The Oxford compact English dictionary defines enclosed as an area that is “shut in on all sides” and “secluded from the outside world”.
Its enclosed nature suggests that an individual entering, exiting or working in such an area will face certain constraints and restrictions.
Defining confined spaces
The thing to make everyone aware of is that a confined space is not necessarily a cramped place.
Contrary to the connotations of ‘confined’ conjuring pretty close quarters, in the hazardous sense it can actually be any place, such as a chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by the nature of its enclosure, there are risks identified within the enclosed space.
So, with regard to detecting gas, what are the risks to be identified and controlled? Well, the main risk is an employee losing consciousness or being asphyxiated as a result of gas, fume, vapour or a lack of oxygen in the working environment.
Employers have a duty to protect their employees and ensure their safety and health. Employers should understand and be aware of the hazards associated with confined spaces. They should carry out an assessment of the risks to health and safety and ensure that their employees and others will not be exposed to a risk that cannot be controlled at a level that is acceptable.
This prompts the next question, of course: what should be included in this hazard identification and assessment of the risk of gas, or lack thereof, within a confined space?
Conditions within and around the confined space should be assessed to identify what gases could be either present or introduced into the confined space by whatever work is to be carried out in or near the confined space. These could be obvious such as those produced from cutting or welding, but could also be less obvious, such as cleaning with chemicals. This work does not necessarily have to be carried out directly in the confined space, it could be an area associated with the confined space.
I have been involved with a situation where chemicals were being used to clean a chute from a hopper. The operatives carrying out the work were not affected by the fumes, but operatives inspecting the inside of the hopper three floors away were overcome. This was due to the direction of airflow into the hopper coupled with the enclosed nature of the hopper. This hazard had not been recognised or risk assessed. The end result was the risk was not controlled to an acceptable level.
You should never forget to also assess which gases are potentially not present; i.e. oxygen.
The most acceptable environment within a confined space will be fresh air. This is defined as a complex mixture of gases. The three main constituent gases in air are oxygen, nitrogen and carbon dioxide.
Their composition is generally accepted as being, by volume:
- Oxygen 20.93%
- Nitrogen 79.04%
- Carbon dioxide 0.03%
Air itself is colourless, odourless and tasteless, supports combustion and is necessary for respiration. As air is a combination of gases, it is possible for individual gases to be isolated through particular processes.
Note should be taken of the previous contents of the confined space, such as:
- Toxic substances encountered
- Flammable substances encountered
- Residues from chemical substances
- The presence of rust, which absorbs oxygen, can be a cause of oxygen deficiency
- Sludge, which can release gases, fumes or vapour when it is disturbed when people enter or carry out work in the confined space
It is always advisable to consider previous records of entries into the confined space for factual information on the environmental conditions encountered on those entries, especially when you consider what the risks could be.
Risks within a confined space include:
- Serious injury to any person at work arising from a fire or explosion
- The loss of consciousness of any person at work arising from an increase in body temperature
- The loss of consciousness or asphyxiation of any person at work arising from gas, fume, vapour or the lack of oxygen
- The drowning of any person at work arising from an increase in the level of liquid
- The asphyxiation of any person at work arising from a free flowing solid or the inability to reach a respirable environment due to entrapment by a free flowing solid
Oxygen in a confined space
It is essential that oxygen is present within a confined space as a constituent of the air within the space. There are, however, two concerns associated with oxygen in or around confined spaces. These are oxygen enrichment and oxygen deficiency.
This can occur when certain processes are being undertaken in or near confined spaces. Examples can be the use of oxygen leaking from cylinders used in welding or its production from chemical processes. It can also occur when other gases such as nitrogen are removed from air. Oxygen has a specific gravity of 1.1 and is therefore heavier than air. The increase of oxygen content results in a considerable increase in the flammability of combustible material. This may also alter the ‘flammable range’ of flammable gases.
