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Valve Stem Seal Type
Created on 2024.08.06
1. Corrosion of stainless steel stem by packing material
Stainless steel stems, especially those made of chrome 13 series steel, are often subject to corrosion on the surfaces that come into contact with the packing. This corrosion often occurs during the storage phase prior to use, when the packing is saturated with water after a hydrostatic test. If it is put into use immediately after the hydrostatic test, corrosion will not occur. Theoretically, the corrosion of stainless steel stems in wet packing is the result of a deoxidized environment on the surface of the stem surrounded by packing. This environment affects the activation and passivation properties of the metal. A number of small anodes are generated at the hypoxic sensitive points on the surface of the stainless steel oxide protective layer, which, together with a large amount of residual blunt metal where anodic action occurs, causes the metal to act as a galvanic cell. The cathode field of graphite commonly used as an anode material acting on the stem steel enhances the galvanic cell current strength, which greatly exacerbates the corrosion of the original corrosion point.
2. Compress the structure of the filler
Most compression fillers use its fibers as the base material due to the properties of asbestos. It is basically unaffected by most media, temperature and time, and is a good conductor of heat. The disadvantage of asbestos is that it has poor lubricity, so it is necessary to fill with lubricants that do not hinder the properties of asbestos, such as graphite powder and mica powder. Since the mixture is still permeable, a liquid lubricant is also added.
PTFE has the characteristics of minimal shrinkage, lowest shrinkage, and a small coefficient of friction. It has high corrosion resistance for most corrosive media. The working temperature of polytetrafluoroethylene filler at the filler is between -150~260°C. It is a high-performance, versatile stem packing over this temperature range.
Flexible graphite has the characteristics of high temperature resistance, it also has a small coefficient of friction and resistance to most corrosive media, and the working temperature at the packing can reach 600°C, so the valves at the high temperature of power stations, petrochemical and other departments use flexible graphite filler.
3. Stem seal packing form
Lip filler
Due to the softness of the lip sheet, the lip filler will expand laterally and adhere to the retaining wall under the action of medium pressure, and this extended filler can be used in compression filler, not relatively hard materials. The disadvantage of lip packing is that its sealing effect is only one-way.
Most of the lip fillers used for valve stems are made of pure PTFE or filled PTFE. However, there are also those made of rubber or leather reinforced with fibers. It is mainly used in hydraulics. Most of them are V-shaped with lip packing used as valve stems. This makes it easy to expand even if it is installed.
Extruded filler
The name extruded filler applies to fillers such as o-rings. After installation, the flanks of this packing are extruded, and the elastic deformation of the material maintains its lateral preload force. As the medium enters the packing chamber from the bottom, the packing moves towards the gap between the stem and the support seat, blocking the leak path. When the pressure in the packing chamber drops again, the packing regains its original shape.
Thrust packing
Thrust packing consists of a packing ring or a gasket between the bonnet and stem shoulder, which can move axially relative to the packing ring. The initial stem seal can be provided with a width auxiliary shaft seal such as a live-shrink packing or by a spring that forces the stem shoulder against the thrust packing, and the subsequent media pressure forces the stem shoulder to make closer contact with the packing.
Sealing of diaphragm valve stems
The diaphragm valve stem is sealed by a flexible, pressurized valve cover that connects the stem to the closure. As long as the diaphragm does not fail, this seal prevents any media from escaping into the atmosphere through the stem. The material of the diaphragm varies depending on the purpose of the valve, and can be made of stainless steel, plastic, or rubber.
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