Abstarct: The main purpose of this project is to compute the wasted gas, which realeased during the purging operation. The operation is carried out reguallry in natural gas pressure reducing stations. A numerical simulation has been used to compute the amount of wasted gas which evacuated from purging valves. The simulation has been carried out using using Fluent pakage software. Natural gas as working fluid is assumed compressible real gas. Fluid is treated turbulent and unsteady flow. To make the solution and results more similar to actual condition, the valve is continuously opened during the simulation. Initial condition, is assumed steady flow in pipeline. Three dimentional modeling is used to simulate the valve and connected pipe, and effects of varisue condition including, inlet pressure, pipeline diameter and valve diameter variation are studied. The numerical results show that due to high pressure difference between inside the pipeline and atmosphere, discharging gas is supersonic at outlet section of valve and the Mach number is greater than one. This is due to a convergent-divergent nozzle creation in discharging pipe. Also results show that outlet mass rate has liner relation with pipeline pressure, second degree relation with valve diameter and has fourth-degree relation with valve to pipeline diameter ratio. Two correlations have been developed for or calculating the amount of wasted gas in steady state and unsteady state condition. Unsteady state correlation is valid for valve opening time and steady state correlation could be used while the valve is completely opened.