The practice of using flammable natural gas to clean power plant piping - which led to the fatal explosion at Connecticut-based Kleen Energy on February 7 - has been commonly used across the gas-fired power generating industry, CSB investigators say.
The explosion, which killed six workers and injured at least 50 others, occurred during a “gas blow” - a planned effort to clean out new fuel-gas piping leading to combustion turbines by directing high-pressure natural gas through the pipes and out of vents located near ground level, adjacent to the power generation building. The gas accumulated above the lower explosive limit and ignited, causing massive damage to the new billion-dollar facility, which was nearing completion.
The ignition source for the blast has not been determined. Investigators from the U.S. Chemical Safety Board in Washington, D.C., say the construction site had many possible ignition sources. Investigators also note that gas blows could self-ignite if debris ejected from piping impacted other objects nearby, creating sparks - adding to the risk of the practice.
The plant was designed to use a “combined cycle” to efficiently generate electricity. In this type of facility, natural gas is combusted to drive massive turbines; then, residual heat is recovered from the exhaust gases to produce additional power through the use of steam turbines. Newly constructed pipes frequently have debris and other contaminants that can damage gas turbine blades, necessitating some form of pipe cleaning prior to startup.
CSB investigators will present the new findings, derived from a survey of 62 representatives from the combined-cycle gas power industry, at professional society meetings in Maryland and Connecticut this week. Thirty-nine survey respondents (63 percent) indicated their companies had at some time used flammable natural gas to blow out piping. Only one of those 39 respondents said a flare was used to safely combust the gas prior to venting to the atmosphere.
According to the survey, using natural gas to clean pipes remains the most common single practice in industry, employed by 37 percent of respondents. The other respondents reported using nitrogen, which is nonflammable, or inherently safer alternatives such as air, steam or cleaning pigs. Just 18 days after the explosion at Kleen Energy, on February 25, the CSB stated that natural gas blows were “inherently unsafe” and urged industry to seek alternatives.
“The industry survey confirms that there are readily available safe alternatives to using flammable natural gas for pipe cleaning,” says CSB Investigator Dan Tillema, P.E. “At the same time, a disturbing number of companies continue to use natural gas, which creates the serious risk of a fire or explosion.”
“Venting any significant amount of natural gas into a workplace is an open invitation to disaster,” says CSB Chairman John Bresland. “With more than 120 new gas power plants slated for completion in the next five years, there is an urgent need to ensure safety during the construction and maintenance of gas piping. The CSB will be considering recommendations to promote safer practices in industries that use natural gas as fuel.”
In February 2010, the CSB issued urgent safety recommendations to the National Fire Protection Association (NFPA) to prohibit indoor venting of natural gas during purging operations within the national fuel gas code, known as NFPA 54. However, the explosion at Kleen Energy the same month occurred during outdoor venting of a massive quantity of gas. In any case, power plants are exempt from the national fuel gas code, which is developed and maintained by nongovernmental consensus committees administered by the NFPA and the American Gas Association, an industry group.