Torispherical formed heads w/small knuckle radius

[VTechie79]

I have done some searching and have been unable to come up with an answer to the following quandaries...

[li]API 620 and ASME Section VII, div 1 does not allow for formed heads with knuckle radius to diameter ratios (r/D) of less than 6%. What is the reasoning behind this and why was 6% chosen as the limit?[/li]
[li]Are there any accepted standards or codes that give a method to calculate thickness or allowable pressure for formed heads with r/D less than 6%?[/li]
Any guidance would be appreciated.

 

[ChrisF90]

VTechie79,

ASME Section VIII, Div. 1 does not prohibit the use of a torispherical head with knuckle radius to diameter ratios (r/D) of less than 6%. It simply states that the thickness for a torispherical head that meets that requirement can be determined by formula (2) of UG-32. See excerpt:

"UG-32(d): Torispherical Heads with ts/L ~ 0.002. The required thickness of a torispherical head for the case in which the knuckle radius is 6% of the inside crown radius and the inside crown radius equals the outside diameter of the skirt [see (i)] shall be determined by..."

For a torispherical head with a knuckle radius to diameter ratios (r/D) of less than 6%, see U-2(g). I would recommend you have an experienced engineer design the head per ASME Sec. VIII, Div. 2, Part 5 (using Div. 1 allowables if the head is for a Div. 1 vessel).

Hope that helps!

 

[JStephen]

See UG-32(j).
For the API-620 approach, the thickness becomes unreasonable with smaller radii, so there's not a specific requirement, just a recommendation.