API 620 on F&D Heads

[crosby84]

Hey everybody.

I'm trying to do 620 calcs on an F&D head. I want to see how the tmin per 620 compares to the tmin per ASME. To give some context, I'm looking at a tank with no nameplate or design info that has an F&D head, and the operating pressure is within 620 range.

I've attached a sketch from Solidworks that I used to get the dimensional values. Based on my calculations, the point of consideration (some random point that I picked in the knuckle) is in tension in the meridional direction (makes sense) and in compression in the latitudinal direction (this makes no sense to me). How can it be in compression in this direction? Please see attached and thank you to anyone who can help.

I used API 620 12th ed.

Regards

 

[JStephen]

The knuckle area is normally in compression tangentially in an F&D head.
An important point: API-620 will often give unreasonable results when used for an F&D head shape- see the note about making the knuckle larger to help with this effect. If thickness is reasonable and the pressure is low, the design may work out. Otherwise, it may not, but doesn't mean the design is bad.

 

[pdiculous963]

acalvane - when internal pressure is applied to a F&D head, the knuckle wants to "stretch" out to maintain continuity between the cylinder and the crown. This causes the knuckle to go into hoop compression. The picture below shows a F&D head under internal pressure; there is the exaggerated displaced shape with the original shape superimposed. The contours plotted are hoop stress. Everything in grey is in tension, the other colors are in compression.

It is known that knuckle buckling is a potential failure mode of F&D heads. This is documented in WRC Bulletin 444 among others. The rules in ASME Sec. VIII Div. 2 (paragraph 4.3.6.1) explicitly consider knuckle buckling in determining the MAWP of dished heads. WRC Bulletin 501 documents the development of these design rules.

 

[crosby84]

Thank you both for your replies. pdiculous963 that was very informative.

Regards