Industrial wood dust fires are regular events. Between 1980 and 1998 the National Fire Protection Assn. says 900 fires per year were started by wood-dust combustion. These fires resulted in one death every two years, 13 civilian injuries a year and an estimated $12 million a year in direct property damage. The coding changed in 1999, which is why the NFPA is unable to provide more current data.
It's a fact the wood dust generated by all types of wood production presents an unspecified amount of risk. A dust explosion or fire results when the normal manufacturing process produces a spark or ember, which is then picked up by the pneumatic dust extraction system and transported along with highly combustible wood chips and dust. If that spark were to reach the collector or storage bin, a fire or explosion could result.
The typical dust collection system contains all the elements needed for a potential catastrophic event. Dust and chips that are collected are the fuel, the airflow in the ducts provides the required oxygen, the filtration media distributes the particles more or less evenly and the dust collector or baghouse provides a vessel of confinement. Introduce a spark into the system, and disaster is just waiting to happen.
"There are statistics, and then there is real life," says Ole Sorensen of Flamex Inc., Greensboro, N.C. Many times manufacturers might have a small fire they don't report to authorities or their insurance providers, he says. "We hear about a fire on average of about once a week," says Sorensen.
The best defense
Spark detection represents the best initial defense against dust fires or explosions. These systems detect sparks and glowing embers, and activate countermeasures to eliminate the cause of the fire so that it never happens. Virtually any woodworking production facility can benefit from the installation of these systems.
"We do systems for companies that have just one pipe going out to the dust collector all the way up to multi-system panel board plants," says Andy Clarke, Clarke's International Inc., Eugene, Ore. "If you have a dust collector, no matter what the size of your plant or the number of people working there, your local fire department or local codes will tell you if you need a system," says Clarke.
The NFPA publishes the basic regulations that cover spark detection equipment. NFPA 664 has established the minimum requirements for fire and explosion prevention in woodworking establishments. Its standard applies to woodworking operations that occupy areas of more than 5,000 square feet where dust-producing equipment requires a dust-collection flow rate of more than 1,500 cubic feet/minute.
All the major companies, of course, have spark detection equipment, says Sorensen. "Smart people purchase this equipment because of the peace of mind it provides: the safety issues, the protection of assets and the protection of continuation of business," he says.
The size of the operation is critical. "If you talk about a reasonably large or medium to large facility," explains Bob Barnum with GreCon Inc., Tigard, Ore., "they're going to employ spark detection." That takes in virtually any production woodworking operation.
Reduce the risk
The pressure to install spark detection equipment really comes from the authority having jurisdiction. That might be a local fire department, local city, county or state code or the insurance carrier.
Despite the overwhelming evidence that spark detection equipment is one of the best investments a woodworking company can make to protect assets and employees, some owners think they just don't need it. "Some people say we've been running for 20 years and never had a problem," says Clarke. These people, he says, rationalize their decision because the return on the investment isn't the same as the return they get on other equipment in their plants. "The one time they do have a problem," he adds, "they could lose everything."
There are three major components to any spark detection system. The control console manages and guides the system; the detection mechanism looks for the spark or ignition source and the extinguishing system normally employs a water mist.
Detection is accomplished using an optical sensor mounted to the blowpipe that recognizes the infrared radiation given off by sparks or glowing particles. The most common method of extinguishment uses a water mist spray in the duct. Strategically positioned nozzles produce a wall of water inside the duct to extinguish sparks or embers. The time between detection and extinguishing the sparks is a fraction of a second.
The control console oversees all operations of the spark-detection system. All data from sensors located in the ductwork are compiled and processed by the control console. The console activates the appropriate response, either a mist spray or in some instances an abort gate that diverts the spark and logs the time, date, location and duration of the event, along with the countermeasures that were initiated.
In addition, today's control systems will produce reports for specific zones or machines. "You can pull up a report on a specific piece of equipment, say a sander in your plant, and look at all the alarms you had in the last six months or just the night before," explains Clarke.
Installations are unique
The equipment used for spark detection is fairly standard and all manage to do about the same thing. The actual design and engineering of a specific spark detection system for a facility is never off-the-shelf. "We have standard equipment that we custom design to fit the installation or to meet the needs of the end user," says Barnum.
Neither the engineering nor installation for a spark detection system is terribly complex, however. Calculation of the distance between the detection device and the spray apparatus is critical, but easily calculated. "It is a function of the diameter of the ducts and the water pressure," says Sorensen. "Typically we say 0.3 seconds between detect and spray as a ball park." Once the velocity of the air in the blowpipe has been determined, it's a fairly routine calculation to determine locations, he explains.
Installation is normally conducted by outside contractors or by a company's own maintenance crew.
"Most plants, if they have their own electrical and maintenance crews who are already familiar with the production machinery, are going to install the spark-detection system themselves," says Barnum. Detailed schematics are provided for each installation along with a punch list.
Before the installation can be certified, factory technicians come out to inspect the work and approve it for operation.
Spark detection systems can be retrofit on virtually any existing dust collection system, and this is a fairly common occurrence. Problems sometimes arise when companies buy up systems that are put on the market when other plants shut down. Sometimes these legacy systems go in without the correct engineering. "They don't fully understand how to do this or how critical these calculations are," says Sorensen. The results of such efforts, literally, flirt with disaster.
As with any production system, regular maintenance is required to keep spark detection systems operating at peak efficiency. The biggest problems woodworking manufacturers have with their spark detection equipment usually can be traced back to a lack of maintenance. "As long as the detectors and spray nozzles are working right, the system is going to operate properly," says Clarke.
If a plant hasn't had any incidents for a few months, there's still a need to know that equipment is operating properly. "You need to inspect and test the water lines regularly," he says. "You want to make sure that the water is available at the valves when it is required.
"Assuming that woodworkers already have a spark detection system, the message is to maintain it," says Barnum. "And if they don't have one, the message would be, to get one."
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