Compression Ring for API-650 tanks

[MikeG7]

Concerning requirements for a compression ring for a tank with a small internal pressure, is the compression ring mandatory? All the figures of F-2 show a ring. My understanding (please confirm it) is that if "participating area" of roof and shell is sufficient, ring is not required.

2nd part of the question: is the internal pressure acting on the inside surface of the roof the cause of the compression?

Thanks
Mike

 

[JStephen]

I agree with your first item, that an additional piece need not be added where the area is available in the shell and roof. I think this would apply to Appendix F tanks only, that non-Appendix F tanks will require a top angle. Note that in Detail "i", the compression ring could be the same thickness as the roof.
And second question, yes.

 

[MikeG7]

JStephen, thanks. Does not seem logical to me that App.F tank may not require it if shown by calculation that it is not needed, but non-Appendix F tank must have it? Please clarify further.
Thanks

 

[JStephen]

I think it's just an inconsistency in the standard. If you have ideas on how to resolve that, forward them on to the committee.

 

[MikeG7]

JStephen, would you say that it is "common practice" to include the the top angle (or similar section) than to omit it? If it was not omitted on a vessel would that be generally against common practice?

Then just one aside question, is the top angle considered a top wind girder also? Are they one and the same or considered independently?

Thanks