PROTECTIVE CONCEPTS IN COMBUSTIBLE GAS AND VAPOR DETECTION
Dealing with the actual or potential presence of combustible gases and vapors in the workplace involves a complex series of decisions. Decisions related to “what to do” should be implemented by means of an integrated explosion protection plan based on a hierarchy of priorities. The most desirable approach – when feasible – is to implement engineering controls and practices that completely prevent the release or formation of explosive gases and vapors. This is often referred to as “Primary Explosion Protection.” If it is impossible to completely control or eliminate the possibility of an explosive atmosphere, the next best solution is to implement use of equipment and techniques to monitor for the presence of gas, and to take appropriate action when explosive gas is determined to be present. Equipment used in hazardous locations that are subject to the potential presence of explosive gas must be designed and certifi ed as safe for use in the intended area for the intended purpose.
Combustible gas hazardous locations are areas where the atmosphere contains, or has the reasonable potential for containing, fl ammable gases and vapors. A fl ammable concentration of gas is one that is capable of being ignited if a source of ignition is present. The presence of fl ammable gas may be a consequence of leakage, chemical process, displacement, microbial activity (such as decomposition or fermentation), or some other specifi c process or activity. In order for an explosion to occur there must be suffi cient oxygen, an ignitable concentration of gas, a source of ignition, and suffi cient molecular energy to propagate the explosion chain reaction. Equipment that is used in areas that potentially contain fl ammable gas must be designed so that hot surface temperatures, electrical discharge, and other forms of stored energy, (such as inductance and capacitance) associated with the equipment are not capable of causing ignition of the fl ammable gas, given the type and severity of the hazardous conditions in which the equipment is installed or operated.
It should be noted that this article is treating these concepts on a somewhat simplistic level. For instance, oxygen in air is not the only oxidizer capable of reacting with another gas in an explosive chain reaction. The fuel used to propel the Proton rocket on its way to the International Space Station consists of dinitrogen tetroxide (N2
O4 ) and dimethyl hydrazine (CH3 NHNHCH3 ). But at
most industrial sites, the oxidizer is the oxygen in the ambient air. Similarly, there is more than one type of hazardous location with the potential to contain an explosive atmosphere. A location may be hazardous because of the presence of combustible dusts, or ignitable concentrations of fl yings or fi bers. This article is focused exclusively on locations that are hazardous because of combustible gases and vapors in concentrations suffi cient to lead to a potential explosion hazard.
The lower explosion limit (LEL) is the minimum concentration of gas or vapor in air that will support combustion, if a source of ignition is present. If the gas is present in air in concentrations less than the LEL it is too “lean” to be ignited. The upper explosion limit (UEL) is the highest concentration of a gas or a vapor in
air capable of burning explosively when a source of ignition is present. Above the UEL the concentration of gas in air is no longer capable of being ignited because the ratio of fuel molecules to oxygen molecules is too high for the explosion chain reaction to propagate. In this case the mixture is too “rich” to be ignited. The fl ammability range is the concentration range between the LEL and the UEL concentration. Gas can burn explosively when present in concentrations within the fl ammability range when a source of ignition (such as arc, fl ame or heat) is present.
The minimum ignition energy (MIE) is the minimum energy suffi cient to ignite a particular fl ammable gas in air, when the gas is present in the ideal ratio of fuel to air for the gas to be most easily ignited. Different gases, and different mixtures of fl ammable gases, have different LEL and MIE values. Even when gas is present in concentrations within the fl ammability range, sources of energy less than the MIE are not capable of igniting the gas.
Combustible gas hazardous locations are defi ned a little differently in North America compared to the UK and Europe. In North America, the most widely used hazardous location classifi cation scheme is based on the National Electric Code (NEC) NFPA® Articles 500 - 506. The NFPA®
70, scheme divides hazardous locations
into three classes that are based on the characteristics of the fl ammable materials. “Class I” includes gases and vapors, “Class II” includes combustible dusts, and “Class III” includes fl ammable fi bers and “fl yings.” The classes are further divided into divisions based on the risk of fi re or explosion the class of material represents, and the probability of being present in in a potentially hazardous quantity.
In the UK and Europe, the hazardous location classifi cation scheme is based on “Zones” that are defi ned by International Electrotechnical Commission (IEC) and European Committee for Electrotechnical Standardization (CENELEC) standards. The most important difference is that the IEC/CENELEC system is based on dividing the level of hazard probability into three zones rather than two divisions.
The G999 is a compact, one to six gas atmospheric monitor with optional wireless communication used to transmit real-time monitoring results to a remote collection point. The instrument is certifi ed as an intrinsically safe “Ex i” device for use in Zone 1, gas group IIC, temperature class T4 hazardous locations.
Type of Hazardous Material
NEC / NFPA® Class / Division
IEC / CENELEC Zone
Gases and vapors Class I, Division 1 Zone 0 Zone 1
Class I, Division 2 Zone 2
Combustible dusts Class II, Division 1 Zone 20 Zone 21
Fibers and fl yings
Class II, Division 2 Zone 22 Class III, Division 1 (Not yet Class III, Division 2
determined)
OCTOBER / NOVEMBER •
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