What actions could have been taken to prevent this explosion?
It’s highly plausible that this operation had been conducted multiple times without a visible incident occurring with electrostatic sparking taking place in previous operations without a combustible dust being present in the spark gap when discharges occurred. This is a common feature of process operations that have suffered from the consequences of a fire or explosion caused by static electricity.
The first place to start is to determine why electrostatic charge was “permitted” to accumulate on the chime. In this case electrostatic charge accumulated on the chime because the chime was electrically isolated from a true earth ground. Had the chime been connected to the grounded process vessel, charge would not have accumulated on the chime. Excess electrostatic charges would simply have found their way to earth. So in accordance with industry guidelines like NFPA 77 and IEC 60079-32-1, the isolated metal component should have had a connection to a verified ground (in this case the process vessel) with a resistance of 10 ohms or less.
Both IEC 60079-32-1 (13.4.1) and NFPA 77 (126.96.36.199) & (188.8.131.52) state:
“Temporary connections can be made using bolts, pressure-type earth (ground) clamps, or other special clamps. Pressure-type clamps should have sufficient pressure to penetrate any protective coating, rust, or spilled material to ensure contact with the base metal with an interface resistance of less than 10 Ω.”
Devices like the ones highlighted in the pictures below can be used to connect the drum to the grounded process vessel. At minimum a grounding clamp with FM/ATEX approvals, like the example in the picture on the left, should be specified so that any physical impedances like paint coatings and product deposits are fully penetrated to make contact with the base metal. The picture on the right shows a device that will inform the operator when he/she has achieved a connection resistance of 10 ohms or less between a metal drum and the process vessel. This is indicated via a pulsing green LED mounted in the grounding clamp that provides the operator with a simple GO / NO GO instruction to tip the powder into the process vessel.
The use of a plastic drum inside the EX/HAZLOC area also needs addressing. Charging of resistive powders is impossible to eliminate unless specific changes are made to the powder to enhance its electrical conductivity. Very often this is either impractical or not possible to achieve. However, using a plastic container constructed out of a low conductivity material like polyethylene is not recommended in codes of practice as the charge generated during the transfer process remains on the surface of the drum, even if attempts are made to try and ground the plastic drum. Using plastic objects that are poor conductors carries a significant risk of induction charging of other objects in the EX/HAZLOC area. Process equipment and operators if they are in contact with, or in close proximity to, charged plastic objects, can become electrostatically charged. Ideally a metal drum should be used and connected to the grounded process vessel so that no charge is permitted to accumulate on the metal drum.
Last, but not least, all process operators should be grounded through static dissipative footwear that can effectively permit any charge generated during the operation through their own movement bleed off their bodies to the ground. This will ensure they do not carry the risk of discharging static parks from their bodies onto grounded objects.
Codes of practice like IEC 60079-32-1 and NFPA 77 outline what proactive measures can be taken to minimise the risk of a fire or explosion caused by discharges of static electricity. The majority of hazards can be controlled through the installation and proactive use of static grounding devices. Devices ranging from basic grounding clamps right through to ground status indicators with output contacts for interlocking with processes can be specified for a wide range of processes.
Please note this case study is referenced from a third party source and is not in any way linked to the operations of Newson Gale customers.
Mike O’Brien, Head of Marketing and Sales for Newson Gale
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.