In recent decades, with the development of modern science and technology, there is more and more demand for cryogenic valves in engineering projects.
Cryogenic valves are used in fertilizers, LNG and petrochemical industries.
In addition to liquid nitrogen and other liquid inert gases, the medium controlled by cryogenic valves is not only flammable and explosive, but also gasification when heated or flashing, resulting in rapid volume expansion, which can lead to leaks and explosions.
Based on the characteristics of the media and the requirements of the application of the valve at low temperature, the design, manufacture, test and installation methods of cryogenic valves are different from those of ordinary valves.
Standard and definition of cryogenic valves
Different standards for low-temperature valve have different definitions.
British valve standard BS6364 "low-temperature valve" applicable to the medium temperature range of -50℃~-196℃;
The United States standard MSSSP-134 "low temperature valve and its body/bonnet and the length of the requirements" to the low temperature of the interpretation of "for standard practice, Low-temperature range between -100℃~ -195℃";
China National standard GB/T 24925 "low-temperature valve technical conditions" standard applicable to the medium temperature range of -29℃~-196 ℃;
Shell valve Standard shell Mesc SPE 77/200 "cryogenic and ultra-low temperature valves" is suitable for medium temperature range of -30℃~-196 ℃.
The liquefaction temperature of different gases at atmospheric pressure is generally divided into six kinds of temperature levels abroad.
The first level is 0~-46 ℃,
The second stage is -47~-60 ℃,
Level Three is -61~-70 ℃,
Level four is -71~-101℃,
Level Five is -102~-196℃,
Level six is below 253 ℃.
General -46~-150℃ called low temperature,-150 ℃ below called ultra-low temperature.
The definition of cryogenic valves in the petrochemical industry is defined by the design temperature of the conveying medium, which is generally applied to the medium temperature-40 ℃ below the valve called the cryogenic valve, applied in the medium temperature-101 ℃ below the valve called ultra-low temperature valve.
Design features of cryogenic valves
1, low-temperature valve material selection Low-temperature Valve working medium not only the temperature is low, and most or toxic, or flammable, explosive, and strong permeability, so decided to the valve for many special requirements. At low temperature, the mechanical properties of steel are different from normal temperature, and the most important index of low temperature steel is its low temperature impact toughness. The low temperature impact toughness of the material is related to the brittle transition temperature of the material, the lower the brittle transition temperature of the material, the better the low-temperature impact toughness of the material.
The metallic materials of carbon steel, such as core cubic lattice, have low temperature and brittle phenomena, and the impact toughness of austenitic stainless steel and other surface core cubic lattices is not affected by low temperature.
Low-temperature valve body, bonnet and other pressure-resistant parts of the material, usually with good low-temperature strength of the toughness of materials, but also to consider the weldability, machining performance, stability and economic factors. At design time, the most commonly used are -46℃, -101℃ and -196℃ three cryogenic levels.
-46℃ Low-temperature Grade general selection of low-temperature carbon steel, -101℃ and -196℃ Low-temperature Grade general selection of 300 series austenitic stainless steel, this stainless steel has moderate strength, good toughness and better processing performance.
2, low-temperature valve structure design
1) Bonnet structure design One of the most obvious features of a cryogenic valve is that its bonnet is generally a long neck structure, GB/t 24925 "low-temperature valve Technical conditions" There is also a clear provision of "low-temperature valve, globe valve, ball valve, butterfly valve cover should be based on the use of different temperature requirements designed to facilitate the cooling of the long neck bonnet structure, To ensure the temperature at the bottom of the stuffing is kept above 0 ℃. The design of the lengthened bonnet structure is mainly designed to keep the valve operation handle and the packing letter structure away from the cryogenic zone, not only can avoid the temperature too low cause frostbite to the operator, can also ensure that the stuffing letter and pressure sleeve in the normal temperature to prevent the sealing performance of filler to reduce, prolong the service life of filler. Because in the low temperature state with the temperature reduction, the packing elasticity gradually disappears, the leak-proof performance decreases, because the media seepage causes the filler and the stem place freezes, affects the stem normal operation, simultaneously also can because the stem moves up and down the filler scratches, causes the serious leakage. Therefore the low-temperature valve must adopt the long neck bonnet structure form. In addition, the long neck structure is also convenient for winding cold insulation materials to prevent cold energy loss.
As the Low-temperature pipeline generally has a thicker thickness of the cooling layer, the long neck valve cover is easy to keep the cold, and the packing gland in the cooling layer, in favor of the need to tighten the gland bolt or add filler without damaging the cooling layer.
Installation requirements for cryogenic valves Because of the special structure of low-temperature valve, the installation of Low-temperature valve has its special requirements. Because of the low temperature valve of the long neck bonnet structure features, low-temperature valve in the installation of the stem stem direction must be in a vertical 45-degree angle range, and should try to avoid installation in the vertical pipeline. Otherwise low-temperature medium will fill the valve cover of the lengthened part, resulting in valve filler failure, and will be cold to the valve handle, to the operator to bring personal injury. For the pressure relief structure of low-temperature valve, in the installation of valves, pay special attention to the valve pressure relief direction requirements. The direction of valve pressure relief should be marked on the process flow chart and reflected in the pipeline axis mapping.
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