GT3 Upgrades Boost Safety, Reliability

New cooling and fire suppression systems mean less maintenance and lower operating costs



Geoff Oldfather, (520) 586-5465, C: (520) 444-3473,

J.D. Wallace, (520) 586-5157, C: (520) 235-4203,


BENSON – Arizona G&T Cooperatives (AzGT) recently completed work on GT3, one of AzGT’s workhorse natural gas peaking units at Apache Generating Station. A new cooling system incorporates three larger, more efficient fans, and the spray-bar cooling system now uses purified water to prevent scale build-up on the fans.

“These projects upon completion position AzGT to serve its Members and customers more reliably and with lower maintenance costs,” said Mike Nelson, AzGT executive director of power production. “The new system has greater capacity; we’ve got three fans instead of two. It’s a much better design with a much larger cooling capacity.”

GT3 was installed at Apache Generating Station in 1974. The original design included a two-fan heat exchanging unit that runs oil through finned pipes to lubricate the turbine and generating bearings. The cooling fans blow air across the finned pipes, dissipating heat into the air.

To control temperatures, a spray-bar system was eventually added to the unit, which sprayed water onto the cooling fins. However, scaling built up on the fin pipes and cooling fans, which required extra  maintenance. The new project changed inlet air-cooling water from raw to purified water, reducing the potential for scaling and related maintenance.

A new fire suppression system was also installed. It uses a bank of cylinders holding compressed carbon dioxide, which replaces the old system of halon gas. The new system is more effective, less expensive to maintain and operate, and safer for people and the environment.

The projects required a tremendous amount of work over a short period, culminating with a test start of the upgraded GT3 in January. GT3 has been returned to standby service as a peaking unit, which means it will be used as a backup to Apache Station’s main units when electricity demand peaks in AzGT’s service area—usually during hot summer months when people need more electricity for air conditioning.


About Arizona G&T Cooperatives

Arizona Electric Power Cooperative (AEPCO) and Sierra Southwest (Sierra) together comprise the Arizona G&T Cooperatives ( AEPCO owns and operates the 605-megawatt (combined gross) Apache Generating Station, located at Cochise, east of Benson. AEPCO also owns, operates and maintains 841 miles of electric power transmission line—including line owned in part with other utilities—and 33 substations to provide wholesale electric power from Apache to six Member distribution cooperatives in southern Arizona, western New Mexico, northwestern Arizona and California.

Sierra is the vehicle to develop new ways to serve the renewable energy needs of AzGT Member cooperatives and customers, and helps maximize solar and other renewable tax credits. Sierra has initiated two utility-scale solar projects; the 20 MW Apache Solar project on AEPCO property adjacent to and northeast of Apache Generating Station, and SunAnza, the 2 MW solar array adjacent to the Anza EC headquarters in Anza, California.

Combined, the distribution cooperatives that receive AEPCO’s wholesale power serve more than 161,000 meters representing more than 420,000 individual residential, commercial, agricultural, and industrial member/consumers.

The Class A Member cooperatives that receive wholesale power from AEPCO include Duncan Valley Electric Cooperative, Duncan; Graham County Electric Cooperative, Pima; Mohave Electric Cooperative, Bullhead City; Sulphur Springs Valley Electric Cooperative, Willcox; Trico Electric Cooperative, Marana; and a California member, Anza Electric Cooperative, Anza, California.

These Member cooperatives own AzGT and, by extension, AzGT’s Member distribution cooperatives are owned by their members—the people at the end of the line who use the power.