Effect of transient temperature on 304 stainless steel LPG tank structure using numerical simulation approach
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The use of numerical simulation approach to investigate the effect of transient boundary temperature on an LPG tank structure was investigated. Here, both transient thermal and structural system were coupled in ANSYS software version 19.2 to create an interaction between the thermal and mechanical load on the tank structure. The focus of this paper is to identify stress hotspot which may eventually lead to stress-corrosion using a non-linear solver. Literature has proven that temperature gradient acting on a material is a possible cause for failure in most engineering structures due to stress induced corrosion. In this study the effect of a time dependent change in the temperature of the material (304 stainless steel) was investigated. The temperature was set to increase from cryogenic to 30 °C, and the pressure which represents mechanical load was also implemented at the wall boundary. Results obtained showed that stress was concentrated at the principal plane connecting the tank roof to the cylindrical structure. However, a failure analysis was conducted were the mechanical load was increased to 3 × 1043 Pa. It was found that the material failed after 1,000,000 s time steps and the tensile yield strength obtained from the stress–strain curve was lower than the material standard value. This can be explained with the concept that the action of temperature disrupted the material microstructure, hence, reduced the material stiffness to fracture. The stress–strain curve was validated with the standard plot for the 304 stainless steel material type.
KeywordsFailure analysis Stainless steel LPG storage tank Extreme temperatures Transient thermal analysis Transient structural analysis
The application of 304 stainless steel materials in recent years for the construction of engineering structures are of increasing demand in the industry [2, 5]. Carbon steel materials being harder, good heat distributor and easy to sharpen compared to stainless steel is less preferred for use to build pipelines, metallic implants, bridges, storage vessels, offshore platforms and many more. The wide application of 304 stainless steel materials is derived from its corrosion, chemical resistance and the ability to prevent stain from contaminants in its surrounding . Therefore making 304 more preferred than carbon steel.
In this paper, a typical liquefied petroleum gas (LPG) vertical storage tank made from a 304 stainless steel material was used for the analysis. LPG is a type of gaseous hydrocarbon liquefied at room temperature and pressure of magnitude 101,425 Pa for use in cooking, heating homes and as fuel source . It is mainly composed of propane and butane at a specified proportion . In The United Kingdom, LPG is obtained from 100% propane and are classified as Grade A.
Studies has proven that the thermodynamic property of the fluid is greatly influenced by temperature and pressure from the surrounding. Examples includes boiling point, thermal expansion, and vapor pressure. The action of pressure and temperature gradient on the fluid, causes work to be done and therefore contributes in increasing the internal energy of the system. From the real gas theory, the gas molecules moves randomly in a container and collides in-elastically with the wall of the container there-after. The intensity of the molecular vibration is directly linked to the amount of thermal energy absorbed by the fluid. The rise in the vibrational frequency of the gas molecules is sourced from heat transfered from the surroundings and mechanical work done on the system. The result of the gas–wall interaction transfer momentum energy to the material microstructure. Hence, stress is generated on the material and could eventually contribute to failure at the long run.
The American Society of Metals postulates that the 304 stainless steel material will deform at a load of 213 MPa. Most a times, the load acting on LPG storage tanks in the refinery or processing plant could exceed this theoretical limit at a pro-long time interval, and could cause the material to experience fatigue stress and eventually fail or collapse [9, 13].
Thermal load resulting from heat action was found to be a common cause for stress concentration and failure of structures built from a 304 stainless steel material. Adnyana  recently conducted an interesting research concerned with the failure analysis of stainless steel heat exchanger tubes for use in a petrochemical plant, published with the Journal of Failure Analysis and Prevention. He investigated on a case where the shell and tube of the heat exchanger failed after a year of maintenance work. Metallurgical examination, chemical analysis, hardness testing and microscopic examination approaches were carried out by Adnyana  to identify the cause of failure. However, his study informed that the heat exchanger tubes exposed to high heat levels failed due to stress-corrosion cracking. The thermal stress exerted on the material was as a result of the consistent change in the temperature gradient at local points. Furthermore, Maharaj and Marquez  looked into the failure of a stainless steel pipe elbow used in the transportation of purge gas. The material type was an SA-312 TP04 stainless steel. It was found that the material failed due to local stress at the welded points, and can be accounted by the extreme steady-state piping vibration at welded points were the thermal stress was experienced. The microstructural and vibrational evaluation techniques were implemented for the evaluation for the possible cause of failure.
