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Delving into the underlying principles of gas detection
Ever wondered what makes your gas detector function? Declan Chukwuma Umege educates us on the principles of gas detector actions and their industrial applications.
Flammable, toxic and oxygen enriched atmospheres create potential hazards for employees, plant, assets, the environment and the reputations of companies. The use of early warning devices such as gas detectors gives crucial extra time to implement remedial or protective actions. Constant monitoring systems should, therefore, be integrated into every plant or process.
Hazards
Specific to each gas, there is a limited band of gas concentration that will produce a combustible mixture. At levels below the lower explosive limit (LEL) there is insufficient gas to produce an explosion, while at levels above the upper explosive limit (UEL) the mixture has insufficient oxygen to combust. An increase in pressure, temperature or oxygen content will generally broaden the flammability range.
Some gases can be dangerous to life at very low concentrations. The measurements most often used for the concentration of toxic gases are parts per million (ppm) and parts per billion (ppb).
More people die from toxic gas exposure than from explosions caused by the ignition of flammable gas.
We all need oxygen to live, but too much or too little can prove fatal. Normal ambient air contains 20.9% oxygen. When this dips below 19.5% the air is considered oxygen deficient, with concentrations below 16% considered unsafe for humans. Conversely, at increased oxygen levels the flammability of materials and gases increases. At 24% items such as clothing can spontaneously combust.
Areas requiring detection include: The oil and gas sector Semiconductor manufacturing Chemical plants Power stations Waste water treatment plants Boiler rooms Tunnels
Detection principles
The following sections outline the various gas detection types available.
Electro-catalytic
Nearly all modern, low cost combustible gas detection sensors are electro-catalytic. They consist of a very small sensing element sometimes called a bead. They are made of an electrically heated platinum wire coil, covered with a ceramic base and an outer coating of a palladium or rhodium catalyst dispersed in a substrate of thoria.
This type of sensor operates on the principle that when a combustible gas mixture passes over the hot catalyst surface, combustion will occur and the heat created will increase the temperature of the bead. This in turn alters the resistance of the platinum coil and can be measured by using the coil as a temperature thermometer in a standard electrical bridge circuit. The resistance change is then directly related to the gas concentration in the surrounding atmosphere and can be displayed on a meter.
Semiconductor sensor
In the same way as catalytic sensors, semiconductor sensors operate by virtue of gas absorption at the surface of a heated oxide. In fact, this is a thin metal-oxide film deposited on a silicon slice by much the same process as is used in the manufacture of computer ‘chips’.
Absorption of the sample gas on the oxide surface, followed by catalytic oxidation, results in a change in electrical resistance of the oxide material and can be related to the sample gas concentration. The surface of the sensor is heated to a constant temperature of about 200-250°C to speed up the rate of reaction and to reduce the effects of ambient temperature changes.
Semiconductor sensors are simple, fairly robust and can be highly sensitive. They have been used with some success in the detection of hydrogen sulfide gas and they are also widely used in the manufacture of inexpensive domestic gas detectors; however, they have been found to be rather unreliable for industrial applications, since they are not specific to a particular gas and can be affected by atmospheric temperature and humidity variations.
They also need to be checked more often than other types of sensor, as they have been known to lose sensitivity unless regularly checked with a gas mixture. In addition, they are slow to respond and recover after exposure to an outburst of gas.
Thermal conductivity
Thermal conductivity is suitable for the measurement of high percentage volume (%V/V) concentrations of binary gas mixes. It is mainly used for detecting gases with a thermal conductivity much greater than air, such as methane and hydrogen. Gases with thermal conductivities close to air, such as ammonia and carbon monoxide, cannot be detected. Gases with thermal conductivities less than air, such as carbon dioxide and butane, are more difficult to detect as water vapour can cause interference. In the absence of air, mixtures of two gases can also be measured using this technique.
The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment. If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. If the thermal conductivity of the sample gas is less than that of the reference then the temperature of the sample element increases. These temperature changes are proportional to the concentration of gas present at the sample element.
Infrared
Many combustible gases have absorption bands in the infrared (IR) region of the electromagnetic spectrum of light. The principle of infrared absorption has been used as a laboratory analytical tool for many years, but since the 1980s electronic and optical advances have made it possible to design equipment of sufficiently low power and smaller size to make this technique available for industrial gas detection products as well.
