FIORDA Case Study – Corrosion Attack on Primary Reformer Tubes
Summary Incident FIORDA Database, No: 7
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| No: | 7 |
| Upload Date |
08/15/2018 |
| Plant Code | AMOXY |
| Incident Code | AMOXY07 |
| Technology | Ammonia |
|
Plant section |
Primary Reforming |
| Main Equipment | Primary Reformer |
| Sub-Main Equipment | Primary Reformer Catalyst Tubes |
| Operation phase (during event) | Normal Operation |
| Operating parameters (during event) | Unknown |
| Medium | NG Fuel |
| Risk category | Operational |
| Hazard type | Corrosion |
| Failure Cause | Poor control procedures |
| Failure Mode | Contamination |
| Hours of operation | 17520 |
| No of failures last 6 months | 65 |
| No of failures last 12 months | >1 |
| No of failures last 24 months | >1 |
| Warning signs | Unusual spots |
| Event description | Ammonia plant primary reformer was retubed by replacing HK-40 tubes with Nb- modified HP alloy having the same outer diameter but reduced wall thickness. After about twenty four months of the plant startup, during routine visual inspection plant operator observed unusual spots on some of the reformer tubes. During turnaround, severe degradation and loss of material was observed at isolated locations on several tubes. Out of 300 tubes, 65 tubes were replaced as their wall thickness was reduced significantly. |
| Immediate response action | Routine observation |
| Findings | A combination of burner geometry anomalies, over firing and insufficient or poor combustion air flow distribution created conditions for poor combustion, irregular flame patterns and high levels of CO. The corrosion products formed are consistent with sulphurdising with hydrogen sulphide in the fuel gas of 0.6% and high partial pressures of S2 of 10-8 atm at high temperature. Increasing temperatures leads to a higher rate of sulphidation. The mixed oxides and sulphides present in the corrosion products indicate that the flue gas around the affected tubes was not constant and was alternating between oxidizing and sulphurdising. Microscopic examination, chemical analysis, combustion survey and the high sulfur content in the corrosion products reveals that the reformer tubes failure occurred due to Sulfidation. |
| Consequence Primary | Primary reformer tubes replaced |
| Consequence Primary cost | Unknown |
| Consequence Secondary | Extensive TAR associated with loss of production |
| Consequence Secondary cost | Unknown |
| Consequence Details | |
| Comments | Many of the burners flame were observed to be unstable rather long straight and narrow and were moving towards the reformer tubes. A number of burners were observed to have the flame slowly moving away from the burner because of irregular air supply probably due to partial blockage of nozzles. Some haziness was also observed at the lower level indicating after burning due to insufficient combustion air in some parts of the primary reformer. Prior to reformer re-tubing client was not cautioned about the HP micro alloy tubes susceptibility under reducing environment to sulfidation attack due to the presence of sulfur in the fuel gas. |
| Consequence | Catastrophic |
| Likelihood | Unlikely |
| Risk Level | High (10) |
| Prevention Safeguards | Implement a MOC procedure
Implement a MOC procedure that require a HAZOP study for any significant change in plant design. Use keywords and COMPOSITION during the HAZOP workshop. Invite a material specialist from the supplier to respond to the technical queries related with equipment specifications.
Develop internal guideline and company standards Develop internal guideline and company standards that require a BOD (Basis of Design) to be developed for any major design change. in BOD include all stream parameters and composition that may influence the affected equipment. (e.g high content of Sulphur from NG fuel gas shall be given to tubes manufactures as BOD document, so the manufacturer can perform a material analysis and consequence modelling for their tube materials).
Plant monitoring and reformer tubes inspection Plant monitoring and reformer tubes visual inspection during shifts.
Plant survey Perform plant survey and inspection conducted by the contractor or equipment supplier with purpose to collect and validate data presented by the Client in the Basis for Design document. |
| Mitigation Safeguards | Unknown
Unknown |
| Corrective Recommendation | Reduce Hydrogen Sulfide
Reduce Hydrogen Sulfide contents in the fuel gas to reformer. Modification was conducted to pass the fuels gas A modification was conducted to pass the fuels gas through sweetening plant before mixing with purge gas. It reduced the Hydrogen Sulfide contents from 0.6% up to 0.02% in the fuel gas.
Improve the air distribution To improve the air distribution the burners combustion air dampers and fuel valve were fully opened.
Burners wind box pressure survey Burners wind box pressure survey was conducted to ensure equal distribution of combustion airflow to the individual burner.
Reformer tubes temperature mapping Frequent reformer tubes temperature mapping and visual inspection is being conducted to detect any abnormality with in the fire box.
Excess O2 in the furnace discharge Excess O2 in the furnace discharge has been increased from 0.8% to ~1.8 to 2.0 % in both ammonia plants.
Air ingress check Air ingress check through convection zone was conducted by smoke test to eliminate the air ingress to get the representative sample of the flue gas. |
| Lesson Learned |
With change in fuel gas quality and increasing the combustion air, after six months of service no corrosion attack has been observed on reformer tubes. And the appearance of new corrosion spots on second reformer tubes are on the decreasing trend. |
