Serve and Detect (August 15, 2010)
When the bodies of the final four missing workers were recovered days after a massive explosion at the Upper Big Branch Mine in Montcoal, West Virginia in April, the event gained the unfortunate distinction of becoming the most deadly mining accident south of the border in 40 years. The fatality toll was 29,
The four-day rescue effort was complicated by the fact that rescue teams entering the mine encountered “unfavourable atmospheric conditions,” says a statement from Massey Energy Company, the Richmond, Virginia-based owner of the mine. The conditions required that bore holes be drilled about 330 metres into the mine and exhaust fans used to make conditions safe enough for teams to proceed.
Like many explosions at coal mine operations, it is believed that a buildup of methane gas played a role in the West Virginia accident. Whatever the final conclusions, the fatal incident illustrates the importance of proper gas monitoring and what can go wrong when hazardous gases reach dangerous concentrations.
For those looking to minimize any potentially catastrophic consequences — in coal mines as well as a host of other work environments — there is a range of gas monitors, accessories and sensors to fit the bill.
ONE, TWO OR THREE
In general, monitors fall under two main categories: fixed and portable products. A third option is stand-alone monitors, which are similar to fixed gas detectors, but can also be transported from one work site to another.
Depending on the conditions and activities of a workplace, choices range from single to multi-gas detectors, specific sensors (such as catalytic bead and infrared that monitor the lower explosive limit [LEL] of combustible gases), and photoionization detectors, or PIDs, which are used in workplaces where volatile organic compounds (VOCs) are present.
Purchasing decisions should be guided, first and foremost, by the workplace environment. “Typically, where the biggest mistake will be made with a piece of equipment or a piece of technology is that it’s not properly applied to the application,” suggests Dave Wagner, director of product knowledge at Industrial Scientific Corporation in Oakdale, Pennsylvania. Wagner notes mistakes run the gamut from uncertainty over which gases are present to which ones require monitoring and may interfere with the proper functioning of a sensor, such as chlorine in a catalytic bead sensor.
Joe Glorioso, sales application product manager for Mine Safety Appliances Company (MSA) in Pittsburgh, says a standard detector monitors “the big four”: oxygen, combustibles, hydrogen sulphide (H2S) and carbon monoxide (CO).
Asked about using fixed or portable monitors, Glorioso says a rule of thumb is that fixed gas detectors are mainly used for monitoring an area while portables are used for personnel. “In my opinion, there are really no set black-and white guidelines,” he says. “In many cases, you really should have both,” he adds.
Greg Reeves, president of Oakville, Ontario-based Arjay Engineering Ltd., says fixed gas detection systems are ideal where “there could be continuous emissions of a gas,” such as in a parking garage, chemical plant or warehouse, or in areas where an unexpected or catastrophic leak could occur, including a refinery.
Glorioso agrees, saying fixed monitors offer 24/7 coverage in workplaces where there’s “a lot of ground to cover.”
He would count himself a supporter of teaming both fixed and portable monitors. “You can spread out your permanent instruments, but your portable instruments, again, will protect that worker,” Glorioso notes. “Say you spread out your fixed instruments every [15 to 30 metres], depending on where the leak occurs, the temperature conditions [and] the wind, it could all affect where that gas cloud dissipates to.”
On their own, offers Reeves, portable monitors are a good choice for confined spaces, “especially if you’re going into spaces where you don’t know what gases may be there.” The big four gases are typically targeted, but added protection from “unknowns” may be achieved by using what he calls a background broad-range sensor. Two examples include a broad-range VOC sensor and a solid state sensor.
Consider a situation in which workers in a sewer are unaware that a few blocks upstream a worker at a dry-cleaning service has just dumped some percoethylene. “You may not necessarily alarm because it is not one of those [four] target gases,” Reeves says.
Ross Humphry, president of Canadian Safety Equipment Inc. in Mississauga, Ontario, agrees, pointing out that sewers are already prone to oxygen deficiency and the presence of methane and H2S gases. Add to those expected hazards that industries in the area “can deposit waste solvents and chemicals [and this] can create a soup of toxics that may not be detectable by the sensors in the instrument you have chosen,” Humphry cautions.
Circumstances can certainly change. He notes that utility manholes, which also have the potential for oxygen deficiency, but usually do not contain materials that generate methane and H2S gases, can have these gases migrate into them. “Not all confined spaces are equal and the gases found in them can vary dramatically based on the types of processes that can create the gases,” Humphry says. In some cases, he notes, positive pressure supplied air may be considered a safe alternative.
