API 650 external pressure load for Appendix H tanks?
API 650 external pressure load for Appendix H tanks?
(OP)
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)?
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)?





RE: API 650 external pressure load for Appendix H tanks?
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
RE: API 650 external pressure load for Appendix H tanks?
RE: API 650 external pressure load for Appendix H tanks?
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.
RE: API 650 external pressure load for Appendix H tanks?
Joe Tank
RE: API 650 external pressure load for Appendix H tanks?
RE: API 650 external pressure load for Appendix H tanks?
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
RE: API 650 external pressure load for Appendix H tanks?
cheers,
gr2vessels
RE: API 650 external pressure load for Appendix H tanks?