Kumar et al.  also investigated on the failure analysis of stainless steel pipes for use in the petrochemical industry. In his investigation, hydrogen was transmitted through the pipeline to a reactor. Leak points were identified on the pipe surface. Reactors emits high levels of heat energy, as a result the transmission pipes were exposed to the heat intensity. From his findings, the action of heat on the material had an influence on the pipe structural integrity. Specific areas were observed to have experienced local rise in temperature gradient and caused stress to be generated. As a result, the pipe surface was found to corrode due to local stress.
Fuller et al.  demonstrated that the failure analysis of an AISI 304 stainless steel shaft can be achieved using the conventional 14-step failure analysis approach. The approach involves mechanical testing, nondestructive testing, metallography, chemical analysis, but does not include detail transient thermal analysis coupled with the model structure. This study showed that the steel shaft failed at specific areas due to intergranullar stress cracking. The failure rate was rapid at heat affected zones.
Furthermore, Reinders et al.  investigated on the effect of pressure and temperature increase of LPG stored in a thermally coated pressure vessel exposed to surrounding fire out break from the boiling liquid expanding vapour explosion (BLEVE), using experimental approach. The environment temperature of the LPG tank was varied from cryogenic in LNG-tanks to over 1000 °C in a fire. The results obtained showed that extreme temperatures was the main cause why the tank structure behaviour deviated from normal. However, a 2-D Computational Fluid Dynamics (CFD) approach was used by Scarponi et al.  to study the behaviour of LPG tanks exposed to wild-land fires and, similar conclusion with Reinders et al.  work was made.
Critical review of the literature therefore informs that, heat has a negative impact on stainless steel material modulus of elasticity, as a result, it is possible that tools and structures made from stainless steel materials could fail due to heat induced stress. This forms the basis for our investigation were we seek to identify regions exposed to intense heat from the surrounding condition and, estimate the stress induced by the heat action on the material using a numerical approach. The numerical simulation approach involves coupling both thermal and structural system in ANSYS software, to model thermal and structural interaction. Here, a vertical LPG storage tank is used as the case structure for the investigation.
Despite the extensive studies on the failure analysis of stainless steel materials from literature, the application of a coupled transient thermal and structural system approach is rare in literature. Therefore, this paper focuses on performing failure analysis on a typical LPG storage tank model made from a stainless steel material under the influence of transient thermal and mechanical load. The study is subdivided into two specific objective. The first is to identify areas with the potential for failure under a relatively low temperature range, and to further perform failure analysis on the structure under a relatively high static temperatures and pressure. The two key measurable parameters for this study are the Von Misses stress, and Total Mechanical and Thermal equivalent strain.
2.1 Simulation approach
Fluid material properties
LPG (propane) grade A
504 kg m−3
Fluid base temperature
44.09 kg kmol−1
Specific heat capacity
1549 J kg−1 K−1
2.7014E + 05 J K−1
− 1.0386E + 08 J mol−1
Isotropic thermal conductivity
0.0177 W m−1 k−1
stainless steel material properties
7750 kg m−3
Derived from young modulus
1.9E + 11 Pa
1.693E + 11 Pa
Coefficient of thermal expansion
1.7E − 05 K−1
Tensile yield strength
2.07E + 08 Pa
Compressive yield strength
2.07E + 08 Pa
Tensile ultimate strength
5.8E + 08 Pa
tank model design considerations
Height of tank
Height of fluid
Height of roof
Tank bottom thickness
Body Wall thickness
Roof wall thickness
Tank surface area
Tank interior volume
2.2 Heat transfer process modelling
Fundamentally, there exist three modes of heat transfer. That is heat transfer by conduction, convection and radiation. In some case, a combination of these could take place simultaneously . In this paper, heat transfer by conduction and convection is expected to be experienced. The heat energy transferred to the fluid is sourced from the environment, such that, the tank structure conducts the heat and transfer the heat to the LPG by convection. In the process of heat transfer, heat is loss due to the material thermal resistance and thermal conductivity efficiency . Heat flux, rate of heat transfer by conduction, wall thermal resistance and conductivity plays a significant role in the heat transfer process. It can be estimated from the Eqs. (2), (1), (3) and (4) respectively.