These sensors have a number of important advantages over the catalytic style. They include a very fast speed of response, typically less than 10 seconds, as well as low maintenance and greatly simplified checking, using the self-checking facility of modern micro-processor controlled equipment. They can also be designed to be unaffected by any known poisons, they have a failsafe and will operate successfully in inert atmospheres and under a wide range of ambient temperatures, pressures and humidity conditions.
The technique operates on the principle of dual wavelength IR absorption. Light passes through the sample mixture at two wavelengths, one of which is set at the absorption peak of the gas to be detected, while the other is not. The two light sources are pulsed alternately and guided along a common optical path to emerge via a flameproof ‘window’ and then through the sample gas. The beams are subsequently reflected back again by a retro-reflector, returning once more through the sample and into the unit. Here a detector compares the signal strengths of sample and reference beams and, by subtraction, can give a measure of the gas concentration.
This type of detector can only detect diatomic gas molecules and is therefore unsuitable for the detection of hydrogen.
Open path flammable infrared
Traditionally, the conventional method of detecting gas leaks was by point detection, using a number of individual sensors to cover an area or perimeter. More recently, however, instruments have become available which make use of infrared and laser technology in the form of a broad beam, or open path, which can cover a distance of several hundred metres. Early open path designs were typically used to complement point detection, however the latest third generation instruments are now often being used as the primary method of detection.
Typical applications that have had considerable success include pipelines, perimeter monitoring, offshore platforms and liquid natural gas (LNG) storage areas.
Open path detectors actually measure the total number of gas molecules, i.e. the quantity of gas, within the beam. This value is different to the usual concentration of gas given at a single point and is therefore expressed in terms of LEL values.
Open path toxic infrared
Optical open path and point detection of flammable gas is now well established and has been widely accepted in the petrochemical industry, where it has proven to be a viable and reliable measurement technology. The main challenge in adapting this technology to measure toxic gases is that very low levels of gas must be reliably measured. Typically, flammable gases need to be measured at percentage levels of concentration. Average toxic gases, however, are dangerous at parts per million (ppm) levels, i.e. a factor of 1,000 times lower than for flammable gas detection.
To achieve these very low sensitivities it is not possible to simply adapt the technology used in open path flammable IR gas detectors. Open path toxic IR detectors need to utilise a different measurement principle, in which the instrument probes individual gas lines as opposed to a broad spectral range. This is facilitated by the use of a laser diode light source.
The output of the laser is effectively one single wavelength and so no light is wasted and all of the light emitted is subjected to absorption by the target toxic gas. This provides a significant enhancement of sensitivity compared with open path flammable gas detection techniques. Further enhancements are achieved by the use of sophisticated modulation techniques.
Electrochemical sensor
Electrochemical sensors can be used in a wide variety of safety applications as they are compact, require very little power, exhibit excellent linearity and repeatability and generally have a long life span, typically one to three years.
Commercial designs of electrochemical cells are numerous, but share many of the common features described below.
Three active gas diffusion electrodes are immersed in a common electrolyte, frequently a concentrated aqueous acid or salt solution, for efficient conduction of ions between the working and counter electrodes.
Depending on the specific cell the target gas is either oxidised or reduced at the surface of the working electrode. This reaction alters the potential of the working electrode relative to the reference electrode. The primary function of the associated electronic driver circuit connected to the cell is to minimise this potential difference by passing current between the working and counter electrodes, the measured current being proportional to the target gas concentration. Gas enters the cell through an external diffusion barrier that is porous to gas, but impermeable to liquid.
Optical scanning system
This is based on the use of an absorbent strip of filter paper acting as a dry reaction substrate. This performs both as a gas collecting and gas analysing media and it can be used in continuous operation. The system is based on classic colourimetry techniques and is capable of extremely low detection limits for a specific gas.
It can be used very successfully for a wide variety of highly toxic substances, including di-isocyanates, phosgene, chlorine, fluorine and a number of the hydride gases employed in the manufacture of semiconductors.