It is important to choose the correct gas detector, but choice of sensors is equally critical. Bob Henderson, president of GfG Instrumentation in Ann Arbor, Michigan, says one of the more significant developments in recent years is that manufacturers continue to expand their product lines, offering end-users more sensors from which to choose.
Take infrared sensors, for example. Ideal for certain combustible gases, Reeves says that these are “very selective and precise,” and can be tuned to the specific wavelength absorption of a target chemical. “Depending on that wavelength, you can determine which gases it will or will not respond to, and you can make that wavelength very loose or very tight,” he reports.
Reeves says infrared sensors tend to be used for refrigerants, which include variations of the compounds trifluoroethane, tetrafluoroethane, chlorodifluoromethane and trichlorofluoromethane (freons are used in heating, ventilation and air-conditioning systems).
Other working environments in which infrared sensors might be a solid choice include offices, breweries an
d greenhouses, where carbon dioxide (CO2) is used to either monitor indoor air quality or to control air flow.
The wavelength of a CO molecule is long and ideal for a large infrared sensor, while CO2 has a very short wavelength, Reeves says. As such, the sensor for CO2 can be small, usually no longer than the length of a finger, he adds.
“You can get an infrared sensor full of electronics and interface for $300 to $400,” notes Reeves. “If you move up into the longer wavelengths, like refrigerants, which require the longer wave path, they come into the thousands.”
Henderson notes that the purchase price will go up by approximately $1,000 to $1,100 for an instrument equipped with an infrared-type combustible sensor, versus one with a traditional LEL, or catalytic bead, sensor.
Besides the higher cost, Wagner suggests that infrared sensors carry another disadvantage compared with catalytic bead — the inability to detect hydrogen.
Reeves further cautions that users of infrared detectors should be aware these may not pick up “gases that may give you nuisance or cross-sensitivity. And you may not want to ignore those other gases.”
For workplaces that have opted for infrared sensors, but also have hydrogen in their applications, Henderson suggests there are a few options: use an electrochemical sensor to measure the hydrogen or provide a traditional LEL sensor for hydrogen and employ the infrared sensor for everything else.
HARD TO TELL
Humphry notes there are some issues of concern when it comes to catalytic bead sensors and combustible gases. The sensor is “unable to distinguish one combustible gas from another and can only provide accurate gas levels when they are calibrated with a known gas or are programmed with a variety of different gas responses and are then told which calibration curve to use,” he explains.
Glorioso sees a couple of advantages in choosing infrared over catalytic bead sensors. These include a longer sensor life expectancy (infrared sensors can last five-plus years compared with about two years for catalytic bead), and there is no need for oxygen to be present within the workplace atmosphere to detect gas.
With traditional LEL sensors, Glorioso explains that an oxygen sensor is usually placed right next to the first sensor to ensure that enough of the gas is available to provide a good reading.
In addition, equipment manufacturers note that infrared sensors are more responsive to large hydrocarbons, require less frequent calibrations and cannot be “poisoned” like traditional sensors.
Catalytic bead sensors, says Reeves, are “notoriously poisoned by silicone.” If there is silicone in the air, he reports that “the vapour starts to stick on the sensor and it basically suffocates the sensor.”
An ammonia electrochemical sensor may also become poisoned. “It has a hard time recovering after it gets a heavy dose of ammonia,” says Reeves, adding that because there is no chemical reaction occurring with the infrared option, it is not as susceptible to these recovery issues.
With the long list of gases that can be found on the job, the possibility exists of interference gases or cross-sensitivities of gases. Beyond silicone and sulphur, Wagner says chlorine is an inhibitor on a traditional LEL sensor.
“A chlorinated compound in a high enough concentration would tend to prevent a traditional combustible gas sensor from being able to respond. It won’t give you false alarms; more a false sense of security,” he suggests.
Likely the most common — and possibly the most troublesome — of cross-interference is hydrogen’s effect on a CO sensor, says Wagner. “That’s something that is fairly hard to avoid.” Cross-interference aside, the ease and frequency of calibration is a factor for some when purchasing a monitor.
Manish Gupta, marketing manager with Draeger Safety Canada Limited in Mississauga, Ontario, offers an example: some units require monthly calibration while others do not have to be calibrated for two years. Manufacturers can even set units “so that if you don’t calibrate it within 30 days, the unit will shut down,” Gupta reports.
