Corrosion failure analysis of spent caustic drainage line pipe

A mid country oil refinery requested for the analysis of spent caustic 4 in. damaged line pipe leading to spent caustic storage tank which failed after installation by localize corrosion attack within 8 months. The refinery also reported that the failures were frequent and were occurring at same region between continuous flow line (CFL) and intermittent flow line (IFL) at 6 O’ Clock position. The line duty cycle was half filled used as and when required basis. The chemistry of spent caustic solution, operating conditions, metallurgy and electrochemical behavior of line pipe in spent caustic was carefully reviewed. The design flow was also examined and certain flaws in design were identified. It was concluded that by modifying the diameter of 4 in. diameter existing line pipe in the design, the problem was solved. The original suggestions have been validated after more than 3 years of satisfactory service of said line pipe.