"Y" coefficient fot Thickness calculation for B31.3

[Stileman]

Please, can anyone let me understand the meaning of the Y factor in ASME B31.3 para 304.1.1?
I mean, for "Y" I got two choices, a choice from table 304.1.1 (t < D/6) or by a formula (t >= D/6).

But, how can I know if "t" is < or > than D/6 if I still have to calculate "t"?
And then, in the formula 3b, I got "d" internal diameter...how can I know the value of "d" if I don't know the value of "t"??

I really won't sleep tonight, it's driving me crazy!!
Thank you all!!
E.

 

[JohnGP]

Stileman,

You seem to be trying to make life difficult for yourself. With standard pipes you are hard pressed finding thicknesses that are >D/6 except in the max thicknesses in the smaller diameters. Start off with the value from the chart, and see where you end up. Make adjustment if necessary.

For calculating "t", use formula (3a) which is based on outside pipe diameter. If you are going to use standard pipe then the outside diameter is constant for a given pipe nominal size, regardless of thickness.

Am I missing something here?

John