Iec 61779 6 1999

Microsoft Word 1779 6x doc INTERNATIONAL STANDARD IEC 61779 6 First edition 1999 06 Electrical apparatus for the detection and measurement of flammable gases – Part 6 Guide for the selection, installa[.]

Scope

This section of IEC 61779 provides essential guidance on the selection, installation, usage, and maintenance of electrically operated group II apparatus designed for detecting and measuring flammable gases It adheres to the standards outlined in IEC 61779-1 to IEC 61779-5 and serves as a practical resource for users The focus is on apparatus, instruments, and systems that detect the presence of flammable or potentially explosive gas or vapor mixtures in air, utilizing electrical signals from gas sensors to generate meter readings, activate visual or audible alarms, or trigger other devices.

When operating in classified areas, it is essential to install and use equipment in a manner that prevents the ignition of combustible gas-air mixtures Compliance with IEC 60079-10 standards is necessary to ensure safety in these environments.

For the purpose of this standard, flammable gases shall include flammable vapours.

1.1.2 This standard applies only to group II apparatus intended for use in industrial and commercial safety applications, involving areas classified in accordance with IEC 60079-10.

For the purpose of this standard, apparatus includes a) fixed apparatus; b) transportable apparatus; and c) portable apparatus.

This standard does not apply to apparatus designed solely for detecting non-flammable toxic gases, laboratory or scientific equipment meant for analysis or measurement, devices intended for underground mining, apparatus used in explosives processing and manufacturing, tools focused exclusively on process control, or equipment aimed at detecting potentially flammable atmospheres caused by dust or mist in the air.

Normative references

This section of IEC 61779 references several normative documents that are integral to its provisions For dated references, any amendments or revisions do not apply, but parties involved in agreements are encouraged to consider the latest editions of these documents In the case of undated references, the most recent edition is applicable Additionally, IEC and ISO members keep updated registers of valid International Standards.

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU.

IEC 60050(426):1990, International Electrotechnical Vocabulary (IEV) – Chapter 426: Electrical apparatus for explosive atmospheres

IEC 60079 (al parts), Electrical apparatus for explosive gas atmospheres

IEC 60079-0:1998, Electrical apparatus for explosive gas atmospheres – Part 0: General requirements

IEC 60079-10:1995, Electrical apparatus for explosive gas atmospheres – Part 10:

IEC 60079-19:1993, Electrical apparatus for explosive gas atmospheres – Part 19: Repair and overhaul for apparatus used in explosive atmospheres (other than mines or explosives)

IEC 60079-20:1996, Electrical apparatus for explosive gas atmospheres – Part 20: Data for flammable gases and vapours, relating to the use of electrical apparatus

IEC 61779-1:1998, Electrical apparatus for the detection and measurement of flammable gases

– Part 1: General requirements and test methods

– Part 2: Performance requirements for group I apparatus indicating up to 5 % methane in air

– Part 3: Performance requirements for group I apparatus indicating a volume fraction up to

– Part 4: Performance requirements for group II apparatus indicating a volume fraction up to

– Part 5: Performance requirements for group II apparatus indicating a volume fraction up to

For the purpose of this part of IEC 61779, the following definitions apply.

2.1 aspirated apparatus combustible gas detecting apparatus that obtains the gas by drawing it to the gas sensor – for example by means of a hand-operated or electric pump

2.2 catalytic sensor sensor, the operation of which depends upon the oxidation of gases on an electrically heated catalytic element

2.3 clean air air that is free of flammable gases and interfering or contaminating substances

2.4 continuous duty apparatus combustible gas detecting apparatus that is powered for long periods of time, but may have either continuous or intermittent sensing

2.5 continuous sensing mode of operation in which power is applied continuously to the sensing element and readings are taken continuously

2.6 diffusion apparatus apparatus in which the transfer of gas from the atmosphere to the gas sensing element takes place by diffusion, i.e there is no aspirated flow

The 2.7 electrochemical sensor operates by detecting changes in the electrical parameters of electrodes immersed in an electrolyte, which occur as a result of redox reactions involving gas on the electrode surfaces.

2.8 electromagnetic radiation absorption sensor sensor, the operation of which depends upon the absorption of electromagnetic radiation by the gas being detected

2.9 explosion protected apparatus apparatus incorporating a type of protection covered by the IEC 60079 series of standards

An explosive gas atmosphere is created when a mixture of flammable gases or vapors combines with air under normal atmospheric conditions Once ignited, combustion can rapidly propagate through the remaining unconsumed mixture.

NOTE 1 – This definition specifically excludes dusts and fibres in suspension air Mists are not covered by this standard.

A mixture with a concentration exceeding the upper explosive limit is not classified as an explosive atmosphere; however, for area classification purposes, it is prudent to treat it as an explosive gas atmosphere in certain situations.

NOTE 3 – Normal atmospheric conditions include variations above and below reference levels of 101,3 kPa and

20 °C provided the variations have a negligible effect on the explosive properties of the flammable materials.

2.11 explosive range range of gas or vapour mixtures with air between the explosive (flammable) limits

2.12 fixed apparatus apparatus which is intended to have all its parts permanently installed

2.13 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

2.14 group II apparatus electrical apparatus for places with a potentially explosive atmosphere, other than mines susceptible to firedamp

2.15 infrared absorption sensor sensor, the operation of which depends upon the absorption of infrared radiation by the gas being detected

2.16 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

2.17 lower flammable limit (LFL) volume ratio of flammable gas or vapour in air below which an explosive gas atmosphere does not form, expressed as a percentage (see annex A)

NOTE – This is also known as lower explosive limit (LEL).

2.18 open path infrared sensor sensor capable of detecting gas at any location along an open path traversed by an infrared beam

Portable apparatus refers to lightweight devices designed for easy transport and use in various locations These battery-operated tools include hand-held devices weighing less than 1 kg for single-handed operation, personal monitors that are similar in size and continuously operate while attached to the user, and larger equipment weighing up to 5 kg that can be operated while held by hand, suspended with a shoulder strap, or carried with a harness Some of these devices may feature a hand-directed probe.

