Prmary Reformer Catastrophic Failure, this paper published in AICHE in 2011...
Root Cause Analysis of Primary Reformer Catastrophic Failure: A Case Study Naveed Ramzana , Shahid Naveeda , Muhammad Rizwanb , Werner Wittc a
Departmentof Chemical Engineering, University of Engineering and Technology Lahore,Pakistan Fauji Fertilizer Corporation, Pakistan c Lehrstuhl Anlagen und Sicherheitstechnik,Brandenburgicshe Technische Universität,Burger Chaussee 2 Lehrgebäude 4/5 ,Cottbus 03044,German Contact: Email: [email protected]
Abstract: This paper describes the investigation of an incident of catastrophic failure of a primary reformer in an ammonia plant. The lessons learned from the root cause analysis of the event are of interest. The reformer is one of the critical unit operation in an Ammonia Plant. Under normal operating conditions, reformer tubes get aged and progressively develop ‘creep’ in metal leading to a possible catastrophic failure. This event of total failure occurs when one tube fails and results in destroying the neighbouring tubes. Such a situation may occur during start-up. A Failure Diagnosis Model is developed and several hypotheses were tested to determine most likely causes. The analysis revealed that due to short supply of Nitrogen the start up procedure of the reformer was modified on expert’s advice shortly before the operation. This change was not reviewed by the whole team. The important parameters in view of the changed procedures were not monitored. This communication gap resulted in severe damage to the reformer melting almost all of the tubes and convection zone coils. It may be observed that the Primary Reformer amounts to 25% cost of whole Ammonia plant. The systematic root cause analysis thus clearly identified the reason of catastrophic failure as incomplete analysis and non conformity of procedures.
Failure diagnose model, ammonia plant, start up, reformer tubes, convection
1 Introduction: Natural gas is the most important raw material for production of ammonia as it is the cheapest source of Hydrogen. It is converted catalytically with steam. This reforming reaction is known as steam reforming or Oxylysis . The reaction of Methane and Steam in ammonia plant takes place in a tubular reactor known as Primary Reformer in which tubes are packed with catalyst. The primary reformer is the heart of Ammonia plant and represents largest expenditure, i.e. capital and energy costs. As the reforming reaction is highly endothermic that’s why the frequency of primary reformer burn downs is very high . Consistently this happens during start-up or shut-down because plant operators deviate from the standard operating procedures This is one of the root causes of reformer burn downs. Conditions during shut downs and start ups are so transient that these generate additional stresses due to the high rate of temperature changes. Regular visual inspection of furnace tubes is necessary during start up and shut down. It gives clear picture of what is going on inside the furnace and that the tubes are not being overheated! Individuals must also take additional precautions for the increased risk of localized heating due to transient temperature and pressure changes within the radiant box . Reliable and smooth operation of a reformer especially depends upon the effective monitoring of catalyst tubes. Most accidents result from failures or malfunctions within a system. A system consists of people, equipment, material, and environmental factors . The components of a system and its environment are interrelated, and a failure in any part can affect the other parts. Some of the causes of incidents may be human error, instrument malfunctions and natural calamities / sabotage. Following are the common causes of tubes failure [5,6]: o
Over – Heating
Stress Corrosion Cracking
Flue Gas Mal–distribution
2 Incident Back Ground: The incident took place in a company that operates a modern integrated ammonia-urea complex to produce nitrogenous fertilizers in Pakistan. The ammonia plant is based on Kellogg’s steam reformation process and natural gas is the main raw material. The plant was commissioned in August 1998 and just after two years in October 2000, its primary reformer tubes melted and convection coils damaged during start-up activities.
Design life of reformer tubes is 100,000 hours (11 years approx.) whereas operational life in this case was just 2 years. The design capacity of the plant is 600 M.tons /day ammonia and 1050 M.tons /day urea. Description of Primary Reformer: The primary reformer is a typical Kellogg’s top fired box type heater having 156 catalyst tubes. The furnace consists of 72 Arch John Zink Burners located at top and fired with natural gas. The catalyst tubes have advanced metallurgy consisting of HP Micro-alloy i.e. 25 % Cr, 35% Ni, 1% Nb with traces of Ti, Zr, W, Cs. Heat duty of primary reformer is 31.09 Kcal / hr also having extensive heat recovery in the convection section. Figure 1 gives the schematic diagram of primary reformer.
Figure 1. Schematic diagram of Primary reformer
Incident Details On 16th October, power failure occurred due to malfunctioning of certain instruments at the utilities unit which resulted in a total plant shut-down. After problem rectification, primary reformer start-up activities were iniated. Heavy leakage was observed from a gasket at the waste heat boiler steam drum during its pressurization. The system was depressurized and shut down to rectify the leakage problem. On 17th October 2000, after completion of all maintenance activities, the reformer start-up activities were started with modified operating procedure to save significant time of about eight hours. Every thing was going normal but reformer outlet temperature was not increasing and was on lower side in spite of firing almost 60% of the burners. Reformer outlet temperature indicator
(TI) was suspected to be faulty. On physical inspection of the reformer, it was observed that most of the tubes were melting down. Immediate shut-down was carried out. Initial investigations concluded that reformer outlet temperature indicator malfunctioned and did not show the actual picture which resulted in reformer failure. The incident was accompanied by long shut-down of almost six months for replacement of reformer tubes and maintenance of the convection zone coils.
3. Failure Analysis 3.1 Data Collection Questions: (i)
Has this failure mode / mechanism ever occurred before? Many incidents of the same nature occurred at various locations in the world and these resulted in severe economical and production losses. Table 1 describes some of the incidents of the same nature .
