Selection of Electrical Equipment for Explosive Gas Environments
The selection of industrial electrical equipment is mainly aimed at long-term reliable operation and good workability in harsh industrial environments. Energies When electrical equipment is used in explosive gas atmospheres, special care must be taken to prevent explosion of electrical equipment due to sparks or hot surfaces. . Therefore, the purpose of the selection is to reduce the probability that an electrical device may cause an explosion to a minimum allowable.
The conditions causing the explosion are that both explosive gas and ignition conditions must exist. In a certain place, the possibility of the existence of explosive gas is high, the structure of the electrical equipment should make the possibility of producing the ignition source as small as possible. On the contrary, for those places where the explosive gas is unlikely, the ignition source can be allowed to The probability of existence is slightly higher. Therefore, electrical equipment and actual dangerous conditions must be carefully matched so that on the one hand it is ensured that the safety level requirements of electrical equipment are met, on the other hand, the complexity of unnecessary costs and system design of electrical equipment is avoided. Determination of hazardous conditions and explosive gas atmospheres Provides guidance for the implementation of tests on special electrical equipment. Based on this data, studies were conducted on the relevant selection procedures for electrical equipment with appropriate protection levels. 2 Hazard Category 21 External hazards As follows: non-explosive hazardous locations: air mixtures are not expected to occur in large amounts, do not require the structure and use of electrical equipment where special precautions are taken. Explosion Hazardous Locations: Explosive gas air mixtures are expected to occur in large quantities or may occur in large quantities, requiring The structure of the device or Use a place where special precautions are taken Explosion Hazardous areas Divided areas are as follows: Zone 0: Explosive gas Air mixture Continuous or long-standing zone 1 Zone: Explosive gas during normal operation Zones where air mixture may occur Zone 2: Explosive gas Air mixture not Areas that may occur, or if present, but only for a short period of time, should be determined by the relevant experts, and use the classification map to indicate that the classification of explosion hazardous locations and their scope should be used. The map of explosive hazardous locations should also be classified. Point out the temperature class and gas group related to explosion-proof electrical equipment. 1. In well-designed and well-maintained open-air equipment, explosion hazardous locations are mainly 2 zones, and the isolation zone is zone 1 and zone 0 is rarely.
22 External hazards of electrical equipment The internal hazards in work vessels and auxiliary piping systems, etc., are not limited by the above classification of hazardous locations if it is simply because there is normally no air at atmospheric pressure. However, when electrical devices are used in such devices, the electrical devices can be compared to the hazards specified in the site classification to determine the degree of danger. For example, hydrocarbons are processed in a container under normal operating conditions. In abnormal situations, such as factory commissioning or shutdown, the risk is equivalent to that in Zone 2. When flammable substances are stored in open containers and steam When space is related to the surrounding atmosphere, this danger is equivalent to the danger in Zone 0.
23 Internal hazards of electrical equipment Sometimes flammable substances can inevitably infiltrate into electrical equipment enclosures, especially into measurement and control instruments for process variables. Depending on the structural integrity, the severity of the internal hazards of a device containing flammable substances that may be the source of release depends primarily on whether the internal release of the device occurs under normal conditions or only under abnormal conditions.
It will be appreciated that the consequences of the release of flammable materials in the enclosure may be more severe than the consequences of the same release in open air conditions because the internal release material is usually left in the enclosure. Leaks uncovered in the open air will gradually increase the concentration in the outer shell before the atmosphere reaches flammability, and the addition of this concentration can only be slowly reduced by aeration and diffusion.
On the other hand, it should be recognized that internal release usually occurs in a small enclosure, and when the release continues, the gas concentration will rapidly reach the explosion limit.
