A Study On The Pressure Gradient Method For Insulation Performance Test Of Vehicle-used LNG Cylinders

LNG Welding insulated cylinder was used as LNG fuel container on large vehicles which has a thermal insulation structure with high vacuum and multi-layer winding.It is a special equipment,and must be periodically inspected following ‘ Special Equipment Safety Law of the People’s Republic of China’.However,as matter of fact that there are no corresponding standards for its periodic inspection and assessment in China,and the inspection work has not been carried out yet.The main reason is that there is no suitable detection method for the assessment of this thermal insulation performance.Referring to the relevant manufacturing standards,only the static daily evaporation rate can be measured,but it is too much complicated and difficult to implement in Welding insulated cylinder on vehicle.Therefore,a new detection method of thermal performance for LNG Welding insulated cylinder on vehicle has always been the key research issues by experts at home and abroad.In this paper,a gradient method-pressure gradient method – which can indirectly detect the thermal insulation performance of insulated cylinders for vehicle LNG is proposed.The method is based on the characteristics of the natural pressure rise of the cylinder,and directly utilizes the medium inside the bottle as the test medium.The rate of pressure rise(pressure gradient)in the bottle is used as a characteristic index to evaluate the thermal insulation performance.In this paper,the correlation between pressure gradient and daily evaporation rate is designed and equivalent conversion mathematical model is established.With LNG and liquid nitrogen as medium,based on the experimental study of liquid nitrogen as experimental material,the weighing method and the pressure gradient method of static evaporation rate is analyzed.The results show that the model is in good agreement with the experimental results,and the correctness of the model,the stability and reliability of the pressure gradient method are verified.By the experiment and equivalent conversion mathematical model analysis,the factors that influenced the equivalent static daily evaporation rate were analyzed.The results showed that the rate of pressure rise in the cylinder was affected by ambient temperature.And the effect of filling rate on the detection results was basically consistent with weighing method.Also,there was extremely small influence on the inner cylinder by atmospheric pressure.The reason was analyzed in this paper,why the equivalent static daily evaporation rate measured by the pressure gradient method was smaller than that by weighing method.The absorbed heat was converted into part in equivalent conversion mathematical model,also a small part into the temperature rise of the inner liner and other energy forms.Whatever,the experimental results indicated that the tendencies of pressure gradient and weighing method were consistent,and the deviations were within the controllable range.So,pressure gradient method can provide a data reference for the establishment of inspection standards.