Utility executives, entering an era of caution rather than construction, are studying ways to extend the life of existing generating facilities beyond the traditionally alloted 40 years.

Across the river from the District of Columbia in Alexandria, Potomac Electric Power Co. engineers and technicians have been poring over the company's Potomac River Generating Station, trying to ascertain how long--and at what cost--that plant's life might be prolonged.

With five units completed between 1949 and 1957, the Potomac River plant would be facing the end of its life in the 1990s if past were prologue. But a number of factors now argue for keeping geriatric generators alive.

Among the compelling reasons are high construction and capital costs and the long lead time that a new plant requires--approximately 10 years. In addition, for the first time since shortly after the Depression, demand for electricity has been going down instead of up. As a result, demand forecasts have been scaled down and utility executives are much more conservative about putting a lot of money into a new plant.

Still another factor, according to industry officials, is that the technology of coal-fired power plants has not changed dramatically enough to promise improvements in efficiency of the magnitude new power plants once produced.

"All those things mean that a new plant coming on line does so at tremendous costs and no longer promises improved efficiency" large enough to merit the high costs, said Pepco Senior Vice President Paul Dragoumis.

"Companies have been in the midst of large construction obligations over the last five years," said Douglas Bauer, senior vice president for economics for the Edison Electric Institute. "Now they're starting to catch their breath and beginning to look ahead and they're saying, 'Is there any way we can mitigate large increases in capital obligations by extending the life of existing facilities?' "

Bauer said that about 15 to 16 percent of current capacity "must or should come out of service in the 1990s." He said that utilities currently plan to retire about two-thirds of that capacity and are looking for ways to keep the rest of it running.

Pepco began studying the Potomac River facilities two years ago and appears to be further along than many utilities toward a systematic program for meeting future demand by plant life extension. Most utilities are in the "mulling over" stage, Bauer said.

The final decision has not yet been made on prolonging the Potomac River plant's life in the sense that the utility company's comprehensive energy plan still envisions a new plant in the 1990s. That may change with the next draft of the plan, however.

"We think the economics of the thing is a shoo-in," Dragoumis said.

"About two years ago we began to look seriously at using this Plant Life Upgrade Study (PLUS)," said Pepco Vice President John Rasmussen, who is in charge of generating, engineering and construction. "We don't want to just have superannuated power plants out there. We want good quality plants.

"What the corporation has decided is to take the car to the garage and see if we can keep it running," Rasmussen said. "Bill Herrman, director of planning, will decide in terms of the comprehensive energy plan whether to spend the money that takes or to buy a new car."

Pepco found that the heat rate (an efficiency measure) was declining by about 2 percent a year in the units it examined; maintenance costs were going up about 5.2 percent a year, and the amount of time that the plant was available and operating was declining by about 1 percent a year.

"Those indicators showed us what one would expect--sort of a graceful degradation of the performance of the units," Rasmussen said. The first studies searched for "any kind of fatal flaw where fixing it would be more expensive than cost-effective," he said. Pepco workers looked at generator insulation and turbine shells, ran metallurgical tests and otherwise tried to get a sense of the shape of the units.

"There are hundreds of different things, different components that you have to address separately," said Rasmussen. The only major part that needed replacement right away was the last stage of a superheater in Unit One that will cost $1.65 million.

"The surprising thing is that it's in good condition," engineer Robert Carelock said.

"The good news is that there are no fatal flaws, no catastrophic situations facing us," Rasmussen added.

Eventually, Pepco will examine all of its coal-fired units, according to officials. Some units, such as oil-fired units at Benning Road and Buzzard's Point, were ruled out as candidates for extension because their foundations were crumbling and because they depended on more expensive fuels.

Besides fatal flaws, the wrong type of fuel can make a plant too expensive to continue. The cost of meeting air pollution controls, the cost of acquiring a site for a new plant and the cost of maintenance are all items that enter into such decisions.

Vendors, contemplating the same future uncertainties as electric company utilities, are adjusting in similar fashion. Instead of building as many new power plants, major companies such as Westinghouse now emphasize plant-life extension.

"Since utilities were planning on much higher growth, and because it takes 10 years to build a power plant, there were many units in the system that were not really needed for the load growth," said Gene Cattabiani, executive vice president of Westinghouse.

"When demand was growing rapidly what you did was to go out and buy a new plant. In a growing market you approach in in one direction; in a shrinking market, you approach it in another," Cattabiani said.