The Incidents:
Backside cracks were found universally in the austenitic stainless steel (316LMod) loose liners of urea reactors in recent years, and led to urea reactors leakage.
In order to discover the leak of the inner lining in time, the 0.6MPa circulated steam is flushed between the intensity layer and the loose liner during the operation of urea reactors. And the composition of steam in the outlets is monitored at any time.
In the past 5 years (2007-2012) backside cracks were found in the loose liners in more than 20 urea reactors. Some backside cracks are larger than 1000mm.

The Causes:
Characteristics of backside cracks in urea reactors loose liners are generally as follows:
(1) Leak areas mainly appear in the base metal zone. In the backside of liners, there are traces corresponding to the leak detection groove which cracks mainly expand along.
(2) The cracks expand very quickly. The shortest time of causing a urea reactor putting into production to leak was only 14months.
(3) The leak source in the internal surface was small (pinhole) while in the backside it was large (crack). The closer to the surface of the backside, the longer the cracks extended. In addition to the leak source, more backside crack areas in which the inner lining was not penetrated yet were found during the inspection.
There were several types of cracking in austenitic stainless steel, such as chloride ions (Cl-) SCC, wet hydrogen sulfide (H2S) and chloride ions SCC, pure wet hydrogen sulfide SCC, and polythionic acid SCC. For polythionic acid SCC, cracking was intergranular, and it was transgranular for others.
Analyses on the sample revealed that, the backside cracks were caused by the interaction of wet hydrogen sulfide (H2S) and chloride ions (Cl-) stress corrosion cracking (SCC), and H2S played a dominant role. The source of H2S and chloride ions was the leak detection steam.

The Consequences:
Urea reactors leakage.

The Lessons Learned:
UreaKnowHow.com’s advise it to never flush a leak detection system with steam, water or condensate. And to apply the AMMO LASER Leak Detection System. Refer to: https://ureaknowhow.com/ammo-laser-leak-detection-system/

Also refer to: https://ureaknowhow.com/2012-cao-analysis-on-backside-cracks-in-316lmod-inner-linings-of-urea-reactors/