Numerical Simulation of Leakage and DiffusionProcess of LNG Storage Tanks
Authors: Li Xue , Chen Bing , Rasouli Vamegh , Zhou Ning , Zhang Qian , Liu Xuanya , Huang Weiqiu 
Published: January 2021

Background

The previous researches mainly focused on the potential hazards associated with LNG leaks and the level of the influence of external environmental factors on the dispersion effect of LNG spills. Few considerations were given to phase change. Therefore, in order to investigate the evolution process of LNG liquid pool and gas cloud diffusion, the effect of phase change on dispersion during LNG release is studied to analyze the behavior characteristics of LNG liquid pool expansion and gas cloud diffusion, and the effect of the leaking aperture on the gas cloud diffusion process is also studied.
Methods

The Eluerian model and Realizable k-ε model were used to numerically simulate the liquid phase leakage and diffusion process of LNG storage tanks. The homogeneous Eulerian multiphase model was adopted to model the phase change process after LNG leaks to the ground. The Eulerian model defines that different phases are treated as interpenetrating continuum, and each phase has its own conservation equation. The average diameter of LNG droplet and NG bubble were set to 0.01 m. The standard k-ε model and realizable k-ε model are commonly used to describe turbulent motion. However, the realizable k-ε model can not only effectively solve the problem of curved wall flow, but also simulate free flow containing jets and mixed flows. In addition, the realizable k-ε model had higher accuracy in concentration distribution by simulating Thorney’s heavy gas diffusion field test. Therefore, the realizable k-ε model was selected for gas diffusion turbulence.
Results

The diffusion of the explosive cloud was divided into heavy gas accumulation, entrainment heat transfer and light gas drift. The vapor cloud gradually separated into two parts from the whole "fan leaf shape". One part was a heavy gas cloud, the other part was a light gas cloud which spread with the wind in the downwind direction. The change of leakage aperture had a greater impact on the whole spill and dispersion process of the storage tank. The increasing leakage aperture would lead to 10.3 times increase in liquid pool area, 78.5% increase in downwind dispersion of methane concentration at 0.5LFL, 22.6% increase in crosswind dispersion of methane concentration at 0.5LFL and 249% increase in flammable vapor cloud volume. Within the variation range of the leakage aperture, the trend of the gas cloud diffusion remains consistent, but the time for the liquid pool to keep stable and the gas cloud to enter the next diffusion stage was delayed. The low-pressure cavity area within 200 m of the leeward surface of the storage tank will accumulate heavy gas for a long time, forming a local high concentration area.
Conclusion

Within the variation range of leakage aperture, there will always be a local high concentration area within 200 m downstream of the storage tank. In the field near the storage tank, the clouds settle and accumulate towards the ground in the state of gas-liquid two-phase flow, and the density of the cloud is gradually lower than the air in the far field, manifesting as light gas diffusion. The methane concentration in this area is high and lasts for a long time, so it should be the focus area of alarm prediction.