Failure analysis of an air-cooled heat exchanger in natural gas dehydration unit

Corrosion and erosion can have a synergistic effect on failure related to heat exchangers in gas refineries. This work presents failure analysis of outlet piping of an air-cooled gas cooler in a natural gas refinery. Visual inspection as well as thickness measurements were employed to determine the severity of metal loss. Chemical composition analysis and hardness measurements were performed in order to make sure of authentication of base and weld metals. Metallographic investigation indicated that no metallurgical changes have been occurred. Assessment of service condition revealed that temperature drop of gas stream passing through the gas cooler resulted in water condensation followed by corrosion. On the other hand, calculations of critical velocity of fluid revealed that gas velocity exceeded the critical velocity which intensified the erosion and flow induced damages. Thus, it was concluded that although general CO2 corrosion plays a role on the metal loss, flow induced localized corrosion and erosion were the main causes of failure.