Practical Experience in the Early Detection and Assessment of Vessels with Htha Degradation

High Temperature Hydrogen Attack (HTHA) is a damage mechanism with a significant impact in the refinery industry, internationally. The mechanism results in gradual and relatively slow degradation of the vessel material during the incubation time period, but more rapid degradation thereafter. The paper gives an overview of this damage mechanism in the context of inspection surveys to identify degraded areas, and engineering assessments to assess fitness for continued service. The basic mechanism of HTHA is described, with reference to the influence of temperatures and pressures. The various forms of attack, as well as the process of HTHA degradation, and the influence of a number of factors, are described. The detection of attack using advanced NDT and risk-based approaches are described. Available inspection techniques for HTHA detection and quantification are reviewed. Specific attention is given to Advanced Backscatter, Velocity Ratio, and Metallurgical Replication and Fast Fourier Transformation techniques. Typical reporting formats are presented. Practical examples regarding the mapping of the survey results for a specific vessel are given. The Fitness For Service (FfS) and Remaining Life assessment of equipment with identified damage, is discussed in the context of the API RP 579. The alteration and rerating of equipment that has suffered from HTHA are considered when FFS of HTHA equipment, is discussed