A gas leak refers to an unintended leak of natural gas or another gaseous product from a pipeline or other containment into any area where the gas should not be present. Gas leaks can be hazardous to health and the environment. Even a small leak into a building or other confined space may gradually build up an explosive or lethal concentration of gas. Leaks of natural gas and refrigerant gas into the atmosphere are especially harmful due to their global warming potential and ozone depletion potential.
There are many types of gases which may cause acute and chronic health effects to humans. The gases usually enter the body through inhalation. In petroleum refineries and petrochemical units, harmful hydrocarbon, hydrogen sulphide, and benzene leaks can potentially occur. In thermal power plants, leaks of carbon dioxide and carbon monoxide from unmaintained furnaces and stacks are common. Incidents of chlorine gas leaking from water treatment plants, and ammonia gas leaking from fertiliser units have also been recorded. The harmful gases are classified based on the health impact they may cause, such as those which are toxic, poisonous, irritant, corrosive, and carcinogenic in nature.
As a proactive measure, detection of gas leaks with as much advance warning as possible reduces the severity of said leak and will greatly help with mitigation measures. Gas leak detection is the process of identifying potentially hazardous gas leaks by sensors. Additionally, a visual identification can be done using a thermal camera. These sensors usually employ an audible alarm to alert people when a dangerous gas has been detected.
Exposure to toxic gases can also occur in operations such as painting, fumigation, fuel filling, construction, excavation of contaminated soils, landfill operations, and entering confined spaces. A gas detector is a device that detects the presence of gases in an area, often as part of a safety system. This type of equipment is used to detect a gas leak or other emissions and can connect to a control system so a process can be automatically shut down. A gas detector can sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. This type of device is important because there are many gases that can be harmful to organic life, aka, humans and animals.
“in petroleum refineries and petrochemical units, harmful hydrocarbon, hydrogen sulphide, and benzene leaks can potentially occur”
The following aspects should be considered when planning leak and gas detection:
- Human factors • Objectives of detection systems
- Types of detectors required
- Maintenance of detectors
- Management of detector systems The following issues may contribute towards a major accident or hazard:
- Unrecognised high-risk areas, where detectors could be used
- No detectors or the wrong types in place in high risk areas
- Detectors incorrectly positioned and installed on site
- Poor level of maintenance and control of detection systems
- Reliance on ineffective detectors
Further contributory factors for an assessor to consider concerning leak and gas detection are as follows.
The appropriateness of the types of detectors being used; for example, UV detectors, IR detectors, smoke detectors, intrinsically safe detectors, heat detectors, specific substance detectors, or explosimeters, in terms of the environment in which they are located and their ability to perform the duty expected.
The effectiveness of using the detectors in terms of their positioning relative to the possible leak sources, taking account of dispersion and dilution of the released gases/vapours. The effectiveness of the detectors in terms of the types of substances to be detected must also be considered; for example, flammable substances, acid gases, smoke, explosive substances, toxic substances, as well as the concentrations required. Detectors may also be chosen that can react to more than one substance.
Types of devices
The types of protective devices linked to the detection systems must also be considered when assessing. These can include alarms, warning lights, reaction quenching systems, isolation systems, fire retardant systems, plant shutdown systems, trip devices, and emergency services.
The reliability of each detector will also come into question. This should cover the range of detection, response time of detection, level of maintenance, calibration frequency, performance testing frequency, and proof testing.
The detectors should be clearly seen, heard and understood, (with appropriate warning signs, lighting, and noise recognition), on plant, in the control room and off-site (if appropriate).
The procedures to respond to alarms, as a result of a leak/gas being detected (emergency evacuation plans, fire drills, risk assessing existing emergency evacuation plans), to confirm that the release has actually occurred and to record and investigate false alarms and take action to change the system to maintain the confidence of operators.
Level of risk
The level of risk associated with each potential leak source (risk assessments, risk-rating systems) and the reduction in that assessed risk value achieved by the use of detectors. The provision and
“from a risk-mitigation perspective, there is a hierarchy of risk that should be considered when designing a plant’s hazardous-area gas detection system”
accessibility (to operators, maintenance staff etc) of a sufficient site plan which maps all potentially hazardous areas (zones 0, 1 and 2, segregation of compatible hazardous substances).
