Tank Overturning/Stability

[sajk14]

Hello all,

With regards to the 3 uplift criteria in section 5.11.2.1 ( API 650 12th Edition Addendum 2 ).

For the value of MDL is it acceptable to include the weight of the shell insulation if the tank is lagged?
Similarly can we also include the weight of any roof insulation in the value for MDLR?

Thanks in advance for your thoughts.

 

[JStephen]

If the wording in API-650 isn't clear enough to tell, I'd consider it at designer's option. That said, I'd personally omit it, figuring the insulation might never get put on, or might be removed at some point in the future, etc.

 

[IFRs]

Might be a good technical inquiry?

http://rfi.api.org/Index.aspx

 

[JStephen]

Two other things. One, you'd have to assume that insulation was designed for the same wind speed as the tank- if it blows off the tank, it's not there any more! Secondly, more often than not, when I'm doing the tank calcs, I don't know the insulation weight yet. I can plug a conservative number in for roof design, though.

 

[IFRs]

I typically don't include insulation - I don't consider it dead load. I don't include stairways, railings, nozzles, manways, etc either. Squeaking by to save anchors or thickened annular plate, etc seems silly when the owner is investing large dollars in a major piece of capital equipment that is expected to outlast the terminal manager. If you are THAT close, suck it up and make the tank stable by itself.

My humblest of opinions only...

 

[sajk14]

Thanks JS & IFRs...you both make some good points as usual.

I also prefer to take the more conservative approach and have the tank stable on its own merit and not be 'clutching at straws' so to speak...however i will send in a Tech Enquiry to API just out of interest...(thanks for the link to the enquiry form IFRs).

I will let you know of any outcome from API. How long do they typically take to respond?

 

[JAS]

Insulation weight shall not be considered for both MDL & MDLR because it is the "Moment of the Joints" due to "Nominal weights of Shell & Roof". You can consider the extra weight, if there are permanent attachments acting on the Shell and Roof, for instance Platforms/Stiffeners/cleats, but not Insulation.
Any dead & removable weights like Insulation actually lowers the chance of Anchoring and its sizes/quantity.

 

[sajk14]

Another area of debate is in Annex F where the term DLR (which is applied as a force not a moment) is used for calculating max design pressure, required compression area and calculated failure pressure.

Again if the roof is insulated could this additional weight be included in the relevant Annex F formulas?

 

[JStephen]

I would assume the tank was pressure-tested prior to being insulated, so no to that one.
On the technical inquiries, kind of a mixed record. Figure a few months, never quick enough to help with a particular tank, it's always for the next one. I've had one request rejected due to "not approving specific methods" or whatever, when in my judgment, it didn't fall in that category at all. I've had a couple of inquiries that apparently just disappeared- no response ever, on what seemed to me a perfectly reasonable question.

 

[IFRs]

Questions that you think deserve an audience after failure to get a response can be asked in person (SCAST meets 2x each year), or brought to a committee member who can ask it for you if they feel it has merit.

 

[JStephen]

I noticed just now that in API-620, dead load is specifically defined to include insulation unless otherwise noted.

 

[sajk14]

Hmmmm interesting JStephen....to save me some time what section or clause number in API 620 is this referenced?

It will be interesting to see what API come back with regarding my technical query under API 650 if it is actually allowed in API 620.

Will keep you posted on this either way!

 

[JStephen]

That's in 5.4.1(a), definition of dead load.

 

[Mech2325]

sajk14

The definition of MDLR as per API 650 "is the moment about the shell-to-bottom joint from the nominal weight of the roof plate plus any attached structural" states to use the nominal weight and any attached structure. As a general rule, what i think, whenever you are designing for wind, do design it for the most stringent condition, which is having the least weight. Whereas, when you are designing it for seismic, do consider the maximum weight in your calculations since, for seismic, this is stringent one.

Having said that, i would suggest you to "not consider" the weight of insulation in your calculations for anchorage check against wind loading. But, do include all the structures since they would increase the projected area of the wind.

Also, when you check the anchorage against seismic loading (Appendix E), do consider the weight of the attached insulation.

Regards,
Mech2325

 

[sajk14]

Thanks mech2325 for your comments and input on the subject.

I will let you all know what answer API reply back with following the tech enquiry sent.