Isolating a confined space is a process that removes the space from service by:
§ Locking out electrical sources, preferrably at disconnect switches remote from the equipment.
§ Blanking and bleeding off pneumatic and hydraulic lines.
§ Disconnecting belt and chain drives and mechanical linkages on shaft-driven equipment where possible.
§ Securing mechanical moving parts within confined spaces with latches, chains, chocks, blocks, or other devices.
Effective communication among entrants, attendants, and supervisors is essential and must occur continuously to be effective. It is critical to have the correct communications equipment for the space being entered. Workers inside have to be able to communicate among themselves, as well as with the attendant outside of the work space. In the event of an emergency, communications equipment allows help to be summoned quickly.
Effective communication can take many different forms.
§ Voice communication is highly effective when distance and noise levels permit. If the distance is too far or the noise level too loud, other methods must be used to maintain contact.
§ Hand signals can be used effectively if the entrant and the attendant are able to maintain visual contact.
§ Hard wire communication and hand radios can also be highly effective.
§ Because of normal movement, rope pull signals are not recommended. Ropes can also be ineffective if the entrant becomes incapacitated and is not able to signal in the case of an accident.
All workers involved in a confined space entry should be knowledgeable about the communication equipment and trained in its use. Testing needs to be done routinely prior to space entry as well as immediately after entry. All communication equipment must be intrinsically safe, especially in flammable atmospheres.
Ventilation is the process of continuously moving fresh, uncontaminated air through a confined space. Ventilation dilutes and displaces air contaminants, assures than an adequate supply of oxygen is maintained during entry, and exhausts contaminants formed by processes such as welding, oxy-fuel gas cutting, or abrasive blasting. Oxygen levels must be maintained within a range of 19.5 to 23.5 percent.
Ventilation by a blower or fan may be necessary to remove harmful gases and vapors from a confined space. There are several ventilating methods and equipment to choose, depending on the size of the confined space openings, gases to be exhausted, and source of makeup air.
Equipment selection considerations include the:
§ Physical structure of the space,
§ Space’s previous contents,
§ Existence of natural drafts,
§ Number and location of any openings, and
§ Nature of any contaminant-producing tasks that may be performed in the space.
Because ventilating may create the potential for static electricity, always follow appropriate bonding and grounding procedures.
Under certain conditions where flammable gases or vapors have displaced the oxygen level, but are too rich to burn, forced air ventilation may dilute them until they are within the explosive range. Typically, an opening is made in the top or side of the space and clean air s blown into it. Dilution works best with low toxicity and concentration levels and when the contaminants are well distributed.
Ventilation should be continuous where possible, because in many confined spaces, the hazardous atmosphere will form again when the flow of air is stopped. Be sure that the source of air intake is not placed where it can draw carbon monoxide (as from an idling vehicle parked close by) or other contaminants into the space.
A common method of ventilation requires a large hose, one end attached to a fan and the other lowered into a manhole or opening. For example, a manhole would have the ventilating hose run to the bottom to blow out all harmful gases and vapors. The air intake should be placed in an area that will draw in fresh air only.
It is important to understand that some gases or vapors are heavier than air and will settle to the bottom of a confined space. Also, some gases are lighter than air and will be found around the top of the confined space. Therefore, it is necessary to test all areas — top, middle, bottom — of a confined space with properly calibrated instruments to determine what gases are present.
If testing reveals oxygen-deficiency or the presence of toxic gases or vapors, ventilate the space and retest before entry. Allow sufficient time for diffusion during the ventilation/purging process. If ventilation is not possible and entry is necessary, such as for an emergency rescue, workers must wear appropriate respiratory protection.
Never trust your senses to decide if the air in a confined space is safe. You cannot see or smell many toxic gases and vapors, nor can you determine the level of oxygen present.
Atmospheric testing is required to evaluate the hazards of the permit space and to verify that acceptable entry conditions exist. The person performing the testing must know how to operate and read the test instrument. A monitor should be calibrated before every use or as required by the manufacturer to ensure that the device provides accurate measurements.
Draw air samples through a weep hole or other small entry port leading into the space. When combustible or flammable gases could be present, use a non-sparking probe. If possible, do not open the entry portal to the confined space before this step has been completed. Sudden changes in atmospheric composition within the space could cause violent reactions or dilute the contaminants, giving a false low initial gas concentration.
