Should I change the size of the vent pipe? 1

[nouanda]

Dear Experts,

I am working on a feasibility study, which requires me to check the compatibility of an installed PSV with new process data.
This valve is 4"x6" and has an orifice of 4" (12.57 sq.in, out of API standards).
The set pressure is 100 barg.
The discharge flange is connected to a 6" vent pipe, to send the gas 163m away (this is the length of the pipe).
At maximum flow, I find a head loss of 67bar through the vent pipe, so the flow is non-critical.
I find a required orifice of 10sq.in, which is quite close to the installed orifice.
If the flow were critical, the required orifice would be 8sq.in, and the margin would be more comfortable.

My question is:
Is it reasonable to enlarge the vent pipe (ie 8"), without changing the valve, using a 6"x8" reducer?
I think it is not a clean solution but it may reassure the client??

Thanks

Nouanda

 

[Latexman]

Good advice JoeWong!

nouanda - I have NO experience in gas wells, but if it's reasonable, now is a good time to question the assumption that the control valves from all 13 wells fail open simultaneously. That approach, if it is within your client's acceptable risk criteria, may help the situation tremendously.


Good luck,
Latexman

 

[786392]

The approach (!!) mentioned by Joe is definitely very challenging/complicated.But can be an optimum!
For this a thorough study of all wells scenarios and'hazops have to be taken at length to minimize the consequential serious implication within the acceptable tolerance limits.
A thoughtful process involving key stake-holders and decision makers will be needed.

Best Regards
Qalander(Chem)

 

[JoeWong88]

Latexman,
Thanks for your compliment...

JoeWong
Chemical & Process Technology

 

[nouanda]

Hi all,

Some news about this issue (yes, I'm still working on it!)

Finally, it has been decided with the client to decrease the maximum flow down to 417000Nm3/h, and to respect a criteria of Mc 0.9.

Our approach changed a little:
We decided to divide our pipe into differents pieces of different sizes (increasing from valve to vent).
We calculated the minimum pressure at the end of each piece, so that the velocity is below Mc 0.9, and then we calculated the length of a piece to respect this pressure.

All of this was calculated backwards from the final point: the release at 1.013 bar abs.

Yet, the sizes are still quite big for such a small PSV(not less that DN500), but we found a result that satisfy the client. So now, let's see what he will decide (install this pipe or not?)!

 

[786392]

Thanks for the update Dear!It is good feedback.
Wish you and your client a safe,successful& sustainable outcome indeed!

Best Regards
Qalander(Chem)