Selection of control valve packing material
Update time: 21-10-31 Views: 973
Packing is a dynamic sealing filling material used to fill the space of the stuffing box to prevent the medium from leaking through the valve stem or the inner wall of the stuffing box.
Packing is an important component of the Control Valve, it is the key to prevent the leakage of the medium. To achieve the ideal sealing effect, the material, structure, and reasonable installation method of the packing must be considered. This article only discusses the material of the packing.
The commonly used packing for Control Valves are divided into the following types according to their materials:
1)Teflon molding packing
2)Teflon rod, wire, tape packing
3)Flexible graphite ring shaped packing
4)Flexible graphite and metal braided packing
5)Asbestos packing
Teflon molding packing: PTFE molding packing is generally designed as a V-shape, which is mostly used for straight-stroke control valves. PTFE has a very low coefficient of friction and is one of the most commonly used packings for control valves. When used in standard bonnets, the temperature of the medium should be controlled at -18~232℃. When used in extended or cooling fins bonnets, the temperature of the medium should be controlled at -101~427℃. In general, when long-term use is required, the temperature of the packing part should be controlled. The temperature should be controlled below 180°C, and the temperature of the packing part should be controlled below 200°C during intermittent use. If the temperature exceeds this temperature, the PTFE packing will soften and lose its sealing effect. Polytetrafluoroethylene packing cannot be used for molten alkali metals, and high-temperature fluorine, hydrogen fluoride and other media.
PTFE rod, wire, tape packing: PTFE rod, wire, tape packing is made by pressing polytetrafluoroethylene into rods, wires, and ribbons, which are mainly used for online maintenance. During the use of the control valve, the leakage of the medium from the packing part often occurs, and it is not allowed to stop for maintenance under many working conditions. When in use, loosen the gland flange and gland, and evenly wrap PTFE rods, wires, and tapes under the packing gland, and then reset and tighten the packing gland and packing pressure plate, which can effectively prevent Leakage. It can also be woven into rectangular strips of PTFE rods and tapes for use as molding packings. Mainly used for rotary control valves. When using, pay attention to connecting with wave ports as much as possible, and the cuts of each layer of packing should be staggered. The operating temperature of PTFE rods, wires and tape packings is basically the same as that of PTFE molded packings.
Flexible graphite ring molding packing: The flexible graphite ring molding packing is formed by graphite pressing, and can be used for both linear and quarter-stroke control valves. It has the advantages of good sealing performance, strong chemical inertness, and corrosion resistance. The biggest advantage of the flexible graphite ring shaped packing is that it can withstand high and low temperatures of -196~815℃. The disadvantages of flexible graphite ring shaped packing are that the friction force is much greater than that of PTFE shaped packing; the second is that graphite material is easy to stick to the valve stem, causing scratches in the packing area and increasing resistance to cause external leakage; The third, flexibility Graphite ring shaped packing is not suitable for aqua regia, concentrated sulfuric acid, concentrated nitric acid and other media.
Flexible graphite and metal braided packing: Flexible graphite and metal braided packing can be used on linear and quarter-stroke control valves. It is braided from copper wire, stainless steel wire, Monel wire, Inconel wire, etc. and graphite wire. It is currently one of the commonly used packings for medium and high-end control valves.
Its various physical properties are similar to those of flexible graphite ring shaped packing. Since it is woven from flexible graphite and metal wire, the metal wire can scrape off the graphite material adhering to the valve stem when the valve stem moves. However, flexible graphite and metal braided packing easily cause voids in the stuffing box, leading to leakage. Therefore, in general, flexible graphite and metal braided packing are used in conjunction with flexible graphite ring shaped packing. The flexible graphite ring shaped packing is installed at both ends, and the flexible graphite and metal braided packing are placed in the middle, so that the disadvantages of the two can be overcome and the advantages are combined.
Asbestos packing: Asbestos packing has good sealing performance, corrosion resistance, and a friction coefficient lower than that of flexible graphite. Asbestos packings are divided into pure asbestos and composite asbestos. Composite asbestos is mainly composed of asbestos and rubber, asbestos and polytetrafluoroethylene, asbestos and graphite. Because composite asbestos packing is pressed into silk (rope) shape by asbestos and other materials Yes, so in most cases, the control valve can be repaired online without disassembling the control valve. Many domestic and foreign companies press composite asbestos into a ring shape and use it as a molding packing.
Precautions:
1)Great harm to the environment. Therefore, European and American countries have banned the manufacture and use of asbestos packings. If products are exported to the above countries, they must not use asbestos packings.
2)The sealing of the packing part of the control valve not only depends on the choice of packing material, but also the dimensional accuracy and surface roughness of the stuffing box, the dimensional accuracy and surface roughness of the valve stem are also the key to the packing seal.
