37 replies to this topic

[Chapaco001]

Hi everybody !

I got a question on atmospheric storage tank venting requirements. As y'all know, venting is required whenever there's liquid movement inside the tank or sudden gas contraction in the vapor space caused by rainfall. The latter is referred to as "thermal inbreathing".

Now, our friends at the API described in their RP 2000 (about 10 years ago) a way to calculate the flowrate of thermal inbreathing. But being 10 years old, it was time to update this RP, and so they did. It's now Standard Practice 2000 (as new as November last year). The calculation for liquid inbreathing was also updated. But to my surprise the flowrates went up a whopping 400% (yes, four hundred percent). Of course, there's no explanation given or at least I could not spot it.

Does anybody of you have a clue what this increase is all about ? I mean, it can have a huge inpact on the design of control valves, nitrogen supply units etc.

Thank for your attention !

cheers, Chapaco .

[ankur2061]

Chapaco,

Well, at last we have a new version of API 2000 (after 11 years). I have yet to have a look at it. However, if what you are telling is correct, then we do have a problem on our hands because prior to the release of the new edition, no atmospheric tanks have been designed with what you have mentioned. And these tanks seem to be working fine.

I am curious to know, before I lay my hands on the new edition, if the calculations for 'Total Inbreathing' & 'Total Outbreathing are as per the old post I have posted on the 'Relief Devices Forum'. Here is the link:

http://www.cheresour...nk-vent-sizing/

Can you confirm if the new API 2000 matches with what I have mentioned in my post.

Regards,
Ankur.

[CMA010]

API 2000 has been a standard since at least 1992. The latest API 2000 / ISO 28300 incorporated calculation of the venting requirements according EN 14015. The old API 2000 calculations are part of informative appendix A of the latest API 2000 / ISO 28300.

There is a presentation of Groth available on the internet which shows the differences between the in- and outbreathing requirements according the API STD 2000 of 1998 and according the latest API 2000 / ISO 28300.

[Dacs]

I haven't had the chance to see the latest revision for API 2000, but if I remember correctly, there's a section in API that states the basis for the thermal inbreathing calculation.

If there's a change (400% is quite big), maybe there must be some changes in the basis.

[ankur2061]

Dear All,

Today I not only had a look at the normal venting requirements in API STD 2000/ISO 28300, but even prepared a spreadsheet for the same & yes as mentioned by Chapaco the 'thermal inbreathing' has increased by a whopping margin.

My spreadsheet example used a tank size of 3180 m3.

As per the old API 2000 (1998) the thermal inbreathing rate as per Table 2B for a tank of 3180 m3 volume reads as 536 Nm3/h of air.

As per the new API 2000, with the following new inputs of Latitude (< 42 deg), avg, storage temp. of 40 deg C, uninsulated tank & stored liquid considered as 'Hexane or similar', I get an inbreathing value of 1839 Nm3/h of air.

This is 3.43 times higher than the rate as given in the older API 2000.

The new API is based on original work done in Europe & published as EN 14015:2005.

The novel terms in the thermal inbreathing & outbreathing equations are the 'Latitude' of the tank location which is based on the distance from the equator (essentially in someway it includes the solar radiation/rainfall factor at the given latitude) & the credit or discredit of insulation for the tank. Annex E of the new standard tries to give some explanation but the equations are not explained & thus the lack of understanding.

This is an explosive new topic & very important as far as vent or PVRV sizing of atmospheric tanks. An importanrt question arises:

Were the earlier normal venting rate calculations wrong & are the earlier vent & PVRV's underisized, considering the phenomenal rise in thermal inbreathing rates?

Only the API STD 2000 commitee members can tell. Any learned member of the committee willing to share his/her thoughts??

Also time for inputs from our senior members, Art, Paul Ostand, Doug, Zauberberg, Harvey, Guido, Fallah & others.

Regards,
Ankur.

[Chapaco001]

Thanks for your responses !

At the time I wrote this post, I also sent an email to API asking for clarification on the subject. If/when I receive an answer from them, I'll make sure it is posted here.

CMA010: could you provide with a link to this presentation of Groth ? Might just have the background we need.

cheers, Chapaco

[sheiko]

CMA010: could you provide with a link to this presentation of Groth ? Might just have the background we need.

cheers, Chapaco

http://www.ori.milan...resentation.pdf

Edited by sheiko, 06 March 2010 - 01:52 PM.

[Dacs]

I'm quite worried on this new development.

We just recently finalized our N2 capacity for our project, in which a large chunk goes to N2 blanketing for tanks as large as 3000 m3.

I'm updating my N2 inbreathing calculations based on this new standard and I may have to present this issue to our clients, knowingly that this will impact our bottomline.

And it doesn't look good. One tank that I just recently recalculated as per ISO 28300 gives a whopping 220% increase in required inbreathing!

[ankur2061]

I'm quite worried on this new development.

We just recently finalized our N2 capacity for our project, in which a large chunk goes to N2 blanketing for tanks as large as 3000 m3.

