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Double Jeopardy?


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#1 gpl

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Posted 11 May 2022 - 12:50 PM

Hi,

 

I'd appreciate your thoughts on the following re: double jeopardy:

 

Steam is flow controlled to a reboiler.  Overhead gas from the column goes to a cooler, KO drum, and compressor.  Excess gas to the compressor suction is vented via a pressure controller and control.

 

If the steam flow controller is put in manual and the valve is fully opened, there is a very significant increase in flow exiting the column.  For this scenario, would it be correct to consider this flow with the compressor off and pressure controller in operation... or with the compressor off and pressure control valve fully closed?

 

thanks,

 

gpl



#2 latexman

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Posted 11 May 2022 - 01:19 PM

It depends on your company's definition/rules of what is (and isn't) a credible scenario.  That is, IF your company has a written policy on that.  A good company will.  Otherwise, you probably follow API 521, which isn't much help, because they say, "Ultimately, the user shall decide whether a scenario is credible or not."

 

So, it's up to you.

 

Now, if the steam is put in manual, is that "announced"?  As a consequence of excessive steam flow, like higher overhead flow or higher column pressure or dP, is that announced?  If compressor is off, is that announced?  If pressure controller is not operational, is that announced?

 

See what I'm getting at?  How intelligent is your DCS and alarming?

 

And, how is the operating disciple?  If the operators put steam in manual. will they stay with it until it's back in auto, or will they go to lunch and play a hand of cards until their lunch time is over?

 

All this would go into my thinking of what to say is credible or not.  Of course, the conservative answer is compressor off and pressure control valve fully closed, right?



#3 Bobby Strain

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Posted 11 May 2022 - 03:51 PM

It takes a lot of information to address these issues. You are not likely to get an answer from us. It should be addressed within your organization, not some remote query. Is this a new facility not yet in operation? Nor finished construction?

The steam flow control valve can fail open without being placed in manual control.

Good Luck.

 

Bobby


Edited by Bobby Strain, 11 May 2022 - 03:54 PM.


#4 fallah

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Posted 12 May 2022 - 01:24 AM

Hi,

 

I'd appreciate your thoughts on the following re: double jeopardy:

 

Steam is flow controlled to a reboiler.  Overhead gas from the column goes to a cooler, KO drum, and compressor.  Excess gas to the compressor suction is vented via a pressure controller and control.

 

If the steam flow controller is put in manual and the valve is fully opened, there is a very significant increase in flow exiting the column.  For this scenario, would it be correct to consider this flow with the compressor off and pressure controller in operation... or with the compressor off and pressure control valve fully closed?

 

 

Hi,

 

In order to get a proper response please upload the sketch/PFD of the system you described along with the relevant control loops...



#5 breizh

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Posted 12 May 2022 - 06:16 AM

Hi ,

To add to the previous comments , you may want to share with us the safety matrix with existing interlocks . 

At least you should have a look at it together with the Hazop study if existing facility .

Note : Consider to review the case : loss of energy with the consequences on your system ( equipment and DCS) .

Good luck

Breizh 



#6 gpl

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Posted 12 May 2022 - 08:29 AM

Hi All,

 

Thanks for your comments.  This is for a new plant currently being designed.  HAZOP has been concluded, but LOPA is not yet complete.  The control system includes a safety PLC for SIFs.  Both the steam flow controller and compressor vent pressure controller are part of the basic process control system. 

 

If I understand your comments correctly, we should defer to the LOPA  requirement.

 

regards,

 

gpl  



#7 latexman

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Posted 12 May 2022 - 09:21 AM

Maybe. If the worst case scenario for this PRD is due to a single independent failure, like fire case or loss of cooling or tube rupture, it is a moot point for the two (and three?) independent failures you asked about, right? In that case, the PRD sizing and record documentation can progress before LOPA. But, as always, it depends. I suspect the reason you are concerned is the OP is about the worst case scenario. Is it?

#8 gpl

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Posted 13 May 2022 - 08:30 AM

Yes, steam supply valve going fully open results in highest flow for PRD.



#9 astro

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Posted 17 May 2022 - 09:55 AM

I'd suggest reviewing API STD 521 in detail. There's a lot to read and absorb.

 

I'd also suggest that the question here is more about "capacity credit" rather than "double jeopardy". API STD 521 7th Ed, 4.4.8.2   Capacity Credit

In evaluating relieving requirements due to any cause, any automatic control valves that are not under consideration
as causing a relieving requirement and that would tend to relieve the system should be assumed to remain in the
position required for minimum normal processing flow. In other words, no credit should be taken for any favorable
instrument response.

...

Although controllers actuated by variables other than the system pressure can try to open their
valves fully, credit can be taken for such control valves only to the extent permitted by their operating position at
normal minimum flow regardless of the valveā€™s initial condition.

 

Most processes can be run in manual. This is often a requirement for startup. So, it's sensible to consider that point that potentially makes credit for control action invalid.

 

Even where double jeopardy is relevant, clear understanding of the standard's definition along with the possibility of latent failures that can void double jeopardy requires careful thought.






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