Study of Joule-Thomson cooling effect due to leakage in compressed gas pressure vessels

Abstract

The reliability of pressure vessels in Compressed Natural Gas (CNG) ship is very important because there is very high pressure inside the CNG container (up to 250 bar). A key way to maintaining the integrity of pressure vessels is the principle of Leak-Before-Failure (LBF). However, the temperature of leaking CNG will drop due to Joule-Thomson cooling effect when CNG passes through the crack. The localized cooling effect on the steel plates may affect the ductile to brittle transition failure. Hitherto, the LBF is applied to piping system without considering the temperature change of steel material caused by Joule-Thomson cooling effect. Simulation and experiments thus are desirable to verify the effect in a narrow leaking crack on pressure vessel. By this means, the application of LBF may need to be corrected. The narrow leaking crack can be considered to comprise many short and equivalent parts along the through-wall direction. Each part can be considered as a small throttling type nozzle. An iterative calculation program in MATLAB has been developed in this thesis. Due to the similarity on Joule-Thomson effect, argon is chosen as a substitute for CNG with safety in mind. The program is developed to calculate the pressure, velocity, density, temperature, viscosity, thermal conductivity and heat transfer coefficient of leaking argon through the crack by considering the Joule-Thomson effect. The calculated results of temperature, pressure and heat transfer coefficient of argon along the crack depth are used as boundary conditions in COMSOL FEA program for the heat transfer and thermal stress simulation. In MATLAB iterative calculations, the initial argon pressure inside the pressure vessel is the maximum pressure of 91 bar during test. The initial temperature is room temperature of 30 o C. Solutions of pressure, velocity and density of leaking argon of each small space inside the crack are firstly obtained after inputting the crack dimensions and roughness parameters. The pressure of leaking Argon drops from 91 bar at the entrance of the crack to 9.5 bar at the exit of the crack. The velocity of leaking Argon through the crack does not change much. The density of leaking Argon drops from 150.7 kg/m 3 at the entrance of the crack to 16.8 kg/m 3 at the exit of the crack. Then the temperature of leaking argon inside the crack is calculated. The temperature of leaking Argon decreases from 30 o C at the entrance of the crack to 0.04 o C at the exit of the crack.