Risks are greatly amplified by remote working conditions, in which workers might have limited access to emergency services and help when needed. In a remote location there can be no room for error as even the smallest mistakes can be fatal. Procedures must be well rehearsed, the workers should be skilled and healthy, and the equipment must be well maintained. In the event of an emergency, rescue must be fast and remote sites must be appointed with all the essential safety equipment - as well as having people onsite who are well trained in how to use the apparatus.
Carbon monoxide, chlorine, hydrogen fluoride, hydrogen sulphide and ammonia are some of the most common gases produced. Typically, these types of gas are found in the production of iron and steel, chemicals and petrochemicals, oil refining, natural gas sourcing, refrigeration and water treatment.
Certain environments require finely tuned methods of detection and precaution, simply because of the greater risk they pose. Petrochemical plants, for example, carry an increased risk of serious gas leakage, which could lead to an explosion or fire that would not only damage equipment, but would also put lives at serious risk.
Nuclear power stations use gases, such as CO2, as coolants to transfer heat to generate steam and to prevent overheating of the reactor. Any loss of CO2 could reduce the efficiency of heat removal and could possibly lead to overheating. In both of these situations, gas detectors are used not only to keep the process operating safely, but to keep people safe by detecting any gas releases.
Some gases are relatively harmless, but it is vital that gases identified as toxic or flammable are controlled and managed safely. There are a number of factors that affect the need for speed of detection. These include the type of gas, its temperature, pressure, quantity and toxicity, its proximity to employees and the public, the effectiveness of counteractive measures taken against it, and the speed and effectiveness of medical intervention available.
Whether an engineer is repairing buried cables, a manager is entering a small plant room or a subcontractor is inspecting the inside of a pipeline, despite working in widely different industries these personnel all face the same hazard. Gas related injury poses a serious threat in any confined space where the free movement of air is limited.
Fatalities occur as a result of poor training and an ignorance of the dangers of confined space working. Tragically, panic driven rescue attempts also contribute to the problem, with around 60% of confined space deaths involving people trying to rescue those already trapped or injured.
Some confined spaces are easy to identify; for example, enclosures with limited openings such as reaction vessels, enclosed drains, sewers, storage tanks and silos, boilers, combustion chambers in furnaces, and ductwork. While the oil and gas, utilities and manufacturing industries might seem the obvious locations for confined space working, confined areas can in fact occur in any void, including plant rooms, service ducts and poorly ventilated rooms.
Confined spaces pose a wide variety of potential hazards. They are often poorly ventilated and not only can they contain gases and other harmful substances, but rescue or escape can be problematic.
A confined space is usually defined as any space that is large enough for someone to enter to perform assigned work, has limited means of entry or exit, and is not designed for continuous employee occupancy. Employers are required to evaluate the risks these workplaces pose to their employees and then monitor to prevent them. In most cases, both the assessment and the safe working system will require testing of the atmosphere with gas detection equipment.
The risks can be divided into three categories: combustible gas, toxic gas, and high or low oxygen levels. It is of course the duty of employers to find alternatives to manned work in these areas wherever possible. In many cases, however, such work cannot practicably be avoided, and so the priority must be to make it as safe as possible.
“arguably, the most common hazard when working in confined spaces is the presence of poisonous gases, fumes or vapours”
Combustible gas risks
For combustion to occur, the air must contain a minimum concentration of combustible gas or vapour. This quantity is called the lower explosive limit (LEL). At concentrations equal to or greater than this, combustion will occur in the presence of a suitable ignition source such as a spark or hot surface. For most combustible gases and vapours, the LEL is less than 5% by volume, and a combustible atmosphere is usually described as hazardous at 10% LEL.
Typically, storage vessels that have contained hydrocarbon fuels and oils present a danger. Other dangers come from fuel leaks, such as in pipelines, gas cylinders and engine-driven plant. Formed by decaying organic matter, methane’s odourless gas collects in pockets underground and presents an almost universal danger for workers in pits, sewers and other subsurface locations.
Toxic gases and vapours
Confined space workers may be exposed to any of a large number of toxic compounds depending on the nature of the work and its environment. A risk assessment should be made detailing which toxic substances a worker may be exposed to in any given work situation.
When generators, for example, are used in a confined space, carbon monoxide in the exhaust fumes creates a serious poisoning risk. Workers near to traffic on roads may be exposed to carbon monoxide and nitrogen dioxide from vehicle exhaust fumes. The decomposing action of bacteria in organic matter releases toxic hydrogen sulphide and carbon dioxide, both of which are common subsurface hazards.
Poisonous gases and fumes
Arguably, the most common hazard when working in confined spaces is the presence of poisonous gases, fumes or vapours. Toxic gases or vapours can be formed by the processes of cleaning, welding or painting, which, in an unventilated space can poison and suffocate the worker. In excavation work, contamination could result from hazardous substances previously deposited in the ground, or from natural sources, such as carbon dioxide produced by limestone.
Some gases may also become trapped in residues such as sludge, scale, or animal waste.
These may not be identified by atmospheric tests, but could be easily disturbed and released by workers. In addition, toxic gases will not necessarily originate from within the confined space itself, as fumes may also filter in from outside sources such as nearby processes or car exhausts.
The normal concentration of oxygen in fresh air is 20.9%. An atmosphere is hazardous if the concentration of oxygen drops below 19.5% or goes above 23.5%. If the concentration falls to 17%, mental and physical agility are noticeably impaired, with death coming quickly if it drops only a few percent more. At these levels unconsciousness takes hold so rapidly that the victim will be unaware of what is happening.
