Incendive electrostatic sparks usually result from the lack of a thorough and detailed risk assessment, unintended changes to equipment during routine maintenance and unsafe operator working practices. To prevent electrostatic discharges igniting combustible dust atmospheres companies should risk assess their processes and equipment to ensure any potential sources of ignition are identified and managed correctly. In operations that use solvents for suspending powders in blending and conveying systems (and for powders that give off their own flammable vapours) surrounding work spaces will be zoned or classified as being potentially flammable and combustible atmospheres. All potential sources of internal and external static discharges from process equipment situated in zoned & classified areas must be accounted for and managed in the appropriate way.
Earthing (grounding) of multiple isolated parts with a single monitoring system
If they are not sufficiently bonded and grounded isolated components in conveying and dust collection systems are capable of holding large amounts of static electricity. Isolated components usually result from design oversight or after maintenance teams reassemble fittings without re-establishing static bonding connections. Pipes, valves, blowers, hoppers and other components engaged in powder transfer processes can be isolated from each other due the insulating properties of parts like rubber gaskets or through normal wear and tear. The most secure means of preventing charge build-up is to bond and ground components to a reliable high integrity ground point. The NFPA and CENELEC state these bonding connections should have a resistance to earth of less than 10 ohms. To manage the uncertainty of knowing whether or not components can become isolated during processing operations, dedicated earthing equipment can be specified to monitor all potentially isolated points in the conveying system. If a component loses its ground connection, or experiences a rise above 10 ohms resistance in the bonding circuit, operators can be alerted to the potential hazard immediately, either through automatic shut down of the operation or by hazard strobes and sounders.
The same kind of earthing device can be used to ground and bond components in systems like fluid bed dryers which experience vibration effects that can lead to momentary sparks gaps between components that make up the overall assembly. Isolated charged components have the capability to discharge to fully bonded or grounded components within the structure of the machine. The important thing to do is fully assess the potential of components to become momentarily isolated as static sparks can release large amounts of energy in milliseconds.
Charges accumulating on the surface of mixing and blending machines can be dissipated using discrete purpose designed earthing systems. These systems provide dual protection dissipating static from the vessel wall preventing internal discharges into the potentially combustible atmosphere present in the vessel and preventing external discharges into the potentially flammable or combustible atmosphere surrounding the machine. Continuous monitoring of the earthing circuit combined with output contacts, that can be deployed to shut down the process or alert personnel to the hazard, maximises the safety of the process and workers in vicinity of the machine.
Powder filling operations often produce clouds of combustible dusts that have the potential to disperse in oxygen above their MEC limit. Spark discharges and Propagating Brush Discharges (PBDs) can ignite the resulting dust cloud. It is critical to ensure that conductive and semi-conductive powders are not deposited into containers or bags that insulate the resulting charges.
Type C FIBC bags can mitigate against these risks by conducting charge from the powder through conductive threads in the bag to the ground connection point on the bag. As charges are dissipated from the surface of the powder the risk of static spark discharges to nearby conductive objects and uncontrolled PBDs over the powder surface is reduced. To compensate for normal wear and tear on bags it is important to ensure the bag maintains its capacity to dissipate charge and also ensure the ground connection between the bag and known earthing point is functioning correctly. Dedicated FIBC bag earthing systems can be specified that ensure the resistance of the bag is compliant with the requirements of the equivalent European standard. Should the bag lose it ground connection, the system will draw the attention of operators to this potential hazard.
Vacuum truck operations are particularly vulnerable to incendive static spark discharges. The movement of charged powder from source to collection chamber can induce large charges on lances, hose connections, the hose itself and components within the collecting chamber. A range of deflagration incidents have been reported in vacuum truck operations, particularly in situations where components on hoses and lances have become isolated and discharged static sparks into the surrounding atmosphere or within the vacuuming system. The American Petroleum Institute recommends that all connecting metal parts of the vacuum collection system are conductive to less than 10 ohms and that the vacuum truck itself is connected to a fully verified ground point. Truck-mounted bonding systems, containing flashing LEDs can be specified helping operators observe 10 ohm or less connections to pre-installed earthing points. Another system, currently in development, will enable operators to confirm a full ground connection using a truck-mounted mobile ground proving system. This ground breaking system will eliminate the time and uncertainty of using meters to measure and establish safe ground connections in locations where pre-installed earthing points do not exist.
What to do | To prevent uncontrolled electrostatic discharges posing a fire and explosion hazard in powder processing operations, a thorough static audit conducted by qualified personnel should be carried out. The audit should focus on investigating and identifying situations where charge has the potential to accumulate on conductive and semi-conductive components. For situations where potentially isolated components are identified dedicated earthing equipment should be installed to monitor and control the release of static electricity, thereby removing a primary source of ignition in combustible dust atmospheres.
Mike O’Brien, Managing Director for Newson Gale
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