RPS97
Mechanical
- Dec 7, 2020
- 1
Greetings!
I've been asked to do a Design by Rule according to ASME BPVC 2015 Section VIII division 2 on a jacketed vessel type 2 operating at ~150°C (jacket covering a portion of the cylindrical shell and one head), and the covered parts will be subjected to external pressure.
As the vessel material is SA-240 gr. 304, the allowable stress F_ha has to be adjusted considering the tangent modulus according to 4.4.3.1. So far so good.
But when I began calculating the required thickness of the cylindrical part, I noticed that decreasing the unsupported length actually decreases the allowable external pressure. That happens because the tangent modulus decreases so fast that the higher predicted elastic buckling stress apparently isn't worth it. Am I doing anything wrong here? Or is it just that 304 steel doesn't quite fit ASME material model?
For reference, I used (ASME Section II part D):
Yield strength - 154 MPa
Ultimate tensile strength - 456 MPa
Young's Modulus - 186 GPa
Thanks in advance.
EDIT: After a good night of sleep I think I might have come up with the answer. The predicted buckling stress F_he is proportional to Young's Modulus, so the adjusment is actually replacing it for the tangent modulus, which is not constant and varies with the stress itself. That implies that an iterative equation needs to be solved in order to find F_he. Hopefully someone can confirm this procedure!
I've been asked to do a Design by Rule according to ASME BPVC 2015 Section VIII division 2 on a jacketed vessel type 2 operating at ~150°C (jacket covering a portion of the cylindrical shell and one head), and the covered parts will be subjected to external pressure.
As the vessel material is SA-240 gr. 304, the allowable stress F_ha has to be adjusted considering the tangent modulus according to 4.4.3.1. So far so good.
But when I began calculating the required thickness of the cylindrical part, I noticed that decreasing the unsupported length actually decreases the allowable external pressure. That happens because the tangent modulus decreases so fast that the higher predicted elastic buckling stress apparently isn't worth it. Am I doing anything wrong here? Or is it just that 304 steel doesn't quite fit ASME material model?
For reference, I used (ASME Section II part D):
Yield strength - 154 MPa
Ultimate tensile strength - 456 MPa
Young's Modulus - 186 GPa
Thanks in advance.
EDIT: After a good night of sleep I think I might have come up with the answer. The predicted buckling stress F_he is proportional to Young's Modulus, so the adjusment is actually replacing it for the tangent modulus, which is not constant and varies with the stress itself. That implies that an iterative equation needs to be solved in order to find F_he. Hopefully someone can confirm this procedure!