About Gas Monitoring  

Gas leak detection is an important part of every gas safety programme. Many industrial processes require the use of potentially toxic gases and other processes may generate hazardous gases as a bi product. Such hazards can be managed through careful equipment maintenance and regular monitoring for early signs of leakage. 

One of the primary concerns of a gas detection system is the protection of workers within the facility from the effects of toxic gas as well as oxygen depletion. Areas for protection are where gases could potentially build up and highly toxic or flammable combustible gases are stored, handled, transported or processed.  

Environmental Monitoring

For environmental monitoring the concentration of gases that need to be monitored is determined by the concentrations which are hazardous. Toxic gas sensors are normally ranged slightly above the short term exposure level (15min reference period). Gemlog equipment also provides a warning when the long term exposure limit is exceeded (8 hour TWA reference period).

Example: Carbon Dioxide (CO2) Long term 0.5% & Short term 1.5% Gemlog sensors are ranged to 2.0%

For further information consult HSE EH40/2005 Workplace exposure limits - www.hseni.gov.uk/eh40_2005.pdf  (Please note this link and document title was correct October 2012)

Flammable gas warning alarms are normally set at 20%LEL sometimes 10% LEL, with the second alarm at 40%LEL.

Oxygen depletion sensors are normally set to warning at 19% and full alarm at 17% oxygen. Gemlog Controls also offer an optional Oxygen enrichment alarm at 23%, this optional alarm may be deactivated at the controller.

Controller/Sensor Locations

There are no standards governing gas detection controller location. However we would suggest they should be mounted outside the room(s) being monitored and ideally in an adjacent corridor or room at typically head height.

When it comes to sensors BS EN 50073:1999 states “Sensors should be located in positions determined by those who have knowledge of gas dispersion, the process plant systems and equipment involved and in consultation with both safety and electrical engineering personnel”.

The following information is required in order to identify the correct gas detector type and locations for any application:

  • The most suitable detectors to provide fast and reliable operation
  • The limitations of sensors, for example cross-sensitivity to other gases which may be present

Other important factors to be considered:

  • Sensors must be easily accessible for testing and maintenance
  • Sensors and cables must be protected against mechanical damage
  • The sensor technology selected must not be adversely affected by other substances in the environment
  • Sensor accessories must be carefully selected to ensure correct operation in the chosen position, for example: spray deflectors should be used in wet areas, sun shades may be fitted if detectors are exposed to direct sunlight.

When installing gas detectors it is advisable to ensure that the sensor orifice is not exposed to liquid or dust contamination by positioning the unit downwards. Spray deflectors should be used when detectors are installed outdoors, or in indoor areas subject to wash-down operations.

There is no way of determining the maximum coverage of any one sensor, there are too many variables. The most obvious being the shape of the room concerned. However 50 to 100m2 per sensor is a reasonable general guide, adding more sensors at points where leakage could occur or in "L" shaped rooms.

Having considered suitable locations for gas sensors, the mounting height has to be decided on. In general, for gases lighter than air the detectors should be above the area where leaks are likely and for gases/vapours heavier than air the sensors must be at floor level or in inspection pits or ducts into which heavy gas/vapour may flow.

Gases will not separate out into discrete layers according to their densities. If they did, air would not exist as a homogeneous mixture but the heavy carbon dioxide would be at ground level with oxygen, which is heavier than nitrogen, next with a layer of nitrogen on top. If gases behaved like this, they would in fact be behaving like liquids. It is 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 may affect the gas dispersion.

Also, the nearer in density to air a gas is, the more easily it will flow with air due to draughts and ventilation etc. Therefore a compromise with gases like carbon monoxide and also gases only slightly heavier than air such as hydrogen sulphide and nitric oxide, is to mount the detectors at a height as close as possible to the breathing areas of personnel being protected (typically 1.5 to 1.8metres).

When monitoring oxygen depletion, it is necessary to consider what might be displacing it. If helium is displacing the oxygen, the detectors should be mounted at a high level.

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