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Throttling Valve Outlet Pressure In Choked Flow Condition


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#1 go-fish

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Posted 22 September 2022 - 02:04 PM

I am unclear about the outlet pressure across a manual depressurisation vent valve in gas service releasing to atmosphere.

The globe valve has high pressure hydrogen on inlet at 80 barg and the outlet is connected to an open vent piping releasing at about 30m height. What will be the valve outlet pressure when an operator depressurises the system volume by opening this valve?

1. Choked condition pressure

OR

2. Atm pressure plus the line losses in the discharge piping at choked flow rate

Due to high inlet pressure, my understanding is that the valve will reach choked flow condition and the flow rate won’t increase any further. If the outlet pressure is Option 1 then how is this pressure dissipated as the vent tip is atmospheric pressure?

#2 latexman

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Posted 22 September 2022 - 02:53 PM

It is both.  Just inside the valve at the smallest flow area, it is 1.  Just inside the valve after the smallest flow area, it is 2,  There is a shock wave between the two.  I.e. a pressure discontinuity!



#3 go-fish

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Posted 22 September 2022 - 03:42 PM

Ok, so the pressure discontinuity will be inside the valve body itself and the pressure at the outlet flange will be Option 2. Right? Is it this pressure discontinuity which will be converted into noise? I mean the conservation of energy will still apply.

I am calculating rho x velocity^2 for FIV screening and PWL for AIV screening criteria per Energy Institute Guidelines for the valve outlet piping. The results are highly dependent on the outlet pressure and Option 2 due to higher pressure drop across the valve is resulting in much higher values than Option 1.

#4 latexman

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Posted 22 September 2022 - 03:48 PM

Yes. Yes. Idk about your 2nd paragraph.

#5 Bobby Strain

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Posted 22 September 2022 - 10:35 PM

You must also check the pipe discharge for choked flow.

 

Bobby






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