Partial Inerting - A Cost Efficient weapon against Dust Explosion

Hey everybody, hope you are enjoying my blog and finding the information useful in your field of work. Today we are talking about a topic which is rather less explored and understood in many industries as of now i.e. "Partial Inerting".

Let's start first with understanding Inerting. Inerting is defined as the replacement of "oxidant" in a system by a non-reactive, non-flammable gas, to make the atmosphere within the system unable to propagate flame. Inerting may be achieved by using a non-flammable gas, which will neither react with the fuel nor with the oxidant. There are many gases used for inerting such as Nitrogen, Carbon Dioxide, Flue Gases, Noble Gases (Argon, Helium), etc.

P.S: Some material may react with these inert gases under some conditions. Hence, correct selection of gases is critical.

Now let's discuss the concept of PARTIAL INERTING to prevent dust fire and explosion. Partial Inerting means dilution of oxidising atmosphere with an inert gas but not up to the extent of LOC i.e. oxidant concentration lies between LOC and atmospheric concentrations. In other words, for partial inerting, the oxidising atmosphere (most often air) in which the explosible dust is dispersed is mixed with a fraction of inert gas (e.g. nitrogen) considerably smaller than that required for complete inerting (i.e. below LOC). 

Explosion Severity Test and St Class : Modified Hartmann vs 20 Litre (St 0H/1H/2H vs St 1/2/3)

Hope you have enjoyed the posts till now. The response has been overwhelming with the number of emails regarding the queries and some regarding the requests for upcoming posts. I would again like to request to all of you that, I am always in search of some good topics related to Process Safety, Dust Explosion, Electrostatic Hazards, Hazardous Area Classification or any other process safety-related concern. Do send in your queries and requests here in comments or on himanshuchichra@gmail.com. Let's continue with the present discussion topic now.

It has recently been told to me by one of the clients’ that their explosion severity test states the results as St 1H in place of St 1/2/3. His question was simply that should I go ahead and provide an explosion vent corresponding to St 1? Now, this is a tricky one.

Dust Explosion - Which Powder should be tested? Can I use the properties from internet for my powder?

"Nothing has happened in my facility and it has been running for 20 years"; "My powder cannot explode, we use this at home commonly"; "Are you still wondering if you have combustible dust?" Just because nothing has happened yet doesn't mean it's not about to.

This week I received an interesting query about which powder should be tested. Interestingly, there was an additional statement attached to the question, "I have searched over the Internet and have found my powder explosive properties. Can I use these properties for my site assessment?"

Some of the powders which can explode include food materials (including sugar, spice, starch, flour, etc.), plastics, wood, paper, rubber, pesticides, pharmaceuticals, chemicals, coal, metals, etc.

According to OSHA, "any combustible material can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, under certain conditions, it can become explosible. Even materials that do not burn in larger pieces, given the proper conditions, can be explosible in dust form."

For liquids and gases, flammability data are well understood & usually established and available at least under atmospheric pressure and normal temperature conditions. However, for powders/ dusts is not available and is dependent on multiple parameters such as particle size, moisture content and also on particle shape/ morphology.

Usually, the properties which shall be tested for each of the unit operations differ depending on their requirement for that unit operation. We will pick this topic in our next blog.

Now let us look at each of these parameters:

• Particle Size: It has to be noted that explosion properties of a powder usually worsen with decrease in particle size. This means that finer the particle size easier it will be to burn and consequences can be worse as well. Powder used in industries is a distribution of smaller and larger particles. Whenever it disperses, the larger particles will fall to the ground because of more weight, however, the fines will remain suspended in the air forming a dust cloud. As per IEC standards, a particle size of less than 63 microns is used for conducting various tests. As per standard thumb rule, a particle size of more than 500 microns is not considered to be capable of propagating dust explosion.

• Moisture Content: First and foremost impact of using water is that it suppresses the dust cloud formation. Also, increased moisture content act as an inert and makes the ignition of powder more difficult and also impacts explosive properties of a powder. As per IEC standards, the powder is dried to moisture content of less than 10% before testing.

• Particle Shape and Morphology: Even with a change in particle shape and morphology from crystalline to amorphous, the material properties change significantly.

Hence the answer to the question is, one should test the finest and driest sample being handled on their site. Don’t depend on the Internet properties as one doesn't know what condition it has been tested for, what was the particle shape, size, etc.

Also, it is imperative to test the powder if a vendor is changed or if the manufacturing process is modified or if physical characteristics of the final material change or if any material has a combination of various particle sizes.

This will give you worst possible characteristics of the powder based on which assessment should be carried out and measures should be provided.

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