The Design Of Cryogenic Valve For LNG And The Research About Material’s Properties At Cryogenic Temperature

The consumption of liquefied natural gas is growing by the rate of10%per year. It is one of the world’s most rapidly growing energy markets, It is the main work of "12th Five-Year" about adjusting the energy structure.. The high-speed development of liquefied natural gas contributed to the pace of developing the LNG cryogenic valves,also it promoted the research of materials’performance and phase at low temperature.This article describes the calculation of the LNG cryogenic valves. The article designs the structure of LNG cryogenic valves according the characteristics of liquefied natural gas. The nickel-base hardfacings were deposited on AISI304by plasma transferred arc welding. The impact test and tensile test were employed to research mechanical properties of AISI304and deposited sample at room temperature and low temperature. The optical microscope (OM), scanning electron microscopy (SEM-EDX), X-ray diffraction (XRD), microhardness tester were used to research the influence of cryogenic treatment to the microstructure and mechanical properties of nickel-based alloys and AISI304.Also the deformation of the material in the low temperature and the law of the size’s changement at cryogenic temperature are researched.The results through the above research are as follows:(1) Because the LNG is flammable, explosive, and temperature is too low,so the long-necked valve bonnet,the drainingboard, parts of decompression, anti-static structure, triple seal structure, fire protection and other special structures are needed for the LNG cryogenic valves.(2) The cryogenic treatment can enhance dimensional stability of AISI304at low temperatures, At the temperature of-196℃, the shrinkage at longitudinal direction of AISI304rod is0.05%,the shrinkage at dameter direction is0.79%. AISI304still have high plasticity and toughness at cryogenic temperature.both the impact fracture at room temperature and low temperature are ductile fracture. The dimples are shallower and smaller when the temperature is lower. After cryogenic treatment the hardness has a slight increasement,the microscopic morphology did not change significantly.(3) The Ni40was deposited on AISI304by plasma transferred arc welding. The connection of base metal and surfacing is well at low temperature.there isn’t cracking and other defects in the fusion line; the hardness of Ni40increases with the decreasement of the temperature.When the groove is opened at different locations,the impact energy of the specimens are different;there is little variation between the impact toughness of room temperature and the impact toughness of low temperature. Fracture of Ni40at normal and low temperatures are quasi-cleavage fracture;Afte cryogenic treatment small amount of precipitate formed at the intergranular organizations of surfacing layer,the small phase at the intergranular organizations decreased.(4) The Ni60was deposited on AISI304by plasma transferred arc welding. The connection of base metal and surfacing is well at low temperature, there isn’t cracking and other defects in the fusion line.The hardness of Ni60climbs up and then declines with the decreasement of the temperature.There is little variation between the impact toughness of room temperature and the impact toughness of low temperature surfacing trial samples at room temperature and low temperature impact toughness Fracture of Ni60at normal and low temperatures are quasi-cleavage fracture Afte cryogenic treatment small amount of precipitate formed at the intergranular organizations of surfacing layer,the small phase at the intergranular organizations decreased.