Definition
As explained in the article on Explosive Atmospheres, minimum ignition energy, abbreviated to MIE, is the least amount of energy required in an electrostatic spark discharge to cause ignition of an explosive atmosphere formed from mixture in air of flammable gas, vapour or dust/powder. MIE is a measure of the sensitivity of a substance to ignition by electrostatic discharges. The lower the MIE, the more easily a substance will be ignited by electrostatic discharges.
The importance of MIE in selecting safe static protective FIBC
Common flammable gases and solvent vapours (e.g. methane, pentane, toluene, xylene, etc.) have MIE in the region of 0.2 mJ to 0.3 mJ, making them sensitive to electrostatic ignition. Some substances have even lower MIE, but these are not normally present in processes involving FIBC operations. The substance with the lowest MIE that is likely to be present when FIBC are emptied is methanol, with an MIE of 0.14 mJ. Hence, this is the value that is used in the safety qualification testing of Type D FIBC specified in IEC 61340-4-4 (also see Proving the Safety of CROHMIQ). The concept is to test under worst-case conditions likely to be found in industry. Qualifying Type D FIBC under realistic worst-case conditions ensures that they are safe in all other conditions.
When choosing the safe type of static protective FIBC, it is necessary to know the sensitivity to electrostatic ignition of both the packaged product and the environment in which the FIBC will be emptied. Using this information and the safe use tables published in IEC 61340-4-4, IEC TS 60079-32-1 or NFPA 77 (see the article on Standards & Legislative Frameworks), or the Static Protective FIBC Selection Flowchart, the safe type of FIBC can be determined. If the package product contains residual solvent, or is expandable polystyrene (EPS) containing pentane, it is not necessary to know the exact MIE. If any flammable solvent vapour or gas is present in or around FIBC, then Type A FIBC and Type B FIBC must not be used. Only Type D FIBC and securely grounded Type C FIBC* are safe for use when flammable solvent vapours or gases are present.
If the only flammable substance present in or around FIBC is a powder, then it is necessary to know the MIE of the powder to determine if it is safe to use Type B FIBC. If the MIE of the powder is less than 3 mJ, then Type B FIBC are not safe to use. Type B FIBC are only safe for use with combustible powders, such as sugar, flour, corn starch, etc., with MIE greater than 3 mJ. For powders with MIE less than 3 mJ (e.g. bisphenol-A, finely ground sulphur, coal dust, etc.), only Type D FIBC and securely grounded Type C FIBC* are safe for use.
*WARNING: Type C FIBC must be securely grounded before and during filling and emptying operations. Isolated Type C FIBC are extremely dangerous and are known to have caused explosions.
CROHMIQ Type D FIBC provide full protection against electrostatic explosion risks without the need for grounding.
Where to find MIE values
MIE data for common gases, solvent vapours and dusts/powders can be found in published literature, in product safety data sheets (SDS) and in various online resources, including the CROHMIQ webpage Minimum Ignition Energy Data by Product. Apart from the specific information in SDS, most available data is for single substances. Data is not so readily available for mixtures of substances, or for more specialist and less commonly used substances. In such cases, it may be necessary to measure the MIE of the substances concerned.
The basic principle of measuring MIE is to create an explosive atmosphere by mixing gas or solvent vapour with air inside an ignition chamber, or dispersing dust/powder with air into an ignition chamber, and then passing a spark of known electrical energy between two electrodes located inside the ignition chamber. Energy is increased until ignition occurs.
Although the basic principle is relatively simple, accurate measurement of MIE requires specialist knowledge and experience, and carefully designed and calibrated apparatus. Measurements must be done in accordance with recognized standards such as ASTM E582 (gases & vapours) or ISO/IEC 80079-20-2 (dusts/powders), which specify the design, calibration and operation of the apparatus.
Laboratories providing MIE measurement services
The independent laboratories listed in the links below offer services for determining MIE of gases, vapours and dusts/powders. Please note that this list is not comprehensive; there are other laboratories that offer similar services, which may be found via online searches.
Dekra – powders and combustible dusts
Stonehouse – gases and vapours