April 2013, Vol. 68 No. 4
Features
Priorities In Dealing With Gas Line Hits
While some projects are more dangerous than others, most construction work involves risks that can cause serious injury and death.
Underground utility construction often involves installations of pipe and cable in easements crowded with multiple utilities — telecommunications, water and sewer pipes, power cables and natural gas lines. Accidentally damaging any utility is bad: essential services are interrupted, costly repairs are required, work on the project that caused the hit is delayed and most seriously, the safety of crew members and the public are exposed to risk.
Hitting a power line endangers construction crew members and others in the area of the accident, but damaging a gas line can result in a major disaster. Consider the 1998 incident in downtown St. Cloud, MN, that killed four people, injured 15 others, and destroyed three buildings, damaged another five so badly they had to be demolished, and required extensive repairs for 14 other structures.
The cause? A 1 1/8-inch plastic gas line was severed when workers, drilling a vertical hole to install an anchor for a guy line to support a utility pole, struck a large buried slab of concrete, bending the anchor rod and causing it to strike the gas line. Escaping gas migrated to an unoccupied building where it was ignited about 20 minutes after the pipe was struck.
Procedures are in place to prevent accidental utility hits: yet still they occur. When that happens, construction personnel must respond properly and without delay.
Jerry Gann, district director, Houston Northwest District, CenterPoint Energy, Houston, TX, provides some insights and “dos and don’ts” when excavator damage to a gas pipeline occurs.
“When a gas line is damaged and there is a release of gas, the two most important things are to protect life and property,” said Gann.
“For construction personnel on the job site,” he continued, “the first steps for crew members to take are to remove any ignition sources that could ignite escaping gas, including turning off engines, electrical devices and gas lights — anything that could cause a spark — evacuate and secure the area, and call 911 and the gas service provider.”
There also are important ‘Don’ts’:
• Do not backfill an excavation where the pipe was hit;
• Do not fill in a bore hole in which the pipe was struck;
• Do not attempt to put out a fire if escaping gas ignites; and
• Do not attempt to repair the damaged pipe.
Gas properties
In regards to the first ‘don’t,’ Gann said leaking gas will take the path of least resistance which can be a water or sewer pipe, the space between a tree root and surrounding soil or other available conduits. However, an open excavation allows the gas to escape into the outside air where it will rise and vent into the atmosphere, helping to dissipate from the site of a leak.
A basic understanding of the characteristics of gas can help workers better understand how to deal with a gas line accident.
Gas is a naturally occurring hydrocarbon composed of methane (CH4), which is principally found in underground formations of porous rock. It is colorless, odorless, tasteless, flammable, non-toxic and lighter than air. In its natural state, it is not possible to see or smell it. For safety reasons, Mercaptan, a chemical odorant that smells a little like rotten eggs, is added to natural gas so that it can be smelled if there is a gas leak.
Natural gas has a limited range of flammability of 5 to 15 percent gas-to-air mixture. The ignition point of gas is high — 1,000 to 1,200 degrees Fahrenheit.
Gann said that by understanding the characteristics of natural gas, construction personnel can make informed decisions about how to react when a gas pipeline is damaged.
“Making sure the public is safely evacuated, securing the area and calling 911 and the gas company are key front lines of defense to ensuring the situation does not get worse,” he said. “Once the emergency responders and gas company personnel are on the site, the excavator should set up liaison activities with those groups to disseminate key information and offer any needed assistance. Gas company personnel will make the necessary repairs to shut off the flow of natural gas.”
The best way to avoid all the problems resulting from accidental utility hits is to prevent them such as simply calling 811 before digging.
Accurately locating and marking underground facilities is the first step in preventing damage to buried infrastructure, and the nation’s 811 One-Call System provides a means for locating and marking underground facilities before construction begins. On private property projects or where utilities are operated by organizations who are not members of One-Call, other methods are necessary to identify and mark the locations of underground lines.
Most high-pressure transmission gas lines and liquid lines are marked with signs. But most gas distribution and service lines are unmarked and contractors and utility crews must depend on 811 One-Call or other locating sources. Contractors often make these locates themselves and also verify markings made by One-Call.
Lessons learned
So what is to be learned from all of this?
First, said Gann, prevention is the main key by calling the 811 One-Call before excavating, then digging carefully around the markings.
In the event of accidental pipeline damage, remember (in this order) to protect life, then property, he emphasized.
“Remembering those two things will help guide your actions in the critical first few moments of a gas leak,” Gann said. “When making the area safe, the characteristics of natural gas will remind you it is colorless, will go up in the air like a balloon and if covered up will migrate taking the path of least resistance. So take the appropriate actions and set up barricades accordingly.
“Finally, working with emergency responders and gas company personnel will help expedite the safe repair of the pipeline. For more information about pipeline safety, visit the local gas company’s or pipeline operator’s website,” he suggested.
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