Table 1. Incidents of catastrophic failure of primary reformer 
Canada, Terra Industries
140 out of 168 tubes
Neighboring tubes Canada, Northern Albarta
All the tubes were Wet Catalyst
Were there any plant upsets prior to failure? Yes. Plant emergency shut-down was carried out twice: First due to power failure and second time due to leakage
from outlet gasket of waste heat boiler steam drum during its pressurization. Frequent change of conditions during emergency start-up and shut-down activities has direct effect on primary reformer. (iii)
What was the equipment condition prior to failure? Process fluid was not yet introduced in to the reformer. Actual indication of process side temperature was not available and temperature indicator was suspected faulty. The furnace side operating pressure was normal however fuel pressure and flow was on higher side.
What was the equipment condition on last overhaul? Equipment was newly installed just two years back and no major overhaul was carried out as its condition was found good during last inspection.
3.2 End of Natural Life Questions: (i)
How long had the unit been in service or operating? The unit was in service since commissioning for approximately only two years. No significant maintenance job was carried out in available shut-downs as there was no requirement for it. Its design life is approximately eleven years.
Time since last routine / planned maintenance was carried out? Last routine / planned maintenance was carried out one year before the incident.
Time since reformer operation remained normal? Reformer operation remained normal since its commissioning.
Are these periods acceptable and compatible with design criteria and plant production requirements? No it has completed just 20% of its design life and nothing abnormal was observed during this period. So this is not the reason or cause for failure, so proceed further for root cause analysis.
3.3 Modification Questions:
(i) Have there been any changes in its design? No. There had been no modification to its design neither in its upstream / downstream equipment. (ii) Have there been any changes in operating procedure? Yes. The operating procedure was changed from the original. Therefore re-assessment of the operating procedure is carried out. 4. Analysis of Operations: The operating procedure was modified. 4.1 Normal Operating Procedure: In the normal start up procedure at the plant, N2 gas is passed through primary reformer and a heating rate of 50 0C per hour is maintained at reformer outlet temperature indicator. This N2 passes in a close loop i.e. from outlet after low temperature shift converter; it is recycled back in to primary reformer. This cycle continues until 350 0C temperatures is obtained at reformer outlet. To increase reformer outlet temperature, more burners are ignited.This is the standard operating procedure at all Casale designed Ammonia plants.
4.2 What was the modified operating procedure? Due to emergency shutdown, sufficient N 2 inventory was not available at site for start up. At least 8 to 10 more hours were required for N2 inventory make up. To save production loss, the start up procedure was initiated. Other, fertilizer plants in the country were consulted and they were not using nitrogen gas during start up therefore start up activities continued. Furnace firing was started in the absence of nitrogen gas and reformer outlet temperatures were monitored to have a 50 oC per hour heating rate. Reformer outlet temperatures were not increasing so firing rate was increased. During this period many alarms appeared on DCS for convection zone temperatures. The alarm was inhibited to avoid any inconvenience to the control panel operator because he was busy with the steam drum level control. Since there were no changes in these outlet temperatures the firing rate was further increased and 56 out of 72 burners were fired which represents about
70% of the heat input without any fluid flow through the reformer. The boardman instructed the plant operator to have a physical check of reformer. The operator found that there were no reformer tubes inside the furnace - they were melting down. Consequently to save just 10 hours of production loss, they lost the whole furnace tubes and even many of the convection zone coils. 5. What went wrong? The furnace was being fired and reformer outlet temperatures were being monitored without introduction of any nitrogen through the reformer. So in this case, convection zone temperatures were the true indicator and they were ignored. In plants where nitrogen gas is not used during start up, they monitor convection zone temperatures until the introduction of steam through the reformer. Due to absence of any flow through the reformer its outlet temperature did not increased and high heat increases with no process flows resulted in high tube temperatures and finally melting of tubes. 6 Conclusion and Lessons Learned: Failure analysis showed that the cause was in the operating system i.e. the start up procedure was not properly communicated or understood by the operational team. This lack of communication caused a reformer catastrophic failure. The lessons learned included: (i)
Be super vigilant during Reformer Furnace Start Up.
Have a regular visual Inspection of Reformer tubes during Start up.
Stop & Think. Especially when changes are made that deviate from normal operating procedure. Any change in procedure should be thoroughly reviewed and well discussed with operating personnel.
Don’t assume that instrument readings are wrong. Ensure reliability of instruments before start up.
Analyze all related parameters during start up.
Firing rate should be increased gradually.
Critically analyze Tube Metal Temperatures during startups.
References: 1. I. Dybkjaer, Tubular reforming and auto thermal reforming of natural gas-an overview of available processes, Fuel Processing Technology, 42(1995), 85-107. 2. D. H. Timbres and M. McConnell, Primary Reformer Failure, Proceedings of 47th Safety in Ammonia Plants & Related Facilities Symposium, AIChE Autumn Meeting, Montreal (2002), AIChE Technical Manual, 42(2005). 3. PW Farnell, Heat transfer effects in the tunnel region of a top-fired reformer, IMTOF 1997. 4. I. Cameron, R. Raman, Process Systems Risk Management, 1st Ed., Elsevier Academic Press, (2006). 5. S. K. Bhaumik, , R. Rangaraju, M. A. Parameswara, T. A. Bhaskaran, M. A. Venkataswamy, A. C. Raghuram and R. V. Krishnan, Failure of reformer tube of an ammonia plant, Engineering Failure Analysis, 9(2002),553-561. 6. C. M. Schillmoller, HP-Modified Furnace Tubes for Steam Reformers and Steam Crackers”, NiDi Technical Series No 10 058 Canada, (2000).
7. Coronado Bay, AIChE safety symposium, September 2002.