A Study on the Accumulation and Distribution of Condensate Water in a CBM Field Gas Pipeline System

Abstract

Abstract This paper aims to determine the lowest pigging frequency and to reduce costs of pigging in CBM field gas pipeline system, by simulating the movement of water condensation in the dendritic gathering and transportation pipelines considering hydraulic and thermal character. The water content as a function of time and the distribution of liquid accumulated along the flow line have been simulated. As a result, optimized operation schemes have been proposed. Numerical model was established by considering the influence of temperature, pressure and terrain on gas flow and the saturated water content. The model which was validated by the PipePhase software contains three sub-models, namely the pressure drop model, the temperature drop model and the water film formation model. Then the migration model of water film was established to analyze the accumulation and distribution of condensate water depending on the flow rate and the terrain. Finally, the effects and sensitivity of decisive parameters on the amount of liquid were studied. And the precipitation and distribution of condensate water at the diameter-changing point were analyzed. Thus, total fluid volume and its distribution under different operating and environmental conditions were obtained. It was found that in the entire line, the liquid phase was mostly attached to the pipe wall by liquid film. The liquid film had a slight movement resulting from gravity and the carry of gas phase. Condensation occurred mainly in the first 1/3 length of the pipeline. At the end of the pipeline, the liquid stopped accumulating when the gas temperature kept constant and the pressure drop maintained. More condensate water and longer accumulation region were found in smaller diameter pipeline. And a significantly increased liquid holdup was also found in the diameter-reduced pipeline. Therefore, it is suggested that condensate tank or water drain valve or other facilities should be placed at the low-lying section in the first 1/3 length of the pipeline. The maximum acceptable pressure drop was taken as the target value to determine the longest pigging period under different working conditions. The liquid decreased gradually with the increase of inlet pressure, while with the decrease of temperature. The liquid volume is more sensitive to inlet temperature, and then ambient temperature, inlet pressure and flow rate. Thus it is suggested that the outlet pressure of compressor should be increased or an air cooling device should be installed.