Technical Notes Index

• Siting of Flammable Gas Sensors
  (TN1002)
• Point LEL Detectors
  (TN1001)

SITING OF FLAMMABLE GAS SENSORS - Technical Note
TN1002

COMBUSTIBLE GAS SENSORS : HOW MANY & WHERE
This technical note has been written to help users of Combustible Gases who intend to install a Gas Monitoring System to ensure Plant Safety.

Economics compels to reduce the number of Sensors. Reduction compromises safety. Proper site survey, good understanding of principles involved and objective analysis will lead to an effective & economical system.
Every detection system is based on the fact 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.
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 area 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 being true for vapours heavier than air; they will 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 any air currents.

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

• a) Source Detection/Point Detection, in which the sensors are located immediately adjacent to the likely sources of hazard
  
• b) Perimeter Detection, in which the sensors are located to surround the whole area or plant from which the hazard may arise.
  The approach adopted is dependent upon the size and nature of the site concerned and upon 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 gasleak 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.

1.1 FACTORS TO BE CONSIDERED

Each location of gas sensor must be considered individually. Among the considerations determining the number and locations of gas sensors are:-

• Density of Gas: Lighter than air gases obviously rise, e.g. Hydrogen, Methane, etc., or mixtures in which these predominate; hot gases with density close to air. Outdoors, these 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.
  i) Comparable-with-air gases 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.
  ii) Heavier-than-air gases, e.g. 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.
  Flammable mixtures are more rapidly formed when leakage velocities are low.

• b) Location of Sources of Leakage : 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.
  
• c) Location of Sources of Ignition : It is advisable to have sample points between the likely sources of gas and possible ignition sources.
  
• d) Special Geometrical Features : Pits, trenches, cable ducts, manholes, drains, etc. are obvious places where heavy gases can collect.
  
• e) Ventilation and Wind : 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.
  
• f) Volume to be monitored : 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.
  
• g) Miscellaneous Factors: Some additional factors that may establish temporary or permanent conditions that should be taken into account in the siting of gas detectors and sensors are given below :

Factors affecting the location of gas detectors and sensors

• 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

Do's and Don'ts

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 :

• a) The sensor should be protected from splashing/hosing down by water, or bombardment by grit or dirt.
  
• b) The sensor needs to be protected against winds or draughts.
  
• c) Always use a Splash Guard with dust filter to protect from strong winds & water splashes.
  
• d) The sensor should not be installed in close proximity to an uneven source of heat, although a moderately hot location may be acceptable.
  
• e) It must be installed with care. (For instance, it must not be forced onto a badly fitting junction box).
  
• f) 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.
  
• g) 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.
  
• h) As far as possible, stainless steel sensors should be used.
  
• i) Do not install Sensors in front or in close proximity of windows, ventilators, doors etc.
  
• j) Always install Sensor facing downward to protect the Sinter disc.
  
• k) Protect the Sensor by an Impact Guard if impact is likely.
  
• l) Do not subject the Sensor to vibration. Choose vibration free location.
  
• m) Calibrate all Sensors every 3 months and maintain record.
  
• n) Install Sensors after all construction, welding, painting operations are over.
  

TIP

Conduct a periodic survey of all possible gas accumulation pocket areas for a month and record these in a log book. This survey report will lead to proper siting of Sensors at most vulnerable points.
Survey should be conducted - 1) at different times of the day to account for variation caused by temperature and wind cycle during the day 2) during plant running & stopped (if not a continuous plant) conditions 3) at different times of a year to account for weather variation during the year.
Use an Intrinsically Safe Portable Explosive Meter with auto sampling facility to conduct above survey.

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This technical note is based on information obtained from reference literature, manufacturers' data and are our own experience.
Further information can be obtained on request.

Point LEL Detectors - Technical Note
- TN1001

This technical note has been written to put into proper perspective the fundamentals and the differences between the two main technologies used for LEL (Lower Explosive Limit) detection and measurement.

• Catalytic Combustion (CC) Sensor
• Semi Conductor (SC) Sensors

Catalytic Combustion (CC) Sensor

Detection Principal

When a combustible gas comes into contact with the gas sensor element, the gas is oxidized, even if its concentration is still below the lower explosive limit (LEL). The oxidation reaction generates heat, and temperature of the platinum wire rises. This temperature rise is directly proportional to the concentration of the gas, and since the resistance of the wire also changes in proportion to the rise in temperature, the gas concentration can be measured by using a bridge circuit to measure the difference in potential between the sensor element and temperature compensation element.

