In a statement last Thursday, the U.S. Chemical Safety Board (CSB) recommended that power plants and industry discontinue the gas venting practice that resulted in the massive explosion on Feb. 7 at the Connecticut Kleen Energy plant, which was not yet online.
The explosion in Middletown, Conn., killed six and injured 26 people. CSB Lead Investigator Don Holmstrom noted that “Just three days prior to this tragic accident, the Chemical Safety Board recommended changes to the National Fuel Gas Code to prevent disastrous explosions involving gas purging.”
According to Holmstrom’s press statement, “This accident occurred during a planned work activity to clean debris from natural gas pipes at the plant. To remove the debris, workers used natural gas at a high pressure of approximately 650 pounds per square inch. The high velocity of the natural gas flow was intended to remove any debris in the new piping. At pre-determined locations, this gas was vented to the atmosphere through open pipe ends which were located less than 20 feet off the ground. These vents were adjacent to the main power generation building and along the south wall.
“This cleaning practice is known within the natural gas power industry as a ‘gas blow.’ Industry personnel have indicated to CSB investigators that gas blows are a common practice during the commissioning of new or modified gas pipes at their facilities. [Ed. note: Two large engineering, procurement, and construction contractors told POWER magazine on background that their standard practice is to use compressed air whenever possible to clean out the gas pipes near the combustion turbines.] CSB investigators have reviewed gas utility records for the morning of the accident. These records together with written pipe cleaning procedures and witness testimony confirm that the gas blows occurred intermittently over the course of the morning. At the same time that gas blows were underway, there were potential ignition sources present in the surrounding area, including inside the power plant building. There were many construction-related activities underway inside the building. Determining the exact ignition source is not a major focus of our investigation at this point. In most industrial worksites, ignition sources are abundant and efforts at accident prevention focus first and foremost on avoiding or controlling the release of flammable gas or vapor.
“Initial calculations by CSB investigators reveal that approximately 400,000 standard cubic feet of gas were released to the atmosphere near the building in the final ten minutes before the blast.
“That is enough natural gas to fill the entire volume of a pro-basketball arena with an explosive natural gas-air mixture, from the floor to the ceiling. This gas was released into a congested area next to the power block building. This congested area likely slowed the dispersion of the gas. The gas built up above the lower explosive limit of approximately 4% in air and was ignited by an undetermined ignition source.”
The Chemical Safety Board is an independent federal agency that investigates and reports to the public on the causes of major chemical accidents at industrial sites across the country.
Sources: CSB, Hartford Courant