FAQ | Dust Explosion Hazards
Since ASTM explosivity testing is done in a closed system or bomb on a static cloud of dust, do these results apply for a non-static concentration of dust that is in high velocity motion in air, such as in a pneumatic conveying system?
A good question, but best begin by understanding the actual lab test conditions and comparing them to plant conditions.
The same test equipment design – used to measure maximum dust explosion pressure (Pmax) and Kst value – is the 20-liter sphere (see Stonehouse Explainer article on Kst here). It is essentially a pressure vessel in which we create a dust cloud, ignite it, and measure pressure rise over time. The dust cloud is created by first evacuating the pressure vessel, then opening a full port valve to introduce the dust sample. A timer sequence then activates a pair of chemical ignitors (detonators) according to a set delay time. At time of ignition, the dust cloud has a degree of turbulence. Actually, most dust clouds will be somewhat turbulent – otherwise there would not be a dust cloud!
When the original research on dust explosion test methods was done in Switzerland and Germany, the delay time and turbulence level in the 20 litre sphere were a focus of attention because turbulence does increase explosion rates of pressure rise – and Kst value – and the maximum explosion pressure, Pmax. In practice, the actual test turbulence level is a little arbitrary – but levels were deemed at the time to be reasonably representative of process plant conditions (e.g. turbulence in a silo during pneumatic filling or perhaps in a reverse jet bag house). Tests of explosions in 1m3 vessels and eventually in full sized process plant showed this to be reasonable. Of course, the observant of you will appreciate that in truth, although turbulence level is a little bit arbitrary in the test in as much as it is the same for ALL POWDERS tested in the test equipment – and therefor Kst value is a very useful comparator to use, showing which powders are more strongly explosible than others. There has been academic work that has been critical of this (for example, ref. 1) but we would observe that investigations of real industrial incidents – explosions – have not yet, at any rate, shown the standard explosion relief design methods to be unsound.
For completeness, it is probably worth pointing out that at high levels of turbulence, the tried and tested formulae used for explosion vent design may not always work. Indeed, in pressure piling situations (e.g. explosions in long pipes), even relatively weak dust deflagrations can transition to detonations – with much more devasting potential than the simple explosion protection design can accommodate.
How do you handle product testing for a facility when there are over 150 different raw material products that are used? They can't expect us to test them all!
As one of the US’s major dust explosion contract test laboratories, this is a question that we hear regularly and sometimes the sentiment expressed here can inhibit actioning a dust testing program. “What can of worms are we opening here?”
There is good news. In all likelihood, there will only be a need for limited testing; but ‘what to test?’ and ‘which tests to perform?’
Most times when we get this question, we explore the status of any Dust Hazards Analysis (DHA) on our client’s facility – according to the NFPA 652 standard on combustible dusts. This has to be completed by September 2020 this year anyway, so it makes sense as a logical first step. This approach would allow for the evaluation of each hazard situation and selection, not only of representative sample(s), but also the applicable test(s). It should be noted that which test(s) to perform depends – amongst other factors – on the likely ignition sources that are identified during the DHA as well as the prevention and/or protection measures that will need to be taken to ensure safety of people and the facility.
In our view, the Dust Hazard Analysis should include the entire list of the client’s materials:
- to see if various materials could be grouped together so that a small number from each group would need to be tested,
- to study the materials’ SDS’s for information on explosibility/combustibility that might help us exclude some materials from testing,
- to determine average quantity of each material used – we could perhaps initially focus on materials that are used in larger quantities and higher frequencies,
- to determine if the testing of some samples from the dust collectors and any central dust removal system alone would be sufficient in obtaining the necessary data for the DHA – instead of testing all possible powders.
Sometimes, to narrow the list of samples for testing, one could perform ‘screening’ tests which would obviously be quicker and cheaper to perform and would then allow us to perform ‘full’ tests on a smaller number of materials.
Example: You have a dust collector serving multiple points and collecting different powders and you want to check that the explosion relief vent area is large enough. To check this, we will be looking for the maximum credible explosion severity (Kst) value powder (mix) that can occur in the dust collector. We’d begin by looking at your list of powders. Are they all explosible? Do we have data on the same materials? Any information in the SDS?
This review may identify a few ‘red flag’ materials. Next, we’d review powder usage data. Are any powders used in such small quantities that it is not credible that they could significantly contribute to the overall worst case Kst in the dust collector. Are powders used in sequence or will there always be mixtures in the collector?
Eventually we come down to a short list of most commonly used powders with the highest Kst values. Depending on our initial findings, we would then advocate testing a few worst-case powders – or even sampling at certain times from the dust collector when worst case conditions are likely to be prevalent.
In summary, you as the owner or operator of a facility with potentially combustible dust are responsible for undertaking the following steps:
- Determining the combustibility and explosibility hazards of the materials processed at your facility
- Conducting a Dust Hazard Analysis (DHA) – Identifying and assessing fire, flash fire, and explosion hazards
- Managing identified fire, flash fire, and explosion hazards
- Establishing written Safety Management Systems
We keep getting requests from our clients for Dust Hazards Analysis (DHA); however, DHA is only one (albeit an important) component of the requirements of NFPA 652 and the authority having jurisdiction requires that you are in compliance with all the requirements of NFPA 652.
DHA is required if materials handled and processed have been identified as combustible and/or explosible. DHA is a systematic review to identify and evaluate potential fire, flash fire, and explosion hazards associated with the presence of combustible particulate solid(s) in a process or facility.
Please note that NFPA 652 requires that the DHA is conducted by an expert with the demonstrated ability and credentials to effectively identify, assess, and recommend practical measures for controlling the hazards related to processing and managing combustible particulate solids.
First off, NFPA 652 only requires us to determine if the dust cloud under the right conditions is “explosible (Go/No Go)? In other words, would a dense cloud of dust, dispersed in air and subjected to an energetic ignition source, be capable of causing a flash fire or explosion?
NFPA 652 permits you to determine the combustibility or explosibility of your powders/dusts based on one of the following methods:
- Historical facility data or published data that is deemed to be representative of current materials and process conditions
- Laboratory analysis of representative samples
- You are also permitted to assume a material is explosible, forgoing the laboratory analysis
Note that the absence of previous incidents is not allowed to be used as basis for deeming a particulate non-combustible or non-explosible.
Besides combustibility (Go/No Go) testing does NFPA 652 require me to do any other testing or data determination?
NFPA 652 does not specifically require any testing other than possibly Explosibility (Go/No Go). However, in order to perform any meaningful Dust Hazard Analysis (DHA) it is essential that one has applicable information/data on the ignition sensitivity, explosion severity, and electrostatic properties of dusts that are being handled. Typical tests that might be considered include:
- Minimum Ignition Energy
- Minimum Ignition Temperature (Cloud and Layer)
- Minimum Explosible Concentration
- Limiting Oxygen Concentration
- Maximum Explosion Pressure (Pmax) and Kst
- Electrostatic Chargeability and Volume Resistivity
Note that depending on the nature of your operations and processes you often do not need to perform all of these tests. Usually, you can utilize a step-by-step approach to testing whereby the result of the initial test(s) determines if any additional testing is required.