Corrosion Circuit
Definition
A group of equipment items and piping segments that are exposed to the same process environment and are therefore expected to experience the same corrosion or degradation mechanisms at similar rates. Corrosion circuits (also called corrosion loops or integrity circuits) group equipment for efficient inspection planning — representative CMLs in the circuit can provide condition data applicable to all items in the circuit.
Guide
Context
Corrosion circuits are the organizational unit for inspection planning in static equipment integrity management. Grouping equipment by corrosion circuit ensures that inspection effort is allocated efficiently and that all equipment in a similar environment receives appropriate attention. A single corrosion circuit may contain a dozen piping segments but only require CMLs on the most vulnerable locations.
Examples
- Corrosion circuit CC-CU-01 (Crude Unit overhead): includes the overhead condenser, overhead accumulator, overhead piping from column top to accumulator, and reflux return piping. Active mechanisms: NH4Cl corrosion, CO2 corrosion.
- A new process chemical injection point creates a new corrosion sub-circuit downstream because the corrosion environment changes at the injection point.
Applicable Standards
- API 570
- API 581
- API 571
Related Terms
A broader grouping than a corrosion circuit that encompasses all equipment sharing common integrity management requirements, not limited to corrosion. Integrity circuits consider all active damage mechanisms, operating conditions, materials of construction, and inspection requirements to group equipment that can be managed under a unified inspection and maintenance strategy.
Condition Monitoring Location(CML)A designated, permanently marked point on equipment where thickness measurements or other condition assessments are taken at regular intervals. CMLs (also called TMLs — Thickness Measurement Locations) are positioned at locations where degradation is expected to be most severe or most representative — areas of high velocity, turbulence, impingement, deadlegs, injection points, and heat-affected zones. Each CML has a unique identifier and documented coordinates.
Damage MechanismA 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.
Corrosion RateThe rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.
Risk-Based Inspection(RBI)A methodology that uses risk assessment to prioritize and plan inspection activities for pressurized and other equipment. RBI combines Probability of Failure (driven by damage mechanisms and equipment condition) with Consequence of Failure (driven by process hazards and safety impact) to rank equipment by risk and allocate inspection resources where they will have the greatest impact on risk reduction.
Where this appears in Reliatic
This concept is used in the Inspections module.