December 13, 2001
This article combines examples of fires caused by inadequate protection from welding with information on heat-resistant textiles and how they can be used to provide that protection.
While an arc welding unit was being used on the second floor in a Kaukauna, Wisconsin, warehouse, sparks dropped through an opening to cardboard boxes below, igniting the boxes.
There was no fire watch on the first floor, and when the fire was discovered 15 minutes later, employees could not put it out. They called the fire department but were too late to save the two-story building of ordinary construction. It was a $1.6 million loss.
This real-life incident is one of 28 recapped in the latest version of the National Fire Protection Association's (NFPA's) Standard 51B, Fire Prevention During Welding, Cutting and Other Hot Work. Some of the incidents resulted in the loss of lives, and the collective property damage amounted to more than $10 million. They serve as graphic examples of what can happen without safe work practices.
Even though hot-work permits call for covers and fire-resistant tarpaulins to be used within 35 feet (11 meters) of the job (see Figure 1), almost all these fires could have been prevented at a fraction of the cost of just one fire-related loss had heat-resistant fabrics been used on all the jobs. Fabrics are easy to use, versatile, and a cost-effective way to protect the work environment from fire caused by hot work.
Because the fibers and coatings are considered high-tech, people involved with hot work should understand the basics of heat-resistant fabrics. With this information, they can specify the proper fabric for the hot-work application at hand.
Using heat-resistant fabric for welding protection is not a new concept. Until the '70s, one fiber stood out as the product of choice — asbestos. Once the Occupational Safety and Health Administration (OSHA) began restricting its use in the workplace for health reasons, however, a variety of alternatives began to emerge, from simple to space-age.
Although product choices vary, only two basic fibers and a few coatings can be considered for hot-work applications.
Fiberglass. Fiberglass is a synthetic fiber made from a mixture of sand (silica), clay, and limestone, which is melted and spun into continuous filaments of yarn, then woven into fabrics ranging in weight from a few ounces to a few pounds per square yard. Regardless of weight, fiberglass melts at about 1,400 degrees F. In its uncoated state, it is best used for spark protection.
Silica. Silica starts out as fiberglass but goes through a chemical treatment that removes almost all impurities, leaving an essentially pure silica content and much higher heat resistance. The melting point of this fabric is about 3,000 degrees F, with a continuous-use temperature of 1,800 degrees F.
For the best protection, a minimum 96 percent silica content is recommended. Because of its superior heat resistance, silica often is specified for the toughest hot work, including hot-slag protection.
Note that coated fabrics should not be used in a confined space for worker safety reasons. In such applications, only coating-free fabrics should be considered, and fabric manufacturers are able to supply such products upon request.
Vermiculite. Vermiculite is a mineral-based coating applied to fiberglass fabric to boost its heat resistance by about 50 percent and provide some added durability. Vermiculite-coated fiberglass fabrics generally are used for heavy-duty curtains or where moderate weld spatter is anticipated.
Wetting down vermiculite-coated fabrics is not a good idea, as it can become slippery underfoot.
Silicone Rubber. When used outside, plain fabrics can lose their strength as a result of exposure to the elements, such as moisture or wind. And even some indoor applications require the added protection that silicone offers, such as oil and abrasion resistance. These coatings will degrade at about 500 degrees F but will not support combustion and usually have provided protection for the fabric by the time they burn off.
Silicone coatings come in a rainbow of colors to meet any safety standards in place, and they can be applied to almost any fabric on one or both sides. Silicone-coated fabrics are used extensively as vertical shielding on outdoor scaffolding on which hot work is anticipated.
Neoprene Rubber. Neoprene rubber protects fabric to about 225 degrees F and usually is applied on the working side of fiberglass fabric. Although not as heat-resistant as the silicone rubber, neoprene-coated fabrics work extremely well as floor coverings and blankets in situations that require extra durability.
