【SeaChuan Safety Discussion】A040: Should explosion-proof flexible hoses be installed mandatorily?
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The explosion-proof principle of explosion-proof flexible hoses: When a combustion explosion of flammable gases occurs inside the connected flameproof instruments, part of the energy generated by the explosion (heat and pressure) is absorbed by the explosion-proof flexible hose, while part of the pressure is released outward through the connectors as well as the conduit; during this release process, the heat energy is further absorbed by the metal components of the connectors. Ultimately, this achieves the goal of preventing the ignition (explosion) of external flammable gases. The connectors of explosion-proof flexible hoses are made of metal, and the connections are breathable; the gaskets are also metal, serving the purpose of allowing for venting and absorption. It can be seen from this that the explosion protection of explosion-proof hoses relies essentially on the extension of the explosion chamber, thereby reducing the localized release of explosive energy. Advantages and disadvantages of explosion-proof flexible pipes: Explosion-proof flexible connection pipes have advantages such as flame resistance, corrosion resistance, water resistance, aging resistance, and a robust structure; in addition, they can also protect cables from damage. However, since the explosion-proof flexible tube is not sealed at the instrument connection points, its biggest drawback is water ingress into the instruments; in mild cases this can cause damage to the instruments, while in severe cases it can lead to loss of control over them, resulting in safety accidents. This is also the main reason why explosion-proof flexible hoses are not popular among instrument maintenance personnel. “\"Explosion-proof flexible hoses\" are a sensitive topic in safety inspections; most field maintenance technicians are at a loss when it comes to these hoses. However, some safety experts insist that only explosion-proof flexible hoses can truly meet the requirements for explosion protection. So what exactly is a \"explosion-proof flexible tube\", and why do safety experts admire it so much?
From a regulatory perspective, is it necessary to use explosion-proof flexible hoses? According to SY/T 4129-2014 \"Technical Specifications for the Construction of Automation Instrumentation Projects in Oil and Gas Pipelines\", section 7.3.4 states that when unarmored cables or wires are installed inside the protective conduit, galvanized pipes should be used throughout the length starting from the instrument enclosure; explosion-proof flexible hoses should be used to protect the connections between the instruments and the junction boxes； When armored cables are installed inside the protective tube, the bending radius of the protective tube at turns should be greater than 10 times the diameter of the protective tube. 7.3.4 When unarmored cables or wires are installed inside the protective tube, galvanized tubes should be used throughout the length starting from the instrument box; explosion-proof flexible hoses should be used to provide protection between the instruments and the junction boxes ； When armored cables are installed inside the protective tube, the bending radius of the protective tube at turns should be greater than 10 times the diameter of the protective tube. According to SH/T 3521-2013 \"Technical Specifications for the Construction of Petrochemical Instrumentation Projects\", section 8.4.7, when the protective tube is connected to on-site instrument panels, instrument boxes, junction boxes, wire conduit boxes, and other components, sealing measures must be taken, and the tube must be fixed firmly. The outlet of the protective tube should be approximately 250 mm below the inlet for the instrument equipment. A flexible tube is recommended for connecting it to the sensing elements or local instruments; if a flexible tube is not used, the end of the tube should be shaped like a flared tip or equipped with a cable guard. When the protective pipe is laid from top to bottom, a drain tee should be installed at the lowest point. GB 50093-2013 \"Code for Construction and Quality Acceptance of Automatic Instrumentation Projects\" 7.4.8 Flexible pipes should be used to connect cable conduits to sensing elements or field instruments, and waterproof bends shall be provided. When connecting to on-site instrument boxes, junction boxes, wiring boxes, etc., it must be sealed and firmly fixed. Explanation of the provision: 7.4.8 To prevent dust, water, or other liquids from entering the sensing elements or field instruments, flexible pipes can be used for connection; in some projects, cable conduits are employed with a disconnected connection to the sensing elements or field instruments, and seals are used when the cables enter these elements or instruments. According to Clause 10.4.1, Item 11 of GB/T 50892-2013 \"Code for Design of Instrumentation and Control Systems in Oil and Gas Fields and Pipeline Projects\", the sealing of cable entry points is intended to prevent explosive gases from entering the instrument junction boxes or wiring boxes, thereby avoiding the risk of explosion and loss of the explosion-proof properties of electrical equipment. Sealing devices for cable entry points can include explosion-proof gaskets, explosion-proof sealing washers, sealing stuffing boxes, and explosion-proof sealed flexible connectors. HG/T 20512-2014 \"Code for Design of Instrument Piping and Wiring\": 8.1.13 For the protection of the cables at the entrances and exits of field instruments, junction boxes, and local instrument panels/cabinets, one of the