Generator and Turbine Fitness-For-Service Evaluations and Repair Support

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Generator and Turbine Fitness-For-Service Evaluations and Repair Support

DATE:

April 19, 2021

BY:

Andrew Wodoslawsky, Hydro Mechanical Engineer, Hydropower & Hydraulic Structures, Black & Veatch

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Generator and Turbine Fitness-For-Service Evaluations and Repair Support

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Hydropower owners and operators face integrated challenges: improve equipment reliability, minimize unplanned outages, improve generation, and reduce costs.  

Because of evolving market forces and operational needs, hydropower owners/operators look to extract more value from existing assets. Questions they are asking themselves include:  

  • What is our actual safe power operating limit? 
  • What component is the weakest link? 
  • Where is the best investment of limited resources?

The rise in intermittent renewable energy sources is one market driver that is increasing the need for hydropower assets to operate in a cyclical mode of operation. A result is increased starts and stops and frequent torque fluctuations that reduce generator life expectancy. Generation equipment cannot be expected to last forever, and rapid aging leads to difficult questions such as:  

  • “Which cracks or deviations are concerning?” 
  • “Which ones require immediate action?” 
  • “Which symptoms are benign?”

To make the best decisions for their organizations, owners can commission fitness-for-service evaluations and independent technical reviews. When executed by experienced professionals, such evaluations and reviews can deliver risk-informed and actionable results that help owners understand, restore or improve performance and machine life. In some cases, this has saved both downtime and expense.  Some examples follow. 

FITNESS-FOR-SERVICE EVALUATION 

Black & Veatch was contracted to perform a fitness-for-service evaluation for an owner that had found two very large and numerous smaller cracks in a cast-steel rotor spider. The cracks were found while preparing to return the unit to service after a scheduled maintenance outage. To provide recommendations for a safe return of the unit to service, Black & Veatch first directed materials sampling and testing, then constructed a three-dimensional (3D) finite element model of the rotor spider assembly using measurements and photos provided by the client, and then simulated behavior of the spider under standstill, runaway, normal operation, and motoring (synchronous condensing) using finite element analysis (FEA), with and without the cracks. The analysis revealed that only some of the cracks presented operational risk.  

Black & Veatch then prepared a rotor spider weld repair specification that targeted only the cracks that were a threat and provided on-site support and supervision during the third-party repairs. Also modelled and verified were the safety of “mouseholes” that would be introduced in the spider structures as part of the weld-repair. Following the repair, Black & Veatch provided a detailed and graphical record of the re-assembly of the unit to support the client’s new initiative of updating standard procedures for the powerhouse. The unit was successfully repaired and returned to commercial operation. 

INDEPENDENT TECHNICAL REVIEW 

Black & Veatch also helped a different owner who had decided to replace a turbine guide bearing and corresponding turbine headcover, to improve accessibility to the bearing components for easier and safer maintenance. However, the new bearing geometry introduced a larger cantilever from the head cover up to the centerline of the guide bearing, reducing the lateral stiffness that kept the shaft line centered. As a result, the owner contracted with Black & Veatch to perform an independent technical review of the ability of the new, locally designed headcover to survive the worst hydraulic transients. Black & Veatch also reviewed the ability of the combined new headcover and bearing to keep the shaft line’s critical speed within industry tolerances, given the reduced lateral stiffness.  

Delivered on a fast turnaround, a “lean” structural stress 3D finite element analysis (FEA) determined the locally designed and manufactured head cover was safe against transient events and simultaneously verified the new lateral stiffness of the bearing. Black & Veatch performed a two-dimensional (2D) finite element rotor dynamics analysis and found the design change would decrease the margin of safety against resonance, as was expected, but that the safety margin was still acceptable by industry standards. The two analyses allowed for a quick and high-confidence “green light” to make the desired improvements with only minor modification of the specifications of critical hardware. The machine was modified and returned to commercial operation without delay. 

After finding cracks in the structural welds of its rotor spider, another owner contracted Black & Veatch to perform a fitness-for-service evaluation and provide recommendations. Black & Veatch started with a detailed physical inspection of the rotor spider, directing technicians during their non-destructive examinations to identify and characterize the cracks. Next, the rotor spider, rim and poles were constructed in a 3D finite element model, and behavior was simulated at normal and fault conditions. The results clearly differentiated between cracks originating from the original manufacture and cracks originating from a specific fault condition. The analysis allowed Black & Veatch to prepare a detailed and targeted repair plan and specification. The cracks deemed most critical in the unit were repaired, and the unit was returned to commercial operation. 

SERVICE EVALUATIONS AND TECHNICAL REVIEWS CAN HELP SET INVESTMENT PRIORITIES AND SAVE COSTS 

The actual risk level posed by discovered defects or design changes is not always obvious. It’s why there is no substitute for a high-quality fitness-for-service evaluation or independent technical review to understand the cause and future implications of a defect or design change. Once the cause and implication are understood, then the solution’s place of priority among all other competing interests can be soundly judged.  

In these specific case studies, the evaluations saved critical funds compared to performing full equipment replacement. Downtime was also improved as the equipment was able to be quickly repaired and returned safely to service instead of waiting months for new equipment to be fabricated.