【Instrument Explosion Protection】Knowledge of intrinsically safe explosion protection technology for instruments that instrument professionals should know

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This post was last edited by 955559 on 2018-10-24 at 16:18. In many chemical processing operations, it is necessary to handle certain flammable and explosive process media. To ensure the safety of human life and the property of production equipment, explosion-proof technology has been applied in various industries and related fields, giving rise to a range of industry-specific, ** and international standards, which continue to evolve alongside the development of industry. For automated instruments, the commonly used explosion-proof types are intrinsically safe, flameproof, and increased safety types. Thanks to the rapid development of electronic technology and the continuous emergence of low-power electronic devices, intrinsically safe explosion-proof technology has seen wider promotion and application. In particular, since the intrinsically safe explosion-proof design, compared to other types of explosion protection, not only features a simple structure and a wide range of applications but also is easy to operate and maintain, intrinsically safe explosion-proof instruments that use the suppression of ignition source energy as an explosion prevention method have been accepted by manufacturers and users. Principle of intrinsically safe explosion-proof technology: Intrinsically safe explosion-proof technology is actually a low-power design technique. For example, in a hydrogen (IIC) environment, the circuit power must be limited to around 1.3 W. It can be seen that intrinsically safe technology is well suited for industrial automation instruments. Since electric sparks and thermal effects are the main sources that can trigger explosions of explosive gases, intrinsically safe technology achieves explosion prevention by limiting these two potential sources of ignition. Under normal operating and fault conditions, as long as the energy of the electric sparks or thermal effects generated by the instrument remains below a certain level, the instrument cannot ignite explosive hazardous gases and cause an explosion. It is actually a low-power design technique. The principle involves restricting energy, thereby reliably keeping the voltage and current in the circuit within acceptable limits. This ensures that electrical sparks and heat generated in cases of normal operation, or in situations such as short circuits or component failures, do not cause explosions of any hazardous gases that may be present in the vicinity. Generally, in a hydrogen environment, that is, an environment with a high level of danger and prone to explosion, the power must be limited to below 1.3W. The International Electrotechnical Commission (IEC) stipulates that in Zone 0, areas with a high level of hazard, only intrinsically safe explosion-proof technology of category E*a may be used. Therefore, intrinsically safe explosion-proof technology is a safe, reliable, and widely applicable explosion-proof technology. Intrinsic safety instrumented systems can be divided into E*a and E*b depending on the level of safety and the location of use. The explosion protection rating of E*a is higher than that of E*b. Class E*a intrinsically safe instruments prevent combustion and explosion of circuit components under normal operating conditions as well as when there are two faults in the circuit. In Type IA circuits, the operating current is limited to below 100mA, making them suitable for Zones 0, 1, and 2. Class E*b intrinsically safe instruments prevent the components in the circuit from catching fire or exploding, both under normal operating conditions and in the event of a fault in the circuit. In Type IB circuits, the operating current is limited to below 150mA, making them suitable for Zones 1 and 2. Features of intrinsically safe explosion-proof technology: 1. There is no need to design and manufacture complex, large-sized, and bulky flameproof enclosures; as a result, intrinsically safe instruments feature a simple structure, small size, light weight, and low cost. According to available information, the cost ratio for establishing an intrinsically safe and flameproof switch transmission circuit is approximately 1:4. 2. Maintenance, calibration, and replacement of certain components of the instrument can be carried out while it is still powered on. 3. High safety and reliability. Intrinsic safety instruments will not see a reduction in their safety and reliability due to human-induced factors such as the loss of fastening bolts or rust and scratches on the shell joints. 4. Since intrinsically safe explosion-proof technology is a type of \"low-voltage\" technology, the use of intrinsically safe instruments can prevent electric shock accidents among on-site engineering technicians. 5. Wide range of applications. Intrinsic safety technology is the only explosion-proof system suitable for Zone 0 hazardous areas. 6. For simple devices such as thermocouples, they can be connected to the intrinsically safe explosion-proof system without special certification. It can be seen that, compared to any other explosion-proof design, the use of intrinsically safe explosion-proof technology confers significant technical advantages on industrial automation instruments. The application of intrinsically safe explosion-proof technology in process automation engineering: An intrinsically safe explosion-proof system consists of three components: intrinsically safe field instruments, intrinsically safe cables, and intrinsically safe associated equipment. On-site instruments include various primary sensing instruments installed in hazardous areas. Intrinsically safe field cables, represented by two-wire transmitters, come with dedicated grounding wires and are distinguished from other cables by their durable pure blue color. Associated equipment includes Zener safety barriers, isolated safety barriers, and other types of protection devices with current-limiting and voltage-limiting functions. It can limit the energy that penetrates into the intrinsically safe equipment on site to safe levels, thereby ensuring the safety of the equipment, personnel, and production processes on site. Intrinsic safety explosion protection certification: 1. Intrinsic safety explosion protection represents a holistic concept of explosion protection: the field devices and safety barriers that make up a system must undergo explosion protection certification by **authorized certification bodies. Additionally, a joint certification issued by these bodies for the intrinsic safety instruments and safety barriers is required to confirm the safety of that intrinsic safety circuit. When the field equipment is simple, it does not require intrinsically safe certification; it can still form an intrinsically safe explosion-proof circuit in combination with a safety barrier that has already obtained such certification. Simple devices refer to contact switches, thermocouples, thermal resistors, light-emitting diodes, and bridges, etc.; such devices do not contain energy storage elements. 