12 replies to this topic

[aju_1807]

Hello,

 

This is with reference to latest edition of API-2000 (7^th edition), as a user I have a query on basis of inbreathing calculation to be used for tank. As covered in API2000 section 3.3.2.3.2, the thermal inbreathing of a tank is function of tank volume and latitude at which tank is located. The basis of the equation VIT = C *Vtk ^0.7 * Ri is detailed out in Annexure G.3.

 

As per Annexure G.3, the equation for general method has been worked out based on following assumptions :

 

• Tank sizes                                :           Any sizes
• Initial Temperature                    :           55 deg C
• Rainwater Temperature             :           15 degC
• Free Convection in tank            :           4 W/m2K
• Maximum rate of temperature   :           Calculated – Considering worst case rain intensity of 225 kg/m2h

 

 

Experiment was conducted based on above conditions and the equation in section 3.3.2.3.2 is derived based on above worst boundary limits. However, as per Figure G.4, it provides rate of temperature change for a given tank with respect to different rain intensity.

 

Query/Proposal for section 3.3.2.3.2 modification :

 

As per API2000 section 3.3.2.3.2, the thermal inbreathing is calculated based on worst case rain intensity of 225 kg/m2h. However, the thermal inbreathing rate needs to be calculated based on worst rain intensity experienced for the specific geographical location. The Figure G.4 to be used for determine the maximum rate of temperature with respect to tank volume and rain intensity. The same rate of temperature to be used to calculate the additional moles of air required when the tank vapor space contracts from initial temperature of 55 degC to final temperature determined based on Figure G.3 (considering maximum rate of temperature).

 

Above exercise will provide actual thermal inbreathing flow rate to be considered for tank with respect to rain intensity of that particular geographical location. E.g. tank located in Sahara desert and in Thailand with same dimensions will have different thermal inbreathing requirements whereas as per API2000 sction 3.3.2.3.2 this is function of tank volume and latitude only (tank on same latitude may have different rain intensity).

 

Suggestion :

 

The user should be allowed to use Fig G.4 for performing thermal inbreathing requirement instead of calculating it based on API2000 section 3.3.2.3.2, since this will also take rain intensity into consideration which is presently calculated based on default rain intensity of 225 kg/m2h.

 

Appreciate feedback from experts here!

 

 

Regards

AJ

 

 

[breizh]

Hi,

Did you check the G5 annex with sensitivity study ?

Hope this going to help you .

Breizh 

[aju_1807]

Hi Breizh, 

 

Yes i checked and my question is in reference to that only. If sensitivity is performed using different rain intensity then API2000 section 3.3.2.3.2 providing thermal inbreathing should not be based on one equation whcih is actually based on rain intensity of 225 kg/m2h (by default) which will not be applicable for country like Algeria, UAE etc where there is hardly any rainfall!

Edited by aju_1807, 03 August 2021 - 04:51 AM.

[breizh]

Hi,

Are you sure about the rain fall in those countries ?  Do you have data to support ?

 

Good luck

Breizh 

[Pilesar]

Either you conform to an API standard or you do not. What is stopping you from calculating venting requirements according to sound engineering practice? If you size a vent using your engineering judgment with less capacity than under the API procedure, then do not claim it conforms to the API standard. 'Adequate tank venting' and 'conforming to API standard' are not necessarily the same thing. The API standard contains general, simple rules that may not apply to all locations and conditions.

[aju_1807]

Hi,

Are you sure about the rain fall in those countries ?  Do you have data to support ?

 

Good luck

Breizh 

 

Rainfall data for those countries are available and its as low as 30 kg/m2 hr which is too low compared to original assumptions of 225 kg/m2h !

[latexman]

I lived in Dubai, UAE in 1991-1992.  Yes, I saw it rain only once during that two year period, but that one time it rained very, very hard and flooded all the streets.  I'll keep using the API 2000 equation.

 

Is your data averaged or is it for peak events over history?  Something seems off.

[aju_1807]

Either you conform to an API standard or you do not. What is stopping you from calculating venting requirements according to sound engineering practice? If you size a vent using your engineering judgment with less capacity than under the API procedure, then do not claim it conforms to the API standard. 'Adequate tank venting' and 'conforming to API standard' are not necessarily the same thing. The API standard contains general, simple rules that may not apply to all locations and conditions.

 

I do agree and here we are complying with what is quoted in API. Only difference is different rain intensity. The base equation in report is based on higher rain intensity which is a defauly equation which we all use. Only thing i am trying to explore is why to impose that on country where we dont have a rainfaull. For sure, we can do our own engg calculation which is in indirect compliance with code. However, sometime client prefer to stick with API2000.

 

Anyway thanks for your response.

 

Anyway thanks for your response.

[aju_1807]

I lived in Dubai, UAE in 1991-1992.  Yes, I saw it rain only once during that two year period, but that one time it rained very, very hard and flooded all the streets.  I'll keep using the API 2000 equation.

 

Absoulety right, it was a case in year 2007-2008 as well. However, whatever the worst rainfaill has been recoreded will be a starting point for such calculation and not average rainfall which is expected to be negligible. Even if we consider worst recorded rainfall in UAE, it will far too off from rain intensity of 225 kg/m2h (which is equivalent to alomost 9 in/hr) !!

 

Just have a look at Fig G.4 of API2000 and see the difference rain intensity makes for small capacity tanks (in particular)!

[latexman]

I think 99.99% of Engineers just use the equation.  It's been like this for decades.  I haven't heard of any incidents where higher than average rainfalls have caused tank failures.  Do you have any documented cases?

[aju_1807]

I think 99.99% of Engineers just use the equation.  It's been like this for decades.  I haven't heard of any incidents where higher than average rainfalls have caused tank failures.  Do you have any documented cases?

 I agree, no one challenges this and in fact we dont even see the assumptions involved in deriving this equation. I was curious to know teh experience because in my case consideration of rain intensity for smaller tanks play a huge role in determining PVRV size and numbers.

 

We have not used rain intensity either but since the DT curve for different rain intensity is available in API2000 itself, thougjht of checking with experts on their experience of using Fig G.4 for thermal inbreathing rate.

 

Anyway thanks for your response. I will try sending this to API commitee to see their views on this.

[latexman]

Thank you for this interesting topic!  I would love to see/hear about the feedback you get from API.  I suspect other folks here would too.  Please share their follow-up with us when you get it.  I would especially like to hear about documented API investigations into incidents where the root cause was determined to be local weather/rainfall deviations from average.  To me, this might be the proof we need to be concerned about local weather deviations from average.  I understand your proposal is more technically correct than using an average equation for everywhere, but do the deviations rise to the level of a real world problem given the safety factors built into the API Code, or not?

[Pilesar]

Additional background about API 2000... there were significant changes made in 2009 when the 6th edition was published. The new calc methods resulted in much larger required flow capacities for inbreathing. You are not alone when you express surprise at the required design flows calculated. To be fair to API, tanks do not very frequently rupture under positive pressure. Implosion under vacuum is more typical. If you are required to certify that the tank system is designed according to API 2000, then you have to do the calcs according to the API standard. I would not hold my breath waiting for a useful answer from the API committee that would reference anything other than what they already wrote in the standard.