There are no specific Australian standards or regulations governing gas detection installations, unlike for fire detection. In theory, everyone may install gas detection systems regardless of their knowledge. However, there are general guidelines as such documented in AS1668.2 which prescribes requirements for gas detection systems for carparks. Similarly, AS1667 describes technical requirements for refrigerant gas detection systems which should be adhered to.
Sensors are to be installed at locations that are selected in close consultation with the persons that are familiar with the potential accumulation of gas accumulation, leakage or dispersion. They will have knowledge of the processing or manufacturing plant systems and its equipment involved. The system will be designed and installed in consultation with both safety and electrical engineering personnel.
Designing your Installation
The following information should be considered in order to identify the correct gas detector type and locations for any application:
- Application, hazardous zone or not
- Quantity of gas detections required
- Detector height to be fitted
- Cross reference to other gases
- Accessible for testing and maintenance
- Sensor technology
- Environmental circumstances
In general, for gases lighter than air the detectors are fitted at high level above the source of a possible leak. Examples are detection of Hydrogen, Ammonia, Methane, etc. Gases heavier than air should be fitted at low level, examples are Chlorine, Carbon Dioxide, Propane, etc.
We should take in consideration that gases do not exist as homogeneous mixture. If they behaved like this, they would in fact be behaving like liquids. Hence it's better to view gases as tending to rise if they are light and tending to sink if they are heavy and to think about other phenomena which might affect the gas dispersion.
In addition, pressure and temperature would influence the gas its density. For example, if carbon monoxide, which is only slightly lighter than air, is under pressure and is suddenly released into the atmosphere a drop in temperature is caused resulting in an increase in density. This may cause the carbon monoxide to fall to floor.
Air is a mixture of gases, typically:
- Nitrogen 77.2 %
- Oxygen 20.9 %
- Water Vapour 0.9 %
- Argon 0.9 %
- Carbon Dioxide 0.03 %
- Other Gases 0.07 %
Gases with relative simular density to air or slightly heavier will easily flow and circulate with air. Therefore a compromise, is to mount the detectors at a height as close as possible to the breathing areas of personnel being protected, typically 1.5 to 1.8 metres above the finished floor level (AFFL).
When monitoring deficiency of oxygen, it is necessary to consider what might be displacing it and from that point decide the mounting height of the detector.