Monitoring combustible dust minimum explosive concentration
Q: Why is it important to know my combustible dust minimum explosive concentration (MEC), and is there a way to continuously monitor the MEC?
Minimum explosive concentration (MEC) is defined in NFPA 652 as the minimum concentration of a combustible dust suspended in air, measured in mass per unit volume (for example, grams per cubic meter), that will support a deflagration, such as an explosion or flash fire.
NFPA 652: Standard on the Fundamentals of Combustible Dust (2016)
requires any facility that handles combustible dusts to determine each dust's explosibility characteristics by testing it. Typically, this involves determining the dust's KSt
, MEC, and minimum ignition energy (MIE), which is usually measured in millijoules (mJ), but other characteristics may also be important. These determinations are also critical information to have to perform a dust hazards analysis (DHA).
The KSt and Pmax test results are used in determining explosion vent sizing, methods of chemical suppression and isolation, and other measures used to protect a dust collection system, building, and personnel by prescriptive methods found in NFPA 68 and 69. The MEC and MIE values are critical for determining the hazards and risks involved when processing and handling the combustible dust. Thus, MEC and MIE are key factors in properly performing a DHA.
By knowing your combustible dust's MEC and MIE values, you can more effectively determine the potential hazards. These values also apply directly to any risk assessment that would be used in conjunction with a DHA. For example, if the MEC for your dust is greater than 1,000 grams per cubic meter and the MIE is greater than 1,000 mJ, then the risk of an electrostatic discharge creating an ignition of your combustible dust cloud is very low. However, if your MEC is less than 50 grams per cubic meter and the corresponding MIE is under 25 mJ, then the risk of an electrostatic discharge creating an ignition is much higher.
If your process involves operations that can generate dust clouds (either under normal or abnormal conditions) inside or outside the process equipment, then it's critical to know how much dust is required within that dust cloud to have an explosible concentration. Controlling this hazard becomes more of a factor if the required MEC concentration for safe operation is low as it will directly affect housekeeping, dust collection, bonding and grounding, and other process requirements. Conversely, if the required MEC concentration is comparatively high, the level of housekeeping, dust collection, or both may be effectively reduced.
However, effectively providing reliable and accurate continuous monitoring of the airborne dust concentration in a building or enclosure, such as a silo, hopper, or bucket elevator hasn't been a simple matter. Fortunately, given the importance of knowing your combustible dust MEC, the market is beginning to catch up to the demand for monitoring.
To provide continuous or spot monitoring of dust concentration levels, you can use inductive electrification, light scattering, ultrasound, or other methods. Inductive electrification involves an induction-based alternating current (AC) signal created when particles move in the air/gas stream and pass nearby the sensor, creating certain frequency bands. The dust will have its own frequency which can be detected. This isn't the old triboelectric method where a direct current (DC) signal is created by actual particle contact. The inductive electrification method is more sensitive and can handle very low dust concentrations. The light scattering methodology can be either forward or backward scattering and involves measuring the intensity of the light scattered by the presence of dust particles in a solution of air/gas stream. Ultrasound is similar to light scattering except it uses ultrasonic sound waves rather than light.
My experience has been exclusively with the inductive electrification and light scattering methods. I have used the inductive electrification method successfully, and it has proven to be reliable, accurate, and adaptable to the actual processes where dust concentration continuous monitoring is critical to plant operations. However, you should investigate which method is best adapted to your needs. It's important to note that the inductive electrification device is now certified as intrinsically safe in Class II, Division 1 (E,F, & G) locations.
The inductive electrification approach's simplicity allows its use for monitoring airborne dust concentrations in the ullage space in hoppers, bins, or silos, and in broken-bag detection for dust collectors and confined spaces. Other methods may also be adapted. The costs involved in this approach do vary, and it may be feasible to have an on-site demonstration in an actual application to provide more assurance the costs are justified. Each device type is capable of wireless monitoring with typical PLC plant control schemes.
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