Explosions in enclosures

Light or no confinement Heavy confinement Combustion explosions in enclosures (no pressure)... 

For explosions in enclosures involving high explosives, solid propellants, high explosive with combustible materials in contact, or combustible mist, dust, or gaseous explosive mixtures, the long-duration gas pressures caused by confinement of the products of the explosions can be the dominant loads causing structural failure. 

The explosion-proof enclosure is designed such that an explosion in the interior of the enclosure containing the electronic circuits will be contained. The enclosure will not allow sufficient flame to escape to the exterior to cause an ignition. Also, a surface temperature rating is given to the device. This rating must indicate a lower surface temperature than the ignition temperature of the gas in the hazardous area. 

Explosion-proof enclosures are characterized by strong metal enclosures with special close-fitting access covers and breathers that contain an ignition to the inside of the enclosure. Field wiring in the hazardous environment is enclosed in a metal conduit of the mineral-insulated-cable type. All conduit and cable connections or cable terminations are threaded and explosion-proof. Conduit seals are put into the conduit or cable system at locations defined by the National Electric Code (Article 501) to prevent gas and vapor leakage and to prevent flames from passing from one part of the conduit system to the other. 

This is a location which is not permanently contaminated but is likely to be prone to fire hazards during processing, storage or handling of explosive gases, chemical vapour or volatile liquids, although under careful and controlled conditions. Eor such locations in addition to a flame- or explosion-proof enclosure, type Ex. d, an increased... 

Equipment described as explosion-proof is equipment installed in enclosures that will withstand internal explosions and also prevent the propagation of flame to the external atmosphere. As the gases generated by the explosion expand, they must be cooled before reaching the surrounding atmosphere. 

The surface temperature of explosion-proof enclosures cannot exceed that of high-temperature devices, Equipment can be tested by nationally recognized testing laboratories and given one of 14 T ratings, as indicated in Table 17-2. This equipment may exceed the 80 percent rule."... 

Arcing contacts in Division 2 areas must be installed in explosion-proof enclosures, be immersed in oil, be hermetically sealed, or be non-incendive. High-temperature devices must be installed in explosion-proof enclosures. Fuses must be enclosed in explosion-proof enclosures unless the fuses are preceded by an explosion-proof, hermetically sealed, or oil-immersed switch and the fuses are used for overcurrent protection of instrument circuits not subject to overloading in normal use. 

Figure 17-16 depicts typical devices containing arcing contacts enclosed in explosion-proof enclosures. Figure 17-17 shows typical explosion-proof alarm devices. A telephone instrument suitable for Class I, Divisions 1 and 2, Group D classified areas is shown by Figure 17-18. 

Seals are required at entries by conduit or cable to explosion-proof enclosures containing arcing or high-temperature devices in Division 1 and Division 2 locations. It is not required to seal IM in. or smaller conduits into explosion-proof enclosures in Division 1 areas housing switches, circuit breakers, fuses, relays, etc., if their current-interrupting contacts are hermetically sealed or under oil (having a 2-in. minimum immersion for power contacts and 1-in. for control contacts). 

Seals are required where 2 in. or larger conduits enter explosion-proof enclosures containing taps, splices, or terminals in Division 1 areas (but not Division 2 areas). 

Except for conduit or cable entries into explosion-proof enclosures containing arcing or high-temperature devices (as described in Item I above), cables that will leak gas through the core at a rate of less than 0.007 ft /hr at 6 in. of water pressure need not be sealed if they are provided with a continuous gas/vapor-tight sheath. Cables with such a sheath that will transmit gas at or above this rate must be sealed if connected to process equipment that may cause a pressure of 6 in. of water at the cable end. 

To retard corrosion and to facilitate future maintenance (e.g., allow the non-destructive removal of threaded Junction box covers), all threaded connections should be lubricated with an antiseize compound which will not dry out in the environment. If lubricant is applied to the threaded (or flanged) portion of covers of explosion-proof enclosures, the lubricant must have been tested and approved as suitable for flame path use. It is cautioned that some lubricants contain silicone, which will poison most catalytic gas detector sensors and should not be used near gas detectors. 

C 22.2, No. 30 Explosion-proof Enclosures for Use in Class I Hazardous Locations... 

Gleim, E. J. and Marcy, J. R, A Study to Determine Factors Causing Pressure Piling in Testing Explosion-Proof Enclosures, RI. 4904, U.S. Bureau of Mines. 

Starke, R. and Roth, P, An experimental investigation of flame behavior during explosions in cylindrical enclosures with obstacles. Combustion and Flame, 75,111-121,1989. 

Confined Explosions. In situations where the vapors are confined within a building, vessel, or other such enclosure, flammable materials with flash points below the temperature within the enclosure may have the potential for an explosion. Similarly, in confined situations, combustible materials, regardless of temperature, can pose a potential for explosion if dispersed as an aerosol, mist, or dust. 

Precautions recommended include use of cold zinc and total enclosure of such processes [2]. The possibility of explosions of zinc dust suspended in air is presented as a serious hazard [3], A serious dust explosion in the air filter unit of a zinc grinding mill was initiated by a spark from an explosion relief panel [4], See Zinc chloride, below... 

A recent review on the topic of the relatively long-term gas pressures which develop for explosions within enclosures appears in Ref. 16. That material is summarized here. 

The prime method of protection from a gas explosion in the enclosure is through gas detection and... 

Advances in instrumentation, such as diode arrays and ruggedized interferometers, have made IR and Raman instruments readily available for process work. NIR hardware has always been used more for production and quality control than laboratory and research work. They, too, have become smaller, faster, more rugged and, in 1980s dollars, less expensive. Explosion-proof enclosures allow close proximity to reactors containing solvents and can be operated in dusty locations (raw material handling situations). 

Inexpensive, rugged fiber optics, equipped with every type of probes, make all these instruments amenable to processing measurements in real time. Recently, several manufacturers have been introducing wireless units that are smaller, more rugged, and faster than conventional spectrometers. Many process instruments from a decade ago were simply lab instruments fitted with explosion-proof enclosures and placed in factories. They are now being built specifically for the process environment. 

Clean agent systems can also be used for explosion prevention and suppression where flammable materials are confined. Clean agent fire extinguishing systems are used primarily to protect hazards that are in enclosures or equipment enclosures. Some typical hazards that could be protected by clean agents are ... 

Positive displacement pumps can be built to run on 440-V three-phase service, 220-V service or 110-V single-phase service. The site of electrical service may be restricted in some fuel production and distribution areas. Often, explosion proof enclosures and associated equipment must be used when locating equipment in a hazardous area. If explosion-proof equipment is required, the cost of the equipment will increase significantly. 

Resins are used in electromc/electncal applications (connectors, circuit boards that are vapor and wave solderable, microwave transparent radomes, integrated circuit chip carriers, miniature switches, explosion proof enclosures, lamp reflectors, and high-precision fiber optic components). Polyetherimide is used for medical components (hat require all forms of sterilization. Other uses are found in the transportation field, dual-ovenable cookware, as well as bearings, fasteners, and advanced composites. 

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