Calculation of pressure variation due to fluid discharge

[Chestermiller]

I can accept the initial step decline and depressurization wave upon the opening of of the of the pipe discharge to due an instantaneous flow change from the opening of the venting valve; but, I have doubts about there being following successive similar stepwise pressure declines. I have been involved in many liquid flow and blowdown tests for measuring and certifying the performance of both modulating and snap open relief valves using highly sensitive pressure transducers and at no time have I observed on any test indications of pressure stepdown type discontinuities recorded or by visual and audible observation of discharge flow rates that would be inherent with any discontinuous pressure drop(s) during the blowdown periods.

This is a very valid point. The model I described above is, of course, highly idealized, and there are many kinds of physical mechanisms unaccounted for which can sooth the response. One obvious example is viscous dissipation. Even in gas shock waves, viscous dissipation is an important contributor.

Still, at short times (up to a few minutes), I would expect the idealized model to do a pretty good job near the orifice. However, after that, when the reflected wave returns, I would say a subsequent step change would be either extremely modulated or entirely unlikely.

 

[vepexu]

Thank You all for help, especially Chestermiller.

If no one else has anything to add, I consider this topic closed.