The control valve is one of the terminal control elements of the automatic control system. Because of the high temperature and high pressure conditions in the chemical plant, some of the mediums are highly corrosive and toxic, and are flammable and explosive. When the valve packing leaks, not only Resulting in the waste of raw materials, but also causing serious pollution to the environment and even causing life-threatening accidents such as fire, explosion and poisoning. Control valve part of the valve by the valve and valve body composition, the valve parts including the valve core, the valve stem, stuffing box and the bonnet, etc., which stuffing box components for sealing the valve stem ... Control valve packing Old leak how to do? 1. Introduction The control valve is one of the terminal control elements of the automatic control system. Due to the high temperature and high pressure conditions in the chemical plant, some of the mediums are highly corrosive and toxic, and are flammable and explosive. When the valve packing leaks It will not only result in the waste of raw materials, but also cause serious pollution to the environment and may even lead to life-threatening accidents such as fire, explosion and poisoning. Therefore, the control valve packing leakage should cause enough attention, in the design and selection of a reasonable selection of packing is very important. In view of the packing seal of the control valve, combined with many years of work experience and relevant information, through the analysis of the structure of the stuffing box of the control valve, introduces the characteristics and applications of PTFE and flexible graphite packing, and reasonable choice of packing A brief introduction. 2, the role of control valve packing and control valve: the valve part of the valve from the valve body and the composition of the valve includes the valve trim, stem, stuffing box and bonnet, etc., of which stuffing box components for the valve Rod seal, is a flexible method to prevent the process medium by reciprocating or rotary movement in the stem surface leakage, it is an integral part of the valve body, the valve stem seal almost always use stuffing box to achieve. Control valve packing is a dynamic sealing packing material, usually installed in the packing cap on the valve cover, its role is to prevent the controlled medium due to stem movement caused by leakage. Commonly used filler material is divided into two main categories: PTFE and flexible graphite. 2.1 Polytetrafluoroethylene (PTFE) Polytetrafluoroethylene (PTFE) is a polymer made of tetrafluoroethylene by polymerization, with excellent chemical stability, corrosion resistance, tightness, high lubricity and non-stick, electrical insulation Sexual and good anti-aging ability. Its corrosion resistance even more than glass, ceramics, even for strong acid, alkali, strong oxidizer also has good corrosion resistance, is an ideal sealing material. However, its poor temperature resistance, PTFE began in 200 ° above a very small amount of cracking, compression, heat a creep, which affects the sealing performance, and is not suitable for molten lye or fluoride occasions. Commonly used PTFE packing: (1) PTFE molding packing: it is made of PTFE woven in bulk, is an open material, good flexibility, durable, good sealing effect , Easy to replace, is the most widely used material. (2) V-type PTFE filler: generally made of polytetrafluoroethylene bar turning processing, packing structure was V-shaped, at both ends of the compact case, due to the PTFE small friction coefficient, lubrication Role, good sealing performance and so on. V-shaped packing ring is characterized by: the pressure in the valve medium, the outer lip of the packing open, always close to the stuffing box wall, static seal. Also under the action of pressure, the lip of the inner ring of the packing will always be close to the valve stem to ensure the dynamic sealing when it is opened, so that even if the valve stem moves up and down, the same sealing performance can be guaranteed. (3) PTFE - graphite filler: By adding some glass fiber, graphite, molybdenum disulfide, in order to improve the creep resistance and thermal conductivity of PTFE, but the hardness becomes larger, corrosion resistance decreased, sealing performance decreased. 2.2 Flexible Graphite Flexible graphite materials are non-cellulosic materials, it is to remove impurities in natural flake graphite. After the strong oxidation mixed acid into graphite oxide. Thermal decomposition of graphite oxide released carbon dioxide, the rapid expansion of the volume, into a loose texture, soft and flexible flexible graphite. Is a long life, good sealing performance of the material. 2.2.1 Features (1) Has excellent heat resistance and cold resistance. Flexible graphite from -250 ℃ degree of ultra-low temperature to +600 ℃ high temperature, the physical properties of almost no change; (2) has excellent chemical resistance. In addition to corrosion of nitric acid, concentrated sulfuric acid and other strong oxidizing medium, flexible graphite has almost no corrosion in other acids, alkalis and solvents; (3) It has good self-lubricating property. Like natural graphite, flexible graphite is easy to slip when subjected to external force, so it has self-lubricating property and good sealing performance. (4) Due to its porous porosity and curl, flexible graphite has good resilience and flexibility. When the shaft or sleeve due to manufacturing, installation, there is eccentric radial runout occurs when there is sufficient floating performance, even if graphite cracks, but also very close together, so as to ensure the binding close to prevent leakage. 