A7. H2 sensors positioning
The correct location of reliable sensors is crucial for timely detection and warning of hydrogen leaks before an explosive mixture is formed. Recommended locations for sensors include [2, 5, 6]:
- Evaluate and list all possible leak or spill sources to be monitored (valves, flanges, connections, bellows, etc) and provide valid justification for sources not monitored;
- At hydrogen connections that are routinely separated (for example, hydrogen refuelling ports);
- Locations where hydrogen could accumulate;
- In building air intake ducts, if hydrogen could be carried into the building;
- In building exhaust ducts, if hydrogen could be released outside the building.
The following points should also be considered [4]:
- An understanding of how a gas leak disperses is required to choose the correct location to install the detection device(s). Hydrogen, being less dense than air, will rise when released and disperse rapidly.
- When thinking of the location of hydrogen sensors/detectors, take the response time into consideration.
- The LEL used shall be the LEL of the gas or gas mixtures.
- When positioning detectors, local airflow also needs to be considered. Intuitively hydrogen detectors should be placed above a potential leak source however airflow may carry the hydrogen 'downstream', away from the detector and before reaching the ceiling. In that case detection may be delayed or even prevented.
- Temperature can also have an effect on the dispersion of a gas. As hot air rises a layer of lower density air forms at the ceiling creating a 'thermal barrier' which may slow the diffusion of leaking hydrogen enough to delay detection at the sensor.
- A combustible gas detector that meets the above requirements should be provided for all indoor or separately controlled gas compressors.
- When hydrogen is stored as a cryogenic liquid and leaks, its density is initially greater than air causing it to settle to the ground before heating up, becoming lighter than air and eventually rising.
- Dilution of hydrogen increases the further the detector is from the site of the leak. As a result the actual hydrogen concentration can be higher than the concentration indicated by the detection device when the device is located far from the leak site. For this reason detectors should be placed close to a potential leak site and should be sufficient in number to cover the installation.
- It is recommended that a hydrogen sensor be placed at the most elevated point in an enclosed space.
- If a forced ventilation system is installed then a sensor should be placed where the ventilation is applied.