Features

• i) Output is proportional to the gas concentration, with a virtually linear relationship up to the LEL.
• ii) High precision and excellent reproducibility.
• iii) Virtually unaffected by ambient temperature and humidity.
• iv) Some Pellistors may be poisoned by silicons, sulfides and chlorides.

Semi Conductor (SC) Sensors

Detection Principal

The outside of the sensor is formed an n-semiconductor consisting of a metal oxide (SnO2) which has been aged by a long period of sintering. Inside this is a semiconductor sensor with a pair of palladium coils. A potential is applied to one of the coils, which also serves as a heater, and it is heated by the sensor current (joule heat) to approximately 350° C.

If power is supplied to the semiconductor in the presence of air, atmospheric oxygen is absorbed as negative ions onto the surface SnO2 molecules. The electrons inside the sensor are repelled by the negative ions on the surface, and are pushed towards the center, reducing the size of the channel. If a combustible gas such as H2 is then absorbed, it reacts with the oxygen on the surface, reducing the concentration of oxygen ions, and increasing the size of the channel so that the current can flow freely again.

Features

• i) Output is non-linear and gradient is small at 0-100% LEL range.
• ii) Very sensitive at ppm levels.
• iii) Responds to non-combustible gases and vapours also.
• iv) Output is affected by variation of temperature and humidity.
• v) Can go to sleep (loss of sensitivity) and recovery after exposure is very slow.

Comparison

CATALYTIC COMBUSTION SENSOR

• Only sensor which positively detects combustible gases. Other sensors infer the presence indirectly. Process-involved in detection detects ‘only and all’ combustible gases. So, it is a true LEL sensor.
• Oldest (60 Years), most understood and most used all over the world. Manufacturing process and specifications are fairly standard amongst different manufacturers. Inhibiting and poisoning agents are well known and poison-resistant sensor are also available. Very large number of manufacturers and suppliers available all over the world.
• Used only for LEL detection and measurement. It is an application specific product
  
• Expected working life varies between 3 to 10 years depending upon conditions of use. This technology has worked in large varies of applications worldwide.

SEMI CONDUCTOR SENSORS

• It depends upon the reducing property of combustible gases. So, it responds to all reducing vapours including commonly prevailing toxic gases like CO, CO2, smoke and even humidity (H2O)
  
• Relatively new (20 years), immature, not well tested and documented. Manufacturing process and specifications vary largely amongst different manufacturers. First development took place in Japan where it is largely used for domestic LPG alarms. Very few suppliers worldwide.
  General technology. Different materials and processes make sensor selectively respond to different gases. So manufacturing process is complex and inconsistent and hence unreliable.
  
• Expected life is claimed to be between 5 to 10 years but lack of variety and length of experience of this technology and diverse claims by different manufacturers demands caution and further scrutiny.

Do's and Don'ts

• Look for missing specifications while comparing Sensors of different makes.
  
• Select a company with good application knowledge. Most of the problems are due to improper system design, incomplete application understanding and not because of sensor.
  
• Distinguish between failures of sensor and of measurement system. Sensor, though vulnerable, is a standard component and is well understood. It is generally made a convenient scapegoat as the system supplier has no value addition in this component.
  
• Choice of Sensor is governed by application also. Don't choose a Sensor based on tall claims or lack of knowledge.
  
• Educate yourself. Half knowledge makes the purchaser vulnerable. Or, hire an advisor.
  
• Choose the right Sensor. Neither over specify nor under specify your need.
  
• Choose the right quantity & location. (Ask for our Technical Note on ‘Siting of Sensors’)
  
• Install a standard Transmitter with Sensor to ensure System compatibility with Sensors of different makes.
  
• Understand and implement maintenance needs of the Sensor.
  
• Install the Sensor after all construction, welding & painting processes are over.
  
• Calibrate all Sensors every 3 months and maintain record.
  
• Use Semiconductor Sensor for Alarm Only application and use CC Sensor for monitoring and measurement with Alarm application.
  
• Protect the Sensor from water, splashes, winds, droughts, impact, vibration.
  
• Sensors may fail to Safe Condition, hence regular sensor checks must be performed. Use standard Test Kits and Calibration Kits.

We invite your comments on the content and style. Please register yourself in our mailing list to receive further issues in this series. This technical note is based on information obtained from reference literature, manufacturers' data and are our own experience. Further information can be obtained on request.

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