Photo source - http://www.csb.gov/

This is a subject that does not, although it should, get much attention,dust explosions. In any industry where there are large amounts of dust present, either in the air or in the machinery being used, there is a potential for an explosion.

Surprisingly even the most innocuous of dusts can become combustible and explosive when in high enough concentrations in the surrounding air under the right (or wrong) conditions. Even dust produced from solid materials, which in solid form do not burn, such as iron, steel and aluminum can become explosive in a dust form. Who would imagine sugar or flour mills are particularly susceptible and highly volatile environments for dust explosions. Several years ago in the USA there were a series of explosions in such factories and dust collectors resulting in loss of life and substantial property damage. Some of these explosions were truly catastrophic and resulted in raising to the ground complete factories. Since then many governmental regulations, standards, laws and recommendations have been implemented to try to prevent future occurrences.

We all know the 3 requisites for a fire to occur, and be sustained, are fuel, heat and oxygen. For a combustible dust explosion 2 more ingredients are required, dust laden air in a confined space such as a factory, dust collector, vessel etc and an airborne dust concentration level above the lower MEC level of the particular dust.

MEC. All dusts are different and are reactive in different ways, and all dusts can be considered explosive to higher or lesser degree. The different ways in which dust react results in differing concentrations of dusts being required to be airborne to create a combustible dust explosion hazard. For example magnesium dust is highly explosive and a small concentration of magnesium dust in air can be a highly explosive cocktail. Whereas sand dust is inherently a lot less explosive and therefore would require a much higher concentration in air in order to become an explosion hazard. MEC refers to Minimum Explosible Concentration of the airborne dust measured in oz/ft³ or kg/m³. All dusts have their own MEC rating.

Combustible dust explosions can occur when a vessel containing dust over and above its MEC level comes into contact with an ignition source such as fire, flame, spark, cigarette, electrical fault spark etc. This can create an explosion. Often this primary explosion will be in a piece of equipment, such as a dust collector, containing dust over and above the MEC level. If you're lucky, at worst, maybe a dust collector is destroyed and, if located outside, hopefully no further damage or injury to building or personnel occurs.

The big risk with this type of explosion is a hidden danger. As mentioned early this type of explosion can level buildings and factories. You may wonder how an explosion in a small dust collector can manage to destroy a complete factory. Well it is not the dust collector explosion that does the damage. The explosion in the dust collector is the start of a chain of events. Obviously an explosion in a small dust collector located outside of a building is not going to create so much damage. The hidden danger with these types of explosions is the secondary explosion that can occur.

In any factory processing fine, powdery and dusty materials, or materials products that produce dust, there is a likely hood of accumulation of build up of dust on plant and machinery, ducting, false ceilings, trunking, piping, hidden areas etc. The initial explosion may be in a relatively small piece of equipment. That small initial explosion can generate a large shock wave through out the building or factory that will disturb accumulated dusts and disperse them into the air creating a confined dust cloud. If MEC level of the now dispersed dust in the cloud is exceeded the whole building now becomes an explosive environment. As there is already an ignition source from the original explosion, the resulting explosion can be devastating and much larger than the initial explosion. Unfortunately if a secondary explosion is likely to occur there is little time to react to prevent it. This time between initial and secondary explosion can be less than one second.

We are involved in the abrasive blasting industry so we will only concern ourselves with dusts applicable to this industry. Abrasive blasting generates larges amounts of dust. Although more dangerously volatile materials such as magnesium and aluminum are blasted, most blasting is performed on steel items using either steel grit, slags or mineral sands. There are two sources of the dust being created, firstly from the abrasive breaking down and eventually becoming dust, and secondly dust created from the surface of the item being blasted. The main contributor of the dust is abrasive, however this is not the complete story. The explosiveness of the dust must also be considered, ie small amounts of a highly explosive dust such as aluminum can be as or more dangerous than large amounts of a more inert dust such as steel grit dust. The potential explosiveness of the dust ties back to the MEC level mentioned earlier.