Determining detector location
From a risk-mitigation perspective, there is a hierarchy of risk that should be considered when designing a plant’s hazardous-area gas detection system. There are a number of simple and quite often obvious considerations that help to determine detector location.
Weight versus air
To detect gases that are lighter than air, such as methane and ammonia, detectors should be mounted at a high level and preferably use a collecting cone. To detect gases that are heavier than air, such as butane and sulphur dioxide, detectors should be mounted at a low level.
Wind and weather
Consider how escaping gas may behave due to natural or forced air currents. Mount detectors in ventilation ducts if appropriate. When locating detectors consider the possible damage caused by natural events, e.g. rain or flooding. For detectors mounted outdoors it is preferable to use the weather protection assembly. Use a detector sunshade if locating a detector in a hot climate and in direct sun.
Consider the process conditions. Butane and ammonia, for instance, are normally heavier than air, but if released from a process line that is at an elevated temperature and/or under pressure, the gas may rise rather than fall. Detectors should be positioned a little way back from high pressure parts to allow gas clouds to form. Otherwise, any leak of gas is likely to pass by in a high-speed jet and not be detected.
Ease of access
Consider ease of access for functional testing and servicing. Detectors should be installed at the designated location with the detector pointing downwards. This ensures that dust or water will not collect on the front of the sensor and stop the gas entering the detector. When setting open path infrared devices, it is important to ensure that there is no permanent obscuration or blocking of the IR beam. Short term blockage from vehicles, site personnel, birds etc can be accommodated. Ensure the structures that open path devices are mounted to are sturdy and not susceptible to vibration.
Principles of gas detection
Modern combustible gas detectors have to be much more accurate, reliable and repeatable than this and although various attempts were made to overcome the safety lamp’s subjectiveness of measurement (by using a flame temperature sensor for instance), it has now been almost entirely superseded by more modern, electronic devices.
Today’s most commonly used device, the catalytic detector, is in some respects a modern development of the early flame safety lamp, since for its operation it also relies on the combustion of a gas and its conversion to carbon dioxide and water.
Nearly all modern, low-cost, combustible gas detection sensors are of the electrocatalytic type. They consist of a very small sensing element sometimes called a ‘bead’, a ‘Pellistor’, or a ‘Siegistor’. They are made of an electrically heated platinum wire coil, covered first with a ceramic base such as alumina and then with a final outer coating of palladium or rhodium catalyst dispersed in a substrate of thoria. This type of sensor operates on the principle that when a combustible gas/air mixture passes over the hot catalyst surface, combustion occurs and the heat involved increases 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 or some similar indicating device. To ensure temperature stability under varying ambient conditions, the best catalytic sensors use thermally matched beads. They are located in opposing arms of a Wheatstone bridge electrical circuit, where the ‘sensitive’ sensor (usually known as the ‘s’ sensor) will react to any combustible gases present, whilst a balancing, ‘inactive’ or ‘non-sensitive’ (n-s) sensor will not.
Inactive operation is achieved by either coating the bead with a film of glass or de-activating the catalyst so that it will act only as a compensator for any external temperature or humidity changes. A further improvement in stable operation can be achieved by the use of poison resistant sensors. These have better resistance to degradation by substances such as silicones, sulphur and lead compounds, which can rapidly de-activate (or ‘poison’) other types of catalytic sensor.
An electro-chemical sensor consists of at least two electrodes (a measuring electrode and counter electrode) which have electrical contact in two different ways: on the one hand via an electrical conductive medium called electrolyte (a pasty like liquid to transport ions); on the other hand via an outer electric current circuit (a simple copper wire to transport electrons). The electrodes are made of a special material which also has catalytic characteristics enabling certain chemical reactions to take place in the so-called 3-phase zone, where gas, solid catalyst and liquid electrolyte are present. The electron grabbing oxygen being needed for this reaction comes from the ambient air. Further electron grabbers are known, e.g. chlorine, fluorine, ozone and nitrogen dioxide. Thus, the current of sensors being used for these gases flows in reverse direction. The current can be measured by means of a microamp meter. Gas specific electrochemical sensors can be used to detect the majority of common toxic gases, including CO, H2S, Cl2 and SO2.
