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 An Intrinsically Safe A new LVDT-based position monitor represents an intrinsically safe version of a well-established device. Martin Aronow and Jackson Szczyrbak, The need for safe electrical equipment for use in hazardous environments has been generally recognized since the latter years of the nineteenth century, when electric lights, motors, and signaling were introduced to the mining and textile industries with the unhappy consequence of occasional fires and explosions. Today, combustibles such as acetylene, hydrogen, and other gases; alcohol, gasoline, and other liquids; metal dust, carbon and coal dust; flour and grains; and fibers and flyings are common in many industrial sites worldwide. In response, standards have been developed for classifying these hazardous areas, and techniques have been developed to prevent explosions. Prevention and Control
Reducing risk depends on preventing ignition. For fires or explosions to occur, there must be a sufficient concentration of flammable material, oxygen, and an ignition source of sufficient energy to start the process. Minus any of these elements, the hazard is zero. It is therefore reasonable to conclude that the easiest way to reduce such hazards is to separate the flammable element from the ignition source. But this is not always feasible. A more practical approach is to isolate the ignition source from a potentially explosive material by encapsulating it or immersing it in an inert substance such as oil or an incombustible powder. Another technique is to hermetically seal the ignition source inside the walls of a vessel and keep the combustible material outside. There is also the flameproof method, which encloses the electrical equipment in a housing that can withstand internal explosions without igniting anything on the outside. The joints of such housings have flanges designed to prevent flames and hot gases from escaping, and the housing's exterior surfaces are maintained at a temperature below the ignition level of the gases outside. This way, an ignition source and a combustible mixture can coexist and possibly cause an explosion--but it stays inside the enclosure. The advantages are that large amounts of electrical energy can be safely handled, and that appropriate housings are readily available. On the negative side, these enclosures are bulky, heavy, expensive, and hard to install. All wiring into and out of the housings must be via conduit that requires special fittings and seals, and the electrical source must be shut off before the enclosures can be safely opened.
In the explosionproof technique, purging, or pressurization, is used to prevent a potentially hazardous concentration of combustible material from accumulating. In this technique, an incombustible atmosphere is maintained inside the electrical enclosure at a pressure higher than ambient, thus isolating the equipment from the hazardous environment outside. Should a combustible substance manage to enter, the resultant explosion would be contained without rupturing the enclosure. And if the byproducts of an explosion did manage to escape, the cooling effect of expansion into the ambient atmosphere would prevent their causing additional combustion. The intrinsic safety method is based on limiting the electrical energy in a given apparatus to a level incapable of causing ignition under any normal or fault condition. This is accomplished by installing an energy-limiting interface (safety barrier or galvanic isolator) in the wires between the hazardous and nonhazardous areas. This approach also limits the discharge of energy storage devices such as capacitors and inductors contained in the electrical equipment. In the U.S., the accepted methods of reducing the risk of fires and explosions in hazardous environments are codified and kept up to date by consortia of insurance companies such as Factory Mutual and Underwriters Laboratories. The Canadian Standards Association is the governing
Generally speaking, explosionproof is the method of choice for apparatus handling or using significant amounts of power. Intrinsically safe is preferred for hazard management and risk reduction in low-power circuits such as instruments and controls. Neither technique is predicated on the impossibility of an unwanted event, but rather on quantifying the probability of failure using the mathematics ordinarily associated with reliability and quality control. Both are also based on the requirement that two or more highly unlikely events must take place simultaneously before a failure can occur. The exponential probability distribution allows us to estimate the probability of success (or non-failure) over a time period equal to or greater than time t: PS = where: PS = probability of failure-free operation over a time period
t = a specified period of time m = mean time between failures (MTBF) l = failure rate = 1/m A New Solution As the instrumentation industry develops, more measurement and control objectives can be accomplished with smaller and smaller amounts of power, making the intrinsic safety approach both practical and economically feasible. TRW Automotive Electronics has developed an intrinsically safe LVDT-based linear position transmitter for a variety of hazardous environments. LVDTs offer several advantages:
The loop-powered, 2-wire HCT-IS operates with loop resistances up to 700 The HCT-IS transmitter consists of a primary and two secondary coils, hermetically packaged against corrosive environments and external pressure and coaxially wound on a cylindrical form with secondary coils connected in a series-opposing configuration. A freely moving, magnetically permeable core couples the magnetic flux generated by the primary when an external AC voltage is applied. Because the secondary coils are connected series opposing, there is a particular core position where the output across the combined secondaries is zero. Moving the core in either direction will make the output in one coil increase and the other decrease. There is a 180° phase reversal at null. The coil design makes the change in the vector sum of the voltages proportional to the distance from the null or zero point. Six standard ranges are available, from Candidate applications include monitoring control valves that handle flammable liquids or gases (see Figure 1, page 38), gas turbine vane position, roller gaps in metal and plastic manufacture (see Figure 2, page 39), and pinch and feeder rolls coated with graphics whose ink contains volatile solvents. Keep in Mind Because no approach is entirely without risk, sustaining safe conditions requires correct installation, continuous monitoring of the process, and maintenance, whatever the safety technique selected:
Martin Aronow is Senior Applications Engineer and Jackson Szczyrbak is Principal Engineer, Sensors and Components, TRW Automotive Electronics, 100 Lucas Way, Hampton, VA 23666-1573; 757-766-4494, fax 757-766-3979, martin.aronow@lucasvar ity.com (Martin), 757-766-4355, fax 757-766-4459, jack.szczyrbak@lucasvarity.com (Szczyrbak).
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and was designed to minimize the effects of changes in loop resistance and EMI on position indication. Output is the industry-preferred 4–20 mA. The 2-wire loop configuration allows its use with a single barrier and a minimum of wire or cable. There are no wipers or switch points.
