August 2011

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Gas Detection (1 of 2)

Always check to see the guidelines that any particular municipality follows, or solicit the advice of your “local expert”!

Steven R Calabrese
Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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Basic Principles

In this, part one of a two-part series on gas detection, we’ll discuss basic principles and lay down the groundwork for the second part of the series, which will deal with the technologies of gas detection and the practical implementation of gas detection systems. I suppose in order to understand what we’re detecting and how/why we’re doing it, we need a fundamental knowledge on the properties of hazardous gases, concentration levels, and exposure limitations. So I give you…Part One:

Gas Hazards

There are three types of gas hazards that we’re typically concerned with: flammable gas hazard, toxic gas hazard, and asphyxiant gas hazard. Of the three, we’ll spend most of the time during this series discussing and dealing with the first two. Not to say that the third is any less of a concern in terms of safety, but covering the basics of the first two will go a long way in covering the third.

Flammable Gas Hazard: Combustion is the defining term for this hazard. In order for combustion to take place, three factors must be present: fuel (gas), oxygen, and a source of ignition. Think about lighting a propane grill. You turn the fuel on, strike the igniter, wave your hand over the grill (careful!) and whoosh, you have your flame.

There is a range of gas-to-air concentration that will result in a combustible mixture. Not enough gas and the mixture is considered too lean. Not enough oxygen (too much gas) and the mixture is too rich. Like the carburetor in your automobile; flood it with fuel and try to start it (good luck with an older model!). The range of concentration that results in a combustible mix is defined by an upper and lower limit. The upper limit is referred to as the Upper Explosive Limit (UEL) and the lower limit is referred to as, you guessed it, the Lower Explosive Limit (LEL).

In monitoring for flammable gas hazards, we’re generally establishing two alarm levels. Reaching the first alarm level would have the effect of turning on a ventilation system, so as to forcefully remove the gas from the air and bring the concentration back down to a safe level. This is established as a percentage of the LEL, typically around 25%. Reaching the second alarm level would indicate that the ventilation system has either malfunctioned or the concentration of flammable gas has exceeded the capabilities of the ventilation system. In this case the condition must be annunciated by an audible/visual alarm. The second alarm level is set at no higher than 50% of the LEL.

Examples of combustible gases, in addition to the two mentioned above (propane and gasoline fumes) include hydrogen, methane, ammonia, and oxygen (really!),

Toxic Gas Hazard: toxic is a synonym for poisonous, as in life-threatening, as in “that doesn’t smell good, I’m getting out of here!” Actually, while some toxic gases do have strong odors associated with them, it is the ones that don’t that tend to be the most deadly. Carbon monoxide (CO) is, for all practical purposes, odorless in concentrations that can be lethal. Indeed, more fatalities occur due to exposure to toxic gases than due to explosions causes by flammable gases.

Toxic gases are measured in parts per million, or ppm. To define safety limitations in ppm, we need to discuss a couple of additional terms. The first, known as Time Weighted Average, or TWA, is the level at which a person can be exposed to the toxic gas for an eight hour period of time (40-hour workweek), and not incur any adverse health effects. The second is the Short Term Exposure level, or STEL. This is the time weighted average (maximum) concentration level at which a person can be exposed to the toxic gas for a fifteen minute period of time without suffering from any adverse health effects.

So in monitoring for toxic gas hazards, we see that it’s not just the concentration level in ppm that’s important, but also the amount of time that one is being exposed to the toxic gas. As with flammable gases, we typically establish two alarm levels for toxic gas hazards. The first is set for the TWA. Reaching this level should invoke the ventilation system, however no audible/visual alarm is required. Just ventilate the space until the concentration level drops back below the TWA. The second is set for the STEL. At this point the condition must be annunciated.

[an error occurred while processing this directive] Gases that pose toxic gas hazards, in addition to carbon monoxide, include nitrogen dioxide (diesel fumes), chlorine, ammonia, and hydrogen sulfide. To put some values to a couple of these gases, I did a little product research and found a carbon monoxide and a nitrogen dioxide detector, made by the same manufacturer. These detectors have on-board alarm relays, for both activation of a ventilation system, and annunciation via a separate audible/visual alarm. The ventilation relay setpoint for the CO detector is 35 ppm (averaged over 5 minutes), and the annunciation relay setpoint is 200 ppm (after ten minutes). By contrast, the ventilation relay setpoint for the NO2 detector is adjustable between .5 and 3 ppm, and the annunciation relay setpoint is adjustable between 1 and 5 ppm. The important thing to take away from this is that, always consult an expert in gas detection for your application, especially when adjustability of an alarm relay is offered.

Asphyxiant Gas Hazard: An asphyxiant gas is one that can deplete the level of oxygen in the air. Air that is oxygen-deficient is a hazard seeing that we as humans need oxygen in the air to live. Simple asphyxiants deplete the oxygen in the air by displacement; the oxygen is displaced by the asphyxiant. In small enclosed spaces this can become a real suffocation hazard. Many refrigerants used in chiller systems are considered asphyxiants. In a chiller room it is therefore important to be able to monitor and alarm a high refrigerant level in the room, perhaps caused by a leak in the refrigerant line. Carbon dioxide (CO2) at high concentrations is considered an asphyxiant. Normal outdoor air concentrations of CO2 are typically around 700 ppm, whereas normal “acceptable” indoor concentrations are under 1,100 ppm. By contrast, levels reaching upwards of 50,000 ppm will deplete the oxygen in the air and are considered a threat.

Tip of the Month: Safety guidelines vary from state to state, and internationally as well. In the US, there are several organizations that publish safety guidelines, including the Occupational Safety & Health Administration (OSHA) and the National Institute for Occupational Safety & Health (NIOSH). Each of these organizations has its own terminologies that may differ from each other, and from what’s been written herein. Always check to see the guidelines that any particular municipality follows, or solicit the advice of your “local expert”!


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