2.20 relative density density of gas or vapour relative to the density of air at the same pressure and at the same temperature (air is equal to 1,0)

2.21 release rate quantity of flammable gas or vapour emitted per unit time from the source of release which itself could be a liquid surface

2.22 remote sensor sensor which is not integral with the main body of the apparatus

2.23 sample line pipeline by means of which the gas being sampled is conveyed to the sensor

2.24 sampling probe separate sampling line, that may or may not be supplied with a portable apparatus that is attached to the apparatus as required.

The sampling probe is typically a short, rigid device, often around 1 meter in length, and may feature a telescopic design It can also be connected to the apparatus via a flexible tube for enhanced functionality.

2.25 semi-conductor sensor sensor, the operation of which depends upon changes of the electrical conductance of a semiconductor due to chemisorption of the gas being detected at its surface

2.26 sensor assembly in which the sensing element is housed that may contain associated circuit components

The sensing element of a sensor is the component that responds to the presence of a flammable gas mixture, resulting in a physical or chemical change This change can be utilized to trigger either a measuring function, an alarm, or both.

2.28 single point sensor sensor capable of detecting gas at a single point location

A release point is defined as the location from which flammable gas, vapor, or liquid can escape into the atmosphere, potentially creating an explosive gas atmosphere.

2.30 spot reading apparatus apparatus intended to be used for short periods of time as required (typically 5 min or less)

The 2.31 thermal conductivity sensor operates by measuring the heat loss through conduction from an electrically heated element in the target gas, and comparing it to a similar element in a reference gas cell.

2.32 transportable apparatus apparatus not intended to be portable, but which can be readily moved from one place to another

2.33 upper flammable limit (UFL) volume ratio of flammable gas or vapour in air above which an explosive gas atmosphere does not form, expressed as a percentage (see annex A)

NOTE – This is also known as upper explosive limit (UEL).

2.34 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)

The measuring principles of various sensors are given below as well as typical advantages and disadvantages of each.

Catalytic sensor

The catalytic sensor operates on the principle of oxidizing flammable gases at the surface of a heated catalytic element, such as a filament or bead This oxidation process leads to a change in the temperature of the sensing element, which varies with the concentration of the detected gas Consequently, the resulting change in electrical resistance is measured to determine gas levels.

A reasonable concentration of oxygen in the order of 10 % or greater is required for catalytic sensors to operate.

Catalytic sensors are designed to directly detect flammable gases, while other sensor types, as discussed in sections 3.2 to 3.6, infer the presence of flammable gases based on their response to different gas properties.

Since oxidation depends upon the presence of oxygen, detection apparatus should use only this type of sensor for gas concentrations up to the lower flammable limit.

The catalytic sensor may be used in either a) diffusion mode; or b) aspirating mode.

The sensors detect flammable gases by a process of combustion and are suitable for the detection of a wide range of flammable gases but with possible variations in sensitivity.

The main disadvantages with catalytic sensors are the following: a) Range limitation

The catalytic sensor operates on the principle of catalytic oxidation and requires adequate oxygen to function effectively It is designed for detecting gas and air mixtures that are within the lower flammable limit.

WARNING – ABOVE THE LOWER FLAMMABLE LIMIT, A CATALYTIC SENSOR

MAY RESPOND AMBIGUOUSLY AND IN SOME CASES THE INSTRUMENT

MAY ERRONEOUSLY INDICATE THAT THE FLAMMABLE GAS AND AIR MIXTURE

IS BELOW THE LFL. b) Interfering gases and vapours

Monitoring atmospheres with gases that dilute or displace air, such as nitrogen or carbon dioxide, can lead to low or zero responses from catalytic sensors Additionally, steam-laden environments may cause issues due to the saturation of the sintered flame arrestor from condensation High levels of inert gases like argon or helium can also disrupt the thermal balance of the sensor, leading to misleading readings of combustible gases Furthermore, catalyst poisoning can inhibit sensor performance.

Catalytic sensors can be permanently or temporarily inhibited by specific airborne contaminants, leading to a diminished or nonexistent response to gas detection.

NOTE – For this reason, it is therefore important that all catalytic gas detection apparatus is regularly tested in accordance with 9.2.

This inhibition may be permanent or temporary according to the nature of the contaminant.

Permanent inhibition, commonly referred to as "catalyst poisoning," can occur due to exposure to materials like silicones, tetraethyl lead, sulfur compounds, and phosphate esters Additionally, certain substances, including halogenated hydrocarbons, may lead to temporary inhibition.

Some catalytic sensors are resistant to contaminants and do not need extra protection, while others may benefit from activated carbon or filters However, caution is necessary when using carbon filters, as they can effectively block contaminants but may also hinder the detection of hydrocarbons other than methane and significantly slow down response times.

Their performance may also be affected by the level of humidity in the atmosphere.

To mitigate the effects of inhibition, sensors can operate in intermittent mode However, this method may lead to false responses, especially when sensors encounter high gas concentrations during the "power-off" phase of their cycle It is crucial to exercise caution when utilizing portable devices for leak detection or similar tasks.

The manufacturer’s guidance should be sought where the presence of contaminants is suspected, or where temporary or permanent inhibition is experienced.

Thermal conductivity sensor

The thermal conductivity sensor operates based on the heat loss through conduction from an electrically heated resistance element, such as a filament, bead, or thin film resistor, situated within a gas sample stream of constant velocity or a diffusion chamber This process leads to a measurable change in electrical resistance.

This type of sensor is best suited for detecting individual gases of high or low conductivity relative to air.

Thermal conductivity sensors are effective for monitoring gases with significantly different thermal conductivity compared to air, particularly at high concentrations above the lower flammable limit (LFL) These sensors should not be utilized for measuring gas concentrations below the LFL, except for gases like hydrogen, which these sensors can detect with high sensitivity.

