Gas-fired combined cycles are a significant percentage of the country’s power generation capacity and the low price of natural gas means these facilities are running at higher and higher capacity factors.
A huge wave of these facilities hit the industry in the 1997-2002 time frame and now these plants have around a decade of service under their belts. One of the three main subsystems of these plants is the heat recovery steam generator (HRSG) and the premier meeting for discussing problems and solutions is the HRSG User Group Conference.
A big topic of discussion at the annual confab last week was attemperator failures and limitations with their controls. The objective of an attemperator is to control high-pressure superheater and reheater steam temperature to within design limits. One way to prevent failures is to improve the attemperator control scheme or at least make sure the control system in place is working as intended.
The simplest control logic used at a majority of combined cycles is a temperature feedback loop, but this has led to excessive dead-band at many facilities. So-called cascade control is much more complicated, according to information presented at the conference, but also far more effective. Many plants with simple temperature feedback have modified the control loop for cascade control – 64% of plants surveyed have simple feedback control today, but that figure was 73% three years ago. Some plants are demonstrating feedforward control, such as by using gas turbine fuel demand as a signal, which users report may take the hysteresis out of the control loop.
Survey data presented indicates that 36% of facilities experience attemperator control instability. While this is down from 44% in 2009, it confirms that many plants need to review their situation.
If your facility is part of the one-third of the combined cycle fleet experiencing problems with the HRSG attemperator, the specialists at Hurst Technologies can help. We’re exclusively focused on I&C and control systems. We can review the design and the control logic; assess the condition of the instrumentation, such as the thermocouples; recommend effective modifications; write and review bid specifications; design new control systems, and see the project through startup.
The specialists at Hurst Technologies, Corp assisted NRG Energy with the burner management system (BMS) upgrade for Limestone Generating Station Units 1 & 2. Both are 800-MW units fired with Texas lignite. After successful completion of this project, NRG asked Hurst to undertake BMS upgrades for the 600 MW, PRB-fired Units 7 & 8 at W. A. Parish Generating station, one of the largest electric power stations in the country.
Included in the scope of work were the following:
- Write bid specification
- Review bids, develop bid comparison matrix, and resolve differences
- Recommend complete replacement or BMS processor upgrades
- Review new system logic
- Conduct factory acceptance test
- Supervise installation
- Conduct site acceptance test
- Lead startup with new system
Under Hurst’s guidance and recommendations, both plants were able to avoid total BMS replacements and new equipment purchases, and instead simply replaced the processors in the existing control system. This saved $100K for each 600 MW unit and $300K for each of the 800 MW units. All work was accomplished without having to extend any scheduled outages.