The sealing performance of valves is one of the most important indicators to measure the quality of valves and is also the main factor that determines the life of valves. Any type of valve, regardless of its working principle, structural form, and application, has the problem of sealing, so it is a common problem in studying valves.
The sealing parts of the valve mainly include: (1) the seal between the end flange of the valve and the pipeline; (2) the seal between the middle flange of the valve and the valve bonnet; (3) the seal between the stuffing box and the valve stem; (4) the seal between the valve opening and closing parts and the valve seat (also called the opening and closing parts sealing pair seal).
If the parts mentioned in (1), (2) and (3) above leak, the medium will overflow out of the system, which is called external leakage. The leakage of the medium between the opening and closing parts sealing pairs mentioned in (4) usually does not overflow the system, so it is called internal leakage (but some safety valves, steam traps, etc. may also overflow out of the system).
Generally, the external leakage of the valve is not allowed for the system. For some valves, internal leakage of the valve is allowed as long as the leakage amount meets the relevant standards.
