Multiple Corrosion Mechanisms in a Crude Distillation Overhead System

Troubleshooting efforts were devoted to addressing atypical corrosion activity in an atmospheric tower overhead system. These efforts indicated that three different sections of the overhead were being simultaneously attacked---each section having its own distinct corrosion mechanism. The areas affected and their mechanisms were as follows: 1) Corrosion resulting from ammonium chloride deposits in a "dry" pressure relief valve; 2) Wet sulfide corrosion on 70/30 Cu/Ni exchanger tubes; and 3) Velocity accelerated corrosion at the outlet elbows of these exchangers. The severity of these mechanisms correlated well with variations in unit operating conditions. Several techniques were used to identify the mechanisms involved, including electrolyte simulation modeling, metallurgical analysis of weight loss coupons, UT/X-ray measurements, and hydrogen permeation measurements. Recommended mitigation plans and their effectiveness upon implementation will also be discussed.