Fire shelter overhaul
This summer some firefighters will carry a redesigned wildland fire shelter developed by the U.S. Forest Service's Missoula (Mont.) Technology and Development Center. The new generation shelter will provide distinctly improved protection from radiant heat and flames.
However, no fire shelter can guarantee safety. It's critical that firefighters understand that there are limits to the protection fire shelters can provide. The new shelter isn't meant as a replacement for constant situational awareness, nor is it an excuse to take risks on the fireline. A firefighter's highest priority is, as always, to stay out of situations that can lead to entrapment.
Wildland firefighters have carried fire shelters since they were developed during the 1960s. More than 1,100 firefighters have deployed the shelters, which are credited with saving more than 300 lives and preventing hundreds of burn injuries. The original or standard fire shelter was designed to reflect radiant heat — direct flame contact can damage it. In the past fatalities have occurred when flame contact was severe.
In January 2000, MTDC was asked to develop an improved fire shelter with the goal of maintaining the level of protection from radiant heat while improving protection from direct flame. Other considerations included material strength, durability, flammability, weight, bulk, toxicity and cost.
The first step in the development process was to devise tests that evaluate the performance of the prototype shelter materials and designs. Field testing provided some valuable information on fire shelter performance, but the expense and difficulty associated with accomplishing the tests and the variability of the field test conditions led us to conclude that repeatable lab-based tests would provide more reliable information on fire shelter performance. To assist in the development of the lab-based tests, MTDC gathered information on the fire environment during field tests in Montana and Canada's northwest territories.
With assistance from the departments of human ecology and mechanical engineering at the University of Alberta in Canada and private laboratories in the United States, small-scale laboratory tests were designed to allow screening for strength, flammability, thermal performance and toxicity. Full-scale tests were developed to measure the strength, durability, flammability, thermal performance and toxicity of the overall shelter designs.
Interest from private industry in the development of a new shelter was high. Most of the private companies involved submitted materials rather than designs. Although these companies had expertise in fire protection, they had little background in the requirements of a fire shelter. So MTDC developed a fire shelter design that would allow the center to test promising materials in full scale.
Most of the materials selected for full-scale testing were sewn into shelters using the MTDC design. One company, Storm King Mountain Technologies, submitted fully designed shelters for testing. The Storm King shelters were tested in the form in which they were received. In all, more than 60 materials and combinations of materials were considered with 17 different materials selected for testing as full-scale prototypes.
The Federal Fire and Aviation Leadership Council, made up of fire managers representing the departments of Agriculture and the Interior and state agencies with responsibilities for wildland fire management, selected the final shelter design in June 2002.
The shape of the new generation shelter differs from that of the standard fire shelter. The shelter is now shaped like a half cylinder with rounded ends. The new shape has a number of advantages: The rounded design reduces the surface-area-to-volume ratio, decreasing the material needed to provide enough volume inside the shelter. The new materials also improve protection from flames, but they weigh twice as much as the old materials. If the new materials had not been used efficiently, the new shelter could have weighed more than twice as much as the standard shelter.
In addition, the new shelter is narrower and longer than the standard shelter. Overall the new design offers significantly more protection than the standard shelter, but it may feel smaller to users. The new model was designed and tested using subjects up to 6 feet, 4 inches tall because military studies show that less than 1% of the military population is taller than that. Because some firefighters are taller, MTDC is designing a version of the new shelter to fit them.
The new design's reduced ratio of surface area to volume means improved protection because the shelter has less surface area to absorb radiant heat. The rounded ends also solve a problem noted during field testing of the standard fire shelter. Video taken during test fires showed that the flat ends of the standard shelter could reflect heat onto adjacent fuels, igniting them before the flame front arrived. Flames from adjacent fuels could damage the shelter, reducing protection just before it's most needed. The rounded ends of the new shelter design scatter radiant heat to the atmosphere, rather than focusing heat on fuels next to the shelter.
Composed of two layers of material, the new shelter weighs 4.2 pounds. (The standard shelter weighs 3.4 pounds.) The outer layer is woven silica laminated to aluminum foil, and the inner layer is fiberglass laminated to aluminum foil. The outer layer of foil reflects radiant heat and the woven silica slows heat transfer to the inside of the shelter. The inner layer of foil prevents heat from being re-radiated inside the shelter and helps keep gases out. When the two layers of material are sewn together, the air gap between them provides additional insulation.
Seams across the top of the shelter support its main shell and also help keep the outer foil layer in place by preventing peeling at the seams. When the shelter is heated to 500°F, the adhesive that bonds the foil to the silica breaks down, allowing the layers to separate. In turbulent conditions the foil can tear and peel away from the silica layer.
To speed deployment, the new shelter has shake handles — if the user grasps the handles and shakes, the shelter will unfold quickly. As with the standard shelter, there is no “head end” of the new version; it can be deployed in either direction.
