MONITORING HAZARDOUS ATMOSPHERES

MONITORING HAZARDOUS ATMOSPHERES

BY CRAIG SCHROLL, CSP, CET

Atmospheric monitoring is crucial to safe entry operations in confined spaces. Atmospheric hazards can kill and have killed. All fire service organizations should have the capability to monitor the atmosphere in areas in which their personnel will be operating. Among the types of responses that commonly require monitoring of the atmosphere are confined space rescues, trench rescues, hazardous materials releases, carbon monoxide detector activations, and carbon dioxide fire control system activations. Atmospheric hazards that may exist within the confined space can be divided into three major categories: oxygen deficiency or enrichment, flammable materials, and toxic materials. All could pose a life-threatening situation. Atmospheric hazards demand detailed evaluation and assessment prior to entering the space and must be continuously monitored during entry operations.

Atmospheric hazards can have two primary sources. The hazard may be present in the space prior to the entry operation (an oxygen deficiency caused by a lack of ventilation within the space or a flammable vapor hazard created by the presence of residues of flammable liquids within the space, for example). Also, residues of toxic products that may have been previously stored in the space may present a toxic atmospheric hazard for entry personnel. Responders may also create contamination within the space by disturbing residue, sludge, and scaling that may be on the inside of the tank and may contain some of the tank`s previous contents that had not been released.

OXYGEN DEFICIENCY OR ENRICHMENT

Priority should be given to monitoring the oxygen content within the confined space. Oxygen deficiency is the primary cause of death in confined spaces. To be considered safe, the oxygen concentration within a confined space must be within the range of 19.5 percent and 23.5 percent. The atmosphere is considered oxygen-deficient when the oxygen concentration falls below 19.5 percent and poses a direct threat to entry personnel.

Oxygen-deficient atmospheres may be the result of stagnant air caused by a lack of ventilation or the consumption of oxygen by reactions taking place in the space–the oxidation process of rust, for example. Biological actions–such as the breakdown of organic materials through bacterial action–may also consume oxygen. An oxygen deficiency can also be caused by introducing an inert gas such as nitrogen in the atmosphere.

Oxygen enrichment (elevated concentrations of oxygen) within the atmosphere also can present problems. When the oxygen concentration in the space exceeds 23.5 percent, the atmosphere is referred to as “an oxygen-enriched atmosphere.” In addition to the adverse effects of breathing in an enriched-oxygen atmosphere, the potential for and magnitude of fires or explosions are greatly increased. Chemically, a fire is an oxidation reaction. When enriched levels of oxygen are present, these reactions may be more likely to occur, and with greater force.

FLAMMABLE MATERIALS

Fire or explosion is the second leading cause of death in confined space operations. Flammable gases or vapors can lead to fires or explosions within or just outside the confined space and can pose inhalation risks for responders. For example, solvent vapors present in a high enough concentration can have a narcotic effect on an individual, leading to drowsiness, a reduced capacity to exercise judgment, and other safety problems. Exposure effects may occur well below the levels that will present flammability problems. This aspect is covered under the section on toxic materials. The generally accepted safe level for flammable gases or vapors is 10 percent of the lower explosive limit (LEL).1

All flammable vapors and gases have a lower and upper flammable limit (UEL). These flammable limits set the boundaries of the flammable or explosive range. These vapors are ignitable only within the flammable range. If an ignition source were introduced into a concentration of vapors below the lower flammable limit, the mixture of gasoline vapors and air would not burn. Below the lower flammable limit, the mixture is too lean–not enough fuel vapors are present. The mixture would not ignite above the upper flammable limit because it is too rich–there are too many fuel vapors compared with the oxygen in the air. Our target for flammable atmospheres is to maintain an environment below 10 percent of the lower flammable limit. This allows a 90 percent safety margin before the bottom of the flammable range is reached.

TOXIC MATERIALS

Toxic gases or vapors may come to be within the confined space through three primary methods:

They may be a product or the residue of a product in the space.

They may result from activities of the entry crew or from materials the crew has taken into the space.

They may be generated by the natural decay processes within the space.

Toxic materials pose a difficult challenge in assessing the atmosphere within a space. To adequately monitor for toxic gases or vapors, these gases or vapors must first be specifically identified. When the presence of the toxic is the result of the previous contents of the space or operations conducted within the space by the entry crew, identifying the toxic material is relatively easy. However, toxic gases and vapors that could be generated by the decomposition of materials within the space may be more difficult to identify. There is no generally accepted single value for levels of toxic gases or vapors. The safe level of atmospheric contaminants must be assessed based on the knowledge of the specific chemical. One of the more common references available to determine safe levels of exposure is the “Z Table” in U.S. Occu-pational Safety and Health Administration Standard 1910.1000. This table establishes permissible exposure limits for a wide variety of materials. Another commonly used guide for determining safe levels of exposure to toxic materials is the “American Conference of Governmental and Industrial Hygienists Guide to Threshold Limit Values.” The American Conference of Governmental and Industrial Hygienists (ACGIH) is a private nonprofit organization.

