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How to solve the leakage of the inlet control valve packing?

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.

2 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.
2.1 Features
(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 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.

The main cause of packing leakage
There are many reasons for the leakage of the control valve in the actual use of the valve. On-site maintenance should be specifically analyzed and judged according to the actual use of the valve, thereby fundamentally eliminating the cause of the leakage and solving the safety hazard. According to the law of leakage, the main reasons are summarized as follows:
(1) The material and type of filler are not suitable: if the PTFE filler is used in the working condition of 150°C-200°C, the creep will occur during long-term operation under critical conditions, and the sealing performance of the filler will be reduced by the action of high-pressure medium; For mediums with strong permeability such as liquid ammonia, tar, and fuel, weaving-type fillers are used. These conditions are affected by high-temperature and high-pressure media and are prone to leakage.
(2) Improper installation method of the packing: After the packing is loaded into the stuffing box, the axial pressure is applied to the packing, and due to the plasticity of the packing, it generates radial force and is in close contact with the valve stem, such as the packing is installed. Tightly loose, uneven force, so that the contact is very uneven, and some of the contact is loose, or even contact, causing the filler to leak.
(3) During the use of the control valve, there is relative movement between the valve stem and the packing. This movement is called axial movement. During the use process, with the influence of high temperature, high pressure and strong fluid medium, the control valve stuffing box is also a part with more leakage. The main cause of the leakage of the packing is the interface leakage, and leakage of the textile packing (the pressure medium leaks outward along the small gap between the filler fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the contact pressure of the packing, the aging of the packing itself, loss of elasticity, etc., and the pressure medium leaks outward along the contact gap between the packing and the valve stem.
(4) The control valve frequently moves, the valve stem bends, wears, corrodes, and the smoothness of the seal causes the filler to wear and leak.
(5) The control valve changes during the working condition. For example, during the heating process of the device, the control valve has a cold state to a hot state, the hot medium changes greatly, and the valve stem is affected by the thermal expansion, so that the packing gap becomes larger. The packing leak is severe.
(6) The packing gland is not tight, skewed, or the horizontal installation of the control valve may cause the valve stem to be in poor contact with the packing, and the gap is too large or too small.

Preventing leakage of fillers
(1) In order to facilitate the filling of the packing, chamfer the top of the stuffing box and place an erosion-resistant gap at the bottom of the stuffing box. A suitable metal guard ring (the contact surface with the packing cannot be inclined) to prevent the packing from being pushed out by the medium pressure. .
(2) Improve the surface precision and finish of the stem and stuffing box. If the sum of the friction coefficients between the various movable parts is zero, the force acting on the packing gland can be uniformly transmitted to the entire packing without any attenuation. The sum of the actual frictional forces is present, it is impossible to be zero, and the radial force acting on the filler decreases as the distance from the gland increases or decreases. The greater the frictional force, the greater the pressure attenuation. Once the sealed pressure exceeds the force on the packing, it begins to leak. Therefore, the stem and stuffing box should not be scratched, pitting and worn during maintenance, and the finish should be good.
(3) The selection of suitable materials must have the ability to resist temperature changes, and have the ability to resist creep, resist relaxation and resist oxidation. Under normal circumstances, the conditions meet the preference for PTFE, and graphite is not satisfied. Mixed fillers can also be used: one is a mixture of graphite filler and tetrafluorofiller; the other is a mixture of O-ring and V-type filler. Graphite fillers can be used for the highly permeable medium.
(4) When packing, it should be added once in a circle, and pressed tightly with the pressing tool, and pressed firmly to make the packing force uniform. The open packing should be placed 90° or 120° up and down. The number of fillers to be added should be no leakage. Adding less filler gland will enter the stuffing box and cause leakage.
(5) For the PTFE filler that acts as a spring, the gland screws should be tightened symmetrically and should not be skewed. Other types of packing do not have to be tightened too tightly to leak.
(6) After the new valve or the newly-regulated control valve is used, the filler should be inspected for leaks. If there is any leakage, it should be treated in time to prevent the leakage of the packing from increasing.

Problems to be aware of during use
(1) The 45° incision is used when the molding filler is cut, and the incision is 90° or 120° out of each turn during installation.
(2) Use PTFE forming filler under high pressure, pay attention to cold flow characteristics.
(3) The flexible graphite ring is used alone, and the sealing effect is not good, and should be used in combination with the graphite braided filler.
(4) Graphite filler can not be used for strong oxidants, such as concentrated sulfuric acid, concentrated nitric acid and other media.
(5) The dimensional accuracy, surface roughness, stem size accuracy and surface roughness of the stuffing box are the key factors affecting the sealing of the forming packing.

In many years of on-site practical applications, there are many reasons for the leakage of the control valve. As long as we diligently observe and carefully analyze the causes of the leakage, the problem of packing leakage can be eliminated; however, it should also be noted that some control valves The causes of filler leakage are complicated and there are many leakage factors. It is also necessary to continuously analyze and explore in future practical work.

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