In recent decades, with the development of modern science and technology, there has been an increasing demand for cryogenic valves in engineering projects. Cryogenic valves are widely used in fertilizer, LNG and petrochemical fields and they are suitable for medium with temperatures between -40℃ and -250℃. Cryogenic valves include cryogenic ball valves, cryogenic solenoid valves, cryogenic gate valves, cryogenic globe valves, cryogenic safety valves, cryogenic check valves, cryogenic butterfly valves, cryogenic needle valves, cryogenic throttle valves, cryogenic pressure reducing valves, cryogenic electric control valves and cryogenic pneumatic control valves. In addition to liquid nitrogen and other liquid inert gases, most media of cryogenic valves are not only flammable and explosive, but also gasified when the temperature rises or flashing occurs, resulting in rapid expansion for volume, which may easily lead to leakage and explosion. The design, manufacturing, testing and installation methods of cryogenic valves are different from those of ordinary valves because of the characteristics of the medium and the requirements of adapting to the use of valves at low temperatures.
1. Standards and definitions of cryogenic valves
Different standards have different definitions for cryogenic valves.
(1) British valve standard BS6364 Valves for Low Temperature indicates that the medium temperature range of cryogenic valves is from -50℃ to -196℃.
(2) The American standard MSSSP-134 Requirements for Cryogenic Valves and Valve Bodies or Bonnet Extensions explains that the standardized low temperature is between -100℃ and -195℃".
(3) The Chinese standard GB/T 24925 Technical Conditions for Cryogenic Valves is applicable to the medium temperature range of -29℃ to -196℃.
(4) Shell valve standard SHELL MESC SPE 77/200 Valves for Low Temperature and Ultra-low Temperature is suitable for the medium temperature range of -30℃ to -196℃.
(5) There are generally the following six temperatures in foreign countries according to the liquefaction temperature of various gases under normal pressure:
Grade one: from 0 to -46℃
Grade two: from -47 to -60℃
Grade three: from -61 to -70℃
Grade four: from -71 to -101℃
Grade five: from -102 to -196℃
Grade six: below -253℃
Generally, temperatures between -46 and -150℃ are called low temperature, and a temperature below -150℃ is called ultra-low temperature.
The definition of cryogenic valves in the petrochemical industry is based on the design temperature of the conveying medium. Generally, valves used for the medium temperature below -40℃ are called cryogenic valves, and those are applied to the medium temperature below -101℃ are called ultra-low temperature valves.
2. Design features of cryogenic valves
Material selection of cryogenic valves
The working medium of the cryogenic valve not only has low temperature, but also has great permeability, which determines many special requirements for the valve's material. The mechanical properties of steel at low temperatures are different from those at room temperatures. In addition to strength, the important index of low-temperature steel is its low-temperature impact toughness. The low-temperature impact toughness of a material is related to the brittle transition temperature of the material. The lower the brittle transition temperature of the material is, the better the low temperature impact toughness of the material becomes. Metal with body-centered cubic lattice like carbon steel has low-temperature cold brittleness, while the impact toughness of austenitic stainless steel with face-centered cubic lattice is not influenced by low temperatures.
Cryogenic valve bodies, valve bonnets and other pressure-resistant parts are usually made from materials with good low-temperature strength and toughness. At the same time, factors such as weldability, machinability, stability, and economy must also be considered. Three low temperatures -46℃, -101℃ and -196℃ are commonly used in the design. Low-temperature carbon steel is generally used for a temperature of -46℃. 300 series austenitic stainless steel is generally used for temperatures between -101℃ and -196℃. This kind of stainless steel has moderate strength, good toughness and good processing performance.
The structural design of petrochemical cryogenic valves
(1) The structural design of valve bonnets
A notable feature of cryogenic valves is that their bonnets are generally long-necked. In GB/T24925 Technical Conditions for Cryogenic Valves, there is also a clear stipulation that bonnets of cryogenic gate valves, globe valves, ball valves, and butterfly valves should be designed with a long neck that is beneficial for cold preservation according to different temperatures, ensuring that the temperature at the bottom of the packing box remains above 0℃. The design of the extended bonnet structure is mainly to keep the valve's operating handle and the packing box away from the low-temperature area, which can not only avoid the frostbite of the operator due to too low temperature, but also ensure that the packing box and gland can be used at normal temperatures. Prevent the sealing performance of the packing being reduced and the service life of the packing is prolonged. The packing's elasticity gradually disappears as the temperature decreases at low temperatures, and the leak-proof performance decreases. The leakage of the medium causes the packing and the valve stem to freeze, which affects the normal operation of the valve stem. Meanwhile, the packing will be scratched because the valve stem moves up and down, causing serious leakages. Therefore, the cryogenic valve must adopt a long-necked valve structure for bonnets. In addition, the long-necked structure also facilitates the winding of cold insulation materials to prevent loss of cold energy. Because low-temperature pipelines generally have a cold insulation layer with great thickness, it is beneficial to the cold insulation of the long-neck bonnet. The packing gland is outside the cold insulation layer, which is beneficial to fastening the gland bolts or adding packing at any time without damaging the cold insulation layer whenever needed.
BS 6364, MSS SP-134 and SHELL MESC SPE77/200 standards all specify the length of the valve bonnet. Among them, BS 6364 stipulates the extension size of cold boxes 15 to 500, and stipulates that the small extension length of the non-cold box should be 250 mm; MSS SP-134 indicates size requirements for cold boxes and non-cold boxes extension 15 to 300. In comparison, the length of the non-cold box is longer than specifications in BS 6364, while that of the cold box is shorter than specifications in BS 6364. SHELL MESC SPE 77/200 does not distinguish between cold boxes and non-cold boxes, and specifies the length of 15 to 1200 in different temperature ranges.
