Inspectioneering Journal

99 Diseases of Pressure Equipment: Stress Raisers

By John Reynolds, Principal Consultant at Intertek. This article appears in the January/February 2004 issue of Inspectioneering Journal.
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Speaking of stress raisers, they are another insidious type of flaw that can and do lead to equipment failures. Stress raisers (aka stress intensification sites) can be mechanical or metallurgical notches. Undercutting, physical weld flaws, mismatched thicknesses, and sharp geometric intersections can all become stress raisers. But so too can so-called "metallurgical notches" like one finds at the edge of a weld where the cast structure of the weld pool meets the wrought structure of the heat-affected zone. Equipment in many static services without significant cyclic stresses can tolerate the presence of some stress raisers because of hefty code design margins. But equipment in services with significant fatigue stresses (thermal and mechanical) are especially vulnerable to such notches and deserve special QA/QC attention during design, specification, fabrication, and repairs.

Pressure swing absorber vessels are sensitive to many types of notches and should be fabricated with a minimum of stress raisers, including flush grinding longitudinal and circumferential seams, better than average peaking control, and blending of the toes on any filet/ nozzle welds. Dearators which are prone to corrosion fatigue are another class of vessels that deserve special attention to avoid stress raisers. Vessels and piping in services that give rise to thermal fatigue also deserve special QA/QC to avoid stress raisers that might initiate cracks from thermal cycling. There is also a commonly used variety of integrally reinforced seton nozzles that require attention by the engineer, inspector and welder to make sure that the manufacturer's instructions for weld profiles are carefully followed, otherwise the weld profile combined with the section thickness mismatch may give rise to fatigue cracking years later when it's not even obvious that fatigue stresses are present.

Do all the people involved in fabrication and repair of your equipment in cyclic services know how important it is to avoid stress raisers in order to provide long term reliability and integrity?

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