10 replies to this topic

[ryn376]

If the flow nozzle in the system in the attached drawing has an inlet pressure, P₂, of 165 psig, what will be the pressure after exiting the nozzle at P₃ (after pressure recovery, downstream of the VC) if the vessel downstream is at atmospheric pressure (P₄)? Assume air is flowing through the nozzle at 60^oF.

 

 

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[Zauberberg]

It can't be anything else than the P4 plus pressure drop from the point of interest to the tank.

[ryn376]

Why would it not be the critical pressure? 

 

Pc/P2 = (2/ (k+1)) ^ (k/(k-1))

[Zauberberg]

And where the transition between the 'critical pressure' and the actual tank pressure would happen then?

[ryn376]

From what I've read, it seems the pressure would dissipate as a shock wave in the downstream tank.

[Zauberberg]

Following that logic, the pressure in the flare header downstream of a relief valve would be equal to the calculated 'critical pressure' (considering the upstream, relieving pressure) and not the back-pressure from the flare stack.

 

The shock wave phenomenon you are referring to occurs in sonic flare systems, but not across a flow element.

[fallah]

ryn376,

 

You're right...the flow through the nozzle will be choked, then the pressure downstream of the VC will be critical pressure of the air at defined conditions, i.e. around 0.55*180 psia=99 psia

 

This pressure will be dissipated as shock wave, noise and vibration between nozzle downstream and atmospheric tank...

Edited by fallah, 04 February 2015 - 11:57 AM.

[Zauberberg]

Fallah, that would require designing flare header for the highest of all critical pressures of the relieving sources, and we know this does not happen in reality.

[ryn376]

I just want to make sure I have this correct in case of a more extreme example where flange ratings would have to be specified. For example, if upstream of the nozzle the pressure was 2500 psig (requiring 1500# CS flanges), the downstream flanges would have to 600# instead of 150# because the pressure would be ~1370 psig; if the pipe wasn't long enough to drop the pressure to atmospheric?

Edited by ryn376, 04 February 2015 - 12:32 PM.

[fallah]

Zauberberg,

 

In a choked flow through a PSV, as a nozzle, the critical flow pressure will be at the exit plane of the valve flow nozzle while the pressure at the valve outlet flange is the back pressure which is lower than the critical pressure and based on which the flare header and subheaders would be designed...

[fallah]

ryn376,

 

If the PSV tailpipe would be long, back pressure will be increased and even might become higher than the critical pressure (non choked flow) and in this situation the outlet flange rating might has to be upgraded accordingly...

Edited by fallah, 04 February 2015 - 12:40 PM.