Relief valves: Outlet : Why do we avoid sonic flow 3

[jamesbanda]

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..

 

[zdas04]

I'm not sure you are reading your spec correctly. I've seen specs that limited velocity in the flare header (generally to below 0.6 Mach since that is the point where density begins changing very rapidly with increases in velocity). That doesn't mean that you are limiting the PSV discharge velocity.

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

 

[Chance17]

In practice, there is no absolute spec number.
I have seen applications up to 0.75 Mach.
It is simply a question of how large a safety factor one wishes to apply.

 

[atayto]

I beleive at mach figure of 1 you will definetely have problems with acoustic induced vibration. You can have a rupture in your PSV when it is releiving. This is process safety issue. BP standard is 0.8 for PSV tailpipe.

 

[zdas04]

Virtually all non-pilot gas PSV's are designed for critical flow (i.e., velocity at the discharge port is 1.0 Mach). The standards that people write for their companies are for the tailpipe, ignoring the transsonic region.

David

 

[denniskb]

The mach limitation of 0.7 to 0.85 is imposed on the discharge velocity from the vent pipe well downstream of the PSV (i.e. at the end of the vent to atmosphre or vent connection into a flare header) so that this does not become the flow limiting point in the system. It is critical that the PSV itself determine the relieving flowrate. If the vent pipe becomes choked (Mach 1) then this becomes the flow limiting component and not the PSV. This is also the reason that a well designed vent pipe will increase in size directly after the PSV as this point could also become choked if it were located away from the PSV.

Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[zdas04]

denniskb,
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