Calculate Fatigue Life of NPT Thread

[MrPDes]

I have thermowell that fastens into a blind flange using a 1/2" NPT tapered thread. There is only pressure cycles. No external loads.

The vessel is designed in accordance with ASME VIII Div 1 and the fatigue assessment in accordance with Div 2.

I am trying to find the most labour efficient way of assessing the fatigue life of the thread.

I would like to use "Fatigue screening based on experience" in accordance with Paragraph 5.5.2.2 of ASME VIII Div 2. I would have thought there would be some established industry guidelines that provide a fatigue life based on industry experience for such a common component. However there doesn't appear to be any such guidance.

All I can find is Paragraph 314.2.2(c) of ASME B31.3 allows the use of this thread and application for severe cyclic service and ASME PVHO recommends that 6000lb NPT socket fittings be welded into vessel shells instead of 3000lb.

Conducting an extensive FEM analysis on a perfect ideal thread may be great for an academic project, however in practice there is so much fabrication variability of the thread of two supposedly identical fittings. FEM analysis seems like allot of work for an approximate result that needs a fatigue penalty factor of 4.

Does anyone have any advise for the best way of assessing fatigue in NPT thread?

 

[Josep Maria Perdigo]

The German standard AD Merkblätt S1 (simplified analysis for cyclic loading) provides the study of bolts for flange connections. It is interesting to note that this analysis is only necessary when the screws are disassembled and mounted frequently. The stress concentration factor is 5.