Classification of areas - Explosive gas atmospheres Classification of areas - Combustible dust atmospheres Equipment protection by pressurized room "p" IEC 60079-29-1 mod 2007 Explosive
Gas properties and other physics
3.1.1 ambient air normal atmosphere surrounding the equipment
Clean air refers to air that is devoid of gases or vapors, including flammable, toxic, or environmentally harmful substances, which could affect the sensitivity and performance of equipment.
3.1.3 concentration amount of the gas or vapour of interest in a specified amount of the background gas or air, expressed in suitable units
Typical units for measuring concentrations include volume fraction (v/v), molar (moles per mole – m/m), percentage of the lower flammability limit (LFL) of a specific substance, as well as parts per million (ppm) and parts per billion (ppb) by volume.
3.1.4 dose total amount of substance absorbed or trapped, proportional to the concentration and the duration of exposure
3.1.5 explosive gas atmosphere mixture with air, under normal atmospheric conditions, of flammable substances in the form of gas or vapour, which, after ignition, permits self-sustaining flame propagation
3.1.6 explosive range range of gas or vapour mixtures with air, with concentrations between the lower and upper explosive (flammable) limits
3.1.7 firedamp flammable mixture of gases, consisting mainly of methane, naturally occurring in a mine
3.1.8 flammable gas gas or vapour which, when mixed with air in a certain proportion, will form an explosive atmosphere
Note 1 to entry: For the purposes of this standard, the term "flammable gas" includes flammable vapour
3.1.9 flashpoint lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture
LFL volume fraction of flammable gas or vapour in air below which an explosive gas atmosphere does not form, expressed as a percentage (see IEC 60079-20-1)
Note 1 to entry: This is also known as lower explosive limit (LEL)
3.1.11 optical radiation ultra-violet, visible or infrared regions of the electromagnetic spectrum
3.1.12 relative density density of gas or vapour relative to the density of air at the same pressure and at the same temperature
Note 1 to entry: Air is equal to 1,0
3.1.13 release rate quantity of flammable gas or vapour emitted per unit time from the source of release which itself could be a liquid surface
A source of release refers to the specific point or location where flammable gas, vapor, or liquid can escape into the atmosphere, potentially creating an explosive gas atmosphere.
[SOURCE: IEC 60050-426:2008 426-03-06, modified by removal of Note]
3.1.15 toxic gas gas that may be harmful to human health and/or the performance of persons due to its physical or physico-chemical properties
UFL volume fraction of flammable gas or vapour in air above which an explosive gas atmosphere does not form, expressed as a percentage (see IEC 60079-20-1)
Note 1 to entry: This is also known as upper explosive limit (UEL)
3.1.17 vapour gaseous state of a substance that can reach equilibrium with its liquid or solid state in the temperature and pressure range of interest
This standard simplifies the scientific definition by stating that the substance must remain below its boiling or sublimation point under ambient temperature and pressure conditions.
3.1.18 ventilation movements of air and replacement with fresh air due to the effects of wind, temperature gradients or artificial means (for example fans or extractors)
3.1.19 volume fraction v/v ratio of the volume of a component gas to the volume of the gas mixture under specified conditions of temperature and pressure
Note 1 to entry: This is also known as volume ratio.
Types of equipment
3.2.1 alarm only equipment equipment which generates an alarm signal but does not have a meter or output giving a measure of the integral concentration
3.2.2 aspirated equipment equipment that samples the gas by drawing it to the gas sensor – for example by means of a hand-operated or electric pump
3.2.3 continuous duty equipment gas detecting equipment that is powered for long periods of time, but may have either continuous or intermittent sensing
Diffusion equipment facilitates the transfer of gas from the atmosphere to the gas sensor through random molecular movement, operating without any aspirated flow.
3.2.5 explosion protected equipment equipment incorporating a type of explosion protection covered by the IEC 60079 series of standards
3.2.6 explosion protection measures applied in the construction of equipment to prevent ignition of a surrounding flammable atmosphere by the equipment
3.2.7 fixed equipment equipment that is intended to have all its parts permanently installed
3.2.8 group I equipment equipment for mines susceptible to firedamp
3.2.9 group II equipment electrical equipment for use in places with an explosive gas atmosphere other than mines susceptible to firedamp
3.2.10 portable equipment spot reading or continuous duty equipment that has been designed to be readily carried from place to place and to be used while it is being carried
A portable equipment is defined as battery-powered devices that include various types of tools and monitors This category encompasses hand-held equipment weighing less than 1 kg, designed for one-handed operation Additionally, it includes personal monitors that are similar in size and weight to hand-held devices, which operate continuously while attached to the user Furthermore, larger equipment that can be carried by hand, with a shoulder strap, or a carrying harness is also considered portable, and may feature a hand-directed probe.
3.2.11 sample (sampling) system equipment which typically draws more than one sample, conditions them as necessary, and presents them to a sensor in aspirated equipment
Note 1 to entry: This usually refers to fixed equipment where multiple samples from different detection points are presented sequentially to one or more sensors;
3.2.12 spot reading equipment equipment intended to be used for short, intermittent or irregular periods of time as required (typically 5 min or less)
3.2.13 transportable equipment equipment not intended to be portable, but which can be readily moved from one place to another
Sensors and detectors
3.3.1 catalytic sensor sensor, the operation of which depends upon the oxidation (combustion) of gases on an electrically heated catalytic element
Electrochemical sensors operate by detecting changes in the electrical parameters of electrodes immersed in an electrolyte, which occur as a result of redox reactions involving gases on the electrode surfaces.
