Calculating stresses in API 650 tank filled past shell/roof junction 3

[MFarris]

I have been asked to analyze the stresses caused by filling an API 650 storage tank above the roof to shell joint.

The tank(s) are mounted on concrete ring wall foundations but are NOT anchored to the foundation by use of anchor chairs & bolts.

Where would the max. stresses occur in this case, at the floor to shell joint or at the roof to shell joint? How would they be calculated, and what would be any indications of overstress other than a complete failure?

As far as I can tell, the code does not address this issue.

Yes, I know that you're not supposed to overfill a storage tank, but our operations people have done it in the past and there is no easy solution to address this problem instrumentation-wise.

Any help or suggestions would be greatly appreciated.

 

[JohnGP]

Maximum stresses will depend on the existing design of the various elements. But you can check how big the problem is by analysing the tank against API 650 Code requirements for the condition of the tank filled to overflow level.

The bottom course required thickness can be confirmed using section 3.6.3 or 3.6.4 of the Code with H set equal to the overflow level.

The Roof-to-Shell junction can be checked against Appendix F requirements with regard to an internal pressure equivalent to the liquid head above the junction (to overflow level).

Only if either check revealed that Code boundaries were exceeded would you need to lie awake at night worrying, and to consider doing a more detailed analysis.

Regards,
John

 

[JoeTank]

Depending on tank size and details...
Roof seam could fail
Top angle could fail
Roof to shell weld could fail
Upper shell could could buckle
Shell to bottom weld could fail
Bottom seam could fail


Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[Ashereng]

and there is no easy solution to address this problem instrumentation-wise.

Sorry, I can't help with the stress calcs.

I may be able to help with the instrumentation part.

Why can't instrumentation help prevent an overfill?

 

[JoeTank]

API-650 does not address the overfill case, but it does have procedures for checking the shell and the shell-to-roof joint. It doesn't address the check for the shell to bottom joint because uplift at the base of a tank is prohibited at this point. What you need is a Storage Tank Engineer to assist you on such matters. The knowledge needed will come from API-650, API-620 and basic tank engineering knowledge. Hope this helps.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JStephen]

You can analyze the allowable pressure in the roof and top angle by the methods in API-620. However, they don't address what happens to the floor-shell connection when the shell begins to uplift.

Some research along this line has been done in connection with frangible roof studies. Both API and WRC have books dealing with that topic.