17 November

Why I have NOT had my powders tested to determine their Explosion and Fire Properties

There are still facilities handling and processing dusty powders that are operating with no knowledge of the fire, explosion, or thermal stability properties of the materials they process. Why?

One thing is clear. There are a number of well-established, national and international standards applicable to the control of fire and explosion hazards in handling combustible dusts. In the USA, of course, we have NFPA 652 – Standard on Fundamentals of Combustible Dusts [Ref 1]– the catchall standard that supports another handful of industry- and commodity-specific NFPA standards on this topic [Ref 2]. Pretty much all standards and guidelines around the world require that some form of dust explosion/ fire hazard assessment be performed on powder handling facilities. Most people would agree that to properly perform such a hazard assessment (termed a Dust Hazard Analysis (DHA) in NFPA 652) requires some knowledge of fire/ explosion/ thermal stability properties of the powders that are handled. Yet there are still companies that have no such data (and indeed, some that have not even performed their Dust Hazard Analysis.

Why is this?

As process safety consultants and operators of a dust testing laboratory, we find ourselves meeting many companies to talk about explosion hazards at their facilities. Almost always we are in the privileged position of being taken into the confidence of our clients, with most of them wanting to ‘do the right thing’ to ensure the protection of their people, the community, and their businesses. Whatever we find at their facility, we will always do our utmost to help them; that’s our job. This situation though, gives us a unique insight into the thinking and motivation of facility operators/ owners across the whole range of industries; from the 5-man, back-room outfit, hand-mixing a few powders with no dedicated safety officer, through to multinational corporations with multiple ‘directives’, or ‘guidance’ from head office and a need to reconcile these with national standards. That thinking varies a lot. This is what we hear:

What we hear FROM YOU

Let’s take a look at what you tell us about why you have NOT obtaining dust test data! We’ve grouped the comments we hear into 5 themes:

  1. Lack of awareness or understanding 
  • “We didn’t realize that the dusts we produce could explode and cause risk to life and disruption.
  • We keep the dust inside our equipment and hadn’t appreciated that secondary dust explosions outside the process equipment could still occur and it is those that often cause most damage.
  • We know chemicals can produce dust explosions, but we are only handling food-stuffs.”

When some of our senior process safety consultants began their careers, 20 or even 30 years ago, lack of awareness of dust explosion was much more apparent than it is today. There was even surprise that dust explosions could destroy whole facilities and in the 1990’s, arguments were raging about what dust explosion tests (if any) should be performed – and even how dust explosion properties should be determined. The much-lauded Kst value, for example, obtained in the 20-liter sphere test apparatus, was not always accepted in the USA; historical ‘Hartmann Bomb’ pressure data was sometimes preferred.

Now, North American industry is much more aware, and rarely do we see disbelief in our clients that a dust explosion could lead to the destruction of an entire facility, and that dust combustibility data has an important role to play in preventing this. There is never-the-less education still needed. The work of the United States Chemical Safety Board, AIChE CCPS, NFPA and others – and consultants such as ourselves, have all helped increase awareness. But for the doubters that still remain it’s worth noting that the vast majority of powders handled and processed by industry are explosible, and that although it is sometimes the exotic chemicals that cause the biggest dust explosions in practice, it is still the food industry that has more dust explosions than any other industrial sector [Ref 2].

  1. It’s the supplier’s responsibility

“We buy our powder from ‘xyz’ and we’ve asked them if their powder is an explosion risk (or) asked them for fire and explosion data, but:

 They sent us their safety data sheet which said ‘flash point – not applicable’ or ‘flammable range – no data available’

  • They’ve not sent any information
  • They said we need to obtain test data ourselves”

NFPA 652 is unambiguous and totally explicit on where responsibility lies: The responsibility for determining whether materials are combustible or explosible lies firmly with owner/ operator of the facility.

What’s more, where dusts are determined to be combustible or explosible, additional testing shall be performed, as necessary, to obtain the data necessary to support the DHA. Such data should also be obtained by the owner/ operator of the facility.

Many companies are unfortunately still providing Safety Data Sheets that were designed for flammable gases or liquids, so often the usual Section 9, ‘Physical and Chemical properties’ section is sent out containing either meaningless or at worst misleading statements. How does ‘flammable range – no data available’ actually help us? We’ve seen this statement on many powder SDS’s where the powder is a known dust explosion risk!

Of course, it’s a commercial matter between client and supplier as to who should pay for this data to be obtained. From our experience it is almost always the client that pays and this DOES often make most sense. The problem is that data needs to be obtained on powder that is representative of what is to be found on plant. Particle size will vary in different pieces of process plant, for example, and powder moisture content will likely vary too. Compare the powder sampled from a hopper to the same material found in a bag house to understand this point. Both particle size and moisture content can strongly affect dust explosion properties. explosibility properties and sampling from plant is usually more reliable than historical or ‘stock’ data supplied by a supplier on their powder. It is also not going to be subject to SDS interpretation uncertainty!