Although special procedures applicable to all forms of electrical equipment are not possible to formulate, the following rules can be used as a general guideline: when no internal release of flammable substances is likely to occur even under foreseeable anomalies, such electrical equipment can be considered as A typical example of no abnormal release is a Bourdon tube manometer with such a mechanical stiffness. Over-range metering that meets any foreseen overpressure does not deactivate the manometer. (See) Flammable substances may occur under predictable anomalies. When the internal release, the electrical equipment should be considered to have abnormal release. Typical examples are thin-walled metal diaphragms or skin tigers. Due to repeated bending or accidental exceeding the rated pressure limit, mechanical failure may occur (see). In order to ensure that there is no abnormal release within the housing, flammable substances from The risk of ignition in the container system must be minimal. During the escape period of flammable substances, operating within the operating conditions and subject to the operating conditions, abnormal release of various seals, rotary or sliding seals, various flanged couplings, non-metallic flexible conduits shall be assumed to be in use. A continuous leak occurs after a period of time.
In places where air is thin, it is also necessary to distinguish between restrictive release and non-restrictive release. Here, restrictive release refers to a release of any release of flammable substances to a value within the range of the dilution in the protective gas device. If necessary, restrictive release can be achieved by placing a damper in the pipeline that carries flammable substances to the electrical equipment.
Unrestricted release means any release that is limited by a value within the range of dilutions that are not released from the protective gas device. Normally, the internal hazard caused by anomalous release is basically the same as that in a dangerous zone at explosion in Zone 2, whereas the release is normal. Internal hazards are basically the same as those in explosion hazardous locations in Zone 0. However, there may be special circumstances beyond this rule, such as the use of inert gas for positive pressure or thin air. 3 Actual hazards After determining the external and internal hazards of process equipment and electrical equipment, the most dangerous conditions should be selected. The basis of electrical equipment.
For example, when an electrical device with an internal hazard that is substantially the same as Zone 2 is installed in a non-explosive hazardous location or Zone 2, the internal hazard determines the actual hazardous condition, and when the same electrical device is installed in Zone 1 or Zone 0, the external hazard The actual dangerous conditions are determined.
4 The use of industrial electrical equipment If the external hazard is divided into Zone 2 hazardous locations or internal hazards are similar to Zone 2 hazards, as long as the electrical equipment does not generate arc ignition or spark ignition during normal operation, and no hot surface is induced. In the case of burning capacity, high-quality industrial electrical appliances can often be used, and the use of this concept mainly in non-hazardous locations must take into consideration the possibility that electrical equipment may become a source of ignition under abnormal conditions, but in the case of hazardous areas in Region 2, such ignition sources and explosive gases are simultaneously The danger is negligible.
The suitability of industrial electrical equipment in Zone 2 shall be assessed by qualified personnel. Since only the normal conditions of use of electrical equipment are considered, this assessment is relatively easy. Especially in the field of low-current technologies, such as process equipment, if not all, most of the industrial electrical equipment can be used immediately, because these electrical equipment usually does not have the problem of heat generation, and most of the circuits are fully enclosed if normal operation will Sparks, such as accident-indicating switches, whose maximum safety circuit parameters can be found in published intrinsically safe ignition characteristic curves, without using safety factors that do not meet normal conditions should be considered to have hingH. If the industry is not possible to use non-sparking electrical appliances Special-purpose party stamp1 5 explosion-proof forms have the possibility of abnormal release of electrical equipment law to ensure that electrical equipment does not explode, these special methods are called "explosion-proof form"
Various types of explosion-proof are usually indicated by characters. The meanings of these symbols and their special explosion-proof forms are briefly explained as follows. See d explosion-proof enclosures. A type of explosion protection In this type of explosion-proof, explosion-prone electrical equipment components are enclosed in a single enclosure. The shell can withstand the explosion pressure of the internal explosive mixture without damage, or prevent the internal flame from propagating through any joint surface and each structural opening in the shell to the external explosive atmosphere. Increased safety against excessive temperatures and arcs and sparks Possibility to achieve a type of explosion protection provided by the addition of safety h: Hermetically sealed airtight enclosures contain devices for potential ignition measures to prevent mechanical damage.