Errors can occur when the apparatus is improperly calibrated for specific gases, when using a flow-sensitive thermal conductivity sensor with unstable gas sample flow, or when the gas sample is not conditioned to eliminate water vapor and other interfering substances Additionally, variations in ambient temperature without proper control can lead to inaccuracies, and the presence of other gases with different thermal conductivities may neutralize the signal, potentially resulting in a zero reading.

The sensor is capable of measuring high concentration and is independent of the oxygen level in the gas-flow.

Thermal conductivity sensors have several disadvantages, including a lack of selectivity for individual gases and limited sensitivity, which can result in detection limits exceeding the lower flammable limit (LFL) unless the gas's thermal conductivity significantly differs from that of air The varying thermal conductivities of flammable gases complicate detection; lighter gases like methane and hydrogen are more conductive than air, while heavier gases are less so, making responses to gas mixtures unpredictable without knowledge of their proportions Additionally, these sensors depend on thermal conduction and convection from a heated filament, leading to potentially misleading results if gas flow conditions are not adhered to as per manufacturer guidelines Some instruments are also sensitive to orientation due to their reliance on convection.

Electromagnetic radiation absorption sensor – usually infrared

This type of sensor operates on the principle of energy absorption from a beam of electromagnetic radiation by the target gas Most current instruments function within the infrared (IR) spectrum.

Detection apparatus utilizing electromagnetic sensors can be classified into three main categories: a) specialized analyzers equipped with sampling systems; b) compact, self-contained electromagnetic detection devices designed for use in potentially explosive environments; and c) light "pipes," such as fiber optic cables, that transmit electromagnetic light from a control unit to a remote sensor cell.

In cases a), b) and c) the absorption of electromagnetic radiation by the gas is detected by spectrometric means and produces an electrical signal to provide indications of gas concentrations and alarms.

Electromagnetic sensors may be used for the detection of most flammable gas(es) in any specified range of concentrations up to 100 % (v/v) with the exception of hydrogen.

Open-path electromagnetic radiation detection apparatus differs from other types by measuring the path integral of gas concentration along an investigative beam rather than at a specific point This allows for the detection of gas presence over a broader area However, it cannot differentiate between a high concentration of gas in a short segment and a low concentration over a longer segment, which leads to non-compliance with IEC 61779-1, IEC 61779-4, and IEC 61779-5 standards.

The sensor independently measures the concentration of a specific gas, regardless of oxygen levels, by selecting the appropriate wavelength for detection Most flammable hydrocarbon gases absorb infrared radiation, allowing for concentration measurements from 0% to 100% v/v through careful optical path length selection While the response time can be quick, it is often limited by the introduction of gas into the optical path and may be further affected by weather protection housings, gas filters, and hydrophobic barriers.

Advanced sensors equipped with sophisticated signal processing offer self-diagnosing and self-calibrating features, minimizing the need for human intervention when malfunction alarms are absent These sensors provide high stability, eliminate ambiguity at concentrations exceeding the Lower Flammable Limit (LFL), and are immune to poisoning effects Additionally, modern technologies reduce maintenance requirements through automatic calibration self-checks, while built-in warnings help detect failures in the infrared source or excessive contamination of the optical system.

Infrared sensors are specifically calibrated to detect certain gases or a limited range of gases, meaning they will not identify gases outside their calibration bandwidth Therefore, devices with these sensors should only be utilized for detecting gas mixtures they are calibrated for Additionally, when filters are employed to maintain the cleanliness of optical components, they may become obstructed in excessively dirty environments.

Serious errors are likely to occur with some detectors due to the presence of water vapour or interfering gases or both.

This sensor is not suitable for measuring hydrogen.

Some types of infrared sensors, particularly open-path designs, are sensitive to misalignment caused by shock and vibration.

Semi-conductor sensor

The operation of a semiconductor sensor relies on variations in electrical conductance caused by chemisorption when the heated semiconductor sensing element interacts with gas The concentration of gases is determined by measuring the changes in conductivity.

Semi-conductor sensors may be used in either diffusion mode or in a sampling system.

This type of sensor is normally used for the detection of a specified gas in a nominated range of concentrations.

Semi-conductive sensors are sensitive to a wide range of gases and produce large signal changes at low gas concentrations.

Semi-conductor sensors designed for detecting flammable gases often lack specificity and can be affected by variations in humidity and the presence of interfering gases Additionally, these sensors may experience drift in both their zero and span readings Notably, certain gases, such as NO₂, can generate negative signals.

NOTE – Normally, the manufacturer will give guidance on substances that will inhibit operation of the sensor or produce false indications.

Electrochemical sensors

The principle of operation of the electrochemical sensor depends upon the change of the electrical parameters of electrodes when a specific gas is present The change in the electrical

Licensed to MECON Limited for internal use in Ranchi and Bangalore, this document is supplied by the Book Supply Bureau The parameters arise from a chemical redox reaction occurring on the surface of electrodes situated in an electrolyte, with both electrodes and electrolyte typically enclosed within semi-permeable membranes.

Electrochemical cells are compact, require little power, and have a high sensitivity to certain gases.

Electrochemical cells have several disadvantages, including their inability to detect simple hydrocarbons such as methane, ethane, and propane Additionally, these cells can gradually degrade over time and may be affected by the presence of interfering gases Regular adjustments are required to address drifts in zero and span, and their response and recovery times are relatively slow, often exceeding 30 seconds Furthermore, the properties of the electrolyte can limit their operation in low temperatures below -15 °C and high temperatures above 50 °C.

Flame ionization detectors (FID)

The flame ionization detector operates on the principle of ionizing organic compounds, creating an ion cloud that moves between electrodes in the ionization chamber This movement generates an electric current, which is directly proportional to the concentration of gas or vapor present in the gas stream.

This type of sensor is used for the detection of gas/air mixtures up to the lower flammable limit.

This type of sensor is used where high sensitivity, wide measuring range, small measuring uncertainty, poison resistance and fast response time are of main interest.

