Selection principle of non-metallic valve seats' or clacks' materials
For non-metallic sealed valves, non-metallic valve seats and valve clacks are one of the key factors affecting valve usage.
The following are the common non-metallic materials and their characteristics:
1. Carbon fiber: heat resistance, corrosion resistance, wear resistance, high thermal conductivity.
2. Fluororesin (PTFE): excellent corrosion resistance, suitable for seal of strong corrosive fluid.
3. Special fiber (aramid fiber, inorganic fiber): corrosion resistance as good as with asbestos, white, suitable for anti-pollution valves.
4. Graphite (flexible graphite): heat resistance, corrosion resistance, high thermal conductivity, suitable for high temperature and high pressure valves.

Here are some basic selection principles of common non-metallic materials.
1. In the application of media conditions, the material should have chemical stability. That is, the selected non-metallic internal parts' materials will not transform or lose effectiveness due to the corrosion, chemical dissolution, aging, high temperature melting of the media in the media environment.
2. In the application of media conditions, the material should have the basic mechanical properties. For example, PTFE has good corrosion resistance and good self-lubricating properties, but it has a clear tendency of cold flow. Thus, it should not be used under higher pressure conditions and we should take measures to prevent cold flow when it is under high pressure conditions. 3. Take into account the economy principle at the same time.

Selection principles of stem packing
The following points must be considered when we select the packing.
1. The production of packing
2. The design pressure of valve
3. The design temperature of valve
4. The minimum operating temperature
5. The size of the packing
6. The material of the stem
 
The general selection principles of the stem packing are given as follows:
1. The stem packing should have good self-lubricating performance.
2. The stem packing should be able to meet the operating requirements of the medium temperature, that is, at the operating temperature, the packing should not be or less become soft, carbonized, ashing and fluffed and will not shrink or harden during the service life.
3. In the application of media conditions, the stem packing should have chemical stability, that is, the packing should be able to resist the corrosion of the media.
4. The stem packing should not adversely affect the stem, the stuffing box, etc. For example, when the stem is made of austenitic stainless steel, the amount of harmful elements such as chloride ion and sulfide should be limited in the packing.
5. When there is the fireproof requirement on the valve, the selected stem packing and structure shall meet the requirements of the fire safety test.
6. Take into account the economy principle at the same time.