Introduction to intrinsic safety principles and applications

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Introduction to Intrinsic Safety Principles and Applications Hazardous Areas: In industrial process control, when it comes to raw materials that are prone to combustion and explosion, due to damaged or leaking containers, the air contains volatile explosive gases, dust, etc. These areas are called hazardous areas. For example: Petroleum and its derivatives, hydrogen, gas, flour and other substances will cause explosions once the conditions are right. 　　 　　 intrinsically safe: The working principle uses safety barrier technology to limit the electrical energy provided to field instruments within a safe range that neither generates sparks sufficient to detonate nor generates a temperature rise on the surface of the instrument sufficient to detonate. In accordance with international standards and China * * Standard, when any failure occurs in the equipment connected to the safety zone side of the safety barrier (except for voltage failure not exceeding 250V), the intrinsically safe explosion-proof method (E * ) to ensure explosion-proof safety on site. 　　 　　 Intrinsically safe equipment failure classification: E * Class A intrinsically safe equipment cannot ignite an explosive gas mixture when one or two faults occur during normal operation. E * Class B intrinsically safe equipment cannot ignite a mixture of explosive gases when a fault occurs during normal operation. 　　 　　The explosion-proof mark of the instrument must be marked on the nameplate and sample or product manual of the explosion-proof instrument. The practical significance of understanding the above basic knowledge of explosion prevention is to identify the explosion protection signs of the instrument, so that the installation area of ​​the instrument and the explosive gases that can be involved are clear at a glance. 　　Example 1: E * a IIC T6 The meaning of this riot sign is: 　　Sign Content Symbol Meaning Riot Prevention StatementEx: Comply with certain explosion-proof standards, such as China * * Standard anti-riot method Ia: Adopt ia level intrinsically safe explosion-proof method. Can be installed in Zone 0 gas category IIC: This permit involves IIC explosive gas temperature ℃ group T6: The surface temperature of the instrument should not exceed 85℃. Example 2: EE * a IIC The meaning of this explosion-proof mark is that it complies with European standards and adopts Class IA intrinsically safe explosion-proof instruments that can involve IIC explosive gases. The absence of a temperature ℃ group indicates that the instrument is not in direct contact with explosive gases. 　　 The field instruments in the above two examples match the safety barrier and can be installed in the most dangerous situations involving the most dangerous gases. 　　Example 3: Ex de(ib)IIC T4~6 This instrument complies with certain explosion-proof standards and adopts explosion-proof and increased-safety ib class intrinsically safe explosion-proof methods. It can involve IIC gases, and the surface temperature of the instrument does not exceed 85~135℃. For example, a certain electromagnetic flow transmitter uses explosion-proof power supply, adds a junction box, and the signal is ib intrinsically safe. When the upper limit of the ambient temperature range is 40~80°C, the surface temperature of the instrument shall not exceed 85~135°C. 　　Example 4: Division1 ； Class I Group A ； T6 This meter complies with the explosion-proof standards of the United States or Canada. It can be installed in Division 1 and can involve Group A gases. The surface temperature of the meter does not exceed 85°C. This is the most advanced anti-riot instrument certified by US and Canadian standards. Usually text is used to describe the specific anti-riot methods. 　　 　　Intrinsically safe explosion-proof method The intrinsically safe explosion-proof method uses safety barrier technology to limit the electrical energy provided to field instruments within a safe range that neither generates sparks sufficient to detonate nor generates a temperature rise on the surface of the instrument sufficient to detonate, thus eliminating the source of detonation. 　　 For instrumentation and control loops, limiting energy first means limiting voltage and current. And because capacitors and inductors can store and release electrical energy, capacitors and inductors must also be limited. 　　 In practice, people use spark experimental devices to determine the electrical energy limiting parameters for different hazardous categories of gases through experiments. International standards and China * * Commonly used electric energy detonation curves given in the standard include voltage and current detonation curves, voltage-capacitance detonation curves, and current-inductance detonation curves. Based on these curves, and referring to the insurance factor of 1.5 times, one can determine the electrical energy limiting parameters for a given circuit when a certain type of gas is involved. 　　 For example, when it comes to Class IIC gases (such as hydrogen), the pressure limit value is usually set to 28V for standard 24VDC power supply circuits (such as variable supply gas, electrical converters, solenoid valves, etc.). Check the voltage and current detonation curve according to this voltage limit value, and consider the insurance factor of 1.5 times to determine the current limit value at this time. It can be determined that the current limit value at this time should be 119mA. According to the 28V voltage limit value and considering the 1.5 times insurance factor, after checking the voltage capacitor detonation curve, it can be determined that the loop capacitance value should be limited to 0.13μF. According to the current limit value of 119mA and considering the insurance factor of 1.5 times, after checking the current inductance detonation curve, it can be determined that the loop inductance value should be limited to 2.55mH. 　　In order to limit the surface temperature of the instrument, in addition to limiting the open circuit voltage and short circuit current of the loop, the maximum power of the loop must also be limited. 　　 Intrinsically safe explosion-proof circuits are always composed of an intrinsically safe field instrument and a safety barrier as a circuit energy-limiting related device.