ASME B31.3 Wall Thickness - Y Coefficient Clarification

[ColossalCrumpet]

Morning everyone. I understand this section of B31.3 relatively well. The only thing I'm having trouble wrapping my head around is the whole Y coefficient aspect! We're trying to calculate wall thickness, so how on earth do we figure out if t < D/6? Am I right in just assuming a schedule from B36.10 and going from there, or is this incorrect? Mostly, the Y coefficient is obtained from the table, I've not yet come across a situation where I've had to calculate Y.

I'm just having a really hard time getting myself out of this confusion. It seems strange to assume wall thickness to determine Y Coefficient when we're actively trying to determine wall thickness via this whole calculation. I hope this makes sense and someone can clear it up for me. Thanks!

 

[david339933]

You got it. Pick a t and go with it. See how it plays out and change it if required.

 

[LittleInch]

I think you're confusing yourself.

Just use the Y coefficient from the table and input all the other figures to get t.

If t is > D/6 then start again to get t calculating the Y factor.

So you don't assume anything. You calculate t, check to see if it is < D/6 and if so then you find the nearest standard schedule you can from B36.10.

If t> D/6 then start a new calculation.

Does that make sense?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ione]

The formulas reported in the standard are for thin walled cylinders. For thick cylinders you have another approach

 

[Krausen]

The Y coefficient comes into play if you ever transition from "thin shell theory" to "thick shell theory" I believe. It can be an iterative approach in design sometimes. If you end up with a "thick shell" wall thickness in design (t > D/6), not accounting for the Y coefficient will get you into trouble, as your estimation of "thin shell theory" is no longer valid & the Y coefficient must be accounted for to correct it. Stresses under "thin shell theory" vs "thick shell theory" are not equivalent. Typically most piping designs fall under "thin shell theory", which simplifies the matter