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Atmospheric Crude Distillation Unit Design For Vacuum


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#1 ankur2061

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Posted 20 April 2014 - 02:38 AM

Dear All,

 

I would like to know that if atmospheric CDU needs to be designed for vacuum besides overpressure. I have had conflicting accounts about CDU being designed for vacuum with some experiences stating that it need not be designed for vacuum.

 

My understanding is that if CDU is designed for a steam-out operating during maintenance and / or shutdown, a Full Vacuum (FV) condition needs to be specified in the column datasheet.

 

Unfortunately not having much experience in the core petroleum refining sector, I don't have suitable references related to the design pressure conditions of an atmospheric crude distillation unit.

 

I would appreciate if members having the relevant experience could share their experience related to design conditions of an atmospheric CDU with proper justification.

 

Regards,

Ankur



#2 aroon

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Posted 21 April 2014 - 01:29 AM

Hi Ankur,

 

In my opinion, full vacuum condition is not required for CDU design. The reasons are below:

  1. CDU is very big unit, which consist of many equipment in open communication. Even if we consider the design of only tower with FV, contribution in capital investment will be very significant.
  2. There are certain administrative procedures which can be used as safety measures during steam-out operation. Many of the refineries are mitigating the concern related to steam out during their safety review by taking credit of administrative procedures. The reason behind this may be spending too much cost on re-rating is not good option.
  3. Some design uses water seals to handle this type of issue.
  4. Simultaneous collapse of steam for whole big system is some time seems to be unrealistic. However, for conservative analysis, this approach is generally not considered. Second thought can be given on the significant system volume and settle out calculation.

 

Hope this might be giving you some direction to move ahead. The question itself is very tricky and having both (Yes and No) the answers correct. Now the decision should be based on the refinery approach towards ultimate safety i.e. whether they are having good faith on their administrative procedures or not. Myself too appreciate some more views from some experts ..........



#3 ankur2061

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Posted 21 April 2014 - 01:51 AM

Aroon,

 

Appreciate your inputs and applying admministrative measures to prevent vaccum formation is a good way to avoid design for vacuum. However, if your unit is located in an area where availability of quality personnel and strict observance of administrative measures is somewhat questionable then I suppose it may not be possible to take credit for administrative procedures. In a nutshell if I am looking for an idiot-proof design, can I still ignore designing the CDU for vacuum.

 

I would be very cautious in a design where I am dealing with a client who cannot ensure that administrative measures will be rigorously and correctly implemented. I would certainly hesitate not to provide that bit of "extra" in the design which would prevent equipment failures even if the administrative measures fail partially.

 

I hope to have more views on this thread.

 

Regards,

Ankur.



#4 PingPong

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Posted 21 April 2014 - 03:36 AM

I am sure that any reputable engineering company will design a crude distillation column, plus sidestrippers, ovhd condensor(s) and overhead drum(s), for full vacuum. My company does it as long as I can remember. I am also sure that any reputable oil company would insist on a full vacuum design, if the engineering company would not do it themselves.

 

Not only because the CDU is steamed out during a refinery turnaround, but also because the columns use stripping steam during normal operation.

 

The whole point of applying mechanical design pressures (and temperatures) is that one does not have to rely on people to always act correctly over the whole lifetime of the plant. Using administative procedures to assure that equipment will never fail due to exceeding its pressure (or temperature) limitations may sound good on paper, but is rather naive.

 

Note also that sometimes companies decide to use a start-up ejector to evacuate the unit during each start-up so as to speed up the process of inerting the unit. Such client requirement can come up at the end of the engineering when mechanical design conditions cannot be changed anymore. Or client wants to add such start-up ejector during the first turnaround. 



#5 ankur2061

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Posted 21 April 2014 - 05:20 AM

PingPong,

 

Many thanks for your inputs. I would be particulary interested in knowing if companies like Shell, Conoco, Chevron, BP etc. insist that the atmospheric CDU and associated equipment (condensers, drums) etc. need to be specified for full vacuum. That will strenghten my argument in favor of specifying "FV" for the atmospheric CDU and the peripheral equipment.

 

Regards,

Ankur.


Edited by ankur2061, 21 April 2014 - 05:20 AM.


#6 aroon

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Posted 21 April 2014 - 07:21 PM

 

Appreciate your inputs and applying admministrative measures to prevent vaccum formation is a good way to avoid design for vacuum. However, if your unit is located in an area where availability of quality personnel and strict observance of administrative measures is somewhat questionable then I suppose it may not be possible to take credit for administrative procedures. In a nutshell if I am looking for an idiot-proof design, can I still ignore designing the CDU for vacuum.

In this case, I would say you must specify "FV".






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