Different types of pressure cylinders behave differently when ex-posed to fire. Depending on the gas and the relief valve used, a cylinder might vent rapidly or rupture violently, said Steve Selk, investigation manager for the U.S. Chemical Safety and Hazard Investigation Board.
"Whereas an LPG cylinder might BLEVE, an oxygen cylinder may fail differently," Selk said. "At high temperatures the oxygen might attack the metal of the cylinder leading to a failure that way."
Learning more about those differences is only one avenue of the CSBs investigation into the damage wreaked June 24 when fire and explosions swept through a St. Louis facility for transferring industrial gases from bulk storage tanks into smaller pressure cylinders for sale. Propane, acetylene and various non flammable gases were involved.
Other than to raise aerial ladders to protect exposures along the edges of the facility, St. Louis firefighters could do little to save it.
Fragments of shattered pressure cylinders were hurled into surrounding residential neighborhoods, resulting in a widespread evacuation, Selk said. The CSB investigation has been focused primarily on damage that occurred offsite, he said.
"We found dozens of fragments, some weighing as much as 100 pounds, throughout the neighborhood," he said. "One hit a church. Three fragments hit homes. One fragment knocked a hole about two feet in diameter in the wall of one home. Cars were hit by debris and set ablaze."
One fragment found by CSB investigators landed 900 feet from the plant. Other fragments reportedly landed even further away, Selk said.
"The damage to the immediate area certainly raises some interesting issues with regard to the effect that thermal flux has on these cylinders," Selk said. "It also raises some interesting issues about the design of relief devices for pressure cylinders."
The facility had bulk storage tanks for helium, argon and carbon dioxide, but no bulk storage for flammable gases. Other gases on hand also included propane, propylene and oxygen. The majority of pressure cylinders at the facility contained acetylene. The facility also had a small amount of toxic material such as hydrogen sulfide, sulfur dioxide and chlorine.
At about 3:15 p.m. on the day of the fire several individuals observed a flame directed upwards in the southeast corner of the facility's outdoor storage area, Selk said. The facility was immediately evacuated.
"There was no fixed fire protection outdoors," he said. "When workers evacuated the facility the fire began to grow. That's all we know right now."
Investigators are studying any simularities between the St. Louis incident and disasters at similar facilities in Tulsa, OK, in August 2003 and Miami, FL, in March 2004, Selk said.
Technically, the term describing when one cylinder failure contributes to others in a congested area is "fratricide," he said.
"One wonders if fixed fire protection could have kept things cool and prevented failures that soon got out of control," Selk said.
A statement issued by the owners of the facility reports that asbestos has been found in the ashes lying on the ground at the plant and in some intact pieces of debris outside the plant. All tests for airborne asbestos and other potentially hazardous substances conducted by the Missouri Department of Natural Resources (DNR) have been at acceptable levels. Asbestos is not hazardous to human health unless it is airborne.
Tests conducted by the DNR since the fire have shown the debris on the ground at the company's facility contains ashes, carbon silicate and asbestos. Carbon silicate and asbestos are used inside acetylene cylinders, many of which were destroyed in the fire. Use of asbestos in acetylene cylinders ceased in 1985. The traces of asbestos lying on the ground are not considered hazardous.
The company also announced it is working to relocate its facility away from residential neighborhoods. Selk said local attention focused on continued operations at the site is to be expected.
"Any time such a large number of fragments is directed at high velocity through a residential neighborhood I wouldn't be surprised that there is community concern." o