St 1 dust - Dangerous or not?

Hope all of you readers are doing well. It is the time of Diwali here in India and everywhere one can see people in the festival mood. Be careful and safe & have a wonderful Diwali.

Today I am trying to clear another misconception which I have come across while talking about St 1 dust. Many of our clients have told me that we are handling St 1 dust which is also the so-called "WEAK EXPLOSION" hence the risk is low. 

I always give them the example of Imperial Sugar Explosion in 2008 where a St 1 Sugar dust lead to 14 fatalities or Didion Milling Co. explosion in 2017 where St 1 grain dust was to blame for 5 fatalities, a dozen injuries and $1.8 million fine.

Combustible dusts are ranked into one of four classes i.e. St 0, St 1, St 2 and St 3. The level of explosion violence increases as we go from St 0 to St 3. The only dust that has zero risks of the explosion is a St 0 dust. However, all other dusts have an explosion hazard including the St 1 dust which can compromise containment on a piece of equipment, or blow out the walls of a building. Grains, Sugar, Fluor, etc. all fall in St class 1 dust.

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.

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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