Be sure to allow enough time for the instruments to respond to full scale. Assess as many space conditions as possible from the outside; but if entry is necessary for some part of the assessment, respiratory protective equipment may be needed for entrants’ safety. All entrants must be permitted to observe the testing and review the results before they enter the space.
For safety’s sake, take at least three sets of readings:
1. Before ventilation,
2. After ventilation, and
3. The entrant’s reading during the initial entry survey.
Additional or continuous monitoring may be needed. Because of the way test instruments operate, atmospheric monitoring must be performed in a specific sequence.
Oxygen tests must always be made first because most combustible gas meters are oxygen-dependent. Too little oxygen may cause a low combustible gas reading. Too much oxygen can cause a combustible gas meter to explode if gases and vapors are present in ignitable quantities.
Oxygen concentrations are generally measured over a range of 0 to 25 percent oxygen in air, with readings being displayed on either an electronic readout or an analog meter. Oxygen indicators are calibrated with uncontaminated fresh air containing a minimum of 20.8 percent oxygen. With some models, an alarm is activated when oxygen levels drop below 19.5 percent and above 23.5 percent.
§ Flammable and combustible gases
Flammable and combustible gases are measured next because the risk posed by fire or explosion is more immediate and life-threatening than exposure to toxic gases and vapors.
§ Toxic gases and vapors
Toxic gases and vapors, which are commonly found in confined spaces, are measured last. A toxic sensor requires that the specific toxic substance be identified in advance. Each substance has a specific level to ensure entrant safety and the sensors are specific to these levels. Substance specific detectors should be used whenever actual contaminants have been identified.
The results of the atmospheric testing will have a direct impact on the selection of protective equipment necessary for the tasks in the confined space. It may also dictate the duration of worker exposure to the environment of the space, or whether an entry will be made at all. Evaluation and interpretation of this data and development of the entry procedure should be implemented, based on the evaluation of all serious hazards.
The atmosphere of a permit space which may contain a hazardous atmosphere should be tested for residues of all identified contaminants using permit-specified equipment. This will determine if residual concentrations at the time of testing and entry are within the range of acceptable entry conditions. Results of testing (i.e., actual concentration) should be recorded on the confined space permit in the area provided adjacent to the acceptable entry condition.
The measurement of values for each atmospheric substance should be made for at least the minimum response time of the test instrument specified by the manufacturer.
When monitoring for entries involving a descent into atmospheres that may be stratified, the space should be tested a distance of approximately four feet (1.22 m) in the direction of travel and to each side. If a sampling probe is used, the entrant’s rate of progress should be slowed to accommodate the sampling speed and detector response.
In addition to atmospheric hazards, a confined space must also be assessed for physical hazards. These hazards include those associated with hazardous energy releases, grinding equipment, dry particles that can engulf an entrant, communication problems, noise, temperature, and size of openings into the space.
Engulfment in loose materials is one of the leading causes of death from physical hazards in confined spaces. Engulfment and suffocation are hazards associated with storage bins, silos, and hoppers where grain, sand, gravel, or other loose material are stored, handled, or transferred. The behavior of such material is unpredictable, and entrapment and burial can occur in a matter of seconds.
In some cases, material being drawn from the bottom of storage bins can cause the surface to act like quicksand. When a storage bin is emptied from the bottom, the flow of material forms a funnel-shaped path over the outlet. The rate of material flow increases toward the center of the funnel. During a typical unloading operation, the flow rate can become so great that once a worker is drawn into the flow path, escape is virtually impossible.
A condition known as “bridging” can create additional hazardous situations. Bridging occurs when grain or other loose material clings to the sides of a container or vessel that is being emptied from below, allowing a hollow space to be created. The bridge of material over the space may collapse without warning, entrapping workers who are standing below or on top of the bridge and who are unaware that the surface is unstable.
Bridging can occur in storage bins, silos, and hoppers that contain ground grains, soybean meal and other meals, or other loose materials such as cement, limestone, coal, and sawdust. The diameter of the storage vessel and the moisture content of the stored materials are factors that contribute to bridging.
