First, the safe use of explosion-proof electrical products is as important as the safety performance of the product.
For a long time, China's national standards related to explosion-proof electrical safety, mainly explosion-proof electrical basic standards (GB 3836 series-based manufacturing inspection standards) and explosion-proof electrical product standards. The main bodies of implementation of these standards are product manufacturing companies and explosion-proof electrical product quality inspection agencies. However, whether or not explosion-proof electrical products can be used in hazardous locations to ensure explosion-proof safety not only depends on the design, manufacture and inspection departments providing products with high explosion-proof safety, but also depends on the safe use of product users. Even if the performance and quality of explosion-proof electrical products are not properly used, they will not only fail to play an anti-explosive role, but may even become horrible killers that endanger security. Many tragic explosions have proved this. For example, the rubber seal in the cable entry device of explosion-proof electrical products is an important part to maintain the explosion-proof performance of the product. This part is often overlooked in use and cannot be correctly selected, installed, and maintained. For example, some users did not press it after repair, some of the selected seal ring size and material does not meet the standard requirements, and some repairs simply do not install the seal ring. As a result, the flammable gas in the hazardous area can enter the junction box cavity unimpeded. Once the junction in the terminal box junctions arc. In the event of a spark, it may ignite a flammable gas and cause an explosion. An example of such an explosion caused by the incomprehensible use of explosion-proof electrical products is a large proportion of the total accident statistics.
This shows that the safe use of explosion-proof electrical products and the quality of manufacture that reflect the safety performance of explosion-proof electrical products are equally important. This is just like when we are sick, we usually need the doctor to carefully inspect the various symptoms of our body and then guide us to the correct medication. Quality-qualified drugs must be combined with proper use to achieve safe symptomatic treatment. Otherwise, they may be counter-productive. Quality drugs not only cannot safely treat diseases, but may even endanger the safety of the body.
The safety use of explosion-proof electrical products as we mentioned generally includes the following aspects: correctly classify explosion hazardous locations; properly select and install explosion-proof electrical products; and properly maintain and repair explosion-proof electrical products.
From the above, it can be seen that explosion-proof electrical safety is a systematic project that includes design, manufacture, inspection of explosion-proof electrical products, classification of hazardous locations, selection and installation of explosion-proof electrical products, maintenance and inspection, and related quality management personnel. Training and other links. The negligence of any link may be a hidden danger of an explosion.
It is true that many user departments also recognize the importance of the safe use of explosion-proof electrical products to ensure explosion-proof safety, but suffer from the lack of a complete set of national standards for a long time. Therefore, they can only be developed based on experience and some incomplete information. Some department standards and regulations implement the technical production management for the safe use of explosion-proof electrical products. Therefore, the formulation of a set of national standards that can be integrated with international standards as soon as possible has always been an urgent concern of all parties.
In recent years, with the development of explosion-proof electrical standardization work in China and the need to strengthen integration with international standards, China’s explosion-proof electrical standards have also evolved from past product manufacturing inspection standards to include manufacturing inspections, site classification, and selection. A complete explosion-proof electrical safety standard system consisting of standards for installation, maintenance and inspection. The "Classification of Hazardous Locations (GB 3836 14-2000)", "Electrical Installation of Hazardous Locations (GB)", and Overhaul of Electrical Apparatus for Explosive Gas Environments (GB)" are the first of their three national standards. The standard form completely stipulates the technical and management requirements for the safe use of explosion-proof electrical products by user departments, and is of great significance to the safe production of explosion hazardous locations.In addition, as these three standards are implemented in line with international standards, It also created favorable conditions for China's petrochemical installations to enter the international market, as well as the explosion-proof production safety and technical management of imported equipment.
Classification of hazardous locations is the most important basic work for the safe use of explosion-proof electrical products
The above-mentioned three criteria for the user sector, the first and most basic work is the classification of the explosion hazardous locations involved in this department.
This is like a patient who visits, first of all through a thorough examination and the doctor's analysis of the symptoms after diagnosis. Accurate diagnosis is the basis for symptomatic treatment and safe drug use. This article specifically introduces the analysis of the national standard “Hazardous Location Classification†(GB).
