In current large and medium-sized ethylene plants, cryogenic separation is mostly used to separate hydrogen, methane and other components in cracked gas; cryogenic separation technology is also mainly used in coal-to-methanol purification plants. In order to meet this requirement, cryogenic valves are widely used. Only by understanding the design requirements and special structure of cryogenic valves can there be no problems during the selection and installation process.
Cryogenic valves, especially ultra-low temperature valves, have extremely low operating temperatures. When designing this type of valve, in addition to following the general valve design principles, there are also some special requirements.
According to the conditions of use, the design of cryogenic valves has the following requirements:
1.1 Valves should not be a significant source of heat in cryogenic systems. This is because the inflow of heat not only reduces the thermal efficiency, but also causes the internal fluid to evaporate rapidly, causing abnormal pressure increase and causing danger if the inflow is too much.
1.2 Low-temperature media should not have a harmful impact on the operation of the handwheel and the sealing performance of the packing.
1.3 Valve assemblies that are in direct contact with low-temperature media should have explosion-proof and fire-proof structures.
1.4 Valve assemblies operating at low temperatures cannot be lubricated, so structural measures need to be taken to prevent friction parts from being scratched.
2 Material selection for cryogenic valves
2.1 Cryogenic ball valves body material
2.1.1 Factors that should be considered when selecting the main material. Among metal materials, except austenitic steel, copper, aluminum, etc. with face-centered cubic lattice, general steel will exhibit low-temperature brittleness at low temperatures, thereby reducing the strength and strength of the valve. service life. When selecting the main body material, you must first select materials suitable for working at low temperatures. Aluminum does not exhibit low-temperature brittleness at low temperatures. However, because the hardness of aluminum and aluminum alloys is not high, and the aluminum sealing surface has poor wear resistance and scratch resistance, its use in low-temperature valves has certain limitations. It is only used in low-pressure and Selected for small diameter valves. In addition, the material selection of cryogenic valves should also consider the following factors: 1) the minimum operating temperature of the valve; 2) the mechanical properties required for metal materials to maintain working conditions at low temperatures, especially impact toughness, relative elongation and Structural stability; 3) Good wear resistance at low temperature and without oil lubrication; 4) Good corrosion resistance; 5) The welding performance of the material must also be considered when using welded connections. 2.1.2 Selection of materials for the valve body, valve cover, valve seat, and valve disc (gate plate). The general principles for selecting materials for these main parts are: when the temperature is higher than -100°C, ferritic steel is used; when the temperature is lower than -100°C, ferritic steel is used; Austenitic steel is used when the temperature is ℃; materials such as copper and aluminum can be used for low-pressure and small-diameter valves. Select appropriate materials based on the lowest operating temperature during design. 2.1.3 When the temperature of the valve stem and fasteners is higher than -100℃, the valve stem and bolts should be made of alloy steel such as Ni, Cr-Mo, etc., and should be appropriately heat treated to improve the tensile strength and prevent thread seizure. Injury etc. When the temperature is lower than -100℃, it is made of austenitic stainless acid-resistant steel. However, the low hardness of 18-8 acid-resistant steel will cause the valve stem and the packing to scratch each other, causing leakage at the packing. Therefore, the surface of the valve stem must be plated with hard chromium (plating thickness 0.04-0.06mm), or nitrided and nickel-phosphorus plated to improve surface hardness. In order to prevent the nuts and bolts from seizing, the nuts are generally made of Mo steel or Ni steel, and molybdenum disulfide is coated on the thread surface.
2.2 Selection of low-temperature valve gaskets and packing materials In the design of low-temperature valves, on the one hand, the structural design ensures that the packing works at a temperature close to the ambient temperature. For example, a long-neck valve cover structure is used to keep the stuffing box as far away from the low-temperature medium as possible. , on the other hand, the low temperature characteristics of the filler should be considered when selecting the filler. Asbestos fillers impregnated with polytetrafluoroethylene are generally used in cryogenic valves. Flexible graphite is a recently developed excellent sealing material. Cryogenic valves can also adopt a bellows sealing structure without packing, usually using multi-layer bellows. Gaskets for cryogenic valves must have reliable sealing and recovery properties at normal temperature, low temperature and temperature changes. Since gasket materials will harden and reduce plasticity at low temperatures, gasket materials with small performance changes should be selected. When the operating temperature is -200°C and the low-temperature maximum operating pressure is 3MPa, the asbestos rubber sheet of long-fiber white asbestos is used. When the operating temperature is -200°C and the maximum operating pressure is 5MPa, a spiral wound gasket made of acid-resistant steel tape sandwiched with asbestos, or a spiral wound gasket made of polytetrafluoroethylene and acid-resistant steel tape. Spiral gaskets made of flexible graphite and acid-resistant steel are ideal for low-temperature valves at -200°C.
3. Special structure of cryogenic valves. Cryogenic valves mainly include gate valves, globe valves, ball valves, butterfly valves, check valves, etc. Their main structures are roughly the same as general valves.
3.1 Valve body The valve body should be able to fully withstand expansion and contraction caused by temperature changes. Moreover, the structure of the valve seat will not be deformed due to temperature changes.
