This article will be limited to sawmills which receive raw logs, cut the logs producing green lumber, kiln dry the stuck lumber, and plane and dress the dry lumber to specified dimensions. Many of the hazards associated with sawmill operations are also found in allied woodworking industries including; plywood mfg., oriented strand board mfg., wood pellet mfg., furniture making and further processing of wood products, etc.
Sawmills typically come in two flavors, hardwood and softwood processing. Although there are some inherent differences between the two, they will not generally be of concern with respect to fire and explosion hazards discussed herein.
Sawmills receive green logs by truck nearly continuously throughout their operational day. These logs are stacked to form a log deck, which may cover several acres in area, with logs stacked 10 ft. high or more.
From the log deck, raw logs are pulled and automatically checked for inclusion of metals (spikes, nails, or similar) which could damage mill equipment, and sent to the ring de-barker, electronically measured, and then sent on to the sawmill. In the mill the de-barked log is trimmed and cut by a series of saws which are computer controlled to optimize the board ft. of lumber produced from each individual log. Often saws and milling equipment are on elevated platforms connected by conveyors and similar handling equipment.
The cut green lumber is sent to a sorter/stacker where like boards are sorted into bins. The like boards in each bin are stacked together with small wooden “stickers” between each layer of boards allowing air to circulate around the lumber. “Stuck” lumber is then placed into a steam heated drying kiln where it resides until the moisture content is reduced to the desired range.
The dry lumber then goes to the Planning Mill where it is surface and edge planed and cut to final dimensions. The dry lumber is stacked, banded, and shipped to the customer.
Hazards and Protection
The most obvious hazard associated with this process is fire. If a structure or a process handles or stores wood or sawdust products, complete sprinkler protection is indicated.
While green logs stored in a log deck can and will burn, they are generally not a major fire hazard, and it is not possible to install fire sprinkler protection above a log deck, or many other outdoor structures associated with a mill. Good practices here are to provide fire hydrants on 200 – 300 ft. spacing around the log deck, and to maintain well defined aisles for vehicle access within the log deck areas. This will allow for manual firefighting and ready access to all areas of the log deck. The provision of fire hydrants should also be extended throughout all areas of the mill yard to permit ready access for manual firefighting.
Hydraulic fluids are a common hazard in pretty much all sawmills, where they are typically used in the cutting, handling, and conveying systems. It is not unusual for a mill to have in excess of 100 pieces of hydraulically operated equipment. Hydraulic reservoirs for this equipment can range from 25-1000 gallons, or more, but are most typically in the range of 100-500 gallons.
If ignited, a hydraulic leak under pressure can literally become a large blow torch with no effective means of fire protection possible. To counter this, the solution is to provide an automatic shutoff for all hydraulic equipment. This can be accomplished by providing heat detection with an interlock to shutoff and depressurize the hydraulic unit on heat detector activation. It is also obvious that sprinkler protection should be installed for any and all areas where hydraulic equipment is located.
While fire is a clear and obvious concern, a less obvious, but potentially more catastrophic concern is a dust explosion. Sawdust is created throughout the milling process, and is handled and moved away from points where it is created, to points where it is stored and collected. This is typically accomplished through the use of drag chains, conveyors, pneumatic handling systems, baghouses, cyclone collectors, etc.
Sawdust in and of itself is not explosive, but when it is dispersed as a cloud within the air, it can become devastatingly explosive. The air suspension of sawdust is an inherent condition in much of the dust handling systems such as baghouses, cyclone collectors and pneumatic ductwork, and thus this equipment is often inherently prone to dust explosion. There are two conditions which are very significant in the potential for ignition and the energy output of a dust explosion: these are dust particle size, and dust moisture content. In both cases, less is more, i.e. the smaller the particle size, the less energy is required to ignite and the more explosive force will be generated if ignited. The same is true for % moisture content with dusts having less moisture content are more susceptible to ignition and having higher explosive forces generated. Thus the highest dust hazards in most plants are found at the planner mill building and associated equipment where kiln dried lumber is handled.
The most accurate way to determine explosability of particular sawdust products is to perform testing through a qualified 3rd party testing laboratory.
There are many ways to reduce or eliminate dust explosion hazard potentials. These can include high speed spark detection and water spray. These systems can detect a spark and provide quenching waterspray within milliseconds prior to the spark entering a zone where an explosive dust concentration is present. Explosion suppression systems can also detect a spark and release chemical agents that will terminate an incipient explosion within a piece of equipment where an explosive dust concentration is present. Abort gates and venting systems are also important and can reduce explosive pressures before that can damage equipment.
Housekeeping is probably the number one issue and safety factor within sawmills. Good housekeeping practices equate to greatly reduced risk of fire or explosion. The most common housekeeping issue is sawdust which escapes the equipment. The presence of sawdust throughout a building, particularly on structural elements such as beams or girders can result in devastating explosions. A common occurrence is for a small initial explosion to occur within a localized piece of equipment, this minor shock is then strong enough to shake the building thus releasing a great deal more dust into an air suspension. This much larger amount of dust now in suspension can allow for a very large secondary explosion involving the entire building. Thus housekeeping, particularly for elevated surfaces, is critical. Never use compressed air to blow down dusts, as this creates the hazard itself. An explosion proof vacuum system should be used instead.
Although sawdust in the green mill areas are less of a concern from an explosion standpoint, they are very much a concern from a fire standpoint. Over time dust will settle onto building structural members where they will dry and harden causing the building structure to become a combustible fire hazard. This provides an avenue for very rapid firespread and development where even sprinkler protection may be ineffective.
Wherever possible buildings should be designed to eliminate horizontal surfaces (beams, girders, cable trays, conduits, electrical panels etc.) where dust may collect. This can be done by boxing beams and girders, and providing smooth ceilings. With pneumatic dust handling equipment, the preference is to design equipment to operate under negative pressure (vacuum) such that any leakage will not cause dispersion of dust into an area. Where dusts do collect, cleaning practices should keep dust deposits to less than 1/16 in. on any and all surfaces. In older and less well designed plants this may mean continuous cleaning throughout the operating shift.
Another housekeeping concern at sawmills is hydraulic oil leakage. With a great number of hydraulically operated equipment there are bound to be leaks. If left unattended these often result in several inch deep pools of hydraulic oil caught in the drip pans, which typically are fitted below hydraulic equipment. A strong program of inspection, maintenance, and cleaning is required to keep ahead of this issue.
There are many other hazards associated with sawmill operations, however the ones above are of primary concern.
Risk Logic is very experienced in the Property Loss Prevention measures at a Sawmill facility. Pease reach out to us if you have any questions
ADDITIONAL READING RESOURCES
OSHA Combustible Dust website: https://www.osha.gov/dsg/combustibledust/index.html
FM Data Sheet 7-76 – Prevention and Mitigation of Combustible Dust Explosions and Fire
NFPA 654 – Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
NFPA 664 – Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities
NFPA 499 – Classification of Combustible Dust
NFPA 68 – Deflagration Venting Systems
NFPA 69 – Explosion Prevention Systems
NFPA 91 – Exhaust Systems