Proper Detector Tube Use Ensures Reliable Indication of Gas Concentration

Transcription

1 Proper Detector Tube Use Ensures Reliable Indication of Gas Concentration Edward Ligus Draeger Safety, Inc. Abstract: Detector tube technology has been around since the early twentieth century, and can now be used to detect hundreds of different gases and vapors. Still, proper use of these devices is required to ensure reliable readings. This white paper explains detector tube technology and how it should be used to get reliable concentration readings from grab samples. A Brief History Driven by the needs of the coal-mining industry, detector tube technology was first patented by two Americans in 1919 as a means of detecting carbon monoxide (CO). In those days, mine fires were a constant threat, producing odorless and colorless CO gas. The risk of fire has been reduced by current mining technology, but CO is still a threat due to fire and the use of diesel-powered mining equipment. Although the presence of CO is certainly a threat, the potential for explosion from methane gas may be the more relevant concern to the coal-mining industry. There are many other industries where toxic gases and vapors pose a risk to workers. Some of the more common exposure environments include oil and gas drilling operations, refineries, chemical plants, metal manufacturing industries, semiconductor processing, hazardous material clean-up sites, fire-fighting operations, and many others. The detector tube method lends itself well to the diversity of gases and vapors found in any workplace scenario. There are hundreds of detector tubes that can identify and quantify the concentration of hundreds of gases and vapors. Detector tubes are capable of detecting more gases and vapors than any other on-the-spot field method available, making them applicable to virtually any industry. 1

2 Advantages and Disadvantages Widespread use of detector tubes is the result of their versatility, which includes: Excellent portability and ease of use Detection of more than 500 gases, vapors, and aerosols Ability to detect either a specific chemical or a general family of gases Detection of concentrations from a few pats per billion (ppb) up to volume percent levels Accuracy from ±5% to ±25% A few examples of these detection capabilities will illustrate the wide range of applications that detector tubes serve. Concentrations of dimethyl sulfate can be detected down to 5 ppb. Carbon dioxide and hydrogen sulfide can be quantified from parts per million (ppm) ranges up to high volume percent levels. Many aerosols can be detected, such as sodium and potassium cyanides, chromic acid, and oils. Hydrogen cyanide, which is commonly found in the combustion byproducts of burning plastics in structure fires, can be detected down to a few ppm. The entire detector tube methodology is simple to use, requiring only basic skills. Many tubes provide a direct readout of the concentration, which is the length of a stain on a scale, typically calibrated in ppm or other standard units. Their measurement results are highly reliable, using technology that has been proven over the past 90 years. Although detector tubes have many advantages, there are disadvantages that should be considered. There are detector tubes that require refrigeration throughout their shelf life. Generally, such tubes must be maintained at or below a temperature of 50 F to ensure accurate results. Another storage issue is photosensitivity. Like chemical reagents, all tubes must be kept out of direct sunlight for prolonged periods. They should not be removed from their package until ready for use, and should be handled carefully to prevent breakage. In addition, detector tubes do not provide a permanent record of results. This is to say the color change that takes place inside the tube is not stable. The tube must be read immediately following the last stroke of the sampling pump. Over time, which could be hours or days, the color change may extend 2

3 throughout the length of the tube or it may retreat, becoming shorter than the original result. Some tubes may revert back to the original color entirely. If a permanent record is necessary, the user must take a digital photo of the reading for future reference. Tube Design and Usage A detector tube is a hermetically sealed glass tube (Figure 1) containing an inert carrier material mixed with one or more reagents that undergo a colorimetric reaction when specific contaminants are drawn into the tube. The layer in which the colorimetric reaction takes place is the indication layer. The length of the color change, or the intensity of color change as compared to a standard, indicates the amount of contaminant present [1]. Prior to use, both tips are broken away from the ends of the tube using a tube opener or the pump used to pull the air sample through the tube. Figure 1. Representation of a typical detector tube with identifying and usage information. As shown in Figure 1, the air contaminant to be measured is indicated on the tube (water vapor in this example). The manufacturer s literature that comes with the tube may need to be consulted to determine measurement units, which could be ppm, ppb, mg/ml, volume %, etc. Each tube is designed for a specific air volume that must be pulled through the tube in the direction indicated by the arrow. 3

