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Nitrogen Requirement Calculation


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#1 malikch06

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Posted 19 April 2011 - 12:15 AM

dear engineers!
i have to calculate the nitrogen blanketing requirement for highly pressurized vessels storing hydrocarbons. i have searched in api 2000, & 521, but didn't find the exact matter,,,, any one having knowledge of this is requested to please share his thoughts..

Regards
Engineer Sohail Malik

Edited by malikch06, 19 April 2011 - 12:25 AM.


#2 riven

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Posted 19 April 2011 - 02:11 AM

Get this article

Artikel:
Author: GEORGE R. KINSLEY.JR
Title: Properly purge and inert storage vessels
Journal: CEP CHEMICAL ENGINEERING PROGRESS
Year:2001 Vol. 97 Nr. 2 Pag. 57-61

#3 breizh

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Posted 19 April 2011 - 02:41 AM

hope it helps
Breizh

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#4 proinwv

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Posted 19 April 2011 - 07:32 AM

Malik, I do not understand why you cannot find what you require in API 2000.

API 2000 will establish your required flow for inbreathing and outbreathing and emergency conditions. Using that information, along with your operating cycle, and your experience to add a safety factor (if needed) should give you the information you require.

You might look on my website for other information. www.ostand.com

Paul

#5 malikch06

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Posted 21 April 2011 - 12:54 AM

dear sir....
first of all, i wud like to thanx u for replying,
sir actually,, i am not facing any difficulty in using api 2000, actually my problem is that we are sizing nitrogen system for highly pressurized vessels (more than 250 psig in some cases), while api 2000 gives the standards regarding atmospheric pressure and slightly above, storage tanks,, please explain this ambiguity.. and also the differnence in between blanketing & padding,
Regards
Engineer Malik



#6 proinwv

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Posted 21 April 2011 - 06:37 AM

Malik

I cannot necessarily explain why the writers of the standard did what they did, but I do know from experience that most tanks that are blanketed are done so at very low pressure. Blanketing is normally done to prevent ingesting atmospheric air, which would not be a normal condition in your case.

Blanketing, padding, inerting are all terms used for the same process.

Where in the standard do you feel that it deviates from what you need to find out?

#7 malikch06

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Posted 21 April 2011 - 07:44 AM

sir,
actually in refinery practises the vapor pressure do not exceed too much above atmospheric pressure (at ambient temperature),,
((and one more thing in addition to your mentioned purpose for blanketing is to avoid formation of vapors))
but in our case vapor pressure is far above atmospheric pressure,, thats why we intend to apply pressurized nitrogen (in the range of 250psig) in order to avoid vaporization...., and here comes the problem that our api's standard tell only about atmospheric storage tank.............., r u getting my problem??, or may i xplain my conditions more specifically.
waiting for ur reply
Engineer Sohail Malik




#8 proinwv

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Posted 21 April 2011 - 07:51 AM

Of course you are correct. The title of the standard tells us that. Sorry for my missing the obvious!

To know what the writers wanted to address, I suggest that you contact the API and see if you can talk/write to the committee chair or others on the committee.

I would also contact any company that may have sat on the meetings for this. I do know that Protego in Germany were active on the committee.

I hope that this helps. We would hope that you will be able to share any information that you might find on this.

Good luck

Paul

#9 Art Montemayor

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Posted 21 April 2011 - 09:11 AM


Engineer Sohail Malik:

I hope that you are familiar with basic chemistry and physics and can relate to Dalton’s Law of Partial Pressures: the total pressure exerted by an ideal gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. This is expressed in an equation as follows

Ptotal = P1 + P2

Where,

Ptotal = the total pressure exerted by the system, psia
p1 = the partial pressure of the first component, psia
p2 = the partial pressure of the second component, psia.

In your presumed case, the first component is nitrogen and the second component is your stored hydrocarbon liquid.

However, it is important to note that Dalton's law is not exactly followed by real gases or vapors. The deviations from Dalton’s Law encountered with mixtures of real gases are considerably large at high pressures. Under those conditions, the volume occupied by the molecules can become significantly larger compared to the free space between them. This effect will substantially change the pressure exerted by both of the real gas components in actual practice.

Although you do not describe the specific application you are contemplating, I believe that you basically are trying to apply an inert, static “blanket” of nitrogen gas on top of a high pressure, liquid hydrocarbon stored in a pressure vessel. If that is the case, then the George R. Kinsley article furnished is not applicable to your operation.

I have applied nitrogen blanketing to pressure vessels containing LPG, liquid Butane, and liquid Ethane. If you have LPG, the mixture of the various hydrocarbons makes the estimated pressure results very complex and difficult.

