ASME vs FEA 1

[rmerlob]

I tried posting this yesterday and somehow it just didnt get posted, here it goes:

I posted this on the autodesk forums:

So, im trying to compare results from calculating a torispherical head according to ASME VIII with the model dimensions, with an 8 mm thickness the ASME calculation validates it to 250 psi, running Inventor FEA doesnt, Im doing a symmetrical analisys for 1/4 of the head, I get a stress of 334.5 MPa .

Material is A516 gr 70, 260 MPa YS, 485 UTS, since its over 70 ksi UTS , ASME recomends allowable stress 20000 psi for their formula (see spreadsheet attached)

I would expect FEA to actually predict what SF ASME is going for with the formula but since Im getting a stress higher than yield, something must be up.

Any insights?

EDIT: simulating the entire head with a fixed constraint at the bottom head gives the same results as the 1/4 head, 351 MPa max Von Mises

Link:

http://forums.autodesk.com/t5/Autod...nd-Inventor-FEM-results-analisys/td-p/3281429

Discussion at that forum practically concluded that fea is not giving inaccurate results, but somehow, ASME code clears this head for 250 psi while FEA doesnt

So could anyone check the spreadsheet over there and comment?

Is hard for me to believe that ASME code will allow for yielding in a pressure vessel head, yet i cant see where im wrong.

Thanks

 

[Karloss12]

Your FEA method's may not be the best but your final membrane stress in the crown looks about right.

I remember reading some reputable article some time ago about FEA with ASME VIII. The article spoke of an example where a nozzle needed to be located in the knuckle of a head and that this required an FEA analysis.

A simple ASME head calculation was done to get the minimum thickness of the head without the nozzle. Then when they went to do an FEA analysis on the head (with or without the nozzle in it.) the bending stresses in the knuckle were higher then what was allowed.

The article came to the conclusion that the ASME VIII head design method simply calculated the membrane stress of the crown. And then through years of experience standard shapes (Tori, Ellipsoidal) were developed so that the knuckle didn't rupture, even though unknown to them at the time the stresses in the knuckle were high. (Remember we haven't always had FEA or strain gauges connected to computers.) And it still considered safe today, so why change it.

So, a head designed to ASME may have excessive knuckle stress and be acceptable, however as soon as you have to do an FEA on the knuckle of the head then you have increase the thickness of the head to make its knuckle stresses compliant with the code.

Feel free to correct me if I'm wrong.

 

[Karloss12]

Have a look at research paper ASDU 67017: Elastic Stresses in a Torispherical head.

I'm assuming the diameter of your head is the same as the crown radius. I get a maximum inside knuckle principle stress of 280MPa. (Slightly more then yield.) And on the outside surface -170MPa. It claims to be accurate to within 10% of experimental results.