4 There may be more than one layer of chemicals inside a detector tube. Figure 2 illustrates the difference between single- and multi-layer tubes. For some reagents, a prelayer is needed to facilitate the proper reaction. In some tubes, a drying layer is added that acts as a moisture trap. This is used when the indication layer would be adversely affected by moisture content in the sample. Typically, too much water vapor results in a tube reading that is too low. In other cases, a precleanse layer is used to reduce cross-sensitivity. The precleanse layer selectively traps unwanted gases or vapors that would influence the reading. Otherwise, these superfluous gases would react in the indication layer, thereby causing an abnormally high reading. A classic example is the benzene specific tube, which has a precleanse layer to trap other similar aromatic compounds such as toluene, xylene, and ethyl benzene, while allowing benzene to pass into the indication layer. Figure 2. Single vs. multilayer tubes: some tubes have prelayers to facilitate the required chemical reaction and color change in the indication layer. A chemical conversion layer may also be necessary. This is used ahead of the indication layer when the gas or vapor to be measured does not react readily with the indication 4

5 layer reagent. The chemical conversion layer converts the gas or vapor into something that will react with the reagent and produce a color change in the indication layer. More elaborate detector tubes may employ an ampoule design (Figure 3). Usually, ampoules are incorporated into a detector tube when it s not possible to place all the necessary reagents in a single tube due to chemical instability that adversely affects shelf life. To avoid this issue, the manufacturer uses an ampoule (a tube within the detector tube) to house part of the reagent until the tube needs to be used. The ampoule contents may be in the form of a solid granular material, a liquid, or a paraffin (releasing a vapor). The ampoule may be broken prior to the measurement or immediately after the air sample is drawn through the tube. Tube instructions will clearly specify the proper order for a valid measurement. Figure 3. Ampoule tube construction depending on the reactant gas and indication layer, the reagent ampoule will be broken just prior to or right after the air sample is introduced into the tube. In all cases, detector tubes are designed to be used with a dedicated sampling pump (Figure 4) that draws a quantified volume of air through the tube. The number of pump strokes (n=2 in Figure 1) required to pull the correct volume of air is indicated in the 5

6 operating instructions and is often imprinted on the detector tube. Every manufacturer calibrates their pumps and tubes to work together as a unit. Therefore, interchanging pumps and tubes from various manufacturers is not recommended, as such practice may result in erroneous results. As alluded to earlier, detector tube storage and operating temperature ranges should not be exceeded. A typical operating range is 32 F to 104 F. Low temperatures typically slow or stop some chemical reactions. Temperatures below the specified operating range in the manufacturer s instructions will result in abnormally low readings or there may be no color change at all in the reagent. High temperatures act as a catalyst in most chemical reactions, which usually result in detector tube readings that are too high. However, in some cases, the tube s reagent system may contain volatile compounds that can be vaporized due to high temperatures. This could result in a low reading, which may also be difficult to see because of a faint color change due to loss of the chemical indicator. Here are some options for handling low and high temperatures: Low temperatures down to 4 F: wrap fingers around the tube to warm it High temperatures up to 140 F: wrap a chilled cloth around the tube Sampling in hot flue gases: a hot air probe can be used for exposures of up to 3 minutes Engine Exhaust Sampling: use a motor vehicle probe Storage temperatures typically are specified as no higher than 77 F, but some manufacturers specify no higher than 50 F. In all cases, storage temperatures should remain above freezing (32 F). 6

7 Figure 4. Typical detector tube pumps; upper units provide automatic volume control; lower units are manual sampling pumps. Test Standards The Safety Equipment Institute (SEI) administers a third-party voluntary verification testing program according to the ANSI/ISEA (2003) Standard. The detector tube verification testing is done by the Bureau Veritas Laboratory. These tests establish basic accuracy, minimum stain length, maximum channeling and other characteristics. 7

8 A variety of other standards organizations recognize detector tubes as viable tools for conducting workplace measurements. Among those are The American Conference of Governmental Industrial Hygienists (ACGIH ), the National Institute for Occupational Safety and Health (NIOSH), and the Occupational Safety and Health Administration (OSHA). The American Petroleum Institute (API), in a document referred to as the Garrett Gas Train, specifies hydrogen sulfide and carbon dioxide Draeger tubes for testing drilling mud. The Compressed Gas Association (CGA) specifies the use of certain tubes to check for contaminants in compressed gases, such as Grade D breathing air. Common Detector Tube Applications The prevailing use of detector tubes tends to fall into one of the following categories: Occupational health and safety Process control and technical gases Public safety Leak detection Occupational Health & Safety Detector tubes are widely used in various industries to check for contaminants dangerous to the health and welfare of a person working in the sample area. In these applications, the detector tube is used to check the ambient concentration of a toxic gas or vapor against established standards. Detector tubes are particularly useful when there is no electrochemical sensor available or when very low (ppb) or very high (Vol %) concentrations are present. In terms of personal safety, various standards are applied in determining the permissible exposure limit (PEL) to a particular contaminant. The most prominent standards used for this purpose are those published by OSHA. This agency updates and publishes an extensive list of its PELs [2], which are available in the Federal Register. Another prominent standard is published by ACGIH. This is updated on an annual basis, and the current (2010) version lists Threshold Limit Value (TLV ) occupational 8

