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Debate: Will Low Point Pocket Remain Wetted? Sketch Provided

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#1 terrel.lobo


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Posted 12 September 2017 - 08:17 PM

Hi Folks,


I'll start by saying i'm of an electrical background (working instrumentation) not a chemical/process engineering background.  I was having a debate with some colleagues and was wondering if I could get some opinions from the fine folks on this site.


Please see attached sketch.  In the sketch, we have about ~40kPag of operating pressure at the elevation of the ESV (based on static head of 65-80deg C water level in the degasser).  The red line is a 20" pipe.  The level in the degasser is controlled by the LCV on this 20" line.  There is a 1" tubing branch that goes to an Instrument (AIT).  The AIT is located in a low point pocket as shown.  Both the 20" pipe and 1" tubing return to an atmospheric tank (Caisson of an FPSO Vessel) above the water level of said tank (i.e. Draft level of the FPSO Vessel).  There's potentially some variable back pressure (max 5 kPag) in the caisson.  The 1" tubing run has a check valve on the caisson inlet.  All the elevations are shown.


Under normal operation, both 20" and 1" lines are 'full' and flowing towards the caisson.


The debate:

Question 1:


With the ESV closed (assuming tight shutoff of ESV), will the low point pocket that the AIT sits in remain wetted or will the contents of the low point experience a "siphon effect" or be forced to the caisson?  


I am of the belief that the low point pocket of the AIT will still be wetted.  I do think that the ~4.5m vertical 1" tubing may drain slowly as air enters/displaces it from the caisson but I don't believe it to be enough to create a vacuum on the low point pocket.  i.e. There's no driving force.


Another argument I heard was that upon closure of the ESV, there would be ~40 kPag downstream of the ESV - supposedly enough to force liquid out of the low point pocket.  This doesn't quite make sense to me.


Another argument I heard was in the case of the LCV passing while the ESV was closed.  i.e. assumption that 20" line was draining to the caisson.  i.e. We have air in that 20" line causing potential siphon effect.  Again, i'm not quite sold on this being a concern.



Question 2:  



I sketched another 1" ESV upstream of the AIT.  If this were closed in normal operation.  Would the ~4.5m of vertical tubing be enough to "suck" fluid out of the low point pocket?  I assume no. (I think of the straw analogy of me holding the inlet end closed).



Pretty high level questions - hopefully I've given enough info.  Any thoughts or educated input to this debate would be appreciated.  Wondering if I've missed the mark on my belief/assumptions.






Attached File  Sketch.png   18.92KB   0 downloads

Edited by terrel.lobo, 12 September 2017 - 08:46 PM.

#2 Bobby Strain

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Posted 12 September 2017 - 10:31 PM

I wonder what is the point of your question. Does it matter what happens?



#3 terrel.lobo


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Posted 13 September 2017 - 06:20 AM

Hi Bobby,


Ah, sorry, some context. 


In this particular example, the AIT (analyzer) is measuring oil in water content but has a caveat:  It requires water in the sampling chamber at all times or it has the potential to damage itself.  This is not a problem in a planned shutdown scenario as an operator can turn off the analyzer.  In the case of an unplanned shutdown event (ESV closed), I'd rather not leave it in the hands of the operator.  I believe my tubing design would retain water in the low point pocket during unplanned events.  I have colleagues that agree and colleagues that disagree. 

The potential for siphon effect and/or draining effect was an item that came out in a design review.


Please note, I'm looking at other automated means to protect the AIT from a dry chamber incident.  These come at a cost and schedule impact.


[Edit: Another thought one may have is use a different AIT instrument but I'm working with what I have]

Edited by terrel.lobo, 13 September 2017 - 07:24 AM.

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