Detection specificity and sensitivity are achieved through the use of specially formulated chemical reagents, which react only with the sample gas or gases. As sample gas molecules are drawn through the system with a vacuum pump, they react with the dry chemical reagents and form a coloured stain specific to that gas only. The intensity of this stain is proportionate to the concentration of the reactant gas, so the higher the gas concentration, the darker the stain. By carefully regulating both the sampling interval and the flow rate at which the sample is presented to the system, detection levels as low as ppb can be readily achieved.
Fixed or portable
Portable instruments
Generally, portable gas detectors are compact, robust, waterproof, lightweight and can be easily carried or attached to clothing. They are also useful for locating the exact point of a leak that was first detected with a fixed system.
Portable gas detectors are available as single or multi gas units. The single gas units contain one sensor for the detection of a specific gas, while multi gas units usually contain up to four different gas sensors, typically for oxygen, flammability, carbon monoxide and hydrogen sulfide.
Products range from simple alarm-only disposable units, to advanced fully configurable and serviceable instruments with features such as datalogging, internal pump sampling, auto calibration routines and connectivity to other units.
Recent portable gas detector design advances include the use of more robust and lightweight materials for their construction. The use of high power microprocessors enables data processing for instrument self-checking, running operating software, data storage, and auto calibration routines. In addition, modular designs allow simple routine servicing and maintenance. New battery technology has provided extended operating time between charges in a smaller and more lightweight package.
Future designs are likely to see the integration of other technologies such as GPS, bluetooth and voice communication, as well as the incorporation of gas detection into other safety equipment.
Fixed point systems
The most common method employed to continuously monitor for leakage of hazardous gases is to place a number of sensors at the places where leaks are most likely to occur.
These are often then connected electrically to a multi channel controller located some distance away in a safe, gas free area with display and alarm facilities and event recording devices. This is often referred to as a fixed point system. As its name implies, it is permanently located in the area, for example, an offshore platform or oil refinery.
The complexity of any gas detection system depends on the use to which the data will be put. Data recording allows the information to be used to identify problem areas and assist in the implementation of safety measures. If the system is to be used for warnings only, then the outputs from the system can be simple and no data storage is necessary. In choosing a system, therefore, it is important to know how the information will be used so that the proper system components can be chosen.
In toxic gas monitoring, the use of multi point systems has rapidly demonstrated their potential for solving a wide variety of workplace exposure problems and is invaluable for both identifying problems and for keeping workers and management aware of pollutant concentrations in the workplace.
In the design of multi point systems, considerable thought should be given to the various components and their interconnection. When using catalytic detection sensors, for example, the electrical cable connections to the sensors have three cores, each 1mm2, carrying not only the output signal, but also power to the electrical bridge circuit, which is located at the sensor to reduce signal voltage drop along the cables.
In the case of toxic gas monitoring systems, the atmosphere is often sampled at locations remote from the unit and the gases are drawn by pumps to the sensors through a number of synthetic material, narrow bore tubes. Care in design of such systems will include selection of suitable sized pumps and tubes, a sequential sampling unit for sampling each tube in turn and filters to stop particulates or water cutting off the flow of gas.
The bore size of tubing can be critical, since it needs to be large enough to allow rapid response times for standard size pumps, while at the same time should not be so large as to allow excessive dilution of the sample by air. Each sampling point must be connected to a separate tube and if a number of points are connected to a single, central sensor, it will be necessary to purge the sensor with clean air between samples.
The controllers used in fixed systems can be centrally located or distributed at various locations in a facility according to the application requirements. They come in a control panel as either a single channel or multi-channel configuration, the latter being useful where power, space or cost limitations are important.
Control units include a front panel meter or display to indicate the gas concentration at each sensor and will also normally have internal relays to control functions such as alarm, fault and shutdown. The number of alarm levels available varies between controllers, but typically up to three levels can be set, depending on statutory requirements or working practices within the industry. Other useful features include alarm inhibit and reset, over-range indication and analog 4-20mA outputs.
It is important to remember that the main purpose of a gas detection system is to detect the build up of a gas concentration before it reaches a hazardous level and to initiate a mitigation process to prevent a hazard occurring. If the gas concentration continues towards a hazardous level then executive shut down and hazard warning alarms are initiated. It is not enough to just log the event or measure the gas levels to which personnel have been exposed.
Published: 12th Aug 2014 in Health and Safety Middle East