The sensor is suitable for the measuring range from ml/m 3 (10 –6 ) up to the lower flammable limit.

The operational principle of the sensor is non-selective, as it responds to all organic carbon compounds, generating a signal To ensure accurate readings in environments with various gases or vapors, the sensor must be calibrated for the specific gas or vapor to which it is least sensitive.

These sensors are not suitable for inorganic gases such as H 2

To maintain a consistent gas sample pressure, it is essential to keep the air and flammable gas levels stable However, it is important to recognize that flame arrestors in the sampling line can become contaminated, which may lead to challenges in maintaining a steady sample flow.

This clause, along with clauses 5 and 6, emphasizes the importance of documenting plant and site information, as well as the decisions that need to be made A typical checklist for these decisions can be found in Annex B.

General

When choosing flammable gas detection equipment, it is essential to consider specific features that may require careful handling and accurate interpretation of the results.

Each of the various types of sensor has inherent limitations, as described in clause 3.

This standard encourages the exploration of various detection principles beyond those outlined in sections 3.2 to 3.6, promoting innovation in detection technology However, it is crucial that any detection principle employed ensures the apparatus performs adequately for its intended use Evaluating the apparatus against the performance criteria set forth in IEC 61779-1, IEC 61779-4, and IEC 61779-5 can serve as a reliable basis for assessment in suitable situations (refer to annex D).

Selection criteria

When selecting gas detection apparatus, it is essential to consider several key criteria: the specific gases to be detected and their concentration ranges, the intended application such as area monitoring or personnel safety, and whether the device should be fixed, transportable, or portable Additionally, the classification of the intended usage zones according to national regulations, the environmental conditions in those areas, and any specific features of the apparatus that may require careful interpretation of its output are crucial Furthermore, one must account for time dependency and interactions with safety devices, as well as calibration requirements, including zero checks.

A gas detection and measurement system must be designed to ensure that the total delay time is less than the maximum allowable for its intended application Key factors to consider include the delay time of the sampling system, the sensor's response time, the delay in data transmission lines, the delay of alarm devices and switching circuits, the time required for executive action devices like shut-down valves to function, and the potential release rate of flammable gas.

4.2.2 Gases that the apparatus is required to detect

The gas detection apparatus must be highly sensitive to the specific gases it is designed to identify, while also being capable of accurately measuring a wide range of gas concentrations.

Reference should be made to the manufacturer's information to determine the suitability of particular detectors.

WARNING  THERMAL CONDUCTIVITY, INFRARED, ELECTROCHEMICAL,

AND SEMICONDUCTOR SENSORS MAY BE SENSITIVE TO CERTAIN NON-FLAMMABLE

GASES, IN ADDITION TO THE RANGE OF FLAMMABLE GASES WHICH THEY ARE

INTENDED TO DETECT FOR EXAMPLE SEMICONDUCTOR SENSORS MAY BE SENSITIVE

TO WATER VAPOUR OR TO COMBUSTION PRODUCTS IN ADDITION TO FLAMMABLE

GASES ADVICE SHOULD ALWAYS BE SOUGHT FROM THE MANUFACTURER

CONCERNING THE EFFECT OF INTERFERING GASES ON PARTICULAR SENSORS.

THESE CAN ALSO BE SENSITIVE TO NON-FLAMMABLE GASES AND INSENSITIVE TO

Determining the concentrations of individual flammable gases in a mixture is typically not feasible with the apparatus outlined in this standard Generally, the sensors described in sections 3.1 to 3.6 react to the majority or all flammable components present in a mixture, without differentiating between them.

When monitoring areas with multiple gases, it is advisable to choose a detector calibrated to the gas it is least sensitive to It is crucial to ensure that these sensors maintain sufficient sensitivity to other present gases If this is not feasible, consider using separate sensors, each calibrated for the specific gases expected in the environment.

Gas detection equipment intended for use in hazardous areas must be certified for the specific gases it may encounter This certification should ensure compliance with the appropriate gas group classifications, such as IIA, IIB, or IIC, as well as the relevant temperature class, in accordance with IEC 60079-0 standards.

WARNING – GAS DETECTION APPARATUS OF THE TYPES COVERED BY THIS

STANDARD ARE NOT NORMALLY DESIGNED OR CERTIFIED FOR USE IN OXYGEN

ENRICHED OR DEFICIENT ATMOSPHERES AND THEIR USE IN SUCH ATMOSPHERES

SHOULD BE AVOIDED FOR EXAMPLE PARTICULAR CARE SHOULD BE TAKEN WHERE

OXY-ACETYLENE WELDING OPERATIONS ARE BEING CARRIED OUT IN AN AREA

PROTECTED BY GAS DETECTION APPARATUS; SHOULD AN UNLIT OXYGEN ENRICHED

ACETYLENE JET BE ACCIDENTALLY DIRECTED AT ANY OF THE GAS SENSORS, AN

UNCONTROLLED IGNITION COULD OCCUR; OXYGEN ENRICHED ACETYLENE IS A

4.2.3 Intended application of the apparatus

4.2.3.1 Fixed apparatus and fixed systems

Fixed apparatus should be selected where it is required to provide permanent gas monitoring in selected areas of a plant or other installation.

Fixed apparatus typically includes sensors or sampling points situated in hazardous areas, along with associated equipment that can be found in either hazardous or non-hazardous locations, such as a control room All components of the system are permanently installed.

There are three primary types of fixed apparatus utilized in hazardous environments The first type features both the sensor and control unit located within the hazardous area, which can be either combined or provided as separate components The second type consists of individual sensors placed in the hazardous area, linked to the control equipment situated in a safe area.

The sampling apparatus is ideal for most industrial applications, especially when a quick response is necessary It typically includes multiple sampling points in hazardous areas, linked to a sensor via aspirated sample lines This setup is often more advantageous than traditional methods in situations with relatively stable process conditions or when environmental factors make sensor accessibility challenging.

