Refining Process Unit Damage Mechanism Susceptibility and Inspection Planning
Thinking by unit
Unit questions reward knowing the environment each stream creates. Crude units combine chloride hydrolysis to HCl in the atmospheric tower overhead, ammonium chloride deposition and under-deposit corrosion, sour water, naphthenic acid corrosion, sulfidation, and extensive CUI. FCC units add catalyst erosion and sulfidation. Cokers combine thermal fatigue of coke drums with erosion and sulfidation. Hydroprocessing units present wet H2S cracking, ammonium bisulfide corrosion, ammonium chloride deposition, and high-temperature hydrogen attack. Catalytic reformers involve high-temperature hydrogen and chloride, carburization, and polythionic acid SCC at shutdown. Amine units present amine SCC and erosion-corrosion. Sulfur recovery units present sulfidation. Boilers present caustic SCC, corrosion fatigue, flow-accelerated corrosion, and under-deposit corrosion. Cooling water systems present MIC, pitting, and SCC of stainless. Storage tanks present soil-side corrosion, CUI, and brittle fracture.
Inspection planning
Use corrosion loops and damage mechanism reviews to assign mechanisms to each circuit, then pick NDT that detects the expected morphology: profile radiography or UT for thinning, shear-wave UT or wet fluorescent magnetic particle for cracking, and replication or specialized UT for metallurgical damage. Shutdown and startup are damage events in their own right. Set inspection intervals from the mechanism rate or threshold and confirm with integrity operating windows.
Traps
The usual trap is assuming a mechanism belongs to only one unit, or forgetting utility and shutdown conditions. Worked example: a hydrotreater reactor effluent air cooler suffers ammonium bisulfide corrosion and ammonium chloride deposition on the water-wash side; the inspection plan must include water-wash quality monitoring and localized UT at injection points, not just general thickness surveys.