Factors influencing dust explosions

Influence of particle size, humidity, inflammable gas, oxygen concentration and turbulence on dust explosions

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Section summary
1. Introduction
2. Particle size
3. Humidity
4. Oxygen concentration
5. Flammable gas
6. Turbulence

Bulk Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust

1. Introduction

Dust explosions parameters like MIE, MIT, Pmax, Kst...etc... are measured in laboratory in standard conditions. However, a certain number of factors influencing dust explosions and making real cases different from the theoretical values are to be taken into consideration.

2. Particle size

Powder dust cloud explosion risks increase when the size of the particles reduce. It is indeed easier to put in suspension small particles to create a cloud in the right dust / gas proportions, but it also makes the explosion more powerful as reducing the size of the dust, for a same quantity, will dramatically increase the specific areas : this will help the powder to burn very fast.

On the other hand, having larger particles may allow to inhibit the explosion. Reports show that, from a particle size of 200 to 500 microns, the explosion becomes unlikely. WARNING : the process operators must be very careful since, if the initial particle size distribution may appear safe, process operations on the powder may create fines, which makes in practice any powder / bulk solids potentially dangerous. That is also why the MIE is measured at a defined particle size, to account for dust generation in the process. These phenomena must be taken into account in risks analysis.

3. Humidity

It looks logical that a higher humidity of the powder will reduce the likelihood of an explosion. Indeed, humid particles will be much more difficult to inflame as the water needs 1st to be evaporated to allow the particle to start burning, requiring much higher energy to trigger an explosion. Humidity can also help in agglomerating particles.

Literature reports that particles with a humidity of more than 30% will most probably not trigger an explosion. This situation is however quite rare and related to very specific applications.

4. Oxygen concentration

Referring to the explosion pentagon, the presence of air, and thus oxygen, is required to trigger the explosion. A lack of oxygen can actually inhibit the explosion risk. It is a strategy quite often applied to avoid dust explosion risks. Of course, the system must be designed to monitor the oxygen content, regulate it, and shut off in case it cannot reach the target value. Having low oxygens concentration is typically done by inerting with nitrogen or carbon dioxide.

5. Flammable gas

In a majority of cases, the dust is dispersed in air. One must however be careful in understanding every process aspects that could generate a flammable gas. Either because the process is such that the gas is not air or a mixture, or because the dust itself is releasing some materials.

The presence of a solvent, a flammable gas, will drastically change the explosion properties and will increase the likelihood of explosion and / or the violence of the explosion.

- Lower MIE for the dust cloud in suspension with air + a flammable gas
- The gas may also be more easily flammable when mixed with dust than only with air

6. Turbulence

The turbulence of the dust cloud is also a factor known to influence on the effects of an explosion. It is very difficult to modelize but it should be noted that the pressure increase Kst will be higher as the turbulence is higher, for example with a high speed mill.


Source
# [Laurent] Securite des procedes chimiques, Andre Laurent, Tec et Doc, 2003, page 234-240




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