I have received a calculation note for ball valve from our supplier and the calculation for body wall thickness was made according to ASME B16.34 and also with ASMEVIII Part I.

Please can you tell me if ASME VIII Part 1 is applicable for valves calculation since this standard is intended to Pressure Vessel construction.

I think that ASME B16.34 conditions are sufficient to size valves. Please correct me if I am wrong.

You're not wrong.  Sect VIII Div. 1 are design 'rules' for unfired pressure vessels.  Period.  The calc's in VIII-1 are simplified, because the rules keep your design safe.  If you want to design-by-calculations a valve body, you need a Finite Element Analysis with a very small grid-square size.  Huge set of calculations.  

Just follow B16.34.  Like VIII-1, it is Rules & Experience driven.

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