An occupancy with inherent hazards is wood processing. With flammable liquids, wood dust and heavy combustible loading it is imperative to adhere to strict property loss prevention standards. In a study over a ten-year period of the wood processing / furniture manufacturing industry, fire and explosion was observed to be the prominent losses in over 51% of all reported property claims accounting for almost 75% of the total liability loss in dollars.
Two typical problem areas in furniture manufacturing facilities are wood dust handling and lacquer spray finishing. Even the with best dust removal systems, housekeeping for wood dust removal is an on going issue. Dust fires may start when a piece of tramp metal (nail) gets stuck in a hog trap causing a spark and igniting the dust. Lacquer spray finishing can be ignited from overheated motors, a spark from a tool or a maintenance worker’s torch. Excess accumulation of overspray, improper storage of solvent soaked rags and poor ventilation may be some of the contributing factors to a fire.
Housekeeping for Wood Dust
Process areas which are susceptible to dust or lacquer accumulation provide a very hazardous combustible occupancy. The dust and lacquer can accumulate in hidden spaces such as above machinery and on roof framing. A detailed dust housekeeping program to keep accumulations to a minimum is necessary. Wood dust accumulations should not exceed 1/8 inch and wood dust combined with lacquer or another resin type finish should not exceed 1/16 inch.
The best method is to vacuum whenever possible to limit the amount of dust. A central pneumatic vacuum system would offer the best form of dust housekeeping. The vacuum system would have to be intrinsically rated for Class II locations as described by the National Electric Code (NEC). This equipment is constructed to exclude ignitable amounts of dust from the equipment enclosure. Approved equipment of this type also has been evaluated to assure hazardous surface temperatures do not exist. Equipment listed as suitable for Class II locations is dust-ignition proof and designed to meet Articles 50 and 502 of the NEC.
Unfortunately, Class II rated vacuum systems are uncommon in the wood working industry. Also, the time it would take to vacuum all of the dust from the rafters makes this safer method of removing fugitive wood dust impractical in some cases.
The most common method used to remove wood dust and keep accumulations at an acceptable level is the compressed air blow down method. However, this is the least desirable method since the wood dust is in suspension and a potential fire and/or explosion may occur. Again the source of the air must be rated for Class II locations.
The following should be used a guideline for wood dust blowdown procedures:
1. All electrical equipment should be shut down during and two hours after the blowdown.
2. All production and machinery should be stopped.
3. All ignition sources should be controlled.
4. Ensure that no hot surfaces exist.
5. All employees in the building should be made aware that a blow down is occurring so they do not try to operate the machinery.
6. All cutting and welding should be avoided in the building until two hours after the blowdown occurs.
7. Limit the extent of the blowdown to small areas at a time with as low a volume and pressure of air as possible.
8. Do the blowdown frequently enough to avoid an accumulation of 1/8 inch (3.2 millimeters) of wood dust and 1/16 inch (1.6 millimeters) of lacquer dust.
9. Assign housekeeping crews responsible for specific areas and/or systems and include production crew members in the clean up crews to add experience identifying fugitive dust locations (e.g. rafters, top of offices, top of machinery).
10. Stagger the cleanup schedule and crews to cover off days.
Training should be conducted for all employees regarding the above-mentioned Wood Dust Blowdown Procedure.
Finishing Room Protection
Another hazardous area in a wood processing plant is the Finishing Room. Lacquer spray finishing is very common in furniture plants. Nitrocellulose coatings are particularly hazardous since the overspray is difficult to control and when overspray particles are ignited, the flame will spread quicker than sprinklers can operate. Lacquer overspray combined with wood dust provide for a very hazardous area.
The following is a guideline for spray finishing areas:
1. No open flame or spark producing equipment within 20 ft. and all spray finishing operations should take place within the spray booth.
2. All wiring, electrical switches and light fixtures within 10 ft. of any spray booth should be vapor proof type.
3. Ventilation for each spray booth should be adequate to remove all vapors and mist from the atmosphere. Ventilation should be on at all times when spray finishing is taking place.
4. Hoses used in spray finishing operations should be in good physical condition without any evidence of cracking.
5. Tools used to clean spray booths should be made of non-sparking material such as brass or aluminum.
6. Bags should be placed over the sprinkler heads to prevent finishing material from collecting on the heads inside the spray booths or exhaust ducts. The bags should be changed at least weekly. The sprinkler heads should be checked weekly and any contaminated heads replaced.
7. Flammable liquids at any spray booth should be limited to the absolute minimum amount necessary for a one shifts supply.
8. Containers in use in finishing operations should be kept closed. Plastic containers are not acceptable for flammable liquids.
9. Spray booths should be cleaned as often as necessary to prevent accumulations of combustible residue on walls, ceiling, floor, baffles and ducts. Ducts should be cleaned all the way to the top to prevent the accumulation of dust on the roof. Accumulations should not exceed 1/8 inch in any area. Maximum cleaning is one per shift.
10. When flammable liquids are dispensed from 55 gallon drums, the drums should be equipped with Factory Mutual Approved safety bungs with all openings tightly sealed around pumps and hoses.
11. Rags that are saturated with flammable materials should be collected in closed top containers during the day. At the end of the workday all drums should be removed from the work area and either tightly sealed or thoroughly soaked with water.
12. Where many small containers of finishing material are used, such as in repair area, Factory Mutual Approved flammable liquid storage cabinets should be used.
13. Aerosol cans used for touch up at any spray booth or touch up area should be limited to the absolute minimum necessary for one day’s production. These cans should be stored in a Factory Mutual Approved flammable liquid storage cabinet.
14. All areas used for storage of finishing materials such as bulk storage rooms should be equipped with low level mechanical ventilation with the fan, which should run continuously to vent vapors from the room.
15. Ceiling joist, pipes, lights, and other overhead structures should be inspected monthly for lacquer dust to prevent significant accumulations. Proper records should be kept on hand. Roof areas and areas above machinery should also being inspected for dust accumulations.
For additional assistance in minimizing hazardous operations for wood processing and wood working facilities, please contact us.