The nature of confined space work may make it difficult to separate the worker from hazardous forms of energy such as powered machinery, electrical energy, and hydraulic or pneumatic lines. Activation of electrical or mechanical equipment can cause injury to workers in a confined space. It is essential to de-energize and lock out all electrical circuits and physically disconnect mechanical equipment prior to any work in confined spaces.
The release of material through lines which are an integral part of the confined space pose a life-threatening hazard. All lines should be physically disconnected, blanked off, or a double block and bleed system should be used.
Falling objects can pose a hazard in confined spaces, particularly in spaces that have topside openings for entry through which tools and other objects may fall and strike a worker. Traffic hazards from forklifts and street traffic, even inattentive pedestrians, can pose a danger for entrants. Operational processes in close proximity to the space may create hazards or release toxic substances that migrate into the space.
Interior surfaces that are uneven, wet, or slick can cause falls in confined spaces. In addition, wet surfaces can provide a grounding path and increase the hazard of electrocution in areas where electrical equipment, circuits, and tools are used. Workers in some confined spaces need to be aware that objects may fall on them, particularly in spaces with topside openings and where tasks are being done overhead.
Grinding equipment, agitators, steam or steam fittings, mulching equipment, drive shafts, gears and other moving parts pose a danger if they are not locked or blocked out prior to entry.
Extremely hot or cold temperatures can make work inside a confined space hazardous. Heat stress increases fatigue and the inability to concentrate. Jobs requiring manual dexterity are more difficult in cold environments. Entrants working in spaces with great temperature variances should wear appropriate protective clothing. If a confined space has been steam cleaned, it should be allowed to cool thoroughly before any entry is made.
Noise within a confined space may be amplified because of the design and acoustic properties of the space. Excessive noise is not only harmful to a worker’s hearing, but can also affect communication, such as causing a warning to go unheard. This is especially true when hearing protection is being worn.
Biological hazards such as molds, mildews and spores frequently found in dark, damp spaces can irritate the respiratory system. Bacteria and viruses, found in sewage treatment, expose the entrant to a variety of illnesses. In addition, rodents, snakes, spiders and other insects, as well as bird and animal feces can present serious health hazards.
The presence of dust may pose dangers to entrants in the form of respiratory hazards. Dust also has the potential to cause combustion or an explosion and cause reduced visibility and slippery surfaces within the space. Other conditions, such as inadequate lighting and the presence of radioactive matter within the space must be evaluated before entry.
Entrants are in the most danger because of the many, often invisible, hazards associated with permit space environments. In order to properly identify the hazards of confined spaces, each space needs to be individually surveyed. By assessing the specific space, as well as the work to be performed, appropriate measures can be taken to avoid tragic incidents.
In general, the hazards of confined spaces are categorized as either atmospheric or physical. Atmospheric is usually the most lethal because dangerous atmospheres are not always detectable through the senses. Also, the atmosphere in a confined space may be extremely hazardous because of the lack of natural air movement. This characteristic of confined spaces can result in:
§ Flammable/combustible atmospheres, and/or
§ Toxic atmospheres.
Normal air has an oxygen content of 20.8 percent. An atmosphere is considered “oxygen-deficient” when there is less than 19.5 percent available oxygen. Any atmosphere with less than 19.5 percent oxygen should not be entered without an approved self-contained breathing apparatus (SCBA).
When the oxygen level drops below 17 percent, an entrant may experience rapid breathing and an accelerated heartbeat. As the oxygen content decreases, other physical effects become evident including poor muscle coordination, rapid fatigue, and intermittent respiration, nausea, and an inability to perform tasks. At concentrations less than six percent, there is rapid loss of consciousness, and death occurs in minutes.
Oxygen deficiency occurs from chemical or biological reactions which displace or consume oxygen from the space. Oxygen consumption takes place during:
§ Combustion of flammable substances as in welding, cutting, or brazing; and
§ Bacterial action, such as in the fermentation process.
Oxygen deficiency can result from bacterial action in excavations and manholes which are near garbage dumps, landfills, or swampy areas. Slow chemical reactions such as in the formation of rust on the exposed surface of metal tanks, vats, and ship holds will also consume oxygen in a confined atmosphere.
