MSA Senscient ELDS 1000 series, Руководство по эксплуатации

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Installation Design & Engineering
32
Senscient ELDS™
Rev 15
GB
3.2. Siting and Mounting
3.2.1 General
When designing an installation for a Senscient ELDS™ OPGD it is important to give consideration
to where it is to be located with respect to gas leak hazards, what potential sources of problems
may be encountered in this location; and how the unit is to be mounted and supported.
3.2.2 Location for Best Coverage
Guidance on the positioning of gas detectors to provide the best detection coverage is contained
in national Codes of Practice. It is recommended that the installation designer consults these
Codes of Practice. Senscient would advise that the best current methodology for siting all types of
fixed gas detectors is based upon the expert use of gas dispersion modelling, performed for the
particular area(s) of plant or facility where potential gas leak hazards exist. There are a number of
organisations with the gas dispersion modelling tools and expertise necessary to perform this
work. In general, for OPGDs the following positions usually provide the best results:

Running parallel to the physical perimeter of an area of plant or facility containing potential gas
leak hazards.

Forming a continuous ‘ring-fence’ surrounding an area of plant or facility containing potential
gas leak hazards.

At sufficient distance from any potential leak sources for dispersion to produce a gas cloud of a
size that will reliably be intercepted by the beam-paths of the OPGDs employed.

Between potential leak sources and any known or likely sources of ignition.

For natural gas: beam-path parallel to the ground / floor and above the height of most of the
valves and flanges in the vicinity.

For toxic gases: beam-path parallel to the ground / floor at ‘breathing height’ (~ 1.4m - 1.7m).

For gas mixtures significantly denser than air: beam-path parallel to the ground / floor and
below all potential leak sources*.

For gas mixtures significantly lighter than air: beam-path parallel to the ground / floor and
above all potential leak sources*.
* In most instances the hazardous gases leaking from a plant or facility are mixtures of a
number of gases with different chemical and physical properties. In a gas mixture the
constituent gases will retain most of their chemical properties, but the physical properties of
the gas mixture will approximate the sum of the physical properties of the constituent
gases.
For sour natural gas, the above principle means that this gas mixture will be both toxic and
flammable; whilst the density of this mixture will tend to be dictated by its methane content.
Only accurate gas dispersion modelling will determine the precise physical properties of a
leak of sour natural gas, but typically this mixture’s density will be similar to, or even slightly
lower than that of air. Only leaks of very cold natural gas or of neat hydrogen sulfide are
likely to be slightly denser than air.
CAUTION
Dispersion modelling provides little support for the practice of placing hydrogen sulfide detectors
close to the ground. In the majority of instances leaks of sour natural gas or hydrogen sulfide will
not sink; whilst the toxicity hazard is greatest at the height where such gas can be breathed in.