Heater Condition and Life Assessment Evaluations
Timing of furnace retubing can be critical in minimizing equipment costs and maintaining productivity. Consistently replacing heater tubes on a premature basis can cost a medium-size refinery up to $1 million per year or more in unnecessary capital costs. Optimization of heater retubing schedules can only be achieved by implementing a set of criteria that are realistic and based on sound engineering analysis. Specific tube replacement criteria are required for each type of heater and tube material, e.g., criteria for crude heaters may not be appropriate for coker charge heaters. When replacement criteria have been established, implementation of a comprehensive baseline-condition assessment and heater inspection program to support the criteria will ensure its success. What is required is an overall heater tube-life management program that allows tube life to be optimized, tube replacement to be scheduled well in advance, and productivity interruptions to be minimized. The program should also identify heaters that are consuming their service life at a high rate so that inspection priorities and schedules can be adjusted. Finally, the program should have the capability of predicting the effect of higher tube skin temperatures, corrosion rates, and operating pressures on tube remaining life and equipment reliability.
Elements of a comprehensive tube-life management program include the following:
Prioritization (ranking) of heater tubes by initial estimates of remaining life
Inspection prioritization/identification of critical heaters
Heater inspection/condition assessment
Creep testing of tube samples (not required but highly recommended)
Accurate determination of remaining life by probabilistic analysis
Recommendations for optimum tube replacement dates consistent with plant
operational and productivity goals
Economic analysis of retubing costs vs potential costs associated with increased
profits at a higher risk level
Condition assessment of tubes in a fired heater is an on-site inspection that encompasses a combination of the following:
Thorough visual inspection of the heater tubes
- Tube distortion ? sagging, bowing, contact between tubes
- Tube surface ? scale, pitting, evidence of overheating
Thorough metallurgical inspection of the heater tubes
- Field metallurgical replication ? in-situ metallography to determine level
of thermal degradation
- Hardness measurement
Diametral strain measurement of tubes to determine level of creep
This type of assessment is often conducted to complement a calculational assessment and helps determine whether damage mechanisms not modeled are operative.
Calculational Remaining Life Assessment
Currently two deterministic techniques to assess remaining life are being used for fired heaters: API 530 appendix E and the Omega approach in API 579.
The approach in API 530 is based on stress-rupture (Larson-Miller) and is the more traditional, tried and tested approach. The API 579 approach is strain-rate-based, relatively new, and is currently gaining acceptance.
Both approaches are deterministic, i.e., single-value approaches, that necessarily use worst-case input values. As a result, assessments using both approaches are usually rather conservative. However, such assessments are relatively quick and therefore low cost.
Use of probabilistic assessment techniques is described briefly in section 184.108.40.206 of the API document, API 579 (January 2000) that covers fitness for service of equipment operating in the refining and petrochemical industry.
Monte Carlo simulation is then used to simulate the tubes and their operation by randomly selecting values from these distributions for use in the API 530 Appendix E based remaining life calculations. Repeating this process many times simulates the "life" of a tube and, by making the necessary assumption that historical operation will continue into the future, allows prediction of total life of the tube. In this way, the uncertainty inherent in deterministic type assessments arising from the use of single, worst-case values is eliminated.
Heater Tube Life Management (pdf)
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