By: Eric Scheibler, Consulting Engineer, Quest Integrity Group
and Susan Smelley, General Manager Hydropower Generation, TVA
Presented at the CEATI Hydro Workshop in Palm Desert, CA March 19-20, 2014
The Raccoon Mountain Pumped Storage Facility (RPS) in Chattanooga, TN is among the world’s largest pumped storage hydroelectric power stations. Tennessee Valley Authority (TVA) operates RPS with four 425 MW reversible Francis units providing 1600MW of generating capacity. TVA wanted to develop a long-term inspection program for critical equipment associated with the pressure boundaries along the waterway at RPS. The scope of this project encompassed components along the waterway beginning with the Draft Tube Liner including the Spherical Ball Valve, Spiral Case, Wicket Gates, Headcover, Mechanical Shaft Seal, Wheelpit piping, Draft Tube and associated Manways. This presentation covers the methodology that was used to develop inspection recommendations and intervals for RPS operating under power generation mode.
The project consisted of performing multiple types of analyses including: computational fluid dynamics (CFD), finite element analysis (FEA), and fitness-for-service assessments. Laser scanning of one of the Francis units runner blade was performed to obtain the actual blade geometry as its span-wise profile, taper, and twist are not made available through traditional engineering drawings. A CAD model of the Francis unit was created using the laser scanned data and engineering drawings. The developed CAD model was used to create both CFD and FEA models for the Francis units. A CFD model was developed to obtain steady state pressure distribution along the main flow path and headcover of a Francis unit. The pressures computed from CFD were then prescribed as boundary conditions for the FEA models where stresses were calculated at various critical components within the unit. The stresses from the FEA analyses were extracted and used as primary stress inputs for critical crack size computations and fatigue life estimates for the fitness-for-service assessments. This approach was performed in accordance with API 579-1/ASME FFS-1 Fitness-for-Service guidelines and provided TVA with quantitative means to plan for regular inspections and end-of-life budgeting.