This can occur when a confined space is infrequently opened up to the atmosphere and rusting has occurred. This rusting is a slow form of combustion and absorbs oxygen. Again, other processes such as burning and welding can use up the oxygen content. There are many examples of fatalities occurring due to this reduction in the oxygen content. Unconsciousness, and subsequent fatalities, can result when the oxygen content drops to around 16%. The lowest acceptable level of oxygen content should not be below 19% in order to provide a reasonable level of safety.
What is important is that any change in the oxygen content of the air within the confined space should be well understood and the impact of this variation fully appreciated.
Assessments on the effect of chemicals to be used in a confined space should be fully noted and understood to ensure the assessed risk of the chemical and its impact on those employed to use it are controlled to a level that is acceptable.
There was a very upsetting example of an employee using chemicals to clean an aviation fuel tank. There was a complete lack of understanding of the hazard and risks associated with this cleaning chemical. A very unfortunate employee working with the chemical found that he was so hot inside the fuel tank that he decided to unplug the light to remove this as a source of the heat. When he unplugged the light, this caused a spark that ignited the vapour from the cleaning chemical resulting in an explosion, severely burning the person concerned. His injuries were sufficient to prevent him from ever working again. Following an investigation, it was revealed that the cleaning chemical was not required; warm soapy water did as good a job of cleaning the tank. This should have been realised sooner with an effective assessment of the risk of utilising the chemical within the confined space.
Detecting for gas, vapour or fume
Every confined space can, and must, be tested prior to persons entering it. This can be achieved by the use of tubing or a probe and drawing a sample to the detection device. An alternative to this is to place or lower a suitable detection device into the space and allow it to record the environmental conditions. A suitable gas monitoring device will retain peaks and troughs of gas readings obtained from within the confined space. These will be available to those assessing the risks.
Where the assessment of risk suggests a requirement to test the environment within a confined space, the space should be opened, ventilated (mechanical and/or natural) and tested prior to any person entering the space and, if required, tested again after a predetermined period ascertained by the control measure from the risk assessment. Testing should be carried out on every occasion the confined space is re-entered to ensure nothing has changed within the space. The reason for this is that small changes in barometric pressure, either increasing or decreasing, can alter the atmosphere within a confined space. When persons are working within the confined space the atmosphere should be constantly monitored, particularly if mechanical ventilation is a requirement of the risk assessment. Constant monitoring will also be required if the work being undertaken is particularly hazardous, i.e. cutting and welding.
Detection instruments can be supplied in various combinations of sensors. These vary from one sensor devices for a specific gas to multi-sensor devices for a variety of gases. They may also vary depending on the design role of the software attached to the sensor. This can be as simple as recording the readings and alarming if trigger points are reached. Others can record peaks and troughs for each period of testing. Some will allow full data logging of everything that is detected by the device over a period with the capability of downloading the information onto a computer for its retention and analysis.
The choice of device used to test the atmosphere will depend on what is known about the confined space and the potential gas, vapour or fume likely to be present (do not forget the possibility of oxygen content enrichment or deficiency). Chemical detector tubes may be an appropriate device, but where possible, portable electronic devices that constantly monitor the atmosphere in which the person is working should be considered. Whatever the choice of device, it is most important that it specifically monitors and detects the gas, vapour or fumes identified by the risk assessment.
Order of testing
The priority gas to be ascertained as to its presence is oxygen, due to the influence that it has on both the flammability and toxicity of other gases. Flammable gases should next be tested and this should be followed by toxic gases, fumes and vapours. It should also be remembered that some gases, e.g. carbon monoxide, are both toxic and flammable. In this example the toxic risk from carbon monoxide is the greater of the risks, but there are many examples where the flammable risk from the gas will be the greater.
Testing should be carried out by a competent person who knows and understands the characteristics of the gases, fumes and vapours being detected. They should also know and understand both the relevant legal requirements and standards. They must be capable of interpreting the results and implementing any necessary actions resulting from the readings.