FID sensor, the operation of which depends upon the ionization of the gas being detected in a hydrogen flame
FTA sensor, the operation of which depends upon the change of temperature of a flame by the gas being detected
3.3.5 infrared absorption sensor sensor, the operation of which depends upon the absorption of infrared radiation by the gas being detected
3.3.6 paramagnetic oxygen detector sensor, the operation of which depends upon the magnetic properties of oxygen
PID sensor, the operation of which is based on the ionisation of gaseous compounds by ultraviolet (UV) radiation
3.3.8 remote sensor sensor that is not integral with the main body of the equipment
3.3.9 semiconductor sensor sensor, the operation of which depends upon changes of the electrical conductance of a semi- conductor due to chemisorption of the gas being detected at its surface
3.3.10 sensor assembly in which the sensing element is housed that may also contain associated circuit components
The sensing element is a crucial component of a sensor that responds to the presence of flammable gas mixtures It generates a physical or chemical change, which can be utilized to trigger either a measuring function or an alarm, or both.
3.3.12 single point sensor sensor capable of detecting gas at a single point location
A thermal conductivity sensor operates by measuring the change in heat loss through conduction from an electrically heated element in the gas being analyzed, in comparison to a similar element situated in a reference gas cell.
Supply of gas to instruments
3.4.1 measuring point measurement point location of a single sensor aspirated by diffusion or by a probe
3.4.2 point detection equipment detection equipment located at a measurement point
Note 1 to entry: This normally refers to a complete single point sensor, rather than a sampling point, and is in contrast with open path equipment
3.4.3 sample line means by which the gas being sampled is conveyed to the sensor including accessories
3.4.4 sampling point point from which sample is taken in an aspirated equipment
Note 1 to entry: Typically this term is applied to fixed aspirated systems
Note 2 to entry: Typically the equipment at this point will consist of a filter, but there may be other sample conditioning equipment as well
3.4.5 sampling probe separate sample line which is attached to the equipment as required, that may or may not be supplied with the equipment
Note 1 to entry: It is usually short (e.g in the order of 1 m) and rigid (although it may be telescopic), but it may be connected by a flexible tube to the equipment.
Signals and alarms
The alarm setpoint is a predetermined or adjustable setting on the equipment that defines the concentration level at which the system will automatically trigger an indication, alarm, or other output functions.
3.5.2 alarm signal audible, visual, electronic or other signal generated by the equipment when an integral concentration of gas in excess of a preset value is detected
3.5.3 continuous or quasi-continuous sensing mode of operation in which power is applied continuously to the sensing element and readings are taken continuously or at regular and frequent intervals
3.5.4 fault signal audible, visual, or other type of output which provides, directly or indirectly, a warning or indication that the equipment is defective or out of adjustment
3.5.5 indicating devices means for displaying values or states in analogue or digital form
3.5.6 inhibition signal audible, visual, or other type of output which provides, directly or indirectly, a warning or indication that normal operation has been suspended
3.5.7 intermittent sensing mode of operation in which the power or flow to the sensor is applied intermittently according to a predetermined cycle and readings taken at the predetermined cycle
3.5.8 latching alarm alarm which, once activated, requires a deliberate action to deactivate it
3.5.9 safety function function (inclusive from gas sampling to signal output of the gas detection equipment) to enable the overall system to achieve a safe state.
Times, checks and equipment behaviour
Drift variation over time refers to the changes in the readings produced by equipment when monitoring a consistent gas volume fraction or a stable concentration distribution, including clean air, under constant ambient conditions.
3.6.2 functional check application of test gas or other means of obtaining a response from the sensor to check its function
Note 1 to entry: This may include the generation of an alarm This check is performed without adjustments of sensitivity
Note 2 to entry: This is also known as a “response check” or “bump test”
3.6.3 initial calibration first calibration for a specific substance, measuring range and application carried out by the manufacturer before delivery, or on site, before starting the operation
substances which lead to temporary or permanent loss of sensitivity of the sensors
Recalibration involves periodic checks and adjustments of the sensor's zero signal and sensitivity using a known calibration gas mixture This process ensures that the parameters, type of gas, measuring range, and specific application established during the initial calibration remain unchanged.
3.6.6 recovery time time interval between the time when an instantaneous decrease in gas concentration is produced at the sensor input and the time when the response reaches a stated indication
3.6.7 selectivity response of the equipment to the gas of interest compared with the response to other gases
Note 1 to entry: If there is high selectivity to the target gas, the results will be less ambiguous and the cross- sensitivity to other gases will be low
3.6.8 sensitivity ratio of change produced in the equipment by a known concentration of gas or vapour
Note 1 to entry: Depending on context, this can refer to the minimum change in concentration of gas or vapour that the equipment will detect
Note 2 to entry: High sensitivity implies that low concentrations can be measured
3.6.9 span reading on the normal test gas of the equipment
The response time, denoted as tx time interval, refers to the duration measured under warmed-up conditions It is defined as the interval between the moment an instantaneous switch occurs between clean air and the standard test gas at the equipment inlet, and the point at which the response achieves a specified percentage (x) of the stabilized signal for the standard test gas.
Note 1 to entry: Not applicable to spot reading equipment
3.6.11 zero gas gas recommended by the manufacturer which is free of flammable gases, and interfering and contaminating substances, the purpose of which is calibration/adjustment of the equipment zero
Terms exclusive to open path equipment
3.7.1 albedo proportion of incident light scattered back from a surface
A beam blocked signal is an audible, visual, or alternative output that alerts users when the optical path is obstructed or when the detected signal is insufficient for the equipment to operate properly.
3.7.3 gas calibration cell sealed enclosure with transparent ends which can be filled with test gases
3.7.4 integral concentration mathematical integral of the gas concentration along the optical path
Note 1 to entry: It is expressed in units of concentration multiplied by distance, e.g LFL metre for flammable gases or ppm metre for toxic gases
Note 2 to entry: 100 %LFL x 1 metre = 1 LFL metre;
MOR length of path in the atmosphere required to reduce the luminous flux in a collimated beam from an incandescent lamp, at a colour temperature of 2 700 K to 5 % of its original value
An open path optical system refers to an optical path that crosses the monitored area of the atmosphere, allowing gases to move freely within that space.
