Issues with wood dust collectors - FROM A DSEAR ATEX PERSPECTIVE:
Issues with wood dust collectors - FROM A DSEAR ATEX PERSPECTIVE:
What are the important factors regarding ATEX DSEAR in wood-dust dust collector design:
We carry out a number of DSEAR risk assessments on wood dust extractors where the dust collectors are made from a thin 0.8mm or 1mm galvanised sheet with no bracing or strenghtening, they have however been fitted with an ATEX explosion relief panel.
In addition, the fans are fitted on the product / inlet side; blowing product into the dust collector. The wood dust collector manufacturer has in many instances wrongly certified the dust collector as suitable for an ATEX Z22 or Z21 which has been copied directly from the explosion relief panel. The dust collector manufacturer may have failed to realise a number of important points;
1. The finess and structure of the wood dust is crucial: in general the finer and more uniform in profile, the more likely the wood dust will explode, rather than burn.
In general if > 40% of the woodust size is below 200 micron?? the explosion risk can greatly increase.
2. The moisture content is crucial: the lower the moisture content - the higher the explosive risk ie < 8% moisture ? TBC is sufficient for an explosive event; note the moisture content can vary due to environmental conditions.
3. What are likely quantaties of fine wood dust being collected?
4. What is the air volume of the dust collector?
5. Assuming the dust is 'explosive' can the initial ignition be contained to create a sufficient pressure rise within the vessel ie dust extractor, to create an explosion (pressure rise) rather than fire.
So what should be done if you have one of these types of dust collectors fitted in your factory, or are being offerred one as a dust control solution for your woodwork shop?
The very first thing to do, if you have existing dust collectors, is to have dust classification test carried out on a representative sample of wood dust. This is a relaively low cost test, 1 sample price £350 (with discounts for 2 or more samples). Approx 0.5kg ssmple is required, sealed in box or bag / ensuring moisture content will be un-changed.
R&B Indistrisl's dust classificstion test consists of;
1. A sieve test of the complete sample
2. A moisture content test
3. Various ignition tests to determining whether the sample will ignite and sustain combustion or not.
From the results it is possible for us to determine the suitability of the dust / product with respect to the dust collection system design. For instance:
- In many instances the fitting of explosion relief panels is not a requirement from a DSEAR / ATEX perspective and no further control mitigation measures are required: here the dust would be generally course and flammable but not an explosive risk.
- In other cases additional control measures may also be recommended, such as flame suppression / water deluge systems in order to reduce risk to a satisfactory level.
- In other applications the incorrect dust collector has been specified / supplied and in these circumstances a complete re-design of the system should be carried out.
In many applications it will be difficult to determine the above characteristics of the dust prior to the actual collection of dusts from the woodworking machines. However, the risk increases depending on:
- If the dust contains a reasonable % of mdf, hardwood.
- Types of wood working machines in use ie sanders, x cut / table saws, ....... tend to create finer dusts.
In these circumstances a correctly specified ATEX certified dust collector should be specified / installed. This can include some or all of the following:
- A wood dust extractor built with the correct vessel strength / bracing etc. This includes the discharge / bin design.
- Strengthened duct to the explosion valve.
- Correctly sized explosion relief panels; taking into account such things as explosive dust properties, vessel strength, and volume, ....
- Water deluge or other suppression system.
Another important consideration should be the amount of fine dust layering within the workplace or within the clean side of the dust collector. This can create a secondary dust explosion far greater than an initial fire / explosion.
These are the results from a recent dust testing which was carried out on the wood dust from the Mill.
Without predjudice, I have provided the following information which I hope is useful?
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. Only weak explosions are likely where the mean particle size are outside of these limits. Note the higher the quantity of fine dust below 80 micron, the higher the risk.
The dust sample received had approx. 20% below 125 micron with approx 8% < 90 micron, at 11% moisture w/w.
This would indicate that if ignition took place, only a weak explosion would occur, with the mix of dust received. If dust separation can occur where the fine dust can then form a cloud in air, there will be a higher risk of explosion.
At Mill Brook Beds it could be conceived that an explosive fine dust formation, could occur during the cleaning period (when dust collector shut down) i.e. due to the separation of fine / course dusts within the dust collector / filters etc, although the agitation will not be as much compared to either shaker or reverse pulse jet systems. At the same time as the dust cloud formation there would also need to be an ignition source: The most likely cause for an ignition would be either:
- Tramp wood say, caught in a wood work machine saw blade which is heated to ember temperature and then sucked into the dust collector.
- Metal impact on fan impellor i.e. via nail or metal object, or again tramp wood caught in impellor & heated to ember temperature before releasing into hopper.
- Static ignition either from filters or plastic collection bags (low risk)
We would recommend that the plastic collection bags are replaced for antistatic type and placed within an earthed metal collection bin; currently if a fire were to occur this could spread due to the splitting of the collection bags.
When replacing filters ensure antistatic filters are fitted.
It must be ensured that dust / product is not allowed to build up within the duct which could also facilitate the spread of fire to other areas.
In addition, if the above scenarios are likely or foreseeable then we would recommend a heat / smoke detection & deluge system should be fitted into the dust collector. A magnetic separator within the duct leading to the dust collector should also be considered.
If funds are available a new ATEX dust collector should be considered, however we feel the risk of explosion is low with the types of control measures specified above.
The above is also dependant upon:
- Processes (connected to LEV) do not change, i.e. if more sanding or processes that create more fine dust are envisaged.
- The type of wood used does not change, i.e. very little hardwood / MDF dusts.