Venting of a 32 km NG pipeline - Field measurements and CFD simulations
This paper describes two separate field experiments involving the venting of natural gas pipelines. The pipelines were 32 km and 60 km in length respectively. The studies were carried out as part of an investigation of the phenomenon of Low Temperature Excursions (LTE) under the aegis of the Energy Pipelines Cooperative Research Centre (EPCRC), Australia. When a highly compressed gas is allowed to escape from a pipeline, the large drop in pressure is accompanied by a significant drop in the temperature of the gas. The general impression is that the pipeline material is also cooled to a comparable extent. This has often led to over-specification of the properties of the pipeline material. Theoretical and CFD studies have shown that although the gas undergoing severe decompression can indeed attain very low temperatures, the adjacent pipe wall does not experience cooling to a comparable extent. This appears to be in part due to the short time scales involved, and due to frictional effects at the gas-pipe wall interface . In the field experiments described, in-situ measurements of gas pressure and pipe wall temperature were carried out. One of the field experiments also involved thermal imaging of the vent pipe surface. The measurements showed that even the most susceptible parts of the vent pipe were not excessively cooled during the event, i.e. much less than the cooling experienced by the gas at the corresponding location. CFD simulations of the highly transient flow in one of the field studies suggest that this may be due to a combination of the time scales involved, and frictional dissipation in severe pipeline decompression, i.e. rapid decompression from an initially high pressure level. Based on the above findings, work is in progress to improve the temperature calculations in computer applications.