Preventing Explosions in Hazardous Areas

Unless process media are completely inert, the potential exists for fire, explosion, corrosion, and/or environmental damage in case of an alarm situation. Of these disasters, explosion and fire are often the deadliest to plant personnel.

Explosions can be prevented in several ways. One way is by limiting the amount of electrical energy available in hazardous areas. Controlling electrical parameters such as voltage and current requires the use of energy limiting devices known as intrinsically safe (IS) barriers. IS barriers limit the levels of power available in a protected barrier. If a spark or excess electrical heat cannot occur, neither can a fire or explosion.

Intrinsically safe components

An intrinsically safe circuit contains three components: the target device, IS barrier, and wiring. Devices within the protected area can be categorized as simple (contacts, resistors, thermocouples, RTDs, etc) or complex (transmitters, relays, solenoids, etc.).

Complex devices often have complicated circuitry that can store excess electrical energy and are normally certified “intrinsically safe” by safety testing and certification organizations, such as Underwriters Laboratories.

Selection of proper IS barriers requires calculation of both the open-circuit voltage and short-circuit current of simple devices. For complex devices, both allowed capacitance and inductance values must be calculated. Results are then compared to ignition curves that have been calculated for a wide variety of flammable/explosive media (gases, vapors, airborne dusts or fibers, etc.) to determine if the available energy is below the amount needed for ignition.

Brute force approach

Explosion-proof enclosures provide a brute-force method of preventing or controlling potentially explosive situations. These heavy, cast-usually but not always-devices feature sealed and securely fastened access doors. They protect the normal power level devices within them from coming into contact with an explosive atmosphere.

Even under fault conditions, an explosion or fire usually cannot occur because of limited air for combustion within the sealed container. If an explosion does occur, the housing is strong enough to contain it.

Although there have been many refinements in explosion-proof enclosure design, the fact remains that they are bulky, can be difficult to mount because of their weight, and are not the handiest of housings to access. Additionally, seals, gasketing, and purging systems require inspection and maintenance if their integrity is to be trusted.

The fact remains that in industries where high voltages and currents are routinely encountered and process systems are rarely reconfigured, explosion-proof enclosures remain a practical method of preventing an industrial tragedy.