March 15, 2006

O&M staff keep their cool at Alaskan plant

Peter S. Jackson, PE, Tetra Engineering Group Inc. and John Kotrotsios, Chugach Electric Association Inc.

Thermal generation neighbors

The newest, indoor ABB-11DM units (Figure 4) at Beluga were installed in the late 1970s, just as utilities began to deploy the first combined-cycle plants in search of higher efficiencies and lower heat rates. Remarkably, the units incorporate some design features common in today's modern HRSGs. Among them:

4. An oldie but goodie. The gas turbine powering Beluga Unit 6 is an ABB-11DM installed in the late 1970s. Courtesy: Chugach Electric Association Inc.

• The use of gas exhaust bypass stacks to accommodate simple-cycle operation.
• Interstage attemperation sprays to control main steam temperature.
• Dedicated sprays to control the temperature of bypass dumps to the steam turbine condenser.
• Motor-operated drain valves on high-temperature modules.
• External deaeration systems.

The two GE Frame 5s at Beluga are capable of black starts, thanks to the diesel engines with which they were equipped. This makes the other units black-start-capable, but only indirectly.

Other challenges are provided by Mother Nature. Mount Spurr, an 11,000-foot volcano about 30 miles to the west of the plant, last erupted in September 1992. The event threw enough ash into the air to force the closing of Anchorage International Airport. As a consequence, Beluga was completely isolated for a while and had to dip into its inventory for the plant's necessities: food for the operators and inlet air filters for the gas turbines.

Not to be outdone, Mount Augustine, about 200 miles to the south, erupted in January of this year. This event has not yet produced any ash fallout at the plant, but it has diverted commercial and local air traffic from their normal routes. Two other volcanoes are situated between Spurr and Augustine: Mount Redoubt, which last erupted in 1990, and Mount Iliamna, which has remained dormant over the past century. Because of these and other natural hazards Chugach Electric keeps Beluga well-stocked with supplies at all times.