Thermal stress inPressure vessel

[Dedithea]

hi all, i'm new in ANSYS and need for help..
I'm using ansys workbench to simulate thermal stress in pressure vessel. the problem is, the von mises stress is way too high.
in the simulation, the inside temp of PV is 80 celcius, and the pressure inside PV is atmosferic (about 0.1 MPa), materials is SS304 which has yield stress 215 MPa.
the result shows that von mises stress is ranging from 6.3 to 450 MPa.. Von mises stress is higher than the yield stress of material which cause deformation in PV. But how could it be, since the temp and pressure inside od PV is really low. I couldnt imagine atmosferic presure PV to deform..
Does anyone have any sugestions for this problem??

thanks

 

[sidk1984]

Hi Dedithea,

Regd your problem of high vonMises Stress and deformations.
1. if possible, please spend some time to review your methodology, inputs ( units too, like coefficient of thermal expansion and Young's Modulus)
2. General idea, temperature profile will be exported as nodal loads to structural system which due to inclusion of thermal expansion co-efficient will cause change in length and in presence of constraints will lead to deformations, strain and stresses .
3. Assuming its linear model , and if these high stress regions are observed near to constraints, you can probably neglect them. if constraint region is problem , try to see ASME B&PVC , DBA methodology ( if not wrong its listed in Section III , not sure about sub-section )which is applicable for linear analysis , and they have suggested rules and allowable stress values near to constraints.
4. try to approximate your analysis with some sample rough calculations if possible , like using ' Alpha*DelT*L' as to evaluate a rough value of simplified dummy structure similar in length and thermal profile.

Hope it may help you.

sid