Combustible dusts test methods

The standard gives requirements for the dust sample in chapter 4.

4.2 Characterisation of sample. The sample shall be representative of the material as it appears in the entire process operated.

NOTE Many unit operations such as extract systems will separate dust into finer fractions than seen in the main processing equipment and this is accounted for when taking the sample.

If the sample is not representative of the material as found in the process then sample preparation shall be carried out to apply the worst case conditions. […]

4.3 Preparation of sample. If it is not possible to test the sample as received, or if the sample is no longer representative of the process material then it may be necessary to condition or alter the sample for testing. This may include

– grinding/sieving,

– drying and

– humidifying.

Any apparent changes noted in the properties of the dust during preparation of the sample, for example, by sieving or owing to temperature or humidity conditions, shall be stated in the test report.

NOTE 1 Sample preparation such as grinding and sieving, or drying can alter the material characteristics. Where finer fractions are present in a facility it is appropriate to take fractions of less than 63 μm to give the most easily ignitable mixtures. When the sample is a mixture of substances, the sample preparation can result in a change to the sample’s composition.

In Chapter 5 it gives indication how to execute the test in the Hartmann tube and in the 20-litre sphere. The particle size is an important characteristic of the dust.

5.2.2 Determine particle distribution. For material which does not contain combustible flyings check the particle size distribution:

• If there are no particles less than 500 μm in size then the material is not a combustible dust.
• If there are any particles less than 500 μm in size then continue the test procedure in a Hartmann tube to determine whether it is a combustible dust.

The standard mention to use a fraction of “less than 63 μm” in order to determine if a certain dust is combustible. But that is only in the case “the sample is not representative of the material as found in the process”.  As a consultant trying to understand if a dust, used in an industrial environment, is combustible may seem simple, but it is not. Risk assessment is key, as it often happens.

If you are faced with a situation whereby a certain dust is taken from a bag straight from the supplier, opened and poured into a hopper, then you can assume the size of the dust in the bag as representative. If the lab test says that dust is not combustible, for example because of its large particle size, then the area you are analysing is not a dangerous one from an explosion point of view.   

However, if the same dust is injected and transported in a pneumatic piping, you need to assess how different is the dust “at the end” of the transport. For example, in that case, we recommend the lab. Test the dust “at the final destination”.

In case you are not sure what type of dust size is present inside the process you are analysing, then you need to follow the advice from this standard and “take fractions of less than 63 μm to give the most easily ignitable mixtures”.

As a confirmation of this approach, hereafter what VDE 2263 Part 6 (Dust fires and explosion protection in dust extracting installations) states regarding the risk assessment in dust filters and extracting installations in general.

5.3 Explosion hazard. […] 5.3.1 Characteristics required. […] The [dust] sample under examination shall represent worst case conditions (e.g. fine-dust fraction during dedusting, chemically unchanged). If this fine-dust fraction is not available, dry fine dust having a grain size < 63 µm shall preferably be used.

That is the way the language in CLC/TR 60079-32-1 (Electrostatic Hazards – Guidance) should be read

9 Static electricity in powders. 9.1 General. According to experience the ignitability of bulk material ranging from fine dust to granules or chips increases with decreasing particle size and decreasing minimum ignition energy (MIE). Explosion hazard assessment should always be based on the minimum ignition energy of the finest particle size fraction that may be present. This fraction is usually obtained by sieving a sample through a 63 µm sieve