3.2 The ball valve cover adopts a long-neck valve cover structure. Its purpose is to protect the stuffing box. Because the sealing of the stuffing box is one of the keys to the cryogenic valve. If there is any leakage. It will reduce the cooling effect and cause the liquefied gas to evaporate. This is because at low temperatures, as the temperature decreases, the elasticity of the packing gradually disappears, and the leak-proof performance decreases. Due to medium leakage, the packing and the valve stem freeze, which affects the normal operation of the valve stem. At the same time, the valve stem will also Moving up and down will scratch the packing, causing serious leakage. Therefore, cryogenic valves must adopt a long-neck valve cover structure. In addition, the long-neck structure also facilitates wrapping of cold insulation materials to prevent the loss of cold energy.
3.3 Disc Gate valves use flexible gates or open gates; stop valves use plug-shaped valve discs for their flat seats and needle valves. These structural forms can maintain reliable sealing regardless of temperature changes.
3.4 Valve Stem The valve stem needs to be chromium-plated, nickel-phosphorus-plated or nitrided to increase the surface hardness of the valve stem and prevent the valve stem from interlocking with the packing and packing gland (gland), damaging the sealing packing and causing stuffing box leakage.
3.5 Gasket When selecting a gasket, the low-temperature properties of the gasket material should be considered, such as compression resilience, preload force, fastening pressure distribution and stress relaxation characteristics.
3.6 Stuffing box and stuffing The stuffing box cannot be in direct contact with the low temperature section, but is located at the top of the long-neck valve cover so that the stuffing box is far away from the low temperature and works in a temperature environment above 0°C. In this way, the sealing effect of the stuffing box is improved. In the event of leakage, or when low-temperature fluid directly contacts the packing and the sealing effect is reduced, grease can be added from the middle of the stuffing box to form an oil seal layer to reduce the pressure difference in the stuffing box as an auxiliary sealing measure. The stuffing box mostly adopts a two-stage packing structure with a middle metal isolation ring. However, some use other types such as general valve stuffing box structure and double stuffing box structure with self-tightening valve stem.
3.7 Upper sealing cryogenic valves are all equipped with an upper sealing seat structure. The upper sealing surface must be overlayed with cobalt-chromium-tungsten carbide and polished after finishing.
3.8 Valve seat, valve disc (gate plate) sealing surface The closing parts of the cryogenic valve adopt a cobalt-chromium-tungsten carbide surfacing structure. Due to the large expansion coefficient of polytetrafluoroethylene and becoming brittle at low temperatures, the soft seal structure is only suitable for cryogenic valves with temperatures above -70°C, but polytrifluoroethylene can be used for cryogenic valves with temperatures of -162°C. 3.9 Middle flange bolts
3.9.1 Bolts should have sufficient strength. This is because bolts often break due to fatigue when working under repeated loads.
3.9.2 Because bolts can easily cause stress concentration at the thread root, bolts with full thread structure are used.
3.10 Measures to prevent abnormal pressure rise After the valve is closed, some liquid will remain in the valve cavity. As time goes by, the liquid remaining in the valve cavity will gradually absorb heat from the atmosphere, return to normal temperature, and revaporize. After gasification, its volume expands violently, increasing by about 600 times, thus generating intense pressure and acting on the inside of the valve body. This situation is called abnormal pressure rise, which is a phenomenon unique to cryogenic valves. When abnormal pressure rise occurs, the gate plate will be pressed tightly against the valve seat, causing the gate plate to fail to open. At this time, the high pressure will push out the middle flange gasket or destroy the packing; it may also cause the valve body and valve cover to deform, significantly reducing the valve seat sealing; or even the valve cover will rupture, causing serious accidents. In order to prevent abnormal pressure rise, the following measures are generally adopted in the structure of low-temperature valves:
3.10.1 Set up a pressure relief hole, also known as a pressure balance hole or an exhaust hole, that is, drill a small hole on the inlet side of the elastic gate or double gate. hole, used as a pressure balance hole on the inner cavity and inlet side of the valve body. When the pressure in the valve cavity increases, the gas can be discharged through the small holes. This method is relatively simple and has been widely adopted. Use a pressure relief hole to prevent abnormal pressure rise. When designing the valve body, there should be an arrow indicating the direction of fluid flow; when installing, pay attention to the position of the pressure relief hole to ensure that the pressure relief hole leads to the side of the medium inlet. The pressure relief hole is opened Be more careful when on the gate. The location of the pressure relief hole depends on the valve structure. Some are on the valve body; some are on the gate plate.
3.10.2 Set an outlet pipe on the valve or install a safety valve to discharge abnormally high pressure. Usually a safety valve is installed on the valve cover. When the pressure rises to a certain value, the safety valve opens and discharges abnormally high pressure to ensure the safety of the valve body. An exhaust valve can also be installed at the lower part of the valve body to drain out the remaining liquid in the middle cavity of the valve body to prevent abnormal pressure rise.
4. Installation requirements of cryogenic valves After understanding the design requirements and special structure of cryogenic valves, the following principles must be followed during the specific installation process:
4.1 When the fluid is a liquid, the valve stem of the cryogenic valve should be installed upward to prevent some liquid from remaining in the valve cavity after the valve is closed. The liquid will vaporize and cause abnormal pressure increase, making the gate unable to open. Moreover, the valve stem of the cryogenic valve is relatively long, so attention should be paid to it during installation.
4.2 For low-temperature gate valves with pressure relief holes, the direction of the pressure relief holes should be marked on the valve body, and attention should be paid to the marking not being covered by the cold insulation layer.
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