NOTE – Apparatus and systems of the kind described in c) above should not be confused with those for process control, which do not come within the scope of this standard.

Gas detection apparatus may be designed to produce any or all of the following:

2) audible and/or visual alarms;

3) outputs to initiate actions such as process shutdown and automatic safeguard actions, for example process control, ventilation, elimination of ignition sources, etc.

Where item 3) is required, additional hardware may be necessary.

Each installation should be considered in its own right, in consultation with the manufacturer and safety authorities and in compliance with any mandatory local safety regulations.

Transportable apparatus should normally be selected for such purposes as monitoring temporary work areas ("hot" work) and areas where flammable liquids, vapours or gases may be transferred.

Portable apparatus should normally be selected for such purposes as leak detection, verification of gas-free conditions, safety checks and similar applications.

Portable devices typically operate in diffusion mode; however, for leak detection or gas monitoring in confined spaces that are difficult for the user to access, a static sample probe or a hand- or mechanically aspirated sample probe is essential.

When using portable equipment in environments where gas concentrations may exceed the lower flammable limit (LFL), it is essential to choose apparatus specifically designed for such conditions.

When choosing portable or transportable equipment, key factors to consider include its size, weight, and durability, as well as its power supply needs Additionally, the type of indication required and the visibility or audibility of alarms play a crucial role in the selection process.

Miscellaneous factors affecting selection of apparatus

Certain flammable gas detection devices can be affected by external radiofrequency interference, leading to issues like calibration errors, zero drift, and false alarms To avoid these problems, it is essential to choose equipment that is resistant to such interference.

Gas detection devices are primarily utilized in unclassified, zone 1, and zone 2 areas, while certified intrinsically safe equipment according to Ex ia standards can be employed in zone 0 areas These detectors are specifically designed for identifying combustible gases in air with about 21% oxygen by volume For guidance on system safety and instrument performance in low or high oxygen environments, refer to the instruction manual or consult the manufacturer for advice.

If it is necessary to detect mixtures in concentrations above the LFL, use an instrument or dilution accessory designed for this purpose.

Nature of a release

The build-up of a flammable atmosphere is significantly influenced by various chemical and physical parameters related to the release These parameters include intrinsic properties of the flammable material as well as specific characteristics of the process involved For clarity, the impact of each parameter is considered while keeping all other parameters constant.

5.1.2 Release rate of gas or vapour

The greater the release rate the larger the extent and/or rate of build-up of the flammable atmosphere.

The release rate itself depends on other parameters, namely: a) geometry of the source of release

This is related to the physical characteristics of the source of release, for example an open surface, leaking flange, etc.; b) release velocity

The release rate from a source is directly proportional to the release velocity For products within process equipment, this release velocity is influenced by both the process pressure and the geometry of the release source.

A significant release rate combined with a high velocity will generate a momentum jet which will affect the behaviour of the released gas at least in the vicinity of the source.

Gas escaping at high velocity, such as from a pressurized line or container leak, initially acts as a momentum jet moving away from the release point As the distance from the source increases, the jet's momentum diminishes, leading to dispersion influenced first by buoyancy effects and later by gas diffusion patterns.

The concentration of flammable vapour or gas in the released mixture influences the release rate; d) volatility of a flammable liquid

The relationship between vapor pressure and heat of vaporization is crucial; when vapor pressure is unknown, the boiling point and flashpoint serve as useful indicators.

An explosive atmosphere cannot form if the flashpoint of a flammable liquid is significantly higher than the maximum temperature of the liquid A lower flashpoint increases the potential for a flammable atmosphere to develop Some liquids, like certain halogenated hydrocarbons, lack a flashpoint but can still create an explosive gas environment In such instances, it is essential to compare the equilibrium liquid temperature at the saturated concentration of the lower flammable limit with the maximum liquid temperature Liquids must be considered when their temperature exceeds TF-5K, where TF represents the flashpoint.

NOTE – Under certain conditions, the mist of a flammable liquid may be released at a temperature below its flashpoint and still produce an explosive atmosphere.

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU. e) liquid temperature

The vapour pressure increases with temperature, thus increasing the release rate due to evaporation.

The temperature of a liquid after it has been released may be increased, for example by a hot surface or by a high ambient temperature.

A lower Lower Flammable Limit (LFL) volume ratio of flammable gases or vapors in air leads to a faster accumulation of a flammable atmosphere With the same release rates, gases with lower LFL values reach their ignition concentration more rapidly than those with higher LFL values.

LFL and UFL both vary with temperature and pressure but normal variations in these parameters do not appreciably affect the limits A useful reference is IEC 60079-20.

Improving ventilation efficiency reduces the buildup of flammable atmospheres Conversely, obstacles that hinder ventilation can accelerate this buildup However, certain barriers, such as bunds, walls, or ceilings, can also restrict the extent and rate of flammable atmosphere accumulation.

5.1.5 Relative density of the gas or vapour which is released

The behavior of gas released with minimal initial velocity, such as vapor from a liquid spill, is influenced by buoyancy and is determined by the gas's relative density compared to air.

Gases or vapors that are much lighter than air will rise, while those that are significantly heavier will settle at ground level The accumulation of a flammable atmosphere at ground level increases with higher relative density, whereas the vertical spread of a flammable atmosphere above the source grows with lower relative density.

For practical purposes, a gas or vapor mixture with a relative density under 0.8 is considered lighter than air, while a relative density over 1.2 is deemed heavier than air Mixtures with relative densities between these values should be evaluated for both characteristics.

Mixtures of high and low density gases with air exhibit minimal density variation, and once these gases are combined, they do not separate; instead, they can only become further diluted.

5.1.6 Source temperature and/or pressure

The absolute density of a gas or vapor release can be significantly influenced by its temperature and pressure before release, especially when these conditions differ greatly from the ambient temperature and pressure This variation can impact the behavior of the release near the source.

5.1.7 Other parameters to be considered

Other parameters such as climatic conditions and topography may also have to be taken into consideration.