SHELL MESC SPE 77/200 has a wide selection of extended lengths by considering comprehensively, which is more convenient and reliable. If the bonnet is used in key situations at low temperatures, it can be designed according to the SHELL MESC SPE 77/200 standard or the special length required by the design unit. In addition, when the length is selected, it is necessary to consider whether the thickness of the designed cold insulation layer is greater than this length, and if so, it should be lengthened to match the thickness of the cold insulation.
1. Standards and definitions of cryogenic valves
Different standards have different definitions for cryogenic valves.
(1) British valve standard BS6364 Valves for Low Temperature indicates that the medium temperature range of cryogenic valves is from -50℃ to -196℃.
(2) The American standard MSSSP-134 Requirements for Cryogenic Valves and Valve Bodies or Bonnet Extensions explains that the standardized low temperature is between -100℃ and -195℃".
(3) The Chinese standard GB/T 24925 Technical Conditions for Cryogenic Valves is applicable to the medium temperature range of -29℃ to -196℃.
(4) Shell valve standard SHELL MESC SPE 77/200 Valves for Low Temperature and Ultra-low Temperature is suitable for the medium temperature range of -30℃ to -196℃.
(5) There are generally the following six temperatures in foreign countries according to the liquefaction temperature of various gases under normal pressure:
Grade one: from 0 to -46℃
Grade two: from -47 to -60℃
Grade three: from -61 to -70℃
Grade four: from -71 to -101℃
Grade five: from -102 to -196℃
Grade six: below -253℃
Generally, temperatures between -46 and -150℃ are called low temperature, and a temperature below -150℃ is called ultra-low temperature.
The definition of cryogenic valves in the petrochemical industry is based on the design temperature of the conveying medium. Generally, valves used for the medium temperature below -40℃ are called cryogenic valves, and those are applied to the medium temperature below -101℃ are called ultra-low temperature valves.
2. Design features of cryogenic valves
Material selection of cryogenic valves
The working medium of the cryogenic valve not only has low temperature, but also has great permeability, which determines many special requirements for the valve's material. The mechanical properties of steel at low temperatures are different from those at room temperatures. In addition to strength, the important index of low-temperature steel is its low-temperature impact toughness. The low-temperature impact toughness of a material is related to the brittle transition temperature of the material. The lower the brittle transition temperature of the material is, the better the low temperature impact toughness of the material becomes. Metal with body-centered cubic lattice like carbon steel has low-temperature cold brittleness, while the impact toughness of austenitic stainless steel with face-centered cubic lattice is not influenced by low temperatures.
Cryogenic valve bodies, valve bonnets and other pressure-resistant parts are usually made from materials with good low-temperature strength and toughness. At the same time, factors such as weldability, machinability, stability, and economy must also be considered. Three low temperatures -46℃, -101℃ and -196℃ are commonly used in the design. Low-temperature carbon steel is generally used for a temperature of -46℃. 300 series austenitic stainless steel is generally used for temperatures between -101℃ and -196℃. This kind of stainless steel has moderate strength, good toughness and good processing performance.
The structural design of petrochemical cryogenic valves
(1) The structural design of valve bonnets
A notable feature of cryogenic valves is that their bonnets are generally long-necked. In GB/T24925 Technical Conditions for Cryogenic Valves, there is also a clear stipulation that bonnets of cryogenic gate valves, globe valves, ball valves, and butterfly valves should be designed with a long neck that is beneficial for cold preservation according to different temperatures, ensuring that the temperature at the bottom of the packing box remains above 0℃. The design of the extended bonnet structure is mainly to keep the valve's operating handle and the packing box away from the low-temperature area, which can not only avoid the frostbite of the operator due to too low temperature, but also ensure that the packing box and gland can be used at normal temperatures. Prevent the sealing performance of the packing being reduced and the service life of the packing is prolonged. The packing's elasticity gradually disappears as the temperature decreases at low temperatures, and the leak-proof performance decreases. The leakage of the medium causes the packing and the valve stem to freeze, which affects the normal operation of the valve stem. Meanwhile, the packing will be scratched because the valve stem moves up and down, causing serious leakages. Therefore, the cryogenic valve must adopt a long-necked valve structure for bonnets. In addition, the long-necked structure also facilitates the winding of cold insulation materials to prevent loss of cold energy. Because low-temperature pipelines generally have a cold insulation layer with great thickness, it is beneficial to the cold insulation of the long-neck bonnet. The packing gland is outside the cold insulation layer, which is beneficial to fastening the gland bolts or adding packing at any time without damaging the cold insulation layer whenever needed.
BS 6364, MSS SP-134 and SHELL MESC SPE77/200 standards all specify the length of the valve bonnet. Among them, BS 6364 stipulates the extension size of cold boxes 15 to 500, and stipulates that the small extension length of the non-cold box should be 250 mm; MSS SP-134 indicates size requirements for cold boxes and non-cold boxes extension 15 to 300. In comparison, the length of the non-cold box is longer than specifications in BS 6364, while that of the cold box is shorter than specifications in BS 6364. SHELL MESC SPE 77/200 does not distinguish between cold boxes and non-cold boxes, and specifies the length of 15 to 1200 in different temperature ranges.
SHELL MESC SPE 77/200 has a wide selection of extended lengths by considering comprehensively, which is more convenient and reliable. If the bonnet is used in key situations at low temperatures, it can be designed according to the SHELL MESC SPE 77/200 standard or the special length required by the design unit. In addition, when the length is selected, it is necessary to consider whether the thickness of the designed cold insulation layer is greater than this length, and if so, it should be lengthened to match the thickness of the cold insulation.
Next: The Design of Petrochemical Cryogenic Valves (Part Two)
Previous: The Analysis of Low-pressure Sealing of Swing Check Valves (Part Two)
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