Tuesday, November 16, 2010

Pipe Analysis: Fluid Mechanics vs. "The Code"

I'm doing stress analysis on some cryogenic pipe now. And now I suddenly face "The Code", a reality I have until now not had to face.

Coming from a design engineer background, I initially approached the pipe with my copy of Roark's, and Engineering Fluid Mechanics. This approach results in stress values that are then compared, with safety factors, to the ultimate and yield strength of the material. If calculated values exceed either, the design has "failed", and must be beefed up.

The piping designers introduced me to Caesar II, an application that lets you model a piping system, and it gives the stresses, and compares the results to "Code", in this case, ASME B31.3. I did the obvious: test a simple case both ways and compare the results. It... didn't go well. Wildy different results.

After discussion, the conclusion is this: it's apples and oranges. Can't compare them. The Code is based on empirical data from years of field experience. The Code is complicated, non-intuitive. An expert is required to confirm that it is interpreted correctly. So, I must set aside my spreadsheet, MathCad, and books, and yield to our resident expert, the Caesar II application.

One important thing I got out of my failed attempt to actually read "The Code", which by the way is very lengthy and dry: Code covers pressures and pipe configurations. It does not cover momentum forces due to fluid flow, nor does it address the nature of the fluids being carried. You've got to do your homework on material comparability. You've also got to calculate the momentum forces, and add them into your Caesar model, so you'll still need MathCad some.

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