Explosion-proof enclosures

MANUAL hand VALVE. 44 ENCLOSURE. EXPLOSION PROOF... 

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. 

The dust-ignition-proof protection concept excludes dust from entering the device enclosure and will not permit arcs, sparks, or heat generated by the device to cause ignition of external suspensions or accumulations of the dust. Enclosure requirements can be found in ANSI/UL 1203-1994, Explosion-Proof and Dust-Ignition-Proof Electrical Equipment for Use in Hazardous Locations. ... 

Purchase Price Typical purchase prices, including drive motors, of tubular and disk sedimenting centrifuges are given in Table 18-16. The price will vary upward with the use of more exotic materials of construc tion, the need for explosion-proof elec trical gear, the type of enclosure required for vapor containment, and the degree of portability, and this holds for all types of centrifuges. 

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."... 

In Division 1 areas, meters, instruments, relays, and similar equipment containing high-temperature or arcing devices must be installed in approved explosion-proof or purged enclosures. Unless such devices are specifically labeled as suitable for Class I, Division 1 areas, it is best to assume they are not suitable. 

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. 

Confine internal explosions to explosion-proof enclosures and conduit systems. 

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 prevent the accumulation of moisture in conduits and enclosures, drains should be installed at all low points. In classified areas, breathers and drains must be explosion-proof. Figure 17-28 shows typical explosion-proof breathers and drains. 

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... 

Originally the classification of materials was derived from tests of proprietary explosion-proof (flameproof) enclosures. There were no published criteria. Equipment was approved relative to the lowest ignition temperature of any material in the group (Magison 1987). In about 1965 the U.S. Coast Guard asked the National Academy of Sciences (NAS) to form a panel to classify 200 materials of commerce. The Electrical Hazards Panel of the Committee on Hazardous Materials was formed by the NAS. The Panel studied many ways to estimate the hazard classification of materials. The Panel finally reported to die U.S. Coast Guard in 1970 that no workable, predicdve scheme could be defined, and it then proceeded to assign tentative classifications to the 200 materials. 

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. 

An explosion-proof machine is a totally enclosed machine whose enclosure is designed and constructed to withstand an explosion of a specified gas or vapor which may occur within it and to prevent the ignition of the specified gas or vapor surrounding the machine by sparks, flashes or explosions of the specified gas or vapor which may occur within the machine casing. 

If the pumps are located indoors, a Division 1 classification is likely to apply. Motors must be Class 1, group D, explosion-proof, or they may be separately ventilated with clean outside air brought to the motor by fans. Auxiliary devices such as alarm contacts on the motor must be suitable for the area classification. The installed costs, overall efficiencies, and service factors associated with the enclosures that are available will influence the selection. 

Specifically note that Type 7 (explosion-proof) enclosures and their associated conduit systems are neither gas or liquid tight. Consequently, corrosive gases such as hydrogen sulfide and water from rain or internal condensation can accumulate with the enclosure. Premature failure of electrical devices and interconnections often results when preventive measures such as drains, air purges, and dual rated enclosures are not used to remove or exclude these corrosive elements. Type 7 enclosures are intended for indoor use. 

A Type 10 enclosure is designed to meet the explosion proof requirements of the U.S. Mine Safety and Health Administration (MSHA). It is suitable for use in gaseous coal mines. 

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. 

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. 

Motor enclosure Open drip proof Totally enclosed. . Explosion proof ... 

Motor enclosure (circle one) drip proof, TEFC, explosion proof, other... 

Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited for example for low speed reciprocating compressors, but are not made smaller than 50 HP. A variety of enclosures is available, from weather-proof to explosion-proof. 

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. 

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

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