Intrinsically explosion-proof and safety barrier introduction

Transcription

1 Intrinsically explosion-proof and safety barrier introduction Summary of Topics Page 1. Explosion basic knowledge 1 2. Explosion groups 1 3. Zone classification 2 4. Explosion-proof electrical apparatus design standards 3 5. Explosion-proof mark of different countries 3 6. Explosion-proof type of electrical apparatus 3 7. The basic principles of the intrinsically explosion-proof technology 4 8. The feature of the intrinsically explosion-proof technology 4 9. Intrinsically safety apparatus and associated apparatus Intrinsic safety electrical apparatus type Marking of apparatus The basic components of intrinsically safe explosion-proof system Safety barrier Guidelines for use of devices with intrinsically safe circuits Explosion basic knowledge Explosion is that the material produces a high-speed exothermic reaction. In order to have an explosion, the following three components must be present simultaneously: NO. component interpretation 1 Fuels (flammable substances) Flammable concentrations of gases and vapours or solids which have the potential for igniting an explosion 2 Oxidizers air (21 % Oxygen), pure Oxygen, oxygen releasing compounds (i.e. potassium manganate) 3 Possible sources of ignition hot surfaces, flammable or explosive gases, mechanically generated sparks, electrical installations, transient currents, static electricity, lightning, ultrasonic energy... 4 Explosive limits A mixture is only explosive when its concentration falls within certain material specific limits. These limits are called the upper and lower explosive limits and are listed in according tables. 2. Explosion groups GB ordains that combustible materials are divided into the following classes in accordance with the degree of the inflammable hierarchy. Factory type Classification Representative The min. energy of the gases gases detonating the spark Underground the miner Ⅰ Methane 0.280mJ he factory outside miner ⅡA Propane 0.180mJ Inflammable hierarchy Difficultly Page 1 of 12

2 ⅡB Ethylene 0.060mJ ⅡC Hydrogen, acetylene 0.019mJ easily The United States of America and Canada (NEC500) divide the explosive objects spreading in the air into three classes firstly. The CLASS Ⅰ are gas and vapour. The CLASS Ⅱ is dust. The CLASS Ⅲ is fiber. Then divide the gas and vapour into Group: Class group Representative gases or China/IEC/European countries corresponding dusts class (GB3836.1/IEC /EN50014) A Acetylene ⅡC (Hydrogen, Acetylene) CLASS Ⅰ B Hydrogen ⅡC (Hydrogen, Acetylene) C Ethylene ⅡB (Ethylene) D Methane ⅡA (Propane) E Metal dusts Ⅰ(Methane) CLASS Ⅱ F Coal dusts G Grain dusts To be determined(Ⅲ) CLASS Ⅲ Ⅲ No grouping 3. Zone classification According to EN and EN explosion hazardous areas are divided into zones such as flammables gases, vapours, mists and combustible dust. The classification is based on the likelihood that a dangerous explosive atmosphere occurs. The ATEX directive has re-defined the zone division as follows: Likelihood of an China IEC European explosive Likelihood of an Likelihood of an atmosphere : explosive explosive China occasionally atmosphere: unlikely atmosphere: continuously, for or infrequently - for a Description occasionally long periods or short period only frequently Union European Union constant or long occasionally seldom or short term term corresponds corresponds to corresponds to < 10 to > 1000 h/year h/year h/year gases and Zone zone 0 zone 1 zone 2 vapours classification dusts zone 20 zone 21 zone 22 North America Description Under normal operating conditions, it The flammable gas or continuously, intermittently or vapor will release and periodically exist the flammable gas become dangerous or vapor and the concentrations reach only when the a dangerous level. container rupture, the system is failure or the equipment anomalies Page 2 of 12

