## Design by Rule VS Design by Analysis

## Design by Rule VS Design by Analysis

(OP)

Dear All

I am performing calculations for a pressure vessel according to ASME BPVC Sec. VIII Div. 2. First, I modeled the vessel in PV Elite which performs Design by Rule as per Part 4. Of the VIII-2. All the nozzles and shells passed the strength calculations based on the

The question is:

Can I ignore these results and use design by rule as a tools for passing static loads? After all, I am using Design by Analysis for protection against cyclic loading not plastic failure.

Warm Regards

I am performing calculations for a pressure vessel according to ASME BPVC Sec. VIII Div. 2. First, I modeled the vessel in PV Elite which performs Design by Rule as per Part 4. Of the VIII-2. All the nozzles and shells passed the strength calculations based on the

**design conditions**. Then I started to design the vessel based on the guidelines in Part 5 of the VIII-2. First, I used the Ansys workbench software to obtain the stresses in**operating**pressure and temperature conditions. I continued the procedure and the fatigue calculations for components passed and there will be no failure during the service life of the vessel. However, I noticed that despite the fact that these nozzles have passed the Design by Rule conditions they fail the conditions in Design by Analysis. I mean when the calculations are done if I define some SCL’s and obtain stresses P_{m}& P_{L}+P_{b}they fail the protection against plastic failure conditions in par 5.2.The question is:

Can I ignore these results and use design by rule as a tools for passing static loads? After all, I am using Design by Analysis for protection against cyclic loading not plastic failure.

Warm Regards

## RE: Design by Rule VS Design by Analysis

If something passes DBR and fails DBA, I'd like to know the details, as this is a potentially serious problem.

## RE: Design by Rule VS Design by Analysis

After your reply I read the Code and found something:

I think these sentences allow using just some parts of DBA while other aspects of design are based on DBR or maybe I am misunderstanding them.

Warm Regards

## RE: Design by Rule VS Design by Analysis

Nominally, thought, my expectation would be that anything that passes DBR, would pass DBA (standard flanges notwithstanding).

## RE: Design by Rule VS Design by Analysis

TGS4,I do not have access to 2017 edition but I think I got the root problem. I have not decided to design the thickness by DBA. Rather, I want to use DBR in order to design the thickness. Now, if I am supposed to follow all requirements of design by rule then I have to design the component against fatigue failure. And there is no rule in Part.4 regarding fatigue design of a component. Instead, in Part.4 it is always necessary to check the thickness against fatigue using screening criteria in Part.5.So I think what I did was not picking some criteria in Design By

Analysisand ignoring the others. It actually was doing the whole design in accordance with requirements of Design ByRule.## RE: Design by Rule VS Design by Analysis

## RE: Design by Rule VS Design by Analysis

TGS4,I am in trouble interpreting the code for elastic stress fatigue analysis.First of all, do I need to linearize the stresses for fatigue analysis? I do not think it is necessary to linearize the results of stress analysis since for fatigue the total stress is required to be evaluated including stresses resulting from stress concentrations and other local effects.

Then I need to know the validity of this method with respect to the mesh size of finite elements analysis. Since a linear elastic procedure is to be followed, there will be no limit on the stress of the material in material properties and no yielding and subsequent strain hardening is considered for the material. In such cases if there is a sharp edge in the geometry the stresses go to infinity based on linear elasticity theory. When it comes to finite elements analysis this phenomenon shows itself with change in the stress results when the elements size change. Here, the finer the meshes are generated for the analysis, the higher the resulting stresses will be. Even if there is no sharp edge in the model, the areas around a discontinuity are very sensitive to mesh size when it comes to calculating maximum equivalent stress.

If the elastic stress analysis is applied in protection against plastic collapse this problem does not exist since linearization is done and the effect of local stress concentrations are neglected for being a part of peak stresses.

I personally modeled a nozzle in a shell (using solid elements) and ran the finite element model in Ansys workbench and checked the effects of element size on the stresses. The results show that when I used finer mesh the stresses resulting from linearization (membrane and bending stresses) are not much sensitive to mesh size (maximum 1 or 2 MPa in the 150 MPa). However, the maximum stress continues to increase by decreasing the mesh size. It is very important since the expected life of a component can change considerably with the change in the mesh size.

Please help me come out of this doubt.

Warm Regards

## RE: Design by Rule VS Design by Analysis

Suppose that I am designing all the thicknesses based on DBA. Is it possible to do the protection against plastic collapse using elastic stress analysis and protection against cyclic failure using elastic-plastic approach? Or, design against fatigue with elastic stress analysis and design against ratchet using elastic plastic method?

Warm Regards

## RE: Design by Rule VS Design by Analysis

Regarding mixing and matching the methods, it is generally acceptable.

## RE: Design by Rule VS Design by Analysis

At least now I can understand that I am not the only one who has faced this issue. Nevertheless, I think it is not a good engineering practice to not model discontinuities in order to prevent stresses to go high. You see these type of sharp edges are actually present in the real vessel. But using linear elastic model for these points leads to results that are sensitive to mesh size for fatigue. For plastic collapse it is OK because of linearization but in fatigue design the validity of linear elastic stress method is quite questionable.

Warm Regards

## RE: Design by Rule VS Design by Analysis

One method that I've seen to deal with the issue of the discontinuity is described in a document by the International Institute of Welding (IIW) titled: "Recommendations for Fatigue Design of Welded Joints and Components".

In this document it talks about reading the stresses at a couple of points away from the discontinuity, then extrapolating these stresses back to the discontinuity. Using this method, you should be able to refine the mesh enough to converge the stress at the points away from the discontinuity, using these to calculate a peak stress for use in the ASME VIII-2 formulas for fatigue life.

Take a read through the article though, as it discusses a few different options for extrapolation.

## RE: Design by Rule VS Design by Analysis