The chemical industry in particular, but also the food industry and even farming businesses, has to deal with explosion risk. In this article BaSystemen describes briefly and clearly:
- The basics of an explosion
- Dutch legislation on measuring explosion risk
- Options for raising the alarm at an early stage
The basics of an explosion
In an explosion, a very large amount of a combustible substance burns all at once, often followed by fire. The combustible substance may be a gas, in which case we speak of a gas explosion. But the combustible substance may also be a cloud of dust particles that ignites, in which case we speak of a dust explosion. Gases commonly involved in an explosion are listed in the table below. With dust, the risk is not always easy to assess.
|
Gas |
Explosive range |
|
Methane (main component of natural gas) |
4.4 – 16% |
|
Hydrogen |
4.1% – 74.8% |
|
Propane |
2.1% – 9.5% |
|
Petrol |
1.4% – 7.6% |
|
Diesel |
0.6% – 6.5% |
|
Butane |
1.8% – 8.4% |
|
Kerosene |
0.6% – 4.9% |
The lower limit for a dust explosion is less easy to state in black and white. Milk powder, grain powder, flour, wood dust, metal powder, sugar, spices, etc. generally occur in various compositions. You can find the explosive limit in the safety documentation for the substance concerned.
An explosion requires three basic elements, shown alongside in the fire triangle. Namely:
1) Fuel
There must at all times be enough fuel present to cause a fire. However, natural gas may burn without necessarily exploding straight away, just as it does on a gas hob. For that, the lower explosive limit shown in the table above has to be reached. But take note: there must also not be too much combustible gas present. That is called the upper explosive limit, which is also listed above.
2) Temperature
The ignition temperature is the minimum heat needed to make a substance ignite. For methane this is, for example, −188 °C. Every gas has its own specific ignition temperature.
3) Oxygen
The third factor that must be present for combustion is oxygen. Taking methane as an example, the reaction that then takes place is: 2 O2 + CH4 -> 2 H2O + CO2.
(4) Ignition + chain reaction
If one of the above is missing (or is removed), nothing happens. When all three are present, a spark, flame or other ignition source is enough to cause a fire or explosion, whereby the reaction mentioned under point 3 takes place and, because there is now a flame, continues at breakneck speed.
Explosion risk in the Dutch working conditions decree
Directive 1999/92/EC (also known as ATEX 153) sets out the obligations regarding explosion risk. In brief:
- The hazards associated with explosive atmospheres and the particular risks that may arise from them are assessed as a whole and recorded in writing in an explosion protection document.
- If it can be suspected that the atmosphere in a place or space contains substances to such an extent that there is a risk of asphyxiation, narcosis, poisoning, fire or explosion, the employee may only be present in that place or space if investigation shows that the hazard is not present.
- The employer responsible for the workplace coordinates the implementation of all safety and health measures.
ATEX zones must therefore be clearly mapped out and clearly marked. Equipment that is not explosion-proof may not be used there either. A distinction is made between three zones. Namely:
- Hazard zone 0 or 20: category 1 equipment;
- Hazard zone 1 or 21: category 1 or category 2 equipment;
- Hazard zone 2 or 22: category 1, category 2 or category 3 equipment;
The rule here is: the higher the number, the smaller the likelihood or duration of an explosive atmosphere.
Dust explosions can be even more powerful than gas explosions. It is therefore advisable that production or storage sites for flour, grain, animal feed, milk powder, wood dust, metal powders and so on map and monitor the risks of a dust explosion arising. The force of the first explosion can whip up more dust, allowing secondary dust explosions to occur. The result is a chain reaction in which the subsequent secondary dust explosions are usually far more severe than the primary one. The secondary dust explosions can thus race through an entire building at high speed.
If particles smaller than 500 um are present, there may be a dust explosion hazard. In most cases a dust layer thickness of 0.1 mm is already enough to create an explosive dust-air mixture. A practical guideline from the Dutch labour inspectorate (ISZW) is therefore that a dust explosion hazard is present if you can see your footprints on the floor.
Explosion meters (gas and dust)
As mentioned earlier, an employee may – where there is a risk of fire or explosion – only be present in that space if investigation shows that the hazard is not present. We are therefore often asked about measuring equipment for detecting lower explosive levels at an early stage.
For gas there are several options. Four commonly used options are listed below. All devices are ATEX certified and will not act as an ignition source in explosive environments.
- Portable single-gas or multi-gas detector with LEL sensor
- Multi-gas detector with pump for inspecting or clearing risk areas
- Transportable area monitoring with a function to raise the alarm by text message or email
- Fixed point detection for continuous monitoring of ATEX zones.
For indicative dust measurement there is the Microdust Pro 2. This is an indicative dust monitor with a wide measuring range for both the quantity of dust particles and their size. For specific enquiries, please feel free to contact our specialists.
