Gas turbine instrument air enhancement
Blackhawk Station
Owned by Borger Energy Associates LP, Operated by NAES Corp
230-MW, gas-fired, 2 × 1 combined-cycle cogeneration facility located in Borger, Tex
Plant manager: Craig Courter
Key project participants: Steve Nelson, operations supervisor; Bryan Stout, maintenance supervisor; Alan Bull, project manager
Challenge.
While at base-load operation Blackhawk Unit 1 unloaded from 112% to 60%. Normalized load recovered but was followed by an auto-unload and unit trip. After examining the operating data, the site team concluded that the instrument air pressure dropped from 100 to 22 psig during this event. This caused the main fuel valve to slowly close which starved the turbine of fuel, and in turn, resulted in the hydraulic stage fuel valves being opened by control logic to compensate for the low fuel flow.
The air pressure drop was momentary and when it recovered, the main fuel valve opened back up resulting in over-fuel of the turbine. Subsequent damage resulted in having to replace all combustor and hot-gas-path components; peak-period availability was low as a consequence.
Solution.
The site had struggled in the past with moisture in the instrument air supplied to the turbine enclosure. In an effort to deal with the high moisture, the site installed a secondary air dryer that only treated air supplying the enclosure.A packaged system was installed and had worked well for several years. Within the system there is a shuttle valve that switches air flow between two desiccant filters. The root cause analysis of the failure identified that this was the only point that could cause a momentary loss of control air. Disassembly of the valve revealed a high amount of wear and fouling.
This situation prompted an engineering review of the air supply system and safety shutdown of the gas turbines. The first step to addressing the problem was to design and implement a third source of dry instrument air after the auxiliary dryer. This was achieved by plumbing an alternative air line from the main air dryer system.
The original system was configured in a way that main turbine compressor bleed air and auxiliary compressed air would provide pressure to a regulated system. The GT instrumentation would be fed by the auxiliary air system during startup until sufficient compressor bleed air was established. This is achieved by placing the two systems through regulators with the auxiliary air being set at 5 psi less than the bleed air.
In the new configuration, a third air supply is available to the unit; it is set at 5 psi less than the auxiliary air supply. The third system is also integrated into the system downstream of the auxiliary air dryer. This allows the new air system to be unobstructed in the event that the primary and secondary air supply are obstructed.
The second step to completing the upgrade was accomplished by implementing turbine control logic designed by the OEM that will shut down the turbine if the fuel-gas pressure drops to within 100 psi of the combustor shell pressure; the operator receives and alarm. Auto-unload is initiated at 70 psi and a trip is initiated at 40 psi above shell pressure. Fail over testing was conducted following all improvements and the system is functioning well. The site has now implemented both the physical and control logic changes on both units.
Results.
In response to an over-fuel event caused by a drop in instrument-air supply pressure, plant personnel performed and implemented a comprehensive engineering review, repairs and upgrades, and enhancements to the plant’s digital control system, thereby significantly improving reliability and cost avoidance by over-fuel events.
