Secondary Explosion - What are the dangers?

14 June 2016

So you have an explosion protection system in place to protect your process and plant from a potential explosion.  

Your explosion protection system is serviced by professional and qualified service technicians and one area that is commonly reported by service technicians is the build-up of product on the external surface areas of the process.  This has a potential to cause an external or even worse, a secondary explosion.
Throughout history dust explosions have occurred in various manufacturing processes.  Some of the most catastrophic explosions were the result of secondary explosions. Secondary explosions are the main reason that buildings are destroyed and the number one cause of multiple fatalities.
Two well-known cases of secondary explosions are Imperial Sugar in Savannah, Georgia in 2008 and West Pharmaceuticals in Kinston, North Carolina in 2003.  In both cases, the percussion from the initial explosion caused combustible dust that had accumulated on the surfaces throughout the plant to be suspended in the air and ignited causing catastrophic secondary explosions.  A total of 20 people were killed and 76 injured in these two incidents alone.
So what is a secondary explosion?
A secondary explosion occurs when an explosion within a process has not been contained or suppressed by the explosion protection systems enabling the initial explosion to disturb product built up on the external surfaces, thus allowing the product to form a dust cloud and be ignited by the primary explosion.
Secondary explosions occur when there is a rapid combustion of fine combustible dust particles suspended in the air. Secondary dust explosions can occur where any combustible material in dust form is suspended in the atmosphere in a high enough concentration that allows combustion with a source of ignition. This type of explosion is almost always catastrophic.
All it takes is a surface build-up of product on external surfaces such as electric motors, pipes, tops of dust collectors, false ceilings, roof voids, even floor areas.
How much dust is too much?
·        When you can visibly see the dust on the surface areas, you have too much!
·        If you can use your finger to write you name in the dust and you have visible ridges, you have too much!
·        If you cannot tell what colour the equipment is below the dust, you have too much!
A 1mm layer of product build up that has been disturbed and suspended in the atmosphere will certainly create an explosive dust cloud then all it requires is an ignition source. Now let’s imagine that the product is covering the electric motors around your plant and these motors over heats, you now have a potential ignition source for this dust cloud.
Product dropping in a dust collector could create a vibration that suspends the dust and create a dust cloud.  A door being opened allowing a gust of wind entering the area could allow the dust to be suspended in the atmosphere and create a dust cloud. All that is required to create an explosion is a small ignition source.
These are only a couple of examples of potential catastrophes waiting to happen.
Good housekeeping is the key to reducing or eliminating the risk of a secondary explosion.
**Remember to clean up you area as you are never ready for all the elements to come together for a work place incident, but you can certainly prevent or eliminate them if you are aware of the potential hazards.**