The same can be done for units in which a field or “bump” test is not performed before each use. Bump testing involves using a “squirt gas” to ensure the unit is working properly and sounding an alarm.
While regulatory provisions in many jurisdictions demand a bump test or full calibration of portable gas monitors before each use, Humphry suggests this protective measure does not always take place. The requirement “is great in theory, but rare in practice,” he argues.
Although manufacturers have developed automatic tests that can be done quickly, it is up to each individual user to actually complete the checks.
Docking or calibration stations are also available so that users can configure units, for example, to alarm when gas levels reach a certain concentration. In past, says Gupta, users had to return to equipment makers to make any such change. “Now you’ve got the option that you can customize the unit to your own needs,” he says. “You’re not stuck in a mould anymore having to buy what is available and hope it works for you.”
The ability to configure is particularly important should permissible exposure levels be amended. GfG’s Henderson points out that the new threshold limit value (TLV) for sulphur dioxide in the United States is 0.025 parts per million (ppm). That is down considerably from the previous limit of two ppm, he says. “That is going to make detection very problematic for a lot of people.”
A new TLV for H2S, dropping from a permissible level of 10 to
one ppm, is also expected to come into effect soon, Henderson reports.
Gupta notes that, besides targeting specific gases, certain sectors may need products that offer superior sensor stability. Some sensors on the market will compensate for workplace conditions such as temperature, humidity and pressure, he says.
In the mining industry, for example, some sensors will allow for calibration to be done above ground and still work underground. Gupta cautions that that is not always possible with all types of gas detectors.
Sensor life also varies depending on the product. Reeves points out that improved sensor technology for electrochemical cells has allowed some manufacturers to guarantee sensor life for five years or more, whereas just three or four years ago, the lifetime was closer to two years.
Improved technology is also making it possible for nanotechnology to be used in gas detection, says Reeves. For example, he notes that nanosized microcircuits are currently being examined primarily in the United States for terrorism-related applications.
“It’s falling over into the general industrial side to see how the nanotechnologies could be applied to general gas sensing,” he says. Reeves says he expects that such tiny sensors would take little energy to drive them, with minimal battery power and long battery life.
Currently, Gupta reports, many products come with rechargeable batteries that use “trickle charges,” which means that if the unit is left in the charger overnight, it does not overheat or damage the product.
Of course, price is always a consideration, which, unfortunately, can have a negative influence on gas detection buying decisions. “Cheap, throwaway units [are] now cluttering the market,” suggests Humphry, adding that “most end-users just want to buy the instrument and be done with it.”
He advises gas detector purchasers to view the equipment much the same way as they would a motor vehicle. “It has to be serviced and you have to refuel it. There is no escaping the inevitable,” Humphry says.
Although it may not seem to be the case initially, Gupta says that disposable units cost an employer more in the long run. And, in the worst case scenario, a poorly functioning or malfunctioning instrument can mean the difference between life and death.
Jason Contant is editor of canadian occupational health & safety news.
Time is of the essence when it comes to being exposed to hazardous gases. Consider, for example, what hours — or even minutes — may mean when that gas happens to be hydrogen sulphide.
Parts per million (ppm) Time Effects & Symptoms
10 8 hours Permissible exposure level
50 to 100 1 hour Mild eye, respiratory irritation
200 to 300 1 hour Marked eye, respiratory irritation
500 to 700 ½ to 1 hour Unconsciousness, death
>1,000 Minutes Unconsciousness, death
* Adapted from RAE Systems’ “Guide to Atmospheric Testing in Confined Spaces”
Some sources contend that infrared gas detectors offer a number of advantages over catalytic bead units. First, infrared detectors provide a speedy response (typically less than 10 seconds), demand little maintenance and greatly simplify the checking process, through the use of modern micro-processor controlled equipment.
As well, the detectors are designed not to be affected by any known “poisons,” and can operate in inert atmospheres and under a wide range of ambient temperature, pressure and humidity conditions.
The principle behind the units is dual wavelength infrared absorption. This means light passes through the sample mixture at two wavelengths — one of these is set at the absorption peak of the gas to be detected; the other is not. The two light sources are pulsed alternatively and guided along a common optical path and, finally, through the sample gas.
The beams are then reflected back by a retro-reflector, returning once more through the sample and into the detector. The signal strengths of sample and reference beams are compared and a measure of gas concentration provided.
* Adapted from information from BW Technologies by Honeywell