Failure analysis of reactor heater tubes SS347H in ISOMAX unit

Cracking phenomenon is one of the most important problems associated with stainless steel alloys applied in industrial processes. In this contribution, investigations have been done about the cracks detected by UT inspections in a reactor heater tubes SS347H of an isomax unit of a refinery during the overhaul.Studies have shown cracking the reactor tube by polythionic acid, arisen from the formation of sulfide scales on the inner surface of the reactor tube in operation. The reactor was exposed to the atmosphere and humidity during overhaul. Then sulfide scales react with oxygen and polythionic acid is produced, leading to cracking in the presence of residual internal stresses at the sensitized material of the reactor tube.

► Failure of a cracked heater tube has been investigated.
► Due to the presence of sulfur compounds, moisture and oxygen, polythionic acid has been formed at inner surface of the tube.
► Sensitivity of alloy and residual stresses in welded areas were observed.
► Polythionic acid stress corrosion cracking results in failure of heater tube.