Components that are part of a piping system, such as valves, flanges, and other elements, are much heavier (and stiffer) than the pipe that connects to them. How does CAESAR II handle this? By using rigid elements. Let’s take a look at what a rigid element really is and how it affects your piping stress analysis.

CAESAR II is a 3D beam element solver that computes the element stiffness matrices and load vectors and then solves for displacements, forces, moments, reactions, and stresses.   The resulting information is compared against allowable stresses from the codes of construction.  While code standard calculations evaluate the pressure-containing capability of all piping components in a piping system (usually through the specifications of listed components), the code standards do not attempt to evaluate mechanical load on non-pipe components (such as a gate valve).

Why not?  Well, where is a system going to fail first, on the pipe, a valve, or a flange?  Typically, it will be the pipe.  Additionally, the geometry of non-pipe components are too varied and complex, so accounting for non-homogenous surface area calculations to determine failure on a component that has a very low chance of failing compared to pipe is just a moot point.

Enter the rigid element

This parameter can be applied manually using the Rigid selection box on the classic piping input screen.

Click the selection box to turn it on and two important things occur:

The first is that you have a new parameter that the object carries, “Rigid Weight”.  You can assign any number you want to the “Rigid weight” as it pertains to the object you are modeling.  For example, if this element was simulating two flanges and a gate valve, you would sum those individual component weights and enter that value into this field.

The second thing that occurs is that your element is displayed differently in the modeling window.  By manually entering the rigid element, I have converted element

20-30 to a bright green color with some symbolic geometry so that I can quickly recognize this element during my analysis.  Rigid elements can look differently so don’t worry about that too much right now.

So now that we have one, how does this affect the analysis?

Well, the job of the rigid element is to transfer stresses across the element.  When CAESAR II computes the stiffness matrix during an analysis and solves for loads, the rigid will transfer the load from one side of the pipe to the other. This is a very useful property for modeling more complex types of analysis.  For example, let’s take a look at modeling a vessel attached to piping in a CAESAR II analysis.

 

Vessel Example

Here I have a vessel modeled as the large, vertical cylinder, and a nozzle attachment modeled as the two rigid elements connected to the horizontal piping run on the right.  You’ll notice there is a

(

thin, green element coming from the center of the vertical vessel extending to the flanged connection on the vessel side.

What is important to understand here is that rigid elements retain thermal expansion as a property even though their main purpose is to transfer loads during an analysis.  So what I have effectively done in this case is accounted for radial expansion of the vessel by connecting my rigid element to the centerline of the vessel and extending it to the vessel’s radius.

There is more going on here but start to think about how you can apply this tool as a means to get more precise results from your analysis.  Let’s recap and describe a few more properties about this tool:

• Rigid elements main function is to transfer stresses between nodes in a piping system.
• A rigid element can be weightless, or you can define it a specific weight.
• Thermal expansion can be modeled within a specific application.
• The inherent stiffness of the element for analysis is based on ten times the wall thickness of your piping system.
• CADWorx will automatically export all non-pipe sections as rigid elements with their weight that was assigned in the specification with the C2OUT command.
• Rigid elements will not “fail” during an analysis.

Thank you,

-George

 

 

 

 

 

 

Next, we are going to select the edit the material icon, which is the fourth icon from the right in Figure 4.  This will allow you to search all current materials in the database.   In this example I am interested in SA-269, a particular grade of Austenitic Stainless Steel which I need for corrosion resistance.   Following the dialog input, I typed in “269” and searched.  I received 9 separate material types in return in Figure 5.  The first three correspond to material ID 269, which corresponds with SA-403, this isn’t what I want.  The next 6 materials are exactly what I am looking for.  For the 6 records, there are three groups based on Grade.  The three grades are TP304, TP304L, and TP316L.  For the three grades, there are two separate materials based on whether or not you are using B31.3 or not as your design basis.  Right now, I have a clear understanding of what material ID number corresponds to these materials.  The middle column “number,” has three identifications, 343, 351, and 352 for TP304, TP304L, and TP 316L respectively. I can go back to my piping input screen in Figure 1 and type in any of these numbers to get the corresponding material grade I am looking for.   For now though, let’s take a look at the information CAESAR II contains for a material grade.  I’ll check A269 TP304 B31.3 and click OK.

Figure 6 takes us back to the material database editor after clicking OK.  The largest portion of this table describes the reduction in allowable stress versus temperature, which is a direct copy of the values found in the ASME Boiler and Pressure Vessel Code, Section II Part D.   In some situations if you are working with a unique grade, you may need to create your own table of allowable stresses, however for the vast majority of cases we are going to be working with default values.

Thank you,

George

Summary/Conclusions

Obviously, the minimal reduction in forces does not warrant the use of FEATools unless the stresses are approaching the stress allowables.  It is important to first understand what type of conversion is happening here so that we can better understand our results.  The updated SIF’s and flexibilities are coming from ASME 07-02, recommended changes to Appendix D of the B31 codes.  It has been known  that a number of the SIF’s from Appendix D are incorrect and give incomplete guidance.  In addition, the omission of k-factors for branch connections allows for ASME 07-02 to correct these errors.  

Figure 2 below describes the added restraints and springs that are inserted at the branch connection per the guidelines in 07-02 by FEATools.

Each branch is connected by a series of connecting nodes to the center point of the tee, set with k factors from the allowables in ASME 07-02.  The added spring connections have the potential to reduce stress caused by thermal strain in a given piping system.  This type of model fidelity is best applied when other solutions have not been successful or are not ideal in a CAESAR II analysis, such as adding expansion joints or expansion loops.  In this case, an analyst could conclude that FEATools would not reduce the load significantly enough to make that much difference in the overall equipment loads.

Therefore, if we are going to reduce the loads on the pump nozzles, one must explore other options.

Thank you,

George