2.1 Purpose of Classification of Hazardous Areas
Hazardous area classification is a method of analyzing and classifying places where explosive gas atmospheres may occur. The purpose of site classification is to ensure adequate safety and good economy for the selection and installation of explosion-proof electrical equipment used in such explosion hazardous locations. Because of the use of flammable substances in hazardous areas, it is difficult to ensure that explosive gas atmospheres never appear. Similarly, it is difficult to ensure that electrical equipment used in hazardous locations is never a source of ignition. Therefore, the type of explosion-proof electrical equipment with high safety performance should be selected in places where there is a high risk (ie places where there is a high possibility of explosive gas atmospheres). Conversely, for sites where the risk is less severe (ie, places where the potential for explosive gas atmospheres is less likely), types of explosion-proof electrical equipment that are less safe (but still adequately safe) and relatively inexpensive may be selected.
2.2 Main Contents of Hazardous Location Classification
The classification of hazardous locations mainly includes two aspects:
(1) According to the frequency and duration of the explosive gas environment, the hazardous locations are divided into three areas and zero areas: places where explosive gas atmospheres continuously appear or exist for a long time;
Zone 1: Locations where explosive gas atmospheres may occur during normal operation;
Zone 2: In normal operation, it is not possible to have an explosive atmosphere. If it occurs, it happens occasionally, and it is only a short-term existence.
(2) Determining the quantity and scope of the presence of dangerous sites Because it is not enough to determine the area category of a dangerous place, the number of such dangerous places inside and around the equipment must be determined, and the space range of each type of dangerous place must be determined. In other words, it is necessary to accurately quantify the spatial scope of the site.
2.3 Basic Safety Principles for Classification of Hazardous Locations
(1) When designing equipment and installations for handling or storing flammable substances, the category of hazardous locations should be minimized as far as possible. In particular, the number and scope of sites in Zone 0 and Zone 1 should be minimized. That is, as much as possible to make most of the dangerous places have become 2 District.
(2) The equipment used in the process shall be mainly a secondary release source. If this requirement is not met, the release source shall be released to the air with a very limited amount and release rate.
(3) After the categories of hazardous locations have been determined, they must not be changed at will. For the repaired process equipment, it must be carefully checked to confirm whether it can guarantee the safety level of the original design.
2.4 Classification of Hazardous Locations
According to the definition of a hazardous area, a hazardous area is an area where the explosive gas environment appears or is expected to occur in quantities sufficient to require special safety precautions for the structure, installation, and use of electrical equipment.
It can be seen from the definition that whether or not a place is a dangerous place is mainly based on whether or not an explosive gas atmosphere may occur at the place. The conditions for the formation of explosive gas atmospheres are whether the concentration range of flammable gas is within the explosive limit range. The concentration of flammable gases in the air is within this explosion limit (ie, the lower explosion limit and the upper explosion limit). An explosion can occur if the ignition source is encountered. Exceeding this range, a strong ignition source cannot ignite the explosion. In other words, any mixture that has a concentration lower than the lower explosion limit or higher than the upper explosion limit will not cause the flame to spread by itself when it comes into contact with the ignition source. When the concentration is lower than the lower explosion limit, excess air participates in the combustion reaction as an inert medium and consumes a part of the reaction heat, which acts as a cooling effect and hinders the self-propagation of the flame. Conversely, when the concentration is higher than the upper explosion limit, the amount of combustibles is excessive, ie, the amount of air. Insufficiency leads to incomplete chemical reactions. The amount of heat released by the reaction is less than the amount of heat lost, thus also impeding the spread of the flame.
From the above, it can be seen that the quantity of flammable gas in the site is the key to determine the hazard of the site, and the presence of flammable gas in the site mainly depends on the flammable gas release source and the ventilation conditions that affect the accumulation of flammable gas. Therefore, we can devise such a solution to the problem: the site is classified according to the size of its danger - the danger of the site depends on the number of explosive gas environments in the site - the explosive gas environment is the amount of flammable gas, That is, the explosion limit determines that the amount of flammable gas present depends on the ventilation conditions of the source and place of release.
2.5 Location Classification Method
The core issue of location classification methods is the analysis of the probabilistic probabilities of explosive gas environments that may occur in a site. This requires the research and participation of experienced professionals. At the same time, it is necessary to accumulate and collect the operating status and environmental factors of each facility in the site. And other information. Therefore, the site classification should be determined by professionals who are familiar with the flammability properties, equipment and process conditions, and those engaged in safety, electrical, and other related engineering and technical personnel. The specific method is as follows:
(1) Finding and confirming the presence of flammable gases or vapours in the release source site may create an explosive atmosphere. Therefore, first of all, it is necessary to find out whether the storage equipment, processing equipment or pipeline containing flammable substances in the site may release flammable gas or vapor into the space, or whether air may enter the container and mix with flammable gas or vapor to form an explosive atmosphere. mixture.
