Susceptibility
Definition
The degree to which an equipment item is vulnerable to a specific damage mechanism, based on material of construction, process conditions (temperature, pressure, chemistry), design features, and operating history. Susceptibility is typically rated as None, Low, Medium, or High, and drives the selection of damage factors and inspection techniques in RBI assessments.
Context
Susceptibility assessment is the first step in determining which damage mechanisms are active on a specific piece of equipment. It requires knowledge of the equipment's material, the process environment, operating conditions, and any deviations from normal operation. API 571 provides the technical basis for susceptibility determination.
Examples
- Austenitic stainless steel piping operating at 80C with chloride-containing insulation has HIGH susceptibility to chloride stress corrosion cracking.
- Carbon steel operating below 204C (400F) in clean hydrocarbon service has LOW susceptibility to high-temperature hydrogen attack (HTHA).
Applicable Standards
- API 571
- API 581
Related Terms
A specific process or phenomenon that causes degradation of material properties, leading to eventual failure of equipment. Damage mechanisms include corrosion (uniform, localized, galvanic), erosion, fatigue, creep, hydrogen damage, stress corrosion cracking, embrittlement, and other metallurgical or environmental attack modes. API 571 catalogues over 60 damage mechanisms relevant to refining and petrochemical equipment.
Damage FactorA numerical modifier in the API 581 RBI methodology that quantifies the increased probability of failure due to specific active damage mechanisms. Damage factors account for the type, rate, and extent of degradation relative to the equipment's design margins. Higher damage factors indicate more aggressive degradation and higher probability of failure. Total damage factor is the sum of individual damage factors for all active mechanisms.
API 571Damage Mechanisms Affecting Fixed Equipment in the Refining Industry — a comprehensive catalogue of over 60 damage mechanisms that can affect refinery and petrochemical equipment. Each mechanism is described with its appearance, critical factors, affected materials, prevention methods, and inspection techniques.
API 581Risk-Based Inspection Technology — provides the quantitative methodology for calculating probability of failure (PoF) and consequence of failure (CoF) for pressurized equipment. It includes detailed procedures for determining damage factor, management system factor, generic failure frequency, and consequence areas for toxic, flammable, and environmental releases.
Where this appears in Reliatic
This concept is used in the FMEA & Risk module.