3.7.7 optical axis median line of the optical path
3.7.8 optical path path traversed by optical radiation from an optical transmitter to an optical receiver
Note 1 to entry: The radiation may traverse the path once, twice or many times depending on the form taken by the instrument
3.7.9 receiver assembly in which the optical detecting element(s) are housed and which may contain associated optical and electrical components
3.7.10 retroreflector individual or multiple arrangement of reflecting corners of cubes arrayed so that light is reflected back parallel to its incident path
A gas detection transmitter is a fixed gas detection device that generates a self-conditioned electronic signal or output, adhering to industry standards like 4-20 mA or relay It is designed for use with separate gas detection control units or signal processing systems, enabling central monitoring and data acquisition from multiple locations and sources, including various gas detection equipment.
3.7.12 transceiver assembly in which the optical detecting element(s) and optical transmitting element(s) are housed and which may contain associated optical and electrical components
3.7.13 transmittance fraction of luminous flux which remains in a single beam after traversing an optical path of a given length in the atmosphere
3.7.14 transmitter assembly in which the optical transmitting element(s) are housed and which may contain associated optical and electrical components
4 Basic information on the properties, behaviour, gases and vapours, and specific applications of gas detection
Detecting gases and vapours
General
Clause 4 differentiates between gases, which remain in a gaseous state under typical ambient pressures and temperatures, and vapours, which can coexist with liquid at various pressures or temperatures.
Effective operation of flammable gas detection equipment depends not only on its performance but also on its correct usage
The effectiveness of equipment in safeguarding personnel and areas from flammable gases or vapours relies not only on its capabilities and the user's understanding of its limitations but also on the user's knowledge of gas and vapour properties Proper safety measures cannot be guaranteed without this foundational knowledge.
Understanding the density of gases and vapors relative to air is crucial for determining their potential accumulation areas By knowing the direction and velocity of air movement, one can predict how these substances may spread Additionally, physical or chemical factors, including calibration considerations, may impose restrictions on specific applications.
When assessing gas and vapor detection, it is essential to consider both the gases and vapors that must be detected and those that, while not required for detection, may still be present.
The effects of humidity and temperature variations may need to be evaluated, particularly when sampling lines are used, and, more importantly, if vapours other than water are involved
Minor environmental fluctuations, like slight temperature changes, can greatly impact conditions, especially in the presence of liquids that generate more vapors or when vapors condense into fogs or within equipment.
Neglecting the properties of gases and vapours during the selection, installation, commissioning, training, operation, and maintenance of equipment can lead to inaccurate readings This may result in false alarms or inappropriate responses, as well as missed alarms and lack of necessary actions Such oversights can pose significant risks to both lives and property.
Certain gases and vapors can lead to corrosion and deterioration of specific sensors, which often have defined lifetimes and may experience changes in sensitivity over time This is particularly relevant for sensors detecting toxic gases, oxygen deficiency, and flammable gases Consequently, regular functional checks of common flammable gas detectors are essential, typically conducted using a designated test or calibration gas It's important to note that the calibration equipment suitable for one type of gas detection may not be appropriate for another, necessitating proper training for effective use.
A mixture with a concentration above the upper flammable limit is not inherently explosive; however, when diluted with air, it can become explosive, making it prudent to treat it as such Additionally, it is important to recognize that this mixture is typically toxic at significantly lower concentrations.
Safety when monitoring for flammable gases where personnel could be
Personnel must regularly monitor gas detection readings before entering potentially hazardous areas, as these locations may already have asphyxiants or flammable gases present, necessitating prompt warnings of any dangers.
The equipment provides readings solely for the specific location of measurement or the end of the sampling line, if utilized Consequently, hazardous atmospheres may develop just a few meters away from the sampling point To ensure safety, multiple gas tests should be conducted throughout the work area to detect any potential pockets of hazardous gas or vapor.
To effectively identify potential issues, it is essential to conduct certain tests one or two centimeters above the floor, especially in areas where vapors may be present This approach can help detect minor problems, such as small liquid leaks, at an early stage Additionally, all nearby low spots should be thoroughly tested.
The readings are only valid for the time they are taken Circumstances change Frequent readings are recommended, particularly if vapours (see 4.3.3) could be involved and the temperature is rising
In work environments where various gases and vapors are present, it is essential to recognize the potential sensitivities involved Consequently, establishing a low 'alarm point' or 'action point' is necessary to ensure safety and effective monitoring.
To ensure optimal performance, it is essential to regularly check the sensitivity of equipment utilizing catalytic or semiconductor sensors when there is a risk of sensor 'poisons' like silicones, leaded petrol, or acids being present.
When monitoring the atmosphere for flammable gases and vapours, it is crucial to recognize that many of these substances, with the exception of water vapour, are also toxic to personnel Therefore, implementing additional detectors for specific gases and vapours, along with extra safety precautions, may be necessary.
Flammable gas or vapor detection equipment with high sensitivity sensors for specific toxic gases is designed to detect only those particular gases and typically will not identify other toxic substances.
Monitoring for oxygen deficiency is closely related to flammable gas monitoring, as detailed in section 4.4.1 This feature is often integrated into equipment due to various potential causes of oxygen deficiency, some of which involve toxic substances that pose significant risks Consequently, additional detectors and safety measures may be required to address these concerns effectively.
Before entering a hazardous area, it is essential to consult with the plant safety officer or industrial hygienist to assess the potential toxicity of the atmosphere Additionally, selecting a gas detector that can identify a range of flammable and toxic materials present in the area is crucial for ensuring safety.
Different countries have varying systems and values for defining maximum safe levels of potentially toxic substances For detailed information, refer to the ACGIH's TLVs (Threshold Limiting Values) and BEIs (Biological Exposure Indices) in the USA, or the European Commission's recommended TLVs for health hazards of chemical compounds in the workplace Both organizations update their publications annually, and other countries typically base their national documents on one of these sets of data.