When evaluating the potential risks of an explosion one of the criteria is to know the explosiveness of the dust being generated. A good source for this is the GETSIS-DUST-EX data based which can be found here http://www.dguv.de/en/index.jsp

This database lists all dusts commonly found in the abrasive blasting industry. Dusts are listed according to their Kst value and classified into 4 categories:

• St 0 - Non explosive
• St 1 - Weak to moderately explosive
• St 2 - Strongly explosive
• St 3 - Very strongly explosive

Generally in the blasting industry the explosiveness of the dust will be rated as St 0 - St 1. However there are some notable exceptions of reading St 2 - St 3 when blasting aluminum or magnesium castings and when blasting with walnut shells. Potentially very dangerous applications would be blasting aluminium or magnesium parts using an aluminium abrasive. In this instance both the substrate and abrasive dust are volatile, and to ensure dust collector explosions cannot occur wet type dust collectors would be recommended instead of dry type filters.

Bear in mind in a blasting environment there are 2 areas that need to be considered and evaluated, the ventilation dust collector and the blastroom, enclosed area or cabinet in which the blasting is taking place. All of these are confined spaces.

As the saying goes; prevention is better than cure. So it is our aim to prevent and avoid dust collector and secondary explosions as much as possible. There are several measures we can take to achieve this.

Some items to consider for prevention of explosion:

1. The first step for any installation should be to conduct a Hazard and Risk Assessment of the specific set up.

2. Good Houskeeping. Prevent the accumulation of dust in blastrooms and surrounding areas. Abrasive blasting is inherently a dusty business and inevitably there will be a build up of dust in areas around a blastroom. Do not attempt to clean u by blowing with compressed air as this will just spread the dust around. Dust should be cleaned up with vacuum cleaner and disposed of.

3. Dilution ventilation airflow. To ensure the MEC of any dust is not exceeded the ventilation dust collector should be sized to provide adequate ventilation capacity to dilute the volume of dust in the air to safe levels. If the MEC level is not exceeded then an explosion cannot occur. Correctly sized ventilation dust collector will achieve this.

4. Ensure ventilation dust collector is continuously monitored and maintained in good running order. If filters are fully blocked then the dust collector will not function correctly and capacity of ventilation air will be reduced. This reduction in ventilation are flow will effect the dilution capability.

5. Prevent dust accumulation in ducting. The ducting size and set up should be designed to achieve the correct airflow rate through the ducting. If the airflow is too slow dust will not convey and will drop out of the conveying air stream and accumulate in the duct. This becomes a high risk explosion hazard. If airflow through the duct is too high, excessive wear and damage to ducting will occur. Ensure the optimal speed for the dust being conveyed is achieved.

6. In instances where there is a likelihood of sparks being generated and possibly entering the dust collector inlet ducting, spark arrestors can be fitted to the ducting. These devices create a tortuous path for the air stream and any sparks are extinguished before entering the dust collector. These devices are recommended if welding or hot metal spray works are the be performed in a blastroom.

7. Avoid and eliminate hot surfaces, sparks or ignition sources from dust collector or ducting locations.

Some items to consider to reduce damage should an explosion occur:

1. Locate dust collector out of factory and outdoors and fit with explosion venting facing away from factory wall. Explosion vents are panels that create a weak point in the dust collector. Instead of the dust collector exploding in an uncontrolled and unpredictable manner the use of explosion vents will control the location of the explosion. Explosion vents should be positioned away from factory wall and in a direction to prevent damage to other buildings or facilities.

2. Fit fire detection and fire extinguisher and suppression systems to ventilation dust collector. Objective of this is to extinguish ignition source required for secondary explosion.

3. Inlet ducts to dust collector can be fitted with fast acting slide gates. In the event of an explosion in the dust collector, the slides gates instantaneously close and isolate the dust collector from the factory.

Please note the above lists are not exhaustive and are provided just to give you ideas of some of the items to consider. The whole subject of dust explosions in factories and in dust collectors is vast.

If you want further reading on this subject good places to start are the following:

OSHA - https://www.osha.gov/dsg/combustibledust/guidance.html

NFPA - http://www.nfpa.org/

 

Author: Chris Charlton
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