General-purpose FEA is well suited for the dozens of unexpected analysis tasks a company encounters everyday. But a few specialized jobs are more easily handled by focused programs.
Software called NozzlePro4 from Paulin's company (www.paulin.com) provides one way to stress-analyze nozzles. The software is a focused FEA program that makes it easy to model, mesh, and apply boundary conditions, such as temperature, pressure, and external loads, to nozzles.
ASME codes tell what stresses are allowable in a nozzle, but they don't provide a method to calculate them. And a few current methods, such as the Welding Research Council Bulletin 107, produce values that vary widely because they were never intended to be used for the range of geometries and loading conditions encountered. For example, stresses calculated by the FEA program for a cylindrical junction may be up to 270% larger than those calculated with WRC 107," says Paulin. "And for some smaller branch takeoffs, FEA stresses may be less than 25% of values calculated using ASME B31.3. The nozzle-FEA software produces results that improve on many current methods," he adds.
What's more, the FEA software considers the combined loading from pressure and temperature. It works with eight-node reduced integration, curved-shell elements that have proven themselves tolerant of poor shapes.
Even users adept at modeling in an FEA package will find the software useful because it's easy to generate meshed models and it produces automatic ASME reports and plots. A limited demo program is available at no cost from the developer. It's about a 20-Mbyte, 10-min download and it runs on most Windows-based computers. A 63-page Help file comes with the program to explain operations and results in detail.
Briefly, it works like this: Users select a base shell (the main pipe or cylinder), type of nozzle, and SI or English units. The software then presents diagrams that help fill in accompanying fields that define sizes. The software is simple to operate but expect to do a little reading to appreciate and interpret results.
An example model for a cylinder base shell and straight nozzle illustrates the procedure. Five fields describe the base shell and four define the nozzle, but only outside diameter and wall thickness are necessary for this example. Hitting "Plot Only" shows the model mesh and lets users verify that this is indeed what they want. If not, selections in the Orientation menu allow fine-tuning the nozzle's location on the shell.
The Options menu allows changing the mesh and weld size, among other things. Weld size affects nozzle stiffness. "We find the program matches up well with physical test data for weld strength," says Paulin.
The Loads menu allows assigning weights, pressures, and temperature differentials, and another section defines materials. Hitting "Run FE" starts the solver. It took about a minute on a 933-MHz Pentium III computer.
The software includes a predefined set of loads for piping, such as in-plane and out-ofplane loads. "So even if you do not assign loads, the software still has a standard set to deal with," adds Paulin. From that, the software generates stress-intensity factors and finds allowable loads on the nozzle, along with its flexibility and stiffness.
The report is self-explanatory. It includes many calculations from WRC and ASME codes for comparison. Graphical results are more interesting. For instance, the 3D viewer allows rotating, zooming, and clipping the model. A meter reports the stress at specific locations. A Viewer uses an animation component in Windows called DirectX to show how models react to applied loads. (DirectX is also available from the Microsoft Web site).