Some common properties of gases and vapours
Gases and vapours will completely mix with one another through diffusion or agitation, and they do not separate However, it is important to note that certain gases and vapours may undergo chemical reactions when combined.
If a gas or vapour concentration is increasing in an area it is because more of that substance is being released It is not due to settling out
Gases and vapours remain mixed unless a component is chemically removed or absorbed, such as through a charcoal filter For vapours, removal can also occur via condensation when there is an increase in pressure or a decrease in temperature.
The density of pure gases and the effective density of vapors is directly proportional to their molecular mass Mixing gases and vapors does not result in a significant change in volume Consequently, the density of gas and vapor mixtures can be easily calculated using the volume fractions and molecular masses of their individual components If relative density data is accessible, the relative density can also be determined from the volume fractions and relative densities of the components.
Air has a molecular mass of about 29, giving it a relative density of 1 Gases with molecular masses lower than 29 possess a relative density less than 1, making them lighter than air.
Methane, with a molecular mass of 16, is lighter than air, while carbon dioxide, with a molecular mass of 44, is heavier than air A gas mixture, such as landfill gas or mine seam gas, consisting of approximately 53% methane and 47% carbon dioxide, has a density comparable to that of air.
Mixtures of clean air with pure or mixed gases that are lighter than air will exhibit a reduced buoyancy effect, with the degree of reduction proportional to their volume fractions As these mixtures rise, they will gradually become diluted with clean air, diminishing the buoyancy effect until it becomes negligible.
Mixtures of clean air with heavier gases or vapors remain denser than air, but to a lesser extent These mixtures tend to accumulate in low-lying areas such as pits and trenches, where they dilute with clean air until their impact becomes minimal.
When a release source and its surrounding air are significantly hotter than the ambient air, the released mixture can ascend, even if its relative density at ambient temperature exceeds 1 Generally, a temperature increase of 30 K can counteract a relative density that is 10% greater than that of air Conversely, if the release is cooler than the ambient temperature, the opposite effect occurs.
Gases and mixtures with a relative density between 0.8 and 1.2 are typically comparable to air in terms of relative density, allowing them to propagate in all directions due to temperature variations at release and normal turbulence.
All flammable gases and vapors possess a Lower Flammable Limit (LFL) and an Upper Flammable Limit (UFL), which are determined through experimental methods Detailed data for numerous substances can be found in IEC 60079-20-1 It is important to note that these limits cannot be accurately predicted.
NOTE Because this is experimental, different countries specify different values for LFL and UFL values, which have a legal standing As two examples:
– NFPA 30 is a publication used within the USA
– GESTIS is a publicly available database used within Germany
All vapours, excluding water vapour, possess varying degrees of toxicity, with flammable vapours being harmful at concentrations significantly below 25% of their Lower Flammable Limit (LFL), and many being toxic at levels under 1% of LFL Gases, apart from air and oxygen, primarily act as asphyxiants by reducing the oxygen levels in the air Additionally, the toxicity of other gases ranges from mild to severe.
An asphyxiant is a gas that is either non-toxic or only mildly toxic, primarily functioning by displacing oxygen in the lungs In contrast, toxic gases can lead to serious health issues even at low concentrations.
When detecting specific gases or vapours, it is crucial to consider the potential toxicity of other undetected gases or vapours that may be present.
The differences between detecting gases and vapours
General
The major practical differences between the detection of gases and the detection of vapours are outlined below.
Detection of gases
Gaseous substances that remain in their state under applicable temperature and pressure conditions will adhere to the Gas Laws, exhibiting predictable behavior Consequently, basic training may suffice for effective gas detection.
Gases can exist as pure substances or as mixtures, provided they do not undergo chemical reactions The composition of these non-reacting gas mixtures remains constant regardless of changes in temperature or pressure.
Accurate calibration necessitates the use of manufacturer-recommended calibration gas, which typically contains the target gas at a known concentration with a tolerance of less than 5% relative For functional checks, a test gas with lower accuracy may be acceptable, but it is essential to adhere to the manufacturer's guidelines for sensor functional testing.
Routine calibration kits for single point sensors and portable equipment typically include a portable gas cylinder, flow control mechanisms, and adapters for gas detection devices For multipoint sampling systems, larger cylinders are often utilized near the central equipment and may be permanently connected for automatic calibration.
High-pressure calibration and test gas mixtures can be created and stored to accurately represent specific gas detection applications These mixtures often utilize dry or synthetic air as a balance gas, which is crucial for the proper functioning of catalytic sensors and similar devices However, it is important to adhere to safety limits when compressing gas mixtures that contain more than 50% lower flammable limit (LFL) volume fraction.
Highly reactive gases generally have an extended storage life when paired with specially dried nitrogen as the balance gas, provided that it does not conflict with the sensor's compatibility.
To calibrate open path equipment, a cell with optical windows at both ends must be placed in the optical path, containing a calibration gas For practical cell dimensions, the flammable gas concentrations need to exceed the lower flammability limit (LFL) when using air as the balance gas However, the infrared detection principle allows for the use of nitrogen as the balance gas, enabling safe compression of the calibration gas.
When detecting multiple flammable gases or vapors, it is standard practice to utilize a single calibration or test gas along with relative response data For additional information, refer to section 4.3.3.2.
For calibration gas cylinders the minimum storage temperature specified by the calibration gas manufacturer has to be followed to avoid condensation
Gases can vary in density compared to air; some, like hydrogen and methane, are lighter, while others, such as carbon monoxide and hydrogen sulfide, have a similar density to air Additionally, certain gases, including chlorine, carbon dioxide, and propane, are heavier than air.
When conducting sampling, it is essential to consider the gas density at various locations, as this can aid in identifying the source of the release.
Certain flammable gases, including ammonia, hydrogen sulfide, hydrogen cyanide, carbon monoxide, methylamine, and formaldehyde, are extremely toxic even at undetectable levels by standard flammable gas detection equipment While these gases are recognized in IEC 60079-20-1, they can only be identified at higher concentrations relevant to lower flammability limits (LFL) Therefore, if these gases are present, it is essential to use dedicated toxic gas sensors and implement additional safety measures to protect personnel in the vicinity.
