Div. 2, fatigue analysis and exemption per 5.5.2 1

[jtseng123]

Dear all,

A vessel is exempted from fatigue analysis per Div.2, para 5.5.2.2, however, it’s cycles is greater than 10^6. Shall the fatigue analysis is required per 5.5.2.1 (c) ?

We are buying a new air filter duplicated from the past one which has been operating for more than 20 years without problem and without any fatigue analysis. It is a small vessel with legs supported and re-pad will be used to exactly duplicated form the old one. Only pressure fluctuation between 0 -100 psig in minutes at ambient temperature that the total cycles will exceed 10^6. We got stuck whether we shall spending money asking vendor to do fatigue analysis per 5.5.2.1 (c) even vendor stated they have never done any fatigue analysis or designed for cyclic service on such equipment and the equipment will be fine. This is an US vendor.

Can any one help to interpret the code between the two paragraphs ? Thanks.

 

[MrPDes]

Paragraph 5.5.2.2 for exemption due to experience says "the possible harmful effects of the following design features shall be evaluated" which include pipe threaded connections.

Does this mean a tapper threaded 3000lb NPT coupling fitting requires a detailed fatigue analysis, even if the cyclic pressure range is less than 15% of the rated pressure (Exemption per Step 3 of 5.5.2.3)?

With the complex thread tapper, even a university Phd project would not be able to establish the fatigue life using a FEM or other mathematical analysis method.

The only way to find the fatigue life of such a joint is through "experience" in accordance with Paragraph 5.5.2.2.

Where Paragraph 5.5.2.2 says "shall be evaluated", it doesn't explicitly state that a "full fatigue analysis" be conducted? Even many of the most skilled design consultancies would not know how to conduct an accurate fatigue analysis of this complex component.