How long will the cyclone system in a Cat Cracker reactor or regenerator last?
Cyclones systems in Fluidized Catalytic Cracker units (FCC) are typically designed to a creep allowable stress, where the stress field at various locations of the system has been determined by linear elastic analysis. The basic allowable stress for such internal structures (not on the pressure boundary) is commonly taken from B31.3 or API RP-530, where the allowable stress is dependent on the design life, often 100,000 hours, approximately 11 years. Such analyses may take into account stress classification to give an allowance for elevated stresses of secondary nature (self-equalizing stresses), but this practice often under-estimates the actual stress redistribution that takes place as the structure permanently deforms from creep. Once the time in operation has surpassed the design life, it would seem that the creep life of the structure should be consumed and that the cyclone system would be due for a replacement or at least extensive repair in the load bearing components, hanger straps, outlet tubes, plenum floors etc. However, this is often not the case and many cyclone systems have had useful lives in excess of 20-25 years. If the operating temperature is less than the design temperature with only occasional excursions, then the creep damage on the cyclone system will be significantly less compared to the design expectation and the cyclone system will last much longer. However, if the operating temperature is always near design temperature and there are frequent excursions, the creep life will be consumed at a higher rate, but the useful life will still mostly be well in excess of the nominal design life.
One methodology to determine a more accurate creep life prediction is to analyze the cyclone system with “real” operating condition with an explicit creep model using Finite Element Analysis (FEA). Becht Engineering has used the explicit creep model with finite element method to solve several complex high temperature creep problems.
Let’s look as some of the advantages an accurate simulation of the progression of creep damage through FEA can provide and discuss the challenges in putting it into practice.
The finite element analyst will encounter a number of challenges with this approach, but they can be overcome.
Such challenges include:
- A detailed FEA model is required
- Selecting creep properties of the actual materials
- The creep protocol endorsed by Fitness-For-Service standard API-579-1/ASME-FFS-1 is the MPC Omega model. The uniaxial formulation of this model has shown good correlation to test data, but is not incorporated into commercial FEA codes.
- API-579-1/ASME-FFS-1 lists the acceptable creep damage as 80%, but does not offer guidance on allowances for localized effects.
Becht Engineering has experience in the detailed creep analyses described here and employs an implementation of the Omega protocol in the ABAQUS FEA code that has proven both robust and reliable. Recent work includes a cyclone system of a FCC regenerator for a major European refinery and converter vessel cyclones for an African petrochemical plant. Both of these analyses demonstrated that the remaining creep lives far exceeded those predicted by the original design and the useful life will likely be governed instead by refractory degradation and/or cyclone body erosion. This enables the Owner to make favorable adjustments to equipment strategy, inspection planning and future operating envelopes.
Left Figure: Model mesh for studying creep of cyclone outlet tubeRight Figure: Cyclone outlet tube creep damage after simulating historic and anticipated future conditions. Peak damage is at 48%, far less than predicted by original design.
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Magnus Gustafsson has over 10 years experience in the design and analysis of mechanical components and systems in the Petrochemical and Railway industries. His experience includes Fitness-for-service assessments and design support of pressurized equipment including elastic-plastic Finite Element Analysis, transient heat transfer analysis, creep, high-and low cycle fatigue, fracture mechanics, fluid surge and piping flexibility analysis.
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