For several decades, gas turbine combined cycles have been the backbone for natural gas baseload facilities. However, as markets changed and renewables became firmly rooted within generation portfolios, these facilities are now operated for on-demand generation. This means gas turbine combined-cycle facilities have to develop faster ramp rates, enhanced load cycling capabilities and greater durations between planned maintenance.
As these facilities approach 20 to 25 years of operation, utilities are adding rotor replacements to their five-year capital plans to meet recommended long-term service agreements and plant life preservation practices. With advanced technologies now available in turbine design, blade geometry and materials, replacement rotors and turbine upgrades could potentially increase a facility’s megawatt output and efficiency. While this increase could also drive returns up, replacing a rotor without evaluating the potential pinch points for downstream components could severely limit plant operations and even potentially have negative impacts to plant operations and capability.
Potential Downside to Increased Generation Capability
In today’s market where increased capacity and efficiency can provide greater returns, a replacement rotor that also increases output could appear to be a potential boon. However, utilities must consider two main issues that may arise:
- Permit allowances: Typically, power plants receive a specific emissions permit when developed based on a plant’s assumed runtime, fuel source, output and more. Therefore, any facility change that could affect the unit’s output or create a new or additional source of pollution — such as replacing a rotor — must be checked against the plant’s existing permit. Often, significant modifications that can increase the plant output, heat consumption or potential to emit may result in triggering a new source review (NSR). These NSR considerations need to be fully considered to determine regulatory impacts and possible implications.
- System deterioration and limitations: The entire design of a power plant hinges on its anticipated generation, and additional megawatt output on an aging facility could mean increased temperatures and flow rates. So if existing equipment was sized for a specific rotor size, and all the conditions that rotor size creates, an increase might create faster wear and tear on the boiler components and boiler feed pumps — potentially taking years off the life of the facility. Additionally, if existing equipment sizing did not include margin or consideration for future potential increased performance, this equipment could prevent the increased capability associated with a turbine upgrade or rotor replacement from being fully realized.
Pinch Point Considerations
While these issues are essential to maintaining the health and compliance of existing facilities, that does not mean that replacing a rotor is inherently negative. Instead, rotor replacement simply requires an evaluation of what pain points might develop downstream, and how to address them before they become an issue.
This thorough evaluation would consider multiple aspects of a facility that may be at risk due to increased generation capacity, including:
- Boiler feed pump capacity
- Condensate pump capacity
- Electrical load center capacity exceedance
- Emission increases
- Fuel gas system limitations
- Generator limitations
- Heat recovery steam generator (HRSG) pressure drop effects
- Heat rejection system limitations
- HRSG tube bundle temperature limit exceedance
- Power factor reduction
- Pressure relief valve limitations
- Selective catalytic reduction (SCR) catalyst life and capability
- Shifts in steam production
- Steam line velocities and pressure drops
- Steam turbine valve limitations
- Steam turbine steam path limitations
- Water supply system limitations
To combat potential limited resources or experience in the pinch point system evaluation of rotor replacements, utilities can turn to an external firm for supplemental technical guidance. Conducted remotely, especially in today’s environment of social distancing, evaluation specialists look at plant recordings, equipment design information and PI or historical operating data, and compare that information to a new model built around system implications of upgrades. By focusing on the performance of holistic facility operations, these specialists can help plan for downstream modifications early, in advance of unit outages, to provide a seamless replacement and return to operation.
Learn more about the evaluations and upgrades that are critical to keeping aging power plants compliant, competitive, efficient and reliable.