5 replies to this topic

[KeJoSa2010]

Hello,

I am a junior process engineer and have less than a year experience behind me (so please be gentle ;-) ). I have been asked to help determine the governing scenario for a number of PSVs at a particular gas plant. I have a question that I am confident the good people on this forum can help me with...... I am reviewing a PSV attached to an inlet separator (3-phase) which in theory is pretty simple I am sure.........(bare with me, as I said, I am new to all of this).......There is a control valve (PCV) just upstream of the vessel (which fails open) and a block valve just downstream of the vessel on the vapor outlet. There are also level control valves on both the water and the condensate outlet lines (all fail closed). I am analyzing the closed outlet case (blocked flow) and I am a bit confused. I have been told (by someone with much more experience than myself) that the "fail open of the PCV upstream of the vessel will drive the flow for the blocked outlet case". Further they go on to say that the blocked flow result would be the same as failure open of the inlet valve (PCV) as the flow is all sourced from the same place and that there is no blocked outlet case and instead this valve is under control failure.........Ok, I hope this description makes sense. Anyway, I am a bit confused......I don't see how the blocked flow result would be the same as failure open of the inlet valve!! If the block valve is closed the PCV may not fail, correct? Also, if the PCV fails then it does not necessarily mean the outlet flow will be blocked and I cannot see how the two scenarios would produce the same result.

Thanks in advance,

[ankur2061]

KeJoSa2010,

There is a separate relief device sub-forum on "Cheresources" where tons of information is available on relief valve scenarios. Go to that sub-forum and then use the "search" feature of the forum by typing in key words in the search box. Voila! you will find plenty of discussion relevant to you.

"Fallah" one of our star members who contributes regularly on relief devices has plenty to tell you about various relief scenarios. "Lowflo" is another expert on relief devices and systems.

Regards,
Ankur.

[S.AHMAD]

1. Please attach simplified PFD showing the system - PCV and PSV location.
2. Based on my current understanding of your description, blocked discharge case is still valid. However, I need to look at your diagram before jumping into conclusion

[paulhorth]

Can you confirm whether the PCV is controlling the pressure Upstream, or downstream (in the separator)?
If it's upstream, then a blocked outlet (for whatever reason) will cause the upstream pressure to rise which in turn will drive open the PCV, so blocked outlet will result in an open PCV.
Also, consider the case of instrument air failure. This will lead to an ESD which will close all the outlet lines. You have said that the PCV is fail-open, so loss of air will lead to the PCV going fully open. Once again, you have blocked outlet together with open PCV.

Paul

[fallah]

Hello, I am a junior process engineer and have less than a year experience behind me (so please be gentle ;-) ). I have been asked to help determine the governing scenario for a number of PSVs at a particular gas plant. I have a question that I am confident the good people on this forum can help me with...... I am reviewing a PSV attached to an inlet separator (3-phase) which in theory is pretty simple I am sure.........(bare with me, as I said, I am new to all of this).......There is a control valve (PCV) just upstream of the vessel (which fails open) and a block valve just downstream of the vessel on the vapor outlet. There are also level control valves on both the water and the condensate outlet lines (all fail closed). I am analyzing the closed outlet case (blocked flow) and I am a bit confused. I have been told (by someone with much more experience than myself) that the "fail open of the PCV upstream of the vessel will drive the flow for the blocked outlet case". Further they go on to say that the blocked flow result would be the same as failure open of the inlet valve (PCV) as the flow is all sourced from the same place and that there is no blocked outlet case and instead this valve is under control failure.........Ok, I hope this description makes sense. Anyway, I am a bit confused......I don't see how the blocked flow result would be the same as failure open of the inlet valve!! If the block valve is closed the PCV may not fail, correct? Also, if the PCV fails then it does not necessarily mean the outlet flow will be blocked and I cannot see how the two scenarios would produce the same result. Thanks in advance,

KeJoSa2010,
As Ankur (here i appreciate for his attention) mentioned you should submit the query at relief devices Sub-Forum. Also as AHMAD mentioned please attach a simple sketch showing the PCV and PSV situations and inlet/outlet streams around the separator.
Anyway, PCV wide open that can be caused by instrument failure or misoperation will result in downstream separator to be pressurised and the relief load in this case would be the difference between maximum expected inlet flow and total normal outlet flows (other valves are in operating position at normal flow)or in other word the difference between maximum inlet flow and normal inlet flow through the PCV, provided that to be adjusted for relieving conditions.
On the other hand, blocked outlet condition due to vapor outlet valve to be closed may lead to the condition that the controller can try to open the PCV to be fully open and in this case also the relief load seems to be the difference between the maximum inlet flow and nrmal flow through the PCV.
Based on above your colleague that did claim two causes of overpressure are terminated to the same result might refer to such similarity. Of course, IMO the analysis would be more complecated than i stated and need to be performed based on actual and detailed information.

Fallah

Edited by fallah, 11 January 2012 - 08:13 AM.

[KeJoSa2010]

Thanks for all the responses! In the future I will be careful which forum I post my questions.......Is there a way to move the post to the correct forum after being posted? Also, I have attached the relevant PFD for this vessel and included the location of the PSV. The PCVs are controlling the pressure inside the vessel. Again, thank you all for your input, it is much appreciated.

Attached Files

•  3-Phase Sep.pdf   341.41KB   148 downloads

Edited by KeJoSa2010, 11 January 2012 - 12:29 PM.