Static electricity and CO2 extinguishers

Thanks to Mr Harbhajan Singh Seghal for sending this incident:
"During the plant round of Shift Incharge at Compressor House, it was observed that there is smoke and spark on Non-Drive side bearing of Cooling Tower No.1 – Pump No.D. Immediately this pump was stopped. After stopping the pump fire took place at that place which was extinguished by CO2 Fire Extinguisher. While carbon dioxide cylinder was opened for extinguishing the fire, it was observed that there was continuous spark from the surface of the horn (Outlet Black Nozzle) of the Carbon Dioxide Cylinder. This was informed to Fire Section. As per Fire Section, this is due to static current".

A safety bulletin prepared by the safety advisory group mentions the following:
"The Safety Advisory Group, SAG, has been informed of several fatal accidents caused by explosions which occurred while using CO2 during inerting equipment and storage tanks that had previously contained flammable materials. In most cases the flammable
materials were liquids or gases but dust explosions may also be triggered by the same cause.
Examples of fatalities:
• Two navy firemen were killed in an explosion while attempting to inert an 18,9 m3 Jet Fuel tank by use of portable CO2 fire extinguisher.
• Four persons were killed in an explosion on board the tanker Alva Cape while inerting naphtha tanks using CO2 cylinders.
• Twenty nine persons were killed in an explosion while witnessing the demonstration of a newly installed CO2 fire-extinguishing system for a partially filled 5000 m3 jet fuel tank, in Bitburg, Germany.
Subsequent investigations have shown that, during the inerting process, static charges of several tens of kV were generated and accumulated at the end of the piping connected to the tank. Voltage of this nature is sufficient to produce sparks which act as points of ignition for the flammable mixtures. When liquid CO2 expands up to absolute pressures of less than approximately 5 bars, the result is the formation of small particles of solid CO2 (dry ice). As the two-phase solid/gas flows through the piping, static charges are produced by the particles rubbing against other particles, between themselves, piping and equipment. Subsequently, these charges accumulate in the zones that are not earthed/grounded at the end of the pipelines, most often in valves and nozzles. The sizes of these fields, as determined by experiments, can reach values of between 50 and 180 kV/m. Similarly, static electricity can be generated by the dry ice particles after they leave the discharge nozzle.
The pressure and impurities in the CO2, equipment materials in transfer line hoses, etc. all influence the generation of static electricity".
Read the safety bulletin in this link.