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Pressure surges in pipeline systems resulting from air releases.

 

It is a common practice to locate air valves at high elevations along water transmission mains. If there is no air valve at high points water hammer caused by cavitation may occur. If the absolute pressure somewhere in the pipeline is allowed to fall below the vapour pressure of the liquid, vapour is formed in the pipeline. The original column of liquid divides itself into two which, with different speeds and separated by a growing bubble flows on through the pipeline. Eventually the two columns of liquid begin to approach each other causing the vapour bubble to implode as the liquid columns reunite, resulting in a large pressure surge.

The magnitude of the water hammer shock depends upon the instantaneous difference in velocity of the two columns of liquid at the moment when they reunite, which,in the worst case can be greater than the steady velocity in the pipeline. Cavitation in pipelines must therefore be avoided. The risk of unacceptable negative pressures are greatest at high points,downstream of valves an pumps and in long suction lines.

 

A transient flow model was set up With a high point with no air valve installed.The heads on the left of the high point is lowered from 30 m to 6 m in 10 sec then raised back up to 30 m in the next 10 sec. 

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