Simulating Heat Exchanger Tubes with Springs
File: File:PVE-4473
Last Updated: Oct 4, 2010
By: CM
Heat exchangers are often very large models. In order to simplify them for FEA we remove all of the tubes and simulate using springs. This will significantly reduce the number of elements and allow the model to mesh and solve in a more reasonable time frame.
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Creating NPT Connections for Piping Fittings
File: NA
Last Updated: May 7, 2007
LB
Background:
The best way to analyze NPT threaded piping fittings using FEA methods is with the pipe included in the analysis. Pressurizing the pipe transmits realistic stresses to the fitting body. Anchoring a pipe instead of the fitting produces much more realistic stresses and deflections.
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Simplification of FEA by Symmetry
File: NA
Last Updated:
LB
For irregular geometry, classical B31.3 rules cannot be applied. As a result, a Finite Element Analysis (FEA) is required, meeting ASME VIII-2 guidelines as permitted by B31.3.
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Simplification of Multi-Body FEA Models
File: NA
Last Updated:
LB
An analysis can become more complicated than it needs to be when the model is composed of multiple bodies interacting with each other.
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Finite Element Analysis Reaction Forces
File: NA
Last Updated:
BV
Summary
Reaction forces are the resulting loads seen at the restraints of a model being analyzed. They can be used to ensure an analysis is restrained from rigid body motion, and is static or in balance. The reaction forces are equal and opposite to the sum of the applied loads.
This report shows typical methods used for restraining models and compares the resulting displacement and stresses of identical models both in balance and out of balance.
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Error Plots - Bolt Heads & Surface to Surface Contacts
File: PVE-3179
Last Updated: Dec. 13, 2008
LB
Summary:
Error plots show how well the complexity of a mesh matches the complexity of the model. Once the mesh matches the complexity of the model, the reported error is low. As a guideline, Pressure Vessel Engineering uses 5% error as an acceptance criterion.
It is possible to get stresses below 5% in general vessel areas by applying an appropriate mesh size. This report covers two areas where the error cannot be lowered to reach this acceptance criteria regardless of the mesh size used. These areas are: 1) stresses in and around the head of a bolt and 2) stresses at surface to surface contacts.
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The Nuts and Bolts of Stress Linearization
File: 1844
Last Updated: March 2012
LB
(With an Educational and Functional Stress Linearization Utility)
Summary
This article is a guide to how the stress linearization tool works to separate stresses into membrane and bending. Examples are provided along with sample data and a spreadsheet based stress linearization tool. The programming code is explained in the appendixes and can be examined in the spreadsheet.
This article does not discuss whether the stresses are local or global and which allowable stresses should be used for where. This is a topic for a later article.
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Stress Classification Lines (SCL)
File: PVE-3277
Last Updated: Feb 2009
LB
SCL Passing Through Areas of Very High Error
Summary
Stresses at sharp corners rise towards infinity as the mesh size is reduced. However the forces have to balance in a Finite Element model regardless of the mesh size used. In these studies SCL (Stress Classification Line) results are compared a different mesh sizes. This report shows that it is possible to take stress classification lines through these areas of peak stress and get ultimate stresses - however the results will not be as expected.
Two studies are shown. In Study 1, the SCL passes between 2 sharp corners. The stress classification method produces results that do not vary as the mesh is refined. However, it misses the magnitude of the membrane and reports no bending stress. This is a limitation of the stress classification method.
In study 2, the SCL passes through only one sharp corner. This study shows results that converge to a finite value. Again, the reported membrane stress is less than expected.
It is best not to run an SCL through sharp corners (areas of very high error).
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Large displacement solutions
File: PVE-4048
Last Updated: March 2010
LB
This solar reflector uses a vacuum to pull the front and back surfaces together to focus the reflective surface. The deflected surface shape can be calculated using FEA, but the correct shape can only be computed with large deflection theory.
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Evil SolidWorks Factory Default Material Properties
File: PVE-4261
Last Updated: May 18, 2010
LB
Problem:
When a new object is created, SolidWorks specifies that the material is <not specified>. However, if the item is evaluated, it has a factory default material density of 0.04 lb/cuin (1000 kb/m^3) or the density of water. The appropriate material density would be "!Error - material density is not set!".
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Bloating SolidWorks Files
File: File:PVE-4482
Last Updated: Aug 18 2010
By: LB
This SolidWorks part for a weld neck flange has a design table with 132 different configurations in it. The configurations cover changes in size and rated pressure. When the file was first created it was 2,422 KB in size (2.4 MB). With use it has grown in size without any changes to the file.
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When will FEA be Required in a Submission?
File: NA
Last Updated:
LB
Finite Element Analysis (FEA) is required in a submission when code equations cannot be directly applied to the configuration and/or when additional reinforcement needs to be accounted for to obtain acceptable stress values.
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Setting Up Presentation Screen Shots for FEA Reports
File: NA
Last Updated: Oct. 20, 2008
LB
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This article supplements ABSA's (Alberta Boilers Safety Association) requirements on writing FEA reports: ABSA FEA Requirements. In particular refer to the section "Presentation of Results".
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