Abstarct: In this study, after examining the physical and chemical nature of the impurities natural gas flow and factors of production them in the gas distribution network paying to study the steps taken so far for seperation this particles. given the problems that this particles creates for equipment of industry and consumers this fuel and also because of the difficulty and high cost of operation Current separators, in this study, we decided that to resolve this great problem gas industry, paying to investigation of the use of cyclones in pressure reducing stations and as a pre-filter before stations filtration system. for this purpose, Bojnoord city two number pressure reducing station considered as target station and obtained approximate of the density and concentration of contaminants in natural gas streams in this station, in continue afterwards study of the theory term dominant on cyclone flow, command program on the relationship with Matlab software written for standard cyclone. then according to vary the consumed gas volume in station at different seasons, it was decided to use of four parallel cyclone that have good separation efficiency in the range station capacity. so on select three different sizes of Stairmand cyclones and investigate theory figure performance with Matlab. for the numerical solution after validation of simulation results with experimental results and check the results independence of network obtained separation efficiency three cyclone given at different input speed using computational fluid dynamics. finally, after analyzing the results, high efficiency Stairmand cyclone that diameter of main body is half a meter which can offer as a cyclone for this station which enables the flow of natural gas in the range of  to  cubic meters per hour capacity with optimum efficiency will refinement.