Fatigue Investigation of Flawed Welded Joints in Steam Drum

Introduction: A waste heat boiler is used to utilize extreme flue gas temperature to heat contained water. One part of it is the investigated steam drum that, during exploitation, sustained fatigue cracking in the welded joints. The repair strategy encompassed the welding of strips around the affected joints until a replacement vessel was produced. During this time, for safety reasons and legislation compliance, all the flaws’ effects on the cylindrical structure were evaluated.

Method: The steam drum was manufactured from P265GH steel and had been in operation for five years when, during a routine inspection, cracks were found in the circumferential welded joints of the two hemispherical head sections and the shell. The boiler-grade carbon steel welded joints of approximately 20 m in lengthunderwent complete radiographic testing (RT), magnetic particle testing (MT), and in situ metallography for microstructure evaluation and visual examination. Through non-destructive evaluation and surface replication, data was gathered for performing fitness-for-service evaluation as per API 579-1/ASME FFS-1.

Results: On-site microstructure evaluation and crack characterization has been used for the steam drum. The maximal length of the cracks found was 50 mm, measured through RT. Flaws were located in the heat-affected zone of longitudinal weld seams and were parallel to the weld joint. The depth of the flows is detected by MT with a maximal value of 5 mm below the outer surface. Level 1 assessment proved that the flaws are stable and therefore acceptable, and so it is safe to repair and continue operation.

Conclusion: Due to the fatigue nature of the existing cracks in the steam drum, monitoring has been carried out with an assessment of the anticipated crack growth. As per the results and remaining life calculation, it is found that the vessel is safe to be used.

Funding: The author acknowledges support from project BG16RFPR002-1.014-0005.