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API-620 Knuckle Radius

API-620 Knuckle Radius

API-620 Knuckle Radius

Looking for help on how to reinforce a knuckle on a F&D head:API-620 tank

I have a 156" OD tank with a F&D bottom head and legs. The knuckle radius is 4" which is less that the 6% required per 5.12.3 of the API-620 code book.(6% of 156" is about 9.5")  Is there any way to get around this 6% rule by adding a ring? Or is there a way of proving that the 4" would be adequate?

Any suggestions would be greatly appreciated.


RE: API-620 Knuckle Radius

There is no way around the 6% knuckle rule of API-620.

If this were a non-API-620 tank, one could calculate the T1 and T2 membrane components at various locations on the knuckle and use the average as a means of estimating the hoop compression in the knuckle region.  With just a 4% knuckle, I suspect that the results will not be good compared to allowable compression limits.  In this case a stiffener would be useful.

Not withstanding that problem, there will be very high secondary bending stresses in the meridional direction.  A shell analysis program would be required to calculate the magnitude of the bending in this case.  You could also check into the ASME VIII Code appendices for design formulas for non-6% knuckles, but it would just be a first approximation of the stress levels.

And, when it's all over and done, it's still a non-API Code vessel.

Steve Braune
Tank Industry Consultants

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