If the powder is flammable and also composed of fine particles, the ‘powder cloud’ inside the mixer is likely to be flammable. If the mixing or micronizing is undertaken in air, a DSEAR assessment is likely to conclude that a zone 20 is appropriate for the inside of the mixer. If nitrogen is used to remove the presence of oxygen, the space inside the mixer is no longer flammable, but a zone 22 could still be designated to account for potential inerting failure. If there are robust procedures for assurance of inerting, the space may be designated as non-hazardous.
After the zone has been designated, the question remains whether the flammable dust cloud can be ignited. If the zoning is non-hazardous, ignition sources can be tolerated as there is no flammable atmosphere. In a zone 20, the ATEX code for all equipment inside the mixer needs to be 1D or 1GD, with a temperature rating no more than two thirds of the cloud ignition temperature in oC. In a zone 22, the ATEX code inside the mixer can be 3D or 3GD (category 1 and 2 are also acceptable). These ATEX codes are to account for anything in the equipment that might spark or get hot enough to ignite the flammable dust cloud.
Once the ATEX codes are taken care of, the question remains whether the dust cloud can be ignited by any electrostatic effects. The mixer is likely to impart some degree of shear to the powder mix and the powder particles are thus likely to become charged. If the mixer and all its components are metal and earthed, any charge on conductive particles is expected to be immediately dissipated to other particles and ultimately to the earthed mixer.
If the bulk powder is electrostatically insulating, the particles in the dust cloud are likely to become charged. Since insulting materials cannot be earthed, this charge may persist. The charge from the powder cloud is likely to induce an opposite charge on metal components inside the mixer. If these components are earthed, the charge will quickly dissipate. If there are metal components that are electrostatically isolated (not connected to earth), they can become charged by their proximity to the charged powder cloud. These are termed as ‘isolated conductors’, perhaps a metal port in a rubber gasket. If a charged isolated conductor encounters another isolated conductor with a different charge or if it encounters a pathway to earth, a spark discharge is possible. A spark discharge is a sudden singular discharge that may have enough energy to ignite the dust cloud, particularly if the dust cloud has a low minimum ignition energy (MIE). The earth pathway could be an operator, temperature probe or even an earthing rod. The energy associated with a spark discharge can be calculated from the capacitance of the isolated conductor (a measure of its ability to store charge, influenced by the size of the object) and the voltage of the charge. The calculation is:
Energy E = ½ C V2 (equation A.3.2 in BS 60079-32)
Thus in theory, if all the conductive components of the mixer are earthed, a spark discharge is not possible and electrostatics can be eliminated as an ignition source. However, there are two precautions to this approach:
- The process of earthing and checking earthing is not necessarily straightforward. Even if the mixer as a whole is earthed, individual metal parts can become isolated from earth by rubber gaskets, rubber feet or even a gloved hand. It is also worth noting that operators can only be earthed by the combination of static dissipative clothing/footwear and a static dissipative floor. The polyurethane floors common in many pharmaceutical environments are likely to be electrostatically insulating. Since the operator is intended to be earthed, the operator is expected to be an earth source and potentially a discharge mechanism for a spark discharge. Expert advice may be necessary.
- The energy associated with a spark discharge is proportional to the capacitance of the isolated conductor and the square of the voltage of the charge. When low MIE powders are subjected to high shear, the voltage on an isolated conductor is likely to be relatively high. For this reason, earthing has to be rigorous, and may need to be tested before each production run. Alternatively, the use of nitrogen as an inerting medium may be the best way forward, in line with the control hierarchy of DSEAR, which is to eliminate the flammable atmosphere if possible and practical.
