I am some what confused about the unit coversion issue. as per latest ASME SEC VIII DIV 1 mixed units can be used.
consider a situation where pressure, temperature, lenthg all are given in US system and the requirement is to claulate the thickness required for a cylinderical shell, when I use the formula using the US system of units the required thickness for plate turn out to be 1" exactly. Now let suppose all my vendors are providing me plate thickness in mm. now as per non-mandatory guidelines of appendix GG, refer to table in GG-2(e)which gives 1 inch converts to 25 mm. Now if we refer to GG-5, there we are a conversion factor of 25.4 which gives 1 inch = 25.4 mm.
Can some please elaborate which one is better to follow. Can we have problems with AI using the first approach having 1 inch = 25mm which actually gives a deficit of 0.4 mm. Can we do this calculation for shell thickness by converting all pressure, temperature, diameter etc to SI and then calcualte the thickness which will automatically be in SI system. Which approach is a better one.

Dear Farid.

If the calculated required thickness is exactly 1inch, and you carried that calculation using pressure, diameter etc in metric units, you will get a value very very close to 25.4mm. That is assuming that you used the exact engineering conversion factors.

This basic science. So, for the calculations PV Elite uses 4 or 5 significant digits for the conversion.

In my opinion the benefit the conversions given in Appendix GG (i.e. 25mm = 1 in) is to specify minimum code required thickness. For example, say UG-16(b) specifies 0.0625in, and if you convert that to mm you get 1.5875mm, but as this is not a std. metric size, the code modified it to 1.5mm. This is a practical consideration.

For the min. thickness or weld sizes, PV Elite selects the Imperial values or Metric values based on the unit system selected for the file. But, if you want to change that you can do so from the Tools menu -> Configuration -> Metric Constant Selection (bottom right side).

Thanks Mandeep