Relief valves: Outlet : Why do we avoid sonic flow
Relief valves: Outlet : Why do we avoid sonic flow
(OP)
Hi,
I'm looking at some guidelines on relief valve sizing. Our guidelines state we should avoid sonic flow and size at 0.7 of MAC. Why is this the case?
is this to avoid high noise and vibration (e.g. sonic boom?)..
does the flow get limited this case.. ?
i'm sure we should avoid it.. but want the reasoning..
what is critical flow.. is this at mach 1? or below..
I'm looking at some guidelines on relief valve sizing. Our guidelines state we should avoid sonic flow and size at 0.7 of MAC. Why is this the case?
is this to avoid high noise and vibration (e.g. sonic boom?)..
does the flow get limited this case.. ?
i'm sure we should avoid it.. but want the reasoning..
what is critical flow.. is this at mach 1? or below..





RE: Relief valves: Outlet : Why do we avoid sonic flow
Trying to achieve 0.7 Mach at the PSV discharge would be a REAL challenge (since choked flow gives you 1.0 Mach and incompressible flow happens at around 0.6 Mach with decreasing velocity, you would have to put yourself in the horribly unstable transonic region). You either have a very difficult spec or you are reading it wrong.
David
RE: Relief valves: Outlet : Why do we avoid sonic flow
I have seen applications up to 0.75 Mach.
It is simply a question of how large a safety factor one wishes to apply.
RE: Relief valves: Outlet : Why do we avoid sonic flow
RE: Relief valves: Outlet : Why do we avoid sonic flow
David
RE: Relief valves: Outlet : Why do we avoid sonic flow
Dennis Kirk Engineering
www.ozemail.com.au/~denniskb
RE: Relief valves: Outlet : Why do we avoid sonic flow
Excellent explanation, hope you have better luck than I've had in this thread.
Some of the people reading the thread may not know that sonic velocity is a function of temperature and gas composition (not pressure or by implication density). Mass flow rate is a function of upstream density, velocity, and pipe flow area. So if a PSV is set at 1000 psig, and the tail pipe is too small and you lose 800 psig to friction down the pipe, the mass flow rate is now based on the correct sonic velocity, but the wrong (by about a factor of 1/5) mass flow rate and your overpressure risk is really high.
David