Each device (such as tanks, pipes, pumps, compressors, etc.) should be considered as a potential source of release if it contains flammable substances. If they cannot contain flammable substances, it is clear that no explosive mixture will form around them. If such equipment contains flammable substances, but it is impossible to escape or leak into the site, it may not be regarded as a release source (for example, a jointless pipe installed in a certain space).
If it has been confirmed that the device will release flammable substances to the site, the release frequency and duration should be determined first, and the release source rating should be determined accordingly:
1) Continuous Release Sources: Release sources that are continuously released or expected to be released over a long period of time.
For example, the fixed top tank space and exhaust port; open flammable liquid container near the liquid level, etc., should be considered as a continuous release source.
2) Level 1 release source: The source of release that is expected to be periodically or occasionally released during normal operation.
For example, during normal operation, it is expected that the seal of the pump, compressor or valve that releases flammable substances will be released to the surrounding area; the drain port on the container containing the flammable liquid; during normal operation, the flammable substance is expected to be released to the surroundings. Sampling points in the site; In normal operation, pressure relief valves, vent holes, or other openings that are expected to release flammable substances should be considered as a Class 1 release source.
3) Level 2 release sources: In normal operation, no release is expected, if release is only an occasional and short-term release release source.
For example, in the normal operation, it is impossible to leak the seal of the compressor or valve; in normal operation, it is impossible to leak the flange, connecting piece or pipe joint; in normal operation, it is impossible to release the sample of flammable substance to the surrounding place. Points, etc. should be considered as Level 2 release sources.
(2) Determining the type of the hazardous area The type of the area where the hazardous location is divided is mainly based on the release source level and the ventilation condition in the site.
In general, continuous release sources form a hazardous area in Zone 0; Level 1 release sources form a Zone 1 hazardous location; Level 2 release sources form Zone 2 hazardous locations.
At the same time, the area division should be determined according to the ventilation conditions. When well ventilated, the area category of hazardous locations can be reduced. Conversely, if the ventilation is poor, the area of ​​the hazardous area can be increased. This is because the flammable gas or vapor released into the surrounding area will flow or diffuse through the air formed by the ventilation to dilute the concentration below the lower explosion limit.
(3) Determine the regional scope of hazardous locations Affecting the release rate of flammable gas or gas, the lower explosion limit of gas, relative density, ventilation conditions and other factors in the hazardous area area, so after comprehensive analysis of their impact, determine The area of ​​the hazardous area.
2.6 Factors Affecting the Range of Hazardous Areas (1) The greater the rate of release of flammable substances, the greater the amount of combustible material released into the surrounding area per unit time, and the greater the scope of hazardous areas.
(2) Lower explosion limit For a certain amount of release, the lower explosion limit will increase the amount of explosive gas mixture whose concentration reaches above the lower explosive limit, and the scope of the hazardous area will increase accordingly.
(3) Relative density of gas or vapour If the relative density of gas or vapour is less than air, then lighter than air vapour and vapour will flow upward, so that the range of the vertical hazard zone above the release source will decrease with the relative density. Small and expanding; if the relative density of gas or vapor is greater than air, gas or vapor that is heavier than air tends to deposit on the ground, so that near the ground, the horizontal range of the dangerous area will decrease with the relative density. And expand.
(4) Ventilation increases the amount of ventilation, which can reduce the scope of the hazardous area. This is because ventilation can blow or dilute flammable gases or vapors leaking from the site, reducing the scope of the hazardous area. If the ventilation effect is good, the amount of ventilation is large enough, and the ventilation is continuous, for example, there are spare fans, etc., you can reduce the area category of hazardous locations.
In addition, the release of obstacles around the source can affect the ventilation effect and increase the scope of the hazardous area. On the other hand, if obstacles (eg, dams, walls, ceilings, etc.) can block flammable gases or vapors from spreading further around, obstacles can limit the extent of the danger zone from expanding further to the periphery.
