Very much like dust collection systems, silos belong to the most dust explosion prone environments. How do explosive atmospheres form in silos? Are dust detectors fitted in the silo a worthwhile solution for explosion prevention? Read the following article to find out our answers to these questions.

What causes a dust explosion hazard in a silo?

Explosive atmosphere

As Mariusz Balicki, who is an explosion safety expert at WOLFF GROUP, explains, considerable dust is incredibly common to be found in silos because they are enclosed structures. The operation of a tower silo involves very frequent filling or unloading that means continually stirring up the loose bulk material inside the silo. These dust particles mix with the air – thus also with oxygen – that fills the remaining volume of the silo, which very often contributes to explosive atmospheres being formed inside the structure. Once such air-dust mixture has been created, it can take a long time for its properties to fall below the Lower Explosive Limit. This is because dust, depending on the particles’ characteristics (shape, weight, size), can remain suspended in the air for up to several hours. This also means it is much easier to have an explosive atmosphere concurrently with an ignition source.

It is worth noting at this point that the greatest danger usually arises when there is no material in the silo but a dust level is still high. This is a highly explosive atmosphere potentially having devastating consequences. As it is unfortunately the case with factory workers, they often mistakenly assume that an empty silo is safe because it contains no product. It has not been uncommon for unaware workers to cause explosions using fire-hazardous tools during, for example, renovation work.

It should be also noted that dusting during silo loading and unloading is not limited to the inside of the silo. Bucket elevators, belt conveyors or other handling equipment in the immediate vicinity of a silo can also be a source of danger.

Ignition source

Special attention should be paid to the risk of ignition of an explosive atmosphere in a silo or its adjacent systems caused by:

• Hot surfaces resulting from friction caused by damaged mechanisms,
• Glowing particles fed into the silo along with the transported material,
• Mechanical sparks generated by the impact of metal parts found in the transported material,
• Sparks from welding and other hot works conducted in the immediate vicinity,
• Incorrect or damaged electrical equipment (e.g. motors or luminaires),
• Metal grinding or cutting during repair works,
• Static electricity discharge,
• Microbial activity.

Is silo dust monitoring an effective solution for explosion prevention?

Now, you know how and when an explosive atmosphere forms inside a silo. At the same time, we are often asked by the industrial sector about dust level monitors. Can they be used to detect elevated dust level in the silo and, once they do, are we able to feed a neutral gas that will prevent the formation of an explosive atmosphere?

Theoretically, such a solution could work as a process consisting of the two phases:

1. A monitoring device detects the moment when a dust concentration level approaches the lower or upper explosion limit.
2. This concentration level produces a signal that activates the release of inert gas volume into the silo, sufficient to neutralise explosive atmosphere formation.

But will such a solution work in practice?

We asked this question to Bartosz Wolff, President of the WOLFF GROUP and a long-time practitioner in the field of explosion safety: Let us emphasise it right away that such a solution will not work. Traditional methods of measuring dust level inside a silo will not be effective, as dust level detectors are able to measure only local dust concentrations, either at a particular point in a duct or, as far as the ambient air dust level monitors are concerned, in specific areas of production floors or rooms where dust level must be monitored for safety reasons. In contrast, containers such as silos with volumes reaching tens or even thousands of cubic metres have dust concentrations that are never homogeneous. For instance, looking again at the picture of the silo above during the filling process, the swirl of dust is travelling around the entire interior, with its concentration levels constantly changing. Are we able to answer the question of where to place the probes so that their measurement is reliable? Unfortunately, no.

So, where will dust level monitors work?

There are several types of dust level monitors we can use in industrial applications. The first type comprises those with probes mounted inside ducts, e.g. in dust collection systems on the clean side of filters.

They monitor dust levels in the air that, theoretically, is to be dust-free and, most often, it is delivered back to a production floor. Once dust is detected, it means that the filter bag is defective. Dust collection system failures present a two-fold safety risk. Firstly, workers are exposed to air with elevated particulate matter content which is a health hazard. Secondly, if a system fails to filter dust that is explosive, it allows an explosive atmosphere to form in the ducts that are close to workplaces. A blast in such a duct can bring about dire consequences. For this reason dust monitors are used on filter’s clean side in order to be able to diagnose damage to filter bags and replace them immediately.

The other type of dust monitors is used for monitoring larger areas than ducts and allows the measurement of local dust levels in rooms, production floors or near sensitive process areas that may stir up excessive dust.

While this article mainly concerns the risk of dust explosion in a silo, it is worth returning to the point where we pointed out that an explosive atmosphere can form not only inside, but also outside the silo during loading and unloading. This is the case when, for example, loose bulk product falls from the belt feeder into the silo hopper, or it is discharged onto a belt feeder or other material handling equipment. Monitoring dust levels in these process areas are certainly required, as numerous explosions in industrial environment have been reported due to increased dust levels around this type of equipment combined with sources of ignition from, for example, hot feeder rollers or hot surfaces caused by the friction of faulty components of conveying equipment. The tragic explosion at the Imperial Sugar factory was one of such occurrences.