1. Introduction
It is expected that the global liquefied natural gas (LNG) supply will increase by about 50% in 2020 compared to 2016. A LNG vaporizing station is a small treatment site for LNG gasification to supply the gas pipeline network. It mainly has the functions of receiving, storing, and vaporizing LNG. The LNG storage tank is the key equipment in a LNG vaporizing station and it plays an important role in the whole process. At present, although LNG storage tank design technology is quite mature, the research on the design of the LNG tank pipes is still less so. The difference between LNG pipelines and ordinary oil and gas pipelines is that LNG poses a low temperature hazard, and once a leakage occurs, any exposed human body will suffer frostbite and the leaked LNG will cause the material to contract, causing further damage to the pipelines and equipment, resulting in more serious leaks. However, LNG will evaporate immediately after it leaks. After mixing with air, it will become a mixture with an explosive limit of 5–15%, so it can easily to explode when it meets a fire source [
1]. Therefore, the requirements on the safety performance of equipment and pipelines in LNG vaporizing stations are higher than those of other types of oil and gas stations.
The stress analysis of equipment can be traced back to the 1950s, when the stress problems of equipment began to be studied in the United States, because of the immature conditions and methods at that time, the development of this aspect was not vigorously promoted. In 1995, Adali et al. used the elasticity theory and the Tsai-Wu failure criterion to calculate the maximum destructive pressure of vessels. Based on that, the optimization analysis was carried out by using the robust multidimensional method [
2]. In 2004, Yang et al. described the process, method and evaluation principle of tower container overall stress analysis from the perspective of pressure vessel analysis and design, which provided a method for tower analysis and design [
3]. In 2005, Teng et al. used membrane stress theory to optimize composite containers [
4]. In 2008, Xu et al. applied the shell-solid connection technique in the ANSYS sub-model method to analyze the stress of the whole and key components of a large-scale chemical plant. The effect of the overall structure on the key components under combined conditions was obtained [
5]. In 2014, Song conducted a stress analysis and load optimization study on compressed natural gas (CNG) equipment [
6]. In 2016, Li et al. applied the Workbench software to simulate a compressor cylinder and used the inflation method to divide the grid. The temperature and thermal stress of the cylinder at different stages of gas morphology were obtained [
7]. In 2018, Lu et al. analyzed the vibration problem of reciprocating pump piping systems based on pressure pulsation theory and put forward some pipeline vibration reduction measures [
8].
From the aspect of stress analysis of the cryogenic pipelines, in 2009, Brown et al. put forward a finite element model to analyze the displacement and stress of underground or subsea LNG pipelines [
9]. In 2011, Wang used the CAESAR II software to analyze the cargo handling pipeline of LNG carriers, the feasibility of setting up horizontal expansion joints was obtained and the setting principle of the pipe support was summarized [
10]. In 2013, Jiao used the CAESAR II software to analyze low temperature ethylene loading and unloading ship piping systems, and proposed to use a spring hanger to solve the pipe void problem [
11]. In 2014, Chen et al. adopted the computational fluid dynamics method to analyze the stress of a LNG storage tank’s feed pipe. The stress of the pipeline at different times before steady state was reached was calculated, and the maximum stress of the pipeline appeared around the gasket of the connecting pipe and the pull rod [
12]. In 2015, Yu et al. considered gravity, temperature, pressure, wind and earthquake loads, and then proposed to use spring hangers to optimize piping systems, moreover, they put forward the method of obtaining the allowable stress in the case of accidental loads [
13]. In 2015, Xu and Hui analyzed the stress of the top inlet pipe of LNG storage tank using ANSYS software, they obtained the stress and deformation distribution of the pipeline and optimized the pipeline by adjusting the radius of curvature of the bend [
14]. In 2016, Wang used the CAESAR II software and GLIF software to do stress analysis of cryogenic pipelines in power stations, compared the results, and then described the advantages and disadvantages of the two types of software [
15]. In 2017, Zhang et al. used the Fluent software to study the precooling technology using boiled-off gas (BOG), compared and analyzed the temperature changes of the wall surface within 12 h of pre-cooling at different flow rates, and calculated the pre-cooling thermal stress and shrinkage displacement of the low temperature pipeline based on the temperature results. The results provide the basis for setting the thermal compensation, precooling speed and precooling parameters of the LNG cryogenic pipeline in the pre-cooling process [
16].
It can be seen that there are a lot of stress analysis objects for the equipment, including tower vessels, compressors and pumps. Most of them are at normal temperature or high temperature. There are few studies on the stress analysis of equipment or pipes under ultra-low temperature conditions. Moreover, the research on the reduction of the stress in the low-temperature pipeline is mostly focused on setting up the spring hangers and modifying the pipeline direction and there is very little research on the type, position and size of the compensator, and the length of the cold tightening unit.
The LNG storage tanks and low temperature pipelines in LNG vaporizing stations all use SS304 material. SS304 has excellent low temperature performance, but the linear expansion coefficient is large. Under the temperature condition of LNG transportation, a pipe length of 100 m will contract approximately 300 mm. Between two fixed points, the stress produced by the cold contraction may be far more than the yield point of the material. Therefore, in the pipeline design process, in order to ensure the normal and safe operation of the low temperature pipeline, effective measures must be considered to compensate the pipe shrinkage and prevent the damage caused by cold contraction. Therefore, this paper mainly analyzes the stress of LNG storage tank outlet pipelines. The analyzed content includes flange leakage calibration, stress and displacement analysis under normal working conditions, stress influencing factors analysis and stress reduction measures analysis. The results can provide the basis for the flange leakage calibration method of the low temperature pipeline, the design of the low temperature pipeline system and the measures of the stress reduction. Designers can then use more reasonable settings for the type and size of the compensator based on the analysis results.