I'm updating my N2 inbreathing calculations based on this new standard and I may have to present this issue to our clients, knowingly that this will impact our bottomline.

And it doesn't look good. One tank that I just recently recalculated as per ISO 28300 gives a whopping 220% increase in required inbreathing!

Dacs,

Considering a tank of 3000 m3, latitude as 37 deg (Seoul), storage tank temperature of <25 deg C & uninsulated tank, the calculated thermal inbreathing rate as per my calculations is 1086.5 Nm3/h of air. However if you provide an insulation of 1" (25 mm) of cellular glass (avg. thermal conductivity: 0.05 W/m-K) the recalculated thermal inbreathing with the insulation is 362.2 Nm3/h of air, which is more in line with what the older API std. gives. You now need to find out the economics between the extra nitrogen blanketing cost & the cost of providing insulation to the tank. I think the answer is obvious, providing insulation wins hands down.

Can I hear the insulation manufacturer's laughing all the way to their banks since the new API 2000 was published.

Regards,
Ankur.

Edited by ankur2061, 03 February 2010 - 12:11 PM.

[Dacs]

ankur2061: I haven't considered using insulation. Thank you for the insight, this is definitely cheaper than increasing the N2 capacity, both on capital and operating costs.

So I guess this warrants further investigation on the drastic changes that were implemented in this new edition of API 2000.

Anyone here can provide any major incidences of tank failure due to vacuum for the last decade or so?

[ankur2061]

Dear All,

To understand more about what 'Latitude' is in the new API 2000, we will have to revert back to our geography lessons.

Here are a few links to understand the relationship of solar radiation & rainfall with 'Latitude:


http://www.blueplane...org/climate.htm


http://www.worldbook.../about_climates


http://www.cnr.vt.ed...limate/cli.html

Hope this helps.

Regards,
Ankur.

[Chapaco001]

Hi all,

As promised, the official response of API on my query. As expected, they hide behind their policy and as such they will not explain anything.

"You have requested for me to - comment on the large increase in venting requirements for thermal inbreathing, for example explaining the rationale behind it.

By policy we do not provide comments on the rationale behind any change in the standard. The committee is a large group of technical experts in venting and pressure relieving and decided to make the revision.

Due to limited committee resources, API cannot respond to questions seeking the rationale for requirements in its standards. These requirements are based upon consideration of technical data and the judgment and skill of experienced engineering and technical personnel representing both users and manufacturers who serve on the standards-writing committees.

Thank you,

Stephen Crimaudo
Senior Associate, Standards
American Petroleum Institute
1220 L Street, NW
Washington, DC 20005-4070
Phone: 202-682-8151
Fax: 202-962-4797
crimaudos@api.org"

[JoeWong]

DACS,

I understood that your project has started prior to implementation of new edition of API 2000. Personally, based on code and standard implementation spirit, you may consider to implement the code & standard as of the time the project is started. However, you may consider to assess the impact of the new edition and provide necessary action and risk reduction action to minimize the likelihood to occur and consequences when it occur.

[JoeWong]

Ankur,

Wonder you would like to update the spreadsheet in

http://webwormcpt.bl...estimation.html

[ankur2061]

Joe,

Splendid idea. I will send it to you for your review. Any changes you feel necessary to improve the content of the spreadsheet or its get-up, please inform me & I will incorporate the same and send it back to you after the improvements for you to post it on your blog.

Regards,
Ankur

[Dacs]

DACS,

I understood that your project has started prior to implementation of new edition of API 2000. Personally, based on code and standard implementation spirit, you may consider to implement the code & standard as of the time the project is started. However, you may consider to assess the impact of the new edition and provide necessary action and risk reduction action to minimize the likelihood to occur and consequences when it occur.

I usually err on the side of being safe and on this case I'm very very tempted to implement the new edition of API for my project. However, we're quite far out of the project that by implementing this revisions will surely impact our cost and schedule

BTW, I got hold of the latest (6th) edition of API 2000 and it's quite identical to ISO 28300 as far as calculations are concerned. It's also noteworthy of mention that the previous calculation methodology has been attached (Annex A) in this edition of API 2000, as an alternative.

That said, I ran some initial numbers for inbreathing and outbreathing for all the tanks that I handle and surprising to say, the thermal outbreathing values have dropped quite a little bit (more or less 10%) from the previous edition. The opposite can be said for the thermal inbreathing (200-300% increase).

Me and my boss are wondering the reason behind the sudden increase in thermal inbreathing values that were generated by this new edition of API 2000.

As I said, I haven't heard of any cases of tank collapse due to vacuum for the last 10 years or so after the publishing of the previous edition of API 2000, so I'm still wondering for the motivation of this drastic change of methodology.

[ankur2061]

Ankur,

Wonder you would like to update the spreadsheet in

http://webwormcpt.bl...estimation.html

Joe,

My sincere thanks for taking the efforts to review, update & post the spreadsheet at your blog.

