Pipe Thickness and ASME 31.3

The provision you're referring to is for thick wall pipe and says (in words) that if the nominal wall thickness is greater than 1/6 the OD you need to use a modified value for the Y factor and pay particular attention to fatigue, thermal stress and the actual failure criterion.Thick pipe is considered to be pipe for which the thickness is greater than one-sixth the OD (t > D/ 6). The condition relates to all pipe for which t > D/6 not just equal to D/6. The definitions for Table 304.1.1 shows that you use a modified value to determine Y. That's because the variation of stress through the wall of the pipe is different for thicker pipe. if you'll look at the stress relationship for thick wall cylinders you find that the departure from thin wall behavior becomes numerically significant for T > OD/6.

The prescribed limits are for thin wall (membrane) equation where we considered (with an approximation) the cross-sectional area of the pipe metal as (Π D t), where the actual cross-sectional area must be (Π/4)(D_o² - D_i²). Therefore, for thick wall (mean higher pressures) we have to use what we call Lame's equation, and this makes the longitudinal pressure stress equation equal to P D_i² (D_o² - D_i²).

● In reality, the hoop pressure stress varies thru the thickness of the pipe. This stress variation is defined by Lame's equations which apply equally for thin and thick walled tubes as per European preference.

● By using thin wall equation as per American codes, the hoop stress is considered the same thru the thickness of pipe wall and equal to PD/2t.

While by using thick wall equation, the hoop stress inside the pipe wall is equal to P(D_o²+D_i²)/(D_o² - D_i²), while the hoop stress outside the pipe wall is 2P D_i²/(D_o² - D_i²), which is preferred to be used as per European codes to be used for both thin or thick wall. For thick wall, the Americans have another equations differ from that equations used for thin wall.

Conclusion. For any calculation per ASME B31.3, you have to apply the rules of that code that satisfy the code conditions of applying the equations of membrane stress:

1. Check that D/6 > t_min (can be checked after calculating t_min)

2. Check that P/SE < 0.385

If the above conditions were not satisfied, you have to use -as per ASME code- the equations for thick wall (high pressure).