API 650 external pressure load for Appendix H tanks? 1

[IFRs]

API 650 11th edition includes an external pressure load of 1 inch of water. Appendix H calls for circulation vents whose open area is 2 square feet per 10 feet of tank diameter.

For designing supported cone roofs: The minimum live load is 20 psf. When this is added to 40% of the minimum external pressure (per Appendix R), the total (22.1) comes closer to the 25 psf in earlier editions. Question: is there any provision to eliminate the external pressure load when the tank is ventilated in accordance with Appendix H (and external pressure is not possible)?

 

[gr2vessels]

IFRs,
The vacuum design of the naturally vented tanks is not mandatory, as it is non-mandatory for the blanketted tanks also, providing the flow rate of blanket gas is minimum equal to the vacuum growth rate. Assuming a minimum venting per Appx H, you don't have to include the vacuum loading in the design of the roof, ie you can set as design condition Pe=0.
cheers,
gr2vessels

 

[IFRs]

gr2vessels - while that makes sense, where is it said so in API 650?

 

[JStephen]

IFRs, after looking at these sections, it looks like you are interpreting the standard correctly.

See also 5.2.5, which says a tank is good for 1" vacuum "without the need to provide any additional supporting calculations". I would be curious as to the intent of this line in the current standard. Does this mean you don't need to include vacuum in your roof design, for example? An inquiry to API might be in order.

 

[JoeTank]

I do not believe that API-650 requires any vacuum. It is my understanding that the 1 oz vacuum is a permissible vacuum (w/o additional calc) should one need to have a slight vacuum incorporated into the design. It would not make sense to automatically include it for a tank with open slts in shell. Isn't the 20psf the design minimum for the roof design under all instances. Agree w/ Mr Stephen as to an inquiry.

Joe Tank

 

[IFRs]

Somehow the question became one of vacuum but API calls for a Pe External Pressure load. While this is similar to internal vacuum, they do not call it such. The section I am referring to is 5.2.1.b which covers external pressure. Then in 5.10.2.1 API says to use Appendix R for tank roof load combinations except (d). Appendix R says to combine Live Snow and External Pressure in various combinations. I think when the roof design load was initially reduced from 25 to 20 part of the rational was that in adding 5.2 psf external pressure the roofs would come out the same so the impact on the industry would be nil but API would be closer to ANSI type loading. Appendix R uses only 40% of the pressure so this is a perhaps somewhat misleading thought. I thought that the agenda item 650-472 included some language that stated the obvious - a tank with large holes can't support vacuum or pressure so there was justification for reducing the roof design load to just 20. I could not find such language in the 11th edition and was wondering if anyone else had.

 

[JoeTank]

IFRs,
There is also an internal pressure provision in the load combinations, but no requirement to design for one. I agree that one could not suck hard enough on a tank with circulation vents to impose a partial vacuum condition. The redeuction from 25 psf was jsut a receognition that many local coades allow lower design loads. You can't go much lower with out ending up with a flimsy mess, especially for an industrial facility that allows access to working folk. The 40% factor comes from common practrice of setting a design pressure above the operating pressure to account for the over-vacuum needed to fully open a weighted pallet style valve. If it varies from that the owner must so state in the design spec.
Joe

Joe Tank

 

[gr2vessels]

Just a quick addition, the Pe is defined as vacuum condition in the Appendix V, where significant vacuum/external pressure (1 psi) is considered. Perhaps, in the view of Appx. V, those formulas containing the Pe can set Pe=0, as applicable. I don't have the "Tank" software, but it would be instructive to know if that Pe can be set =0 or not..
cheers,
gr2vessels

 

[jaydees]

Addendum 1 (which should be released this month) addresses this situation. Appendix H tanks with circulation vents can be designed with Pe = 0. However, all other closed top tanks must be designed with a minimum Pe = 1 inch of water.