Forum Members,

The spreadsheet is available at:

http://webwormcpt.bl...estimation.html

Once again thanks to my friend Joe Wong. Any comments are welcome.

Regards,
Ankur.

Edited by ankur2061, 19 March 2010 - 02:54 AM.

[proinwv]

I have a question on the standard. Referring to page 12 in particular, I am not sure what the term "M-Relative molecular mass of the vapor" means. In particular the word "relative" is throwing me.

I am prepared to be embarrassed by the reply.

Thanks!

[ankur2061]

Paul,

I have never liked the way people try to complicate the english language by inventing new words, phrases & idioms. It shouldn't be a problem if it is related to a course in english literature but it definitely can be very annoying when used in plain old engineering. It appears that the API committee has now some members who specialize in introduction of uncommon words, phrases & terminology in the standards & recommended practices which lead to questions such as the one asked by you.

I was also perplexed with the meaning of relative molecualr mass until I stumbled upon this:

http://en.wikipedia..../Molecular_mass

Although the fact remains that the explanation given in the above link, doesn't do much to give clarity on your question.

Maybe you can find a simpler method to describe what is given in the link.

Regards,
Ankur

[proinwv]

Ankur, I have read the Wikipedia explanation and that is also confusing.

It says,"This is distinct from the relative molecular mass of a molecule, frequently referred to by the term molecular weight and abbreviated as MW, which is the ratio of the mass of that molecule to 1/12 of the mass of carbon 12 and is a dimensionless number. Relative molecular mass is abbreviated to Mr."

It seems that it first says it is the same as Molecular Weight (MW), then it says it is Mr. However, if we look back into the previous editions of API 2000, the same equation is used and there it simply explains that "M" is the molecular weight of the vapor.

Further, looking back I see that API 2000 6th ed Eq. (12) and the 5th ed, Eq. (1A) are the same and 5th ed. defines "M" as the molecular weight of the vapor. With this logic it seems safe to use M=molecular weight of the vapor.

How do you feel about this?

Anyone else out there agree with me?

[ogpprocessing]

Dear All,

I have one question regarding to thermaal inbreathing of stable (non-volatile) products such as demineralised water. According to API 2000 this standard is applicable to petroleum products but can ony be used for other products applying engineering judgements. The venting requirements evaluation is based on hexane product and as you know products like DM water are highly stable in comparison with hexane and so coefficients like 4 or 6.5 as recommended by API 2000 (2009 edition) can not be used for stable products.

So how to evaluate thermal inbreathing portion for non volatile products like DM water?

[ankur2061]

ogpprocessing,

Annex - A - Alternative calculation of Normal Venting Calculations of API 2000 (2009) provides clear guidleines for thermal inbreathing of any liquid. This is what is written for thermal inbreathing:

A.3.4.1.2 The requirement for venting capacity for thermal inbreathing for a given tank capacity for liquids of any flash point should be at least that shown in column 2 of Table A.3 or Table A.4.

For tanks having a volume less than 3 180 m3 (20 000 bbl), this calculation is based on the cooling of an empty tank initially at 48,9 °C (120 °F) at a maximum rate of temperature change of 56 K/h (100 °R/h) and is essentially equivalent to 0,169 Nm3 per cubic metre (1 SCFH per barrel) of empty tank volume.

For tanks having a volume greater than 3 180 m3 (20 000 bbl), this calculation is based on an estimated requirement of 0,577 Nm3/h per square metre (2 SCFH per square foot) of exposed surface area using typical tank sizes for those volumes.

The key term is "liquids of any flash point". Thus table A.3 can be used for your application. Tank volumes between the values given in table can be interpolated.

Hope this helps.

Regards,
Ankur.

[vivekag]

Hi,
Could you please provide me this presentation you mentioned. The link (ori.milan...) does not work. Thanks in anticipation.
Regards
Vivek

[GS81Process]

I'm quite worried on this new development.

We just recently finalized our N2 capacity for our project, in which a large chunk goes to N2 blanketing for tanks as large as 3000 m3.

I'm updating my N2 inbreathing calculations based on this new standard and I may have to present this issue to our clients, knowingly that this will impact our bottomline.

And it doesn't look good. One tank that I just recently recalculated as per ISO 28300 gives a whopping 220% increase in required inbreathing!

Pardon my ignorance as I am not a designer of atmospheric storage tanks... but is the N2 supply sized to accommodate the rated discharge pump flow plus the thermal inbreathing requirement?

Has anyone done a quick calc to check the API 2000 inbreathing requirement versus what one would expect based on the density change in a tank's vapour space due to atmospheric cooling? It would be interesting to see whether the API requirement is excessively "conservative" .

[ankur2061]

The API thermal inbreathing rate is itself based on atmospheric cooling due to a sudden rain shower that would lead to cooling and condensation of the vapors in the tank. The tank cooling assumptions are given in section E.5 of Annex E of API STD 2000.

Regards,
Ankur.