A lot of folks know about flammable liquids and their vapors. A smaller group of folks know about combustible dusts. Yet there is a even a smaller group that knows that when a flammable vapor/gas and a combustible dusts are mixed together, both the vapor and the dust will have a LOWER LEL than if they were in an atmosphere by themselves. Here is what happens and why it is important to consider this phenomenon in your safety systems.
We call these mixtures “Hybrids”. A “hybrid” is where two or more flammable materials of different phases (e.g., dust and vapor) are present in the same mixture. Tests have shown that adding a flammable gas to a dust suspension can greatly lower the ignition energy of the dust. This phenomenon is especially true where the gas is present at a concentration below its lower flammable limit (LFL) or the dust is below its Minimum Explosible Concentration (MEC). Such hybrid mixtures can sometimes be ignited even if both components are below their lower limits. A “hybrid” mixture can be formed by the following:
- Vapor desorption from particulates (such as in resin product receivers)
- Reaction of particulates with atmospheric moisture that produces a flammable gas
- Introduction of a dust into a flammable vapor atmosphere (such as adding a dust or powder to a flammable liquid)
The term “hybrid” applies to any mixture of suspended combustible dust and flammable gas or vapor where neither the dust itself nor the gas or vapor itself is present in sufficient quantity to support combustion but where the mixture of the two can support combustion. Hybrid mixtures pose particular problems because they combine the problems of the high charge densities of powder-handling operations with the low ignition energies of flammable vapors. The MIE of a hybrid mixture is difficult to assess, but a conservative estimate can be made by assuming that the MIE of the mixture is at or near the MIE of the gas alone. Because hybrid mixtures contain a flammable gas or vapor, they can be ignited by brush discharges.
It is ABSOLUTELY CRITICAL that this phenomenon be taken into consideration when designing safety systems such as “inerting” the headspace of process vessels. Engineers need to realize that when combustible dusts are added to vessels in the presence of a flammable vapor or gas, the RISKS are INCREASED by a considerable amount. Without the dust in the mixture, the atmosphere in the vessel may not reach the LEL of the flammable vapor, but when personnel begin to add the combustible dust – the “hybrid mixture” will LOWER the LEL for the flammable vapor AND the combustible dust’s Minimum Explosible Concentration (MEC) will also lower in this atmosphere, making ignition that much easier.
Coupled with the fact that dust is a MAJOR source of static electricity, this is why adding powders to flammable liquids is the #1 cause of flammable liquid process explosions. Keep in mind that most of the common hydrocarbons have Minimum Ignition Energies (MIE) well below 1 mj, just a fraction of the energy generated by the falling powder/dust. With a lowered LEL for the vapor and the combustible dust, this is just a recipe for disaster!