Failure analysis of heat exchanger fluid vapor phase vent lines in a Polyester plant

A Polyester plant is using Proprietary heat exchange fluids (26.5% biphenyl, 73.5% diphenyloxide). The plant hot condensate tank, condensate feed tank, and heat exchanger vapor phase dead end and ventable lines (ASTM 106 Grade B) with isolation valves were suffering frequent leakages with severe gouging and pitting corrosion. Lustrous (shiny) coke deposit was prominent inside the cut sections due to coke formation by low boiler products. Some adhered brown iron oxide (rust) deposits were also visible. A sludge sample was analyzed by XRD which revealed that carbon was the major constituent with γ-Maghemite-C (an iron oxide) as second mineral in order of abundance. The failure was diagnosed as severe localized corrosion and gouging at temperatures of about 200 °C and 53 psi by non-condensable low boilers decomposed corrosive products. The remedy was suggested which has been validated over the 2 years trouble free performance.