Avoid Costly Design Mistakes
Common errors keep plants from getting the most reliable and suitable vessels
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9. Cyclic service. If a vessel will experience an unusual number of thermal or pressure cycles over its design life, this could result in premature fatigue failure (usually at a weld) unless preemptive measures are taken. Fatigue is cumulative material damage that manifests as a small crack and progressively worsens (sometimes to failure) as the material is repeatedly cycled. A 1985 survey showed that fatigue was the second most prevalent cause of failure in industry (25%), closely behind corrosion (29%); in the airline industry, it predominates (61%) [4].
It’s up to the purchaser to instruct the fabricator what design/fabrication practices to follow to avoid fatigue. Cyclic service is usually associated with batch processes and ASME [5] provides the following rules:
Design for fatigue if N1 + N2 + N3 + N4 ≥400 for non-integral (fillet weld) construction and ≥1,000 for integral construction (i.e., no load-bearing fillet welds), or 60 and 350, respectively, in the knuckle region of formed heads, where N1 is the number of full startup/shutdown cycles; N2 is the number of cycles where pressure swings 15% (non-integral) or 20% (integral); N3 is the number of thermal cycles with a temperature differential (ΔT) exceeding 50°F between two adjacent points no more than 2.5 (Rt)
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