| As a country with large energy consumption,the demand for energy in China is increasing with improvement of the quality of our lives and the economy.The government strongly promotes the use of natural gas to alleviate environmental problems because gas is now one of the most widely used clean energy resources.Many LNG receiving stations and factories have been built in coastal areas to ease the shortage of domestic natural gas and it promoted the rapid development of LNG-related industries,but high requirements for the safety and efficient work of LNG-related cryogenic equipment are essential.The low LNG temperature makes it necessary for LNG-related cryogenic equipment to be designed to take the problem of cold loss and heat intrusion into account.The cryogenic valve,one of the important components for adjusting the LNG pipeline system,needs to consider about the loss of refrigeration.On the other hand,LNG is flammable and explosive liquid,so the cryogenic valve should be well sealed.However,physical and chemical properties of most of the polymer materials used for ambient temperature sealing may deteriorate at such low temperature.If the sealing fails,leakage and even more serious production accidents will occur.Therefore,Selection of a suitable sealing material at a low temperature is critical for cryogenic valves.In this paper,from the perspective of controlling the loss of cooling,reducing heat intrusion,the mechanisms of cooling loss of ultra-low temperature valve were analyzed.Through simplification of the physical model of cryogenic layer and differential equations of heat conduction,the function is established.In accordance with relevant nation standards,the thickness of the cold preservation layer was estimated by different algorithms.Meanwhile,the specific working conditions for anti-condensation algorithm and the control-allowed loss algorithm were provided.In addition,due to the installation of cold insulation structure,the natural convection heat transfer coefficient and the heat exchange area between the valve cover and the air change.As a result,original designation of the bonnet length and drip tray position can-not meet the ultra-low temperature requirements that the temperature of the bottom of the packing is above 0℃ and no condensation water is formed on the surface of the drip tray.In this paper,the heat conduction differential equation of the long-neck bonnet under cylindrical coordinate is used to calculate the length of the bonnet after installing the cold preservation layer.The formula of the installation position of the drip tray and the efficiency of the drip tray were also discussed.The length of the bonnet after the installation of the cold preservation layer and the dripping water was obtained.Based on the calculation formula for the installation position of the disk,a cryogenic ball valve(2〞、300Lb)was taken as an example.The finite element analysis software is used to calculate the temperature of the outer surface of the cooling layer,the length of the bonnet,the bottom of the packing and the temperature of the drip tray.High-molecular polyethylene was selected as the low-temperature sealing material.Compression,compression rebound and hardness tests were performed at 25℃,-29℃,-50℃,-110℃,-162℃,and-180℃.The maximum stress,compressibility,resilience,and hardness of polyethylene were examined under different temperatures.Based on the compressive test at series of temperatures,the working temperature ranges of the high-molecular polyethylene as a sealing material was achieved. |
