Increase the service life of valve packing
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In valve design or practical usage experience, the packing for lift-type valves is usually made of asbestos packers, graphite packers, or PTFE V-shaped packers. However, all of these types of packing are limited by the number of times the valve is opened and closed; as the number of openings and closings increases, the packing wears out and the gaps between its components widen. After a certain number of operations – which, according to statistics, is no more than 2000 times – the valve will start to leak at the packing area. After leakage, the packing gland needs to be tightened again. This solution is suitable for situations where the valve does not need to be opened and closed frequently. If the valve is opened and closed frequently (for example, 100,000 to 300,000 times per year), this undoubtedly increases the costs associated with its use and maintenance. Is there a design that can still maintain its sealing property after 1 to 2 million openings and closings of the valve? The answer is yes. One approach is to add a washer beneath the packing, and a spring below that washer (the pre-tension of the spring needs to be calculated); when the packing wears out, the spring will push it back into place, thereby ensuring sealing. At present, this method, through testing, can ensure no leakage after 500,000 to 1,000,000 operations; the user is required to retighten the packing gland during annual overhauls, which only helps to reduce routine maintenance. Another approach is to use hydraulic or pneumatic sealing technologies, thereby changing the original design concept of the valve; oil cylinder seal slip rings are employed at the packing area, along with O-ring seals (it is necessary to calculate the friction force and sealing pressure in order to determine the number of slip rings and O-rings required). With this method, leakage can be prevented even after 2 million openings and closings, but the type 0 seal ring ages over time, resulting in a service life of only 5 years; moreover, its cost is high. This post was last edited by Chuangxin Filler on 2009-3-11 16:53.]
I basically agree with your view, but it still depends on the type of valve. For example, valves such as globe valves, gate valves, and control valves have valve stems that move up and down; therefore, in addition to wear of the packing, it is also necessary to consider the damage caused to the packing by the medium attached to the valve stem, including scratches. Ball valves and butterfly valves are valves that rotate 90 degrees, and in these cases the main issue is wear between the valve stem and the packing. Thus, the approach you mentioned can help solve certain problems. For instance, the stem sealing for API6D ball valves used in pipelines adopts the second design you mentioned. Ball valves used in chemical processing plants generally use packing rings, and adding a flexible packing to the packing gland can also enable self-adjusting functionality of the packing. Please feel free to share any valuable feedback if there are any mistakes: handshake