Testing Essential to Establish Risk for Dust Explosion

Over the course of the past several months involving several risk analyses, it has become obvious that even dusts as common as sugar, for which there is a plethora of published data, are not well understood relative to dust explosion risk. When assessing the risk for a dust explosion, the very first thing that needs to be determined is whether or not the dust being handled is explosible. While that statement may seem rudimentary, it is not as straight forward as it would at first seem. The primary test that should always be run on any dust sample is defined by ASTM E 1226. The protocol calls for certain preconditions such as moisture levels at not more than 5% weight, and particle size not greater than 75 µm (i.e. the material that falls through a 200 mesh screen). The reasons for “pre-conditioning” the dust sample in this way are related to trying to define a “worst case” scenario. The logic is that if the data obtained from the test can be reliably assumed to provide very conservative values, then anything less reactive would be covered. The principle products of the E 1226 test are two values: Kst, and Pmax. Both values are used not only to determine if the tested dust is capable of explosible behavior, but also to calculate explosion protection equipment sizes. So, conservative numbers are desirable.

An often overlooked risk factor is Minimum Ignition Energy (MIE), the test for which is defined in ASTM E 2019. This test is extremely helpful in determining the probability for ignition. Low MIE materials, which I will define as those that have values or 10 mJ or less, can be said to be ignition sensitive. In other words, the lower the MIE, the higher the probability for ignition of the tested dust. Consider that an adult male who walks across a carpeted room on a dry day and gets a shock upon touching a door knob, generated a static discharge spark of at least 9 mJ. It is easy to see why it does not take much energy to ignite some dusts and why this factor needs to be fully understood.

In addition to the MIE value, for a proper understanding of risk it is necessary to know the bulk resistivity of the dust. Unfortunately at this time there is no established test protocol for this value. Without knowing bulk resistivity and just having a low MIE value is not sufficient to obtain a complete understanding of the risk for ignition. Generally materials with low MIE values (= 10 mJ) and high bulk resistivity (~ = 1013 – 1014 ohms) are more susceptible to static discharge energy ignition than dusts with lesser values.

One of the challenges an ASTM committee would have in developing a test protocol for bulk resistivity would be establishing a baseline for humidity. The higher the relative humidity in the test apparatus and sample, the lower would be the bulk resistivity of any given dust. Requiring very dry test conditions could result in unrealistically high numbers and render the information nearly meaningless. In recent experiments we have seen bulk resistivity values vary from ~ 107 ohms to >1014 ohms for the exact same dust simply be changing relative humidity from 0% to 30% to 60%. Published values for this particular dust run the whole range and users of such inaccurate data are not doing themselves any favors.

This last point helps to frame the problem of testing to establish risk. What if the dust sample submitted to the lab does not meet the criteria established by the protocols (or there is no protocol)? What if a dust sample has a particle distribution that is too large and none or only a small fraction of the total falls through a 200 mesh screen? This happens all the time. Should a much larger quantity of dust be sent to the lab so the fines can be screened out? This might not be practical or in some cases even possible. In such cases, should the lab be allowed to try to mill the dust to create sufficient fines for test purposes? What if the material will not mill? What if the material readily builds up a static charge so that it clings to, rather than falling through the screen? What if the dust is handled in a tightly controlled process at a moisture level above that defined in the ASTM protocol? What if the project is a new process for which production samples are not yet available? What if the dust is fibrous with an elongated shape and milling would fundamentally alter the explosible characteristics of the tested material? I could go on with the questions, but believe it should be clear that there is more to this whole business of testing than blindly following protocols, much less on relying on published data that may or may not be pertinent.

In just the past few months I have seen published data that indicated a risk for brush discharge ignition for dusts that upon testing did not pose such a risk. On one project fines were created by first freezing the dust sample using liquid nitrogen and then milling the frozen particles because at room temperature the dust would not mill. Of course the actual risk faced by the owner of the plant did not include fines like those created in the lab to adhere to the test protocol. In numerous MIE tests there have been instances where the risk was overstated by means of an allowable option in 2019 to use inductance. Using inductance results in a sustained spark discharge to the test igniter and introducing inductance lowers the MIE value in the test. This is appropriate only under unusual circumstances that should be identified by a qualified specialist who has carefully assessed the process and instructs the lab. The lab should not be expected to know the conditions in the plant, nor should the lab make decisions on the test with only an eye toward establishing a worst case result.

One of the critical elements in the process is obtaining samples that represent the actual worst case for the dust being handled in the specific process. Many dust materials are quite friable, meaning they fracture into smaller and smaller particles as they are handled and flow through the process equipment. For materials of this type, the sample should be taken from somewhere at or as near as possible to the end of the process; for example, from an aspirating dust collector on the finished goods packaging apparatus. Finished product awaiting shipment in a storage bin or silo, would be another logical place to obtain a sample. There would be nothing to be gained, however, from having a sample taken from vessels and locations like these and then further size reducing it by artificial means in the lab. Remember, the objective is to develop a meaningful measure of the risk associated with the product and the process under evaluation. Testing is an adjunct to this larger consideration and the appropriate test protocols are a useful starting place, but there is no substitute for careful thought by qualified people.

One long standing definition of dust is that which falls through a 40 mesh screen (i.e. = 420 µm) and in fact this criterion perhaps frames the furthest point in the direction of meaningful test results one should consider when establishing risk. In more than one recent project, using this criterion proved to be the only practical option. Finer particles could neither be collected nor milled, and yet using a 40 mesh screen established that a meaningful fraction of the material was combustible dust. Testing yielded results that ascertained the materials under evaluation could behave as explosible dust. So even though the testing did not strictly conform to the E 1226 protocol, the results provided the information needed to establish risk and to adequately size explosion protection systems.

In conclusion it is strongly advocated that for each situation testing should be part of the risk analysis effort. Relying on published data can lead to false conclusions. This can result in over sizing and otherwise wasting money on unnecessary explosion protection schemes on the one hand, or missing an important risk element that can lead to an explosion on the other. Testing is not as clear cut as it may seem and it is extremely important to engage the services of a qualified risk consultant who will specify the tests and coordinate with the lab to make sure that a true picture of the dust risk is obtained. There is no shortcut and the consequences of getting it wrong are at least over spending, and at worst not establishing an adequate basis of safety.

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