Failure Lesson: How Averaged Corrosion Rates Can Hide Localized Injection-Point Damage

A process piping circuit in a gas processing facility receives a chemical injection upstream of a heat exchanger. The circuit material is carbon steel, and the service includes water, hydrocarbon gas, and trace corrosive species. The piping circuit RBI has assigned a moderate corrosion rate based on historic UT readings from mid-circuit CMLs, resulting in a planned inspection interval that appears adequate for the circuit as a whole.

A pinhole leak occurred on the outer radius of an elbow approximately one metre downstream of the chemical injection point. Investigation revealed significant localized wall loss concentrated at the injection-point elbow and a short section downstream, while the rest of the circuit showed minimal corrosion.

The corrosion at the injection point was driven by localized turbulence, mixing zone chemistry, water dropout, and velocity effects — all concentrated at and immediately downstream of the injection nozzle. These effects did not occur mid-circuit where the CMLs were placed. The circuit-averaged corrosion rate from mid-span CMLs was not representative of the actual corrosion behaviour at the injection point.

Targeted CML placement at and immediately downstream of the injection nozzle. Separate circuitization of the injection-point zone as a high-risk sub-circuit with its own corrosion rate and inspection interval. Recognition that injection-point corrosion is a distinct damage mechanism requiring specific inspection targeting, not a variant of general circuit corrosion.

Placement of dedicated CMLs at the injection point and the first elbow downstream. Separate circuitization of the injection-point zone in the RBI system. Use of profile radiography, UT corrosion mapping, or PAUT to assess the elbow geometry and downstream section specifically. Inspection at shorter intervals for the injection-point sub-circuit pending establishment of a reliable site-specific corrosion rate.

Average corrosion rates are only valid where corrosion is truly averaged across the circuit. At injection points, dead legs, low points, and turbulent features, the damage mechanism is location-specific and may be independent of the general circuit corrosion rate. RBI programs must identify these locations explicitly, place CMLs there, and assign separate corrosion rate and interval assumptions. API 570 specifically identifies injection points as inspection focus areas requiring targeted assessment.

API 570 — Piping Inspection Code: injection point inspection requirements. API RP 580 / API RP 581 — Risk-Based Inspection. API 571 — Damage Mechanisms: corrosion at injection points and turbulent zones.