Wood Dust Fire and Explosion
What are the fire and explosion hazards of wood dust?
Wood dust is considered to be explosive if ignition of part of a cloud of wood dust results in the propagation of flame through the rest of the cloud. The burning of an unconfined wood dust cloud produces a flash fire. However, if the wood dust is contained within a full or partial enclosure, the pressure build-up can produce a destructive explosion. Its severity will depend on a number of factors, generally, the larger the volume of the exploding dust cloud, the more widespread its effects will be.
It is important to ensure that wood dust does not escape from collection systems and be allowed to build up within workrooms. If dust does accumulate, any primary explosion which occurs in a collection unit may stir up dust deposits within the building which houses the plant. Burning particles from the primary explosion can ignite the dust cloud resulting from it, leading to a secondary explosion that is usually more destructive than the first.
The explosibility of wood dust
You should assume that all wood dust is potentially explosive. . Wood waste usually has a dust explosion risk where the mean particle size is less than 200 microns, and where as little as 10% of the mixture contains dust less than 80 microns in size.
Explosive Wood dust is commonly produced by:
- fine cutting (eg sanding) – which produces a dust of very fine particle size;
- sawing and machining hardwoods – often producing wood waste containing considerably
- more dust than that from softwood;
- the processing of MDF, chipboard and similar boards by machining and sawing – which can be expected to produce waste containing much fine dust;
- profiling and moulding components on routers, spindle moulders etc.
Sources of ignition for wood dust
Common ignition sources include naked flames, faulty or unsuitable electrics and impact sparks.
The sanding or hogging of off-cuts containing metal may produce friction sparks, which can cause sawdust to smoulder and subsequently be fanned into fires or explosions. Use dedicated collection systems for these operations. Consider spark detection and extinguishing devices where there are significant risks.
Hot work involving the careless use of welding or flame-cutting equipment has resulted in many incidents. To prevent this, plant should be isolated and thoroughly cleaned before work starts. Use cold cutting methods whenever possible.
Electrical equipment should be sited away from dusty areas. If this is not practicable, ensure it is adequately protected.
Collection systems for wood dust
There are three main types of system for collecting wood waste:
- One or more woodworking machines are exhaust ventilated to a nearby collection unit within the workshop which does not form part of any other exhaust ventilation system.
- Many (perhaps all) of the woodworking machines are ventilated to a collection unit, which can be some distance from the machines and may be inside or outside the workshop (see Figure 1).
- One or more woodworking machines are exhaust ventilated to a nearby collection unit. These units deliver the wood waste into a larger collection unit, usually outside the workshop. This is known as a ‘through flow’ system.
Ductwork for wood dust
Make ductwork as short as possible with a minimum number of bends. The design should specify a minimum transport (or conveying) velocity of 20 m/s to minimise dust deposits. Use only conductive materials for ductwork so that any static electricity generated can be discharged to earth. Ductwork needs to be regularly inspected internally and cleaned to prevent any accumulation of dust. Suitable access points/hatches should be provided for this.
Collection units for wood dust
There are a number of different kinds of collection unit and the main types are:
- unenclosed fabric filter sock collector;
- unenclosed fabric multi-sock collector (see Figure 1);
- enclosed fabric single-sock collector;
- enclosed fabric multi-sock collector;
- cyclone; and
- bin or hopper.
Precautions for collection units where there is a risk of dust explosion
Collection units should normally be sited outside, away from areas where there may be people. If units have to be indoors, precautions will depend on the size of the collector; the size and construction of the room it is in; the number of people nearby; and how near they are to walkways and combustible materials.
To avoid the risk from secondary explosion or fire, it is essential to enforce good housekeeping practices to prevent the accumulation of wood dust within the building, eg a formal cleaning regime using appropriate vacuums fitted with HEPA-type filters.
Sock collectors (<0.5 m3/s capacity)
Unenclosed sock collectors would quickly disintegrate if the contents were ignited, but would not produce high explosion pressures or widespread effects. Fire risks may exist so, if unenclosed, do not position them within 3 m of workers, combustible materials or walkways. Alternatively, provide a suitable baffle or deflector plate or enclosure (see below). Enclosed sock collectors should discharge at the top to a safe place i.e. above head height.
Sock collectors (0.5–2.5 m3/s capacity)
Ignition of wood dust can lead to a jet of flame at head height, but an explosion is not likely. Where such collectors must remain within the workroom, provide one of the following precautions:
- Total enclosure within a strong metal cabinet with either an air outlet large enough in area to act as explosion relief or explosion vents. Outlets or vents should preferably discharge to a safe place outside the workroom or, if inside, discharge at least above head height.
- A baffle or deflector plate made of non-combustible material to direct flames or burning material to a safe place.
- Ensure the fan can be turned off from a safe place if a fire starts in the filter. A 3 m separation between the filter and regularly occupied locations is likely to be adequate to protect employees.
Sock collectors (>2.5 m3/s capacity)
Site these outside or enclose them in a strong cabinet fitted with explosion vents that discharge to a safe place.
Cyclones for wood dust
Well-made cyclones of less than 0.5 m3/s volume (rare in woodworking) do not usually require explosion relief panels. Larger low-efficiency cyclones usually have large enough air outlets to act as an explosion vent, but the need for additional explosion venting should be assessed. Larger high-efficiency cyclones do not usually have large enough air outlets to act as effective explosion vents and so additional venting will be necessary. Where cyclone air outlets discharge to an after filter, both the cyclone and the after filter will need explosion-relief panels.
Bins or hoppers for wood dust
Where used to store explosible wood waste, these will require explosion relief appropriate to their volume. They should preferably be outdoors but, if indoors, additional explosion relief may be required on the building itself. There should also be a safe system of work for emptying bins and hoppers.
Interconnected plant for wood dust
Take precautions to prevent an explosion spreading between interconnected units of plant, such as collectors, cyclones, filters and incinerators. Collectors should discharge their collected wood waste through an explosion choke, eg a rotary valve, or directly into strong metal containers clamped firmly to the discharge outlets.
Sizing of explosion relief for wood dust
The simplest and most common method of protecting process plant against the consequences of a dust explosion inside it is to provide some deliberate weakness in the structure in the form of explosion relief vents. Suitably sized and sited vents will ensure any explosion within the plant will be vented safely.
Fire fighting
Consider installing a dry sprinkler system and a C-coupling for attachment to a fire-brigade hose (on new plant). Make sure access doors on silos are big enough to allow access for fire fighting. Use gently applied water (eg a spray or mist), not jets to extinguish fire, to minimise the disturbance of burning wood waste
Additional precautions for wood dust explosions
Users should take the following practical precautions to minimise fire and explosion risks:
Ensure there is a preventive maintenance regime for the entire collection system
- Leave the fan running for some time after the machines have been turned off to ensure the ducts are empty when the air flow stops, and minimise dust fall-out in the ducting.
- Keep the system dust-tight.
- Replace seals, gaskets and covers as necessary.
- Empty containers associated with filters regularly.
- Take care to prevent metal objects entering the collection system.
- Smouldering fires often precede explosions – if a fire is suspected, stop the air flow through the collection system before investigating the problem.
For detailed advice on assessment and specific control measures for your workplace contact our consultants.
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