How to Select Minimum Thickness to be Provided from Reqd Thickness

Use the calculated thickness. Don't compromise the design.

Are you asking for a minimum thickness when the calculated thickness is unusually thin - say less than 1/8"?

Don't know anything about PV Elite, but we had a minimum 3/8" wall thickness on carbon steel pressure vessels, even if the calculated thickness (including corrosion allowance and mill tolerance) was less.

What we have here is a typical situation where the uninitiated doesn't understand the concept of common sense selection of materials.

Let's "design" something without taking into consideration what's available in the market.

In my particular case, I was reviewing a Calculation Report for Heat Exchanger:

Calculated thickness for a shell (which I got from Int pressure + Corrosion Allowance) is 7.5mm, but Nominal Thk was taken to be 12mm.

What all factors should be considered while assigning nominal Thk other than next available plate?

Some pressure vessels, such as horizontal oil separators on screw compressors, support equipment in addition to withstanding pressure. The additional load(s) may require greater thickness. Large distances between vessel supports may require considering the vessel like a beam.

Nominal Thk was taken to be 12mm by whom? I'd think minimum thickness would be next nominal thickness greater than calculated (plus corrosion allowance if applicable).

I don't know much about PV Elite, but I'd like to think design software would calculate minimum dished end finished thickness, and hence minimum original plate thickness, allowing for thinning. Then use next thicker than that.

...and remember to bounce the design off the Engineer/Surveyor for the company responsible for providing burst indemnity insurance cover for the vessel for approval prior to fabrication commencing.