Experimental and computational Fluid Dynamics study of separation gap effect on gas explosion mitigation for methane storage tanks
Introduction
Physical layout of element spacing is one of the primary issues of Liquefied natural gas (LNG) project modeling (Taylor, 2007). The internationally recognized standard NFPA-59A (NFPA-59A, 2016) and European standard EN-1473 (EN-1473, 2016) are often used to ensure code compliance of the plant spacing issue. The separation spacing requirements for tanks are specified based on the empirical calculations by considering the volume of the tanks and the allowable heat flux values in NFPA-59A. Whereas the separation distance between two containers is determined by a detailed hazard assessment in EN-1473 (Raj and Lemoff, 2009).
For large tanks with storage capacity over 265 m3, NFPA-59A requires the safe spacing between tanks no less than 1/4th of the sum of the diameters of adjacent tanks, while only 1 m or 1.5 m minimum distance is specified for tanks smaller than 265 m3 (NFPA-59A, 2016). In terms of the EN-1473, the minimum separation distance should be no less than half of the secondary tank's diameter. Nevertheless, there are no fine subdivisions of separation distances according to the relationship between separation distance and tank diameter in these codes.
In order to better understand the impact of separation distance/gap on tank layout design, engineers and researchers had conducted more studies based on the industrial standards. For instance, a Computational Fluid Dynamics (CFD) study was carried out by Santos and Landesmann (2014) to investigate the separation gap effect on the safety of fuel storage tank farms. A specific minimum safety distance recommendation was proposed. However, the analysis was conducted on storage tanks subjected to fire conditions, the explosion pressure was not taken into account. Zhang et al. (2017) had conducted a more detailed safety analysis on tank farm layout optimizations. An integrated probabilistic framework along with some relevant procedures were developed to optimize the space collocation on the basis of different acceptable thermal radiation. Other literatures focusing on optimizing cost by means of different safety distances/gaps were also reported (Diaz-Ovalle et al., 2010; Jung et al., 2011; Patsiatzis et al., 2004). In the study of optimization of facility layout by Jung et al. (2011), the gas explosion scenarios were modelled by conducting CFD simulation, the flame acceleration simulator (FLACS) was only used as an accessional calibrator to assess the financially optimized layout. The relationship between safety distance and explosion overpressure was not discussed.
Overall, the models in the industry standards and the majority of current studies on safety distance determination are based on the flammability of the contents, thermal radiation data and financial risk management. Little attention has been paid on the consequence of gas explosion, such as the explosion overpressure and its effect on adjacent structures.
In this study, the separation distance effect on gas explosion overpressure was thoroughly investigated. Experiments were conducted on a group of tanks. A series of tank layouts with different separation gaps were designed in the testing. The gas clouds were ignited inside a vented tank. CFD simulation was conducted to qualitatively study the relationship between the separation gap distance and explosion overpressure. Two different gas cloud coverage scenarios were taken into account. The internal and external pressures subjected to different separation gaps were calculated and discussed.
Section snippets
Experimental tank group explosion tests
In the tank group explosion tests, the cylindrical tanks designed according to American Petroleum Institute Standard (API-650, 2007) were used. All tanks are 1.0 m in height and 1.5 m in diameter. Same as the tanks used in previous study (Li et al., 2017), the tested tanks are made of steel Q345B with tensile strength 470 MPa and yield strength 345 MPa. The welding between the tank roof and wall has yield strength 450 MPa and tensile strength 530 MPa.
CFD simulation and validation
Following the authors' previous study on vented methane-air explosion overpressure calculation (Li et al., 2017), the same CFD-based software FLACS (version 10.4) was used in this paper. FLACS uses Navier-Stokes equations and k−ε model for turbulence simulation. Obstacles and walls in complex three-dimensional geometries are represented by using on-grid and sub-grid objects with computed porosity values. The database of chemical kinetics, momentum, energy balance equations, and special schemes
Conclusion and discussion
In this paper, experiments were conducted to investigate the vented gas explosion with three different separation/safety gaps. Different pressure peaks at different combustion time were discussed. The observed internal pressures indicated that the first three peaks before Taylor instabilities were directly proportional to the failure pressure of relief panel. Pressure mitigation due to separation gap was only observed in the fourth peak of internal sensors and other pressure peaks at external
Acknowledgement
The authors acknowledge partial financial supports from Australian Research Council project (No. LP130100919) and China National 973 project (No. 2015CB058003) for carrying out this research.
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