ALGOR SOFTWARE HELPS ZIMMERMANN & JANSEN DESIGN VALVES THAT CAN TAKE THE HEAT

This is a light shading of the disk butterfly valve which was created by Zimmermann & Jansen for use in a petrochemical plant.

The valves used to control the flow of liquids in petrochemical plants must withstand intense temperatures, as well as high pressure. If such a valve failed, the impact on human life and the environment could be devastating. The engineering has to be right, and that means the finite element model has to be as accurate as possible.

A disk butterfly valve consists of two dished heads welded to a concentric ring, with two large squares cut 180 degrees apart. Blocks are welded into these cutouts and machined to accept a shaft. The shaft is then pinned to the blocks.

The Technique

In constructing a finite element model, the most accurate way to represent the thin disks and the thick blocks is to use two different types of elements. Plate/shell elements represent relatively thin surfaces, while brick elements are used to represent thicker, solid objects. Algor's unique COMBSST function makes it easy to create a model which is made of more than one element type.

When Michael Lemeshev of Zimmermann & Jansen designs a butterfly valve for a major petroleum corporation, he uses this technique. The basic design is not new, but each time he makes a valve, the parameters change (valve size, pressure, temperature). Each valve has to be modeled and FEA tested. In this particular case, the stainless steel dish had to withstand 50 psi at 1,450 degrees Fahrenheit.

Michael Lemeshev of Zimmermann & Jansen views analysis results.

Mapping out a Strategy

The model started as a simple wireframe of the dished heads in AutoCAD. "This was where the strategy was mapped out for meshing the model later," said Mr. Lemeshev.

The basic strategy was to create a plate/shell model of the dished heads, create a brick model of the blocks and shaft, and then combine them into a single model for analysis. It would be important for the nodes to match up when the model was combined. Because all the geometry appeared on the screen at the same time, it was easy for Mr. Lemeshev to make sure he was meshing surfaces which connected correctly.

Surfing the Dish

The AutoCAD wireframe was imported into Supersurf, which supports a rich set of surface construction methods. The GPATCH command allowed Mr. Lemeshev to quickly create surfaces from multiple patches bounded by the curves of the wireframe. "The frame model which comes from CAD is relatively simple," said Mr. Lemeshev. "It does not include difficult surface intersection curves. For these, Supersurf must be used."

From the wireframe, Supersurf created the surface and the mesh of the dished heads in addition to the intersection between the heads and the block. Supersurf includes the ability to generate different mesh densities. By setting a parameter and clicking, the entire model can be quickly remeshed at a higher or lower mesh density. "Supersurf is wonderful," said Mr. Lemeshev, "because after adding the surfaces to the model I can change the mesh to my heart's content."

Creating the Block

The surfaces of the block were also created in Supersurf. At this point, Mr. Lemeshev defined the mesh so the nodes on the dished heads would match the nodes on the block. Hexagen then turned the surfaces of the block into a solid, 8-node "brick" model. "Without this feature," said Mr. Lemeshev about Hexagen, "creating the brick model would be nearly impossible." Finally, the shaft was created using Superdraw II's copy/join function, then added to the block.

Multiple Element Types Created

The geometry for each model, plate/shell and brick, was complete and ready to be decoded. When decoding takes place, the user specifies the type of element being used. Mr. Lemeshev specified which parts of the model to decode as plate/shell elements, then decoded the rest of the model as bricks. The brick and plate/shell portions were decoded separately. The COMBSST function then combined the two models.

The Analysis

Once the model was complete, linear stress analysis was performed with a loading of 50 psi. "Initially the results showed the disk to be failing, based on ASME code," said Mr. Lemeshev. "We then added internal ribs to keep the dished heads from buckling." Using Supergen, the ribs were created using plate/shell elements, decoded, and added to the original design. The model then passed the analysis. "No other tests were done to compare the results to real world cases. There has been no need for this, because there were never any failures," said Mr. Lemeshev.

About Algor

Algor's COMBSST function has changed the way Mr. Lemeshev designs disk butterfly valves. He said, "Here at Zimmermann & Jansen we have totally realigned our calculation stage of valve design. Now, a specific amount of time is allocated for the finite element analysis of each valve. Also, as I become more proficient in the use of Algor software, the time required for this stage of calculations is dropping."

"The best thing about Algor software is the modeling capabilities. The automatic meshing is unparalleled. It only requires that I do a bit of planning in the frame creation stage. But this becomes easier and easier with each successive model."

Putting the Pieces Together

The disk butterfly valve was created by Michael Lemeshev at Zimmermann & Jansen in steps. This picture shows the disk in Superview.

In this picture, the shaft and the block are shown.

Here, the rib, which was added to make the disk stronger, is visible.

This last picture shows the results of the final stress analysis.