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Introduction to the packing of the imported control valve

The control valve is one of the terminal control components of the automatic control system. Due to the high temperature and high pressure working conditions in the chemical plant, some media have strong corrosiveness and toxicity, and are flammable and explosive. When the valve packing leaks, not only It causes waste of raw materials, and it also causes serious pollution to the environment, and even causes life-threatening safety accidents such as fire, explosion, and poisoning. Therefore, the leakage problem of the control valve packing should be paid enough attention. It is very important to choose the sealing packing reasonably in the design and selection. For the packing seal of the control valve, combined with years of work experience and related information, through the analysis of the structural form of the control valve stuffing box, the characteristics and application of PTFE and flexible graphite packing are introduced, and the filler of the control valve is selected reasonably. Brief introduction.
Control valve packing action and classification
The valve part of the control valve is composed of the inner part of the valve and the valve body. The inner part of the valve includes a valve core, a valve stem, a stuffing box and an upper valve cover. The stuffing box component is used for sealing the valve stem, and is prevented by an elastic method. The process medium creates a leak on the surface of the valve stem by reciprocating or rotary motion. It is an integral part of the valve body. The valve stem seal of the valve is almost always realized by the stuffing box.
The control valve packing is a filling material for the dynamic seal, which is generally installed in the stuffing box of the upper bonnet, and its function is to prevent the leakage of the controlled medium due to the movement of the valve stem.
Commonly used fillers are mainly divided into two categories according to materials: polytetrafluoroethylene and flexible graphite.
1. Polytetrafluoroethylene (PTFE)
Polytetrafluoroethylene (PTFE) is a polymer compound obtained by polymerizing tetrafluoroethylene. It has excellent chemical stability, corrosion resistance, sealing properties, high lubrication and non-stick properties, electrical insulation and good anti-aging ability. Its corrosion resistance even exceeds that of glass and ceramics. It has excellent corrosion resistance even for strong acids, alkalis and strong oxidants. It is an ideal sealing material. However, its temperature resistance is poor. Polytetrafluoroethylene starts to crack a little bit at 200 ° or above, and is subjected to compression, heat and creep, which affects the sealing performance and is not suitable for molten alkali or fluoride. Commonly used PTFE fillers are:
(1) Polytetrafluoroethylene molded woven packing: It is made of polytetrafluoroethylene bulk woven and pressed. It is an opening material with good flexibility, long-lasting durability, good sealing effect and convenient replacement. It is the most widely used. A material.
(2) V-type PTFE filler: generally made of PTFE bar material turning, the packing structure is V-shaped, under the condition of compaction at both ends, due to the small friction coefficient of PTFE, there is lubrication The effect, good sealing performance and so on. The characteristic of the V-shaped packing ring is that under the pressure of the medium in the valve, the lip of the outer ring of the packing is always close to the inner wall of the stuffing box when the opening is opened, so as to achieve a static seal. Under the action of pressure, when the lip of the inner ring of the packing is opened, it always adheres to the valve stem to ensure the dynamic seal, so that even if the valve stem moves up and down, the sealing performance can be ensured.
(3) PTFE-graphite filler: By adding part of glass fiber, graphite, molybdenum disulfide, to improve the creep resistance and thermal conductivity of polytetrafluoroethylene, but the hardness becomes larger, the corrosion resistance is decreased, and the sealing performance is lowered.
Second, flexible graphite
Flexible graphite materials are non-cellulosic materials that remove impurities from natural flake graphite. After being treated with strong oxidizing mixed acid, it becomes graphite oxide. The graphite oxide is liberated by heat, and the volume expands rapidly, becoming a loose graphite with soft texture and toughness. It is a material with long life and good sealing performance.
Characteristics
(1) Excellent heat resistance and cold resistance. Flexible graphite has almost no change in physical properties from ultra-low temperature of -250 ° C to high temperature of +600 ° C;
(2) Excellent chemical resistance. Flexible graphite has almost no corrosion in other acids, bases and solvents except for corrosion in strong oxidizing medium such as nitric acid and concentrated sulfuric acid;
(3) It has good self-lubricating properties. Like natural graphite, flexible graphite is easy to produce sliding when subjected to external force, so it has self-lubricating properties and good sealing performance;
(4) Because its porosity is curled due to its porous porosity, its resilience and flexibility are good. When the shaft or bushing has a radial circular runout due to eccentricity in manufacturing, installation, etc., it has sufficient floating performance, and even if the graphite is cracked, it can be well adhered, thereby ensuring tightness and preventing leakage.
2.2.2 Types of graphite filler
(1) Filament or braid: This filler is more elastic and can enclose the valve stem and purify the stem surface. The woven graphite filler can overcome the leakage of other types of graphite filler due to wear, but the woven graphite filler is easy to penetrate due to the filament structure leading to voids.
(2) flake graphite: a packing ring pressed by a plurality of graphite sheets whose texture is perpendicular to the surface of the valve stem to form a square cross section, so that the fluid is not easily penetrated, but it also reduces the axial pressure and the radial pressure. The ratio needs to be sealed with a larger pressing force.
(3) Curved graphite: its texture is parallel to the axial direction of the valve stem and is processed by several graphite sheets. The bent flake graphite has a much higher pressure during pressing than it does when it is in operation, so it does not shrink again in the stuffing box. At high temperatures, it is known that it is easy to penetrate. However, due to the ease of processing and low price, it is currently widely used.

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