Economics compels to reduce the number of Sensors. Reduction compromises safety. There is an optimum level which ensures effective safety economically.  Proper site survey, good understanding of principles involved and objective analysis will lead to an effective & economical system. Every detection system or sensor requires that the flammable gas or vapour must get from its source to the sensor. If you could afford an infinite number of sensors, there would be no problems with detection – at the time the gas concentration reached a dangerous level anywhere in the plant, an alarm would be initiated. But is it necessary to be so conservative?

There are no set guidelines as to the proper number of sensors in any given area. Unlike fire detection, where a detection area can be used e.g. 100 Sq. m.  The most important guides for gas sensing or coverage area of a sensor are the dispersion characteristics of the gas, air movements, probability of release of gas, likely sources of leakage, structural considerations and economic factors.

Gas and vapours will disperse into the atmosphere based on their properties. The diffusion rate of the gas into the air is inversely proportional to its density. Gases with a density less than air will diffuse very quickly until the gas becomes diluted. The reverse is true for vapours heavier than air. These tend to settle on the ground level. Gases with a density close to that of air will exhibit little mixing effect and will generally be transported by surrounding air currents.

Generally, one, or a combination, of the following approaches to location of sensors and sampling points should be used

• Source Detection/Point Detection in which the sensors are located immediately adjacent to the likely sources of hazard
• Perimeter Detection, in which the sensors are located to surround the whole area or plant from which the hazard may arise.

The approach adopted depends  on the size and nature of the site concerned and on the degree of protection and speed of response required.

While Source Detection in outdoor sites may provide an early warning of a gas leak in still air, it provides no warning at all if the wind direction is such as to blow the leaked gas away from the appropriate detector. On the other hand, Perimeter Detection alone may give a delayed warning of a gas leak in still air, furthermore, Perimeter Detection alone would need to be extensive in order to provide effective protection for large outdoor sites, such as petrochemical plants. Thus a combination of Source and Perimeter Detection should be used for large outdoor sites and Source Detection alone should be used for small enclosed sites.

These obviously rise, e.g. Hydrogen, Methane, etc., or mixtures in which these predominate; hot gases with density close to air. Outdoors – these gases present little hazard, as they are soon swept away.  Indoors – however, or in restricted areas, such as tanks, basements, mines or sewers, they can form hazardous pockets or layers beneath ceilings or roofs. The sample point should therefore be near the ceiling unless this is so high that the gas will be diluted. As a general rule, it should not be more than about 3 metres above the hazard. Account should be taken of warmer air layering at ceiling level.

These will spread in all directions from the leakage source e.g. Ethylene, Ethane, Acetylene, Methanol Vapour, etc. or low concentrations of gases or vapours already partly mixed in the air. Once gases are mixed they stay mixed unless one component is removed by condensation, selective absorption or chemical action.

These gases obviously fall/flow down. Gases like  Butane and Propane (LPG), practically all liquid vapours and mixtures in which such substances predominate, will fall, and can be held back by walls, trays, etc. in a manner similar to liquids. These present an outdoor hazard too, as a pocket of vapour can be ignited by such causes as a spark from a lorry, train, cigarette stub, etc.

The positioning of sensors for the detection of such heavy gases (or individual components in them) should be 150-450 mm from the floor or lowest surface. In fixing the position, care should be taken to ensure that the sensors are not exposed to flooding by water or excessive dust from the floor.

These will normally be couplings, flanges, pump glands, etc or any point where a connection is made and broken. Siting a sample point close to a probable leak source has the advantage that leaks will be detected quickly. However, in large installations this would require too large a number of points. Where there are a number of rooms or compartments to be monitored each will need at least one point.

If the gas leaks under a high pressure, areas of high gas concentration and vacuum pockets should be avoided. In such cases, sensors should be located at such level that gas dilutes to combustible or below combustibility levels. Gas must be within LEL levels to be detected.

It is advisable to have sample points between the likely sources of gas and possible ignition sources.

Pits, trenches, cable ducts, manholes, drains, etc. are obvious places where heavy gases can collect.

Good ventilation can help to minimise the build-up of toxic and/or explosive hazards. In most cases hazards can be reduced by a combination of ventilation and gas detection. Exhaust ducts (or locations close to them) are good points to monitor the general level in such an area. For outside locations prevailing winds should be considered. Combustible Gas Sensors should be shielded from air blowing directly onto the sintered discs, as flows of 5m/sec. and above can cause errors through differential cooling.

It is difficult to give a rule of thumb. In a room with good ventilation or with reliable agitation of the atmosphere a single point could cover up to 10m x 10m x 3m or even more. However, in stagnant air conditions, a single point could only cover a much smaller volume. Outside, it is normally a question of spacing detectors around an installation with, if anything, a bias based on the prevailing wind.

The speed with which a dangerous concentration could build up is also relevant to the number of sensors required. Flammable mixtures are more rapidly formed when leakage velocities are low.

• Ambient temperature and the effects of temperature on the density of the gas : Example : Heating or cooling systems
• Exhaust air blowing into a room or factory space: Example : Air cooled equipment, ventilation and air conditioning systems.
• Stagnant air pockets : Example : Roofs and ceilings of unusual shape and special construction, deep beams
• Chimney effects: Example : Tall buildings, lift shafts, staircases
• Airborne contaminants, e.g. grease, steam : Example : Plant rooms, well heads, furnaces
• Gas expansion causing a change in density of the gas :High pressure gas leaks

• The sensors, whilst designed to be robust, must be treated with care to ensure reliable service. Not only must they be correctly located with respect to likely sources of gas and vapour leaks, but the installation should also be carried out in accordance with the following principles:
• The sensor should be protected from splashing/hosing down by water, or bombardment by grit or dirt.
• The sensor needs to be protected against winds or draughts.
• Always use a Splash Guard with dust filter to protect from strong winds & water splashes.
• It must be installed with care. (For instance, it must not be forced onto a badly fitting junction box).
• Access should be allowed to the sensor for fitting the adaptor for calibration checks. Consideration must also be given to personnel access for performing such checks. If this is impossible a suction type system should be considered.
• Silicone lubricants or contact cleaners containing Silicones should be avoided in the vicinity of combustible gas sensors. Great care must be taken that the sensor does not get coated or painted in any way. Such materials can affect the performance of the sensors or make them impervious to the gases such that they will not respond.
• As far as possible, stainless steel sensors should be used.
• Always install Sensor facing downward to protect the Sinter disc.
• Protect the Sensor by an Impact Guard if impact is likely.
• Calibrate all Sensors every 3 months and maintain record.
• Install Sensors after all construction, welding, painting operations are over.
• The sensor should not be installed in close proximity to an uneven source of heat, although a moderately hot location may be acceptable.
• Do not install Sensors in front or in close proximity of windows, ventilators, doors etc.
• Do not subject the Sensor to vibration. Choose vibration free location.