Experimental investigation of transients caused by rapid valve opening

Effective transient analysis requires more than a facility with the basic physics of the transient flow and the computational machinery that allows a degree of quantitative prediction. Rather, analysts must have something more: they must have a “feeling” or “sense” of what conditions lead to extreme, difficult or pathological behaviour. Without this design sense, analysts are left effectively blind, forced to grasp in the dark among a myriad of possibilities for those conditions or events that truly govern design and for those means that would be effective to counteract transient loading. This paper serves as a reminder that is not only velocity changes that can be decisive in design, but that pressure alone, and its sudden loss, represents a degree of risk. In the particular case study reported here, a pressurized and static length of plastic pipe is subject to a rapid opening at one end; the resulting dramatic transient, which is obviously in no way hinted at by the initially zero velocity, is reasonably severe and generates a decompression wave that in turn leads to cavitation at the dead-end and subsequent cavity collapse, as well as to an impressive acoustic signal. Such a depressurization event may at least be a partial explanation of why transient failures in large systems have sometimes been progressive (one failure leading to a cluster of failures) and severe under low flow/high pressure conditions within some supply and distribution systems.