1. The structure of compressed packing

Most compressed fillers use asbestos fibers as the base material because of the properties of asbestos. It is basically not affected by most media, temperature and time, and is a good heat conductor.
(1) The disadvantage of asbestos is poor lubricity, so lubricants that do not hinder the performance of asbestos must be added, such as graphite powder and mica powder. Since this mixture is still permeable, liquid lubricant must be added.
(2) PTFE has the smallest shrinkage rate, the lowest shrinkage rate, and has the characteristics of a small friction coefficient. It has high corrosion resistance for most corrosive media. The working temperature of the PTFE filler at the filler; between -150-260℃. In this temperature range, it is a high-performance, multi-purpose valve stem packing.
(3) Flexible graphite has the characteristics of high temperature resistance. It also has a low friction coefficient and is resistant to most corrosive media. The working temperature at the packing can reach 600℃. Therefore, the valves at high temperature in power stations, petrochemical departments, etc. use flexible graphite filler.

2. The corrosion of the filler to the stainless steel valve stem

Stainless steel valve stems, especially those made of chromium 13 series steel, are often corroded on the surface in contact with the packing. This kind of corrosion often occurs in the storage stage before use, because the filler is saturated with water after the hydrostatic test. If it is put into use immediately after the hydraulic test, it will not corrode. Theoretically speaking, the corrosion of the stainless steel valve stem in the wet packing is due to the fact that the surface of the valve stem surrounded by the packing is in a deoxidized environment. This environment affects the activation and passivation properties of metals. Many small anodes are produced on the oxygen-deficient sensitive points on the surface of the stainless steel oxidation protection layer. Together with the large amount of residual passive metal that has an anodic effect, these anodes cause the metal to function as a galvanic cell. Graphite, which is usually used in the filler, acts as an anode material on the cathode field of the valve stem steel to enhance the current intensity of the galvanic cell, thereby greatly intensifying the corrosion of the original corrosion point.