Structured Remaining Life Assessment
Mechanical Dynamics & Analysis (MD&A) assessed both the current condition and remaining life of a 1968 Vintage fossil-fired 445MW D8 steam turbine following 700 starts and 345,000 operating hours. The owner wanted to know if the asset was safe to continue to operate and if it could be considered for repowering for another twenty years with natural gas.
MD&A experts knew what to look for because of their experience: the inherent issues, damage mechanisms, characteristic vulnerabilities, and long-term potential weaknesses of these units. We also have various levels of inspection and analysis to suit the owner’s needs to minimize inspection costs and outage time.
With that experience and knowledge, we launched a meticulous and systematic program to investigate all areas, then objectively determine the realistic remaining life of all major stationary and rotating components.
Our engineering experts used specialized non-destructive evaluation (NDE), creep damage assessment, and material sampling analysis. From the owner/operator, MD&A obtained a complete operating history, as well as projected future operations. A truly objective and dependable remaining life assessment is only as good as the quality of the inputs.
Major components included HP/IP outer casing, HP inner casing, main stop valve bodies, combined reheat valve bodies, control valve steam chests, main steam lead piping, and stationary HP nozzle box. The turbine’s HP/IP and LP rotors, wheels and blading components would also be evaluated.
Investigative methods and procedures included operating data and typical startup temperatures to assess each component’s fatigue life. To evaluate the condition of rotor forgings, phased array ultrasonic inspection was performed for the near bore region and external vulnerable blade fits. Material samples were taken from bore areas to determine the rotor forging fracture properties. The industry standard, EPRI’s SAFER-PC program, is used to analyze the severity of the bore area defects.
Solid models were created for each of the other critical casing components. The models then were used for finite element stress analysis, material fatigue analysis, and fracture mechanics calculations.
Results of all NDE of turbine components, stress analysis, material fatigue and conservative fracture mechanics calculations were carefully reviewed, along with the full-service history of the unit. With consideration of the predicted starts of the unit, recommendations were provided for both ongoing operation and inspection.
All recommendations centered on operational issues and future inspection rationale and specifics. As one example, for the HP/IP rotor where some forging defects were found, recommendations included re-inspection intervals, locations, and techniques, with possible remedies depending upon the findings.
As another example, recommendations for the HP/IP outer casing, after inspections that included FEA cold start simulation and transient thermal analysis, included specific test locations and actions given for the second future turbine overhaul or 20 calendar years, whichever comes first. The unit was determined to be in good condition with a substantial future based on the continuation of our recommended cold and hot startup procedures.
The customer was left with more than a status report and estimated endurance.
MD&A prides itself on offering recommendations to our customers that include a clear and objective roadmap for future operations, realistic unit expectations, precise future component inspection advice, and specific operating suggestions.
This would allow the customer to confidently maximize the full useful remaining life of this unit.
MD&A’s specialized services range from steampath audits to finite element analysis and thermodynamic analysis. We lead the power generation industry with real-world engineered solutions that address issues associated with an aging turbine fleet.
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