Why PG&E Loses Gas in the Winter

Two weeks ago, a gas outage in the small Contra Costa County town of Discovery Bay left thousands without heat during one of the coldest weeks of the year. The cold itself was to blame: After temperatures plummeted to 26 degrees, the utility shut down a pressure regulator station, leaving some without gas for as long as three days.

Unlike a faulty pipe — such as the bad weld on an aging, Cold War-era gas main that fatally erupted in San Bruno in 2010 — this kind of problem often has little to do with equipment age. (The company is in the midst of overhauling its regulator stations and has replaced 1,000 units annually since 2011.) But, oddly, these machines can fail even when they work as intended. It's a matter of physics.

The lines that pipe gas to homes and businesses are fed by much larger transmission lines that can operate at 1,000 pounds per square inch of pressure, more than 10 times that of gas appliances. That regulator is the “gatekeeper” machine that lets just enough gas out of those big lines and into our homes. There are more than 100 such stations in San Francisco alone, serving thousands of customers each.

But when you decrease gas pressure, you lower its temperature — seven degrees for every 100 pounds. (That's why compressed air feels cold when you spray it.) Let off too much at once and water vapor in the gas condenses, freezes, and jimmies up the works, which is likely what happened in Discovery Bay.

It's rare in the Bay Area. “Normally this happens in much colder climates,” says Don Slack, president of Aqua Environment Co Inc., which manufactures regulators for PG&E. “But you can freeze a regulator when it's 60 degrees out” since, by design, temperatures inside the system get much colder than ambient temperatures.

It's an El Niño winter, and freak weather happens. Discovery Bay was 13 degrees below the historic average low when the outage hit. The National Oceanic and Atmospheric Administration forecasts a slightly warmer winter than usual in San Francisco, but also warns that “cold air outbreaks … cannot be predicted.” And that's when there's danger.

PG&E might heat the gas to provide a temperature “cushion,” or mix in methanol (antifreeze, essentially). Gas could also be further “dried out” with absorbents at the processing stage. (Although, “If you got it as dry as you can, nobody could afford to buy it,” says David Fish, senior vice president of Welker Engineering in Texas.)

At deadline, PG&E could not confirm if it's using any of these methods. In a statement to SF Weekly, spokesman Nick Stimmel said only that the cause of the outage is under investigation and that the company “prepares for extreme weather by forecasting where impacts will be most prevalent,” using “advanced modeling” programs to pinpoint potential trouble as weather fronts develop.

But Mother Nature can fool you. The only way to be sure if the utility can beat the winter is to wait.

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