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Tel: 0113 286 0166
Fax: 0113 287 8178
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A key feature of the installation is the correct location
of the sensor head. Several factors must be taken into
account, the most important being the density of the
gas.
Under still air conditions a 'lighter than air' gas
such as methane leaking from a small fracture at ground
level, will plume upwards and outwards approximating
a cone shape. As the gas rises it displaces air from
the immediate vicinity creating turbulence, consequently
rapid dilution begins to occur and unless a sensor is
positioned within the plume, there may be no initial
indication of a leak.
As gas continues to escape, the concentration 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 from the highest point downwards 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 layers 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 leakage may occur around valves, flanges and anywhere
where 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 collect.
When siting a sensor head in such places it is most
important to ensure that there is no likelihood of flooding
by water, or excessive dust which may block the sintered
disc and prevent gas 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 thick layers and again cause a
dangerous hazard. When siting heads outside, prevailing
winds must be taken into consideration and adequate
protection provided to resist wind and rain.
For further information on gas densities relative
to air, select: GAS
DATA
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