Besides fiber type and coating, it is essential to understand that heat-resistant fabrics are available in a range of weights, widths, and thicknesses. Choosing the best product for one or even several different jobs can be difficult with insufficient information.These key points about fabrics can make choosing the right one a less-confusing process:
1. Thicker is better when it comes to heat protection. Workers should get the thickest fabric available in any given weight. Fiberglass fabrics used for welding and cutting range in thickness from 0.015 to 0.125 inch. Although they all can withstand the same temperature, heat will travel through them at different rates because of the thickness.
2. Weight is the true equalizer when comparing fabrics of the same fiber type. Just because two fabrics are the same thickness does not mean they are equal in performance because they may be different densities. Therefore, weight and thickness must be considered together before making a choice. Heat-resistant fabrics generally range in weight from about 10 to 60 ounces per square yard.
3. Besides thickness and weight, the only other consideration is width and length, which is determined by the type of work being done. Fabrics are woven to various widths and then are rolled to 150-foot lengths as the standard.
In addition, fabrics can be cut and sewn with or without grommets for hanging when necessary. A wide fabric in roll form may provide the most flexibility on the job site, while blankets and curtains may be better for specific jobs in which vertical barriers must be created or workers must carry the material from site to site.
The information presented here can be applied to some of the hot-work incidents reported by the NFPA to determine which product would work best in each situation.
Incident:In the Kaukauna warehouse fire, sparks from an arc welding unit used on the second floor fell onto cardboard boxes below, which caught fire.
Prevention:The boxes on the first floor, if not moved, could be covered with a fairly lightweight fiberglass fabric, perhaps 18 ounces per square yard. If purchased in 5- by 150-foot rolls, fabric easily can be cut on the job as needed.
In addition, silica fabric could be used on the floor directly under the welding activity, because heat in this area easily could surpass the capabilities of fiberglass. Because sparks can travel up to 35 feet (10 meters) on a horizontal plane from the work area, the remaining floor space in the work area could be covered with either silica or fiberglass fabric, depending on the anticipated intensity of the welding activity. Although this added precaution may seem costly, fabrics can be used again if not damaged by sparks or slag.
Incident:While an employee was using an oxyacetylene torch to modify a bracket in the boiler room, hot slag ignited canvas and plywood that were being used as a temporary covering over a hole in the wall between the fire-resistive boiler room and the storage room. Fire then spread to waxed cartons and plastic bags in the storage room. Fire fighting was impeded by the windowless walls and thick black smoke. The loss in this case was $650,000.
Prevention:A curtain of fabric could be hung in front of the wall, thereby shielding the canvas and plywood from the hot slag until it cools. Fiberglass with a vermiculite coating would provide adequate heat resistance for such a situation.
Incident:A fire occurred at a rolled paper storage facility when welding was performed on a column adjacent to an aisle with rolled paper storage not more than 5 feet away.
The combustibles were not covered, even though the work permit required that materials be moved or shielded. The rolled paper caught fire and had to be extinguished with sprinklers. It was a $250,000 loss.
Prevention:Curtains made of silicone-coated fiberglass could be hung around the combustibles, protecting them from the sparks. The silicone coating can repel sparks, and and the coated material can be cut and sewn into curtains. Most welding suppliers can have curtains custom-made quickly and economically. Silica fabric could be draped on the combustibles as an added precaution.
Heat-resistant fabrics can greatly diminish fire hazards when jobs are performed that involve oxyfuel burning, arc welding, plasma arc cutting, or similar operations that can initiate fires or explosions.
With this brief review, welders should have enough information to ask their suppliers for samples and more details. Keeping workplaces safe from fire while welding has never been easier, thanks to advanced textile and coating technologies.
National Fire Protection Association (NFPA), P.O. Box 9101, 1 Batterymarch Park, Quincy, Massachusetts 02269, phone 617-770-3000, fax 617-770-0700, Web site www.nfpa.org.
Occupational Safety and Health Administration (OSHA), 20 Constitution Ave. N.W., Washington, D.C. 20210, phone 202-219-8151, Web site www.osha.gov.
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