Crack Analysis of Convective Steam Superheating Tube of 300,000 Ton Synthetic Ammonia Reformer

Analysis of 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater Tube Fracture

Introduction

The 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater is a type of equipment used to preheat feed steam during the ammonia production process. In the current production process, it is particularly important for the superheater to maintain a certain temperature, and severe temperature fluctuations may cause equipment failure. This case study is about a 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater that experienced a tube fracture.

Analysis

The actual production process of this 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater as follows: the feed water enters the furnace after it is heated by the superheater. After the preheating of the superheater, the steam generated is sent to the reaction device. The temperature of the feed water leaving the superheater is different depends on the reaction conditions, and the temperature may be changed due to other factors. As a result, it is necessary to adjust the parameters of the superheater to keep a certain temperature. The convection type steam superheater was in operation for several months before the incident.

The cause of tube fracture in 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater was found to be due to over temperature. After several months of operation, some of the tubes of the 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater had been damaged. The inspection logs during the operation of the furnace revealed that the temperature of the feed water in the superheater was slightly higher than usual during the lower than usual load. It was determined that the signal from the controller was weak and insufficient to control the feed water temperature pressure, leading to over temperature and tube fracture.

Conclusion

This case study concludes that the tube fracture in the 30,000 Tons Synthetic Ammonia Conversion Furnace Convection Steam Superheater was caused by the over temperature due to weak controller signal. To prevent such problems from occurring in the future, it is important to ensure the strength of the controller signals to maintain the optimal feed water temperature. In addition, regular maintenance of the furnace and the tubes should be undertaken to ensure the normal operation of the superheater system.

This report focuses on the analysis of the rupture of a 30,000 tons/year synthesis ammonia converters convolution steam superheater tube. It mainly analyzes the operating conditions, sectional structure and material performance, rupturing phenomenon and reason of the tube.

According to the initial tests, the operating temperature of the converters steamed superheater tubes was 510.6℃ and the operating pressure was 0.9MPa. Its outside diameter was 51mm and the thickness of tube wall was 3mm. Its material is HT-35ST boiler tube.The sections of the tube were round, which was accord with their design requirements.

When the tube were damaged, we found out a black strip appeared on the surface of the tube. This strip was of an elliptical shape with a length of 60mm and width of 22mm. After the metallurgic microscopic analysis and chemical composition analysis, which can be concluded that it was resulted from hydrogen exhaustion corrosion.

We suspected that, maybe because of the surge of temperature in operating, the local stress of steam superheater tube was too high to bear so that it caused the thermal fatigue damage of tube. Since tested values of materials, the mechanical properties and chemical components of the tube were all within the relevant codes and standards, it can be eliminated that the internal problems of this superheater tube, such as chemical and mechanical damages, caused the damage of tube.

So, the reason of rupturing of steam superheater tube can be attributed to the hydrogen exhaustion corrosion which triggered by local thermal stress of thermal fatigue. Vigorous preventive maintenance is urgently required. It is recommended that small modification should be added on the structure, related materials and components, additionally, the operating temperature should be regulated according to the temperature resistance properties of synthetic ammonia converters.

Finally, it is suggested to pay attention to the regular inspection to ensure the safe operation.