Statics on combustible dust events state that hot surfaces, flames/hot gases, mechanically generated sparks, electrical apparatus, and static electricity are the top five most common sources of ignition for combustible dust fires and explosions. The following will discuss some methods of control for each of these ignition sources.
Hot surfaces are often overlooked as a potential ignition source. When dealing with hot surfaces, as potential ignitions sources, the first thing to do is ensure surfaces are cleaned and do not acquire material build up. If this is not possible then a hot-surface ignition temperature of dust layer (LIT) test should be conducted to determine the surface temperature that will ignite a dust layer. The surfaces with accumulations should then be kept a minimum of 112°F lower than the test results. The surface temperatures can be determined with a thermal gun.
Hot Flames or Gases
The approach in dealing with hot flames and gases as an ignition source should be the same whether it is associated with hot work or the process. All combustible dust should be removed or keep from the area. If the combustible dust and flames/gases are part of the process then monitoring, sensors, and interlocks should be in place to ensure the safety of the process.
Mechanically Generated Sparks
Mechanically generated sparks, in terms of an ignition source, are usually handled the same as hot flames and gases: remove and/or keep the combustible dust from the area. Some processes are prone to mechanical spark production. In these cases a detection and extinguishing system for the sparks should be installed as a safety measure, and interlocks as required.
If an air intake is present around a potential spark producing locations, then the air intake should be relocated.
Electrical apparatus is anything that requires power, either wired directly or plugged in. The biggest tool when making sure electrical apparatuses are not an ignition source is a hazardous area classification. A hazardous area classification examines the existing dust layers, dust clouds, and potential (normal operating, maintenance and upset conditions) for combustible dust creation. From this examination a classification is then given to a zone/division. The simplified version of the zone/divisions is based on three main classifications:
1. Zone 20/21 or Division 1: Where dust clouds are likely and/or dust layers greater than 1/8 in. thick are present under normal operating conditions
2. Zone 22 or Division 2: Where dust clouds are likely and/or dust layers less than 1/8 in.(3.0 mm) thick are present under abnormal or upset operating conditions
3. Unclassified: Dust clouds and dust layers are not present
From these guidelines, and others in NFPA 499, the facility areas that handle, produced, and store combustible dust can be classified. From that classification electrical equipment, installations, and apparatuses in those areas must be rated to match those classifications.
The hazardous area classification can often be de-rated through diligent housekeeping and the addition or improvement of dust collection.
The first step in figuring if static electricity is a problem is to find out the minimum ignition energy (MIE) of the material(s). Knowing the MIE determines the severity of the issue. If the MIE is over 1,000 mJ, it is defined as hard to ignite. In this case static electricity is not considered a high risk, but could still be a potential ignition source. If the MIE test comes back with a value less than 3 mJ, then static electricity should be a top priority. All items should be bonded and grounded, including people, equipment, and material chutes. All materials that encounter the material(s) should be static dissipating or of metal construction.
If the MIE is in the 3 mJ to 1,000 mJ, depending on where on that range the material(s) is will depend on how high risk static electricity is. In all cases the way to deal with static electricity as an ignition source is to bond, ground, and utilize static dissipation (or metal) materials.
When dealing with any ignition sources the first defense is to remove and/or control any combustible dust. If the dust is not present, then it cannot ignite.