An engineer repairs buried cables; a plant manager enters a small plant room; a contractor inspects the lining of a sewage pipe. All these personnel face a common danger, despite working in widely different industries. Gas-related injury poses a serious threat in any confined space where the free movement of air is limited.
A confined space is usually defined as any space which is 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 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. However, in many cases such work cannot practicably be avoided, and so the priority must be to make it as safe as possible.
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 which have contained hydrocarbon fuels and oils present a danger. Other dangers come from fuel leaks: pipelines, gas cylinders and engine-driven plant. For workers in pits, sewers and other sub-surface locations, methane is an almost universal danger. Formed by decaying organic matter, this odourless gas collects in pockets underground.
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 of 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 on organic matter releases toxic hydrogen sulphide and carbon dioxide, both of which are common subsurface hazards.
Oxygen – too high or too low?
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; death comes very 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 up oxygen, which means 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 which 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 which would not normally burn become subject to spontaneous combustion under these conditions. Common causes of oxygen enrichment include leaks from welding cylinders and even from breathing apparatus.
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 “sees” a dangerous gas level, the panel raises the alarm by triggering external sirens and beacons. This sort of installation is suited to locations like plant rooms which have sufficient room for the hardware.
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 small instrument is easily carried in a confined space, ensuring that 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 disposable monitors. Activated by the user when first required, they run continuously without maintenance for a set period, typically two years.
More sophisticated but only slightly larger are one-channel detectors with an illuminated display showing measured gas levels. Unlike disposable products, these units are designed for servicing rather than replacement, have rechargeable or replaceable batteries, and generally allow the user to set alarm levels. They may also offer datalogging, a valuable feature which stores recorded gas levels for subsequent downloading and review, and so builds a long-term picture of users’ exposure to fluctuating gas levels.
Often, more than one hazard may be foreseeable in a single area. In such cases, multi-channel instruments are used. These generally monitor up to four gases together, with a typical sensor array for underground work covering combustible hydrocarbons, oxygen, hydrogen sulphide and carbon monoxide. A wide range of other sensors can be specified, making this type of unit suitable for most confined space applications. The slightly larger physical dimensions of a multi-gas detector allow for bigger displays showing a range of gas data from all channels simultaneously, as well as useful information relating to calibration and configuration.
Some multi-channel units incorporate a built-in sampling pump, allowing a flexible sample line to be fed into the space while the monitor remains outside with the user. This easily enables the user to test the atmosphere before entry into the confined space. Obviously, it is important that the sample line is free of kinks and blockages, and that sufficient time is allowed for the gas drawn from the chamber to arrive at the sensor.
Timed interval monitoring is particularly helpful in the oil and petrochemical industries. When a vessel which has held combustible liquids is purged with inert gas, a monitor is set up outside to record falling hydrocarbon levels and indicate when it is safe to open the container to air. The latest portable detectors incorporate an infrared sensor for just this purpose because, unlike conventional sensors for combustible gas monitoring, infrared devices can operate in the absence of oxygen and in the presence of very high hydrocarbon levels.
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. Additional protection may be provided in some cases by a removable rubber “boot”. 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 and simple, one-button operation.