After a gas explosion last month in New York leveled two buildings and killed eight people, an old issue received new attention: aging natural gas pipelines that leak.

It can take decades and billions of dollars to replace old steel and cast-iron pipes with plastic ones, but some utilities are making that a priority.

In Philadelphia, after a long, cold winter, construction crews are back at work. On a residential street there, a utility worker is crouched in a freshly dug hole in someone's yard, cutting through an old 2-inch steel pipe. When he's done, gas will flow to this house through a new, bright yellow plastic pipe.

Look out at the street and there's a line of fresh blacktop next to the curb, where crews also replaced a much larger, 6-inch cast-iron pipe that carries gas to the entire block.

When deciding which pipes to replace first, Joseph Hawkinson, superintendent of construction for Philadelphia Gas Works, says the utility company looks at things like the number of gas leaks and repairs. Each year, the company identifies 25 miles of cast-iron pipe to be replaced, he says — it will take 88 years to complete.

That's more than 2,000 miles of pipe, and the price tag is hefty. Right now it's about $77 million a year, Hawkinson says. The cost is reflected in customers' utility bills.

Philadelphia is just one city facing this issue. Research firm SNL Energy looked at federal data and compiled a list of large utilities with the highest ratio of leaks per mile of pipeline.

"It comes out as a very interesting list, [with] concentrations in New York and Pennsylvania," says Michael Carter, a director with SNL Energy. "But also there's other companies up there that have a number of leaks in Texas or Virginia."

New York's Con Edison tops the list with about one leak per mile of line; No. 2 is Philadelphia Gas Works. Considering that they are two of the oldest distribution systems in the country, that's not too surprising. The longer a pipeline is in the ground, the more likely it will corrode or crack when it's cold. Fixing the leaks is important; on top of the safety concern, gas released into the air is bad for the environment.

With incidents like the explosion in New York last month, it can be tempting to conclude that all old pipes need to be replaced quickly. But it's just not that simple, says Richard Kuprewicz, president of Accufacts Inc., which investigates pipeline accidents.

"I keep telling people over the decades, safety isn't free," Kuprewicz says. "However, the excuse for safety is not an excuse to write a blank check."

The key is striking a good balance between safety and cost, Kuprewicz says. These days, utilities are concluding that more money needs to be spent replacing old pipelines. In Philadelphia, customers' bills now include an extra fee. That's helped the utility boost the number of miles of cast-iron pipes it replaces each year by nearly 40 percent.

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