Classification of valves

Please stop posting these terrible posts that we’ve seen hundreds of times; they seriously undermine the spirit of originality on this forum. Stop copying on the internet all the time!

Control valves cannot be considered as a type of valve that operates automatically based on the properties of the medium itself (liquid or gas); rather, they should be regarded as valves whose operation is controlled through manual, electric, hydraulic, or pneumatic means.

The original poster also has good intentions; I understand!

An Analysis of the Causes of Packing Leakage in Control Valves and Corresponding Solutions / Ren Zhicheng, Jinxi Natural Gas Chemical Co., Ltd. During operation, control valves experience packing leakage due to various factors such as changes in operating conditions and differences in the valve’s structural design. This is the most common fault associated with control valves, and it poses a significant threat to the continuous operation of industrial plants. It is also a common problem in many chemical plants and refineries, leading not only to waste of materials but also to environmental pollution. Therefore, preventing leakage from the control valve packing is an important part of routine instrument maintenance; it is also a problem that is not easy to solve, and it constitutes a topic worthy of our research. Function and classification of control valve packing: Control valve packing is generally installed in the packing box of the upper valve cover, and its function is to prevent leakage of the medium being controlled due to the movement of the valve stem. Common fillers can be mainly divided into two categories based on their material composition: polytetrafluoroethylene and flexible graphite (asbestos products have been banned in many places, so they are not discussed here). Numerous types of fillers have been developed from these two basic materials to meet the needs of different applications. 1. Polytetrafluoroethylene: Polytetrafluoroethylene is a synthetic resin material whose carbon atoms form the backbone, with fluorine atoms symmetrically and evenly distributed around them to create a robust barrier, thereby endowing it with excellent resistance to chemical corrosion. It has stable physical and chemical properties, and its corrosion resistance even exceeds that of glass and ceramics; it exhibits excellent resistance to strong acids, strong bases, and strong oxidizing agents, making it an excellent sealing material ； It also has advantages such as good anti-aging properties, a low coefficient of friction, and excellent sealing performance, making it an ideal material to replace asbestos ； However, its temperature resistance is poor; polytetrafluoroethylene begins to undergo slight decomposition at temperatures above 200°C, and it is prone to creep under pressure and heat, which affects its sealing performance ； It is also not suitable for use with molten alkali solutions or fluorides. Commonly used polytetrafluoroethylene packing:  Polytetrafluoroethylene molded woven packing. It is made by weaving and pressing polytetrafluoroethylene powder; it is an open-type packing with good flexibility, high durability, excellent sealing performance, and ease of replacement. It is the most widely used type of packing.  V-type PTFE packing is generally made by turning PTFE rods. The packing has a V-shaped structure; when compressed at both ends, the low friction coefficient of polytetrafluoroethylene provides lubrication, resulting in good sealing performance. It has two main structures: the 60° V type is used for ordinary valves, while the 90° V type is used for high-pressure valves.  PTFE-graphite fillers incorporate some glass fibers, graphite, and molybdenum disulfide in order to improve the creep resistance and thermal conductivity of polytetrafluoroethylene; however, this increases its hardness, reduces its corrosion resistance, and diminishes its sealing properties. 2. Flexible graphite, also known as expanded graphite, is made from layers of natural graphite and serves as an ideal sealing material. Due to its porous, loose, and curled structure, it has good resilience and softness ； The bonding force between its crystals is very weak; when friction occurs due to external forces, sliding between the layers takes place easily, and carbon molecules move to the surface of the rubbing parts, thereby providing self-lubrication and ensuring good sealing properties ； Expansive graphite has a large specific surface area and surface energy; its surface is covered with a large number of gas or water molecules, which form extremely thin layers of gas or liquid films that prevent the penetration of substances. Its air permeability decreases as its density increases. It generally has better impermeability than other sealing materials ； The main component of flexible graphite is carbon; therefore, it has inert chemical properties, good corrosion resistance, and tolerance to high and low temperatures. However, it has high friction, the control valve exhibits crawling behavior and significant lag, it cannot be used with strong oxidizing agents such as concentrated sulfuric acid and concentrated nitric acid, and it is brittle and prone to breaking. There are mainly three types of graphite fillers:  Filamentary or woven types. These fillers have good elasticity, allowing them to better surround the valve stem and clean its surface. Woven graphite fillers can overcome the leakage caused by wear in other types of graphite fillers, but due to their fibrous structure, they contain voids that make them prone to permeation.  