ALGOR SOFTWARE HELPS TO DESIGN TANKS THAT MEET FEDERAL CODES

When the federal government stiffens safety or environmental regulations, it provides better protection for the public, but it also presents new challenges to industry.

A case in point is when the federal government toughened their regulations for the structural integrity of cargo tank vehicles that transport hazardous liquids and gases. Contracting Associates, Incorporated (CAI), Lubbock, Texas faced the problem of deciding if its designs for these tanks met the new stricter regulations.

CAI engineers turned to Algor software to analyze and compare these designs with the new requirements. The principal business of this consulting engineering firm is analyzing designs for various manufacturing and repair facilities that specialize in building or refurbishing hazardous cargo tanks. CAI also produces new designs for custom-built cargo tanks dedicated to a specific service.

Federal regulations mandate that the maximum principal stress in a cargo tank not exceed one-fourth of the material strength used in construction of the pressure vessel. Due to a bureaucratic oversight more than 25 years ago, the federal regulations have not included the total effect of pressure in the definition of maximum principal stress.

Although cargo tanks have been required to meet ASME pressure vessel code requirements where applicable, the regulations ignored pressure "Hoop" stress in combining all the stresses produced by acceleration, braking, overturn and unsupported beam loading to calculate maximum principal stress. Therefore, when the federal government first announced their intention to include hoop stresses in their revised definition of maximum principal stress (also called "effective stress"), CAI engineers immediately began a process of re-evaluating all designs relating to cargo tanks, including designs that had been standard for forty years.

More Capabilities than Competitors

"We purchased the Algor product after comparing it with another comparably priced product. Other FEA mechanical engineering software limited us to 500 nodes for our models, but we are designing enormous vessels with thousands of nodes. I have used our Algor software for models with 20,000 nodes and more," added Loren Kirkes, engineer at CAI. "The fact that I could run the Algor software on personal computers or UNIX systems was also a definite factor in choosing Algor.

"Because the Algor software already included hoop stress in the equation for maximum principal stress, we quickly and inexpensively figured out that our old design, while in compliance with current regulations, would not comply with the new federal codes," said Kirkes.

Once the Algor software told the CAI engineers that they needed to make changes in their design, CAI used Algor to help redesign and engineer the vessel structures to bring them up to the new code.

One significant change CAI made was in the design of the side cradle. The side cradle distributes loads smoothly in the vessel to avoid high concentrations of stress that could lead to excessive strain. Through much development work, which once again relied heavily on Algor software, the CAI engineers tapered off the side cradles at the ends.

A comparison of the following figures shows the physical differences in design between the old and new side cradles.

Old design of the side cradles, where they are curved at the end.

New design which features a side cradle that tapers off at the end. This tapering reduces stress concentrations in the vessel.

"Dealing with federal codes isn't necessarily a simple task, but having access to the Algor software made it much easier to meet the new code requirements, since Algor already included the necessary tools to analyze the new requirements," said Kirkes. CAI also designs pressure vessels, DOT tank trailers and shipping containers for both large companies and common carriers. Because these tanks often contain volatile materials, the federal government often revises its regulations. CAI must constantly update designs to meet these changing federal codes.

"Algor, frequently updates our software to reflect advances in computational mechanics and the physics of phenomena, such as the importance of hoop stress," said Michael L. Bussler, president and founder of Algor, Inc., which develops and sells software for mechanical engineering analysis and design optimization. To apply basic research to its software development, Algor recently gave a grant of one million dollars to Carnegie Mellon's Computational Mechanics Laboratory for a five-year research project to produce more accurate and powerful numerical algorithms for engineering simulation and mechanical design optimization software.