The effect of different ventilation conditions on the scope of the hazardous area can be analyzed as follows:
1) In the case of natural ventilation and integrated forced ventilation, as described above, although the continuous release source forms a zone 0 area, the release source of level 1 forms a zone 1 area, and the release source of level 2 forms a zone 2 area. However, the actual production site is much more complicated due to the effects of ventilation. For example, good ventilation may make the area of ​​a hazardous area too small to be ignored and may become a less dangerous area category. If ventilation is particularly good, it may become a non-hazardous location. Conversely, if the ventilation is poor, the area of ​​the hazardous area may be expanded and may become a dangerously high area category.
2) In the case of local forced ventilation In general, the use of partial forced ventilation to dilute the explosive mixture is better than natural ventilation and overall forced ventilation. As a result, the area of ​​dangerous places will be narrowed down to negligible levels. Maybe become a less dangerous area category. Even become a non-hazardous place.
3) When there is no ventilated place where there is no ventilation, the source of continuous release will definitely form a zone of 0 zone, and the release source of level 1 may also form a zone of zone 0. The release source of level 2 may also form a zone of area 1. However, under exceptional circumstances, such as when the release is minimal or when monitoring is released, it may also make it a less dangerous category.
4) Obstacles Limiting Ventilation If there is an obstruction in the hazardous area that affects ventilation, the scope of the hazardous area may be expanded or made to be a higher risk category.
When considering the influence of obstacles, special attention should be paid to the relative density of gas or vapor in pits and pits.
5) Classification of hazardous locations in the event of a ventilation failure is based on the premise that the ventilation facility is working properly. If the risk of failure of the ventilation device is negligible (if an additional automatic standby system is provided), it is not necessary to change the hazardous location classification determined on the premise of normal operation of the ventilation device. However, if the risk of failure of the ventilator cannot be ignored, the extent of expansion of the explosive mixture without forced ventilation should be predicted. At the same time, the frequency and duration of failure of the ventilator should be predicted and the class of the site determined accordingly.
If the ventilation unit does not malfunction or if it is faulty even if it is short-lived, the hazardous area expanded due to the failure of the ventilation unit should be designated as a Zone 2 location.
If it is possible to take measures to prevent the release of flammable substances (for example, the automatic stop of the process) when a ventilation device fails, it is not necessary to change the previously determined site classification.
6) Other conditions Climatic conditions, topography and other factors can also affect the scope of the explosion hazardous area. In summary, the following matters should be taken into consideration when determining the hazardous area:
Gases or vapours that are heavier than air may flow into spaces below the ground, such as grooves and furrows; gases or vapours lighter than air may be trapped in high spaces, such as the roof space; if the release source is outside the workshop or near the site Measures should be taken to prevent large quantities of flammable gases or vapours from entering the plant or place; the condition of the ventilation has a great influence on the scope of explosion hazardous locations, and great care must be taken when dividing the area.
2.7 Determine the type and scope of the site according to the schematic diagram of hazardous area classification and the example of hazardous area
At the production site, based only on a rough review of the design of a complete set of equipment or a complete set of equipment, it is almost impossible to identify the parts around the equipment as zones 0, 1 or 2 and it is necessary to carefully analyze the various forms of explosive mixtures. possibility.
In order to determine the frequency of release of flammable substances, release time, release rate, concentration, ventilation, and other factors affecting the category and scope of hazardous locations, it is necessary to use the equipment for process flow containing flammable substances that may form release sources. Careful inspection and analysis. The national standard GB 3836.14—2000 “Electrical Equipment for Explosive Atmospheres Part 14: Classification of Hazardous Locations†provides the “Appendix Schematic of Hazardous Locations†and “Instructions for the Division of Hazardous Locationsâ€. The method provides practical help. However, some of the examples of hazardous area ranges given in the standard are divided under certain conditions. Attention should be paid to their limited conditions when used. The examples given in the standards are merely indicative examples. To use the examples in the standard for actual site classification, the actual special circumstances and special details of various situations must be considered. For example, the hazardous area of ​​an oil well facility is divided into Refers to the general situation. If the oil pressure or pressure in the well is very high, the corresponding range of the hazardous area will expand. In addition, only a few influencing parameters are given in each example, but not all. In general, taking into account that these factors are special provisions, and some are qualitative rather than quantitative, the classification results are more conservative. In other words, if the operating parameters can be specified more closely, a more accurate location classification result will be obtained.