Local Stress at Toriespherical Head

[AlexRG]

I am working on the design of a vessel that has torispherical heads and im having a litlle trouble about it. With a design load of 15 psi the calculated head thickness is around 5/16". This was calculated using the general vessel formulas from table 2-1 of the Pressure Vessel Design Manual by Dennis Moss. the procedure 2-4 of same manual studies the stress at four points of the head . If I use the equations for the stresses in the knuckle I get 4000 psi and 56000 psi in the two directions. If the design temp. its 310ºf for SA-36 steel sheets, the allowable stress its 16700 psi. Then i have to increase the thickness of the head up to 3/4" to be ok. its this equation or guideline stipulated in the ASME CODE??. Im doing it all right ??. the internal Diameter of the vessel its 4000 mm. with a L/r = 16 2/3.

 

[gr2vessels]

AlexRG,

The ASME VIII Div. 1 is your best friend;- check out Clause UG 32(e) for the dished end thickness calculations. If you're not familiar with the Code calculations, perhaps you should ask the help of a pressure vessel designer, since there is much, much more than a single formula for the torispherical head thickness.

gr2vessels

 

[prex]

I suppose you are calculating the discontinuity stress between the spherical portion and the knuckle: you should not worry about this one, if head proportions are within code limits.

prex

http://www.xcalcs.com

Online tools for structural design

 

[rhg]

Please, call a pressure vessel designer .... urgent!!!

Regards
rhg

 

[JStephen]

Alex, I'm not familiar with the specific equations in Moss's book. However, I've found if you try to apply API-620 designs to an F&D head, you come up with similar results. The problem is that where the torus and the sphere are tangent, the calculated stresses (and thus strains) are widely different. Of course, since it's the same piece of metal, this can't be. The general approach to the head design under the ASME code is to apply the head thickness equations of ASME and recognize that there may be some theoretical high stress that is not, and need not be, accounted for. Section VIII gives a design for head thickness based on overall design of the F&D head, and elsewhere, an equation for the spherical portion, used in designing reinforcing in that area. I think there's one of the Code Cases that has a more detailed F&D head analysis, but I'm not familiar with it.