RE: Modeling Hygienic Unions

From: <Ken>
Date: Thu May 20 2004 - 17:26:00 EDT

Chris,

My apologies, my initial posting was not sufficiently clear. I am performing a pipe stress analysis, not a FEA analysis. I do not have to design the union, they are already manufactured. The question is one of applying the unions and demonstrating compliance with the code.

Ken A. Nisly-Nagele, P.E.
Project Engineer, Mechanical
Applied Engineering Services, Inc.
7999 Knue Road
Indianapolis, IN 46250
317-585-8920

-----Original Message-----
From: Christopher Wright [mailto:chrisw@skypoint.com] Sent: Thursday, May 20, 2004 2:56 PM
To: ‡
Subject: Re: [PipingDesign] Modeling Hygienic Unions

>I want to model hygienic unions (tri-clamp type, bolted clamp joint) in a
>stress analysis program (Algor) during the line sterilization process (185
>deg. F). Code is ASME BPE & B31.3. The system is mostly 4 and 6 inch size
>stainless steel tubing.

Use ASME Code Section VIII Div 1 Appendix 24 to generate your design. If you must use Algor, do so, but there are better FEA programs. The basic clamp design rules should be taken in compliance with Appendix 24. It takes a little while to get used to the rules, because the arithmetic is unnecessarily complicated, but it's doable. The general complaint about the rules is that the resulting clamps are over-bolted, but this is manageable with effort.

>So, there are obviously some loads which the unions will accept
>without leakage.

Don't be terribly surprised if you can't find such loads with FEA. You're looking at a pretty thoroughly non-linear analysis between bolting preload and gasket material non-linearity. If you can use an O-ring seal instead of a gasket you can probably make a better guess at a solution, but you have to look very carefully at O-ring behavior. If you can predict, reliably, the loads that will cause flange separation at the O-ring groove to the extent that the O-ring extrudes, you'll have a pretty good model. But because of the real world effects, particularly friction and contact behavior, the design model differs a bit from real world performance. You may find that your perfect solution is too perfect to be practicable.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw@skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)

http://www.skypoint.com/~chrisw

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