Explosion proof issues with instruments

Instruments are widely used in industrial production sectors such as petroleum, chemical, coal, metallurgy, and building materials, and many production processes are carried out in environments with flammable and explosive gases or dust. Accidents that endanger personnel and equipment can occur at any time, and explosion-proof requirements must be imposed on all electric instruments. In a sense, whether the explosion-proof problem can be solved is also the key to whether electric instruments can replace pneumatic instruments for use in hazardous areas.

The research on explosion-proof technology first received attention in coal mines, mainly focusing on the explosion-proof of underground electrical equipment, and gradually developed into the explosion-proof of automation instruments used in various flammable and explosive places. Correspondingly, relevant standards have been formulated both domestically and internationally. The national standard "General Requirements for Electrical Equipment for Explosive Gas Environments" (GB 3836.1-2000) in China is designed for all electrical equipment used in flammable and explosive environments, including automation instruments.

The manufacturing industry of electrical equipment at home and abroad adopts only two approaches in explosion-proof design_：One is to use isolation measures in the structure to isolate the circuit from the surrounding environment, so that the heat generated during normal operation of the circuit and the electric sparks and high temperatures formed under fault conditions are confined within a sealed shell, preventing the ignition of flammable and explosive gases from the outside; Another approach is to limit the energy of the circuit, so that the sparks generated by the circuit, whether in normal operation or in the event of short circuits, open circuits, or other fault states, are not sufficient to ignite flammable and explosive gases, and the temperature generated is not sufficient to cause the flammable and explosive materials to self ignite.

The former approach mainly relies on structural prevention, which can be called "structural explosion prevention"; The latter approach is to fundamentally eliminate the possibility of disasters, and its measures are more proactive, called "intrinsic safety explosion prevention", abbreviated as "intrinsic safety explosion prevention".

The specific measures for structural explosion prevention mainly include the following.

(1) Adopting a tight shell, compliant threads and high-quality sealing gaskets, and adopting a special structure sealing interface on the wire outlet. The magnesium content of the shell material, such as aluminum alloy, should be limited. There should be a certain volume of space inside the casing, except for the circuit components, to allow for the expansion of the internal gas.

(2) Transport clean compressed air into the casing to maintain positive pressure inside, preventing flammable and explosive gases from entering and preventing direct contact with the circuit. This method requires a gas source and pipelines.

(3) By filling the casing with oil, the circuit is immersed in the oil, and its heat is carried away by the oil. Sparks are extinguished by the oil, which also serves to isolate the circuit from the surrounding gas. The oil switch of high-voltage circuit relies on oil to extinguish the arc.

(4) Filling quartz sand in the gap between the circuit and the casing also plays a role in arc extinguishing and insulation. Some fuses use sand filling measures in their ceramic tubes.

In terms of instruments, commonly referred to as "explosion-proof" instruments, most of them adopt the above-mentioned measures. As for oil filling, sand filling, and positive pressure measures, they are rarely used in instruments because they are not convenient enough. A relatively complete explosion-proof instrument does not rely on structural explosion-proof measures, but on the design of intrinsic safety explosion-proof circuits to form an "intrinsic safety" instrument. Electrical equipment and instruments used in flammable and explosive places may not necessarily require special explosion-proof design. According to GB3836.1, electrical equipment with a voltage not exceeding 1.2V, a current not exceeding 0.1A, and an energy not exceeding 20W or a power not exceeding 25mW, approved by the inspection unit, is allowed to be directly used in explosive gas environments in factories and underground coal mines. Some sensors and sensitive components, such as thermocouples, thermistors, photocells, etc., belong to this type of electrical equipment. However, it must be noted that when these simple components are used in conjunction with other instruments, the safety of the accompanying instruments must be considered. Moreover, since supporting instruments are often installed in locations far away from sensors or sensitive components, the impact of faults in signal wires and supporting instruments on hazardous areas should be considered.

It can be seen that simply considering the explosion-proof of the instrument itself is not enough. Even if the internal circuit design of the instrument is complete and meets the requirements of intrinsic safety and explosion-proof, it can only be called an intrinsic safety instrument, and cannot constitute an intrinsic safety system. The vast majority of automation instruments are not isolated and require power supply and signal lines to transmit information. If these wires carry high voltage or high current, they pose a threat to flammable and explosive areas. Even if the voltage and current under normal conditions are small enough to cause disasters, various manifestations under fault conditions should still be considered. For example, although the thermocouple itself is quite safe, its wires are connected to the temperature transmitter. If the temperature transmitter malfunctions, high voltage or high current will be transmitted through the wires to the installation location of the thermocouple. To prevent accidents, the power supply of the temperature transmitter should be isolated to avoid common mode high voltage transmission, and a current limiting resistor should be added to the wire to prevent excessive current in case of short circuit. For differential pressure transmitters, only by using safety barriers can an intrinsic safety system be constructed.

In summary, the circuit of an intrinsic safety instrument cannot ignite the specified explosive mixture under the specified test conditions, regardless of the thermal effects and sparks generated during normal operation or fault conditions. The intrinsic safety system should certainly be composed of intrinsic safety instruments, but this is only a necessary condition rather than a sufficient condition. Measures must also be taken to prevent various external energies capable of igniting explosive mixtures from entering hazardous areas.