2. Principles for explosion protection certification of intrinsically safe circuits. Special requirements for intrinsically safe instruments and circuits: 1. Requirements regarding grounding: Intrinsically safe instrument systems must have a reliable, independent grounding system. The entire automated instrumentation system has four types of grounding: intrinsically safe instrumentation system grounding, signal circuit grounding, shielding grounding, and protective grounding. The grounding of the signal circuit and the shielding grounding can share a single grounding electrode; however, intrinsically safe instrument systems require their own separate grounding system, one that is at least 5 meters away from other grounding networks. It is generally required that the grounding resistance of the intrinsically safe ground be less than 1 ohm. The other two types of ground resistance are generally below 4Ω in accordance with design or specification requirements. The protective ground can be connected to the protective grounding network of low-voltage electrical equipment in electrical engineering. 2. Requirements for connection cables: From the perspective of system wiring, due to the distributed capacitance and distributed inductance present in connection cables, these cables function as energy storage elements. They inevitably store energy during signal transmission, and once an open circuit or short circuit occurs in the line, this stored energy is released in the form of electric sparks or thermal effects, affecting the intrinsically safe performance of the system. Therefore, it is necessary to ensure that the connection cables are not affected by external electromagnetic fields or in contact with other circuits, while also limiting the wiring length and the additional non-intrinsic energy generated by induced electromotive forces. Based on this, the allowable distributed capacitance and allowable distributed inductance of the cables are determined. Explosion-proof testing institutions around the world primarily adopt a method that considers the distributed parameters of cables in terms of lumped parameters. The basic parameters for the intrinsically safe performance of connection cables are as follows: Maximum allowable distributed capacitance of the cable (Ci): (Cc) = (Ck) × L. Maximum allowable distributed inductance of the cable (Lc): (Lc) = (Lk) × L. Here, Ck represents the distributed capacitance per unit length of the cable ； Lk—Inductance per unit length of the cable ； L—Actual wiring length. Intrinsically safe cables are cables with low capacitance and low inductance; they possess excellent shielding and interference resistance compared to other cables, making them suitable for use in explosive hazard areas as well as in other situations where high levels of explosion protection are required. The following points should be noted during use: ① The grounding wires and shielding connection wires within the intrinsically safe circuit must be reliably insulated. ②The grounding point of the signal circuit should be on the control room side; when ground-type thermocouples and instruments whose sensing elements are already grounded are used, no further grounding is required on the control room side. ③The spare core wire of the shielded cable and the cable’s shielding layer should be connected to the signal circuit ground on the same side. 3. Equipment temperature rating: The equipment temperature rating specifies the maximum allowable temperature on the surface of the equipment. This is mainly based on technical and economic considerations. In the vast majority of cases, equipment with lower temperature ratings incurs higher costs in terms of purchase and safety. Through comparison, using intrinsically safe equipment is more effective and economical. Intrinsic safety devices installed directly in hazardous areas require consideration of the device’s temperature rating, whereas associated devices do not need to meet such temperature rating requirements. The equipment’s temperature rating must be lower than the ignition temperature of the flammable substances present in that hazardous environment; otherwise, combustion and explosion may occur. 4. Principles for selecting intrinsically safe electrical equipment: ① Simple equipment. In accordance with the explosion protection standards GB3836.4-2000, electrical equipment with a voltage not exceeding 1.2V, a current not exceeding 0.1A, and a power of no more than 25mW can be considered simple equipment. Their typical characteristic is that the internal equivalent inductance Li = 0 and the internal equivalent capacitance Ci = 0 for such equipment. Such equipment can be applied directly on-site. ②Intrinsic safety electrical equipment. For field equipment installed in hazardous locations, the following issues must be clarified: ● Whether it is an intrinsically safe electrical device that has been designed in accordance with the requirements of DB3836.1-2000 and GB3836.4-2000, and has been approved by an **explosion-proof testing institution. ●Whether the grade specified by the explosion-proof marking meets the safety requirements of the hazardous area in use. ●Whether the intrinsically safe circuit is grounded, or whether the grounded part of the intrinsically safe circuit is effectively isolated from the circuits in the safety barrier interface section. ●The method of signal transmission, as well as the minimum operating voltage of intrinsically safe electrical equipment and the normal operating current of the circuit. Based on clarifying the above issues, select the appropriate safety barrier. ③Principles for selecting safety barriers ● The explosion protection rating of the safety barrier must be no lower than that of the intrinsically safe field equipment. ●Ensure that the terminal resistance and loop resistance of the safety barrier meet the minimum operating voltage requirements of the intrinsically safe field devices. ●The safety parameters of the intrinsically safe side of the safety barrier can meet the requirements of Uoc≤Ui, Isc≤Ii, Ca≥Ci+Cc, and La≥Li+Lc. ●The safety barrier must match the safe polarity and signal transmission method of the intrinsically safe field instruments. ●Appropriate protective measures should be taken to prevent the leakage current of the safety barrier from affecting the proper operation of intrinsically safe field devices. Although intrinsically safe instruments have many special requirements in terms of instrument matching, cable usage, and grounding, with the rapid development of microelectronics and microprocessor technologies, industrial automation instruments are moving toward lower power consumption, greater degree of electronics integration, and smaller size, making it easier to achieve intrinsic safety. In summary, for automated instruments, intrinsically safe explosion-proof technology is an ideal explosion-proof solution, and it will undoubtedly be widely used in intelligent fieldbus instruments as well as in modern industrial automation control systems.