A 64-foot-tall industrial autoclave that separated under 90 pounds per square inch pressure is being blamed for a massive nitrogen release on Dec. 27 in Wichita, Kansas, that blew apart a section of the Beechcraft aircraft plant, injuring 15 workers and contractors.
That pressure is less than it takes to inflate a tire mounted on a big rig, Fire Chief Douglas J. Williams with Sedgwick County Fire District 1 told the county board of commissioners Wednesday.
“We were very, very lucky that this happened during a shutdown period,” Williams said. “The large number of employees that would normally have been there were home for the holidays. Those injured were maintenance personnel and other contractors.”
Initial reports indicated that a ruptured nitrogen line was the cause of the blast. Those reports were incorrect, Williams said.
The blast hurled a 2-ton piece of the spherical autoclave more than 630 feet, taking the corner off a neighboring building in the process. As for the rest of the autoclave, it tried to climb out of a four-foot deep pit dug to protect against exactly what happened.
“The pit is there to prevent the autoclave from rolling across the floor if this occurs,” Williams said. “Even so, it shoved the pit wall and a 12-inch concrete floor on top more than eight feet back from its original position.”
Restricting any closer inspection is the extent of the damage. The blast blew off the building’s roof, bringing the trusses and other debris down on top of the autoclave.
“None of us has had a chance to get inside and take a look at what exactly happened,” Williams said.
An investigation to determine why the autoclave came apart is being conducted by FM Global, insurer for Beechcraft.
The first report of the blast reached 911 dispatchers at 8:02 a.m., Williams said.
The first fire officer on the scene established an incident command. Shortly after that, it was expanded to a unified command that brought the leadership of the various agencies into the command structure.
“We pulled the department heads — health department, sheriff and others – into the command van at one time to form a single command team,” Williams said.
But in those the first critical minutes after the blast, the mission was dictated by training and experience – isolate, evacuate and figure out what happened. Division Chief Tony Tracy was designated rescue group coordinator and initiated the first reconnaissance of the scene.
“The first six units that arrived were assigned to get everybody out, establish a perimeter to keep everyone back and conduct a recon to figure out the situation,” Williams said.
At 8:14 a.m., dispatches ordered the hazardous materials unit to the scene. Even though nitrogen is inert rather than toxic, Williams said the concern was about other critical operations in the plant that might have been upset in the blast.
By far the biggest upset was the giant autoclave in Building 36-C, used to compress composite materials together. When built in 1984, the ruptured autoclave was the biggest in the state. Even today it remains the largest of at least eight autoclaves in use at Beechcraft.
“When the autoclave is filled with nitrogen, they heat it,” Williams said. “Nitrogen expands 700 times when heated, building pressure inside the autoclave. That heat and pressure is used to bond composite materials used to make aircraft.”
However, rather than thousands of pounds of pressure, the autoclave’s normally operates at between 90 and 120 psi. Volume rather than extreme pressure created the problem.
Photographs show the autoclave split open along its diameter where the two halves can be opened and then resealed. Although the two halves only appear ajar, the pressure released demolished at least 20 percent of the three-building unit at Beechcraft designated Plant 3.
Outside, the large tanks of nitrogen gas used to operate the autoclave toppled and collapsed. Inside, debris that went through the wall just missing the main natural gas line, Williams said.
“Had this been hit and breached it would have been a different event,” he said. “It would have been a fire event in addition to the damage done.”
Also notable amongst the debris was a 20-foot-long piece of 10-inch iron that went through the roof of 36C and landed in the building next door, he said.
“Had this been a normal workday that alone would have injured half a dozen people,” Williams said.
By 11:20 a.m., officials conducted their second media conference that day. However, firefighters were already breaking down equipment to return to normal service. At 6:16 p.m. the incident was officially closed and the last of the responders left the scene.
The problem for Beechcraft now is to isolate the damaged portion of the plant so that production can resume.
“It is winter, forcing Beechcraft to put up warm walls because of the sprinkler system requirement for the rest of the building,” Williams said. “They can’t have wet pipes exposed in the damaged area.”
Williams credited the smoothness of the emergency response to ongoing command training in which the district’s chief officers work through difficult scenarios together to build teamwork.
“The initial commander was a battalion chief,” Williams said. “When our division chief showed up there was no argument or turf fight over who was in command. The division chief asked, ‘What do you need?’ and the battalion chief said, ‘you’re the rescue guy – go figure out the rescue problem.’’’
Eight local public safety agencies responded bringing a total of 31 fire units to the scene. Williams praised the Wichita Fire Department and other responders for their assistance.
“It was probably one of the cleanest runs I’ve ever seen,” he said. “Everybody did an excellent job.”
Chief Williams plans file the final after-action report in early February.