9 exposure guidelines for more than 700 chemical substances and physical agents. It also lists more than 50 Biological Exposure Indices (BEIs ) that cover more than 80 substances. In OSHA s case, specific methods and analytical techniques for measuring PELs are not designated in the standard cited in reference [1]. Guidelines are supplied in the OSHA Chemical Sampling Information document, which provides information on health factors and typical monitoring methods for both laboratory and on-site usage. Generally, OSHA accepts data from a variety of sampling methods and technologies as long as they provide results with known accuracy. Typically, detector tubes are not used to determine time weighted average (TWA) concentrations of air contaminants, which are the basis for OSHA PELs. Detector tubes are most commonly used for grab samples or short-term measurements that supply results for a specific point in time. Nevertheless, OSHA and other organizations use detector tubes and reference the results to established PEL values. Process Control & Technical Gases Detector tubes are used in process gas streams to determine if the gas composition ratios are correct for producing the desired end product yield. Tubes are also used to check for the presence of contaminants in process gas streams, which can affect both yield and quality. In the case of natural gas pipelines, tubes can monitor water levels, which could present corrosion problems. In addition, tubes are used to check breathing air quality to help protect the safety of operating and maintenance personnel. Public Safety Gas tubes are used extensively by hazardous material clean-up personnel, civil defense teams, and law enforcement agencies. Some manufacturers such as Draeger have configured specific and specialized tube kits that address the needs of clean-up crews. These include five-tube Simultest Kits for inorganic and organic gases and vapors, allowing the determination of five gases/vapors at once. The Civil Defense Set (CDS) kits are used to determine the presence of chemical warfare materials (nerve, blister, and blood agents). There also are Clandestine Kits for investigating illegal methamphetamine labs. 9

10 Leak Detection Although not a common application, tubes can be used to help locate the source of leaking gases or vapors. Again, this is handy when an electronic sniffer is not available. Reading a Detector Tube The use of a detector tube results in a visual indication of contaminant concentration. To take advantage of a tube s inherent accuracy, certain protocols should be followed. Reading the tube requires adequate lighting but should not be done under intense illumination. The stain or discoloration in the tube can best be evaluated by holding the tube against a light-colored background, such as the tube packaging or a white piece of paper. Indoors, this can be done under standard incandescent or fluorescent lighting conditions. Ambient (indirect) outdoor light can also be used, but tubes should never be held up to the sun to evaluate the discoloration. To evaluate the length of the stain, it helps to compare the discolored tube to an unused tube (Figure 5). Figure 5. Comparison of a used and unused detector tube. When evaluating the contaminant concentration on a tube scale, look for the full length of discoloration (sum of all colors) as far as possible. In some cases, there may be diffuse discoloration. In that case, evaluate the scale as far as any discoloration can be perceived, i.e., the end point of the slightest discoloration. 10

11 In some cases, it may be possible to extend the range of a detector tube. For example, consider an ammonia tube that has a range of 5 70 ppm using a known sample volume of 10 pump strokes. It may be possible to extend that range to ppm by sampling with only one pump stroke. However, this should never be assumed. The tube manufacturer should be consulted to determine if such a scale extension would be valid. In addition, be cautious in trying to make a linear interpolation of a concentration value between scale markings, particularly at the upper and lower ends of the scale. A manufacturer s handbook, product instructions, or website may provide some guidance, but it s best to talk directly with a technical expert at the factory before making linear estimates. Sampling Situations Using a detector tube may require some adaptation to the operating environment. One instance of this is when drawing samples from pressurized lines in process or technical gas applications. Three different approaches to obtaining these samples are the use of a T-fitting or a sample bag by venting to a plastic jug. T-fitting Samples In this method, a T-fitting is installed in the line carrying the gas stream to be sampled. Gas flows though the inline portion of the tee, while the side leg is used for sampling. If this is a permanent installation, a shut-off valve would typically be installed in the sample leg. A short piece of tubing is connected between the sample leg and the detector tube to ensure a gas-tight connection and keep any dead space volume to a minimum. The standard pump is used to pull the sample from the gas stream through the detector tube. Sample Bag A sample bag is used to collect a sample and keep it isolated from dilution or contamination by ambient air. A sample bag is made of chemically resistant material such as Tedlar film, with two valves used as a dedicated inlet and outlet. The gas to be sampled enters the bag through the inlet fitting. To ensure the integrity of the sample, the bag should be flushed two to three times, with gas entering the inlet valve and exiting the outlet valve. Once the bag is filled with the necessary sample volume, the detector tube and pump are connected to the outlet fitting using a short piece of rubber tubing. Then the sample is pulled through the detector tube by the pump. 11

