6 replies to this topic

[Koffi]

Hi,

 

I need help with thermal out breathing calculation for two atmospheric tanks. I am doing a calculation to determine the thermal out breathing rates with a maximum ambient temperature fluctuation of 10 deg C per hour.

 

I have already determined the thermal rate from API 2000 based on the size of the tanks (4,000 m3 and 11,000 m3), however, the basis for the API thermal calculations is 40 deg C per hour as the spec says, 'The API thermal breathing requirements are based on an ambient temperature fluctuation of 40 ^oC per hour. In case the temperature fluctuations in a particular country/environment are significantly higher or lower then the design condition should be modified accordingly.'

 

How do I 'modify' my value to get the thermal rate for a maximum temperature fluctuation of 10 deg C per hour for my case, is it simply multiplying by a factor of 10/40 or is there more to it? Please include reference for this solution if any, thanks.

 

 

Regards,

Koffi.

[latexman]

I'm curious, what data do you have that suggests not to use API 2000's temperature fluctuation?  Are you looking at climate data over a 20, 50, or 100 year period for the "worst case"?  Or, is it limited data?  If the tank gets sucked in, "they" will come looking for the root cause whether it's next year or in 20 years.  Also, is this change really going to make a big difference in the whole scheme of things?  I mean, this change might mean a PVRV that is one pipe size smaller.  Is this critical enough to your project to not follow an established practice?  Just a couple things to think about.  An old anecdote comes to mind to me in this situation, "don't sweat the small stuff", especially when it moves you in the direction of a non-conservative safety decision.

 

If you have good, long term data that supports this, no problem.  I would make the change too, if it was really needed.  However, I have seen several, really bad decisions made on limited climate data, so I am sensitive to situations like this.

Edited by latexman, 20 April 2013 - 08:53 AM.

[Koffi]

Thanks Latexman,

 

I quite appreciate your comments and your concerns were same I voice out to the client. However, they are insistent on going along with it since their historical data justifies their confidence in the decision and it's all documented, so if it ever goes wrong I won't be blamed. In any case, it is easier for the tank to 'suck in' due to temperature drop than to 'vent out' due to temperature increase, even for the same amout of temperature change and in this case, our concern is really for 'out-breathing'. It is required to specify the flare gas recovery compressor, vent gas volume above the compressor capacity will be vented.

 

In any case, my primary question remains unanswered, how do I modify my calculated thermal out-breathing value for a 10 deg C change in temperature. The spec says to modify but gives no clue how to do so, this is my only concern and inquiry. Thanks in advance.

[ankur2061]

Koffi,

 

Where in API STD 2000, 6th Edition is it mentioned:

 

The API thermal breathing requirements are based on an ambient temperature fluctuation of 40 ^oC per hour. In case the temperature fluctuations in a particular country/environment are significantly higher or lower then the design condition should be modified accordingly.'

 

I have a copy and I don't see the statement that you quote. The whole basis for the thermal outbreathing equation as per section 4.3.2.3.2 is based on the geographical location of the tank, i.e. the Latitude of the tank.

 

Can you explain where your read this statement?  Maybe I have missed something.

 

Section A.3.3.4 of Annex A of API STD 2000 does have to say the following about thermal outbreathing:

Regards,

Ankur.

[Koffi]

Hello Ankur,

 

It is mentioned in Shell's Process Safeguarding Guidline, EP 2004-5042, for atmospheric tanks, page extract below highlighted in red.

 

As for API2000, I used the alternative method for out-breathing calculation using the tables. I didn't use the first method based on latitude.

 

Thanks

 

　

3.5. Processing and storage tanks

General

Floating roof tanks do not require any relief and/or venting systems, as those tanks should only be fed by stabilised liquids.  Emergency vents are sometimes installed to cater for process upsets, however, experience shows these provide only a minimal level of protection.

 

Overpressure protection and breathing requirements for low pressure, cone-roof tanks are adequately covered by API Std 2000.  Additional guidance is given below.

 

Low pressure vessels (< 1 bar g) are often treated as "tanks", in particular offshore.

 

Normal inbreathing and outbreathing

Normal inbreathing and outbreathing is determined by the summation of:

• maximum liquid movement into or out of the tank
• thermal breathing
• gas flashing

For storage tanks the maximum liquid movement is determined by the loading and unloading rates.  For processing tanks (e.g. free water knock-out or dehydration) the maximum liquid movement is determined by the level control system. Level fluctuations over a short period of time would normally be relatively small, compared to storage tanks. The fluctuations can also be limited during normal start-up and shut-down by the control system.

 

The API thermal breathing requirements are based on an ambient temperature fluctuation of 40 oC per hour. In case the temperature fluctuations in a particular country/environment are significantly higher or lower then the design condition should be modified accordingly.

 

In case a gas blanket system is installed, failure of that system should not be considered as part of the normal breathing requirements.

[Steve Hall]

Can't you just use the perfect gas law?

 

Thermal Outbreathing = {[(Volume of Vapor Space) (Initial T + T Increase + 273) / (Initial T + 273)] - (Volume of Vapor Space)} / time

Edited by Steve Hall, 22 April 2013 - 08:38 AM.

[Koffi]

Thanks Steve,

 

Yours is the kind of reply I was expecting.

 

I have recalculated my values using the perfect gas law like you suggested and got the same value for the gas volume as I got by multiplying by a factor of 10/40, I appreciate.

 

Thanks Latexman, Ankur, your inputs are appreciated.

 

 

Regards,

Koffi.