3 Zone classification (gases and dusts) Div.1 abnormal operate occasionally. Div.2 4. Explosion-proof electrical apparatus design standards country Standards Code Standards Name Country Standards GB Electrical apparatus for explosive gas atmospheres--part 1:General requirements GB Electrical apparatus for explosive gas atmospheres--part 4:Intrinsic safety IEC IEC :1998 IEC :1999 European Committee for Standardization(CENELEC) EN Intrinsic safety North American (America, Canada) NEC500 Country Standards GB is equivalent to adopting IEC Explosion-proof mark of different countries mark signification Ex Explosion-proof mark of China and IEC EEx European economic community AD Italy MS AE France FLP Great Britain UL FM America E Germany \IEC China National Quality Supervision and Test Centre for Explosion Protected Electrical Products (CQST): CQST is one of the China famous professional testing and certification authorities and engages in safety test, examination and evaluation of Ex electrical products. CQST has made deep research and obtained extensive experience in the field of explosion protection electric safety technique and has a high reputation in the word. CQST has signed the cooperation agreements of mutual acceptance of test results separately with more than ten international wellknown Ex scientific research and test bodies, such as UL (U.S.A.), FM (U.S.A.), PTB (Germany), NEMKO (Norway), testsafe (Australia), CCVE (Russia) and LCIE (France).These cooperation have promoted the international exchange of Ex technique and offered convenient services for sino-foreign Ex product entering into opposing side market. 6. Explosion-proof type of electrical apparatus GB (Electrical apparatus for explosive gas atmospheres) defines as follows: Page 3 of 12

4 code type standards explosion-proof measure application zone d flameproof GB Isolate the ignition source Zone1,Zone2 existing e increased safety GB Try to prevent producing the Zone1,Zone2 ignition source ia intrinsic safe GB Limit the energy of the ignition Zone0-2 source ib intrinsic safe GB Limit the energy of the ignition Zone1,Zone2 source p Pressurized GB Isolate the hazardous substance Zone1,Zone2 and the ignition source o jelly filled GB Isolate the hazardous substance Zone1,Zone2 and the ignition source q sand filled GB Isolate the hazardous substance Zone1,Zone2 and the ignition source n non-sparking GB Try to prevent producing the Zone2 ignition source m pour enveloping GB Try to prevent producing the Zone1,Zone2 ignition source h hermetically sealed GB Try to prevent producing the ignition source Zone1,Zone2 For automatic control and instrumentation, intrinsic safe, flameproof and increased safety types are used commonly. 7. The basic principles of the intrinsically explosion-proof technology In electrical apparatus, electric spark and thermal effect is the main ignition sources that set danger explosive gas off. The intrinsic safety methods are considered protection devices and their function is to avoid possible errors and faults by preventing the transfer of unsafe levels of energy to the hazardous area. Possible faults are: 1) Excessive voltage in the hazardous area. 2) High current levels in the hazardous area (short-circuit). The intrinsic safety method of explosion protection always relates to intrinsically safe circuitry that comprises an intrinsically safe apparatus, an appropriate electrical power source and the connecting cables. In intrinsically safe circuits, an explosive environment cannot be ignited by sparking or a thermal effect when operating normally under prescribed fault conditions. In an intrinsically safe circuit for categroy ia, 2 calculable faults (see definition EN 50020) must not cause an ignition and in category ib only 1 such fault is permissible. Limiting the power supply, total inductance and total capacitance within the intrinsically safe circuitry is the basic principle of the intrinsically safe explosion protection method. The project manager or user has to compare the permissible internal limit values for intrinsically safe electrical apparatus with the permissible connection values of the associated electrical apparatus. 8. The feature of the intrinsically explosion-proof technology Page 4 of 12

