When the road salt seeps, sometimes the manhole covers fly

AP News
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Posted: Mar 24, 2015 6:37 PM
When the road salt seeps, sometimes the manhole covers fly

WASHINGTON (AP) — Call it another form of March Madness: Not flying basketballs, but flying manhole covers.

Scientific literature traces manhole explosions back nearly a century, but a series of such incidents in Indianapolis, host of the NCAA basketball championships, has authorities looking for a quick solution.

Good luck with that.

A combination of power system design, winter road salt, older electrical cable insulation and basic chemistry have triggered underground explosions in older downtowns, launching 350-pound manhole covers high in the air. One Georgia Tech engineering professor calculated the explosions could have the force of three sticks of dynamite.

"They have found a manhole cover on top of a building in a certain downtown city," said Daniel O'Neill, who advises several utilities on the problem. "They are dangerous things. There are hundreds of these things happening every year."

Manhole covers have launched several stories in the air, O'Neill said.

The nonprofit Electric Power Research Institute's lab in Lenox, Massachusetts, has spent the last 25 years setting off what officials there call "manhole events." It's not for fun. Engineers are trying to find a way to keep manhole covers from flying.

"We're disappointed to say we've not yet solved the problem," said Matt Olearczyk, manager of distribution research for EPRI. He said, his team will keep at the problem "or we're going to die trying to fix it."

The EPRI team has come up with partial solutions, such as latching manhole covers to the ground with a hook-and-piston system. When there's an explosion, those covers lift a few inches to let off some pressure, but not so much as to let in oxygen to stoke the explosion.

Experts do know how and why these explosions happen amid thousands of miles of tightly bundled electrical cables.

It starts with the way electrical power is distributed in older downtowns underground. Cables are linked so that if one fails, others take over, O'Neill said.

Cable insulation can fray or kink due to age, wear and tear, high power loads during the summer and corrosive road salt. That exposes wiring, which can spark and smolder. Especially when the insulation is older and consists of an oily paper, that releases gases, including hydrogen, methane, acetylene, carbon monoxide and ethylene, O'Neill and Olearczyk said.

Then, salty or dirty water gives the electricity a path to the ground and the spark to set off explosions, O'Neill said.

That's why O'Neill and Olearczyk say they see more blasts events during the winter and in more northerly cities. The salt is a key ingredient. Consolidated Edison once compared manhole explosions to the streets where road salt was used and found a good correlation, O'Neill said.

The expensive process of replacing the cables with plastic insulated modern cables works well, Olearczyk said.

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Online:

EPRI You Tube video of manhole cover explosions: https://www.youtube.com/watch?v=oxTIoWu__-A

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Seth Borenstein can be followed at http://twitter.com/borenbears