Flake graphite: A packing ring made by compressing several sheets of graphite, whose texture is perpendicular to the surface of the valve stem, resulting in a square cross-section; as a result, fluids have difficulty penetrating it. However, this also reduces the ratio of axial pressure to radial pressure, requiring greater compression force to achieve sealing.  Bent-sheet graphite: Its texture is parallel to the axis of the valve stem, and it is made from several sheets of graphite. The pressure applied during the compression of flaky graphite is much higher than the pressure it experiences in operation; therefore, it will not contract any further within the stuffing box. At high temperatures, it is easy to penetrate, as can be understood from its principle. However, due to its ease of processing and low cost, it is currently widely used. The main causes of packing leakage: In control valves, packing leakage occurs due to various reasons. Therefore, during maintenance, it is necessary to analyze each control valve individually; only in this way can packing leakage be resolved fundamentally and potential hazards eliminated. 1. The filler material and type were not selected appropriately. If PTFE packing is used under operating conditions of 150°C–200°C, it will undergo slight creep over time when working under critical conditions; as a result, its sealing performance deteriorates due to the action of high-pressure media ； For example, fibrous-type fillers are used for highly permeable media such as liquid ammonia, tar, and fuel oil; in these cases, exposure to high-temperature and high-pressure media can lead to leaks. 2. Improper method of installing the filler. After the packing is inserted into the packing box, axial pressure is applied to it via the gland. Due to the plasticity of the packing, a radial force is generated, allowing it to make close contact with the valve stem. If the packing is tightened unevenly during installation, the stress distribution is inconsistent, resulting in very uneven contact; in some areas the contact is loose or even non-existent, which leads to packing leakage. 3. The filler has aged over time and lost its elasticity ； Frequent operation of the control valve causes wear on the packing. 4. The valve stem is bent, worn, corroded, and has reduced surface finish. 5. The operating conditions of the control valve have changed. For example, during the startup and heating process of the device, when the control valve transitions from a cold state to a hot state, or when there are significant changes in the temperature of the cold and hot media at the diverging three-way valve, the valve stem is affected by thermal expansion and contraction, which causes the packing gap to increase and leads to severe packing leakage. 6. If the packing gland is not tightened properly or is misaligned, or if the control valve is installed horizontally, it can result in poor contact between the valve stem and the packing, leading to either an excessive or insufficient gap. Measures to prevent filler leakage: 1. Improve the surface precision and finish of the valve stem and stuffing box. If the sum of the friction coefficients between all the moving parts is zero, the force applied to the packing gland can be transmitted evenly throughout the entire packing without any loss. However, actual friction always exists and cannot be zero; the radial force acting on the packing decreases as the distance from the gland increases. The greater the friction, the greater the pressure decay. Once the pressure inside the seal exceeds the force acting on the packing, leakage begins. Therefore, during maintenance, the valve stem and stuffing box must be free of scratches, pitting, or wear, and should have a high level of smoothness. 2. Appropriate materials should be selected; they must have the ability to resist temperature changes, as well as resistance to creep, relaxation, and oxidation. Under normal circumstances, if the conditions are met, tetrafluoride is preferred; otherwise, graphite is chosen. Mixed fillers can also be used: one option is to combine graphite fillers with tetrafluoro fillers ； The second is the combined use of “O”-rings and “V”-shaped packing. For media with high permeability, graphite filler can be used. 3. When filling the packing, it should be added in layers, and each layer should be compressed firmly using a pressing tool to ensure even distribution of the pressure on the packing. The open-ended fillers should be placed at 90° or 120° offsets from each other. The number of turns of the filler should be such that nothing is exposed. If too little is added, the packing gland will move into the packing box, which can easily lead to leaks. 