonnet Bolts’ and Gaskets’ Materials

The general selection principles of valve bonnet bolts' materials are given as follows.

1. There is a tightening in hot condition process of pipe connection flange bolts when pipe connection flanges are in the beginning period of operation, but there is no need for the valve bonnet bolts to have the tightening in hot condition process. Therefore, the valve bonnet bolts will withstand greater initial sealing pressure than the pipe connection flange bolts do, and the valve bonnet bolts' material should not be worse than the material of the pipe connection flange bolts.

2. The valve bonnet bolts' materials should meet the requirements of the medium temperature. For example, the bonnet bolts' materials of the cryogenic and super cryogenic valves should reach the low temperature dedicated manufacturing standard and material grade.

3. Consideration should be given to the possibility of the bolts' materials having direct contact with the media due to leakage at the valve bonnets. For example, for valves used to transport media containing wet hydrogen sulfide, the possible effects of wet hydrogen sulfide on the valve bonnet bolts' materials should be taken into account when the valve bonnet blots' materials are selected. The materials such as B7M and L7M which can prevent the wet sulfide stress corrosion cracking should be selected as the materials of the valve bonnet bolts under this circumstance.

4. The impact of environmental atmosphere on the valve bonnet bolts' materials should be considered. If necessary, protective measures such as coating can be taken.

5. When the valve bonnet bolts' materials are selected, the effects of the valve bonnet gasket types should also be considered. For example, when the valve bonnet gasket is the semi-metal gasket or metal gasket, the high strength valve bonnet bolts such as B7, L7 and B16 bolts or strain hardened stainless steel bolts such as B8-2, B8M2 and 660 bolts should be considered.

6. It is not required that the valve bonnet bolts' material has the same or similar metallurgical properties as the valve bodies' material, but the possibility of the selected bolts' material adversely affecting the valve bodies' material should be taken into account. For example, for valves made of high-grade materials, the adverse effects of the unknown factors such as galvanic corrosion on the valve bonnet bolts' material should be adequately evaluated at the same time when the known factors such as media, environment and medium temperatures are properly evaluated.

7. Take into account the economy principle.

The valve bonnet gaskets' materials and types should also be considered with the bonnet blots' material. The general selection principles of valve bonnet gaskets' material are given below.

1. Generally, the valve bonnet gaskets' material should not be worse than the pipe connecting flange gaskets'.

2. The valve bonnet gaskets' material should be able to meet the operation requirements of the medium temperature. For example, when the non-metallic gaskets or semi-metal gaskets are used, attention should be paid to the non-metallic materials' operation temperature limits.

3. In the application of media conditions, the valve bonnet gaskets' material should have chemical stability. The corrosion resistance and the chemical stability of the gaskets' material, whether it is a metal gasket, semi-metal gasket or non-metallic gasket, shall not be worse than those of the valve bodies' material in the medium environment.

4. For metal gaskets, their materials' hardness should be 30HB to 50 HB lower than the hardness of the valve bonnets' and valve bodies' materials. If necessary, the hardness of the valve gaskets' materials can be handled by special heat treatment to meet the above requirements.

5. For non-metallic and semi-metal valve gaskets, when there is the fireproof requirement on the valve, the selected gaskets' materials and structures shall meet the requirements of the fire safety test.

6. For the pressure sealed valve bonnet, when a flexible graphite gasket is used, the lower part of the gasket is supposed to be wrapped with stainless steel sheet to avoid the loss of graphite due to being squeezed .

7. For the Class150 gate valve, it should be noted that the valve bonnet may be oval, and the wound valve gasket should not be used in this case.
 
8. Take into account the economy principle.