If there is significant ambient air movement or the release is into enclosed spaces, then the above behaviour will be modified as described in the following subclauses.

Outdoor sites and open structures

Gas dispersion in outdoor sites is influenced by wind speed and direction, with lateral spread reduced upwind and increased downwind, particularly at high wind speeds In the presence of buildings or structures, complex airflow patterns can lead to significant gas accumulation in partially enclosed or restricted spaces Therefore, when planning to install gas detectors in major plants, it is advisable to utilize mathematical models of gas dispersion or conduct scaled wind tunnel tests during the design phase.

Local thermal effects play a crucial role in shaping air flow patterns, which can significantly impact gas dispersion For instance, substantial thermal gradients can occur near hot surfaces Moreover, the gas's relative density is influenced by its own temperature as well as that of the surrounding air.

Buildings and enclosures

In enclosed spaces, the risk of hazardous gas accumulation is significantly higher than in outdoor environments When gas is released indoors, it combines with the surrounding air, creating a gas/air mixture The characteristics of this mixture are influenced by several factors, including the velocity of the gas release, the release location, gas density, ventilation, and any thermal flows present These considerations are crucial for determining optimal sensor placement.

In the absence of ventilation and thermal effects, the release of a lighter-than-air gas creates a gas/air mixture that rises to the ceiling, while the release of a heavier-than-air gas results in a mixture that settles towards the floor.

When a release occurs as a momentum jet, its behavior can change significantly For instance, if a lighter-than-air gas jet is directed downward, the gas/air mixture can spread from the ceiling to below the release point Conversely, if a heavier-than-air gas jet is directed upward, the mixture may extend from the floor to above the release point.

NOTE – If a source of a gas release is present in buildings or enclosures, then there should be adequate ventilation provided.

The ventilation of buildings and enclosures is achieved by "natural means," "mechanical means", or a combination of the two.

When the concentration of a release approaches the lower flammable limit (LFL), it tends to disperse with the normal airflow due to the minimal density difference between the gas mixture and the surrounding uncontaminated air.

Natural ventilation refers to the movement of air into and out of a building through intentional or unintentional openings This airflow is driven by two main factors: pressure differences caused by wind and buoyancy effects resulting from temperature variations in the atmosphere.

This document is licensed to MECON Limited for internal use in Ranchi and Bangalore, as supplied by the Book Supply Bureau It discusses the interaction between the air inside a building and the outside environment, highlighting that when the temperature inside is lower than that outside, natural ventilation can create a downward airflow.

The introduction of gas or vapor into a naturally ventilated space will likely lead to the creation of a gas/air mixture, akin to the process outlined in section 6.3.2.

However, in this case, the gas concentration in the mixture will be lower for a given release rate due to dilution by the ventilation air flow.

When a heavier-than-air gas or vapor is released in an enclosure with natural ventilation that creates an upward airflow, the gas/air mixture can rise above and below the release point In contrast, if a lighter-than-air gas or vapor is released in an enclosure with downward airflow, the gas/air mixture can extend both below and above the release level.

NOTE – Further information on natural ventilation is given in IEC 60079-10.

Mechanical ventilation refers to the process of air movement within an enclosure facilitated by mechanical devices, such as fans This method can achieve significant air flow rates, often exceeding 12 volume changes per hour.

The gas concentration within an enclosure ventilated by mechanical means will, in general, be much less than that resulting from a similar release into a naturally ventilated enclosure.

A well-designed ventilation system ensures that the entire volume within an enclosure is effectively circulated by airflow However, certain geometries can create areas of stagnant air, known as "dead spaces," where gas and air mixtures may accumulate To enhance safety, it is crucial to position detectors in these areas.

NOTE – Smoke tracers may assist in identifying the air movement within an enclosure and the presence of any dead spaces where gas/air mixture may accumulate.

If a sensor is installed in the intake or exhaust duct of a mechanical ventilation system

(depending on where the release might occur), then the alarm set point should be set as low as reasonably practical.

Certain sensors utilize sintered materials as flame arrestors, but high air velocities in ducting can negatively impact the diffusion of air/gas mixtures through the sinter to the sensing element.

Environmental considerations

Environmental operational parameters should be included in the instruction manual.

Where environmental conditions are beyond specified values, the manufacturer should be contacted to ensure that the apparatus is suitable.

6 Installation of fixed gas detection apparatus

Many factors are involved in the selection of detector head quantity and location including industry/national standards which may dictate these requirements.

Basic considerations for the installation of fixed systems

If the instrument or any auxiliary components are installed in a hazardous (classified) location, they should be suitable for the area in which they are installed and so marked.

Two main types of fixed systems are commonly used. a) Systems consisting of remote sensors connected to the control equipment by electrical cables

Industrial applications benefit greatly from systems designed for continuous monitoring of areas where flammable gases may accumulate These systems should be strategically installed to ensure they can provide early warnings of any accidental gas releases or accumulations, taking into account the number and placement of sensors for optimal effectiveness.

Remote sensors must be installed in compliance with national electrical equipment regulations In hazardous areas, these sensors and system components should utilize explosion protection methods as specified by the IEC 60079 standards for the designated zone Additionally, systems that include sampling apparatus are also subject to these requirements.

Static process conditions necessitate the use of systems where rapid response is not critical In setups with timed sequential sampling, the time interval between samples at any point must be short enough to prevent hazardous accumulations of flammable gases Additionally, the length of sample tubes and the flow rate must ensure that flammable gases do not accumulate while the sample travels from the sampling point to the sensor Therefore, it is essential to keep sample tubes as short as reasonably possible.

Location of sensors

A fixed gas detection system must be installed to effectively monitor areas where flammable gases may accumulate, posing significant hazards This system should provide early audible or visual warnings about the presence and location of gas accumulations, enabling actions such as safe evacuation, implementation of fire-fighting procedures, shutdown of processes or plants, and ventilation control, either automatically or manually.

