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Design Calculations in US Customary and SI units

Design Calculations in US Customary and SI units

Design Calculations in US Customary and SI units

Here is an interesing issue that's tripped up a lot of engineers over the past 2 years.

With the publication of the 2004 ASME Code Edition a new "Metric Edition" of Section II Part D was released. This edition stipulates the allowable stresses to use when designs are calculated using SI units.

With this new edition it may no longer be practical to provide design calculations in both US and SI units. In fact, in some cases it is possible to have the design "fail" in one set of units even though it passes (marginally) in the other. This is due to the fact that the allowable stresses have been rounded-off and are not direct mathematical conversions from US to SI. Also, the metric "hard" constants included in Section VIII are often not direct mathematical conversions from US to SI.
Code rules require that the SI Edition of Section II Part D be used for allowable stresses when performing calculation in SI units. Section VIII Division 1 also contains numerous numerical constants for dimensions, temperatures, etc; the numerical values in SI units (contained in parentheses in the Code) may not be an "exact" conversion of the Imperial units, see paragraph U-4. Also, Section VIII Division 1 Nonmandatory Appendix GG, paragraph GG-2(d) describes how the round-off between U.S. Customary and SI Editions is performed.
Because of these Code rules and the differences in the numerical constants it is possible that some designs may "pass" in one system of units yet "fail" in the other. This will occur if some given criteria is marginal. Thus it is no longer possible or practical for designers to provide calculations for same vessel in both systems of units.

For example, UG-20(f)(1)(b) exempts some materials through 1" thick from impact testing to -20° F MDMT. But in SI units the exemption is limited to 25 mm. If you use 1" plate you may lose this exemption if the calculations are performed in SI units.

Similarly, due to the difference in allowable stress values between US and SI due to the round-off some designs made in one set of units may not pass muster when converted to the other units.

Be cautious when asked to provide such dual calculations. It's likely they are no longer practical.

Tom Barsh
Codeware Technical Support

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