12 Venting to a Plastic Jug Gas from a process stream can be vented into a plastic jug that has a series of holes around the sides and bottom. This allows the gas to be presented to the detector tube at ambient pressure by inserting the detector tube into one of the holes in the jug. Then the pump is used to pull the sample through the detector tube. Another situation is sampling for unknown gases or vapors. As mentioned earlier, Draeger supplies five-tube Simultest Kits for Civil Defense Service (Chemical Warfare Kits), HazMat Kits for inorganic and organic gases/vapors, and Clandestine Kits for investigating illegal methamphetamine labs. In addition, there are Fumigant Kits and Screening Tube Kits. Some examples of these are described below in greater detail. Screening Tubes Such tubes are used as a first step in identifying the gas to be sampled. Examples of screening tube products include Polytest, Acid Test, Amine Test, and Polytec with multiple layers. These tubes are used to answer the question of whether the gas is basic, acidic, or organic in nature. Getting this information provides a clue on what to check for next. HazMat Kits These kits are designed with either a general matrix of detector tubes for unknown samples or as HazMat simultaneous test sets for typical hazardous material incidents. In the latter case, the kits can be used for industrial, rail, or highway incidents. With a prescribed set of tubes, these kits provide relatively fast, on-the-spot results to help emergency response personnel do their job. Chemical Warfare Agent Sets These sets provide a qualitative determination of the presence of nerve, blister, blood, and choking agents. Test sets are available for combinations of the following agents: Nerve: GA, GB, GD Blister: Mustard gases, Lewisite Blood: Hydrogen cyanide, Phosgene Choking: Chlorine 12

13 Figure 6. Examples of Simultest sets for indentifying and quantifying unknown samples. The circled areas in the right-hand view indicate positive test results. Figure 6 illustrates examples of Simultest sets. They consist of five detector tubes calibrated as a unit to be used for simultaneous detection of specific gases and vapors. It is important to understand these are not the same tubes sold ten to a package. While these tubes are based on the same indication systems, they have been calibrated on a different volume and opening time 1. This allows the measurements to be done in a fraction of the time it would take using one tube after the other. There are one or two marks on each tube, indicating one times the TLV and five times the TLV. Remote Sampling with Extension Hoses Sometimes an extension hose is needed to reach an area where the sample will be drawn. A typical off-the-shelf length is 10 feet, but manufacturers also offer lengths up to 50 feet. For lengths greater than 10 feet, correction factors may be necessary as specified by the manufacturers. Opening time is the time required for a pump to open completely based on a given pump and tube combination. For example, a low range ammonia tube has a 4 5 second opening time whereas a low range carbon monoxide tube has a second opening time. 13

14 Prior to sampling, the pump and extension hose combination should be leak-tested with an unopened tube to confirm there is no leakage anywhere in the sampling train; otherwise, less than the required volume will be drawn through the detector tube, resulting in a low reading. A simple technique to prevent liquid from being drawn into the tube is to make a loop in the end of the hose such that the detector tube is facing up. Electrical tape can be used on the hose to form and retain the loop. As shown in Figure 4, both manual and automatic pumps are available. Regardless of which type is used, sound practice dictates that the pump and tube combination and any in-line accessory be tested to ensure the leak tightness of the entire sampling train. After conducting the measurement, flush the pump and hose to purge them of any residual gas or aerosol that could cause corrosion in the pump or otherwise shorten the operating life of the system. This is particularly important for acidic samples. If the pump has o-ring seals and check valves, as is the case with piston-type pumps, these should be lubricated according to the manufacturer s specifications. Summary Detector tubes are a proven and reliable method of gas sampling in the workplace. They are capable of detecting and quantifying the concentration of more gases and vapors than any other portable monitoring method. Their use is simple and fast, with well-established accuracy. Because of these features and their low cost, they are probably the most widely used method of collecting grab samples, which are used for a broad range of applications worldwide. 14

15 References 1. Maslansky, C.J. and Maslansky, S.P., Air Monitoring Instrumentation, John Wiley & Sons, illustrated edition, May United States Department of Labor, Occupational Safety & Health Administration, 29CFR1910, Subpart Z, Standard Number , Air Contaminants, Table Z-1, Limits for Air Contaminants, Feb. 28, # # # 15