Without adequate ventilation, the simple act of breathing will cause the oxygen level to fall surprisingly quickly. Combustion also uses oxygen, meaning that engine-driven plant and naked flames such as welding torches are potential hazards. A less obvious risk is the fermentation of rotting vegetable matter, which absorbs oxygen and may create a hazard in agricultural storage units. Steel vessels and chambers that have been closed for some time are similarly dangerous because corrosion may have occurred, using up vital oxygen in the process.
Oxygen can also be displaced. Nitrogen, for example, when used to purge hydrocarbon storage vessels prior to re-use, drives oxygen out of the container and leaves it highly dangerous until thoroughly ventilated.
High oxygen levels are also dangerous. As with too little, too much will impair the victim’s ability to think clearly and act sensibly. Moreover, oxygen enriched atmospheres represent a severe fire hazard. From clothing to grease, materials that would not normally burn become subject to spontaneous combustion in these conditions. Common causes of oxygen enrichment include leaks from welding cylinders and even from breathing apparatus.
The monitoring equipment should alarm at 23.5% to indicate the possibility of an oxygen enriched atmosphere.
“Fixed systems typically comprise one or more detector heads connected to a separate control panel”
Most importantly, training should be realistic and not just drill based. It should relate to practise and familiarity with equipment, surroundings and possible emergency scenarios.
Furthermore, practical training within a simulated confined space environment can overcome many of the initial fears and concerns individuals may have. Regular drills and periodic reviews should ensure that training is always kept up to date and fresh in the minds of operatives.
Confined space workers should be trained in:
• Using rescue equipment - breathing apparatus, lifelines and where necessary, should be trained in their construction and how they work
• Checking the correct functioning and/or testing of emergency equipment - for immediate use and to enable specific periodic maintenance checks
• Identifying defects and dealing with malfunctions and failures of equipment during use
• Identifying emergency procedures of the worksite and locality, including initiation of an emergency response
• Resuscitation procedures and where appropriate, the correct use of relevant ancillary equipment and any resuscitation equipment provided - if intended to be operated by those receiving emergency rescue training
• Emergency first aid and the use of the provided first aid equipment
• Use of fire fighting equipment
• Rescue techniques, including regular and periodic rehearsals and exercises - these could include the use of a full weight dummy
Any system that uses gas can develop leaks. This can be through accidental damage or as a result of general wear and tear, but the common causes are human error, corrosion, tired or faulty equipment, poor maintenance, and accelerated chemical reactions that increase pressure.
Obviously the significance a leak has on a business’s ability to maintain normal activities will vary depending on the type of gas being used, its pressure and temperature, any products or reactions generated by the process, where the leak is located, and the quantity of gas that could be released.
The location and sensitivity of any sort of gas detection device depends on from where and when the gas is likely to escape.
Part of the health and safety management process for selecting gas detection technology involves gas dispersion modelling, using likely locations from which the substance could escape as a template. Varying densities, volumes and temperatures of the gas are tested, along with differing weather conditions, to find out how the gas cloud is likely to form and disperse in the event of a leak. Hazops, also known as hazard and operability studies, examine a company’s equipment and its operation to determine the possible points at which gas may be released.
"simple portable detectors contain a single sensor for a specific gas. They are ideal for protecting workers where a risk assessment has identified only one foreseeable hazard"
Companies operating in high risk industries must take a structured review of any risks to plant, people and the environment - that includes gas detection. The key is to ensure that elements of protection are proportionate to the level of risk involved. A common sense approach should be adopted at all times.
In addition to following the hierarchy of control, the Institution of Occupational Safety and Health (IOSH) suggests organisations consider three main areas to make sure they are protected when using harmful gases:
1. Process control - Ensure that presence, pressure, reactions and concentration of any harmful gases are controlled, so that gas does not escape into the workplace or surrounding environment where it could cause damage or harm.
2. Loss prevention - A process of good design, maintenance, inspection, operation and corrosion prevention should be in place to ensure gases are contained.
3. Prepare for loss - Put in place good gas detection and process monitoring, so that if loss of gas occurs it can be identified, with suitable emergency arrangements in place to protect people and the environment.
Selecting detection systems
Most gas detection systems can be broken down into the categories of fixed and portable.
Historically, the early detection systems for gas were very basic, but the principles remain the same to this day.
Portable instruments and larger fixed systems can be used for confined space monitoring.
Fixed systems typically comprise one or more detector heads connected to a separate control panel. If a detector registers a dangerous gas level, the panel raises the alarm by triggering external sirens. This sort of installation is suited to locations such as plant rooms, which have sufficient space.
Much confined space work takes place in more restricted areas, however, making compact portable units more suitable. Ease of use, such as single button operation, means minimal training is required while increased safety is ensured. Combining one or more sensors with powerful audible and visual signals to warn when pre-set gas levels are reached, portable detectors can be carried or worn wherever they are needed. This ensures that in a confined space, pockets of high gas concentration are not missed.
Simple portable detectors contain a single sensor for a specific gas. They are ideal for protecting workers where a risk assessment has identified only one foreseeable hazard. The most basic products are disposable monitors. Activated by the user when first required, they run continuously without maintenance for a set period of typically two years.
More sophisticated detectors have an illuminated display showing measured gas levels.
Unlike disposable products, these units are designed to be serviceable rather than to be replaced, have rechargeable or replaceable batteries, and generally allow the user to set alarm levels.
Multi-channel instruments can monitor up to four gases together; for example, underground work covering combustible hydrocarbons, oxygen, hydrogen sulphide and carbon monoxide.
Finally, because of the inherent difficulties of working in a cramped space in potentially poor lighting conditions, instruments must be easy to use.
Published: 29th May 2014 in Health and Safety International