following two methods is recommended: 1 Continuous type: The cable sealing joint flexible tube is connected to the galvanized welded steel pipe. 2 Discontinuous type: cable sealing joint plus cable tray, or cable sealing joint plus galvanized welded steel pipe. SH/T 3521-2013 \"Technical Specifications for the Construction of Petrochemical Instrumentation Projects\" 8.8.5 Protective tubes installed in explosive hazard areas shall comply with the following requirements: a) When connecting protective tubes together, as well as between protective tubes and junction boxes, distribution boxes, and wire conduit boxes, conductive anti-rust grease should be applied to the threads ； b) When the protective tube is connected to instruments, sensing elements, electrical equipment, junction boxes, and pull box assemblies, or when it enters instrument panels, cabinets, or boxes, explosion-proof sealing fittings shall be installed, along with proper sealing filling ； c) The distance between the sealed pipe fittings and the instrument box, wiring box, and pull box should not exceed 0.45 m ； d) A flexible tube can be used to connect the sealed fitting and the protective tube. SY/T 4129-2014 \"Technical Specifications for the Construction of Automation Instrumentation Projects in Oil and Gas Pipelines\" 7.3.4 When unarmored cables or wires are laid inside the protective conduit, galvanized pipes should be used throughout the length starting from the instrument enclosure; explosion-proof flexible hoses should be installed to protect the connections between the instruments and the junction boxes ； When armored cables are laid inside the protective tube, the bending radius of the protective tube at turns should be greater than 10 times the diameter of the protective tube. AQ 3036-2010 \"Code for the Installation of Safety Monitoring Equipment at Tank Farm Sites for Major Hazardous Sources of Hazardous Chemicals\" 4.2.5 For the renovation of existing tanks, sensors that can be installed without emptying the tank should be given priority. It must meet safety requirements; the wires should have no damage, exposed wires, or short circuits. Secondary instruments should be installed in a safe area. After the sensor cover is installed, it must be thoroughly inspected, and the anti-tampering device should be tightened in place. It is strictly prohibited to open and perform maintenance on non-intrinsically safe explosion-proof equipment while it is still powered on. When using unarmored cables, the sensor is connected to the cable tray using an explosion-proof flexible tube. During installation, avoid welding and any operations that may generate sparks, in order to prevent fires and explosions caused by electrical sparks, mechanical sparks, and high temperatures. For those that need to be installed inside a tank and may generate sparks or high temperatures, air replacement should be carried out before proceeding with the work. AQ3009-2007 \"Code for Electrical Safety and Explosion Protection in Hazardous Areas\", Clause 6.1.1.3.10: 6.1.1.3.10 In the conduit system, explosion-proof flexible connection pipes equivalent to the explosion protection type of the electrical equipment shall be installed at the following locations: the power inlet of the motor ； Difficulties in connecting the conduit to electrical equipment ； GB 50058-2014 \"Code for Design of Electrical Installations in Explosive Atmospheres\" 5.4.3 The installation of electrical circuits in explosive environments shall comply with the following provisions: 5 In explosive gas environments, electrical circuits using steel pipe wiring must be properly isolated and sealed, and shall meet the following requirements: 1) During normal operation, the area within 450 mm around the housing of all ignition sources must be properly isolated and sealed. 2) Steel pipes with a diameter of 50 mm or more shall be provided with isolation sealing within 450 mm of the connection box to which they are connected.
Among all the specifications related to explosion-proof flexible hoses, only those for new or renovated oil and gas pipeline instrumentation systems stipulate that such hoses \"should\" be used; none of the other instrumentation-related specifications require the use of explosion-proof flexible hoses. Both explosion-proof flexible hoses and cable explosion-proof sealing joints meet the requirements. Only GB 50093-2013, the Code for Construction and Quality Acceptance of Automation Instrumentation Projects, recommends the use of flexible hoses for connections and specifies that waterproof bends should be installed. However, according to the interpretation of this standard (section 7.4.8), flexible hoses can be used to prevent dust, water, or other liquids from entering sensing elements or field instruments. In some projects, cable conduits are used in a disconnected manner from the sensing elements or field instruments, with seals being used where the cables enter these components. In other words, the 50093-2013 \"Code for Construction and Quality Acceptance of Automated Instrumentation Projects\" defines the role of explosion-proof flexible pipes as providing waterproofing and dust protection functions, and their use is not mandatory. Only when AQ 3036-2010 \"Specifications for the Installation of Safety Monitoring Equipment at Sites of Major Hazardous Sources of Hazardous Chemicals\" requires the use of unarmored cables, shall an explosion-proof flexible tube be used to connect the sensor to the cable gland. This is a standard for safety applications; it does not differentiate between intrinsically safe instruments and flameproof instruments, so no further comments are needed.
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