Stress Corrosion Failure of Waste Heat Boiler Tubes in an Ammonia Synthesis Converter

Abstract

In a 600 tpd capacity ammonia plant, the stainless steel tubes of the waste heat boiler of the ammonia synthesis converter falled after 6644 h of operation. Investigation revealed that the boiler tubes made of AISI 321 and 310S stainless steel had suffered stress corrosion cracking due to the conjoint action of stress and a corrosive environment, namely, alkalinity and chloride. The causative factor for the failure was the non-operation of the boiler feed water pump which led to the build up of a high concentration of the corrosive ingredients in the system. The inner tubes had failed first, upsetting the circulation of water in the dead end regions of the outer tubes, leading to accelerated evaporation and subsequent deposition. This led to the stress corrosion cracking of the outer tubes. The inner tube portions embedded within the deposit subsequently suffered general and intergranular attack.