4. For PTFE V-type gaskets that rely on spring action, the gland screws should be tightened as much as possible; for other types of gaskets, there is no need to tighten them excessively, just enough to prevent leakage. The gland flange should be tightened evenly and symmetrically, without any skew. 5. After a new valve or a control valve that has just been overhauled is put into use, it is necessary to check for any leaks in the packing. If leaks are detected, they must be addressed promptly to prevent them from worsening. In summary, through years of practical experience in production, maintenance personnel have successfully resolved the problem of packing leakage in many control valves. However, it should be noted that there are still a few cases of packing leakage in control valves that have not been completely solved, and further exploration is needed in future work. By Ren Zhicheng, Jinxi Natural Gas Chemical Co., Ltd. During operation, control valves can experience packing leakage due to various factors such as changes in operating conditions and differences in the valve’s structural design. This is the most common fault associated with control valves, and it poses a significant threat to the continuous operation of industrial facilities. It is also a common problem in many chemical plants and refineries, leading not only to waste of materials but also to environmental pollution. Therefore, preventing leakage from the control valve packing is an important part of routine instrument maintenance; it is also a problem that is not easy to solve, and it constitutes a topic worthy of our research. Function and classification of control valve packing: Control valve packing is generally installed in the packing box of the upper valve cover, and its function is to prevent leakage of the medium being controlled due to the movement of the valve stem. Common fillers can be mainly divided into two categories based on their material composition: polytetrafluoroethylene and flexible graphite (asbestos products have been banned in many places, so they are not discussed here). Numerous types of fillers have been developed from these two basic materials to meet the needs of different applications. 1. Polytetrafluoroethylene: Polytetrafluoroethylene is a synthetic resin material whose carbon atoms form the backbone, with fluorine atoms symmetrically and evenly distributed around them to create a robust barrier, thereby endowing it with excellent resistance to chemical corrosion. It has stable physical and chemical properties, and its corrosion resistance even exceeds that of glass and ceramics; it exhibits excellent resistance to strong acids, strong bases, and strong oxidizing agents, making it an excellent sealing material ； It also has advantages such as good anti-aging properties, a low coefficient of friction, and excellent sealing performance, making it an ideal material to replace asbestos ； However, its temperature resistance is poor; polytetrafluoroethylene begins to undergo slight decomposition at temperatures above 200°C, and it is prone to creep under pressure and heat, which affects its sealing performance ； It is also not suitable for use with molten alkali solutions or fluorides. Commonly used polytetrafluoroethylene packing:  Polytetrafluoroethylene molded woven packing. It is made by weaving and pressing polytetrafluoroethylene powder; it is an open-type packing with good flexibility, high durability, excellent sealing performance, and ease of replacement. It is the most widely used type of packing.  V-type PTFE packing is generally made by turning PTFE rods. The packing has a V-shaped structure; when compressed at both ends, the low friction coefficient of polytetrafluoroethylene provides lubrication, resulting in good sealing performance. It has two main structures: the 60° V type is used for ordinary valves, while the 90° V type is used for high-pressure valves.  PTFE-graphite fillers incorporate some glass fibers, graphite, and molybdenum disulfide in order to improve the creep resistance and thermal conductivity of polytetrafluoroethylene; however, this increases its hardness, reduces its corrosion resistance, and diminishes its sealing properties. 2. Flexible graphite, also known as expanded graphite, is made from layers of natural graphite and serves as an ideal sealing material. Due to its porous, loose, and curled structure, it has good resilience and softness ； The bonding force between its crystals is very weak; when friction occurs due to external forces, sliding between the layers takes place easily, and carbon molecules move to the surface of the rubbing parts, thereby providing self-lubrication and ensuring good sealing properties ； Expansive graphite has a large specific surface area and surface energy; its surface is covered with a large number of gas or water molecules, which form extremely thin layers of gas or liquid films that prevent the penetration of substances. Its air permeability decreases as its density increases. It generally has better impermeability than other sealing materials ； The main component of flexible graphite is carbon; therefore, it has inert chemical properties, good corrosion resistance, and tolerance to high and low temperatures. However, it has high friction, the control valve exhibits crawling behavior and significant lag, it cannot be used with strong oxidizing agents such as concentrated sulfuric acid and concentrated nitric acid, and it is brittle and prone to breaking. There are mainly three types of graphite fillers:  Filamentary or woven types. These fillers have good elasticity, allowing them to better surround the valve stem and clean its surface. Woven graphite fillers can overcome the leakage caused by wear in other types of graphite fillers, but due to their fibrous structure, they contain voids that make them prone to permeation.  