Low temperature valve

1. The selection of materials for low-temperature valves The working medium for low-temperature valves is not only low in temperature, but also mostly toxic, flammable, explosive, and highly permeable, which determines many special requirements for valve materials. The mechanical properties of steel at low temperatures are different from those at room temperature. In addition to strength, the most important indicator of steel used at low temperatures is its low-temperature impact toughness. The low-temperature impact toughness of materials is related to their brittle transition temperature. The lower the brittle transition temperature of materials, the better their low-temperature impact toughness. Metal materials with body centered cubic lattices such as carbon steel exhibit low-temperature cold brittleness, while metal materials with face centered cubic lattices such as austenitic stainless steel have little impact toughness affected by low temperature. The materials for pressure resistant parts such as low-temperature valve bodies and covers are usually made of ductile materials with good low-temperature strength, while also considering factors such as weldability, machining performance, stability, and economy. When designing for engineering companies, the most commonly used low temperature levels are -46 ℃, -101 ℃, and -196 ℃- Low temperature carbon steel is generally used for 46 ℃ low temperature grades, while 300 series austenitic stainless steel is generally used for -101 ℃ and -196 ℃ low temperature grades. This type of stainless steel has moderate strength, good toughness, and good processing performance.

2. The structural design of low-temperature valves: 1) The most prominent feature of low-temperature valves is that their valve covers are generally of a long neck structure. In GB/T 24925 "Technical Conditions for Cryogenic Valves", it is also clearly stipulated that "the valve covers of low-temperature gate valves, globe valves, ball valves, and butterfly valves should be designed into a long neck valve cover structure that is easy to maintain cold according to different temperature requirements, to ensure that the temperature at the bottom of the packing box is maintained above 0 ℃". The design of the extended valve cover structure is mainly to keep the valve operating handle and packing box structure away from the low-temperature zone, which can avoid operator frostbite caused by too low temperature, and also ensure that the packing box and pressure sleeve are used at normal temperature, prevent the sealing performance of the packing from decreasing, and extend the service life of the packing. In addition, due to the thick insulation layer thickness of low-temperature pipelines, the long neck valve cover is convenient for insulation construction and keeps the packing gland outside the insulation layer, which is conducive to tightening the gland bolts or adding fillers at any time when needed without damaging the insulation layer. The BS6364, MSS SP-134, and SHELL MESC SPE77/200 standards all specify the size of valve cover extension. Among them, BS6364 specifies the extension size of 15-500 cold boxes, and stipulates that the minimum extension length of non cold boxes should be 250mm; MSS SP-134 includes requirements for the extension dimensions of cold and non cold boxes ranging from 15 to 300. In comparison, the extension dimensions of non cold boxes are longer than those specified in BS6364, while the extension dimensions of cold boxes are shorter than those specified in BS6364. SHELL MESC SPE 77/200 does not distinguish between cold box and non cold box, and specifies the length of 15-1200 in different temperature ranges. Overall, the SHELL MESC SPE 77/200 has a wide selection range of extended lengths, which is convenient and reliable to use. If used in low-temperature critical situations, the SHELL MESC SPE 77/200 standard can be referenced for design or designed according to the special length requirements of the design unit. In addition, when selecting the length, it is also necessary to consider whether the thickness of the designed cold insulation layer is greater than this length. If so, it should be lengthened to match the cold insulation thickness. 2) Drip plate structure design: Due to the low temperature medium transmitted inside the valve, in order to avoid or reduce the transmission of medium temperature to the filling material on the valve stem and its upper end, and to prevent these materials from freezing and failure, a drip plate structure can be added to the valve. Some research institutions have conducted experimental verification on valves with a drip plate structure and demonstrated that the upper end temperature of the valve cover with a drip plate is higher. Due to the low temperature on the upper part of the extended valve cover, the valve is usually exposed to the air, and the water vapor in the air will liquefy into water droplets when encountering the low-temperature valve cover. The diameter of the drip plate exceeds the diameter of the middle flange, which can prevent the low-temperature liquefied water vapor from dripping onto the middle flange bolts and avoid the impact of bolt rust on online maintenance. In addition, the drip board needs to be installed on the outer side of the cold insulation layer to prevent condensed water droplets from falling onto the cold insulation layer and the upper part of the valve body, protect the cold insulation layer and prevent the loss of cooling capacity.

3. The structural design of pressure relief components has special requirements for the sealing structure of low-temperature valves with a sealed cavity structure when applied to flammable, explosive, and easily gasified media. Some low-temperature media will increase in volume after vaporization, for example, the volume of liquefied natural gas after vaporization is more than 600 times that of liquid. When the valve is in a closed state and the surrounding environment temperature is relatively high, the low-temperature media inside the valve body absorbs heat from the environment and gradually vaporizes, causing a rapid increase in volume, causing overpressure inside the valve and even threatening the safety of the valve, leading to medium leakage and even causing fire accidents, To ensure the safety of the valve and the factory, this type of valve requires a self relieving structure with a central cavity, which enables automatic relief when the internal cavity pressure of the valve exceeds normal pressure. For example, low-temperature gate valves and ball valves, there may be significant differences in pressure relief design due to different valve sealing principles. However, different manufacturers often have their own unique characteristics in the design of pressure relief structures. 4. The design of anti-static and fireproof structures is particularly important as low-temperature valves are generally used in flammable and explosive media. The anti-static design mainly uses a current guiding method similar to a lightning rod to guide the valve stem and valve body, thereby exporting static electricity to eliminate safety hazards and ensure the supply safety of the entire system. As specified in GB/T 24925, "Valves with soft seats or soft closing insert components for flammable vapors or liquids should be designed to ensure conductivity and continuity between the valve body and stem, and the maximum resistance of the discharge path should not exceed 10 Ω. The design of fireproof structures is mainly aimed at solving the problem of medium leakage caused by drastic temperature changes, and the design requirements of fireproof structures are similar to those of ordinary valves.