At the moment I’m graduating in the Dutch rules for Pressure Vessels. After calculating all the wall thicknesses of shell and dished covers I thought it would be nice to compare this with the ASME Boiler and Pressure Vessel Code.

I only looked at the wall thickness of the shell with a joint efficiency of 1. The biggest different I found was in the allowable stresses.

Does anybody now way the ASME code has such a low allowable stress at low temperatures?
For Example: till 500 °F the allowable stresses are the same for material SA 516-65. After that, the allowable stresses are calculated by the yield strength and the difference with the Dutch rules is negligible. How did the make these rules earlier? Why did they choose for such a low allowable stress for low temperatures? How is the formula Tensile strength divided by 4, nowadays 3.5, (ASME II Part D page 692) brought about?

I hope someone can help me figure this out, or can say where I can find the answer!
If anybody has general information about designing pressure vessels, please feel free to contact me!