The design is made that: any spark or thermal effect that meets the design specification and is electrically and mechanically or under the specified fault conditions cannot cause the specified explosive mixture to ignite. Intrinsic safety protection is divided into two categories: Class ia with specified current and The electrical safety factor of the electrical equipment will not be in normal operation, or by the application of a single fault, or the combination of two faults to produce the ignition device will not be in normal operation, or by the ignition of a single fault generated m : Encapsulant type In this explosion-proof type, potential sources of ignition are encapsulated in flame-retardant solid insulation. This material should be capable of preventing fractures under internal fault conditions. n Non-sparking devices In normal operation, neither ignited arcs nor sparks nor electrical devices capable of igniting hot surfaces are considered non-sparking.
o An explosion-proof form of an oil-filled electrical or electrical equipment component immersed in oil. This method of impregnation causes an explosion above or off the oil p: positive pressure type. This type of explosion protection is divided into three categories if the air inside the enclosure is maintained by Pressure is greater than the pressure of the external environment to prevent explosive gases from entering the electrical equipment. Apply air positive pressure. If positive air pressure is prevented from entering the electrical equipment by maintaining the pressure of the inert gas in the housing higher than the external pressure, apply positive pressure serial dilution of the inert gas. (Artificial ventilation) is the use of a sufficient amount of protective gas (usually air) into the enclosure to disperse and dilute flammable gases or vapours that may be generated from the release of flammable substances from the inside, thereby preventing explosions in explosion hazardous locations. To prevent the external atmosphere from entering the enclosure, an appropriate amount of overpressure should be maintained within the enclosure.
If it is not possible to maintain the supply of protective gas inside the enclosure, visual or audible alarms should be provided to prevent dangerous conditions. Once an alarm has occurred, correct measures must be taken to restore the supply of protective gas. If necessary, manually shut off the power supply of the electrical equipment. If it is not possible to manually switch off the power supply, it must be able to automatically switch off the power supply. q The sand-filled type is filled with fine-grained material. The form is such that the arc generated in the shell of the electrical equipment under the intended use condition cannot ignite the external atmosphere gas. The typical filling material is quartz sand.
r Restricted Respiratory Type An explosion-proof version of the ignition source enclosed in a sealed enclosure for a limited period of time sufficient to prevent the ingress of large amounts of external atmosphere to create an explosive mixture inside, and the external components are not capable of exploding s: Special explosion-proof versions The form cannot be entirely classified into any of the above-mentioned explosion-proof shapes. Several types of explosion protection can be integrated in an electrical device. For an electrical device with this type of explosion protection, the relevant documents should indicate which type of explosion protection the Table 2 of Table 1 indicates that explosion-proof forms that provide suitable levels of safety under special hazardous conditions are not actually necessary or are too good for a particular hazard in these suitable explosion-proof shapes. Therefore, all use details need to be Further analysis, in order to achieve the best choice of explosion-proof forms required for special hazardous locations, should further recognize that, for these problems mentioned above, there is no common agreement on the requirements for automatic cut-off of positive-pressure electrical equipment in case of doubt. A conservative approach should be adopted, and some requirements are expected to relax within the limits in the future Partial release Normal abnormality Category 2 Zones None Zone 1 Zone 0 Zone 2 Zone 1 See Nothing Yes 1 Zone 1 Table 1 0 Zone 1 21 or 0 Zone 1 Graph Symbol Explanation: Ai is suitable for an unsuitable 2 - if internal components or Internal circuits cannot ignite under normal conditions or have explosion-proof forms 6,11,3, or meet requirements.
Note: 1 If internal explosion does not occur in this type of explosion-proof element, it is to satisfy the two explosion-proof forms of h and m. 2 In this case, because other hazards occur first, this release is not important.
