Avoid Costly Design Mistakes

Common errors keep plants from getting the most reliable and suitable vessels

By Chip Eskridge, Jacobs, Mike James, DuPont, and Steve Zoller, Enerfab

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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)½ apart (where R is inside radius of vessel and t is thickness of vessel under consideration) —apply a two-times factor if ΔT exceeds 100°F, a four times factor if more than 150°F, and see Div. 2 for more than 250°F; and N4 is the number of thermal cycles for welds attaching dissimilar materials in which (α1-α2)ΔT (where α is the thermal expansion coefficient) exceeds 0.00034, or for carbon steel welded to stainless steel, the number of cycles where 2ΔT exceeds 340.

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