Michael O’Brien, Product Manager
Recent studies covering plants in the U.S., UK and Germany show that controlling static electricity build-up is the key to preventing combustible cloud explosions.
During one 10-year period a single insurer listed a total of 450 incidents across their client base that were attributed to dust fires and explosions. The total cost of the damages amounted to $580 million, with the average gross loss for dust explosions costing $1.9 million and dust fires costing $1.2 million. ( 3)
Since their report was published the CSB has repeatedly requested that OSHA take more action to regulate the safety of operations processing combustible and flamma- ble powders. The 2008 sugar refinery explosion at the Port Wentworth plant of Imperial Sugar should be a warn- ing to a broad range of industries just how risky and rele- vant dust explosions are. Approximately 70% of all chemical processing industry operations handle powders
in a combustible form at some point in their manufactur- ing process. ( 4) CSB Study 1980 to 2005: Sectors with recorded incidents of combustible dust fires and explosions.
Several contributing factors need to be present to support the ignition of a combustible dust cloud, comprising:
• A dispersed dust cloud-oxygen mixture that is above its Minimum Explosion Concentration (MEC).
German study: recorded sources of ignition in com- bustible dust explosions incidents. The German data, which totaled 426 incidents, provides a percentage break- down of known primary sources of ignition. In this study electrostatic discharges make up 10% of known primary ignition sources. The “unaccounted” category accounts for incidents where no physical evidence has been detect- ed. The prime suspect in the “unaccounted” category is very often electrostatic discharges, but as no witnesses can provide evidence of seeing or hearing a spark, igni- tion sources of this type go unreported and unaccounted for.
Even though the majority of combustible dusts have high- er MIE’s than flammable vapours the amount of energy available from electrostatic discharges within contained environments will ignite the vast majority of combustible dusts. This is because the rate of electrostatic charge generation and accumulation in powder processing opera- tions is extremely high.
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.
Therefore, to prevent electrostatic discharges igniting combustible dust atmospheres companies should risk assess their processes and equipment to ensure any poten- tial sources of ignition are identified and managed cor- rectly.
Isolated components in conveying and dust collection systems are capable of storing large amounts of static charges if they are not sufficiently earthed. Isolated com- ponents usually result from design oversight or after maintenance teams reassemble fittings without re-estab- lishing 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 buildup is to bond and ground components to a reliable high integrity ground point. The NFPA ( 5) and CENELEC ( 6) state these bond- ing 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 oper- ations, dedicated static grounding equipment can be spec- ified to monitor all potentially isolated points in the conveying system. If a component loses its connection to ground, or experiences a rise above 10 ohms resistance in the bonding circuit, operators can be alerted to the poten- tial hazard immediately, either through automatic shut- down of the operation or by hazard strobes and alarms.
The same kind of device can be used to ground and bond components in systems such as 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 pur- pose designed earthing systems. These systems provide dual protection dissipating static from the vessel wall preventing internal discharges into the potentially com- bustible atmosphere present in the vessel and preventing external discharges into the potentially flammable or combustible atmosphere surrounding the machine. Continuous monitoring of the grounding circuit com- bined with output contacts, that can be deployed to shut down the process or alert personnel to the hazard, maxi- mizes the safety of the process and workers in vicinity of the machine.
Powder filling operations often produce clouds of com- bustible dusts that have the potential to disperse in oxy- gen above their MEC limit. Spark discharges and Propagating Brush Discharges (PBDs) can ignite the resulting dust cloud. It is critical to ensure that conduc- tive and semiconductive 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 earth connection point on the bag. As charges are dissipated from the surface of the powder the risk of static spark discharges to nearby con- ductive 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 dis- sipate charge and also ensure the earth connection between the bag and known grounding point is function- ing correctly. Dedicated grounding systems can be speci- fied that ensure the resistance of the bag is compliant with the requirements of the CENELEC ( 6) standard. Should the bag lose its ground connection, the system will draw the attention of operators to this potential haz- ard. 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 cham- ber. 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 surround- ing atmosphere or within the vacuuming system. The American Petroleum Institute ( 7) 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 grounding points. Another system, currently in development, will enable operators to confirm a full ground connection using a truck-mounted mobile ground proving system. This groundbreaking system will eliminate the time and uncertainty of using meters to measure and establish safe ground connections in locations where pre-installed grounding points do not exist.
