Probe Links Gas Venting to Explosions

Federal recommendations to change the National Fuel Gas Code to prevent disastrous explosions involving gas purging came only three days before a Feb. 7 natural gas explosion at a Middletown, Connecticut, power plant that killed six people and injured nearly a dozen others.

The Chemical Safety Board strongly cautions natural gas power plants and other industries against the venting of high-pressure natural gas in or near work sites," CSB lead investigator Don Holmstrom said.

"This practice, although common, is inherently unsafe," Holmstrom said.

The recommendations to change existing codes — approved by a 2-to-1 vote of the CSB board — grew out of the agency's ongoing federal investigation into the June 9, 2009, natural gas explosion at food processing plant in Garner, North Carolina, which caused four deaths, three critical life-threatening burn injuries, and other injuries that sent a total of 67 people to the hospital.

The recommendations urge the National Fire Protection Association, American Gas Association and the International Code Council to strengthen the national fuel gas code provisions on purging. CSB Chairman John Bresland said the board is concerned that companies across the country continue to purge pipes indoors.

"Currently, the codes of the NFPA and ICC do not require gases to be vented outdoors or define adequate ventilation or hazardous conditions, nor do they require the use of combustible-gas detectors during these operations," Bresland said. "The CSB recommendations, if adopted, would urge that these things be done."

An NFPA panel responsible for the fuel gas code voted in February to move forward with the CSB's recommendations on gas purging.

A standard cleaning practice known in the natural gas power industry known as a "gas blow" became the central focus of the Middletown explosion, Holmstrom said. The 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," Holmstrom said. "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."

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, Holmstrom said.

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, Holmstrom said during a Feb. 25 press conference.

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.

A photograph taken shortly before the accident shows the high-pressure gas venting out of one of the open pipe ends.

"Determining the exact ignition source is not a major focus of our investigation at this point," Holmstrom said. "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."

In preliminary findings, CSB investigators determined that the catastrophic explosion in Garner resulted from the accumulation of significant amounts of natural gas that had been purged indoors from a new 120-foot length of pipe during the startup of a new water heater in the plant that made a popular beef-jerky product. During pipe purging, workers feed pressurized gas into a pipe in order to displace air or other gases so that only pure fuel gas remains in the piping when it is connected to an appliance such as a water heater or boiler.

Prior to the accident, management had begun a project to install a new industrial size, gas-fired water heater at the facility. New gas piping was required for this — a 120-foot-long section running horizontally along the roof.

After the piping was installed, it contained air which had to be removed and replaced by natural gas. Natural gas was fed into the pipe and released through one or more openings in the pipe at the other end near the water heater, Holmstrom said.

"In the course of our investigation, we were told by companies, fire and building code officials, and inspectors, that purging natural gas piping into buildings is a common practice," Holmstrom said.

The released gas was vented indoors inside a utility room intermittently over a two-and-a-half-hour period, where the water heater was located. As contract workers tried unsuccessfully to light the water heater, a large amount of natural gas escaped into the building.

That gas found an ignition source and exploded, Holmstrom said.

The deaths and injuries were caused primarily by the widespread collapse of the building structure, including prefabricated concrete roofing slabs that are known as "double tees." Each concrete double tee weighed 11 tons or more; following the explosion, a large number of double tees came crashing down toward the floor more than 20 feet below.

The construction of the building, using pillars, girders, and double tees was such that even a fairly modest explosion would cause big sections of it to collapse, Holmstrom said. Structural damage was the largest contributor to the deaths and injuries that resulted that day.

In addition to the loss of life and the serious injuries, the explosion caused serious and extensive structural damage to the packaging area of the plant. 

The explosion also damaged piping from the plant's large refrigeration system which contained ammonia, a toxic chemical. This release hampered emergency response efforts.

Next on the agenda for investigators is a blast analysis to determine the overpressures involved in the accident. Unfortunately, the vacuum pump room is not expected to be rendered safe for inspection until sometime in March, Holmstrom said.

