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# App 2 Flange "Required Thickness"

## App 2 Flange "Required Thickness"

(OP)
Some COMPRESS users have experienced "painful discussions" with their own customers regarding required thickness of flanges designed to ASME Section VIII Div 1, Appendix 2. Usually the situation is something like this: the required thickness determined by COMPRESS is "inconveniently large" for the final customer and they are unwilling to accept the situation. Perhaps they have experience with similar designs in the past and are basing their comments on this (being unaware of any Code changes), or they may just not be technically inclined.

Often they are looking for a single formula to determine "the thickness". Or perhaps COMPRESS users are looking for "the formula" to determine the required thickness. Unfortunately, it is not so simple as there being a single formula for required thickness of the flange.

As we know, there is no one single, simple algebraic formula available in the Code that says "the required thickness of the flange equals..."

The flange analysis is complex and rather than providing a formula for required thickness the ASME Code requires calculation of the following values:

1- Calculated stress values SH, ST, and SR, as well as combined stress values based on these stresses, for the operating condition must be no greater than the corresponding allowable stresses.

2- Calculated stress values SH, ST, and SR, as well as combined stress values based on these stresses, for the gasket seating condition must be no greater than the corresponding allowable stresses.

3- The flange thickness must also permit the flange rigidity requirements of Appendix 2-14 to be met.

In the end, there are no less than 11 different criteria that the flange thickness must meet. It is not possible to determine mathematically a single formula to define the required thickness. The situation is similar to that for designing a cylinder for external pressure:  There is no formula provided (or available elsewhere) for required thickness of the shell. The design approach indicated in the ASME Code is to begin with an assumed value for thickness, perform the vacuum calculations, then check the result and adjust the thickness if desired to reduce or increase if necessary. The design of a flange follows a similar approach: Begin with an assumed thickness, then check the Code criteria.

COMPRESS automatically determines the thickness that is just sufficient to meet all applicable requirements of Appendix 2. This is done by performing the calculations internally numerous times, with special techniques, to find the "magic thickness" to a desired level of precision. The proof that this value is the "required thickness" can be obtained by entering this value as the nominal thickness and then inspecting the calculations: review the calculated stresses versus the allowable stresses, and review the flange rigidity calculation.

Note that the flange design situation with a bolted cover is relatively simple. If there are two mating flanges the analysis becomes more complicated because the two flanges share the same bolt and bolt load. In some cases the design of one flange is dependent on the mating flange.

The situation in heat exchangers is even much more complex. The tubesheet located between the flanges permits use of different gasket properties and dimensions. In this case the flanges are subjected to different gasket forces but are still linked by the bolt force, this can lead to very complex analyses.

"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support
www.codeware.com

Replies continue below

### RE: App 2 Flange "Required Thickness"

(OP)
A related point on why there may be a difference of opinion on required thickness of a flange:  If the flange is a slip-on type then Code Interpretation VIII-1-83-115 may or may not have been applied.

The Interpretation indicates the values that should be applied for g0, g1, and h. The application or not of the interpretation can make a substantial difference in the flange thickness. Personally, I would always apply the interpretation (and obtain a greater flange thickness as a result).

"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support
www.codeware.com

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