Installation
SITING THE SENSING HEADS
A key feature of the installation is the correct positioning of the sensor.
Several considerations must be taken into account, the most important being the density
of the gas.
Density (air = 1)
Acetone
2.0
Hydrogen-Sulphide
1.18
Acetylene
0.91
Hydrogen
0.069
Ammonia
0.6
Methane
0.6
Benzene 2.8
n-Pentane
2.5
n-Butane 2.0
n-Propane
1.6
Carbon Dioxide
1.53
Town gas
0.4-0.7
Carbon Monoxide
0.96
Xylene
3.7
Under still air conditions, a lighter than air gas such as methane leaking from a small
aperture at ground level, will rise in a plume the shape of which approximates an inverted
cone. As the gas rises, it draws air from the surroundings and creates turbulence. Due to
this rapid dilution of the gas occurs and, unless a sensor is positioned within the plume,
there will be no initial indication of a leak.
As gas continues to escape, the diluted methane rises to ceiling level and begins to layer.
In time the concentration at ceiling level will increase and this, in turn will displace air
downwards.
Dangerous concentrations will therefore, tend to occur at ceiling level and the thickness
of this layer will increase with the passage of time.
Ventilation of the room will of course alter the situation significantly but it should be
remembered that if the ventilator is not at ceiling level, a dangerous concentration can
still occur between the top of the ventilator and the ceiling.
For heavier than air gases such as propane or butane, the formation of dangerous layer
occurs at ground level. These gases tend to behave like water and will run down
gradients and pool at the lowest point.
The number of heads required in individual rooms is determined by the number of
possible hazards in the vicinity.
Gas leakages may occur around valves, flanges and wherever gas pipes are jointed. It
may be possible to cover several probable gas leaks in one room by the careful siting of
a single head. Cable ducts, trenches and manholes are also likely places where a build
up of heavy gases may occur.
When siting a head in such places it is important to ensure that there is no likelihood of
flooding by water, or excessive dust which may block the sintered disc and prevent gas
from reaching the sensor.
When monitoring gases outside, those lighter than air will be quickly dispersed, but gases
heavier than air will tend to form in layers and again cause a dangerous hazard. When
siting heads outdoors prevailing winds must be taken into consideration and adequate
protection given against wind and rain by the use of weather or collector cones.
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