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Gas Safety in Confined Spaces

21 March 2013

A confined space is any space large enough for someone to enter and perform assigned work, which has limited means of entry or exit, and which is not designed for continuous worker occupancy. This covers just about every industry, including utilities, construction, hydrocarbon exploration and processing, petrochemicals, marine, agriculture, food processing and brewing, as well as the emergency services.

Employers must assess the risks these workplaces pose to their employees and endeavour 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 gas risks can be divided into three broad categories: combustible gas, toxic gas, and oxygen depletion or enrichment. 

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). Different compounds have different LELs so it’s vital that detectors are capable of detecting at the correct levels. 

Typically, storage vessels which have contained hydrocarbon fuels and oils present a danger. Other dangers come from fuel leaks: burst fuel containers; pipelines on and off site, gas cylinders and engine-driven plant. For workers in pits, sewers and other sub-surface locations, methane formed by decaying organic matter is an almost universal danger.

Toxic gases and vapours

Confined-space workers may be exposed to many toxic compounds, depending on the nature of the work.  A risk assessment should be made of which toxic substances a worker may be exposed to in any given work situation.

When looking at toxic gases related to specific applications, the water industry for example uses many toxic compounds for cleaning and processing both waste and clean water. Hazards such as chlorine, ozone, sulphur dioxide and chlorine dioxide then pose additional risks both in storage and treatment areas.

Oxygen - too high or too low?

The normal concentration of oxygen in fresh air is 20.9%.  An atmosphere is hazardous if the concentration drops below 19.5% or goes above 23.5%. 

Without adequate ventilation, the simple act of breathing will cause oxygen levels to fall surprisingly quickly. Combustion also uses up oxygen, so engine-driven plant and naked flames such as welding torches are potential hazards. 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.

Gas detector types

Both portable and fixed gas detectors 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 reads a dangerous gas level, the panel raises the alarm by triggering external sirens and beacons. This sort of installation is suited to larger spaces like plant rooms, which have sufficient room for the hardware or remote stations that are usually unmanned. 

However, much confined space work takes place in more restricted areas, making compact portable units more suitable. Ease of use, with one 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. In addition, a compact instrument is easily carried in a confined space, ensuring that pockets of high gas concentration are not missed.

Certain features should be expected in every portable gas detector.  Clearly, life-saving tools for demanding environments must be as tough as possible, with reliable electronics housed in impact-resistant casings.  While the need to leave gas sensors exposed to the atmosphere means that no instrument can be fully sealed, a high degree of protection against dust and water ingress is essential.  Toughness notwithstanding, a well-designed detector will also be light and compact enough to wear for an entire shift.  

Finally, because of the difficulties of working in a cramped space, perhaps under poor lighting, instruments should be easy to use.  No matter how advanced a detector's internal architecture or data management options, personnel in the field should be faced with nothing more daunting than a clear display, simple, one-button operation and loud/bright alarms.

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