API 650 F & S Roof Design Stress with the Design Temp. exceeding 250

[rab1944]

Any suggestion for choosing an allowable stress for an API 650 F and S tank with a design temp. in excess of 250 degrees.

The umbrella roof needs to be designed per API 620 but it tops out at 250 degrees F. and my design temp. exceed this by over 100 degrees F.

 

[gr2vessels]

rab,
Is the design temperature your major problem? Did you check out the API 650's Clause 5.10 and 5.10.3? Also, the temperature and material relevant Appx. M and S (refer excerpt Appx M below)?

"M.3.2 For a maximum design temperature exceeding 93°C (200°F), the allowable stress specified in 5.6.2 shall be modified as follows: The allowable stress shall be two-thirds the minimum specified yield strength of the material multiplied by the applicable reduction factor given in Table M-1 or the value given in Table 5-2 for product design stress, whichever is less."


Also, please refer to the Appendix A, the exerpt below:

"SECTION 5.1.1 SCOPE OF RULES
620-I-05/02
Background: API Standard 620, Tenth Edition, Section 1.2.1, states, “This standard does not cover design procedures for tanks that have walls shaped in such a way that the walls cannot be generated in their entirety by the rotation of a suitable contour around a single vertical axis or revolution.” This statement would appear to indicate that there are no design procedures presented in this standard for umbrella roofs, eccentric cone roofs and bottoms, and bottoms sloped to one side. For example, Sections 1.1, 1.2.5, 5.1.1, and 5.1.5, among others, indicate that subject to stated restrictions, designs that will be as safe as those provided by the standard may be used. This inquiry seeks to confirm this interpretation. I am trying to clarify Inquiry 620-I-10/00.
Question: Are umbrella roofs, eccentric cone roofs and bottoms, and bottoms sloped to one side, among those designs for which API 620 does not present design rules as noted in Section 5.1.1?
Reply: Yes."

Hope it helps,

Cheers,
gr2vessels

 

[rab1944]

What I am seeing as the challenge is that I have an internal pressure that puts me into Appendix F and a design temperature in excess of 350 degrees F. API 650 F.7.3 sends you to API 620 for the roof design and API 620 has a limit of 250 degrees for the design temperature.

I don't see where I can ignore the API 620 temperature limits. In others parts of Appendix F, such as F.7.2 it specifically takes exception to 620's allowable compressive stress but I don't see where we are allowed to take exception to the temperature requirements of 620.

Thank you for the comments and please let me know if I am missing the forest for the trees.

 

[JoeTank]

It was not the intent of API that the design temperature be limited by the API-620 rules. API-650 would permit the temperature limit of 500F for carbon steel materials. Perhaps you should submit an inquiry to API on this matter.

Joe Tank

 

[rab1944]

The tank is stainless, Apx. S. Would that make a difference.
Thanks for the comment.

 

[JStephen]

From a practical standpoint, you should be able to apply the reduction factors from App. S and M to an API-620 tank. You may or may not then be complying completely with the letter of the code, and if not clear, that would be worth an inquiry to API.

One of the previous interpretations to API-650 asked about whether you could pressurize an umbrella roof (as distinct from a dome roof) and the answer was "no". Not so much that it can't be done, but that the axisymmetric design rules in API-620 don't apply in that case.