2.2.2 Types of Graphite Fillers (1) Filamentous or braid-like: This packing is more resilient and can encase the valve stem and purify the stem surface. Braided graphite filler can overcome other types of graphite filler due to wear and tear caused by leakage, but due to the filiform braided graphite filler structure leads to voids, and thus easy to penetrate. (2) Flake graphite: The packing ring, pressed with several pieces of graphite plate, whose texture is perpendicular to the stem surface and has a square cross-section so that fluid does not penetrate easily, but it also reduces the axial and radial pressure Ratio, the need to use greater pressure to seal. (3) curved sheet graphite: its texture parallel to the axial stem, with a number of graphite processing. Curved graphite has a much higher pressure during pressing than it does when working, so it does not shrink further in the stuffing box. High temperature by its principle shows that easy to penetrate. However, due to easy processing, low prices, is currently widely used. 3, the main reason for the filler leakage Control valve in the field occurred in the actual use of a variety of reasons for the leakage of the filler, the site should be maintained for the actual use of the valve to conduct a specific analysis to determine, and then fundamentally eliminate the causes of leakage, Solve the security risks. According to the law of occurrence of leakage, the main reason summarized in the following categories: (1) filler material, type selection is not appropriate: such as the use of PTFE packing at 150 ℃ -200 ℃ conditions, the filler in the critical working conditions will work long hours Occurrence of small creep, by the role of high-pressure media packing seal performance decline; Another example liquid ammonia, tar, fuel and other highly permeable medium used weave type packing, these conditions are subject to high temperature and pressure medium, easy to produce leakage. (2) Improper packing installation method: The packing is filled with stuffing box and then subjected to axial pressure by the gland. Due to the plasticity of the packing, it will generate radial force and come into close contact with the valve stem. For example, if the packing is in installation Tight loose, uneven force, so that this contact is very uneven, and some contact loose, or even contact, resulting in packing leaks. (3) control valve in use, there is a relative movement between the stem and packing, this movement is called axial movement. In use, with the high temperature, high pressure and permeability of the fluid medium, the control valve stuffing box is also the occurrence of leakage more sites. Leakage caused by the filler is the main reason for the interface leakage, for textile packing will appear leakage (pressure medium along the gap between the filler fibers to the outside leakage). Leakage of the interface between the stem and the packing is caused by the gradual attenuation of the packing contact pressure, the aging of the packing itself, the loss of elasticity and the like, and the pressure medium leaks outwards along the contact gap between the packing and the stem. (4) frequent action of the control valve, valve stem bending, wear, corrosion, finish decreased filler wear and leakage. (5) Control valve changes in the use of conditions, such as the device driving the heating process, the control valve has a cold to hot process, the thermal medium changes, the valve stem is affected by thermal expansion, the packing gap becomes larger, The packing leaks seriously. (6) packing gland is not tight, skew, or control valve horizontal installation, will cause the valve stem and filler is not good, the gap is too large or too small. 4, to prevent the leakage of packing (1) in order to facilitate the filling stuffing, chamfering at the top of the stuffing box, placed in the bottom of the stuffing box corrosion-resistant gap appropriate metal guard ring (filler and the contact surface can not be inclined) Prevent media from being pushed out by media pressure. (2) to improve the stem and packing box surface accuracy, finish. If the sum of the friction coefficients between the various movable parts is zero, the force acting on the packing gland will be evenly transmitted to the entire packing without any attenuation. But the actual total friction is there, it is impossible to zero. The radial force acting on the packing decreases with the increase or decrease of the distance from the gland. The greater the friction, the greater the pressure attenuation. Once the seal pressure exceeds the force on the packing, it begins to leak. Therefore, the maintenance of the stem and packing letter can not scratch, pitting and wear, finish better. (3) Selection of suitable materials, must have the ability to resist temperature changes, anti-creep, anti-relaxation and anti-oxidation ability. Under normal circumstances, the preferred conditions to meet the four fluorine, graphite is not satisfied when the election. You can also use mixed filler: First, graphite packing and PTFE packing mixed use; Second, O-ring and V-type filler mixed use. Permeable medium can choose graphite packing. (4) packing material, it should be lap lap, and pressed with one by one compression, pressure jail, the packing force. Open packing should be staggered up and down 90 ° or 120 ° placed, fill the number of laps should be no leakage as appropriate. Add less packing gland will enter the stuffing box, easy to cause leakage. (5) For the role of PTFE spring packing effect, gland screws should be symmetrical and tightened, not skewed. Other types of packing do not need to be too tight, so as not to leak. (6) After the new valve or just overhauling the control valve is put into use, it should be checked whether there is any leakage of the packing. If there is any leakage, it should be dealt with promptly to prevent the packing from leaking more and more. 5, should pay attention to the use of the problem (1) molding filler cut with 45 ° incision, installation of each incision 90 ° or 120 ° phase error. (2) Under high pressure using PTFE molding filler, pay attention to the cold flow characteristics. (3) Flexible graphite ring alone, the sealing effect is not good, should be combined with graphite braided packing. (4) graphite filler can not be used for strong oxidants, such as concentrated sulfuric acid, concentrated nitric acid and other media. (5) Size accuracy, surface roughness, stem size accuracy and surface roughness of the stuffing box are the keys to the tightness of the molded packing. 6, Conclusion In the field of practical application for many years, there are many reasons for the occurrence of packing leakage in the control valve. As long as we diligently observe the reasons for the leakage, the problem of packing leakage can still be eliminated. However, Some control valves produce filler leakage is more complicated, more leakage factors, but also in the future practice of continuous analysis and exploration.
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