Keep in mind that toxic materials may pose a skin-contact hazard as well as an inhalation hazard. Ventilation and the wearing of the proper respiratory protection equipment will protect against inhalation hazards. Entry personnel should wear the chemical protective clothing appropriate for the hazardous substance(s) present to protect against skin-contact hazards.

Toxic exposures are typically measured in parts per million (ppm), parts per billion (ppb), or milligrams per cubic meter (mg/m3). Several key limits are used to assess toxic exposure hazards: permissible exposure limit (PEL), threshold limit value (TLV–these limits are similar to PELs but are not established by regulation but by the ACGIH; the TLV figures are the most commonly used for materials that do not have an established PEL), ceiling limits (the upper end of exposure that should not be exceeded even if the exposure would allow the time-weighted average to stay within an acceptable range), short-term exposure limit (STEL–the quantity of exposure permitted for a short duration–usually based on 15 minutes but may run from five to 30 minutes), and immediately dangerous to life and health (IDLH–these exposure levels, as the term implies, indicate immediate danger, and personnel should never be exposed to them even for a short duration; keep in mind that the “D” in IDLH stands for “dangerous,” not deadly; the IDLH does not represent the immediately fatal concentration).

OSHA establishes permissible exposure limits in the United States by regulation. These limits are time-weighted average exposures intended to represent safe long-term exposure. Individuals may be exposed to these levels of a substance on a continuing basis without ill effect.

Lethal concentrations are typically described using the measure LC50–the concentration of a toxic material that will kill 50 percent of the test subjects.

GENERAL RULES FOR ATMOSPHERIC

MONITORING

Atmospheric monitoring of a confined space should be completed before entering the space. When effective monitoring of the work area cannot be completed prior to entry, special procedures must be used. Monitor all areas in which personnel will be working and anytime people are within the space. Monitor continuously throughout the operation. The current regulations stop short of clearly requiring continuous monitoring. This point is open to interpretation from a compliance standpoint. From a safety perspective, however, continuous monitoring is obviously the best choice.

Monitoring must also cover all areas of the space where entrants may be exposed to the hazards of the atmosphere. Continuous monitoring should be established near the breathing zone of the entrants. Another option in this area is to monitor between the entrants and the most likely source of potential atmospheric problems.

Vapor density for some of the contaminants commonly found in a confined space may vary considerably. Vapor density is a comparative measure. Air is equal to one. A vapor with a density greater than one is heavier than air and would sink in air. A vapor with a density of less than one is lighter than air and would tend to rise in air. This characteristic indicates where within the space the contaminant would most likely be found.

PROCESS

The flowchart in Figure 2 illustrates the basic process of atmospheric monitoring. In confined space entry operations, there will always be a need for atmospheric monitoring (the first point on the flowchart). You must then identify the potential atmospheric problems you may encounter. Always check oxygen and flammability issues. This is the simplest approach to these items because instruments designed for confined space work will usually have both of these sensors. The oxygen sensor may be used in the vast majority of operating environments without the specific identification of other issues that may be present. The flammable sensor is typically a broad range sensor that will detect flammability problems without the specific identification of the flammable involved. Acceptable ranges for these materials are general, which also simplifies monitoring.

Toxic sensors are most often chemical-specific, meaning that potential toxics must be specifically identified before the appropriate monitoring equipment can be selected. Each toxic has a specific acceptable level.

The most effective approach is to calibrate all the monitoring equipment prior to beginning work in the confined space. Manufacturer recommendations vary on how frequently equipment should be calibrated. Your policy and procedures may also be used as guides. The ideal situation is to ensure that instruments are calibrated at the beginning of the shift on each day of use. At a minimum, confirm that all devices

have had recent and proper calibrations.

The flowchart now divides into “oxygen…,” “flammable,” and “toxic.” The acceptable range for oxygen is 19.5 percent to 23.5 percent. You must focus on oxygen first because an oxygen deficiency will cause errors in your flammable readings. If the oxygen is within the acceptable range–below 10 percent of the LEL–turn your attention to the flammable. For toxic contaminants, the PEL is used as the safe range. Up to the PEL is considered safe. The regulations allow the PEL to be exceeded for materials that are not harmful or do not cause incapacitation in the short term, but I strongly recommend that the PEL always be used.