The gases fumes and vapours
Many people at work are exposed to gases, fumes and vapours that can, if not controlled, have a harmful effect on their health. These are called ‘hazardous substances’ and must be controlled.
Properties of gases
Specific gravity/effects of gases
The specific gravity (SG) of a gas is the weight of that gas compared with the weight of the same volume of air at the same temperature and pressure.
Air is given a specific gravity of one. A given quantity of a gas can then be compared against the same quantity of air. Gases with a specific gravity of less than one will rise in the atmosphere; gases with a specific gravity greater than 1 will sink and seek regions of low elevation at the earth’s surface.
Knowing the specific gravity of gases is of great use when working in confined spaces. It will not only give you clues as to where specific gases may accumulate in the confined space, but will also provide information on how their presence may affect safe working within a confined space.
- If the gas is methane where the SG is 0.55, which indicates it is lighter than air, then ensure that you test high up within the space
- If the gas is carbon dioxide where the SG is 1.53, which indicates it is heavier than air, then ensure that you test low down within the space
The main three gases in fresh air are oxygen, nitrogen and carbon dioxide.
Colourless, odourless and tasteless; in its pure state it is heavier than air. The human body requires a constant supply of oxygen in order to survive and this is normally obtained from the atmosphere around us.
Colourless, odourless and tasteless; it does not support life or combustion and is slightly lighter than air (SG is 0.97). It is the main constituent of air (79%) and acts as a diluent for oxygen.
This is colourless, has a slightly pungent or acrid smell and a soda water taste; it is heavier than air with an SG of 1.53. Carbon dioxide plays a major role in respiration and cerebral circulation, and is given off by humans/animals. It is a product of combustion, produced by oxidation/decay of carbon and fermentation processes. In high concentrations it acts as a respiratory and central nervous system stimulant. At excessive concentrations it depresses the central nervous system producing unconsciousness, narcosis confusion, stupor (leading to unconsciousness), respiratory arrest and death.
Other common gases:
- Hydrogen – Colourless, odourless and tasteless. Lightest and simplest chemical element. SG is 0.07. Found in natural gas from oil wells, certain volcanoes. Major constituent of water. Explosive range 4-74%
- Methane – Colourless, odourless and tasteless. SG is 0.55. Produced by decomposition of vegetable matter. Explosive range 5-15%Carbon monoxide – Colourless, odourless and tasteless. Lighter than air. SG is 0.97. Produced by incomplete combustion. It is highly poisonous. It is explosive with a range of 12.5-74%
- Carbon monoxide – Colourless, odourless and tasteless. Lighter than air. SG is 0.97. Produced by incomplete combustion. It is highly poisonous. It is explosive with a range of 12.5-74%Nitrogen dioxide – Reddish brown gas that has an acrid smell and an acid taste. It is heavier than air. SG is 1.16. Produced by diesel exhausts and explosives of the nitroglycerine type. It is highly poisonousNitrogen dioxide – Reddish brown gas that has an acrid smell and an acid taste. It is heavier than air. SG is 1.16. Produced by diesel exhausts and explosives of the nitroglycerine type. It is highly poisonous
- Nitrogen dioxide – Reddish brown gas that has an acrid smell and an acid taste. It is heavier than air. SG is 1.16. Produced by diesel exhausts and explosives of the nitroglycerine type. It is highly poisonous
- Hydrogen sulphide – Colourless but has a sickly sweet taste and smell of rotten eggs. Caused by acidic mine waters acting on certain sulphides when using explosives. Heavier than air. SG is 1.18. It is highly poisonous. Explosive range of 4.3-43%
- Sulphur dioxide – Colourless but has a pungent, suffocating, sulphurous odour and almost intolerable acidic taste. Occurs when there is fire/spontaneous combustion in coal, when rubber is burned, and from diesel exhaust fumes. Heavier than air. SG is 2.26. It is highly poisonous
- Ammonia – Colourless with a pungent smell. Lighter than air. SG is 0.68. It is poisonous. Occurs naturally from bacteria acting on urine and faeces. It is produced commercially for use in refrigeration systems, fertilisers, animal feeds and manufacturing processes to name a few. Ammonia is being used increasingly in refrigeration plant
- Flooding fire suppression gases – Banks of fire extinguisher were commonly used in transformer rooms/substations to flood area with an inert gas to put out fires. The gases most commonly used in these types of systems are carbon dioxide or nitrogen
Ensure the whole area within the confined space is examined and continues to be monitored while persons are in the confined space. If the gas levels alter, ensure you know why and what the effect will be on the persons working in the confined space.
