Work in compressed air

Last updated April 16, 2024

Work in compressed air, compressed air work or hyperbaric work is occupational activity in an enclosed atmosphere at a controlled ambient pressure significantly higher than the adjacent normal atmospheric pressure. There are many parallels with underwater diving, and a few significant differences.^[1]

Applications

Compressed air work is mostly used in civil engineering projects where a raised ambient pressure is used to counteract ingress of groundwater from the surrounding soil or rock by balancing the hydrostatic pressure of the water with an applied air pressure inside an enclosed and sealed working area, such as a caisson, shaft, or tunnel.^[1]

Classes

Traditionally, compressed air work was limited to maximum ambient pressures of between 3 and 4 bars (3.0 and 3.9 atm), but experience with offshore saturation diving shows that higher pressures can be managed at acceptable risk using the techniques developed in that industry, including saturation exposures and the use of breathing gases other than air.^[2]

Compressed air work may be categorised as "low pressure", where staged decompression is not required regardless of exposure time (gauge pressure below 0.7 bar), "intermediate pressures" requiring stage decompression, but below the statutory limit, and "high pressure" where the ambient pressure is above the statutory limit (gauge pressure more than 3.5 bar in the UK – the limit may vary according to national legislation).^[1]

Work at 5.5 bar was necessary during tunnelling operations under Hong Kong, after the tunnel boring machine was damaged after hitting hard pink granite near the shore. The cutting heads seized and part of the top of the tunnel collapsed, flooding the tunnel with seawater. The relatively hot environment made the usual procedure of freezing the surrounding rock with liquid nitrogen impracticable so it was decided to pressurise the tunnel and work at ambient pressure until repairs could be completed. A new set of tables was designed and tested as the 1966 Blackpool decompression tables previously used for tunnelling work had produced significant long tern osteonecrosis injuries. Much of the work and decompression was done using trimix breathing gases.^[3]

Physiology of hyperbaric exposure

The major physiological differences between compressed air work and underwater diving are associated with the air environment in compressed air work and the water immersion of diving operations. This reduces risk as drowning is unlikely, other environmental hazards of hypothermia and hyperthermia are more easily managed, and the worker is not encumbered by a diving suit and helmet, though other personal protective equipment appropriate to the worksite is usually necessary, and the risk of fire may be higher. There is also often a significant difference in the numbers of personnel exposed in the hyperbaric working area. In diving it is seldom more than three, while in compressed air work there may be more.

Decompression

Decompression at the end of a shift is usually done in an airlock between the hyperbaric working area and the outside environment, and may routinely use pure oxygen as a breathing gas to accelerate decompression. Decompression schedules have been developed specifically for compressed air work, but other schedules of acceptable safety record may be used.^[1]

Saturation hyperbaric work

When the pressure of the workplace is relatively high, decompression at the end of a shift can take an uneconomically long time and exposes the worker to daily risk. In these cases it may be both economically preferable and reduce the overall risk to the workers to resort to saturation exposures, in which the workers remain under pressure for a tour of duty which may be for several days or weeks, and are decompressed conservatively just once at the end. In this scenario hyperbaric living quarters and the staff to operate them are needed, but full shifts can be worked. Transportation under pressure in a hyperbaric shuttle vehicle will usually be necessary to transfer workers between the accommodation and the workplace.^[2]

Health and safety

Compressed air work is generally considered a potentially hazardous occupational environment, and may be regulated accordingly. In some jurisdictions legislation mag be specific to compressed air work, while in others it may be combined with diving regulations.^[1]^[4]

Medical aspects

Legislation may require medical screening for candidate compressed air workers, similar to that for diving, and medical surveillance may be required for compressed air workers during and after exposure. Short term health risks include decompression sickness, barotraumas of compression and decompression, and long term risks include dysbaric osteonecrosis.^[1]

Management of decompression sickness

The risk of decompression sickness cannot be entirely eliminated within reasonably practicable procedures, and the contractor is generally obliged to provide emergency recompression facilities on site so that any cases or suspected cases of decompression sickness can be expeditiously treated.^[1]

Related Research Articles

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

Decompression sickness is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompression. DCS most commonly occurs during or soon after a decompression ascent from underwater diving, but can also result from other causes of depressurisation, such as emerging from a caisson, decompression from saturation, flying in an unpressurised aircraft at high altitude, and extravehicular activity from spacecraft. DCS and arterial gas embolism are collectively referred to as decompression illness.

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.

Saturation diving is diving for periods long enough to bring all tissues into equilibrium with the partial pressures of the inert components of the breathing gas used. It is a diving mode that reduces the number of decompressions divers working at great depths must undergo by only decompressing divers once at the end of the diving operation, which may last days to weeks, having them remain under pressure for the whole period. A diver breathing pressurized gas accumulates dissolved inert gas used in the breathing mixture to dilute the oxygen to a non-toxic level in the tissues, which can cause decompression sickness if permitted to come out of solution within the body tissues; hence, returning to the surface safely requires lengthy decompression so that the inert gases can be eliminated via the lungs. Once the dissolved gases in a diver's tissues reach the saturation point, however, decompression time does not increase with further exposure, as no more inert gas is accumulated.

