Explosion-proof areas

Sulfuric acid is stable but very corrosive and hygroscopic. It will draw moisture from the atmosphere. Sulfuric acid should be stored in a tightly closed container in an explosion-proof area. Containers should be stored out of direct sunlight and away from heat. Avoid heat and moisture. Isolate from incompatible materials. See also Section 12. 

Personnel change rooms and non-sterile manufacturing or preparation areas are common examples of controlled areas. Particulate matter in the air should be no greater than 100,000 particles greater than or equal to 0.5 pm per-cubic foot. Air locks must be provided to entrances and exits, surfaces must be easily cleanable, and air supply should be filtered and conditioned. The number of air changes should be at least 20 per hour. For explosion-proof areas where solvents are used, the air supply operates on a once-through basis. 

Special boiling point hydrocarbons are used for quick-drying paints, dipping solutions, and quick-drying adhesives. Their flash point is below 21 C, which means that they must be used in explosion-proof areas. Petroleum ether is a special petroleum distillation fraction (hp 40-60 C). and is used as a solvent in the chemical industry. 

Generally, it is more economical to prevent explosive atmospheres ia rooms than to try to provide explosion-proof electrical equipment. Personnel should never be allowed to work ia a ha2ardous atmosphere. Where such an atmosphere cannot be avoided through control of flammable Hquids, gases, and dusts, access to the area iavolved should be limited and the area segregated by hoods or special ventilation. Electrical equipment on open, outdoor stmctures more than 8 m above-ground usually is considered free from exposure to more than temporary, local explosive mixtures near leaks (86). 

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. 

Today loading piers are mostly cathodically protected with impressed current. At moorings for tankers, cathodic protection rectifiers are installed on extinguisher bridges as far as possible from the hazardous area. Otherwise, they must be of an explosion-proof type. 

Must be air purged, explosion proof, or intrinsically safe to be used in hazardous areas. 

Stores and work areas should be designated No Smoking areas and access controlled. Depending upon scale, explosion-proof electrics and static elimination may be required. 

Stores should be specially designed, constructed of non-combustible material, and located away from other hazards (e.g. brick coal bunkers are suitable for small samples, but purpose-built constructions with explosion-proof lights etc. are required for larger quantities). They should be designated No Smoking areas and be well labelled. 

In Division 1 areas, transformers must be installed in approved vaults if they contain a flammable liquid. If they do not contain a flammable liquid, they must either be installed in vaults or be approved explosion-proof. In Division 2 areas, standard transformers are acceptable, but... 

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. 

In Division 1 areas, motors and generators must be either explosion-proof or approv ed for the classification by meeting specific requirements for a special ventilation system, inert gas-filled construction, or a special submerged unit. Although explosion-proof motors are expensive, they normally are available. Explosion-proof generators normally are not available. 

Lighting fixtures installed in Division 1 areas must be explosion-proof and marked to indicate the maximum wattage of allowable lamps. Alsn, they must be protected against physical damage by a suitable guard im liy location. 

All portable lamps in Division 1 areas must be explosion-proof. Figures 17-19, 17-20, and 17-21 show typical explosion-proof lighting fixtures. 

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. 

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). 

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. 

Cascading effects Check valve Clogged (of filter) Consensus standard Conservation vent Dike, berm Discharge valve Division (in electrical area classification) Downspout Expansion joint Explosion proof Faucet... 

Define plant areas requiring explosion-proof, drip-proof and open motor and associated electrical components. Refer to National Electrical Code and National Electrical Manufacturer s Association Standards. 

It is important that the fire and explosion hazards of an area be carefully examined, because the expense of consistent installation of all the motors, controls, switches, instruments, and wiring can be considerable. Tables 14-8A and 14-8B summarize the National Eire Code for hazardous locations. It is equally important to be consistent and not install explosion-proof motors with nonexplosion proof wiring, because a failure in the conduit can still cause considerable damage. 

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. 

Motors are suitable for use in general purpose areas and in Class I, Division 2, Group C and D areas for a wide range of pump fluid temperatures. For Class I, Division 1 Group C and D U.L listed, explosion proof motore are available. 

Work with flammables should be confined to areas where other work that could cause ignition is not performed. This practice must be carefully policed. Extra ventilation may be needed for such an area. Knowing that most flammable fumes are heavier than air, one laboratory installed an explosion-proof exhaust fan at floor level below the bench where flammables were used. To make the area for working with flammables easier to identify, it could be marked off with red lines on the floor. 

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