Flake graphite: A packing ring made by compressing several sheets of graphite, whose texture is perpendicular to the surface of the valve stem, resulting in a square cross-section; as a result, fluids have difficulty penetrating it. However, this also reduces the ratio of axial pressure to radial pressure, requiring greater compression force to achieve sealing.  Bent-sheet graphite: Its texture is parallel to the axis of the valve stem, and it is made from several sheets of graphite. The pressure applied during the compression of flaky graphite is much higher than the pressure it experiences in operation; therefore, it will not contract any further within the stuffing box. At high temperatures, it is easy to penetrate, as can be understood from its principle. However, due to its ease of processing and low cost, it is currently widely used. The main causes of packing leakage: In control valves, packing leakage occurs due to various reasons. Therefore, during maintenance, it is necessary to analyze each control valve individually; only in this way can packing leakage be resolved fundamentally and potential hazards eliminated. 1. The filler material and type were not selected appropriately. If PTFE packing is used under operating conditions of 150°C–200°C, it will undergo slight creep over time when working under critical conditions; as a result, its sealing performance deteriorates due to the action of high-pressure media ； For example, fibrous-type fillers are used for highly permeable media such as liquid ammonia, tar, and fuel oil; in these cases, exposure to high-temperature and high-pressure media can lead to leaks. 2. Improper method of installing the filler. After the packing is inserted into the packing box, axial pressure is applied to it via the gland. Due to the plasticity of the packing, a radial force is generated, allowing it to make close contact with the valve stem. If the packing is tightened unevenly during installation, the stress distribution is inconsistent, resulting in very uneven contact; in some areas the contact is loose or even non-existent, which leads to packing leakage. 3. The filler has aged over time and lost its elasticity ； Frequent operation of the control valve causes wear on the packing. 4. The valve stem is bent, worn, corroded, and has reduced surface finish. 5. The operating conditions of the control valve have changed. For example, during the startup and heating process of the device, when the control valve transitions from a cold state to a hot state, or when there are significant changes in the temperature of the cold and hot media at the diverging three-way valve, the valve stem is affected by thermal expansion and contraction, which causes the packing gap to increase and leads to severe packing leakage. 6. If the packing gland is not tightened properly or is misaligned, or if the control valve is installed horizontally, it can result in poor contact between the valve stem and the packing, leading to either an excessive or insufficient gap. Measures to prevent filler leakage: 1. Improve the surface precision and finish of the valve stem and stuffing box. If the sum of the friction coefficients between all the moving parts is zero, the force applied to the packing gland can be transmitted evenly throughout the entire packing without any loss. However, actual friction always exists and cannot be zero; the radial force acting on the packing decreases as the distance from the gland increases. The greater the friction, the greater the pressure decay. Once the pressure inside the seal exceeds the force acting on the packing, leakage begins. Therefore, during maintenance, the valve stem and stuffing box must be free of scratches, pitting, or wear, and should have a high level of smoothness. 2. Appropriate materials should be selected; they must have the ability to resist temperature changes, as well as resistance to creep, relaxation, and oxidation. Under normal circumstances, if the conditions are met, tetrafluoride is preferred; otherwise, graphite is chosen. Mixed fillers can also be used: one option is to combine graphite fillers with tetrafluoro fillers ； The second is the combined use of “O”-rings and “V”-shaped packing. For media with high permeability, graphite filler can be used. 3. When filling the packing, it should be added in layers, and each layer should be compressed firmly using a pressing tool to ensure even distribution of the pressure on the packing. The open-ended fillers should be placed at 90° or 120° offsets from each other. The number of turns of the filler should be such that nothing is exposed. If too little is added, the packing gland will move into the packing box, which can easily lead to leaks. 4. For PTFE V-type gaskets that rely on spring action, the gland screws should be tightened as much as possible; for other types of gaskets, there is no need to tighten them excessively, just enough to prevent leakage. The gland flange should be tightened evenly and symmetrically, without any skew. 5. After a new valve or a control valve that has just been overhauled is put into use, it is necessary to check for any leaks in the packing. If leaks are detected, they must be addressed promptly to prevent them from worsening. In summary, through years of practical experience in production, maintenance personnel have successfully resolved the problem of packing leakage in many control valves. However, it should be noted that there are still a few cases of packing leakage in control valves that have not been completely solved, and further exploration is needed in future work.