Mathematical models of air vessels for pressure transient control in water pipelines - a review

Air vessels are used to mitigate both up and down surge events in pipeline systems. When the air vessel swings into action in the event of a pressure surge, the compressed air pressure changes accordingly to suppress the adverse pressure. The rate at which the pressure changes in the air cushion occur is very important in determining the effectiveness of air vessels as surge control devices. This rate is dependent partly on the initial air volume contained in the vessel (hence its size) and the thermodynamic process that the air undergoes. Consequently, the dynamic behavior of the entrapped air in these vessels forms an integral part of the overall scheme and needs to be well understood for a proper transient analysis and sizing of air vessels. The Polytrophic model and the Rational Heat Transfer (RHT) model which happen to be the only two existing models for describing the dynamic behavior of entrapped air in air vessels, have been covered in this review. The limitations of these models have been mentioned and the need for an improved model has been highlighted.