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The Mariner East pipelines carry natural gas liquids, which are different than methane.

Like all hydrocarbons, natural gas liquids start out as molecules that contain carbon and hydrogen atoms.

In parts of Pennsylvania’s Marcellus Shale formation, they are trapped thousands of feet below ground, along with methane.

There’s ethane, propane, butane.

These hydrocarbons are heavier than methane – they contain more carbon and hydrogen atoms.

Some of these hydrocarbons could be liquid deep underground, but all of them turn to gas when they hit the atmosphere.

When wells for natural gas are drilled and tapped, there is often a mix of hydrocarbons that rise to the surface and get pumped into a pipeline.

This initial pipeline system is referred to as gathering lines.

Liquifying reduces the space between each molecule, making the volume of the gas 600 times smaller, and the long distance transportation more efficient

So the gathering lines take all the hydrocarbons to a processing facility, where pressure and cooling separate the methane from ethane, propane and butane.

Once the methane is separated from the other hydrocarbons, and those others are turned into liquids, they are each shipped through large transmission pipelines to the end user.

For methane, this could be a power station, or to individual homes for heating and cooking.

In the case of the Mariner East lines, the majority of natural gas liquids are going to an export facility in southeastern Pennsylvania, where they are sent overseas in large tanker ships and used to make plastics.

If there is a leak in the pipeline and the natural gas liquids escape, they turn back into a gas once they hit the atmosphere.

Those hydrocarbons, which are now a gas, sink down toward the ground, remember, they are heavier than methane and they are heavier than air.

If a methane pipeline leaks, the methane molecules float upward.

And this is why natural gas liquids leaks are more dangerous than just pure methane pipeline leaks.

Natural gas liquids don’t disperse as easily as methane, they tend to stick together and pool in low-lying areas, making it easier for an explosion.

The risk of a pipeline explosion is difficult to assess.

For an explosion to occur, there needs to be just the right amount of oxygen – if there’s too little it won’t ignite.

Think of trying to make a fire, sometimes you need to blow on it for it to catch…

But if there’s too much oxygen, the molecules are too dispersed, think of trying to build a fire on a windy day.

An explosion would also need a spark, or ignition source,

This can be an item used every day like a light switch, a car, a doorbell, a cellphone, an air conditioner, something that produces a charge or heat.

And there needs to be containment – something that encloses the leaking gas, even if it’s a ditch or depression near the pipeline.

If a leak occurs on the Mariner East 2, and an explosion takes place, the consequences could be catastrophic.

According to one risk assessment, conducted by Quest Consultants, the impact could extend half a mile in radius from the point of the explosion, and that entire area could experience a flame cloud.

The pipeline operator, Energy Transfer Partners, and Sunoco have a monitoring system that detects a leak if there’s a change in pressure on the line.

But it’s difficult for the average person living near the pipeline to detect, because natural gas is colorless and odorless.

A leak could make a sound like a small hissing, or large roar. It could also discolor nearby vegetation and cause dirt to blow around, or water to bubble, or it could cause the ground to freeze in warm weather.

Valve stations along the pipeline exist for safety reasons. If there is a leak, the operator can shut off the flow of gas more quickly.

If an explosion occurs, responders shut off the valves, and let the fire burn off.

If a leak does happen, you should leave the area by foot, not car, don’t turn on any electronics nearby and only use your cellphone to call 911 when you know you are safely away from the leaking gas.