PSA 8.5 — High-Pressure Piping: B31.3 Chapter IX, Thick Walls and Bore Fatigue
A 3″ interstage line on a reciprocating gas intensifier, 80 MPa (800 barg) design, forged low-alloy steel, 15 mm wall. The stress analysis is done properly: pressure design to B31.3, sustained 44 % of Sh, expansion 58 % of S_A, nozzle loads inside the machine vendor's envelope. Everything green, everything signed.
Four years later a through-wall crack is found at a branch connection. The crack initiated at the bore , not the outside surface, and it has beach marks — it is a fatigue crack. Nothing in the stress report predicted it.
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