Numerical Simulation And Combustible Region Prediction Of Gas Leakage And Diffusion In Spherical Tank

Spherical tank is the main way to store gas in the petrochemical industry. In the event of leakage, internal inflammable, explosive or toxic gas is likely to cause serious accident. Prediction of the combustible region and diffusion behaviors of the gases after leakage is of great significance to the rescue and reducing losses. At present, the widely accepted prediction model has some shortcomings, especially in the use of computational fluid dynamics model. This model basically simplifies the diffusion process as the gas is ejected into computing area at a certain speed, which does not consider the near flow field and cannot accurately describe the real property of high-pressure compressed gas leakage and diffusion. As a result, a new computational fluid dynamics model considering the near flow field and far-field diffusion was developed to study the characteristics of leakage diffusion in natural gas spherical tank in this study. A combustible prediction model of methane gas leakage diffusion was established.The main research contents and conclusions of this study are as follows:(1) Considering the actual spherical tank, the numerical model of high pressure gas leakage and diffusion was developed. This model could illustrate the feature of leak near flow field and far-field diffusion. The qualitative and quantitative verification of near-field and far-field ensured the validity of the simulation.(2) Using the developed model, the concentration, pressure, velocity and other parameters of near-field and far-field under different conditions(the critical and subcritical state) were examined. When the inner pressure of the storage tank was3MPa the aperture was150mm and the velocity of wind was Om/s, the maximum horizontal and vertical distance on its combustible concentration were respectively47.3m and10.24m. The special form like the mushroom cloud appeared at the end of the jet flow. Gas expansion and compression after leakage were found which leaded the pressure varied greatly. The region of negative pressure formed resulted in the entrainment effect on surrounding gas.The velocity varied greatly and the Mach disk appeared near the leaking hole. On the contrary, for the subcritical state, the range of combustible concentration was narrower and the mushroom cloud disappeared. Gas expansion and compression after the leakage did not occur. The pressure and velocity became stable. Comparing the critical and subcritical leaks of flammable regions, critical leakage of flammable region in both transverse and longitudinal range was more than the critical state. When leakage happened, the jet flow state was stabilized in1.5seconds. As a result, the gravity effect on the leakage diffusion was ignored.(3) The region of combustible concentration of the leakage and diffusion was divided into two parts:the columnar stable jet region and the mushroom region. Three parameters were adopted to evaluate the combustible region:Hmax, Lmax. and Dmax.The effects of the pressure of storage tank, the size of leakage hole and the wind velocity on the combustible concentration spread of methane, the volume, pressure and velocity of the combustible gas were examined. It was concluded that there was almost linear relationship between the pressure, leakage rate, volume of combustible gas and the predicted parameters. When the pressure was beyond a certain range, the wave system of expansion and compression disappeared. With the increase of the inner pressure, the Mach disk became clearer and its size increased. The aperture of the storage tank had proportional relations with the predicted parameters. The volume of leakage had a second power relationship with the size of the aperture while the volume of combustible gas had a power relationship with the size of the aperture. The effect of wind velocity and direction on explosive concentration region was relatively complex. When the wind was greater than5m/s with the wind and wind velocity was more than10m/s on negative direction, Lmax increased. Considering the engineering applications, the impact of wind on Hmax and Dmax could be ignored.(4) The predicting method of the flammable region was built based on the aforementioned results. Through double fitting the tank pressure and leakage aperture as well as the influence of wind, the predicted parameters including Hmax, Lmax and Dmax were quantified. Reasonable prediction in the engineering was obtained.In summary, the simulation of the gas leak and diffusion properties and prediction of combustible region satisfy the engineering needs.