Risk Analysis And Control Of Cold Venting Of Natural Gas Pipeline

With the increasing need of nature gas and its wide distribution in our country, pipeline companies have built more long-distance gas pipelines. The valve chamber plays an important role in the long-distance transmission system, when an accident happens in the pipeline system, the valve chamber is used to cut off the broken part and vent the nature gas in the pipeline, so the following repair work can be done. There are so many valve chambers and they locate in different topographies, the external influence is complex, in addition the inflammability and high pressure of nature gas, some risks must be considered. Due to the large amount and high pressure of venting gas and the influence of obstacles, some accidents have appeared in conventional operations causing huge economic losses and threatening the life of people. It is necessary to identify venting hazard of long-distance gas pipeline valve chambers, to simulate high-pressure gas diffusion, to guarantee the venting operation and people’s property and life.In this paper, on the basis of analyzing venting fault tree, venting gas diffusion in the atmosphere became research focus. Fluid mechanics and jet mechanics were used to study the gas diffusion mechanism and identify main factors. Physical models were built by considering obstacles influence, then PHAST and FLUENT were used to stimulate gas diffusion in different situations. By analyzing stimulation results, some factors were defined:wind, atmosphere stability, initial outlet pressure, outlet diameter. For analyzing the low temperature effect, HYSYS was used to calculate the temperature drop around the venting valve at different pressure and temperature conditions. Amount of gas enthalpy decreasing during venting operations was calculated and the heat transfer between gas and the vertical pipeline was described.The diffusion numerical results and gas diffusion theory are consistent. Related phenomena occurring during gas venting operation can be explained by using the results. The research achieved can bring some guidance to control risks and predict consequence for the valve chambers venting operation in long-distance gas pipelines.