It is important to recognize that certain non-flammable gases, including chlorine, sulfur dioxide, nitric oxide, and nitrogen dioxide, can be highly toxic When these gases may be present, it is essential to utilize dedicated toxic gas sensors and implement additional safety measures.
Other flammable gases, including propane, butane, and LPG, can be mildly toxic or narcotic even at concentrations significantly below their lower flammable limit (LFL) Additionally, non-flammable gases like carbon dioxide and nitrous oxide may pose toxicity risks at levels that do not lead to a substantial deficiency in oxygen.
Detecting gases often cannot overlook the presence of water vapor, which can lead to issues in cold equipment exposed to warmer, humid environments For instance, transitioning from a cold storage area to a typical atmosphere or from an air-conditioned space to a humid tropical climate can cause water to condense on sensors This condensation may result in a temporary loss of sensitivity until the equipment warms up and the water evaporates Electrochemical sensors are particularly affected, as they can experience a rapid drop in oxygen readings from a normal 20.8% or 20.9% to 16% or lower due to water coating the sensor membrane The sensor's sensitivity may take several minutes to recover as it warms to ambient temperature and the condensation dissipates.
Water vapour may also cause significant deviations of the reading of several types of sensors (see Clause 5 and Annex A).
Detection of vapours
Vapours are more complex than gases, as they refer to substances that can exist in both liquid or solid forms alongside their gaseous state under normal or slightly altered temperatures and pressures Their behavior differs significantly from that of gases, often leading to additional challenges Therefore, it is essential to receive specialized training on the properties of vapours in environments where they are likely to be encountered.
The rate of evaporation and the maximum volume fraction of saturated vapor increase with temperature and decreasing pressure While the evaporation rate depends on the surface area of the liquid, it remains unaffected by the total quantity of liquid, as long as some liquid is present Additionally, the maximum vapor volume fraction is independent of the background gas, provided that the gas is at the same temperature and pressure and is not soluble in the liquid.
The volume fraction that can be reached by the vapour at any temperature is inversely proportional to the absolute pressure So an increase in pressure can cause condensation
At constant pressure, the maximum saturated volume fraction of any vapor increases by a factor of 1.5 to 2.0 for every 10 K rise in liquid temperature, while it decreases by the same factor for every 10 K reduction in temperature.
Doubling the absolute pressure results in a temperature decrease of 10 K to 17 K at constant pressure, while halving the pressure leads to a corresponding increase in temperature.
The temperature at which the saturated volume fraction can reach 100 % at the prevailing pressure is the boiling point
At atmospheric pressure, a vapor can only reach a 100% volume fraction at or above its boiling point Below this boiling point, the concentration of vapor in air or other gases cannot exceed 100% volume fraction.
The actual vapor amount will be lower than predicted if clean air continuously flows over the liquid surface or if there hasn't been sufficient time for equilibration However, the maximum vapor amount can be reached in an enclosed space, especially if it has been sealed for a while and the air is gradually stirred by convection or mechanical methods.
Flammable liquids possess flashpoints, which indicate the temperatures at which their vapors can achieve lower flammable limits (LFL) in the air above the liquid surface, assuming that the vapor is not continuously dispersed by air currents.
Mixtures of vapours and gases are subject to limitations, particularly regarding temperature and pressure When a specific volume fraction of vapour in a gas mixture reaches the saturation point, liquid begins to condense, a phenomenon known as the "dew point," which applies to various vapours, including water vapour Below this dew point, the composition of the vapour-gas mixture changes, potentially causing gas detection systems or sensors to register inaccurately low readings from the condensing sample Additionally, when condensation occurs and the mixture returns to cleaner air, re-evaporation can lead to falsely high readings.
Calibration and test vapors for field use are practically limited by the minimum temperature and the required pressure in the gas cylinders, in addition to the restrictions outlined for gas mixtures.
Test kits with a stored pressure in the range 2 MPa to 3 MPa are effectively limited to around
50 % LFL for n-pentane (boiling point 36 °C), about 10 % LFL for n-hexane (boiling point
68 °C), rather less for other substances with similar boiling points, and lower values still for substances with higher boiling points
Pentane and hexane test gases are primarily relevant to the petroleum industry, where these vapors are often key components In contrast, other industries face challenges in creating portable or transportable calibration kits that accurately represent the vapors to be measured outside of laboratory settings.
To address this issue, laboratory tests are conducted to assess how equipment responds to various gases and vapors, using a specific test gas as a reference This process is often time-consuming and costly, typically performed only for a specific model rather than for each individual unit, leading to potential variations among different units.
Routine calibration can be performed in two primary ways: first, by ensuring an accurate Lower Flammable Limit (LFL) readout using calibration gas or vapor, allowing for direct use of relative response data for the substances of interest; second, by providing an artificial LFL readout on the calibration gas or vapor, which approximates the correct LFL when exposed to the target gas or vapor or a specified range of gases or vapors.
Sensor responses can vary over time, especially for those with a limited lifespan or those prone to "poisoning," which results in a loss of sensitivity due to chemical reactions This is particularly relevant for electrochemical and catalytic sensors.
Catalytic sensors exhibit a selective deterioration in their response to methane before other gases or vapors If the equipment can be re-calibrated successfully for methane, it will provide over-readings for all other substances, which is a safety advantage.
In catalytic combustion applications involving methane, it is advisable to use methane test gas for functional checks and calibration, even if a different target gas is present Alternatively, calibration mixtures of propane, butane, pentane, or hexane are recommended, as sensitivity to these gases typically decreases before that of other substances.
Oxygen deficiency
General
Oxygen detectors, whether fixed or portable, are crucial for safety in confined spaces and underground coal mines Portable devices often integrate oxygen sensors with flammable gas and toxic gas sensors, enhancing workplace safety.
