From layer to cloud: calculator
Enter the layer thickness, the bulk density of the dust and the room height to see the result. The calculation runs in your browser; nothing you enter is sent anywhere.
If the dust is raised and fills the room: g/m³
- If the cloud stays in the lower third of the room: g/m³
- Dust per square metre of floor: kg
Layer that brings the room to the explosion limit
- For the full room height: mm
- For the lower third of the room: mm (about sheets of paper)
The calculation assumes that the whole layer is raised and evenly distributed. In reality the cloud is far denser close to where it forms. The result is not a safety limit; the aim is that no visible dust layer forms at all.
Detailed calculation
Every step is shown with its formula and the numbers substituted.
Maximum permissible surface temperature: calculator
Enter the ignition temperature of the dust cloud and of a 5 mm dust layer.
Equipment surface temperature at most: °C
Only one ignition temperature was entered. Both are needed for the applicable limit.
The rule applies where no more than 5 mm of dust settles on the equipment. A thicker layer retains heat and ignites at a lower temperature; the limit then has to be determined separately.
Formula
The calculation is a simple mass balance. If the dust lying on the floor is raised completely and spreads evenly over the height of the room, the average concentration in the air is:
c = ρ · s · φ / H
- c: average dust concentration in the air (g/m³)
- ρ: bulk density of the dust (kg/m³); the density of loosely poured dust, not of the particle
- s: layer thickness (mm)
- φ: share of the floor covered with dust (1 if all of it is covered)
- H: height over which the dust disperses (m)
The units work out by themselves: 1 mm of thickness at a bulk density of 1 kg/m³ is 1 gram per square metre. Read the other way round, the formula gives the layer thickness that brings a room to the lower explosion limit of the dust:
s = limit · H / (ρ · φ)
Worked example
A simple case. A dust with a bulk density of 500 kg/m³ lies 1 mm thick over the whole floor of a room 4 m high:
c = 500 × 1 × 1 / 4 = 125 g/m³
A packing hall. The hall measures 30 × 20 × 5 m. On 15 % of the floor there is a flour layer of 2 mm on average; the bulk density is 450 kg/m³. Assume for this example that the test report gives a lower explosion limit of 60 g/m³.
- If the dust spreads through the whole hall: c = 450 × 2 × 0.15 / 5 = 27 g/m³. Below the limit, which looks like “no problem”.
- If the raised dust first stays in the lower third of the hall: c = 27 × 3 = 81 g/m³. Above the limit.
- Conclusion: an average concentration does not show that a room is safe. The cloud is not uniform; local concentrations are much higher.
Press Fill in the example in the calculator to see the first case.
Where do I find the lower explosion limit?
The lower explosion limit of a dust (its minimum explosible concentration) is given in the dust's test report. It depends on the type of dust, its particle size and its moisture. It should therefore come from a test of a sample taken at your plant, not from a general table. If you have no value, leave the comparison field empty; the calculator still gives the concentration.
The surface temperature rule
Where dust is present, equipment surfaces must not become hot enough to ignite it. Two ignition temperatures are measured for a dust: as a cloud and as a 5 mm layer. The equipment selection standard (IEC 60079-14) gives the maximum permissible surface temperature as the lower of two conditions:
T_max = lower of ( ⅔ · T_cloud ; T_5mm − 75 )
Example: if the cloud ignites at 430 °C and the layer at 310 °C, the limits are 287 °C and 235 °C; 235 °C applies. On equipment for dust atmospheres the surface temperature is marked directly in degrees (for example T135 °C). You can read the rest of the label with the Ex marking decoder.
Where this calculation sits in the explosion protection document
When we prepare an explosion protection document, we use these two calculations in the following sections:
- Classification of dust zones. Whether a deposit could form an explosive cloud is assessed together with the level of housekeeping.
- Cleaning plan. The largest accepted deposit and the cleaning interval are justified with this calculation.
- Ex equipment suitability. The permissible surface temperature is compared with the temperature marked on each item of equipment in the dust zones.
- Inspection plan. Who checks, and how often, that cleaning is done and deposits stay below the limit.
The calculation provides a justification. The decision rests on the dust's test data, the sources of release and the deposits actually found on site.
What this calculation does not show
The tool gives an idea of how large a cloud the dust on the floor could form. It does not show:
- Whether the dust will actually be raised, and how the cloud will spread.
- Dust that settles on beams, cable trays and machines. Secondary explosions are mostly fed by these deposits.
- The situation inside equipment: the insides of silos, filters and conveying lines are assessed separately.
- The zone classification. Zones are determined in the explosion protection document from the sources of release, the level of housekeeping and the equipment.
If you need this for a site in Türkiye, contact us.
Sources
- Turkish Ministry of Labour and Social Security, application guide on explosive atmospheres (warning on deposits thinner than 1 mm)
- European Commission, non-binding guide to good practice for implementing Directive 1999/92/EC
- HSE, HSG103: Safe handling of combustible dusts
- IEC 60079-14 (permissible surface temperature in dust atmospheres), ISO/IEC 80079-20-2 (determination of dust characteristics)
This tool is for information. For design and zone decisions, the current text of the standards and the test report of the dust apply.
Frequently asked questions
Where do I find the bulk density?
It may be given in the product's technical data sheet or safety data sheet. If not, you can measure it: fill a one-litre container with dust without compacting it and weigh it. The mass in grams equals the bulk density in kg/m³.
The result is below the limit. Is cleaning unnecessary?
No. The calculation assumes even distribution; in reality the cloud is much denser in places, and dust keeps accumulating. The aim is not to stay below a certain thickness but to have no visible layer.
How do I measure the layer thickness?
A thin layer is hard to measure with a ruler. A more reliable way is to collect and weigh the dust from a known area: the mass in grams collected from one square metre equals the product ρ · s in the formula. Divide it by the room height and you have the concentration directly.
What if more than 5 mm of dust settles on the equipment?
A thick layer retains heat and ignites at a lower temperature. The 75 K margin is then not enough; the permissible temperature is reduced further according to the standard or determined by laboratory investigation. The first measure is to prevent the build-up.