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. Improper sizing of an air valve could lead to the rapid expulsion of air, which might result in excessive pressure surges at the air valve. Although preventing cavitation at high points requires the rapid inflow of air into the pipeline and therefore a bigger inflow orifice, the use of the same orifice for outflow might result in the rapid expulsion of air. However, using dual-orifice sizes-a larger inflow orifice and a smaller outflow orifice-might prevent undue secondary pressure surges associated with the rapid expulsion of air. This article demonstrates, through two example applications, the positive impact of smaller outflow sizes on pressure surges following the expulsion of air. The study also gives a simplified equation to estimate the magnitude of pressure surges based on pipe characteristics, air-valve characteristics, and pressure inside the valve just before the final release of air.