A fixed gas detection system should be installed that it is capable of
monitoring those parts of a plant where a low oxygen concentration may
accumulate. The system should be capable of giving an early warning of
both the presence and location of the oxygen deficient atmosphere, in
order to initiate one of the following actions, either automatically or
under manual control:
1) Safe evacuation of premises
2) Shut down of process or plant.
3) Ventilation control
4) Appropiate fire fighting procedures
This should be capable of monitoring of every part of the plant where an Oxygen deficient atmosphere may occour. It should be capable of giving the earliest possible alarm. Remote sensors are connected to their associated control equipment by means of appropriate cables as specified in the instruction manual.
In general a fixed system should be installed so that failure or maintenance of individual elements does not compromise the safety of the premises being monitored. Duplication of sensors in key areas is to be recommended.
Fixed sensors should be located so as to detect gas accumulations which
could create a significant hazard. Factors to be taken into consideration
1) Indoor or outdoor site
2) Potential leak source
3) Nature of the gas or vapour to be detected.
4) Nature of possible gas or vapour release (high pressure jet, evaporation, liquid leaks)
5) Presence of confined spaces.
6) Topography of site.
7) Air movements: Temperature effects, Indoor natural ventilation, indoor mechanical ventilation, outdoor wind speed and direction.
8) Population of plant.
The densities of gases with respect to air is given in EN 617791:2000.
It should be noted that high pressure gas leaks can result in a drop in the gas
temperature and subsequent temporary increase in density. Sensors should be installed
close to any potential leak source but not so close that they respond to equipment which
may produce inconsequential leakage.
A well designed ventilation system should result in a number of air changes per hour and thus limit the potential for gas buildup. As a general rule a gas sensor can cover an area of around 50 sq. mtrs, this can be affected by the characteristic of the gas and by the nature of the hazard and is only intended for general guidance. All applications are different and require careful consideration before determining the number and location of sensors.
It should also be considered that the general 50 sq. mtr rule equates to an approximate radius of 5 mtr around the sensor. Wall mounting the sensor effectively halves its operational area as shown below.