5 Intrinsically safe explosion-proof technology is actually a low-power design technology. It is a very good apparatus for the design of industrial automation instrument. Compared with other explosion-proof type, using the explosion-proof electrical technology has following characteristics: 1. Intrinsically electrical apparatus have simple structure, small size, light weight. 2. Installation and maintenance can be with power input. 3. High security and reliability. Do not reduce the security and reliability of the electrical apparatus for outside structural damage. 4. Applicable range is wide range. It is the only explosion-proof technology that can be applied in the zone Intrinsically safety apparatus and associated apparatus a. Intrinsically safety apparatus In the conditions of the national standards, any spark and thermal effects produced by this electric apparatus can not ignite the explosive gas environment It can be used in hazardous area. It can be divided into general electrical apparatus and simple electrical apparatus. General electrical apparatus: Has the energy storage element, need explosion-protection authentication. Such as intrinsically transmitter, approach switch, and so on. Simple electrical apparatus:if Voltage is lower than 1.2V, current is lower than 0.1A,energy is lower than 20uJ or power is lower than 25mW, it is considered as simple electrical apparatus. Do not need explosion-protection authentication, can directly access to the intrinsically explosion-protection system. Typical feature as followers: Internal equivalent inductance Li = 0, Internal equivalent capacitance Ci = 0. Common simple apparatuses are: thermocouples, thermal resistors, resistor, light emitting diode, contact switch, and so on. b. Associated apparatus (safety barrier) It is an apparatus installed in a safety area, connecting the intrinsically electric apparatus and non-intrinsically electric apparatus. Safety barrier limit the energy get in to the hazardous area in the safety rang and ensure that the field equipment, personnel and production are safety. 10. Intrinsic safety electrical apparatus type a. Apparatus groups According to the principle of GB , intrinsic safety electrical apparatus has 2 types as follows: Group interpretation Device Group I For mining underground with a potential hazard due to firedamp and/or combustible dusts. Device Group II For all other locations in which a potentially explosive atmosphere exists. There are three class of A, B and C b. Apparatus Category Intrinsic safety electrical apparatus and associated apparatus can be divided into ia class and ib class according to the places used or connected and the safety standards. Code interpretation Page 5 of 12

6 ia ib Very high level of safety: devices featuring two independent means of protection; even in the event of rare device disturbances, the device remains functional and maintains the requisite level of protection. It can be used in the hazardous areas of zone0, zone1 and zone2. High level of safety: devices featuring one means of protection. Even in the event of frequently occurring device disturbances or equipment faults which normally have to be taken into account the device provides the requisite level of safety. It can be used in the hazardous areas of zone1 and zone2. c. Install temperature class The temperature class indicates the maximum surface temperature of an apparatus and must be lower than the minimum ignition temperature of the flammable material to prevent an ignition. The ignition temperature (defined as the temperature at which a mixture self-ignites during testing) directly relates to the temperature class. see table below. temperature class Maximum surface temperature ignition temperatures of flammable materials T T T T Common explosive gas Hydrogen, acrylonitrile, etc. (46 kinds) Ethane, ethylene, etc. (47 kinds) Petrol, crotonaldehyde, etc. (36 kinds) Acetaldehyde, tetrafluoroethylene, etc. (6 kinds) T carbon disulfide T ethyl nitrate and ethyl nitrite Intrinsically safe equipment for direct installation in hazardous areas requires temperature classification. For associated apparatus temperature classification is not needed. Electrical safety Low High 11. Marking of apparatus Equipment for explosion protected areas must be clearly marked. There are two different types of marking. According to CENELEC marking of an apparatus conforming to EN 50014/20 must provide the following information: Manufacturer s name or trademark Part number EEx-symbol Ignition category (e. g. ia ) Designated group together with the respective subdivision (e. g. IIC) Temperature class or maximum surface temperature (for group II devices only) Page 6 of 12