Certain types of oxygen sensors are sensitive to pressure variations, requiring calibration in clean air before each use or after significant altitude changes, such as climbing a tall building or descending into a mine shaft.
The volume fraction of oxygen in dry air is about 20,9 % v/v A typical oxygen deficiency alarm setting is between 17,0 % to 19,5 % v/v
In a scenario with a concentration of 19.0% v/v, the alarm is designed to activate at a deficiency of 1.9% v/v, which equates to a 10% reduction from the original concentration However, in situations where toxic gases are present, this level of alarm activation may not sufficiently ensure personnel safety.
At a setting of 19.5% v/v, the alarm is designed to activate at a deficiency of 1.4% v/v, which corresponds to a 7% reduction from the original concentration However, this level may not be sufficient for ensuring personnel safety in all situations.
In various applications, relying solely on a combined apparatus for detecting flammable gases and oxygen deficiency may not offer adequate information for operators Oxygen deficiency can occur due to the presence of toxic gases, necessitating the need for additional detection systems specifically for these harmful substances.
There are three basic physical and chemical mechanisms by which oxygen deficiency can occur, described in 4.4.2 to 4.4.4 with reference to the 19,5 % v/v alarm setting example above.
Chemical reaction of oxygen, with solid products
Rusting of steel and corrosion of metals occur when oxygen is removed from the air, resulting in the formation of solid oxides This process often takes place in confined metal spaces.
When the alarm activates at 19.5% v/v, it indicates a physiological equivalent to an altitude increase of approximately 650 meters Typically, this elevation does not pose any health risks to personnel.
Chemical reaction of oxygen, with gaseous products
In simple terms, a 1.4% reduction in oxygen levels can trigger an alarm, typically due to respiration, aerobic bacterial activity, or clean combustion This decrease in oxygen is associated with a safe short-term increase of approximately 0.8% in carbon dioxide levels, posing no immediate or long-term health risks to a healthy individual.
Oxygen deficiency resulting from 'dirty' combustion of materials like wood, paper, coal, and oil can create a hazardous atmosphere Even with 19.5% v/v oxygen present, the air may still be lethally toxic if carbon monoxide levels exceed 0.2% v/v.
(2 000 parts per million), of carbon monoxide produced together with the carbon dioxide
The combustion of plastics like PVC and polyurethane can significantly increase atmospheric toxicity, releasing harmful byproducts such as hydrogen chloride and hydrogen cyanide.
Dilution of the air by displacement by some other gas or vapour
Detection of a gas by oxygen depletion is only recommended under very controlled conditions and is not otherwise recommended
The 1,4 % v/v oxygen deficiency needed to cause the alarm would require an addition of
7 % v/v of the other gas or vapour Thus the problem becomes that of knowing what gas or vapour has caused the deficiency There are several categories: a) Inert gases; not toxic
If oxygen deficiency is caused by the dilution of inert gases like nitrogen, argon, helium, or neon (or water vapor), adding up to 7% v/v of the inert gas to the atmosphere is considered safe This scenario parallels the conditions described in section 4.4.2.
NOTE There are areas where the volume fraction of oxygen is controlled at levels between 12 % v/v and
To minimize fire risks, areas with a 15% v/v dilution will be accessed by personnel Portable oxygen detectors must have alarm levels adjusted according to the oxygen levels in these areas Personnel entering zones with low oxygen volume fractions are required to pass a medical test in accordance with national regulations Additionally, flammable gases present in these areas are not toxic.
In the presence of hydrogen, methane, or ethane, the atmosphere may initially seem safe to breathe, but it poses an explosion risk due to being above the Lower Flammable Limit (LFL) A flammable gas detector, in conjunction with an oxygen detector, would have provided an early warning before reaching this hazardous condition.
Breathing an atmosphere with gases like acetylene, ethylene, propane, LPG, or butane can be fatal within seconds due to oxygen deficiency, even if the concentration is above the Lower Flammable Limit (LFL) In such cases, a flammable gas sensor would trigger an alarm before the toxic effects are felt.
Oxygen deficiency measurement should never be used for the detection of toxic gases
WARNING – Oxygen detectors shall never be used to indicate displacement by CO 2
Reliance on oxygen detectors to monitor the presence of carbon dioxide has led to fatalities.
Specific applications of gas detection
Gas detection as means of reducing risk of explosion
Flammable gas detection equipment, when used in various operational combinations, can significantly lower the risk of explosions This is achieved by removing potential ignition sources, such as non-explosion protected equipment in areas prone to explosive gas atmospheres, or by triggering safety functions that maintain flammable gas concentrations below hazardous levels.
In the event of significant accidental gas releases, there is a high probability of an explosive gas atmosphere extending beyond the designated area classification Therefore, it is crucial to implement measures that eliminate potential ignition sources in these regions.
Gas detection safety functions include disconnecting non-explosion protected equipment when alarm levels are exceeded, increasing ventilation to maintain gas concentrations below 25% LFL, and sending shutdown signals to the Safety System upon detecting flammable gas Additionally, in pressurized rooms, ventilation dampers should close to prevent gas ingress For analyser houses, a variety of provisions outlined in IEC 61285 emphasize the critical role of gas detectors Refer to IEC 60079-13 for guidelines on isolating non-explosion protected equipment within the room.
This main alarm level should be at or below 20 % LFL
In all cases of detection of flammable gas levels at or above 20 % LFL, audible and visual annunciation should be initiated
Indication of low levels of flammable gas (with or without pre alarm) should be used to:
1) initiate troubleshooting and repair activities,
2) activate operation of a ventilation system, to increase the rate of existing ventilation to avoid the disconnection of non-explosion protected equipment, and
3) shutdown processes due to more severe gas leaks which may cause the exceeding of the main alarm level
Flammable gas detection equipment must meet the performance standards outlined in IEC 60079-29-1 or IEC 60079-29-4, specifically tailored for the gases or vapors that may be present in ventilated areas.