A simple asphyxiating atmosphere contains an inert gas (or gases) which does not produce any ill effects on the body. However, in sufficient quantity, an inert gas will displace oxygen and may result in an atmosphere unable to support normal breathing.
If 100 percent nitrogen — a non-toxic, colorless, odorless gas — is used to displace the oxygen in a confined space, it would cause immediate collapse and death to an entrant if the confined space is not adequately ventilated before worker entry. Other examples of simple asphyxiants which have claimed lives in confined spaces include carbon dioxide, argon, and helium.
A flammable atmosphere generally results from the vaporization of flammable liquids, by-products of chemical reactions, enriched oxygen atmospheres, or concentrations of combustible dusts. For combustion to occur, three elements — oxygen, fuel, and heat — must be present in the atmosphere. In the right amounts, these elements create an unrestricted chemical reaction which produces a fire. If one of these elements is missing, or is not present in the appropriate amount, combustion will not occur.
§ Fuel — a flammable gas, vapor, or dust
§ Oxygen — to support combustion
§ Source of ignition — a sparking, heat, pressure, shock, or impact
The proper mixture of fuel and oxygen varies from gas to gas. The flammability range for each gas is measured in terms of the lower flammability limit (LFL) and the upper flammability limit (UFL). Ten percent of flammable gas and vapor lower exposure limits is generally considered a safe level.
Example: The explosive range for methane is between five percent and 15 percent in air. Concentrations below five percent methane are below the explosive range (lean), and concentrations above 15 percent are too rich to support combustion. If a confined space contains 27 percent methane and forced ventilation is started, the introduction of air into the confined space may dilute the methane in air, taking it into the explosive range.
An oxygen-enriched atmosphere (above 23.5 percent) will cause flammable materials, such as clothing and hair, to burn violently when ignited. Never use pure oxygen to ventilate a confined space — always ventilate with normal air.
Most substances (liquids, vapors, gases, mists, solid materials, and dusts) should be considered hazardous in a confined space. Toxic gases may be present in a confined space due to the:
§ Product stored in the space
The manufacturing process uses toxic gases and the product may be absorbed into the walls of the space and give off toxic gases. Also, there may be biological or chemical processes occurring in the product stored in the confined space.
Examples: Hydrogen chloride and vinyl chloride monomerin are used in producing polyvinyl chloride. The product can be absorbed into the walls and give off toxic gases when removed; or when cleaning out the residue of a stored product, toxic gases can be given off.
Removing sludge from a tank or sump — decomposing oganic material can give off deadly hydrogen sulfide gas.
§ Work being performed in the space
The operation being performed in the confined space can release a toxic gas. Toxic gases may be created when acids are used for cleaning the interior of a confined space.
Examples: Hydrochloric acid reacts chemically with iron sulfide to produce hydrogen sulfide which is heavier than air and will settle out at the bottom of a confined space. Hydrogen sulfide is extremely toxic and exposure can cause paralysis of the olfactory system (making the victim unable to smell the gas), loss of reasoning, respiratory failure, unconsciousness, and death.
A welding, cutting, or brazing operation can cause the release of nitrogen, ozone, and carbon monoxide. Painting, scraping, sanding, degreasing can also cause the release of toxic gases.
Cleaning solvents used in many industries for cleaning/degreasing produce vapors which are very toxic in a confined space. Solvent vapors cause unconsciousness by depressing the central nervous system. Some chlorinated hydrocarbon solvents, such as chloroform, have been used as anesthetic agents.
In addition, certain chlorinated or fluorinated hydrocarbon solvents are toxic to the heart and have been associated with sudden death in confined spaces. Methylene chloride can be toxic in confined spaces both because of its solvent properties and also because it is metabolized in the body to carbon monoxide.
§ Areas adjacent to the space
Toxicants produced by work in the area of a confined space can enter and accumulate in the space. Gases that are heavier than air may migrate across a work area and accumulate in the lowest level, such as a maintenance pit.
Some toxic gases such as phosgene or carbon monoxide are particularly insidious because of their poor warning properties. Toxic gases which have been reported to cause death in workers in confined spaces include carbon monoxide, hydrogen cyanide, hydrogen sulfide, arsine, chlorine, oxides of nitrogen, and ammonia.
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