7 Serial number (may be omitted if space is restricted) Test authority, date and file number X after the test certificate number indicates that special conditions must be met (see certificate for special conditions) An intrinsically safe apparatus could have the following marking: Ex ia ⅡC T6 temperature class explosion group type of protection conform to IEC or chian standard An associated apparatus could have the following marking: [Ex ia]Ⅱc associated apparatus explosion group type of protection conform to IEC or chian standard 12. The basic components of intrinsically safe explosion-proof system Hazardous Area Safe Area Intrinsically Safety apparatus cable Associated apparatus (safety barrier) Centralized Control System/ PLC/ Operation Terminal Intrinsically performance certification parameter of intrinsically electrical apparatus: 1 Maximal Input Voltage (Ui):The maximal voltage put on the intrinsically safety apparatus, and can not lead to the intrinsically performance failure (AC peak or DC). 2 Maximal Input Current (Ii):The maximal current put on the intrinsically safety apparatus, and can not lead to the intrinsically performance failure (AC peak or DC). 3 Maximum Input Power (Pi):The intrinsically current maximum input power that may consume in the intrinsically safety apparatus inside when the intrinsically safety apparatus connects with the external power and do not lead to the intrinsically performances failure. Page 7 of 12

8 4 Maximum internal Equivalent Capacitance (Ci): Total equivalent capacitance inductance appearing by the intrinsically safety apparatus. 5 Maximum internal Equivalent Inductance (Li): Total equivalent internal inductance appearing by the intrinsically safety apparatus. Intrinsically performance certification parameter of associated apparatus (safety barrier): 1 Maximal Voltage (AC RMS or DC Um):The maximal voltage put on the associated apparatus (safety barrier), and can not lead to the intrinsically performance failure 2 Maximum Output Voltage (Uo):The maximum Output Voltage that may appear in the intrinsically current when open circuit and the intrinsically safety apparatus voltage (including the Um and Ui ) is maximal (AC peak or DC). 3 Maximum Output Current (Io):The intrinsically current maximum current that come from the intrinsically safety apparatus (AC peak or DC). 4 Maximum Output Power (Po):The intrinsically current maximum power that can from the intrinsically safety apparatus. 5 Maximum external Capacitance (Co):The intrinsically current maximum capacitance that can connects to the intrinsically safety apparatus and do not lead to the intrinsically performances failure. 6 Maximum external Inductance (Lo):The intrinsically current maximum inductance that can connects to the intrinsically safety apparatus and do not lead to the intrinsically performances failure. Patch cable:intrinsically cable is a low capacitance and low inductance cable. Compared with other cable has excellent shielding and anti-interference performance. It suits for explosion hazardous area and having high explosion-proof security requirement. Its intrinsically performance as follows: Maximum distribution capacitance that the cable allow: Cc = Ck * L Maximum distribution inductance that the cable allow: Lc = Lk * L Ck Distributed capacitance per unit length of the cable Lk Distributed inductance per unit length of the cable L The actual length of the cable The various configuration of the intrinsically explosion-protection system must meet the following conditions: 1. The explosion-protection mark class of the intrinsically safety apparatus can not higher than the associated apparatus (safety barrier). 2. Authentication parameters have to satisfy the following inequality between the associated apparatus, intrinsically safety apparatus and the cable. Associated apparatus Demonstration of intrinsic safety apparatus Intrinsically safe apparatus + cable Uo Ui Io Ii Po Pi Co Ci + Cc Lo Li + Lc Page 8 of 12