Ventilation and air movement serve two primary purposes: first, they enhance the dilution and dispersion of hazardous gases to minimize the size of affected areas; second, they prevent the formation of explosive atmospheres, which can impact the classification of a zone.
If the ventilation rate is calculated such that gas-air mixtures in concentrations exceeding
A ventilation rate of 25% of the LFL is crucial for effective protection While gas detection is not mandatory, it is advisable to maintain a high safety level in case the ventilation system fails.
If ventilation rates are considered as low ventilation then a combination of ventilation and gas detection is the means of protection and the recommendations provided should be followed
Ventilation rates must be assessed against anticipated flammable gas leakage rates under different operational conditions This comparison is crucial as it influences how flammable gas detection systems and ventilation are managed.
Guidance for the assessment of ventilation efficiency and availability can be found in IEC 60079-10-1:2008, Annex B
4.5.1.3 Gas detection in “high” ventilated areas
High ventilation is essential in applications to prevent the buildup of flammable gas concentrations The ventilation rate is specifically calculated to ensure that significant amounts of vapor-air or gas-air mixtures do not exceed 25% of the Lower Flammable Limit (LFL), except in designated dilution areas.
The ventilation rate should be calculated based on maximum leak rates expected
The detection of flammable gases is an additional means of protection should the ventilation system fail
If ventilation fails (a fault condition) disconnection of non-explosion protected ignition-capable equipment should occur
4.5.1.4 Gas detection in “medium” ventilated areas
When predicting maximum leak rates is not feasible, an appropriate ventilation rate can be established by estimating realistic leak rates This approach ensures that vapor-air or gas-air mixtures with concentrations above 25% LFL are managed, maintaining a stable zone boundary at the calculated ventilation rate However, it is important to note that higher leak rates may still occur, potentially rendering ventilation alone insufficient as a protective measure.
Minimum ventilation rates and the ventilation arrangement to be considered must be sufficient to prevent accumulation of flammable gas in corners or behind equipment
Incorporating gas detection equipment alongside medium ventilation rates enhances safety by forming a crucial part of the primary safety strategy This system should trigger an increase in ventilation when gas concentrations reach or fall below 20% LFL If the elevated ventilation does not mitigate the vapor-air or gas-air mixtures, the main alarm will activate safety measures to disconnect all non-explosion protected equipment.
Depending on the local conditions an increase of ventilation should be started at pre-alarm levels between 5 % LFL and 20 % LFL
In the event of a failure in gas detection or ventilation systems, it is crucial to indicate the fault Equipment capable of ignition may be allowed to function temporarily if an immediate power loss or automatic shutdown could create a more dangerous situation, as long as repairs are underway and the other safety measures are operational.
4.5.1.5 Gas detection in low ventilated areas
Low ventilation rates that fail to prevent flammable gas levels from exceeding 25% LFL are classified as "low" ventilation This insufficient dilution can lead to a lower Zone classification than would typically be assigned for the release grade, such as a Zone 1 hazardous area from a secondary grade source Continuous monitoring of gas concentration using fixed gas detectors may allow for the use of equipment with a lower equipment protection level (EPL) than usually mandated.
In applications requiring safety measures, both ventilation and gas detection play crucial roles in protection The alarm levels and corresponding actions, such as enhancing ventilation and disconnecting non-explosion protected equipment, must align with the standards outlined in section 4.5.1.4.
4.5.1.6 Gas detection in ventilated areas using recirculation
Reduced outside air exchange (below determined adequate ventilation rates) may be used where recirculation of inside air is employed, provided all the following criteria are observed:
1) The area is monitored continuously by a gas detection system for flammable gases
2) Flammable gas detection is provided in the return air of the ventilation system
3) When the air inlet flow is reduced, the inside air distribution is maintained by the recirculation
4) The gas detection system will stop the recirculation when exceeding the lowest alarm level (less than 20 % LFL) and increase the ventilation rate for outside air
5) The outside air exchange is provided at a rate that will maintain flammable gas levels below 25 % LFL
6) At the main alarm level at or below 20 % LFL the non-explosion protected equipment shall be disconnected
7) If the ventilation fails the fault has to be indicated but no disconnection of equipment is required, provided repair of the ventilation system is initiated and the gas detection system is working correctly.
Gas free work permit
In hazardous areas, equipment that requires temporary bypassing of its protection method for maintenance or troubleshooting can operate under a gas-free work permit This work is permissible as long as additional flammable gas detection equipment is utilized and monitored by trained personnel.
NOTE The equipment for the detection of flammable gas can be of portable, transportable or fixed point type detection
When performing maintenance or repair work in hazardous areas, it is essential to operate portable ignition-capable devices, such as arc welding equipment, only in accordance with work permit instructions Additionally, the use of flammable gas detection equipment must be employed and monitored by trained personnel to ensure safety.
When selecting additional equipment for flammable gas detection, it is crucial to ensure that it is appropriate for use in hazardous areas, adhering to IEC 60079-0 and related standards Furthermore, the equipment must comply with IEC 60079-29-1 to guarantee optimal performance for the specific gases or vapors that may be present in the work environment.
For effective monitoring of flammable gas during permitted work, additional detection equipment must be calibrated to identify the specific flammable gases likely present in the working area at that time This ensures a reliable indication of gas presence and enhances safety measures.
For maintenance and repair work in hazardous areas where protection methods are bypassed, it is essential to implement specific safety measures First, all portable gas detection systems must undergo a functionality check with a known concentration of flammable gas before use, in conjunction with a gas-free permit (bump test) Initial measurements should confirm the absence of flammable gas in the work area, followed by continuous monitoring to ensure no gas or vapor is present Monitoring equipment must be operated by trained personnel, and additional operational procedures should be established to enable emergency responders to effectively manage any detected gas presence.
A gas free work permit will only be issued after successfully completing points a) and b), ensuring that the work location is confirmed gas free This certification is essential before bypassing the method of protection and allowing any sources of ignition into the work area.