The above described equations are applicable in the modeling of heat transfer interaction with the tank wall and exterior, provided that the wall thermal conductivity, resistance, temperature gradient, area and rate of heat loss is known .
2.3 Boundary condition, solver preference and calculation activities
3 Results and discussion
3.1 Heat flux distribution
The plot in Fig. 8 below describes the relationship between temperature, heat flux and time. Be informed that the heat flux or concentration is a function of temperature gradient and the tank wall thickness computed using the Eqs. (1) and (2) in Sect. 2.2. Notice that from the highlighted model equations, the heat flux is directly proportional to the temperature. Comparing this with the plot shown in Fig. 8 below, both profiles correlated proportionally to each other, and hence confirms proper implementation of the heat equations.
3.2 Fluid material deformation
LPG is a type of fluid that that vividly response to a change in the temperature gradient due to its thermodynamic properties. One of this property is its ability to volatilize at lower vapour pressure, conventionally below 2.206 GPa. At this critical point, the fluid begins to observe a non-linear behaviour with respect to the phase formed and displacement of the gas molecules. Detail knowledge about the behaviour of the gas/liquid molecules under the influence of absorbed thermal energy can properly be investigated from fluid molecular dynamics analysis (FMDA). This is not the focus of this paper.
It depicts that the bulk is deformed in an unsteady manner as a result of the uneven heat transfer distribution from the exterior, through the Fluid-wall interface and to the fluid. It was observed that the strain was concentrated at extreme points, than at the centre. This can be accounted from the shape of tank model at the bottom support and roof intersect. The resultant effect of the fluid strain or deformation exerts some amount of stress to the container wall adding to that exerted by the surrounding or boundary conditions. This is further discussed in Sect. 3.4.
3.3 Von Misses stress intensity analysis
Stress acting on a material leading to deformation can be evaluated using the Von Misses stress intensity. From previous discussion, it was deduced that stress is being exerted on the tank surface as a result of the fluid strain and boundary conditions. From design point of view, petroleum products container or storage vessels are designed to better manage pressure and prevent stress concentration at a point. Most LPG tanks are spherical in shape, whiles others are cylindrical in shape. The key difference lies on the concept that, in some refineries or gas processing plant for example the Atuabo Gas Processing Plant (AGPP) in Ghana, the LPG tanks are made spherical because external pressure is required to maintain the gas in the liquid state. This is not the case for vertical or cylindrical LPG tanks were the gas is first liquefied and stored. This represents the key reason why some petroleum refineries decides to store LPG in vertical tanks or spherical tanks. Comparing both designs, the spherical shaped LPG tanks are known to manage pressure better than the cylindrical counterpart. Pressure management of the vessel with respect to their structure, relies greatly on the structure’s shape.
The sharp points with high stress concentration represents the potential are on the tank structure exposed to deformation or failure at an extended period of time . Recall that, the transients thermal and structure system were coupled to evaluate the effect of thermal loads on the structure integrity in terms of deformation or failure.
3.4 Failure analysis and model validation
The results obtained from the numerical simulation was validated with a typical stress–strain plot for 304 stainless steel material. Both plot are observed to profile fairly well and the fracture point identified with a dash line are well aligned. Comparing both plot, the material is seen to have fractured at a lower stress value of about 210 MPa. This can be explained from the point of view that extreme thermal and mechanical loads were exerted on the material for a prolong simulation period of 1,000,000 s. As discussed earlier these forces induces fatigue and disrupts the microstructure of the material and therefore reduces the resistance of the material to fracture.
Findings from this study depicts that temperature has a direct impact on the stiffness of stainless steel materials. It showed that, the higher the intensity of the thermal load, the greater the stress experienced on the material at local points. However, using a different approach compared to literature yielded similar output. The principal plane connecting the model design roof to the cylindrical structure was identified to be the possible area exposed to maximum stress concentration and eventually fatigue failure of the austenitic material. From the failure analysis, stiffness was found to decrease with increasing temperature gradient and mechanical load.
We want to acknowledge GOD for the Grace and knowledge imparted on us and All Nations University College, Ghana for providing the platform to successfully complete this research.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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