6.2.1.2 Considerations for the locations of sensors

WARNING – SENSORS SHOULD BE LOCATED IN POSITIONS DETERMINED

IN CONSULTATION WITH THOSE WHO HAVE A SPECIALIST KNOWLEDGE

OF GAS DISPERSION, AND THOSE WHO HAVE A KNOWLEDGE OF THE PROCESS PLANT

SYSTEM AND EQUIPMENT INVOLVED, TOGETHER WITH THE SAFETY

The rationale for the selection of location of sensors should be formally recorded.

NOTE – Reference may be made to IEC 60079-10 for further information on area classification and gas dispersion.

Factors which should be taken into account in determining suitable sensor locations include, but are not limited to, the following:

When assessing a site for potential vapour or gas sources, it is essential to evaluate both indoor and outdoor areas Key factors include the location, density, pressure, and temperature of potential sources, as well as the distance from these sources Understanding the chemical and physical properties of the gases or vapours present is crucial Additionally, sensors should be placed near low-volatility liquids to monitor potential releases It is important to consider the nature and concentration of gas releases, such as high-pressure jets, slow leaks, or liquid spills, along with the presence of cavities and jets, site topography, and air movement patterns.

1) indoors: natural ventilation, mechanical ventilation,

When considering outdoor environments, it is essential to assess wind speed and direction, as well as temperature effects and the overall environmental conditions of the plants Additionally, understanding the population of plants and their specific locations is crucial, along with identifying potential sources of ignition Detectors must be strategically installed to avoid mechanical or water damage during normal operations, ensuring that they are easily maintainable and calibratable Furthermore, structural arrangements like walls, troughs, or partitions should be evaluated to prevent the accumulation of vapors or gases, while adhering to prescribed locations for optimal safety and efficiency.

For optimal gas detection, sensors should be positioned above ventilation openings near the ceiling for lighter-than-air gases, while those for heavier-than-air gases should be placed below the openings close to the floor.

To effectively detect potential gas or vapor ingress from external sources into a building or enclosure, it is essential to position sensors near ventilation openings These external sensors should complement any existing sensors designed to identify releases occurring within the building or enclosure.

If ceilings or floors are compartmentalized by equipment or other obstructions, sensors should be installed in each compartment.

Any thermally induced flow (for example from hot surfaces on plant or equipment) may affect the distribution of a gas/air mixture.

Installation of sensors

To ensure safety, sensors must be installed in all locations prone to hazardous gas accumulation, which can occur even away from potential release sources Areas with limited air circulation are particularly vulnerable Heavier-than-air gases tend to gather in pits and trenches, while lighter-than-air gases can collect in overhead spaces.

For optimal gas detection, sensors must be positioned near significant gas release sources However, to prevent false alarms, they should not be placed directly next to equipment that may cause minor leaks during regular operation.

In general, on open sites minor leaks may be dispersed without causing a hazardous accumulation.

To effectively detect gas leaks within a specific area, sensors should be strategically positioned around the site's perimeter However, this setup may not offer timely alerts for gas releases Therefore, it is crucial not to rely solely on this arrangement, especially if a leak poses a serious risk to personnel or property within the perimeter.

Sensors shall be connected to their respective control unit, as specified by the manufacturer

(observing maximum loop resistance, minimum wire size, isolation, etc.), and use a cable, wire and conduit system, or other system suitable and approved for the purpose and area classification.

Lubricate all threaded connections, but ascertain that the lubricant contains no substance (for example silicone) that might be deleterious to the sensors.

In many cases, the orientation of the sensor may be specified by the manufacturer.

Adequate drainage should be incorporated into the system design to minimize moisture and condensation in the instrument, detector head and interconnecting cable/conduit system.

Any potential flammable gases introduced into sampling systems should be vented in a safe manner.

Timing of installation during construction operations

Sensors should be installed as late as possible in any programme of construction operations

Before introducing gas or vapors into the system, it is crucial to conduct construction, refitting, or maintenance activities to prevent sensor damage, particularly from processes like welding and painting.

If already installed, sensors should be protected with an air-tight seal to avoid contamination during construction work, and should be clearly marked as being non-operational.

Safety in fixed systems

In the event of a gas detector system failure or when channels are taken out of service, it is crucial to implement additional safety measures to ensure adequate monitoring of plant areas.

To enhance safety, it is essential to implement additional measures such as signaling faults in gas detection apparatus, utilizing portable gas detection devices, increasing ventilation, eliminating ignition sources, interrupting the supply of flammable gases or liquids, and ensuring the rinsing and draining of installation components Furthermore, switching off plants or specific sections of them is crucial for maintaining a secure environment.

A fixed system must be installed to ensure that the failure or temporary removal of individual components does not jeopardize the safety of the protected premises It is advisable to duplicate or triplicate remote sensors and control devices in areas that require continuous monitoring.

6.5.3 Protection against loss of main power supply

Protection against loss of the main power supply should include a) main power supply

The main power supply should be designed so that the unrestricted operation of gas detection apparatus and alarm functions are guaranteed.

Breakdown or fault of main energy supply should be detectable Safety of the monitored area shall be preserved by appropriate measures.

The main power supply shall have a separate circuit with specially marked fuse used only for the gas detection apparatus; b) emergency power supply

In situations where an emergency power supply is necessary for the operation of gas detection systems, it must remain active until normal power is restored or monitoring is no longer needed Additionally, any external power supply used must be appropriate for the specific area, taking into account environmental factors and area classification.

Breakdown of emergency power supply should be indicated by an alarm signal.

Environmental conditions

When selecting fixed apparatus and their sensors, it is crucial to consider the diverse environmental conditions they will encounter over extended periods Careful attention to these factors ensures optimal performance and longevity of the equipment.

Sensors in buildings are typically shielded from harsh weather, but outdoor sensors face challenging environmental conditions It's crucial to consider these factors, as high winds can lead to zero reading drift and may cause temporary sensitivity loss during calibration due to the dilution of the detected calibration gas.

