First, the structure of the compressed packing
Most compressed fillers use their fibers as a binder because of the properties of asbestos. It is basically unaffected by most media, temperature and time and is a good thermal conductor.
(1) The disadvantage of asbestos is poor lubricity, so it is necessary to add lubricants that do not interfere with the performance of asbestos, such as graphite powder and mica powder. Since this mixture is still permeable, it is also required to be filled with a liquid lubricant.
(2) Polytetrafluoroethylene has the characteristics of minimum shrinkage, minimum shrinkage, and low friction coefficient. High corrosion resistance for most corrosive media. The working temperature of the PTFE filler at the packing; between -150 and 260 °C. In this temperature range, it is a high performance, versatile stem packing.
(3) Flexible graphite has high temperature resistance. It also has a small friction coefficient and is resistant to most corrosive media. The working temperature at the packing can reach 600 °C. Therefore, the valves at high temperature in power stations, petrochemicals, etc. use flexible graphite. filler.
Second, the corrosion of the stainless steel valve stem
Stainless steel stems, especially those made of chrome 13 steel, often have corrosion on the surface in contact with the packing. This corrosion often occurs during the storage phase prior to use because the filler after the hydrostatic test is saturated with water. If it is put into use immediately after the hydrostatic test, corrosion will not occur. Theoretically, the stainless steel stem in the wet packing is corroded because of the surface of the stem surrounded by the packing in the deoxygenated environment. This environment affects the activation and passivation properties of metals. Many small anodes are formed on the anoxic sensitive points on the surface of the stainless steel oxidized protective layer. Together with a large amount of residual blunt metal that acts as an anode, these anodes act as galvanic cells inside the metal. The graphite normally used in the filler as an anode material acts on the cathode field of the stem steel to enhance the galvanic cell current strength, which greatly exacerbates the corrosion of the original corrosion point.