Today’s episode of the Dust Safety Science podcast is the second part of a two-part series on analyzing the likelihood of an electrostatic ignition incident after an explosion. In the last episode, Eyal Zadok talked at length about incident investigation and evaluating whether or not electrostatic discharge was the ignition scenario. Today, he discusses five additional steps incident investigators can take to look for signs of electrostatic discharge and applies his investigative strategies to an explosion at a Taiwan plastics plant.
Five Additional Steps for Detecting Electrostatic Discharge
Step One: Analyze Electrostatic Forces in the Equipment
Once he determines which equipment was involved in the event, Eyal analyzes its electrostatic forces to identify the conditions under which electrostatic charges can accumulate. This could happen in the chemicals involved, the plant process, the materials, and equipment parts.
“This is a very preliminary step that lays the foundation for the next steps,” Eyal explains. “From a starting point of view, having the values of electrostatic charge density is essential for understanding the other steps that are needed.”
Step Two: Identify the Potential Types of Electrostatic Discharges
Next, identify the types of electrostatic discharges physically possible within the equipment. This identification can be based on the facility configuration, electrostatic charge density and the electrical potentials that might have accumulated in the equipment.
“If the whole facility is made of plastic materials and the process materials are, from an electrical point of view, insulated materials, spark discharge is excluded,” Eyal says. “On the other hand, if part of the facility is made of plastic and part of it is made of metal and these parts are very close to each other, it could be some kind of brush discharge.”
Step Three: Energetic Analysis of Available Electrostatic Discharge
The next step in the process is an energy analysis of the available electrostatic discharge, which is based on the construction of the facility, the process conditions, and, of course, electrostatic calculations. The aim is to identify points of electrostatic potential failure. How much energy can be released and how does that compare to the materials being looked at?
Step Four: Do an Incident Scenario
After I find the kind of electrostatic discharge that might ignite the material, I prepare a complete scenario of the event together with this initiator – the electrostatic ignition part,” Eyal says. “It’s all based on the findings.”
Step Five: Prepare a Simulation
The final step is to prepare a simulation or a small-scale experiment of the electrostatic event and see if it actually works.
The Taiwan Plastics Plant Explosion Reviewed
On July 7, 2021, an explosion occurred at a polyethylene pellet silo in Taiwan. When asked what considerations should go into evaluating the electrostatic ignition sources, Eyal replied that understanding the physical process of electrostatic ignition in silo filling can make it possible to detect the hazard.
Polypropylene pellets carry a tiny fraction of polypropylene particles, and the minimum ignition of polypropylene dust is about 15 millijoules, making it highly sensitive to explosion. While the silo is filling, pellets are falling down, but the dust remains suspended in the air for awhile.
“These pellets accumulate on the silo bottom, creating a cone shape,” Eyal explains. As time passes, the suspended dust goes down to the material cone. But the next filling causes the suspension of this dust layer. So with the new dust emerging into the silo, the dust concentration in the space above the cone is getting concentrated more and more.”
The pellets all down this cone accumulate electrostatic charge because of the friction and generate electrostatic discharges toward the silo walls, a phenomenon known as cone discharge.
“During the filling of a silo made of concrete or metal, you can see the flashes of the cone discharges, especially at night,” Eyal says.
When the energy released in the cone discharge is greater than the minimum ignition energy of the polypropylene dust, it ignites the suspended dust, leading to an explosion within the silo.
Dr. Martin Glor developed an empirical formula for calculating the energy conditions. This formula is based on the silo diameter and the median of the material particles, enabling investigators to estimate the energetic ability of a discharge to ignite.
“As a summary, I can say that in electrostatic risk analysis for silo filling, we need to have a laboratory test of the dust concentration and the raw material that’s coming, as well as the calculation of the dust concentration and, of course, the calculation of the energy of the anticipated cone discharge,” Eyal explains. “From the findings of this analysis, we find out about the probability for the electrostatic ignition.”
Someone commented on the Dust Safety Science page for this incident, questioning how the issue could have been electrostatic discharge if all of the silos are grounded. According to Eyal, this doesn’t matter- the charge is still being built up along the cone and in the materials.
Conclusion
Eyal emphasised that when you understand the phenomena that occurred within the silo prior to the explosion, the cause becomes more apparent. The key is to understand the materials and the processes involved and use logical steps to reasonably anticipate what type of incident can result.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Eyal Zadok directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/eyal-zadok-975717174/
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Incidents
Static Electricity May Have Caused Explosion at Taiwan Plastics Plant
Previous Episodes
DSS148: Analyzing Electrostatic Ignition Likelihood After An Explosion Incident with Eyal Zadok- Part 1
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