Infrared sensors can be used to measure hydrocarbon. When considering the broad range of flammable gases and vapours prevalent in the atmosphere, one will realise that most of these substances are chemical compounds primarily consisting of carbon, hydrogen, oxygen, and sometimes nitrogen. These so-called organic compounds are called
hydrocarbons. Hydrocarbons have special properties which can be used for infrared measurement of their concentration. All the gases absorb radiation in a characteristic manner, some even in the visible range (0.4 to 0.8 micrometres). This is why chlorine is green-yellow, bromine and nitrogen dioxide are brown-red, and iodine is violet, and so on. However, these colours can only be seen at rather high and lethal concentrations.
Hydrocarbons absorb radiation of a certain wavelength range, approx. at 3.3 to 3.5 micrometres, and since oxygen, nitrogen and argon do not absorb, this can be used for concentration measurement of hydrocarbons in air. An optical system containing a mixture of, for example, methane or propane in air will attenuate an incoming infrared intensity in a predictable way, and for a given gas this attenuation depends on only its concentration. Through air, infrared passes without being attenuated, with no reduced intensity, with no measuring signal. When it comes to gas, infrared passes by being attenuated with reduced intensity and the measuring signal corresponds to the current gas concentration. This photometer principle is the basis of an infrared measuring instrument. With the correlation of measured intensity reduction in the one hand and the gas concentration in the optical system on the other, this principle is defined in the calibration process. A defined gas concentration will always produce the same intensity reduction and thus always the same measuring signal. Most of the flammable gases and vapours are hydrocarbons which are almost always detectable by their characteristic infrared absorption.
The measuring principle is simple. Hydrocarbons absorb infrared radiation (IR) in the wavelength range of 3.3 to 3.5 micrometres (µm), more or less, depending on the absorption spectrum of the considered gas. However, the attenuation of the infrared radiation is very small and a challenge concerning the measuring technique.
Diffusion controlled sensors
In diffusion controlled sensors, the high velocity of the gas molecules causes the gases to expand quickly and also to quickly mix with other gases and never separate again. And as long as there are concentration differences in the whole mixture, the process of mixing is incomplete and does not come to an end. These concentration differences can also act as a micro pump. If the concentration difference is kept constant there will be a continuous flow of molecules into the direction of the lower concentration – and this effect is used for sensors in gas detection technology, the so-called diffusion controlled sensors.
Open path, or line-of-sight (LOS), gas detectors continuously monitor combustible hydrocarbon gas levels between two points at ranges of up to, or in some cases greater than, 120 metres. This detection technology uses a beam of light that travels between two modules. When a gas cloud passes through the beam, the gas concentration is measured. To ensure that the target gas passes through the beam, the modules must be strategically located and properly aligned. The modules themselves, however, need not be in the gas cloud for detection to occur.
Ultrasonic gas-leak detectors
Ultrasonic gas-leak detectors sense the high-frequency sound emitted by pressurised leaking gas. In some applications, acoustic gas detection is faster than other fixed gas-detection technologies because acoustic detectors do not have to wait for gas to contact them in order to “hear” a leak. Acoustic detectors are generally unaffected by rain, fog, wind or extreme temperatures, making them suitable for harsh outdoor environments. Along with these advantages, however, come some limitations. For example, acoustic detectors cannot distinguish specific gas types. Nor can they detect toxic parts per million concentrations or the lowest gas concentration capable of producing a flash of fire in the presence of an ignition source.
The thermal conductivity technique for detecting gas is suitable for the measurement of high (%V/V) concentrations of binary gas mixes. It is mainly used for detecting gases with a thermal conductivity much greater than air, for example, methane and hydrogen. Gases with thermal conductivities close to air cannot be detected, such as ammonia and carbon monoxide. Gases with thermal conductivities less than air are more difficult to detect as water vapour can cause interference, for example, carbon dioxide and butane. Mixtures of two gases in the absence of air 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.
Industrial processes increasingly involve the use and manufacture of highly dangerous substances, particularly flammable, toxic and oxygen gases. Inevitably, occasional escapes of gas occur, which create a potential hazard to the industrial plant, its employees and people living nearby. Worldwide incidents, involving asphyxiation,
explosions and loss of life, are a constant reminder of this problem.
In most industries, one of the key parts of any safety plan for reducing risks to personnel and plant is the use of earlywarning devices such as gas detectors. These can help to provide more time in which to take remedial or protective action. They can also be used as part of a total, integrated monitoring and safety system for an industrial plant.