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Flare System Knockout Drum Mist Eliminator

knockout drum flare mist

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#1 TNCX34

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Posted 13 September 2017 - 06:08 AM

Hello

 

One of our client mention that  "Mist Extractor for knock out drum of the Flare system is not permitted as per API or International standards."

 

Do you have any information regarding to this subjected limitation also we are trying to find such limitation however till now there is no chance to understand this limitation or what we need to use instead of this system if the limitation issue is true?

 

Thanks in advance.



#2 Bobby Strain

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Posted 13 September 2017 - 02:42 PM

That is correct. No potential obstruction should be installed in an emergency relief system. So the vessel must be open, without internals that may obstruct flow. And, if this is a new knockout drum, be careful in sizing it. The procedure in the API recommended practice is not correct. And the API committee has not figured it out yet.

 

Bobby



#3 TNCX34

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Posted 14 September 2017 - 01:11 AM

Dear Bobby,

 

In the light of the given information including molar flow rates, I modeled it on software basis and and also check some documentation such API 12J, accordingly regarding diameter seems not problematic considering lenght appr. 6 times more than diameter! However as for internals, in DS provided there are several internals for that system such as, inlet diverter, mist eliminator and it is not given explanation on it but maybe one baffle, coalescer or foam breaker shown as just dashed line as one line I am not sure what is these internals. System is working such 2 phase diverter and gas is on going to flare and liquid is transfering to recycle tanks. At that moment what is your recommendation, not putting any internals to drum, and how can I provide reference about that issue because I need to give certain explanation and references, unless I am using internals? 

 

Thanks for your kind replies.



#4 Pan Nata

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Posted 14 September 2017 - 03:33 AM

TNCX34,

 

Refer to API 521 for KO Drum of Flare or Vent System.



#5 TNCX34

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Posted 15 September 2017 - 01:20 AM

Dear Pan,
 
I think that I checked the important parts of the documentation and I found one possible reference please look at that;
 
Although horizontal and vertical knockout drums are available in many designs, the differences are mainly in how the path of the vapor is directed. The various designs include the following:
 
a. A horizontal drum with the vapor entering one end of the vessel and exiting at the top of the opposite end (no internal baffling).
 
 
What do you think is it a reference of no internal parts?


#6 Bobby Strain

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Posted 15 September 2017 - 02:31 PM

API 12J is applicable to a process separator, not a flare system KO drum.

 

Bobby



#7 Pan Nata

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Posted 18 September 2017 - 01:03 AM

TNCX34,

 

Mist eliminator/extractor is different with baffle. You are talking about mist extractor, aren't you?



#8 TNCX34

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Posted 18 September 2017 - 03:42 AM

Pan,

 

Main problem is that what do I need to put inside of the vessel I am not sure actually. Client DS show that some internals inside of the vessel however definition is not given, just dashed line shown upper inlet shell to near bottom(horizontal vessel). But client also mentioned that first message of mine as internals do not permitted by API and other standarts. I need to understand what do I need to put inside certainly or nothing. Because this vessel will be ASME U-Stamped project and very strict rules to be followed in next schedule and performing changes will be very difficult at that period. I need to clear all the things as internals that is the case. Main concerns that whether there will be any mist eliminator or baffles or another parts which can be determined as internals? Hopefully you can assist me in this regards. Thanks.



#9 Pan Nata

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Posted 19 September 2017 - 12:56 AM

TNCX34,

 

Could you please upload any drawing of your KO Drum (General Arrangement is better)? to make it clear what exactly internal in your Drum. I've ever designed and commissioned Flare KO Drum as per API 521 and stamped with ASME.



#10 chem saurabh

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Posted 13 April 2018 - 06:24 AM

From your description it seems like a coalescer

#11 Amit J

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Posted 14 April 2018 - 12:00 AM

There shall not be any internals in Flare Knock out drum. this will give more conservative scenario design of flare KOD.

 

Philosophy behind this is that...there shall be minimum pressure drop in flare system being it related to plant safety.

 

Also if pressure drop in flare system increases, it will add to backpressure on upstream pressure relieving devices. This may cross mach no. criteria in some section of flare system thereby jeopardising the plant safety or on the other hand will increase in size of flare header & associated system thus causing CAPEX rise.



#12 Technical Bard

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Posted 15 April 2018 - 10:15 AM

A flare knockout drum should be empty, with the exception of an inlet flow diverter (which is allowed) to reduce splashing.  But there should be no weirs, baffles or mist elimination devices that could restrict the flow.  The vessel should be sizes for a small liquid droplet size for disengagement (300 micron or less).  



#13 Pilesar

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Posted 16 April 2018 - 08:57 AM

When I first read your question, I interpreted 'knockout drum of the flare system' as describing a knockout drum that collects from the entire relief header just prior to the flare itself. I think you are asking about a 'knockout drum that relieves to a flare system.' If the vessel is a knockout drum that relieves to the flare system and it needs a mist eliminator for the outlet vapor, you can often use another vapor nozzle for the relief valve as long as the mist eliminator is not in the relief path. For the coalescer you describe, the mist eliminator usually protects just the primary vapor outlet and there is another vapor nozzles that should be used for the relief valve.



#14 flarenuf

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Posted 29 May 2018 - 04:58 AM

hi

Just to clarify/confirm some points here
 

as Bobby and Technical Bard have said 

"No potential obstruction should be installed in an emergency relief system. So the vessel must be open, without internals ( splash plate accepted) that may obstruct flow."  no deviation from this is allowed.

AMJIT   the omission of internals has nothing to do with "conservative design" it is purely a safety restriction, you do not want anything coming loose in the KO Drum and blocking the outlet to flare.
High pressure drop is allowed in a Flare Header system , in fact utilisation of this allows for smaller  pipe diameters with associated reduction in weight and cost . Obviously the headers have to be sized such that the dP is such that backpressures on source valves are not compromised.

 

flarenuf
 






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