If problems are discovered during atmospheric monitoring, the solution of first choice is always to correct the problem. If it cannot be corrected, personnel must be protected with personal protective equipment. It is always a good procedure to use respiratory protection.

ATMOSPHERIC MONITORING EQUIPMENT

Your atmospheric monitoring capabilities should match your district. Thoroughly review your preplans to evaluate the atmospheric hazards you would most likely need to monitor. You should be equipped for some of these needs; others you may want to cover in other ways.

If you have a hazardous materials team that you can call to assist you, it does not make sense for you to purchase highly specialized items like photoionization detectors, particularly if this type of equipment is expected to be used rarely. You can also work in cooperation with local industry to help meet specialized needs. For example, you may want to get the local industrial company that has a highly specialized need to train your personnel on the use of its monitoring equipment and to incorporate in its emergency plan that someone in its organization bring the equipment to your command post during an emergency.

TYPES OF MONITORING DEVICES

Three major categories of monitoring devices are typically used in confined space work: digital, analog, and colormetric tube.

Digital Instruments

Digital instruments are usually the best choice. They are the easiest to use and to interpret. They also offer the widest variety of accessory equipment. They are commonly available in one- to five-gas models from a variety of manufacturers.

Manufacturer Recommendations and In-structions

Many of us, and I include myself, have a tendency to view looking at the manual as an option of last resort. I strongly encourage you to make an exception to that philosophy when working with atmospheric monitoring equipment (for that matter, all emergency response equipment). Please read and use the manual to help you develop a thorough understanding of the operation of your specific instruments. The inability to use and interpret these instruments effectively may get you or a member of your crew killed.

Digital Instrument Capabilities

Gases that may be monitored with digital devices typically include the following:

oxygen (O2)

flammables as a percentage of lower explosive limit (LEL)2

carbon monoxide (CO)

hydrogen sulfide (H2S)

nitrogen dioxide (NO2)

sulfur dioxide (SO2)

ammonia (NH3)

chlorine (Cl2)

methane (CH4)

nitric oxide (NO)

hydrogen cyanide (HCN)

Flammable Gas Sensors

Flammable gases and vapors are most commonly detected with catalytic diffusion sensors. Fine platinum wire is wound into a coil. Two of these coils are used in the sensor, one active and one reference. These coils are wired into a circuit called a “wheatstone bridge” (see Figure 3). In this circuit, voltage is applied to both coils, heating them to very high temperatures, approximately 5507C. When a flammable gas or vapor comes in contact with the sensor, the active coil burns the material, causing an increase in temperature. The reference coil is isolated from the flammable gas or vapor so that it maintains a constant temperature. The response of one coil and not the other causes a change in resistance in the circuit that is converted into a readout by the electronics of the instrument.

The sensor is made intrinsically safe by using a flame-arresting cover over the chamber in which the coils are located.

Flammable Detection Conversion Factors

The sensitivity of a flammable sensor is dependent on the calibration gas used. When the flammable gases or vapors being sampled are significantly different from the calibration gas, a conversion may be used to improve the accuracy of the reading. (See “Flammable Detection Conversion Factor Table” on page 78.)

The multiplier in the table should be used for improved accuracy of readings. An instrument calibrated to pentane being used to read acetylene would use a 0.7 conversion factor. For example, a reading of 20 percent LEL should be converted as follows:

20% LEL reading times 0.7 conversion factor = 14% actual percent of LEL. Accuracy of conversion is 625 percent of the reading. When using the recommended safe cutoff of 10 percent LEL, it is not typically necessary to calculate this conversion. A sufficient safety margin is built in to avoid the need for making conversions. The conversions allow for additional accuracy when conducting specific problem investigations.

Infrared sensors may be used to monitor flammable gases or vapors when there is an oxygen-deficient atmosphere in the space. These sensors do not require oxygen to operate properly.

Toxic Sensors

Most safety instruments use electrochemical sensors (see Figure 6) for the measurement of toxic gases. Two electrodes, sensing and reference, are used. The electrodes are contained with an acid electrolyte. The TeflonTM membrane is porous to gases and allows them to pass into the contained area housing the electrodes. A chemical reaction occurs in the chamber, which releases electrons. The electronics of the instrument determines the concentration of gas based on the electrons released and provides a readout in parts per million (ppm).

Cross Interfering Gases

Cross interfering gases are gases that cause a sensor to provide false readings. The sensor intended to measure a specific gas such as nitrogen dioxide is also sensitive to another gas. For example, the nitrogen dioxide sensor when exposed to 100 ppm of chlorine will read 90 ppm.

