Comprehensive damage mechanism library aligned with API 571, covering identification, susceptibility assessment, and integration with RBI and inspection planning.
API 571 (Damage Mechanisms Affecting Fixed Equipment in the Refining Industry) is the authoritative reference for identifying and characterizing damage mechanisms in hydrocarbon processing equipment. Reliatic maintains a comprehensive damage mechanism library based on API 571 Fourth Edition, covering over 60 distinct mechanism types. Each entry in the library includes: a description of the mechanism and its metallurgical basis, the materials and service conditions susceptible to the mechanism, typical morphology and appearance (to aid in inspection identification), critical factors affecting the rate and severity of the mechanism, and recommended inspection and monitoring techniques. The library is not a static reference — it is actively used by the RBI engine to determine which damage mechanisms are credible for each asset based on its material of construction, operating environment, and service history.
Reliatic organizes damage mechanisms into four primary categories, each with distinct inspection and management implications. General Thinning: Uniform material loss across a broad area. This category includes general corrosion, oxidation, erosion, and flow-accelerated corrosion (FAC). General thinning is the most predictable category because it progresses at a roughly uniform rate that can be measured and trended. It is the primary target of thickness monitoring programs. Examples: carbon dioxide corrosion, naphthenic acid corrosion, sulfidation, and atmospheric corrosion. Localized Thinning: Material loss concentrated in specific areas, often related to flow patterns, geometry, or local chemistry variations. This category includes pitting corrosion, crevice corrosion, microbiologically-influenced corrosion (MIC), and under-deposit corrosion. Localized thinning is harder to detect than general thinning because spot readings may miss the worst areas. Higher inspection coverage (grid scanning rather than spot measurements) is typically required. Cracking: The formation and propagation of cracks due to mechanical stress, environmental exposure, or a combination. This category includes stress corrosion cracking (SCC), hydrogen-induced cracking (HIC), sulfide stress cracking (SSC), fatigue cracking, and creep cracking. Cracking mechanisms are often the most dangerous because they can progress rapidly to failure without significant material loss, making them invisible to thickness monitoring. Specialized NDE methods (TOFD, PAUT, ACFM) are required. High-Temperature Mechanisms: Damage that occurs specifically at elevated temperatures, including high-temperature hydrogen attack (HTHA), carburization, graphitization, temper embrittlement, sigma phase embrittlement, and creep. These mechanisms can alter the material's microstructure without visible surface indications, requiring advanced inspection techniques like in-situ metallography or backscatter hardness testing.
The effectiveness of an inspection is determined by whether the inspection method can reliably detect the active damage mechanism. Reliatic maintains a mapping table that connects each damage mechanism to the NDE methods capable of detecting it, along with the expected detection effectiveness level (A through E per API 581). For general thinning: ultrasonic thickness (UT) scanning at a grid density of 50mm or less achieves A-level effectiveness. Spot UT at designated thickness monitoring locations (TMLs) achieves B or C-level depending on the number and placement of locations. For pitting and localized thinning: radiographic testing (RT) or automated UT C-scan provides A-level effectiveness. Manual UT spot readings provide only D or E-level effectiveness because they are unlikely to find the deepest pits. For stress corrosion cracking: phased array UT (PAUT) or time-of-flight diffraction (TOFD) provides A-level effectiveness for detecting and sizing cracks. Wet fluorescent magnetic particle inspection (WFMPI) provides B-level effectiveness for surface-breaking cracks. Conventional UT provides C-level effectiveness. For high-temperature hydrogen attack: advanced backscatter UT techniques or velocity ratio methods provide the only reliable detection, achieving B-level effectiveness. Standard UT is largely ineffective (E-level). This mapping ensures that when the RBI engine recommends an inspection, the resulting work scope specifies an NDE method that will actually be effective against the identified threat.
Not every damage mechanism is credible for every asset. Reliatic assesses susceptibility based on three factor groups. Material Factors: The material of construction is the primary determinant of susceptibility. Carbon steel is susceptible to general corrosion, H2S cracking, and caustic cracking but not to chloride SCC (which affects austenitic stainless steels). Alloy composition, heat treatment condition, hardness, and grain structure all influence susceptibility. The platform includes a material-mechanism susceptibility matrix covering carbon steel, low-alloy steels, austenitic and duplex stainless steels, nickel alloys, and copper alloys. Environmental Factors: The process fluid composition, temperature, pressure, and the presence of specific corrodents (H2S, CO2, chlorides, caustic, acids) determine which mechanisms can be active. Reliatic evaluates environmental susceptibility using process data sheets and operating condition envelopes. Changes in operating conditions — tracked through the Management of Change module — trigger automatic reassessment of damage mechanism susceptibility. Operating History: Historical inspection data, failure records, and industry experience with similar equipment in similar service provide empirical evidence of which mechanisms are active. An asset with documented pitting in prior inspections has confirmed susceptibility that overrides theoretical assessments. The susceptibility assessment produces a credible damage mechanism list for each asset, which feeds directly into the RBI probability of failure calculation as damage factor inputs.
Damage mechanisms are the primary driver of the Probability of Failure (PoF) in the RBI calculation. Each credible damage mechanism contributes a damage factor to the total PoF. The damage factor for a given mechanism depends on: the susceptibility level (high, medium, low), the time in service (or number of cycles for fatigue mechanisms), and the number and effectiveness of inspections targeting that specific mechanism. Reliatic aggregates damage factors across all credible mechanisms to compute the total damage factor per API 581. This means that an asset with three credible damage mechanisms (e.g., general thinning, pitting, and external corrosion) will have a higher PoF than an identical asset with only one credible mechanism, even if the individual mechanism severities are the same. The damage mechanism assessment is the technical foundation of the entire RBI program — without accurate damage mechanism identification, the PoF calculation cannot produce meaningful results. This is why Reliatic requires damage mechanism assessment as a mandatory step in the RBI workflow, and why changes to operating conditions or materials trigger reassessment through the MOC process.