Using a more conservative approach, the lower streaming current value of 0.01 µA used in the same calculation yields a potential energy of 2.25 mJ, a voltage of 15 kV, and a spark gap of 5 mm (or some 0.19”).
With the MIE of toluene calculated to be 0.24 mJ a potential discharge with energy using the most conservative assumption of 2.25 mJ is still sufficiently high enough to ignite the flammable vapours produced by the toluene present in this process.
How could this have been prevented?
It is 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 flammable atmosphere 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 bucket. In this case electrostatic charge accumulated on the bucket because it was electrically isolated from the general mass of earth. Had the bucket been connected to a true earth ground charge would not have been accumulated on its surface. Instead 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 bucket 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 (188.8.131.52) & (184.108.40.206) 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.”
As can be seen from this case study, when highly flammable or combustible products are being handled it is essential to specify certified equipment which will protect the safety of your plant and employees.
Static grounding clamps and cables like the ones highlighted in the image below, are ATEX/FM certified to prevent any physical impedances such as paint coatings, product deposits and rust ensuring a good electrical connection has been made to the equipment’s base metal.
For operations requiring continuous grounding, the next level of protection would be a self-testing earthing clamp with visual indication.
The Bond-Rite® REMOTE is a wall mounted static grounding device which encourages the operator to ground the equipment before the process starts. The ground loop monitoring system ensures a positive connection resistance of 10 ohms or less. Pulsing green LED indicators inform the operator with a simple GO/NO GO instruction that the process is ok to start.
ATEX/FM approved grounding clamps
Continuous Ground Loop Monitoring
with visual indication
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.
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.