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Regulators are considered as the main parts of natural gas pressure reduction stations. This study tries to simulate the regulator's compressible flow using a finite element method based on the finite volume and creating a triangular unstructured grid. Due to the complexity of the actual geometry of the problem, Simulation helped to investigation of numerical study of dynamic and thermodynamic behavior of high pressure natural gas during the transition through the regulators with the good approximation. In this work, “seat and plug” regulators are studied. We have verified the result of simulation with Gorgan (the city in north of Iran) CGS. Analyzing properties and quantities of the fluid passing through the regulators, with use of the flow simulation, it will be shown that at some point in the outlet, the fluid velocity reaches more than the speed of sound - supersonic region is created- the Mach number approaches to about 2.5, therefor we found the reason of erosion in some parts of pressure reduction stations. Checking velocity and pressure contours, inside of regulators and after them, confirms that the pressure reduction will result the gas volumetric expansion and then cause increases speed at the throat. Simulation show us after 6 times of pipe diameter from discharging of the regulator, the flow reaches to fully developed regime. This information helps us observe the required distance in deploying the instrumentations, also, the pressure reduction in regulators is associated with temperature reduction. That is why gas is preheated before the pressure reduction at the pressure reduction stations.