Powder processing operations can generate vast quantities of electrostatic charge via the movement of powder. The standard method of charging on powder processing operations is due to tribo-electrification, which is basically the contact and separation of the powder with processing equipment, the powder itself or other factors that can cause charging, like surface contaminants. There are numerous types of equipment that can cause the charging of powders. Such equipment includes, but is not limited to:
Table 1. Equipment used in powder processing operations
The processes carried out by such equipment can lead to varying degrees of electrostatic charge generation. Typical charge quantities, from published literature, are tabulated below. The values are based on the amount of charge, in coulombs, carried per kilo-gram of powder.
Table 2. Charge generated on powders by different powder processing and handling operations (NFPA 77 / CLCTR: 60079-32-1)
A simple calculation will show that a metal drum with a capacitance of 100 pF being filled with 25 kg of charged powder, following a simple pouring operation, could be charged to a voltage of 25,000 V.
The potential energy that could be discharged from the drum in the form of a spark can be estimated to be:
By any standard, the voltage generated by an operation that is known to be at the lower end of charge generating capacity can still generate enough potential spark energies to ignite a broad range of combustible atmospheres. Table 2 lists the minimum ignition energy of a sample of powders when they are at a Minimum Explosive Concentration.
Table 3. MIE of various powders when suspended in a combustible concentration
If the powder is being discharged into a blender or mixer that contains a solvent, the MIE of the hybrid atmosphere could be much lower such that the initial ignition of the solvent vapour could propagate a combustible dust deflagration.
The safety factor that needs to be borne in mind with these calculations is the assumption that the equipment being “electrified” by the charged powder is not grounded. If the equipment is grounded, there is no risk of the equipment becoming electrified by static electricity.
Static Earthing protection in powder processing operations.
“Earthing”, in its truest form, is the method by which a low resistance electrical connection is made between equipment at risk of electrostatic charging and the general mass of the Earth. This connection is normally described as a “true earth ground”. The actual connection to earth is achieved via purpose designed earthing rods, or building structures, that are buried below ground level. These earthing systems are tested by engineers to measure their true earth ground resistances to ensure they are below values required in standards like NFPA 70 “National Electrical Code®” and EN 62305 “Protection Against Lightning”. Some static earthing systems on the market today will actually verify if the equipment they are providing static earthing protection for have a true earth ground capable of conducting static electricity.
In pharmaceutical operations, equipment like powder conveying systems, micronizers, blenders and sieve stacks all make up multiple component assemblies that can accumulate high levels of electrostatic charge should any of the components be isolated from a true earth ground. Connections made with items like bonding straps can provide an intentional bond between metal components or assembly mating surfaces may provide an inherent bonded connection.
Fig. 1. A blender getting charged with a powder. Note that the bucket discharging the powder should be bonded to the receiving vessel or grounded independently.
Regular disassembly for cleaning and maintenance can result in bonding connections being missed or not made correctly when the Regular disassembly for cleaning and maintenance can result in bonding connections being missed or not made correctly when the equipment is reassembled. Vibration and corrosion may also degrade assembly connections so it is imperative to ensure that no parts in the assembly become isolated from a true earth ground reference.
The most effective way of ensuring that equipment used in powder processing operations cannot accumulate static electricity is to provide a dedicated static earthing solution that will monitor the ground connection of components at risk of static charge accumulation and alert personnel to a potential hazard should a component lose its ground connection. This is especially important if the ground connection point to the equipment is not readily visible or easily accessible.