Combustible Nano-Dust: Smaller Particle Sizes Lead to Faster, Stronger Explosions

Credit for launching the nanotechnology revolution frequently goes to Richard Feynman’s 1959 talk to the American Physical Society, “There’s Plenty of Room at the Bottom.” Even if you disagree with this credit, I hope you can agree that Feynman’s enthusiasm for “small science” certainly fed the imaginations of scientists, philosophers, and deep thinkers about a future built upon nanotechnologies.

Others, maybe Eric Drexler, or was it Omni Magazine, forecast a darker future for nanotechnology with predictions of self-replicating nanobots gone rogue, consuming nearly all resources, and leaving only a dark, dead, world composed of nothing but useless nanobots forming endless piles of “grey goo,”

More than a half century later, the nano engines of creation are quietly humming away, the alarms over grey goo have gone nearly silent, and many of today’s industries move steadily toward producing smaller particulate, even if most are still quite a ways from true nanometer particle size.

Particle size is the dominant physical parameter that affects explosion severity and ease of ignition for combustible dusts. And particulate size is trending down in many industries. This is true for pigment, toners, electronics, cosmetics, pharmaceuticals, specialty chemicals, additive manufacturing, food, and many more. As with toner, the reason may be improved quality and new transfer technologies. Other advantages may be related to improving mixing efficiencies and reducing production times.

The deflagration index value, or K_St, is the most common deflagration characteristic of combustible dusts that people name in the workplace, that is, if they can name any of the deflagration characteristics of their combustible dusts. Many people are not aware that a single material has different K_St values (i.e. burning rates), dependent on the mean particle size and the particle size distribution, among other things. Mean particle size is so much easier to discuss than particle size distribution. It’s a single number compared to a statistic report, so it gets used most often when paired with a K_St values. A K_St value without a particle size reference, mean, or distribution, loses meaning over time if your industry is move toward Feynman’s “Room at the Bottom” or as some like to call it, the “nano-verse”.

Other useful explosion characteristic values include minimum ignition energy (MIE), minimum explosible concentration (MEC), and layer ignition temperature (LIT). These characteristics are usually not as well-known as K_St. That’s too bad as these value may also be impacted by reduced particle size, especially when that particle size falls into the nano-verse.

Since some particle size reductions may go unnoticed, especially if your management of change (MOC) program is blind to particle size considerations. And it is not just for future size reductions into the nano-verse. Consider the change from granulated sugar that is roughly 400 microns with a K_St value around 60 bam/s to powdered sugar with an approximate mean size of 50 microns and a K_St value likely north of 160 bm/s.

Is your particulate’s K_St value roughly the same as three or five years ago? There’s a good chance that it is, unless it is now dried to a different moisture content or reduced in size to a different size range or different shape (by you or your supplier), or there’s a new additive in the mix. The unknown factor, as it so often turns out to be, is “What is significant when it comes to these potential changes?” That is a difficult question without site-specific testing data.

So, while there may be advantages to working with a smaller particle size that is specific to each industrial application, there is also an increased combustible dust risk associated with smaller particle sizes that needs to be recognized and managed. And in most cases what we are really looking at is particle size distribution, but we should not overlook particle shape, chemistry, or moisture content. These factors also affect how easily a powder may be entrained in a passing air stream, how long a powder takes to settle, where it may disperse, and whether it makes you sneeze. Granted, that’s a bit off point.

In another example, printer toner particulate sizes averaged 30 to 50 microns in 2008. To improve the appearance of type, manufacturers have worked to produce finer and finer toner particulate. Today, toner manufacturers are pushing the size boundaries to, and even below, 10 microns. Ten years ago, K_St values for many toners were in the Class II range: 200 to 300 bm/sec. Today’s smaller toner particulates have K_St values well over 300 bm/sec. In some cases, the protection strategies employed 10 years ago are no longer adequate. Explosion vent sizes may be too small or suppression or isolation system reaction times too slow.

To my limited knowledge, toner production has yet to venture into the nano-verse. Or has it? Ask the R&D chemists and process engineers in your facility, “Where are we trending in particulate size?” And if they see reduced particle sizes, it may be time to consider improved process safety systems. And if you’re a particulate product supplier, wouldn’t it be nice to give your customers a heads-up when particle size is headed down? Be the unsung hero.

In the sugar and toner examples, we have a dramatic and potentially dangerous increase in the K_St value, but it came with, dare I say, a significant size reduction, from 400 micron down to 50, and roughly 50 microns down to 15. Is that a big difference? The sugar had an 8 to 1 reduction and stayed an St-1 dust. The toner leap into a higher category, from St-2 into St-3 with just a 3 to 1 reduction.

Forget “St” categories, a remnant of plotted curves from an old calculation technique. What K_St value was used in your protection system design? What about the pressure ratings for the enclosure? These are design factors that the NFPA committee includes when stating “establish a program and implement a process to manage the retention of documentation, including process and technology information (design drawings, design codes, and standards used for...and safety systems operation.) at NFPA 652, Sections 8.13.1and A 8.13.1).

Everybody that asks, asks to know the K_St value. Too few also ask for the MIE and the MEC. But it’s not just the K_St and design pressures values we need to consider for our overall protection strategy. The minimum ignition energy (MIE) and minimum explosible concentration (MEC) offer an equally valuable consideration in risk management decision making.

In risk analysis we look at the consequences of an event. How bad could it be? We also look for the likelihood of the event. Could it happen here? Think of the K_St value and the Pmax as your consequences and the MIE and MEC as your likelihood.

The decrease in particle size reduces the required energy to initiate a deflagration. It is easier to meet the conditions necessary for a deflagration or explosion by decreasing these other characteristic value of your dust. These changes lead to a greater K_St value. A smaller particulate also creates a faster and stronger explosion since this greatly increases the maximum rate of pressure rise (dP/dt) max. It may also result in a more powerful pressure wave -- all of which makes the material more hazardous. Table 1 shows a chart the indicates the effect on these characteristic values when particle size or moisture decreases.

Particle size may also “unexpectedly” or inadvertently be decreased by process changes or improvements, some even as inconspicuous as replacing mill components or suppliers. For these reasons it is important to understand the impact that smaller sizes will have on your particulate. A summary of the recommended testing campaign and the effects of particle size on the results are provided in Table 2.

How good is your Bonding and Grounding Program (BGP)? Ever see a wire pigtail dangling from equipment, connected to nothing. Often the answer to the program question is something like “We have grounded equipment, but there is no program.” Fair enough for the diligent maintenance personnel that always reconnect bonding wires. But for those less diligent, or hurried, distracted, unaware, or untrained, a programmatic approach that includes training, inclusion of a re-bonding step in work orders and procedures, and inspection component is another tool for preventing unwanted ignitions. How about the building ground; is it effective? Consider checking it every 10 years.

I have never seen a nanoparticle, not even a pile of them shoved together to create a visible mass. But even before Feynman’s time, small particles had become a feature in science fiction stories. Remember the bacteria that save the Earth from a Martian invasion in H. G. Well’s War of the Worlds? Unseen, unsung heroes.

Bonding wires and grounding connections get disconnected. Equipment can become isolated. Static happens. Pigtails get overlooked.

Inspect for it. Find it. Be an unseen, unsung hero and reconnect it.

Timothy Cullina is senior consulting engineer, Fauske & Associates LLC. For more information regarding hazardous or combustible dust, dust hazard analysis (DHA) testing, or other process safety concerns, contact Fauske & Associates LLC at [email protected] or 630-323-8750, or visit www.fauske.com.

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