Power Supply

The majority of these units are powered by rechargeable batteries. Service life varies from a low of a few hours to a high of approximately 10 hours. Some units have field-changeable rechargeable batteries. A few manufacturers produce models that may be powered with regular dry cell batteries, usually AA, C , or lithium. An ideal arrangement for fire departments is to use a unit that allows all of the above options.

Chargers are available in a variety of configurations. Common chargers usually include 115 volt AC and 12 volt DC. Many are capable of rapid and trickle charging.

Remote Sampling

Remote sampling equipment includes pumps, tubing, and probes. These accessories are commonly available in a wide variety of specific configurations. Filtration devices, for avoiding the intake of dust and water, are also available. A dilution tube is a specialty device accessory for sampling flammable atmospheres in areas of reduced oxygen concentration with a catalytic bead sensor. The dilution tube draws air-containing oxygen into the sample stream in measured amounts, allowing for accurate flammable readings.

Alarms

These instruments are usually equipped with audible and visual indications of alarm and fault conditions. Common fault indications are for low battery, battery failure, and sensor related faults. Alarm options usually include various remote alarm devices. Common devices include audible, visual, and vibrating alarms.

Calibration

Calibration is essential to maintaining the accuracy of atmospheric monitoring equipment. The label on a calibration gas cylinder indicates the specific contents and the expected reading that should be gotten during calibration. Digital instruments may have to be set at the expected level at the beginning of the calibration process. All calibration gas expires; the label should indicate a manufacture or expiration date.

Calibration should be done at least as often as the manufacturer recommends. This varies, but once a month is the most common recommendation. You may calibrate more frequently according to your use experience. Anytime the instrument is behaving in a manner you cannot explain, it should be calibrated to confirm that it is functioning properly or to assist with determining the specific problem.

Maintain calibration records on each instrument. At a minimum they should include the following:

date,

the name of the person doing the calibration,

specific identification of the instrument (serial number or user-assigned unique number),

results of the calibration, and

the service performed or needed.

Analog Instruments

Analog instruments provide a dial readout. These devices are commonly available in one- and two-gas units. There are many excellent devices in this category, but they tend to be more difficult to use and interpret. For example, some types of combustible gas detectors, when testing an atmosphere that exceeds the LEL, will move quickly to 100% LEL and then drop back to zero.

Colormetric Tubes

Colormetric tubes are generally a last- resort option for measuring contaminants that cannot be monitored with electronic devices because none are available. Of the three devices discussed, this type of device is the most difficult to use and interpret. The major advantage of colormetric tubes is the wide range of specific chemicals that may be monitored. The rescuer should seek assistance if the entry will require monitoring with colormetric tubes.

TECHNIQUES

Horizontal Entry Points

Initial monitoring inside the space is done with a probe. Using a rigid probe allows sampling from farther into the space. Check both low and high areas within reach of the probe.

Vertical Entry Points

If the cover is in place and has an access hole, the probe may be used for initial monitoring inside the space (see Figure 7). A flexible tube may be used for remote sampling. When using this technique, the tubing must be lowered slowly into the space to allow sampling at all levels. In a situation where a low-level obstruction may interfere with monitoring, if the tubing cannot be placed on the other side of the obstruction from outside the space, the monitoring will have to be completed by an entrant. The presence of different levels within a confined space can create a similar problem (see Figure 8).

SUMMARY

You must have or have access to the equipment necessary for monitoring, understand the capabilities and limitations of that equipment, and properly apply effective techniques to successfully monitor the atmosphere in a potentially hazardous area.

This article has focused primarily on the application of atmospheric monitoring to confined-space situations. Hazardous atmospheres in these situations frequently lead to fatalities, some of whom all too frequently are would-be rescuers. The first priority of emergency response personnel and their officers is to keep their crew alive. We cannot successfully rescue others if we also become victims. n

qFIRECON 1996

Endnotes

1. Explosive limit and flammable limit terms are used interchangeably.

2. Lower flammable limit (LFL) is another common term.

Figure 1. The flammable limits of gasoline. The goal is to maintain an environment that is 10 percent of the lower flamable limit, allowing for a 90 percent safety margin before the bottom of the flammable range is reached.

Figure 2. The atmospheric monitoring process.

Figure 3. A wheatstone bridge circuit.

Figure 4. Cross-section of a sensor.

Figure 5. Infrared sensors.

Figure 6. Electrochemical sensors.

Figure 7. (Top) Using the probe for initial monitoring within a confined space. Figure 8. (Bottom) Different levels within a confined space. Monitoring will have to be completed by an entrant.

CRAIG SCHROLL, CSP, CET, a certified safety professional and certified environmental trainer, founded FIRECON in 1980 with the mission of helping clients prevent, plan for, and control emergencies. He has more than 20 years of experience in safety and loss-control activities and lectures and writes extensively on these and related topics.