9 13. Safety barrier Some special industrial fields not only need the two-wire transmission for signal isolation and providing power input, but also need the explosion-protection performance about safety spark-proof and reliably avoiding the high voltage and signal contacting. Using the current and voltage double limiting loop, limiting the energy getting into the hazardous area lower than the ration, these can improve the explosion-protection performance of the intrinsically system, at the same time increase the anti-interference ability greatly enhanced the reliability greatly of the system. These field products having specific function are safety barrier. Safety barrier is used in the design of an intrinsically explosion-proof system. It is installed in a safety area. Safety barrier is an apparatus with intrinsically current and non-intrinsically. These current limits the energy get in to the hazardous area by limiting the current and limiting the voltage, then avoid the hazardous energy enter into the hazardous area. Safety barrier is named associated apparatus, it is an important component of the intrinsically system. Because the safety barrier is designed as an interface between the field apparatus and the control room apparatus, no matter the control room apparatus is in normal or fault, safety barrier both can ensure the energy send to the field through it is intrinsically safe. According to signal direction, Safety barrier have two types as follow: Ⅰ. Detection type safety barrier: Detection type safety barriers usually are used with the transmitter at hazardous area. The functions as follow: a. Convert the signal from the hazardous area to standard current or voltage signal, isolate, and transmit it to safety area. B. Limit the risk energy getting into s hazardous area from intrinsic safety terminal. C. Give isolation power distribution to the field transmitter. Ⅱ. Operation type safety barrier: Operation type safety barrier usually are used with the current to pneumatic converter or electric appliance valve positioner at hazardous area. The functions as follow: a. Isolate and transmit the current signal from the safety area to the hazardous area, and driver the field actuating element (current to pneumatic converter ), etc. b. Limit the risk energy getting into s hazardous area from intrinsic safety terminal. C. Give isolation power distribution to the safety area. Safety barrier can be devided into zener safety barrier and isolation safety barrier according to construction. Ⅰ. Zener safety barrier The current uses the rapid fuse, current-limiting resistance or voltage-limiting diode to limit the input energy, thus ensue the energy output to the hazardous area. The principle as follows: Hazardous Area Safe Area Zener safety barrier have simple principle. Its current can achieve easily and the price is cheap. But the Page 9 of 12

10 application reliability is affected greatly for its principle defect and this limits its application range. The reason as follows: a. Installation location must have reliable ground system, and the ground must be less than 1Ω, otherwise it would lose the explosion-protection performance and reduce signal anti-interference capacity directly, affect the stability of the system. b. The instrument from hazardous area must is isolation type, or the signal connected to the ground by the ground terminal of the zener safety barrier can not transmit correctly, and because of signal grounding, it will reduce signal anti-interference capacity directly, affect the stability of the system. c. Zener safety barrier has greater impact on the power supply, at the same time it can be damaged easily for the instability of the power supply. d. Using zener safety barrier, signal negative pole all need to the intrinsically ground, it will reduce signal anti-interference capacity directly, affect the stability of the system., the impact on the DCS system is sharp prominent. Ⅱ. Isolation safety barrier Isolation safety barrier is composed of limiting energy unit, isolation unit, signal process unit, power supply unit, etc. The principle as follows: Hazardous Area Safe Area Isolation safety barrier not only has limiting energy function, but also has isolation function, at the same time in accordance with intrinsically limiting energy requirements. Compared to zener safety barrier, although the price is slightly higher, but its others outstanding advantage give more benefits to user. a. Transformer isolated barriers do not require grounding, because the transformer in the barrier galvanically isolates the hazardous area from the safe area. b. Transformer isolated barriers have an internal voltage regulator. The function necessary to the application is thereby assured.a regulated power supply is only necessary for Zener barriers. If the power supply for Zener barriers is too low, the field device can no longer operate. The same applies to: If the power supply becomes too high, the Zener diodes divert the current and the fuse blows when the current exceeds its maximum rating. c. Any process monitoring signal circuit (ma- or mv-signal) that is connected to a transformer isolated barrier is not grounded. In contrast to this, Zener barriers require a suitable ground. From an instrumentation point of view this is best achieved through a quasi ungrounded system. d. Transformer isolated barriers from MORNSUN can be repaired; Zener barriers cannot, because they are filled with an epoxy resin. e. Transformer isolated barriers have an internal current limit. The fuse is not activated if there is a short circuit at the exterminal. f. Instead of using diagrams and Ohm's Law to determine the proper barrier, each transformer isolated barrier is designed for special applications (i. e. 4 ma~20 ma amplifier, thermocouple amplifier, solenoid drivers, etc.) limiting energy unit Isolation unit signal process unit Page 10 of 12