"In December, for the first time, we were able to do extensive safety analysis after enough support and stabilization had occurred to enter the packaging area which is approximately 80,000 square foot part of the overall factory," he said. "We did extensive analysis of the damage that occurred. One area that we talked about is these 11 ton or heavier prefab concrete roof supports that have fallen down. One of the area's where they have fallen down is where we believe the gas purging activity had taken place, the utility room that we often refer as the vacuum pump room."

Although a direct physical inspection has been impossible, photos of the vacuum pump room as it exists today offer valuable evidence, Holmstrom said. The threaded cap from an open two-inch gas line is clearly visible on the ground.

"It looks as though this was intentional activity," Holmstrom said. "The evidence is the workers were having trouble lighting this industrial water heater and the fact that the cap had not been screwed back to the line might have been the reason." Another photo shows that the pressure gauge had been removed from the line as well as the cap.

"Purging flammable gases into building interiors is a recipe for disaster. At ConAgra, we determined the accident would not have happened had the gas been vented safely outdoors through a hose or pipe." Holmstrom noted that since the June 2009 accident, the Garner plant has instituted strict policies on purging, requiring it be done to safe outdoor locations.

As proposed, the CSB recommendations would urge the NFPA and the American Gas Association (AGA), enact tentative interim and then permanent changes to the National Fuel Gas Code. These would require that purged gases shall be vented "to a safe location outdoors, away from personnel and ignition sources."

In cases where outdoor venting is not possible, companies would be required to seek a variance from local officials before purging gas indoors, including approval of a risk evaluation and hazard control plan. The recommendation would also require the use of combustible gas detectors to continuously monitor gas concentrations; the training of personnel about the problems of odor fade and odor fatigue; and warnings against the use of odor alone for detecting releases of fuel gases.

Holmstrom noted shortcomings in the current codes governing gas purging. 

"Our review of the current codes produced by the National Fire Protection Association, or NFPA, and the International Code Council, or ICC, shows the following:

They do not require gases to be vented outdoors. They do not define adequate ventilation or hazardous conditions. They do not require evacuation of nonessential personnel. And they do not require the use of combustible gas detectors. That is crucial because relying on the sense of smell to detect the odor of fuel gases is unreliable because of odor fatigue. After a period of time, humans are unable to smell the odor of the gas. And new pipes in particular can also absorb the odorant out of natural gas, making it difficult or impossible to smell."

The CSB issued a safety bulletin in October 2009 entitled "Dangers of Purging Gas Piping into Buildings." The bulletin's key lesson is: "Purging new or existing gas piping into a building can be highly hazardous due to the possible accumulation of gas … and the associated danger of fire and explosion." It notes that large numbers of workers are at risk, including plumbers, gas installers, maintenance workers, contract supervisors, and industrial facility managers.

"The CSB has examined several other similar accidents in which gas was purged indoors and not detected," Holmstrom said. "We have determined that workers cannot rely on their sense of smell to warn them of danger, in part because people become desensitized to the odorant added to natural gas and propane. Gas detectors must be used."

Other incidents examined by the CSB include: a 1999 explosion at an auto factory power plant in Dearborn, Michigan, killing six, injuring 38, and causing a $1 billion property loss; a 2008 explosion at a hotel under construction in San Diego, California that injured fourteen people; a 2005 school explosion in Porterville, California, burning two plumbers; and an explosion at a hotel in Cheyenne, Wyoming, in 2007 severely burning two plumbers.

The CSB is investigating possible alternatives to this practice, including the use of air, steam, nitrogen, or water or the use of combustion devices to safely destroy the gas. Combustion devices like flares can safely burn up flammable gas or vapor, preventing the possibility of an explosion.

The type of purging described in that code is different from the gas blows used in the power industry, and power plants remain exempt from the national fuel gas code. However, gas purging as defined in the code has certain similarities to gas blows, in that gas is applied at one end of a pipe and gas is intentionally vented at the other end to the atmosphere.

There is an underlying common theme among the tragic accidents purging-related accidents.

"Companies must ensure that flammable gases are not vented into close proximity with ignition sources and workers," Holmstrom said. "That is a vital safety message from all these tragedies. We encourage the gas power industry to closely study the very positive actions recommended by the NFPA and the American Gas Association committees."

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. The CSB is headed by five board members appointed by the president and confirmed by the Senate.