Monitoring of air inlets
In areas where ventilation air must be free of flammable gases or vapors, such as air intakes for pressurized rooms or internal combustion engines near explosive gas atmospheres, it is essential to install gas detection systems to monitor the air inlet If gas concentrations are detected, appropriate actions should be taken to ensure safety.
1) Shut down the air intake (close dampers and/or de-energize fan supply as appropriate)
2) Shut down the process (to halt the release of flammable gas and in preparation for de-energizing the electrical power system)
3) De-energize non-explosion protected electrical power system or instruments
When installing flammable gas sensors in ventilation air ducts or turbulent flow applications, it is crucial to exercise caution These sensors must be specifically designed, tested, and approved for low-level flammable gas detection in such environments to ensure their effectiveness in detecting flammable gases or vapors within the ventilation system.
Pressurized rooms or houses situated in hazardous areas, specifically Zone 1 or 2, must adhere to electrical explosion protection standards outlined in IEC 60079-13 and/or IEC 61285 It is essential to monitor the clean air supply used for pressurization with flammable gas detectors to ensure safety.
Specific considerations for open path detection
Clause 4 outlines the concentration units for gases and vapours, which include a) percentage volume fraction (% v/v) and b) the lower flammable limit (LFL), also referred to as the lower explosive limit (LEL) The LFL is expressed as a percentage volume fraction specific to a flammable substance in air.
The lower flammable limit (LFL) of methane is 4.4% v/v in air, as specified in IEC 60079-20-1 The percentage of the lower flammable limit (% LFL or % LEL) represents the reciprocal of the safety factor Additionally, parts per million (ppm) volume fraction is primarily utilized for measuring toxic gases, vapors, and flammable gases in low concentrations.
Sensor and sampling equipment for detecting oxygen deficiency, as well as flammable and toxic gases and vapors, typically utilize specific units of concentration These sensors operate with a small, homogeneous sample, often contained within a low-volume cell, making concentration units particularly relevant for accurate measurements.
Open path equipment consists of a transmitter and receiver positioned at varying distances, typically between 1 and 200 meters The gas sample analyzed is the entirety of the gas present in the optical path, which may not have a uniform concentration This setup does not allow for the detection of specific concentration variations, such as a small area of high concentration or a larger area of low concentration The measurement obtained reflects the integral concentration across the path length (e.g., in LFL × m) or the average concentration (e.g., in % LFL(avg)), as illustrated in the accompanying figures.
Figure 1 – Integral concentration over the path length
The three Open Path Monitors shown in Figure 1 detail how three gas clouds with different size and concentration would result in the same reading of 1 LFL × m
Figure 2 – Average concentration over the path length
The three Open Path Monitors shown in Figure 2 details how three gas clouds with different size and concentration would result in the same reading of 5 % LFL (Path Average Concentration)
General
Clause 5 summarizes key aspects of the measuring principles outlined in Annex A For comprehensive details, refer directly to Annex A rather than this clause The titles and numbering of subclauses 5.2 to 5.10.5 correspond exactly to those in Annex A, specifically A.5.2 to A.10.5.
Understanding the measuring principles of gas sensors is essential for engineers and managers when selecting the appropriate gas detector for their specific application Engaging with manufacturers or sellers can provide valuable insights, but it's important to note that the overall performance and functionality of a gas detector depend on more than just the gas sensor or sensing element.
When selecting a gas detector, it is crucial to consider its overall performance and functionality, along with the supporting hardware and software.
This article outlines the measuring principles of different sensor types, highlighting their advantages, typical applications, and limitations It also addresses potential interferences from other gases and the issue of poisoning, which refers to the loss of sensitivity due to exposure to other gases or vapors.
A summary of the most common detection principles is given in Table 2 These are explained in more detail in 5.2 to 5.10 (and in A.5.2 to A.10)
Table 2 – Overview of gas detection equipment with different measuring principles
Catalytic sensor Thermal conductivi ty sensor
Infrared sensor Semi- conduc- tor sensor
Yes No No (No) (No) (No) Yes No Not applicable
Typical measuring ranges of flammable gases
(100) % FS ≤ LFL ≤ LFL ≤ LFL < LFL < LFL Not applicable
Typical measuring range open path
Not applicable Not applicable 0 to 5
LFL × m Not applicable Not applicable Not applicable Not applicable Not applicable Not applicable
Typical measuring ranges for oxygen
Not applicable Not applicable 0 to
(100) % FS (with special sensing elements)
Not applicable Not applicable Not applicable 0 to 100 %
Large molecules (See 5.3) H 2 (See 5.5) Alkanes H 2 ; CO (see 5.8) H 2 ; CO;
Medium (Low) Depends on substance
Medium Low Low Low Low to medium
Interference of non- flammable gases b
(H 2 S); Pb No No Si; Hal d ;
SO 2 (No) (Si) No No No
This article presents a qualitative comparison of principles, highlighting that the values do not consider the time for aspirated sampling lines It includes a table with common examples, focusing on chlorinated hydrocarbons and both organic and inorganic halogenated compounds Additionally, it notes that IP refers to the ionization potential of the substance, while X represents the energy of the detector's UV lamp.
The statements in brackets are conditional and reference should be made to the corresponding subclause
NOTE The term ‘Full Scale’ is denoted as “FS”
When considering alternative detection technologies, it is crucial to evaluate their limitations in accurately determining gas concentration Understanding the hazard potential is essential for safety systems, and technologies that do not meet gas measurement performance standards, such as IEC 60079-29-1 or IEC 60079-29-4, should only serve as supplementary methods For example, while ultrasonic detectors can effectively identify leaks from high-pressure sources, they may signal earlier stages of a leak more strongly than later stages with higher flow rates Thus, ultrasonic detectors should complement gas detectors rather than replace them Similarly, infrared cameras can indicate the location and extent of leaks but do not provide precise gas concentration measurements.