Proper placement of sensors in exposed areas is crucial, and effective weather protection measures must be implemented Environmental factors such as steam, heavy rain, snow, ice, and dust can negatively impact sensor performance Additionally, some materials that are otherwise appropriate for sample lines or weather guards may degrade due to sunlight or other environmental conditions.

Excessive ambient temperatures can lead to detection errors and shorten sensor lifespan Furthermore, when operating at high temperatures, detectors may exceed the manufacturer's specified temperature range, potentially jeopardizing their compliance with certification standards.

Gas detectors functioning at temperatures below –10 °C may fall outside their specified temperature range, leading to reduced accuracy To mitigate this issue in portable applications, it is advisable to store the detector in a warm environment when not in use.

To ensure optimal performance, gas detectors should not be placed directly above heat sources like ovens and boilers Instead, it is advisable to position them at an equivalent height, but at a safe distance from these heat sources.

All detector heads and instruments should be mounted in areas which ensure compliance with the manufacturer’s operating temperature specifications.

Excessive vibration levels in machinery can lead to equipment failure; therefore, it is crucial to ensure that the equipment is designed to endure such vibrations or that appropriate vibration isolation mountings are utilized.

6.6.4 Use of sensors in corrosive atmospheres

Precautions should be taken to protect sensors from damage resulting from exposure to corrosive atmospheres (for example ammonia, acid mist, H 2 S etc.).

Sensors mounted in positions where they may be exposed to mechanical damage should be adequately protected in accordance with recommendations provided by the manufacturer.

Where necessary, adequate precautions, for example by screening, should be made to guard against the effects of external radiofrequency interference.

NOTE – Reference should be made to applicable national regulations regarding electromagnetic compatibility.

Hosing down a plant can significantly damage gas sensors, so it is advisable to avoid this practice whenever possible If hosing is unavoidable, it is essential to take measures to protect the sensors.

Sensors should not be exposed to airborne contaminants which may adversely affect their operation For example materials containing silicones should not be used where catalytic sensors are installed.

Sample lines

To prevent condensation in sample lines, it is crucial to choose materials that minimize gas adsorption Keeping sample lines short is essential, and precautions must be taken to avoid dilution from leakage or diffusion of air or gas into or out of the sampling line.

Access for calibration and maintenance

To ensure optimal performance, sensors must be easily accessible for routine calibration, maintenance, and electrical safety inspections If direct access to the sensors is not feasible, it is essential to implement a remote gas calibration facility as a minimum requirement.

However, in principle, gas detection apparatus should be used and installed in such a way that only authorized personnel can influence the operation of the apparatus.

Commissioning

The installation must be thoroughly inspected to confirm that it meets the required standards and that the methods, materials, and components comply with IEC 60079-0 Additionally, it is essential to ensure that the operating instructions, plans, and records are provided Key items for inspection include these critical elements.

Ensure that electrical connections are securely tightened, inspect for leaks in the sample line and verify proper flow, and examine the flame-arresting systems for clogs or dirt Additionally, check the battery voltage and condition, making necessary adjustments or replacements as outlined in the instruction manual, and conduct a test of the malfunction circuit(s).

After installation on site each sensor should be calibrated according to the manufacturer's instructions Calibration should only be carried out by a suitably competent person.

Where a number of gases are likely to be present, reference should be made to the additional precautions described in 4.2.2.

6.9.3 Adjustment of alarm set points

For detection apparatus that only indicates up to the lower flammable limit, it is crucial to set the alarm point as low as possible to minimize nuisance alarms while ensuring safety.

Adjustments should be carried out in accordance with the manufacturer's instructions.

Operating instructions, plans and records

Instructions on the use, testing and operation of fixed gas detection systems should be made available.

For effective maintenance and record-keeping, it is essential to provide installation plans that clearly indicate the locations of all system components, including control units, sensors, and junction boxes, as well as the routes and sizes of all cables and wires Additionally, diagrams for junction boxes and distribution cables should be included.

The records should be updated when any changes are made to the installation.

It is extremely important that the equipment manufacturer's installation manual be read thoroughly, and the instructions followed completely.

7 Use of portable and transportable flammable gas detection apparatus

General

The various types of portable and transportable gas detection apparatus may be used in a variety of ways according to their particular design and specification.

Hand-held devices are ideal for leak detection and spot checks, while larger portable units equipped with visual and audible alarms can serve multiple functions, including leak detection, spot checking, and local area monitoring, tailored to the user's specific requirements.

Transportable apparatus is designed for temporary area monitoring in locations with a risk of flammable gas or vapor mixtures, such as during the loading or unloading of fuel or chemical tankers, or when conducting temporary "hot work" in classified hazardous areas under a gas-free certificate While not meant for prolonged hand carrying, this equipment is intended to remain in place for extended periods, ranging from hours to weeks.

Portable and transportable devices are often exposed to various climatic and environmental conditions Therefore, users must carefully evaluate the specific conditions affecting the device and ensure it is adequately designed or protected to withstand these challenges.

Initial and periodic check procedures for portable and transportable instrumentation 29

Initial and periodic checks are crucial for ensuring the proper operation of portable and transportable instruments, which are not used continuously It is essential to strictly adhere to the manufacturer's instructions for these checks, paying special attention to key points highlighted in the guidelines.

To ensure accurate measurements, it is essential to verify the zero reading of the apparatus when it is operated in clean air If adjustments are needed, they should be made following the manufacturer's instructions or the specified zeroing procedure.

The apparatus's response to a known calibration gas mixture must be verified and adjusted, if necessary, in accordance with the manufacturer's instructions, ensuring accurate detection of the intended gas.

NOTE – For catalytic sensors the mixture shall contain at least 10 % by volume of oxygen.

Tiêu đề	Guide for the selection, installation, use and maintenance of apparatus for the detection and measurement of flammable gases
Chuyên ngành	Electrical Apparatus
Thể loại	standards document
Năm xuất bản	1999
Thành phố	Geneva

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Dung lượng	502,02 KB