FEA analysis on Hemispherical head 6

[llaren]

Anybody can help me to figure out Pressure vessel head calculation? I run with ANSYS for pressure 8.25Bar. Head thickness is 7mm , min 8mm -1mm corrosion. But Von-mises stress is 202Mpa that is higher than allowable stress 137Mpa. Can I comparable vonmises stress and allowable stress? So that head design is fail? I dont understand how can i compare vonmises stress for failure? But If i calculated with PV Elite , actual stress is 110 Mpa only at thickness 7mm.Is it applicable to calculate vonmises stress in pressure vessel? Highly appreciate if any professional help me. Thank in advance.

 

[brkmech1234]

The stress calculated by PV Elite is hoop tensile stress calculated only in one direction. Vonmises stress is the combined stress accounting tensile in circumferential and longitudinal direction and also shear stress may also be taken into account. I remember that vonmises stress is acceptable close to yield point. I am not sure. Experts in FEA field may comment.

 

[prex]

Both Tresca ans von Mises criteria may be used for pressure vessels, depending on what is specified by the applicable code. The two should give comparable results.
I think there is something wrong with your model or calculation. What is the radius of the head? The stress can be calculated with a simple formula as a function of R, t and P.

prex
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[C2it]

Isn't pressure stress in a spherical or hemi shape, simply same as axial stress in a pipe ? PD/4t or PR/2t ??

 

[llaren]

Hi Prex, thank for your answer,,model is not wrong,,But that vonmise stress need to compare with allowable stress or yield stress? head type is ellipsoidal head and outside dia is 1516mm.Design code is ASME VIII Div.1. Actually we can calculate with p= (SEt/(kD-2t (K-0.1) formula. But if I calculate with that formula. Stress is very less than FEA von mises stress. In ASME code, I didn't found any von mises stress comparison.

 

[llaren]

Dear professionals,

I attached sketch

 

[prex]

Either you have something wrong in your model or you are seeing a local stress (perhaps near the attachment to the shell?).
There is no reference to von Mises or Tresca stress in Div.1, you must go to Div.2 to interpret the results of a stress analysis.
And of course von Mises or Tresca stress would be compared to the allowable stress.

prex
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[jtseng123]

my two cents:
although you have S1 S2 and S3 to get von mises or tresca, since it is purely by pressure, FEA results shall be very close to hand calcualtion by code formula or theoritical formula from text book.
You may have put wrong constraints on the model. Add a long shell section to it and constraint at the bottom of the shell not head.
Check the stress at the middle layer, not top or bottom layer. Pressure creates primary stress. Middle layer represents the primary stress.

 

[TGS4]

Two things:
1) If you are designing a vessel to ASME Section VIII, Division 1, if there are rules provided (which there are for heads, and head-to-shell joints), you MUST follow the rules. See U-2(g). Why are you even doing an FEA?

2) If, for some gawd-only-knows reason, you choose to perform an FEA, go to ASME Section VIII, Division 2, Part 5 to interpret the results.

My guess is that you are reading the maximum membrane-plus-bending von Mises stress from your FEA. That's fine, but it has a different "allowable" compared to S.

Here's my free two cents - you are unqualified to be performing an FEA to the ASME Boiler and Pressure Codes. Leave that type of work to the experts - you're likely to do more harm than good. Find someone who is qualified and have them teach you - based on my experience of training engineers in this field, you are looking at about a year of on the job training.

 

[jtseng123]

Be kind TGS4, don't be so harsh as you did to me last time. Everyone has his first step. llaren may just start to learn FEA and compare with code. The attitude is encouraged. (I review a lot of FEA reports, and took 3 courses at graduate school 20 years ago, but forgot all of them, and never done one at work)
But TGS4 is right, you shall just go by code rules. To interpret FEA and correlated to Div 2 is not an easy task. Some times, garbage in garbage out. I saw a lot of FEA reports that the person does not know what he is doing. But don't give up, one day you will be the FEA expert. . Keep exploring and asking questions, llaren.

 

[TGS4]

jtseng123 - not being harsh, just honest. You ahead and look at my posts from the past 8 years; I've been pretty consistent with my assessment of incompetence/lack of qualification. I'm glad that llaren has come here to have their question answered, and I certainly encourage them to return. But, judging solely from the initial question and the OP's reply, right now they are unqualified. Not to say that they can't become qualified and learn, but it is an assessment of where they are right now.

llaren - please review Part 5 of ASME Section VIII, Division 2 and come back here with any questions that you have.

 

[jamesl]

Did you check the boundary conditions at the enge of the head in your model? If you have wrong contraints on your model, you will get wrong results. If you just want to compare code and FEA, you can run a FEA of a sphere first.