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Pic Control In Debutanizer Reflux Drum Vent


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#1 gimenz

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Posted 05 September 2012 - 03:46 AM

Hi,

Good day.
Please allow me to ask for your advise.

I have this Debutanizer Reflux Drum.
OP: 9barG
OT: 35C

Current system:
The vent line of the vessel is provided with a Hand Control valve.
This controller is for venting the non-condensables that might accumulate and will cause a
high pressure in the reflux drum.

Propose change:
Now, the Owner wanted to change this to PC control.

The thing is, this line is normally no flow (or intermittent) and in my opinion there is no point in changing the configuration to automatic.

My question is this,
-The line is intermittent in operation, is there a benefit in changing this to PC control?

Addition:
I would like to thank for the responses i received in my previous post about a PDC across the control valve.
In the attached drawing, there is a PDC valve, i believe this controls the pressure of the Reflux Drum.
And when there's a sudden increase of pressure, the Hand Controller will be opened.

Thank you for the inputs you may give.
I apologize if Im asking maybe trivial questions.

Sincerely,
gimenz

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#2 Dacs

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Posted 05 September 2012 - 04:24 AM

At first glance, I can propose some sort of split range control that will both control the existing PDV and the proposed PV in lieu of the hand valve.

#3 fallah

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Posted 05 September 2012 - 04:49 AM

gimenez,

As you mentioned the role of the PV on debutanizer reflux drum which connected to flare network controls the pressure of that drum and is normally closed in normal operation. If pressure build up occurrence in the drum, let say due to accumulation of noncondensable gases, will lead to abnormal operating conditions, the PV will be opened automatically to keep the refux drum pressure constant.

Your owner is right and due to safety considerations it is better to change this manual control to PIC control instead continue to rely on operator action will be announced just by high pressure alarm.

Fallah

#4 Robert Montoya

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Posted 05 September 2012 - 03:26 PM

Dear Gimenz the owner is right, function of PDC is to control pressure in the drum due low pressure, this valve is normally open because the condenser is total so I guess that the reflux drum does not operate "inundated". When there is an increase of light hydrocarbons or the condenser fault then the pressure increases in the reflux drum therefore the valve PC should open
Now I suggest you take the opportunity to do the fix I'm appending. I hope your comments.

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#5 gimenz

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Posted 05 September 2012 - 09:20 PM

Thank you all for sharing your thoughts, I sincerely appreciate it.

As Sir Dacs have mentioned in my other post, I try to practice the "why, why why".
For this topic, i tried to look this Debutanizer in another project (same process and same licensor)
and the reference configuration is the same (i guess since it is the same licensor)
so it hit me up that there must be some reason why the licensor is not using a PC control.

but all i could think of, is that the reason could be because the flow is intermittent
and now Owner want to change/consider the PC control.

As for your thoughts above, its clearer that PC control is favorable for safety and proper control of operating pressure of the reflux drum.

Thank you again for the inputs.

#6 Dacs

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Posted 06 September 2012 - 06:58 PM

This stuff is normally being decided during design on ease of automation vs cost/higher likelihood of failure/complexity of operation.

Either will do fine, but maybe the owner have seen the operators have devoted too much time monitoring the operation of this valve or the plant may have some missed calls in the drum going overpressure so he wants to take the human factor out of controlling the drum pressure and have it controlled automatically.

As per the owner's proposal, while it will work, I have some issue about its operation.

If you look at the proposal vs what's in the P&ID, you'd basically have 2 separate controllers working on the same controlled variable (which is drum pressure). While I think a properly tuned controller with enough deadband (for the lack of better word) will work here, I'm afraid that it's possible for both controllers to "fight", like the PDV letting in gas to increase the pressure and the proposed PC venting off gas simultaneously that will end up to both valves chattering.

So I was thinking of having a single controller (split range) controlling both the existing PDV and the HV (to be turned to a PV).

If you need more details with this, let me know :)

Edited by Dacs, 06 September 2012 - 07:02 PM.


#7 Bobby Strain

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Posted 07 September 2012 - 07:29 PM

This looks like something KBR or Jacobs designed. If you analyze your control situation, you will find that the control you have is WRONG (maybe). You might want to fix the whole thing while you are at it. Don't start with a fix, because every process engineer is a control expert, even if they know little of the subject; and you can't necessarily tell about any of us. So analyze what it is you must control, sort through what can be manipulated to achieve control, postulate solutions, test them for proper response. If your client wants a vent with a control valve, let her have it. But do her a favor and give her a better control if you can. She will appreciate it. If you GOOGLE the subject, you will get lots of information. But best to be methodical. It's not rocket science. Start with the basics: provide steady column pressure and, while doing so, be sure that the reflux pump can deliver reflux to the column. You can take it from there. The implemented solution should be a balance between cost of changes and control benefits. Remember that designs like these create opportunities for engineers like us.

Good luck,
Bobby




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