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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Shortcomings of Explosion Venting Design

Hey guys, let me thank you all for making this blog a success and for sharing your thoughts. I hope you are gaining some useful insights for the queries which arise in your mind related to process safety. I can really add more value if, you drop in other queries and I probably can select some of them for the upcoming blog posts.

Now let’s discuss Dust Explosion Venting, which I guess many of you out there struggle with. 

For each dust, maximum overpressure and rate of pressure rise can be measured in an explosion severity test carried out using a 20-Litre Sphere, where a controlled explosion is carried out and the resultant pressure is measured in order to determine the (dP/dt)max and Pmax. This (dP/dt)max is then used to find Kst value utilising cubic law. 

Any vendor who is designing explosion vents will ask for at least Kst value of the powder handled, size of equipment and equipment drawing.

Next question which people ask is "What to do if we handle multiple powders in a single equipment?"

The answer is simple, understand the worst Kst dust being handled in your equipment and design your system according to that. 
Since Minimum Explosible Concentration (MEC or what is called as LEL of dust) is usually as low as 10-30 g/m3 as well for powders. So even if you are only expecting a small amount of a dust inside the equipment, its wise to consider the worst case Kst of the powder. 

Then the question comes “Is my system safe now I have provided explosion venting?”

The answer still depends on whether you have isolated the other systems connected to the equipment or not. I know this sounds alien to many of you, but its very simple to understand. Any pressure will be diverted to the easiest path with least resistance possible. 
On one side we provide explosion vent to the equipment which will burst open at a certain pressure (or a small resistance), however, the connected system is the easiest path for the pressure wave to travel (due to negligible resistance). So the answer is your system is safe only if you prevent the propagation of blast wave to the connected system by utilisation of EXPLOSION ISOLATION (can be active or passive, will be discussing on these in future) designed adequately to hold. 

The next question which arises here is that “Is my plant safe now since I have provided explosion vent and isolated other systems as well?”

The answer is dependent on where the explosion is being relieved. The relieved blast wave with flames or flameless (depends on the type of explosion vent used) has to go to a safe location such that it doesn’t hurt people and plant. I have seen plants, where explosion vent is facing a wall or two explosion vents are facing each other or an explosion vent is reliving on plant’s internal busy pathway. This can escalate the event and might lead to damage to people, equipment and property.

So there is the recipe for explosion protection using explosion venting. The ingredients involve Powder Kst, adequately sized explosion vent (designed as per Kst) relieving to a safe location, Explosion Isolation device to prevent propagation.

Hope this helps you guys in understanding the concept of explosion venting.

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