Chemical explosions from gasoline (petrol), gases like methane or propane, or dust conflagrations are subsonic, localised, over-pressure events that move things away from the center of the explosion, often accompanied by flame.
People are generally aware when they are using conventional fuels that they possess inherent dangers and take more care. We are more interested in the less intuitive problems, so we’re going to focus on the not-so-obvious one from this group: combustible dust. This is important because the presence of combustible dust can form a dust explosion hazard.
Consider this scenario: A hobbyist, in his basement workshop, is using a belt sander to put a finish on some rough plywood. He is generating so much dust that you can barely see the far end of his small workshop. The proportion of airborne sawdust is so high that even the sparks from the belt sander’s motor brushes could ignite the whole atmosphere of the shop. After the ignition occurs, a dust fire or dust explosion will follow.
His risk right now is that he could create a dust explosion in this atmosphere that could injure or kill him, and potentially burn down the entire home. He remains blissfully unaware of the dust explosion hazard because he has done this many times before without consequence. If someone had given him a demonstration of a Hartmann Tube at some point in his life, he certainly wouldn’t be risking that life right now.
The Hartmann Tube: Combustible Dust Testing
But what exactly is a Hartmann Tube? In brief, it is a device to test the explosive properties of industrial dusts, in various air-fuel ratio mixtures.
Typically, a Hartmann Tube will have a sustained ignition source about one-third of the way from the bottom. This is generally a high-voltage arc for consistency, creating a continuous spark. Alternatively, a glowing coil is used as an ignition source.
Beneath that is the sample tray where the particulate matter is placed. This powder sample can be pure, or it can be a mix of dusts, taken directly from the industrial setting.
Below that is a mechanism that will launch the powder sample into the glass tube forming clouds at differing densities, depending on the chosen setup method. This mechanism typically employs pressurized air to disperse the fuel
Limits of the tests
It is worthy to note that the test is limited in it’s ability to mimic a real explosion in an industrial setting. This is because the powder concentration is ignited in an artificial manner. In the real world, a continuous spark is rarely generated.
Anatomy of the Hartmann Tube
Summarizing, the Hartmann Tube consists of the following elements:
- A spark gap (spark ignition) or incandescent coil with appropriate ignition energy.
- Pressurized air.
- The dispersion nozzle.
- A dust sample.
- The cylindrical tube.
- Optionally: a cover for the glass tube.
Let’s take a look at a Hartmann tube in action. Because the reaction is so substantive and quick, the slow motion video below helps us understand what is happening within glass tube of the apparatus.
This video of a modest portable rig can test smaller batches using samples from 0.3 grams up to 3 grams. If a material is tested three times in a row and does not ignite, it is considered non-explosive for those test conditions.
From the video, it becomes clear that a dust explosion is very likely to start dust fires. This is because the powder sample forms a dust cloud when airborne, which rapidly ignites. The dust fire rapidly spreads and causes a dust explosion.
What Is the Hartmann Tube’s Function in Explosion Prevention?
As you have seen in the videos, The Hartmann Tube is used for evaluating the explosibility of dust particles in industrial settings. It is a testing device that helps determine whether a particular dust can pose an explosion hazard when it is dispersed in the air and exposed to an ignition source.
In other words, The Hartmann Tube safely creates small dust explosions inside the tube to see if the material is a dust explosion hazard.
Testing Procedure and Evaluation
During the screening test, various concentrations of the powder sample are used. If flame propagation is observed within the Hartmann Tube, one may assume that the substance is explosible in dust form and can thus create a dust explosion.
This information is critical in industrial settings where combustible dust is produced or handled, such as in woodworking facilities, grain processing plants, metal processing sites, and chemical manufacturing.
Nowadays, a modified Hartmann apparatus is used. The original was invented in the mid-twenties, so the modified Hartmann apparatus features some much-needed design upgrades. This makes a modern screening test easier to perform and more accurate.
The final test results help us figure out where a dust explosion hazard might be present in the workplace.
Screening test and ATEX regulations
The information concluded in the screening test is crucial for correctly categorizing the hazardous area into ATEX zones – zone 20, 21, or 22 – depending on how risky the area is. These ATEX dust zones are mostly found in the following industries:
- Grain processing.
- Food manufacturing.
- Chemical production.
- Recycling facilities.
Fuels – which dusts are a dust explosion hazard?
What can be fuel? Anything based on cellulose, for example, should burn well, including wood or plant powders such as wheat (as flour). Especially in the right concentration. Sugar’s composition makes it a perfect fuel, being a pure carbohydrate with its own built-in oxygen (C12H22O11) supply.
Refining sugar requires that it be processed from large crude grains of “raw” sugar down to smaller and smaller fractions. Ultimately, it reaches the silky-smooth powdered state called “icing sugar” which is as fine as talcum powder.
So does that mean that sugar can cause a dust explosion? Yes, in the next example, you will see how sugar dust can form a very serious dust explosion hazard.
Possible Consequences: Sugar Refinery Dust Explosion
Dust can accumulate in all sorts of places, especially alongside machines that process the product. A sudden breeze from opening an outside door could produce an explosive cloud in just seconds.
This refinery in Georgia was completely destroyed because of some trivial ignition source, and precisely the right (wrong) dust conditions. The devastating dust explosion killed 14 people and left 36 injured.
When a wall of flame and over-pressure air is racing towards you at nearly the speed of sound, you will have no time to react. Your survival rate can be 100% if you simply never allow the dust explosion hazard to arise.
Surprisingly, to most people, solids that are essentially noninflammable in a solid state form a dust explosion hazard when dispersed as a fine powder suspended in the air when exposed to an adequate ignition source.
This video (you can skip the preamble and start at 1:58) shows a simple bench test of powdered metals in a flame to demonstrate flammability. You’ll see zinc, aluminum, iron, magnesium, copper, manganese, bismuth, silicon, chromium, cobalt, lead, and ferrocerium (aka auermetall).
In the “pile test” you can see that chunky material is resistant, but fine materials ignite quite easily. It becomes apparent that size really does matter in this instance. The smallest particles have a huge surface area compared to their mass. They can gain or lose heat rapidly, which is precisely the same reason that larger warm-blooded animals have a distinct survival advantage over smaller animals.
A tiny particle can use up its entire fuel source all at once, creating a hot pocket of expanding gas/plasma that can trigger the particles immediately surrounding it to do the same. This chain reaction is why the conflagration propagates so rapidly from a single spark.
The greatest danger is when the suspended dust is so dense that vision can begin to be obscured over as little as one meter of space. At that point, you are quite literally playing with the potential of a dust fire.
For industrial purposes it is best to control dust at the source, collecting it as it is produced. Ventilation to the exterior is a second option but this technique introduces even more variables that are more complex to control. Eliminating the creation of a dust explosion hazard is always the safest option.
Testing dust explosibility with The Hartmann Tube can help you identify which materials in your process represent a threat to safety. There are various testing bodies that provide this dust explosion hazard screening test.
While a dust explosion is unlikely to happen in most industrial sites, it is important to minimize the chances by adhering to local regulations. People must become more aware of on site hazards.
A dust explosion hazard is often overlooked, but the effects of a dust explosion are often disastrous. You can ensure safety for your employees, and infrastructure by identifying threats before they hurt you.