OSHA Confined Space Rule: Flue gas CO testing with attached NOx filter argon or chemicals used in a firefighting system. Oxygen can also be consumed by rusting metal, ripening fruits, drying paint, coatings, combustion or bacterial activities. Oxygen deficient atmospheres may also be created via the accidental release of flue gases and refrigerants into a space (see ASHRAE Standard 15). Oxygen-enriched atmospheres, with concentrations above 22%, present fire and explosion hazards. Oxygenenriched atmospheres may be produced by certain chemical reactions and in industrial settings caused by leaking oxygen hoses and torches. Oxygen-enriched atmospheres present a significant fire and explosion risk as all the “normal fuel to air ratios” of explosive limits at normal atmospheres are reduced. Why measure flammable gases? The Lower Explosive Limit (LEL, or sometimes the Lower Flammable Limit, LFL) is the “minimum concentration” of vapor in air, below which propagation of a flame does not occur in the presence of an ignition source. Rising above the LEL presents a clear danger of fire or explosion. Additionally, explosive gas concentrations can rapidly change due to air currents and pressure changes. That is why the typical alarm point for explosive gases is 10% of the LEL. Why measure toxic gases? There are many toxic gases in the environment, but within the scope of general HVACR, there are a few that are more prevalent. One of the most common and pervasive air toxins is Carbon Monoxide (CO), which emanates from many sources in and around the home and places of work. CO is generated from improperly controlled combustion that is usually released via a poorly functioning vent system. Because CO has no taste or smell, nor can it be seen, it is often called the “silent killer”. Additionally, it can be misdiagnosed as symptoms that mimic influenza (the flu). CO is a cumulative poison that builds in a victim’s system over the time of exposure. That is why the exposures are tracked as time-weighted averages (TWAs). CO poisoning interferes with oxygen transport and absorption and can effectively debilitate the victim physically and mentally. The OSHA Permissible Exposure Limit (PEL, by TWA) is 50 parts per million (ppm) over eight hours. The ceiling maximum value over 15 minutes is 200ppm. Hydrogen sulfide (H2S) is a colorless, flammable and very dangerous gas. Most people can smell it at fairly low levels (e.g. less than 2ppm) as it has a “rotten egg” smell. It is commonly called sewer gas, swamp gas or manure gas. In nature, you will find it in crude petroleum, natural gas and hot springs. Additionally, hydrogen sulfide is produced by bacterial breakdown of organic materials as well as human and animal wastes (e.g. sewage). It is also prevalent in certain industrial activities such as petroleum/natural gas drilling and refining, waste water treatment and around paper mills. Even though most people can smell H2S, the nose is not a reliable sensor. If one is around certain smells for long enough, one can experience olfactory fatigue, get used to the smell and perhaps let their guard down. Respiratory effects include eye, throat and lung irritation, which is worse for anyone with asthma or other breathing difficulties. Increasing levels of exposure will result in worsening symptoms up to the point of unconsciousness or death. For general industry, the ceiling limit of exposure is 20ppm, with a peak limit of 50ppm for up to 10 minutes, if no other exposure happens during a work shift. Selecting a gas monitor Look for a gas monitor for confined space entry that is designed to be a “personal monitor” for health and life safety applications. Look to the manufacturers and distributors of these products to point out these features and benefits, such as ruggedness, alarm types and strength, battery type and life, ease of calibration and the range of available accessories. The first common question is Do I get a single gas or multi-gas unit? The answer depends on your assessment of the hazards presented to workers currently or in the future. As noted above, there are three or four gases very likely to be encountered in HVACR work; unless you also already have one or two types of single gas monitors that qualify as personal monitors, you are better set to consider a multi-gas monitor. For our industry, the typical three-gas unit has an Oxygen (O2), a Carbon Monoxide (CO) and a Combustible (LEL) gas sensor installed. The fourth gas is most typically Hydrogen Sulfide (H2S). Multi-gas monitors are more efficient than a series of single gas monitors from the aspects of over all size and shared displays, housings, batteries and calibration. This makes multi-gas units less expensive to buy and maintain. Continued on p. 26 8 ICM/July/August 2016
ICM July-August 2016
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