New hold-down straps in a different location also aid deployment. The straps on the standard model crossed the shelter's opening and sometimes snagged when a firefighter was deploying it. The new straps are now alongside the opening, where firefighters can slip their hands through them when deploying.
Radiant and convective testing is critical when assessing a shelter's ability to limit heat transfer. The greatest threats a firefighter faces during an entrapment are burns to the body and inhalation of hot gases, which can cause asphyxiation. The MTDC assessed the inhalation threat by measuring the temperature at various locations inside the fire shelter.
Scientific estimates of the maximum survivable air temperature vary, but dry air temperatures as low as 300°F are considered survivable only for very short periods. Therefore, the MTDC also assessed the potential for burns with temperature and heat flux measurements inside the shelter.
Tests found that the new shelter provides improved protection from both radiant heat and direct flame. In radiant heat tests using full-scale designs, temperatures inside the new generation shelter rose 22% less than temperatures inside the standard shelter after five minutes. The temperature rose an average of 169°F in the new generation shelter compared to 207°F in the standard shelter. In direct-flame tests, temperatures inside the new generation shelter rose 81% less than temperatures inside the standard shelter after 40 seconds. The temperature rose an average of 134°F inside the new generation shelter, compared to 572°F inside the standard shelter.
One way to understand heat flux is to picture it as a measure of the rate at which heat strikes a surface. A heat flux of 5 kilowatts per square meter would lead to second-degree burns in about 40 seconds on bare skin.
In radiant heat tests the average peak heat flux was reduced 59%, from 3.7 kilowatts per square meter for the standard shelter to 1.5 kilowatts per square meter for the new generation shelter. In direct flame tests the average peak heat flux for the new generation shelter was 97% lower, just 1.3 kilowatts per square meter compared to 44.1 kilowatts per square meter for the standard shelter.
Because the new generation fire shelter is deployed somewhat differently than the standard shelter, firefighters shouldn't carry the new shelter until they have been properly trained.
The two layers of material in the new shelter have increased the shelter's bulk. In the past, the folded fire shelter fit inside a hard plastic liner that measured 9- by 5
MTDC has developed a new training video, DVD and pamphlet collection, titled “The New Generation Fire Shelter,” as well as a new practice fire shelter. The materials can be purchased from the National Interagency Fire Center, Attn.: Great Basin Cache Supply Office, 3833 S. Development Ave., Boise, Idaho, 83705; fax 208-387-5573 or 208-387-5548. The new practice shelter is available through the General Services Administration Wildland Fire Equipment Catalog and will soon be available through private distributors.
MTDC has recently published a Tech Tip newsletter that describes a simple way to retrofit a common field pack design. “New Generation Fire Shelter Developed for Wildland Firefighters” is available from MTDC by calling 406-329-3900, or visiting www.fs.fed.us/eng/t-d.php?link=pubs/htmlpubs/htm03512313 to establish a user name and password to access the information.
With a pricetag of $256.75, some state and local partners will be phasing the new shelter in slowly because of its increased cost. The standard fire shelter still meets all agency requirements and will remain in the system for about five years. This shelter design has saved many lives and provides good protection as long as flame contact is minimized. The standard fire shelter will be available until current stocks have been depleted. All training materials will be available until the standard shelter is declared obsolete and removed from service.
The new generation fire shelter provides a significant improvement in the level of protection for wildland firefighters in the event of an entrapment. It has been tested rigorously using the best available testing procedures. But no fire firefighter should think that carrying a fire shelter is an excuse to take chances on the fireline. Using a shelter is a last-ditch effort for survival. It does not guarantee survival.
Leslie Anderson has been an equipment specialist at MTDC since 1997. She is a leader for projects involving fire shelters and fire-resistant clothing. Anderson began working in wildland fire in 1979 and over the years has worked as a Lolo Hot Shot, a Missoula smokejumper and an assistant district fire management officer on the Bitterroot National Forest. She spent two years as a Peace Corps volunteer working in wildland fire management with the Costa Rican National Park Service. Anderson has a bachelor's degree in forestry from the University of California — Berkeley and a master's degree in forestry from the University of Montana.
Tony Petrilli is an equipment specialist in the Fire, Aviation and Safety and Health Programs at MTDC. Petrilli has spent more than 17 years as a firefighter for ranger districts on the Lewis and Clark and Beaverhead National Forests and as a smokejumper for the Northern Region. He is qualified as a division/group supervisor. He is a parachute rigger and understands textiles and their uses in firefighting.
The new shelter is available through the GSA Wildland Fire Equipment Catalog and should also be available soon from private distributors. The complete unit with case and hard liner costs $256.75.
Acceptable Use Policy blog comments powered by Disqus