Ensure the readings are within workplace exposure limits and that they stay within safe limits.
A few points to consider when taking gas readings:
- Roof – Do not put your head above the gas monitor. There have been some occasions where individuals have got into position for taking a gas reading, and their head has been above the gas monitor before putting gas monitor into position. The individual’s face went into a pocket of gas, which rendered them unconscious.
- Floor – Place gas monitor towards floor level before your head. Once again there have been cases where people have bent low and been overcome by gas.
- When taking a gas reading leave the gas monitor in position for a few seconds, as gas needs to be drawn into the unit and then analysed.Be aware that if there are vehicles idling or diesel generators in use in the area you are working in/monitoring, you may get a localised spike in carbon monoxide readings. Try to get people to turn engines off, and monitor the area carefully / more frequently. Also note that vehicles idling or generators positioned near fixed monitors may set them off.
- Be aware that if there are vehicles idling or diesel generators in use in the area you are working in/monitoring, you may get a localised spike in carbon monoxide readings. Try to get people to turn engines off, and monitor the area carefully / more frequently. Also note that vehicles idling or generators positioned near fixed monitors may set them off.Remember that a portable gas monitor is to be used for gas monitoring. As gases can be heavier or lighter than air, you need to place the gas monitor into the relevant areas. People get complacent with gas monitors and leave them on their belt or hung up somewhere out of the way.
- Remember that a portable gas monitor is to be used for gas monitoring. As gases can be heavier or lighter than air, you need to place the gas monitor into the relevant areas. People get complacent with gas monitors and leave them on their belt or hung up somewhere out of the way.Should anyone encounter an alarm condition, they should immediately leave the work area. Failure to follow any of the above warnings may cause exposure to oxygen deficient or enriched atmospheres, toxic substances or explosive environments, which may result in death or serious personal injury.
- Should anyone encounter an alarm condition, they should immediately leave the work area. Failure to follow any of the above warnings may cause exposure to oxygen deficient or enriched atmospheres, toxic substances or explosive environments, which may result in death or serious personal injury.
Look at the picture on the left and ask yourself, is this:
- An enclosed space
- A Confined space
- Neither of the above
The answer? It is in fact a confined space, and unfortunately three people died in it. This again unfortunately is a typical confined space incident.
The first fatality slipped on top of the slurry tank while cleaning it. He fell in and was immediately overcome by an atmosphere that was high in carbon dioxide and low in oxygen. He fell into sludge at the bottom of the slurry tank and disturbed more noxious gases. The next two fatalities were attempting a rescue of their colleague. Environmental conditions had deteriorated considerably due to the sludge at the bottom of the tank being disturbed. Without gas monitors they would have had no idea of the hazards in the tank or of the risks they were taking. In fact once they committed to enter the tank to attempt the rescue their fate was sealed. They had no chance of surviving in that environment.
- Know your confined space
- Identify the hazards
- Identify the risks
- Control the risks
- If the environment needs to be monitored, ensure you have the correct gas monitor
- Ensure you supply respiratory protection if required
- Ensure your rescue arrangements are suitable, sufficient and effective
Remember it is your confined space, your problem, and you have to provide the solution.
Published: 16th Sep 2015 in Health and Safety International