Fires in the woodworking industry are a common problem. If you have been involved in the manufacturing of wood products for any significant length of time you are likely to have experienced this problem first hand. Many companies experience fires frequently and some even are forced to combat fires routinely. The processes that are often employed in this industry are conducive to fires and explosions because the wood materials used are easily combustible and can present an explosion or deflagration hazard when reduced to dust particles. The machinery used in the manufacturing of most secondary wood products often produces the ignition source for fires and explosions to occur.
Obviously, the type of processes that you employ primarily determines the level of risk under which you operate. Abrasive operations such as sanding, planing, wood waste grinding and hot operations such as drying tend to be particularly dangerous in regards to the generation of sparks and fire.
Factory Mutual reports that dust collectors most often are associated with dust explosions. Explosion and deflagration hazards are generally associated with the pneumatic conveying, collection and storage of fine, dry wood dust, however it is possible for an explosion to occur in upstream equipment such as abrasive planers, wide belt sanders, metering bins and doffing bins. While the possibility of a dust explosion should never be taken lightly, fires are a more common occurrence in most woodworking facilities.
Fires that occur in composite board presses tend to be the most devastating in terms of downtime and equipment damage because of the high capitalization of machinery and high volume of production. It is estimated that downtime of an average composite board facility due to a press fire could cost hundreds of thousands of dollars per shift.
Spark detection systems
Spark detection and extinguishing systems were introduced to the U.S. in the mid 1970s. These systems gained rapid popularity in the woodworking industry because they were proven to be a cost effective method of preventing common dust collector fires and explosions. Today, spark detection systems are common in all segments of the secondary wood products industry and are widely accepted and recommended by the industrial loss control community.
Water spray systems for press protection are much newer to the composite board industry. The first system of this type was installed in 1993. Since that time these systems have been rapidly gaining acceptance, with more than 200 composite board presses worldwide reportedly having some form of related protection.
Users of these systems typically justify the implementation of protection at their plant based for one or more of a number of reasons.
• To increase workplace safety
• To protect equipment and facilities
• To minimize or eliminate downtime
• To comply with regulations or codes
• To meet insurance requirements or mitigate insurance costs
Another benefit that spark detection systems in particular can provide is that they can be used as a monitoring tool to alert maintenance personnel of existing or potential problems with processes, machinery and sometimes even personnel. Sudden and dramatic changes in spark activity such as an abnormally high frequency of detections in a particular zone can be a signal of defective equipment, incipient failure of a machine, problems with a process or material and sometimes operator error. For example, spark generation or fire detection may be a warning of the following:
• Tooling wear
• Overheated or bad bearings
• Overheated motors
• Belt tracking failure
• Material handling fan problems
• Excessive vibration
• Abnormal metal-to-metal friction or contact
• Unusually high dryer temperatures
• Boiler back flash
• Material back-up or jam-up
• Material impurity/Contamination with foreign material
• Operator error
• Violations of safety policies by personnel/unsafe practices
By alerting maintenance of potential or existing problems very quickly, minor problems can be addressed before they become a major incident.
Spark detection systems can be specifically designed to maximize their use as a monitoring tool for equipment. The user may want to consider independently monitoring critical areas or equipment that is deemed to be particularly hazardous. For example, where it is desirable to monitor certain individual machines or groups of machines, detectors can be placed on the respective branch ducts so that the user can be alerted of any abnormal or frequent spark activity emanating from that machine. This design can often be implemented with an existing spark detection system with minimal incremental costs.
It is important for the user to understand the degree of flexibility that a spark detection system may be able to provide. Most companies use the system simply for detection and suppression of sparks in pneumatic ductwork. While this is their primary function, these systems may also be able to utilize other types of detectors and devices to provide a greater degree of facility protection.
Additional protection may include the use of:
• Flame detectors (on machinery such as planers and moulders)
• Heat detectors (on bins, baghouses and bearings)
• Daylight detectors (on drop chutes and conveyors)
• Smoke detectors (in return air ducts and Motor control rooms)
• Manual release stations (for manual deluge in a baghouse or bin)
The user should also consider interlocking the system with external devices to provide maximum protection. For example, systems can be electrically interlocked with:
• Abort gates to purge burning material out of the pneumatic system
• Visual and audible alarms to annunciate system activity
• Material handling equipment such as the reverse of an auger to dump any burning material out of the material flow
Ideally, a shutdown of spark-producing equipment can be initiated based on frequent or continual detection however it is generally not recommended to shut down the material handling fans. The NFPA Code 664 for woodworking facilities specifically recommends that fans be kept running to help purge burning material out of the pneumatic system when an abort gate is used. Also, when a fire condition is believed to exist in the dust collector, the NFPA recommends that the fans be kept running so that the accumulated dust on the filtration media does not dislodge and create an explosive dust cloud in the bag house.
Systems can also be interlocked with the dust collection system so that the blowers will not be able to operate without protection.
No doubt these systems are very beneficial in providing a level of safety and protection to the user. It can not be stressed enough that fire protection systems must be maintained like any other type of critical operating equipment.
Periodic testing and inspections are crucial in ensuring reliable and effective operation
Many insurance companies mandate periodic testing and inspections by qualified
technicians. The NFPA has established minimum testing and maintenance frequency standards for Spark Detection and Extinguishing Systems in Code 664. It states that if the automatic external test light feature is used with the system, the frequency of inspections required by the NFPA can be reduced. This points out the importance of using available system features for monitoring and supervision to assist the user maintaining the system.
Such features may include:
• External test lights to verify responsiveness of detectors
• Water flow monitors to verify water flow at each valve
• Heat tape supervision to monitor freeze protection on exterior plumbing
• Tamper switches for ball valves to ensure the water supply is not shut off
• Booster pump supervision to monitor its proper operation
• Event printers or interfacing software for system activity, data retrieval and storage
Our experience has shown time and time again that failure to carry our periodic maintenance will severely compromise the benefit these systems can provide.
Maximizing the benefit from a fire protection system requires management become actively involved in every aspect of the systems implementation:
• System design (What are the protection objectives?)
• Equipment specification and procurement (Does it meet the objectives? What testing features will be used?)
• System layout (Will the components be accessible for future maintenance? Will it minimize installation costs?)
• Installation (Will all codes and regulations be met? Can the water requirements of the system be achieved?)
• Training of personnel (Will the appropriate personnel be trained during initial commissioning by a qualified technician? Will future training be provided later as needed for new employees?)
• Operating policies and procedures (Who tests the system? How is it tested? How is activity recorded and reported to management?)
• Maintenance scheduling (How often is testing performed?)
It also is important that the user practice good housekeeping in the plant and eliminate dust buildup on surfaces. The NFPA states that a dust accumulation of 1/8 inch or more constitutes a deflagration hazard. It is just as important that all pneumatic systems are designed to maintain sufficient air velocities in all material handling ductwork. If conveying velocities are too low, material will accumulate in the dust pipes and thus create fuel for a fire to start. It is critical that the air handling system be balanced so all material will be conveyed through the duct, and the possibility of a ductwork fire is minimized.
We have seen through many years of experience that fire protection systems can provide cost effective protection. Spark detection systems in particular are flexible and can utilize various types of detectors to monitor different process applications. They also can be used as a monitoring tool to indicate existing or potential problems with critical process equipment or machinery thereby increasing their value to the user. The degree of benefit that the user will derive from the system is directly related to the amount of proactive involvement by plant management and staff. This involvement from original system design to daily operation and maintenance is crucial for the maximum benefit of a fire protection system be realized.
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