Hi all,
I am working on a calculation spreadsheet based on Plate and Shell Theory. I want to evaluate the allowable stresses according to ASME VIII, Division 2.
I have a problem with code interpretation, namely with Local Primary Membrane Stress (PL):
ASME Boiler & Pressure Vessel Code, Section VIII, Division 2 – Part 5
5.2.2.2 Stress Categorization
“b) Local Primary Membrane Equivalent Stress (PL)
1) The local primary membrane equivalent stress is the equivalent stress, derived from the average value across the thickness of a section, of the local primary stresses produced by the design pressure and specified mechanical loads but excluding all secondary and peak stresses. A region of stress in a component is considered as local if the distance over which the equivalent stress exceeds 1.1*S does not extend in the meridional direction more than sqrt(R*t).”
Definition of stresses (where S is the allowable stress):
General Primary Membrane Stress: Pm<=S
Local Primary Membrane Stress: PL<=1.5*S
If I understand correctly the membrane stress is considered Local if the axial (meridional) criterion of sqrt(R*t) is fulfilled – counting from stress level of 1.1*S. Then the stress needs to be below 1.5*S.
If the axial (meridional) criterion is not fulfilled then the stress is treated as General and the stress needs to be below 1*S.
But what about the stress area between S and 1.1*S ?
Schematic No.1:
The stress sigma (red circle) in schematic No. 1 , is not in the Local stress criterion region (is below 1.1*S) but is also above 1*S and that means the General stress criterion in not fulfilled. Also the axial (meridional) criterion is not fulfilled in this particular example (L’ > Lmax).
Schematic No.2:
The stress sigma (red circle) in schematic No. 2 , is not in the Local stress criterion region (is below 1.1*S) but is also above 1*S and that means the General stress criterion in not fulfilled. There is no relative stress point exceeding the 1.1*S value to do the axial (meridional) criterion check: “…the distance over which the equivalent stress exceeds 1.1*S does not extend in the meridional direction more than sqrt(R*t)”.
Based on that it looks like only the General stress criterion applies and that means the pipe is overstressed.
If the stress state from schematic no. 1 is acceptable, i.e. the maximum stress is treated as Local and is below 1.5*S and the meridional criterion is fulfilled as well, but the stress state from schematic no. 2 is not acceptable, i.e. the maximum stress does not exceed 1.1*S so it cannot be treated as Local but is above 1*S so is higher than the allowable General stress, we end up with a situation that for a certain load value the maximum membrane stress in the vessel is above 1*S - the vessel is overstressed, however if we increase the load further the maximum membrane stress is above 1.1*S but below 1.5*S (assuming that meridional criterion is fulfilled) - the Local stress criterion applies and the vessel is not overstressed anymore. I do not know how to deal with this criterion "discontinuity". How come that in order to be within code requirements one needs to increase the load (pressure)?
I would appreciate your help with this issue. I would like to know how to treat the stresses between S and 1.1*S region? How to interpret the code guidance?
Best regards
I am working on a calculation spreadsheet based on Plate and Shell Theory. I want to evaluate the allowable stresses according to ASME VIII, Division 2.
I have a problem with code interpretation, namely with Local Primary Membrane Stress (PL):
ASME Boiler & Pressure Vessel Code, Section VIII, Division 2 – Part 5
5.2.2.2 Stress Categorization
“b) Local Primary Membrane Equivalent Stress (PL)
1) The local primary membrane equivalent stress is the equivalent stress, derived from the average value across the thickness of a section, of the local primary stresses produced by the design pressure and specified mechanical loads but excluding all secondary and peak stresses. A region of stress in a component is considered as local if the distance over which the equivalent stress exceeds 1.1*S does not extend in the meridional direction more than sqrt(R*t).”
Definition of stresses (where S is the allowable stress):
General Primary Membrane Stress: Pm<=S
Local Primary Membrane Stress: PL<=1.5*S
If I understand correctly the membrane stress is considered Local if the axial (meridional) criterion of sqrt(R*t) is fulfilled – counting from stress level of 1.1*S. Then the stress needs to be below 1.5*S.
If the axial (meridional) criterion is not fulfilled then the stress is treated as General and the stress needs to be below 1*S.
But what about the stress area between S and 1.1*S ?
Schematic No.1:
The stress sigma (red circle) in schematic No. 1 , is not in the Local stress criterion region (is below 1.1*S) but is also above 1*S and that means the General stress criterion in not fulfilled. Also the axial (meridional) criterion is not fulfilled in this particular example (L’ > Lmax).
Schematic No.2:
The stress sigma (red circle) in schematic No. 2 , is not in the Local stress criterion region (is below 1.1*S) but is also above 1*S and that means the General stress criterion in not fulfilled. There is no relative stress point exceeding the 1.1*S value to do the axial (meridional) criterion check: “…the distance over which the equivalent stress exceeds 1.1*S does not extend in the meridional direction more than sqrt(R*t)”.
Based on that it looks like only the General stress criterion applies and that means the pipe is overstressed.
If the stress state from schematic no. 1 is acceptable, i.e. the maximum stress is treated as Local and is below 1.5*S and the meridional criterion is fulfilled as well, but the stress state from schematic no. 2 is not acceptable, i.e. the maximum stress does not exceed 1.1*S so it cannot be treated as Local but is above 1*S so is higher than the allowable General stress, we end up with a situation that for a certain load value the maximum membrane stress in the vessel is above 1*S - the vessel is overstressed, however if we increase the load further the maximum membrane stress is above 1.1*S but below 1.5*S (assuming that meridional criterion is fulfilled) - the Local stress criterion applies and the vessel is not overstressed anymore. I do not know how to deal with this criterion "discontinuity". How come that in order to be within code requirements one needs to increase the load (pressure)?
I would appreciate your help with this issue. I would like to know how to treat the stresses between S and 1.1*S region? How to interpret the code guidance?
Best regards
