PSA 9.7 — FIV and AIV Screening: The Energy Institute Method, and Reading Somebody Else's Study
A relief valve lets a gas stream down into a 24″ flare header. The header is new, correctly supported, stress-qualified, and it has never visibly moved.
Eleven days after start-up, the 2″ drain branch four metres downstream of the tie-in cracks circumferentially at the weld toe. Nobody heard anything unusual. Nobody saw the pipe swing. The header itself is untouched.
Commit to an answer before reading on: what fatigued that weld in eleven days?
Not a beam mode. The header's first bending frequency is a few hertz; eleven days at 5 Hz is about five million cycles, which is not…
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