Investigation on the effects of entrained air in pipelines

Abstract

The main goal of this work is the development of a computational program for the quantitative assessment of the effects of entrained air in pipeline systems with respect to their operational safety. Likewise, two specific problems are investigated. (1) The effect of entrained air in form of pockets on hydraulic transients, during pump shutdown. It can be considered the most dangerous maneuver within a pumping pipeline. The computations corresponding to this study were evaluated by using the method of characteristics. (2) The numerical simulation of fluid transients caused by the shutdown of pumps, considering air pockets located at the high points of pumping pipeline systems and a water-air bubble mixture immediately downstream of the pockets. The constitutive equations – conservation of the gas mass, of the liquid mass, and the mixture momentum – yield a set of differential equations that will be solved by the method of characteristics. For the homogeneous model presented herein the two phases or components are treated as a single pseudofluid with average properties. It is assumed that there is no relative motion or slip between the phases, as well as for the momentum equation for the mixture. In the same way to the compressibility of the gas, the liquid compressibility and the pipe wall elasticity are included in the system of equations. The equation of energy is not used due to the moderate change in temperature of the mixture during the transient. In the case of negative impacts on the safety and operability of pipeline systems resulting from air entrainment, operational remediation measures will be suggested.

Hier wurden bereits zielgerechte Modellversuche ausgeführt und Lösungsvorschläge für bauliche und betriebliche Änderungen ausgearbeitet. Letztere sollten nunmehr wissenschaftlich vertieft und zu allgemeinen Bemessungsregeln, möglichst auf analytischer Basis, übergeführt werden. Vornehmlich stehen die Bewegungsvorgänge von kompakten Lufteinschlüssen und Wasser-Luft-Gemischen (Zweiphasenkomponenten-Strömung) in Pumpendruckleitungen und deren Verhalten beim plötzlichen Abschalten der Pumpenaggregate im Vordergrund.