1. Anti-corrosion. “Corrosion” is the process where the seal surfaces are eroded by medium. If the surfaces are destroyed, their seal ability will become poor. Therefore, the materials of seal surfaces must resist corrosion. Their anti-corrosion ability depends on their chemical stability.
2. Anti-scratch. “Scratch” means the materials will certainly be damaged due to the friction when seal surfaces move from side to side. So the materials of seal surfaces must have good ability to resist grazing, especially for gate valves
. This ability is usually determined by the properties inside materials.
3. Anti-washout. “Washout” is that when medium flows along the seal surface at a high speed, the surface will be destroyed. The destruction is more obvious in throttle valves and relief valves. Their medium is generally high temperature and pressure steam, which has a destructive impact on the seal surface. Anti-washout is therefore one of the most important requirements when choosing the seal surface materials.
4. Enough rigidity, which at the same time will decrease under specified working temperature.
5. Similar coefficient of linear expansion between seal surfaces and materials. This is important for structure of inserting seal rings, in case of loosing and extra strength under high temperature.
6. Under high temperature, materials are required to have good resistance ability to oxidation and thermal fatigue.
At present, it is difficult to find out materials satisfying all requirements above. Therefore the materials can only be chosen to meet several ones according to different valve types and purposes. For example, special attention should be paid to meet anti-washout demand of seal urface when using high-speed medium; more hard materials should be selected when there are solids existed in the medium.