Material selection of low temperature valve
Application of cryogenic valve:
Low temperature valve refers to the valve that can be used under low temperature condition. Usually, the valve whose working temperature is lower than-40℃ is called low temperature valve. Low temperature valve is mainly divided into: low temperature ball valve, low temperature gate valve, low temperature stop valve, low temperature check valve, etc. All low-temperature material parts of the low-temperature valve must be subcooled before finishing. The main materials are to reduce the shrinkage and deformation of the low-temperature valve under low-temperature conditions. The low-temperature valve of Taiwan-Chen company is one of the indispensable important equipments in petrochemical industry, air separation, natural gas and other industries. Its good quality determines whether it can produce safely, economically and continuously. With the development of modern science and technology, the use of low-temperature valves is becoming more and more extensive, and the demand is also increasing.
Cryogenic valve is suitable for liquid cryogenic mediums such as ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, liquefied petroleum products, etc. The working conditions of cryogenic valve are harsh, and most of its working mediums are flammable, explosive, for substances with strong permeability, the lowest working temperature can reach-269 ℃ and the highest using pressure can reach 10MPa. Taichen low temperature valve temperature range LCB(-46 ℃), LC3(-101 ℃), CF8(304)(-196 ℃). The design, manufacture and inspection of low-temperature valves are quite different from those of General valves. Under low temperature conditions, the tensile strength and hardness of the material are improved, the plasticity and toughness are reduced, and different degrees of low temperature brittleness are presented, which seriously affects the normal work of the valve.
Material selection of low temperature valve:
◎ valve body: ferrite stainless steel can be selected when the temperature is higher than-100℃, austenitic stainless steel can be selected when the temperature is lower than-100℃, and copper alloy or aluminum alloy can be selected for low pressure and small diameter valves.
◎ the valve stem is made of Austenitic stainless acid-resistant steel, which requires proper thermal treatment to improve the tensile strength. At the same time, it must be plated with hard chromium (coating thickness 0.04~0.06mm) or nitrided, to improve the surface hardness.
◎ gasket: when the operating temperature is higher than-196 ℃ and the maximum operating pressure at low temperature is 3MPa, asbestos rubber sheet made of long fiber from asbestos can be used; The operating temperature is higher than-196 ℃, when the highest working pressure at low temperature is 5MPa, stainless steel wrapped gasket with asbestos, stainless steel wrapped gasket with PTFE or stainless steel wrapped gasket with expansion graphite can be used.
◎ fasteners: when the temperature is higher than-100 ℃, the bolt material adopts Ni, Cr-Mo and other alloy steel, which requires appropriate heat treatment to prevent thread bite; When the temperature is lower than-100 ℃, the bolt material can be austenitic stainless steel. Nut materials are generally Mo steel or Ni steel, and thread table is coated with disulfide key.
Technical requirements for low temperature valve:
- ▲ use pressure relief hole to prevent abnormal pressure rise. The location of the pressure relief hole can be set on the valve body or the Ram according to the structure of the valve.
- ▲ the valve body should be able to fully bear the expansion and contraction caused by temperature changes, and the structure of the valve seat part will not produce permanent deformation due to temperature changes.
- ▲ adopt the valve seats and the sealing surfaces of the valve Clack with chromium-tungsten hard alloy overlaying structure.
- ▲ use long-neck valve cover structure that can protect the stuffing box.
- ▲ adopt the valve clack that can maintain reliable sealing regardless of the change of temperature, for example, the gate valve adopts elastic disc and open disc, and the stop valve adopts cone disc, etc.