Table 2 Explosion-proof forms "P Suitable range Dangerous class Positive air pressure Positive air pressure Positive pressure with air Pressurization and dilution Internal release Normal off-normal location Classification alarm Off alarm Off alarm cut off No zone 2 Zone 1 Zone 0 No limit Zone 2 Zone 1 Zone Unrestricted Zone 2 Zone 1 Zone Restriction Unlimited Zone 2 or Zone 1 Zone 2 or Zone 1 An unrestricted Zone 2 or Zone 1 Note 2 Zone 0 A diagram Symbol Descriptions and Notes See Table 1 6 Structure Requirements The form includes the basic approach to safety, but does not specify the structural requirements for electrical equipment. This is the reason for the study of structural standards. The National Standards Association European Electrotechnical Commission for Standardization (CENELEC) and the International Electrotechnical Commission (IEC) have published a large number of such standard tables. 3 describes the current use of various standard texts. The national standards for the same explosion-proof forms require a great deal of difference. The European standard publication (EN-50.014150020) has done a lot of work in harmonizing improvements. These have been published. The standard has been recognized as a national standard for all participating CENELEC countries or recognized by these countries. In addition, it has formally stipulated that when complying with the requirements of the aforementioned European standards and being a recognized experimental institute At the time of certification, the member countries of the European Economic Community cannot restrict the free trade of electrical equipment and make the agreement significantly improve the previously existing trade level. It can be expected that the new design of electrical equipment will be based almost entirely on European standards. However, the use of electrical equipment that meets other standards is also possible.
Several countries have pilot institutes, such as BASEEA in the UK (UK Explosion Protection Electrical Equipment Certification Authority) and PTB in Germany (Federal Institute of Physics and Technology in Germany). As independent organizations, they assess whether electrical equipment complies with specific structural requirements and Regulations stipulate that electrical equipment shall withstand the test and test institute stipulated in the standard and issue a "certificate of conformity" for electrical equipment complying with the standards, and issue "certificate of inspection" for electrical equipment that does not fully comply with all the detailed provisions of the standard, but provide at least The equivalent safety level to this standard.
The above standards are mainly suitable for electrical equipment in hazardous areas in Zone 1 or Zone 0. For electrical equipment used in zone 2 hazards such as n or r, test institutes are generally not required to participate. In this case, the manufacturer can provide corresponding instructions. If such certificates or instructions exist, the user should collect, research and However, as long as they do not involve other aspects of security, such as installation and maintenance procedures, their value should not be overestimated. Therefore, it is unusual for users who are aware of the explosion hazard in their factories to use reliable and competent manufacturing. Electrical equipment produced by the factory does not use a formal certificate. This situation may be encountered in the special case of a new product (if the qualification test has not yet been completed), or where a small amount of special electrical equipment is actually not necessary to perform the usual time-consuming and costly identification steps.
Table 3 Structural Requirements Standards International Electrotechnical Commission European Commission for Electrotechnical Commissioning Standards UK Explosion-Proofing Netherlands France New Standards for American Standards Old Standards General Requirements Part 1 Part 5 Part 2 Part 6 Part 4 Part 7 Part 3 Part 2 Part 3 of Part 3 Note: 1 These National Standards are equivalent to European Standards 1980) only include two forms of explosion protection, excluding European standards in the German publication Implementing D1N national standards and VDE regulations are under study.
In the process of improving preparations.
Selection steps Based on general considerations in hazardous conditions and the above explosion-proof forms, the selection steps are summarized as follows: Step 1: Study the site classification map and install electrical equipment in non-explosive hazardous locations, such as switch rooms or control rooms, as far as possible.
Step 2: Most of the remaining electrical equipment should be installed in the hazardous area of ​​Zone 2 as much as possible. In this case, as long as it can't ignite under normal working conditions, you should first consider using general industrial electrical equipment. Step 3: In the second step, do not Where applicable, from Tables 1 and 2, appropriate explosion-proof forms should be selected. Among the possibilities given in Tables 1 and 2, priority should be given to specific hazards, but not to more dangerous locations.
Step 4: When you have selected “flameproof enclosure†and “intrinsically safeâ€, determine the appropriate temperature class for the electrical equipment and the appropriate gas group. Step 5: Select the type of explosion protection, the appropriate temperature rating, and (If necessary) Appropriate gas groups to determine the industrial availability of electrical equipment If failure, choose suitable electrical equipment that is more dangerous.
Step 6: When there are several possibilities, you should make a final choice by comparing equipment costs and operating costs.
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