11 According to the function, the types of safety barrier commonly used are as follows: switch safety barrier, analog safety barrier, digital safety barrier, resistance thermometry safety barrier, thermocouple safety barrier, frequency safety barrier, and so on. 14. Guidelines for use of devices with intrinsically safe circuits The principle of selecting the intrinsic safety electrical apparatus Simple electrical apparatus: According to the principle of GB about explosion-protection standards, if voltage is lower than 1.2V, current is lower than 0.1A, energy is lower than 20uJ or power is lower than 25mW, it is considered as simple electrical apparatus. Typical feature as followers: Internal equivalent inductance Li = 0, Internal equivalent capacitance Ci = 0. Common simple apparatuses are: thermocouples, thermal resistors, ph electrode, strain gauge, switch, and so on. General electrical apparatus: a. The design whether or not in accordance with the GB and GB requirements and if the electrical apparatus had been approved by the explosion-proof inspection agency which was authorized by the national. b. If the explosion-proof mark class is applicable to the safety requirements of the hazardous area. c. Clear the authentication parameters of Ui, Ii, Pi, Ci and Li. d. If intrinsically safety current ground or if it is isolated effectively between the grounding intrinsically current and the safety barrier interface current. e. What is the method of the signal transmission? f. Clear the minimum operating voltage and loop normal operating current of the intrinsically electrical apparatus. The principle of selecting the safety barrier a. Safety barrier s explosion-proof mark class is not lower than the class of the intrinsically field apparatus. b. Make sure the end and circuit resistances can meet the minimum operating voltage of the intrinsically field apparatus. c. Safety barrier s safety parameters of the intrinsically port can conform to Uo Ui, Io Ii, Co Ci+Cc, Lo Li+Lc. d. The safety and the signal transmission mode between the safety barrier and the intrinsically field instrument must be best match. e. Do protection well. Avoid leakage current of the safety barrier affects the normal operation of the field intrinsically safety apparatus. f. Safety barrier have two types: one is zener safety barrier, the other is isolation safety barrier. The principle of selecting the cables The governing regulations cover installation of intrinsically safe circuits, mounting to external connections, cable characteristics and cable installation. Cables and terminals with intrinsically safe circuits must be marked and separated from nonintrinsically safe circuits or feature appropriate isolation (> VAC). Following an excerpt from the requirements according to EN : a. Protection against external electrical or magnetic fields (e.g. power current cables). b. Prevent conductor splicing of fine wires through wire sleeves. c. Min. cross section of 0.1 mm (also single wires of a conductor). d. Protection against damaging (mechanical, chemical, thermic...) e. Armouring, metal cladding, shielding of cables and lines Page 11 of 12

12 f. Common use of single-core nonsheathed cables of intrinsically and nonsafe circuits in one line is not permitted. g. Separate error assessment when using multi-conductor cables and lines. h. When marking cables by colour, light-blue must be used. Installation of cables Electrical cables are the connection between field and interface devices and are important functional components of an automation system. Therefore it is advisable to observe some frame conditions when installing cables. Cables must be protected against negative environmental influences. Chemical resistance, temperature rating, resistance against ultraviolet radiation and applicable operation standards are just some of the parameters which should be taken into account. By choosing an appropriate installation method it is possible to prevent damages, e.g. through vehicles. When connected to field devices, the cable should be routed in a conduit. If connected to vibrating machines or moving parts, some extra cable should be added as a reserve. The minimum bending radius must also be observed. Cable manufacturers provide information on cable installation and valuable installation hints. Data integrity and reliability depend strongly on the cable types and installation methods. Sensor cables should always be installed separately from the power cables. Electromagnetic interferences must also be avoided. Cable connections to motors which are controlled via frequency converters should be protected according to the manufacturer s safety specifications. If long cables are needed, it is recommended to use shielded and earthed cables. The line resistance of sensors may not exceed 50 Ω (EN ); the maximum cable length is determined by the cable' cross-section: R S L= δ R = line resistance [Ω] S = cable cross-section [mm 2 ] δ = resistivity [Ω mm 2 /m] L = cable length [m] Page 12 of 12