I would advise you of the following facts:

No matter how much nitrogen you inject into your pressurized tank, you will always have to start with the fact that the stored liquid will contribute its physical vapor pressure at the storage temperature. You cannot avoid or fail to take that into consideration. The added nitrogen will add pressure OVER AND ABOVE THAT of the normal, storage vapor pressure of the liquid inside the tank. The more nitrogen you inject into the tank (because you might desire a relatively high N2 concentration), the higher the resulting internal tank pressure you must sustain.

That is why you should furnish us with detailed, specific basic data and information as to what you are proposing to do. If you have a SATURATED liquid stored in the tank and want to inert it with nitrogen, then you will inherit a problem of requiring a much higher-pressure vessel rating. I suspect, because of the high pressure storage pressure, that you are dealing with a saturated hydrocarbon liquid. If that is truly the case, then your basic storage pressure is that of a saturated liquid and it represents the liquid's vapor pressure. Adding nitrogen to it will increase the tank’s pressure and require a higher pressure vessel rating.

You have not explained why you desire to inject nitrogen and I recommend you to submit the detailed information requested above in order to reply specifically to your request.


#10 malikch06

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Posted 22 April 2011 - 01:23 AM

Dear Montemayor,
first of all i wud like to discuss about dalton's law of partial pressure,
sir,, i'v fair idea of it's application & implications ,, and according to my knowledge, the terms in dalton's eqn. are the individual pressures of gases present in the vessel
(not liquid's vapor pressure, if we assume zero vaporization for time being; as the primary purpose of nitrogen blanketing is to avoid vaporization)....
please explain if i am wrng,
now coming back to my problem,,, i wud like to elaborate it further,,
actually we r having storage vessels for "hydrocarbons",of which the normal boiling point ranges within (0 - 10) o C,, and our ambient temperature ranges from (25 - 32) oC, we want to keep that fluid in liquid state,, and in order to keep them liquid we must increase the vessel's pressure to counter their vapor pressure. as boiling temperature will increase then and stops the liquid from vaporizing
(by applying N2 gas, in our case),, one more purpose behind applying (nitrogen) pressure is to use this pressure as pumping agent, as otherwise our fluid can't stay liquid.......,.
and here comes the problem that api standards(2000) only tell about blanketing at near atmospheric conditions....., whatif some one wants to blanket at high pressures,
please assist me if u can,, thanx 4 ur correspondence.

Regards
Engineer Sohail Malik


#11 Art Montemayor

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Posted 22 April 2011 - 08:32 AM




Engineer Sohail Malik:

I am glad you are familiar with Dalton’s Law. Then you should go back and review the Law and you will find that you are wrong in believing that the vapor pressure of liquids does not follow its concept.

Everything has a vapor pressure - especially liquids. And you make a basic mistake in presuming “zero vaporization for time being”. You confirm your mistaken idea by stating that you need the nitrogen in order to “increase the vessel's pressure to counter their vapor pressure. as boiling temperature will increase then and stops the liquid from vaporizing”. This proves that you can’t just assume that your liquid has no vapor pressure – it has a very HIGH vapor pressure, and it is what sets your high tank pressure requirement if you want to preserve it as a liquid. This is very basic knowledge and it is part of phase equilibria taught the first semester of Chemical Engineering courses in university. You should accept the facts that I have explained – not because I say them (or believe them), but because they are basic natural laws that apply to what you are proposing to do. My principal intent here is to help you out and avoid having you create an over-pressurized hazard without being prepared to safely mitigate it.

By not responding to my request and furnishing us your basic data (such as the pressures, temperatures, identity of the fluids, size of storage vessel, scope of work, specifications for the pressure vessel, etc., etc.) you are preventing us from being specific and accurate in trying to help you.

What you basically propose to do is not “blanketing”. The term blanketing is applied to super-cooled liquids (liquids that are cooled below their saturated temperature at the designated pressure. You – by your own description – are trying to inject nitrogen into a storage tank and maintain the contents as a saturated liquid by increasing the tank’s pressure. Depending on the identity of the contents, you may not be able to do it. The injected nitrogen will perhaps go into solution with the liquid and form a binary, teriary, or higher mixture of compounds and the result will be a mixture with totally different thermophysical properties. It may – or may not – condense and exist as a liquid when pressurized with nitrogen.

You should stop trying to apply API 2000. It is a Standard designed for a totally different application and does not apply in your case. I am stating this as an engineer with over 51 years of field experience and am only trying to help you in this proposal. I hope you heed my advice because the chance of a hazard being created exists.

Good Luck.


#12 fallah

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Posted 23 April 2011 - 01:30 AM

The term blanketing is applied to super-cooled liquids (liquids that are cooled below their saturated temperature at the designated pressure.


Or sub-cooled?

Edited by fallah, 23 April 2011 - 01:32 AM.





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