As manager for two simple-cycle peaking merchant units in Illinois, Reliant Resources’ James Mason knows the critical role that reliable gas compression plays in bringing his units on-line smoothly and quickly. The plants are located in Aurora and in Shelby County.
“We have six GE LM6000 gas turbines at Aurora,” says Mason. “They require delivery pressure of 650 pounds. However, the pressure from the natural gas transmission pipelines is 450 pounds, so pressure must be increased to 650 for the turbines to operate.”
Aurora uses an Atlas Copco centrifugal compressor to boost gas pressure. The single-stage compressor features an inlet pressure of 484.3 psia and discharge pressure of 722.3 psia. Other specific requirements include 1,420 HP and flow of 40,200 SCFM. Pressure boosting also helps raise the fuel temperature above the dew point, thus removing moisture-laden gas ahead of gas turbine nozzles. Gas delivered to the Shelby County station requires no boosting because the supplier delivers the gas pressure at 800 psia.
“What’s especially important for peaking facilities is fast, reliable starts and steady pressure with minimum pressure swings,” says Mason. “Operating with different combinations of units on-line, it is important to have smooth transitions between the various flow demands.” For the compressor to deliver the required pressure with no flow to the turbine during turbine start-up, it operates in a re-circulating mode.
The compressor at Aurora was installed in the first quarter of 2001. “I thought the output would be borderline,” says Mason. “But with the actual line pressure and the heat value of the gas, we were able to operate the compressor with six turbines on line with adequate flow and pressure. The compressor needed a change of trim in a valve to assist with low demand to minimize pressure swings. Tuning had to be performed to ensure that the compressor delivered the required pressure across the range of flow demands. The compressor needed some additional engineering at the site but we worked that all out quickly and smoothly.”
A CO2 system is also needed for extinguishing fires, and an enclosure for reducing noise was built around the compressor and ventilation equipment.