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,"
"This practice, although common, is inherently unsafe," Holmstrom said.
The recommendations to change existing codes — approved by a 2-to-1 vote of the
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.
"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
An NFPA panel responsible for the fuel gas code voted in February to move forward with the
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
Industry personnel have indicated to
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,
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
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."
Other incidents examined by the
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
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