Detection of flammable and toxic gases – where to install the sensors?
The biggest mistake is treating gas detectors as smoke detectors and installing them where gas can theoretically accumulate, i.e. under the ceiling in the case of gases lighter than air and at floors in the case of gases heavier than air.
This is a flawed rule and not fully compliant with standards, e.g. PN-EN 60079-29-2, because gas sensors detect much lower concentrations and are used to detect flowing gases, and possibly also accumulating gases.
In small cubatures, the literal approach will work and, for example, in a house gas boiler room with a height of 3-4 m, placing a methane detector under the ceiling will be correct, while in a larger plant boiler room in a room with a height of 10-15 m, placing methane detectors only under the ceiling / roof may turn out to be wrong, because in such a room there are quite large heat gains and constantly working ventilation, which will “entrain” and dilute the methane concentration in the air, and sensors installed at a height of 15 m can detect 10-20% LEL of methane much later or at all, if the leakage is minor.
The result of the above approach will be a significant delay in activation:
- switching on emergency ventilation,
- cut off the gas supply to the building,
and consequently, a higher level of fire and explosion hazards.
According to the PN-EN 60079-29-2 standard, gas sensors should be installed as close as possible to the emission source of a given gas and / or along the path of its actual flow (without the need to use canopies or other hoods), and the sensors should be mounted under the roof or next to the floor ( if the emission is a few meters from the floor) they are only auxiliary detection.
In small rooms, sensors can only be installed under the roof or next to the floor (approx. 30 cm from the surface).
Several times I have encountered the desire to detect “additional” methane or hydrogen in large halls by detectors installed under the roof at a height of 20-30 m, where the ventilation is constantly working due to high heat gains.
This solution makes no sense, even as an auxiliary, because:
- at this height (at a distance of 15-20 m from the emission sources) and with such strong ventilation reinforced by large convection, the detectors will not detect anything, i.e. the concentration of these gases is not likely to exceed 10-20% LEL under the roof (please take into account the roof area),
- Providing platforms or other access for service work in accordance with OSH (cranes are usually not an option) is at least difficult / expensive, and often impossible, and these sensors cannot be serviced from drones.
I mention this because somewhere in the back of the head some designers treat these sensors just like point smoke detectors and believe that, just in case, they should also be installed under the roof.
But what for?