  1. Prohibitive cost
  • “Lab test work is expensive and confusing
  • We are handling tens or hundreds of powders and the cost of testing them all is prohibitive”

The cost of lab testing on one sample of your powder can vary from a couple of hundred to a couple of thousand dollars. The first step is really to establish if your powder is explosible and it is permitted here to use historical facility data or published data that are deemed to be representative of current materials and process conditions. In other words, if you can find relevant data and it says your powder is explosible, then work with that; it costs nothing! If you can’t find the data, then a screening (Go/No-Go) test is going to cost you couple of hundred dollars. Well worth the price to establish if you have a dust explosion risk at all.

If you find you have an explosible powder, then published or historical data MAY provide enough information to undertake your DHA. In practice, though, unless you have data that is truly representative of the powders you are processing today, then you may end up having to test again or assume ‘worst case’ for purposes of your DHA. Assuming worst case could mean expensive over-design of explosion protection or prevention measures and costly changes to plant. In that eventuality, having us test your materials to establish such things as sensitivity to ignition and explosion severity will be a much cheaper option. Spending a couple of thousand dollars on testing could save many tens of thousands of dollars on change to plant. This really is where working closely with your chosen test lab and process safety consultant comes in. A good understanding of plant and operations can provide good guidance on the most appropriate tests to have undertaken and will likely end up saving you a lot of money through careful selection of tests.

Finally, we often come across companies handling 10’s or even hundreds of different powders. You can test everything, of course, as a belt and suspenders solution; or you can have us review your powders, making use of our extensive testing experience and our knowledge of your processes to come up with a short list of powders that are representative of groups or classes of your powders. These can then be fully tested and evaluated to obtain ‘class’ data. This approach usually proves to be highly cost effective to most of our clients that are handling many materials. [Ref 3]

  1. We’ve been operating safely for years/ too busy/ too many other things to focus on

“Our facility has been operating safely for 15 years and we’ve never had anything like a dust explosion

  • That shows it can’t happen here
  • We are clearly doing things right
  • We simply have too much work at the moment but we will get round to it”

NFPA make the following point:

The absence of previous incidents shall not be used as the basis for deeming a particulate to not be combustible or explosible.

Most companies that suffer a dust explosion have never had one before!

Maybe you can find a sympathetic ear as to reasons why you have not yet obtained your test data and completed your DHA’s. There has been a pandemic on and some facilities have reduced staff or shut down or been hit by massive commercial pressures as a result… But beware that the excuse is starting to look a little feeble in our humble opinion! Note that under NFPA 652, the deadline for owners/ operators of facilities to complete their DHA’s was September 2020; over a year ago!

  1. It’s not clear what or where to sample to obtain representative samples for testing
  • “Our powder changes in particle size and moisture content through plant

Not knowing how to obtain a representative powder sample is not a reason to avoid testing [Ref 3]. In practice we would advise you to call our specialists for a discussion on this point. Testing representative samples is a key requirement in ensuring the results can be applied to explosion prevention and protection design on plant, but NFPA 652 allows for sample preparation that includes sieving and drying and testing the finest, driest samples to help deal with some of the uncertainties here. This move towards a worst case can result in over-design of explosion protection, for example, but the effect may be marginal. Let us discuss this point with you.

Wrap-up

In our view any DHA worth its salt will need to take into account data on the explosibility, thermal stability, and electrostatic characteristics of the powders involved. NFPA 652 is now a well-established standard in the USA and from what we can see, lots of companies have obtained some test data on their materials (and have completed their DHA’s); lack of awareness of dust explosion risk is not a valid reason for inaction, and the deadlines in NFPA 652 are now well past.  Indeed, some companies are even at the stage of revalidating the DHA’s that they had performed some five years ago, as required by NFPA 652. It is clear that responsibility for obtaining test data lies with the facility owner/ operator and any questions about how to take representative samples or which tests to perform or how to deal with multipurpose plant ARE readily answered through discussion with your chosen test lab and DHA consultant.

So what are you waiting for…

References:

Ref 1: NFPA 652 NFPA 652 – Standard on Fundamentals of Combustible Dusts, 2016 edition

Ref 2: Stonehouse Resource:  https://stonehousesafety.com/combustible-dust-10-nfpa-codes-and-standards/

Ref 3: Stonehouse Resource:  https://stonehousesafety.com/combustible-dust-sampling-for-dust-combustibility-testing/

 

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