Appendix F - definition of DLS

[sajk14]

Hello All,

Looking in API 650 12th Edition Addendum 1.....

Appendix F Clause F.4.2 shows a formula with the term 'DLS' which is defined as "nominal weight of the shell and any framing ( but not roof plates ) supported by the shell and roof".

I am not clear on what is meant by the term 'framing' supported by the shell and roof? Does this mean we need to allow for weight from things like intermediate stiffening rings, insulation bars etc on the shell?

Also what king of weight from 'framing' would beclassed as for the roof? does this mean the roof structure weight?

Thanks for any feedback

 

[IFRs]

The intent as I understand it is that DLS is the Dead Load of the Shell and includes the tank shell and fixed roof structure that is supported by the tank shell. It does not include the dead load of the fixed roof plates, which is DLR. It does not include dead load of fixed roof structure supported by the tank bottom, for example columns, girders spanning columns, etc. It includes the portion of the roof structure that is supported by the shell. In my opinion, given that the calculation is to determine the maximum internal pressure as limited by uplift at the base of the shell it would seem reasonable to include permanent structure that was permanently welded to the shell and you could take credit for substantive items like stiffening rings. Insulation bars are often bolted to clips welded to the shell and may not belong in this calculation.

 

[sajk14]

Thanks IFRs,

What's confusing is that in the same 12th Edition of API 650, in section 5.11.2.1 regarding the 3 uplift criteria formulas for Wind load on tanks....it states that MDL is the moment about the shell to bottom joint from the nominal weight of the shell only. I would have thought that it should also include the weight of any roof structure that is supported by the shell aswell.

In the previous 11th Edition of API 650, MDL did in fact included the weight of the roof structure aswell as the shell but this has been revised in the latest code?

Another point I thought might be worth mentioning is that the 3 uplift criterias all use nominal weights (i.e. ordered plate thicknesses )for the shell and roof plates. Surely if the tank has a corrosion allowance then this should be deducted from the plate thickness and the weight of the corroded shell and roof be used for uplift? otherwise you may have a tank built today that does not need anchor bolts, however in the future after losing pate thickness from corrosion it will require anchor bolts?

 

[fegenbush]

sajk14,

The issue of corrosion in an oil storage tank is not one of uniform thickness loss across a large plate area. Corrosion in tanks is more often of a localized, pitting nature. It should be noted that storage of corrosive products should take into consideration the large scale effects of corrosion.

 

[sajk14]

Thanks fengenbush,

So to sum up....

If the tank does NOT contain corrosive products then we can use nominal/uncorroded weights for uplift. but if the product in the tank IS corrosive then we should deduct the corrosion allowance from the nominal plate thickness, and hence use the corroded weight of the tank for uplift?