Valve leakage considerations

1. A soft-seal valve refers to a valve in which either the valve core or the valve seat uses a non-metallic material (mainly polytetrafluoroethylene, as well as rubber, etc.) for sealing purposes. Soft-sealed valves offer the best sealing performance, but during pipeline installation and system cleaning, various types of contaminants (such as weld slag and iron shavings) may remain. When these contaminants pass through the control valve, they can easily scratch the soft-sealed valve seat or valve element, leading to an increase in leakage and reduced reliability of the seal. Therefore, when choosing a soft-seal structure, it is necessary to consider the cleanliness of the medium and to thoroughly flush the pipes before operation. Rigid sealing mainly involves metal seals, but there must be a sufficient difference in hardness; materials with the same hardness level will not yield to each other, making scratches likely. 2. Considerations regarding sealing reliability: (1) Soft seals provide good shut-off performance, but they are not suitable for media containing particles; once scratched, leakage will increase sharply. Therefore, hard sealing should be preferred whenever conditions permit. Fully functional ultra-lightweight valves can achieve a tight seal of 10‑7 to 10‑8, while triple-eccentric butterfly valves can achieve zero leakage. (2) Considerations regarding sealing wear resistance: In addition to choosing a hard seal, for cut-off control valves with leakage requirements of 10–6 or better, it is necessary (or required) to weld a wear-resistant alloy on to improve the sealing material’s resistance to wear, erosion, and scratching, thereby extending the valve’s service life. (3) The cut-off control valve for high-temperature media is assembled at room temperature and operates at high temperatures, with temperature changes of several hundred degrees. Thermal expansion changes the sealing specific pressure established during assembly. If it’s too tight, the sealing surface will get stuck, causing severe abrasion and tearing during movement ； If it is too loose, the sealing pressure will be insufficient; in both cases, leakage will increase sharply. The best way to address this is to use a flexible valve seat to absorb the differences caused by these changes. There are usually three types of elastic solutions: ① spring-type ； ②Sheet-type flexible metal ring ； ③An elastic valve seat that combines hardness and flexibility. The latter is the best option: it offers flexibility to counteract thermal expansion, along with stiffness and wear-resistant alloys to enhance the reliability of the wear-resistant surface. (4) For erosion media containing particles, it is necessary to ensure that the valve core is protected from direct erosion by the high-speed medium. Both linear stroke valves and butterfly valves do not possess this function, whereas fully functional ultra-lightweight valves feature eccentric rotation. In such cases, the latter two types of valves should be chosen. 3. Rigid seals are more expensive. 4. Temperature and pressure have a significant impact. 5. The composition of the medium also plays a role

As a supplementary point, if fire protection is a concern, a hard seal should be chosen.

What was said upstairs is incorrect; nowadays, soft seals also come with fire-resistant designs! :L :L :L