The structure of compression packing
Most compression packing materials use the fiber as the base material because of considering the properties of asbestos.It is basically free from most medium, temperature and time, is a good heat conductor.
(1) the drawback of asbestos is that it is of poor lubricity, so a lubricant, such as graphite powder and mica powder, must be added without prejudice to the performance of asbestos.As the mixture is still permeable, a liquid lubricant is added.
(2) polytetrafluoroethylene has the characteristics of minimum shrinkage, minimum shrinkage and low friction coefficient.For most of the corrosive media, it has high anti-corrosion ability.Working temperature of ptfe filler at packing place;Between 150-260 ℃.In this temperature range, it is a high performance, versatile stem packing.
(3) the characteristics of flexible graphite with high temperature resistance, it also has a small friction coefficient and resistance to most corrosive medium, in the packing of the working temperature is 600 ℃, so the power plant, petrochemical and other department of the valves are used by the high temperature of flexible graphite packing.
2. Packing corrosion of stainless steel stem
Stainless steel stem, especially one made of chromium 1, 3 series steel, often corrodes the surface in contact with the packing.This corrosion often occurs in the storage stage prior to use because the filler is saturated with water after a hydraulic test.Corrosion will not occur if applied immediately after a hydraulic test.In theory, stainless steel stem in wet packing is corroded as a result of the deoxygenated surface of stem surrounded by packing.This environment affects the activation and passivation properties of metals.Many small anodes are produced on the hypoxic sensitive spots on the surface of the stainless steel oxide protective layer. These anodes, together with a large number of residual passivity metals that undergo anodization, cause the active cells to act within the metal.The cathodic field of the graphite usually used as an anode in the packing material on the stem steel enhances the current strength of the galvanic cell, thus greatly aggravating the corrosion of the original corrosion point.