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Dust Collection Q&A

  •   We have been asked to perform dry ice cleaning in a cement silo. The only gas present from our side would co2. Is it possible that this could cause a dust explosion with the cement dust. There will be lots of ventilation. Your comments would be highly appreciated.

    Answered February 11th, 2016 by Expert: Diane Cave, P.Eng

    First off, most types of cement have a Kst equal to 0 bar m/sec, which means that it cannot explode.  Check with the manufacturer and see if they know the Kst value of the dust.

    If it does have a Kst rating then there are procedures to follow to minimize the changes of an event.

    In order to have an explosion you must have five things present:
    1.    Containment
    2.    Oxidant (ie Oxygen)
    3.    Fuel
    4.    Ignition source
    5.    Dispersion of dust

    If any one of those five are not present then a dust explosion cannot occur.  By reducing or eliminating any one of the items you reduce the risk or eliminate it completely.

    Containment – You cannot change the containment, as it is your surroundings.

    Oxidant – If the concentration of oxygen is too low than an explosion cannot occur.  The addition of CO2 actually reduces the risk, because CO2 is actually used to suppress fires and explosions.

    Fuel – First off the fuel has to be explosible, which means it has to have a measurable Kst.  It also has to be in the correct concentration.  If it is too lean (LEL – lower explosive limit) there is not enough fuel for an explosion, at or below 25% of the LEL is considered safe.  The LEL is a tested value.  If there is too much (UEL upper explosive limit) fuel and the fuel to air ratios is above the UEL then there is not enough air for combustion to occur.  When cleaning, when the dust cloud starts to get thick (for example – so you cannot see a flash light through it).  Stop.  Wait for the dust to settle, remove the settled dust if possible.  Continue and repeat.  Ventilation also helps with this.

    Ignition sources – Take away all possible sources of ignition.  Static is a big one.  Use not sparking equipment.  Remove smokers from the area.  If your equipment that will be in the storage area has moving parts make sure they are not ceased and move freely and do not become hot spots.  Anything you can think of that could cause an ignition source.  Remove it.

    Dispersion of dust – this you cannot control, because you are going to be putting dust in suppression as is the nature of the task.

    I hope that helps, but again none of the above is required if there is not a measurable Kst.