Typically, a Pressure Control Valve (PCV) system is constructed with 3 to 4 PCV lines in order to prepare for both a future demand increase and a continuous supply of power after an emergency shutdown of the operating line. However, some operation failure cases that do not follow the original design concepts of a PCV system have been reported in the field. In this study, an accurate 1-D PCV system numerical model was built and a 1-D compressible flow theory was introduced for analytic valve modeling to find solutions for this problem. Several numerical analyses were successfully performed to examine the generation and propagation characteristics of the transient pressure and to clarify the relationships between the transient pressure or surge wave and each factor or parameter relating to fluid dynamics behavior in a PCV system. The relationship between the transient pressure and other factors, such as the size of pipe and header linked after a PCV, the pipe size of the added components linked after the header, and the generation time of the transient pressure and influence of the Slam Shut Valve were investigated in detail. Finally, in order to reduce the strength of the transient pressure and to delay its propagation, this study demonstrates the benefit both of enlargement of the pipe linked to the component added after the PCV system, and the installation of a dissipation component between the operation line and the other lines.

-The basic understanding of gas diffusion and technology to predict the diffusion phenomena are needed to prepare against a disaster of leakage of natural gas and to design better consistent and reliable gas supply system in enclosure. The experimental results of British Gas Technology Co. are used in present study as a reference of theoretical study using CFD. The present results of 2D CFD analysis for mass flow rate of nozzle release show good agreement with experimental results within 2.6 % error. 3D CFD analysis for the characteristics of natural gas diffusion in enclosure with various ventilation patterns also gives reasonable agreement with experimental results.

-Natural gas through pipeline is supplied to consumers after its pressure gets down compulsorily. The waste pressure energy of this process can be restored by use of turbo expander which can produce electricity. So, turbo expander conducts two functions -pressure reduction and power generation. The power amount is the enthalpy difference between the inlet and outlet states. The five main factors which affect economic profit are facility price, produced power amount, pre-heating amount, electricity cost, and fuel gas cost. Power generation depends mainly on flow amount because inlet and outlet states are fixed. A methodology to estimate economy in irregular flow pattern is proposed and using this way, a case study was carried out.