When materials such as flour, milk powder, wood dusts, sulphur, magnesium or aluminum powder, epoxy resins, powdered paints and other similar powders are handled, transferred or collected, they are, i...
September 1, 2003 | By Simon Fridlyand, P.Eng
When materials such as flour, milk powder, wood dusts, sulphur, magnesium or aluminum powder, epoxy resins, powdered paints and other similar powders are handled, transferred or collected, they are, in fact, in suspension. That means there is a possibility of an explosion.
Why is that? In order for an explosion to occur, the following conditions are necessary:
An ignition source
The proper dust/air mixture, and
Confinement (usually a room, building or process vessel such as a dust collector where dust is in suspension).
An ignition source could come from electrical motors, appliances, sparks, flames or static electricity. It is essential that the sources of ignition in areas where dust is present be controlled.
All electrical appliances and wiring in places where dust is present must be classified. Labelling is usually used to identify classified equipment. The label marking for dust is as follows: Class II, Div. 1, and Class II, Div. 2, Group E, F or G. By using classified equipment, there is a guarantee that no electrical spark will be generated.
The control of ignition sources includes the control of static electricity. Static electricity is caused by the movement of electrons when dissimilar substances are in contact with each other (such as the pneumatic conveying of powders) and then are separated from each other. The electrons produce electrical charges, which, if they cannot rapidly escape, be neutralized or eliminated, produce a static charge buildup.
The charge will eventually develop enough energy to jump as a spark to a less-charged object. Proper methods of grounding and bonding must always be implemented to eliminate static charges.
Industrial processes where dusts are handled may have enough concentration of dust in the air to create an explosive mixture. The severity of explosibility of dusts is identified by the Kst value. For example, wood flour has a Kst value of 200 Bar metres/second, sugar is 138 Bar m/s, vinyl chloride 98 Bar m/s, and aluminum 415 Bar m/s.
The greater the Kst value, the bigger the destructive ability of the explosion. The Kst value of basic materials can be found in handbooks, however the Kst value of composite-type materials has to be established by testing.
Material Safety Data Sheets (MSDS) usually do not carry Kst value information. This information is absolutely necessary for the design and sizing of dust collectors, cyclones or any other process equipment or even process building. In some instances, a reinforced building construction is required.
A masonry wall can withstand a fraction of a rapid pressure rise generated by explosion. Therefore, buildings where a possibility of explosion exists are designed to be reinforced. They are also provided with explosion relief panels or walls. These panels or walls will open up to the atmosphere well before the destructive force of the explosion reaches its peak and will release the pressure to the outside.
In many instances, a relatively small explosion may happen inside a room or a vessel. This explosion will stir up the dust in the surrounding areas and may cause a secondary explosion. The secondary explosion will have a greater destructive nature then the first one.
The second one may generate a third one and so on. At the end, the entire building may be levelled.
An explosion may travel from one building or vessel to another through connecting pieces, generating subsequent explosions. Usually, explosions are followed by fire, which completely destroys whatever was left. There are cases where multi-building complexes have been completely destroyed because of this effect.
It takes only 50 milliseconds for a dust explosion to become fully developed.
A fuel/air mixture and confinement exist in any dust collector/filter receiver. As a result, dust collector/filter receiver explosions represents 15 per cent of all explosions taking place in industrial establishments.
The best way to manage explosions in collector/receivers is to safely vent the explosions to the outside. This is achieved through properly selected explosion relief venting.
Special attention must be paid to the strength of the vessel/collector/receiver. I have seen that many collectors, for example, have been designed at 20 in. w.c. (water column). This strength is only to withstand the pressure generated by the fan and is not enough to withstand an explosion, even when the explosion relief is provided.
Explosion issues are complicated. Only experienced people should handle them. In Ontario, the Occupational Health and Safety Act under Section 7 of Regulation 851 mandates that a Pre-Start Health and Safety Review should be conducted by a professional engineer to address explosion safety issues. Dust collectors are also subject to the review. This review must be performed before the process or dust collector becomes operational, and the report is to be given to a Joint Health and Safety Committee.
Safety File columnist Simon Fridlyand, P.Eng., is president of S.A.F.E. Engineering of Toronto, a company specializing in industrial health and safety issues and compliance. He can be reached at 416-447-9757.