Combined cycle gas turbine (CCGT) power plants in the U.S. have undergone several major changes in operating methodology over the past 20 years.
Many of these changes – such as the shift to higher ramp rates – have been due to more competitive markets and market demand. For example, many existing CCGT plants were built to handle base loads, but are now being used to deal with load-following that fluctuates through an operating period. This requires shorter start times and unexpected capacity that lead to modifications that impact the combustion turbine generator.
Unfortunately, many plant owners miss the impact higher cycling rates have on steam turbines and steam turbine generators (STGs).
To study this issue, the Electric Power Research Institute (EPRI) conducted telephone interviews from a variety of electricity producers located in 50 facilities in the U.S., England, Japan, and Italy.
During the course of their surveys, EPRI uncovered consistent problems with the steam turbines, generators, plant startup and shutdown equipment, and the plant’s control valves. For the steam turbines in particular, they identified a few core problems stemming from common cycling practices:
- Damage to the turbines’ airfoils
- Resonance that led to failure in last-stage blades
- Cracks and deformation in the turbine casing
- Rotor bow
- Issues managing differential expansion
- Rubbing between radial/axial rotating and stationary components
These issues, in many cases, resulted in equipment failure which led to costly outages. The vast majority of issues reported were serious enough to require repairs, replacement, and modification of key components. Those issues listed above are also only in reference to the steam turbines themselves; EPRI also identified many common problems with the other components in the system.
For example, one problem frequently found with the control valves involved cracking caused by thermal fatigue. Multiple cooling and heating cycles can lead to cracking caused by quenching, which – if left unchecked – will eventually lead to outages.
The cost of an outage is almost always higher than the cost of repairing a minor or moderate deficiency before it becomes a major point of failure. EPRI’s survey hopefully will lead plant managers and operators to identify key problem areas in CCGT plants and their steam turbine systems and make plans to have experts repair, modify, or re-engineer these areas. Ongoing plant maintenance is also recommended.
More information about the EPRI study can be found at PowerMag.com.