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Cracks of significant size were found in two chemical process pressure vessels after around 100 000-h service. The finding warranted fracture mechanics evaluation of further service fitness. This was done in five steps: fracture toughness measurements including determination of aging effects, loading limit identification including estimation of further aging and hydrogen effects, stress-intensity factor calculations, material and cracking mechanism investigation, and recommendations of precautions. The principal reason for crack formation was found to have been hydrogen attack due to local insufficient alloying in repair welds. Sufficient safety margins for further service were provided by a combination of precautions during startup and shutdown operations, low-working stresses, and sufficient fracture toughness as well as a detailed service record. Recommendations for further precautions included more frequent and intensified recurrent inspection, inert gas pressurization, and limitation of process pressures at ambient and lower temperatures during startups and shutdowns.
fracture mechanics, fitness for purpose, high temperature, hydrogen attack, pressure vessels
Senior research officer, Technical Research Centre of Findland (VTT), Espoo,