PetroStreet

How can we calculate the pressure design thickness as per ASME B31.3 when t comes out to be greater than D/6?
It states that you may have to consider the failure theory, fatigue and thermal stresses.
Please advise.

You can use the same formula as given for t < D/6 but yes in addition to this the requirements given for consideration of theory of failure, fatigue and thermal stresses must be followed by the designer.
Alternatively, you can use Chapter IX also for determining the pressure design thickness.

Thanks Irish. Have you any reference where I can find this?

I remember an interpretation to ASME B31.3 where this was mentioned.

t greater than D/6 clearly implies that either the pressure is so high or the allowable stress is much low at design temperature, therefore you have to consider what ASME B31.3 is asking for.
And I agree with irish, an interpretation no. 15 to ASME B31.3 confirms it.