Dysbaric osteonecrosis or DON is a form of avascular necrosis where there is death of a portion of the bone that is thought to be caused by nitrogen (N₂) embolism (blockage of the blood vessels by a bubble of nitrogen coming out of solution) in divers. Although the definitive pathologic process is poorly understood, there are several hypotheses:

Diving medicine, also called undersea and hyperbaric medicine (UHB), is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how relationships of a diver's fitness to dive affect a diver's safety. Diving medical practitioners are also expected to be competent in the examination of divers and potential divers to determine fitness to dive.

Diving disorders, or diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides into conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.

A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of breathing gas for the occupants.

Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on context. Immersion in water and exposure to high ambient pressure have physiological effects that limit the depths and duration possible in ambient pressure diving. Humans are not physiologically and anatomically well-adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives, and allow different types of work to be done.

The decompression of a diver is the reduction in ambient pressure experienced during ascent from depth. It is also the process of elimination of dissolved inert gases from the diver's body which accumulate during ascent, largely during pauses in the ascent known as decompression stops, and after surfacing, until the gas concentrations reach equilibrium. Divers breathing gas at ambient pressure need to ascend at a rate determined by their exposure to pressure and the breathing gas in use. A diver who only breathes gas at atmospheric pressure when free-diving or snorkelling will not usually need to decompress. Divers using an atmospheric diving suit do not need to decompress as they are never exposed to high ambient pressure.

Dive planning is the process of planning an underwater diving operation. The purpose of dive planning is to increase the probability that a dive will be completed safely and the goals achieved. Some form of planning is done for most underwater dives, but the complexity and detail considered may vary enormously.

To prevent or minimize decompression sickness, divers must properly plan and monitor decompression. Divers follow a decompression model to safely allow the release of excess inert gases dissolved in their body tissues, which accommodated as a result of breathing at ambient pressures greater than surface atmospheric pressure. Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent.

Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure.

Decompression theory is the study and modelling of the transfer of the inert gas component of breathing gases from the gas in the lungs to the tissues and back during exposure to variations in ambient pressure. In the case of underwater diving and compressed air work, this mostly involves ambient pressures greater than the local surface pressure, but astronauts, high altitude mountaineers, and travellers in aircraft which are not pressurised to sea level pressure, are generally exposed to ambient pressures less than standard sea level atmospheric pressure. In all cases, the symptoms caused by decompression occur during or within a relatively short period of hours, or occasionally days, after a significant pressure reduction.

The physiology of decompression is the aspect of physiology which is affected by exposure to large changes in ambient pressure, and involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues. Gas is breathed at ambient pressure, and some of this gas dissolves into the blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the tissues is in a state of equilibrium with the gas in the lungs,, or the ambient pressure is reduced until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

The following outline is provided as an overview of and topical guide to underwater diving:

The following index is provided as an overview of and topical guide to underwater diving:

A built-in breathing system is a source of breathing gas installed in a confined space where an alternative to the ambient gas may be required for medical treatment, emergency use, or to minimise a hazard. They are found in diving chambers, hyperbaric treatment chambers, and submarines.

References

1 2 3 4 5 6 7 Compressed Air Working Group (2021). Guidance on good practice for Work in Compressed Air – Based on the Work in Compressed Air Regulations (SI 1996/1656). British Tunnelling Society. ISBN 9780954610654.
1 2 ITA Working Group n°5: Health & Safety in Works, In Association with the British Tunnelling Society Compressed Air Working Group (March 2018). Guidelines For Good Working Practice in High Pressure Compressed Air. ITA REPORT n°010 - V3 (Report). British Tunnelling Society.
↑ Imbert, J.P.; Sidali, Armin (3 April 2024). "Deep Tunnel Workers Go Trimix". InDepth. Archived from the original on 15 April 2024. Retrieved 15 April 2024.
↑ "Diving Regulations 2009". Occupational Health and Safety Act 85 of 1993 – Regulations and Notices – Government Notice R41. Pretoria: Government Printer. Retrieved 3 November 2016– via Southern African Legal Information Institute.

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[CAWG_guidance_2021-1] 1 2 3 4 5 6 7 Compressed Air Working Group (2021). Guidance on good practice for Work in Compressed Air – Based on the Work in Compressed Air Regulations (SI 1996/1656). British Tunnelling Society. ISBN 9780954610654.

[ITA_010_2018-2] 1 2 ITA Working Group n°5: Health & Safety in Works, In Association with the British Tunnelling Society Compressed Air Working Group (March 2018). Guidelines For Good Working Practice in High Pressure Compressed Air. ITA REPORT n°010 - V3 (Report). British Tunnelling Society.

[Imbert_and_Sidali_2024-3] Imbert, J.P.; Sidali, Armin (3 April 2024). "Deep Tunnel Workers Go Trimix". InDepth. Archived from the original on 15 April 2024. Retrieved 15 April 2024.

[SA_Diving_Regulations_2009-4] "Diving Regulations 2009". Occupational Health and Safety Act 85 of 1993 – Regulations and Notices – Government Notice R41. Pretoria: Government Printer. Retrieved 3 November 2016– via Southern African Legal Information Institute.

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