Features Health and Safety Sawmilling
Dust collector hazards

February 24, 2016 - We are often asked, “What sized wood dust collector poses an explosion hazard?” The short answer is any dust collector exceeding eight cubic feet in volume is considered an explosion risk regardless of how often it is used.

For an explosion to occur, the dust collector must have sufficient dust to reach the Minimum Explosion Concentration (MEC). This can be achieved after hours of use, as such, if you have used your dust collector, an explosion risk is possible as the finer combustible dust is caked on the filter bags to aid in the filtration efficiency and can be easily suspended during a cleaning cycle or other disturbances.

At this size and usage, the majorities of small wood shop dust collectors do pose an explosion risk and as such, require explosion protection. The most common explosion protection is breakaway panels or doors and when installed and maintained properly, offer reliable protection at low cost.

Problems occur when no consideration is given to the path of the explosion which is vented through the panels. For a typical small shop dust collector, less than 5,000 CFM, the fireball’s path can exceed 30 feet. If any people, debris or other combustible materials are in the fireball’s path, additional explosions and injury can occur. As shown in Figure 1.

The dust collector explosion panels are directed inside a work area and face the entry door. Obviously serious injury and damage can occur if an explosion were to happen. Another common problem is the installation of explosion panels and doors, which require more pressure to open than the pressure to open the access doors. In these cases, the explosion vents out the access doors, which are not necessarily located for a safe discharge of the fireball.

In small wood shops it is common practice to locate the dust collector inside the building and exhaust the “clean” air into the building to save on make-up air energy cost. Both these practices are extremely dangerous.

For dust collectors located inside, an explosion would be vented inside the building with sufficient fireball pressure to suspend any dust layer into a secondary explosion with devastating consequences. Any dust collector located inside must have the explosion panels vented with a re-enforced welded vent duct to the exterior and the vent duct cannot exceed 10 feet. If a vent duct greater than 10 feet is required, a strength analysis must be completed to ensure that the dust collector enclosure is strong enough to withstand the additional backpressure caused by the vented explosion travelling through the vent duct. See Figure 2.

For dust collectors exhausting the clean air back into the building, the return air ducting must be equipped with a sensor and abort gate, which will close and direct the explosion fireball to the exterior in the event that an explosion occurs in the dust collector. The reaction time from the sensor to the abort gate is around 0.3 seconds, as such, you will need to verify the spacing between the sensors and abort gate at the correct distance based on the air velocity in the return air ducting. It is also imperative to check the velocity on a regular scheduled maintenance to ensure that the velocity is within the range for correct operation of the abort gate. If the velocity increases, the explosion will be past the abort gate before it closes and discharge inside the building with potentially devastating consequences. If the velocity has reduced, the abort gate will close early and still provide protection.

As the consequences of not having correct explosion protection on the dust collector and return air system can be devastating, the recommended arrangement for wood dust collectors is to be located outside, and the clean air should not be returned to the building. The first impulse is to disregard these cautions as many operations consider it cost-effective to return the air and locate the dust collector inside. This is not always the case. In order to properly justify the risk versus cost savings you need to look at the building code requirements on clean efficiency for returning air from a dust collector and the actual payback based on capital cost and on-going maintenance cost versus hours of operation during colder months. All required conditions to be met are explained in NFPA 654, “Standard on explosion and fire protection in wood working,” and NFPA 654, “Standard for the prevention of fire and dust explosions from the manufacturing, processing, and handling of combustible particulate solids.”

NFPA 654 section 7.13.1.6.1 is very specific and requires that conditions must be met, maintained and proven for the recycle of air to be permitted. The requirements that must be met, from NFPA 654 are all of the following:

1. Combustible or flammable gases or vapours are not present either in the intake or the recycled air concentrations above applicable industrial hygiene exposure limits or 1 per cent of the LFL, whichever is lower;
2. Combustible particulate solids are not present in the recycled air in concentrations above applicable industrial hygiene exposure limits or one per cent of the MEC, whichever is lower;
3. The oxygen concentration of the recycled air stream is between 19.5 per cent and 23.5 per cent by volume;
4. Provisions are incorporated to prevent transmission of flame and pressure effects from a deflagration in an air material-separator back to the facility unless a process hazard analysis indicates that those effects do not pose a threat to the facility or the occupants;
5. Provisions are incorporated to prevent transmission of smoke and flame from a fire in an air-material separator back to the facility unless a process hazard analysis indicated that those effects do not pose a threat to the facility or the occupants;
6. The system includes a method for detecting air-material separator malfunctions that would reduce collection efficiency and allow increases in the amount of combustible particulate solids returned to the building;
7. The building or room to which the recycled air is returned meets the fugitive dust control and housekeeping requirements of this standard (Chapter 8);
8. Recycled-air ducts are inspected and cleaned at least annually.

It is important to check on the cost and maintenance of the required sensors required for safe return air before considering the true install and operating cost versus energy cost savings. In most cases smaller wood shops operating five days per week can’t justify the savings and by not returning the “clean” air you will make your facility significantly safer from an explosion risk and returning potentially cancer-causing finer wood particles back into the facility.

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February 24, 2016  By John Bachynski

Example of material in the pathway of the explosion vent. We are often asked