Considerations for Gas Pipeline Blowdown

Blowdown is a planned or unplanned release of pressurized natural gas from stations, equipment (vessels) or pipelines. In high pressure pipelines, the blowdown leads to low temperatures within the fluid and high vent rates due to larger gas inventory volumes. Blowdown is a hazardous operation, and for this reason several methods have been proposed to improve the accuracy in the estimation of the blowdown rate and the blowdown time in pipelines. In general, these approaches include either physical volume or pipe models with numerical and analytical methods of solution. For instance, a very simple approach for the estimation of the blowdown time is presented by American Gas Association (AGA); this approach can be used a as first-approximation to verify the size of the blowdown stack/valve. Another well-known simple method to predict gas-line blowdown times was presented by Weiss, Botros and Jungowski (WBJ) (1988); this method involves the application of correction factors that regards the pipeline as a volume. However, due to the transient nature of the blowdown and the proven accuracy of their formulation, in recent years transient simulations have been performed using commercial simulation software. This work compares simplified approaches and an acknowledged transient method integrating significant effects of fluid mechanics (quasi-steady flow for one phase), heat and mass transfer and rigorous thermodynamics. This transient method is extensively used by process design engineers since it is included as a calculation tool or utility in commercial simulation software. Experimental data taken from acknowledged literature allows estimating the level of accuracy of these approaches. Due to the complexity and sometimes non-availability of the transient models included in commercial simulation software, a novel and innovative simplified hybrid approach is presented in this work. This approach includes novel and improved correlations as well as numerical solutions of a physical model that can be easily translated into a computational code or sequentially structured in a spreadsheet. This method allows for estimating relevant variables associated to the pressure – time computation and the optimal sizing of blowdown stack/valves in gas pipelines, based on recommended gas blowdown times; these times were estimated considering a balance between the maximum permissible blowdown duration and the minimum wall and fluid temperatures that can safely be contained in the pipeline. Finally, comparisons between the results obtained by using commercial software and the novel approach are presented, showing a fair level of accuracy of this method (7.6 % maximum error percentage) considering its simplicity with regard to the transient modelling.