Explosion-proof connections

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. 

Flexible cord approved for extra-hard service, flexible metal conduit, and liquidtight flexible conduit for limited flexibility. A suitable grounding conductor must be provided inside the flexible cord s outer jacket. Flexible conduit must be bonded with an external jumper or an approved internal system jumper external bonding jumpers are disallowed for flexible conduit exceeding six feet. Typical liquidtight and flexible cord connectors and an explosion-proof flexible connection are shown in Figure 17-23. 

A box or fitting must be installed at each conductor sphce connection point, receptacle, switch, junction point, or pull point for the connection of conduit system. In Division 1 areas only explosion-proof boxes or fittings are allowed. General purpose gasketed cover type fittings are allowed in 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. 

These devices are tested only for internal explosions and not for external explosions pressurizing the devices from the outside. As an example, a factory-sealed push-button start/stop station connected to an explosion-proof motor starter cannot suffice as a seal for the motor starter conduit entry. A separate seal must be installed at the point of conduit entry. 

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. 

The explosion-proof design includes the use of conduit with special sealed connections around all junction boxes. 

Explosion proof equipments, buildings and procedures must be enforced when flammable fluids are handled, especially for light hydrocarbons. Explosion atmosphere sensors have to be installed, and connected to high power fans and to fluid reservoirs stop valves. 

Light fixtures, electrical outlets, electrical switches, all electrical connections are explosion proof. 

It is therefore more cost effective to use a large ultrasonic system supplying 80 kW to process liquids at a flow-rate of 10 m /h than to use 5 ultrasonic processors with a power of 16 kW each or 40 processors with a power of 2 kW each. The robustness of the transducer enables its use under heavy-duty industrial conditions. Also, the processor can be designed to be explosion-proof. Like the transducer and the flow cell, the generator is housed in two connected compact stainless steel cabinets. This makes the device self-contained, robust and easy to install. The standard footprint of a 16-kW system is just 600 mm x 1200 mm. 

Any process heat plant design implies piping through the containment to connect the reactor vessel with the chemical plant. The fracture of a pipe could result in the accumulation of a flammable gas mixture in the containment. Precautions must be taken to minimize the risk of a fire or gas explosion such as avoidance of explosive gas ingress, proper detection devices, inerting, sufficient safety distances, appropriate layout of secondary coolant boundary, explosion-proofed wall, plant isolation valve. For the PNP-500, the use of two concentric pipes for the process gas carrying lines were recommended. Alternatives are concrete channels around the gas lines or inerting of the containment [10]. 

The lights in the hood should be shielded from the hood body in, as a minimum, a vapor-proof enclosure. These can also be selected as explosion-proof units should the usage include substantial amounts of very flammable materials. In fact, if a large portion of the work in the hood is expected to involve very volatile flammable materials, the hood should be designed completely as an explosion-proof unit. In any case, all electrical outlets and switches should be located outside of the hood on the vertical fascia panels. Similarly, utilities should be provided by remotely controlled valves in the side panels operated by handles outside the hood. The connections to the utilities inside the hood must be chemically resistant and should be clearly identifiable as to the utility provided. 

Centrifuge, continuous extraction, Alloy 20 including flexible connections, explosion-proof motor, variable speed drive, ammeter, tachometer, excluding pumps, starter, flowmeters and control valves. FOB cost 355000 at aqueous feed rated capacity = 2.2 L/s with n = 0.48 for the range 1.9-20. L-i-M = 1.45. 

The employer shall ensure that all wiring components and utilization equipment in hazardous locations are maintained in a dust-tight, dust-ignition-proof, or explosion-proof condition, as appropriate. There shall be no loose or missing screws, gaskets, threaded connections, seals, or other impairments to a tight condition. 

Some users do or will prefer the "3A"solvents — those which display flash points above 60°C (140°F) so that there is no concern about wastes being hazardous, or electrical connections being rated explosion-proof. 

However, explosion-proof design of electrical connections and wiring does also what one intuitively expects — it acts as a barrier to keep flammable materials and oxygen from an obvious type of spark source. 

There is a second approach — a proven (and low-cost) method by which one can safely use flammable materials in cleaning machines, and not require expensive explosion-proof electrical connections. 

Store and use dichlorosilane only in adequately ventilated areas. It should be used only in a closed system constructed with compatible materials and designed to withstand the pressures involved. Keep away from heat and all ignition sources such as flames and sparks since dichlorosilane will form flammable mixtures with air and other oxidizing agents. All lines, connections, equipment, and so forth, must be thoroughly checked for leaks and grounded prior to use. Use only spark-proof tools and explosion-proof equipment. 

Intrinsic Safety. Division I areas can also use intrinsically safe barriers to provide an "explosion-proof" installation. These barriers must be located and connected in accordance with the rules for intrinsically safe installations, such as ISA RP12.6. 

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