Modern compressed air and gas work

• Published in: Tunnels & Tunnelling International no. NOV; pp. 51 - 53
• Main Author: LAMONT, Donald
• Format: Journal Article Trade Publication Article
• Language: English
• Published: Sidcup Progressive Media Group 01-11-2011; Compelo
• Subjects: Applied sciences; Atmospheric pressure; Building process; Buildings. Public works; Compressed air; Concrete; Construction industry; Construction works; Exact sciences and technology; Legislation; Magnetic resonance imaging; Miscellaneous; Necrosis; Nitrogen; Tunnels, galleries; United Kingdom > UK; Hyperbaric solution; Sinking(boring); Compressed air drilling; Legislation; Well; Tunnel driving; Construction technology; High pressure
• ISSN: 1369-3999

The current UK legislation regulating compressed air work is the Work in Compressed Air Regulations 1996 supplemented by Health and Safety Executive guidance document L96 - 'A guide to the Work in Compressed Air Regulations 1996' along with the addendum to L96 covering oxygen decompression and the use of non-air breathing mixtures. L96 is complemented by guidance in BS 6164:201 1 - 'Code of practice for health and safety in tunnelling in the construction industry', which has a clause dealing specifically with the interaction between the pressurised workings and the surrounding ground. Within Europe, EN 121 10Tunnelling machinery - safety - Airlocks' is the relevant standard for the manufacture of bulkheads and airlocks. Exposure to pressure in tunnelling has led to numerous cases of decompression illness (DCI). Over the past 60 years for which UK records exist, around 0.6 per cent of all exposures have resulted in DCI. This is an average figure which does not reflect the true nature of the problem. Not surprisingly, detailed analysis of the DCI figures has shown that those most likely to experience DCI are the miners. It was not unusual for between a quarter and half of the miners on a contract to experience at least one DCI 'hit'. The predominant manifestation of DCI in tunnelling is Type One decompression sickness or 'the Bends', as it is often referred to. This occurs in over 90 per cent of tunnelling DCI cases. Although often considered to be a 'hazard of the job', and of little consequence, decompression sickness resulted in occasional fatalities throughout the 20th century until the introduction of the Blackpool tables of exposure limits and decompression times in the mid-1960s. Fortunately Type Two DCS, which results in severe neurological symptoms, is relatively rare from tunnelling exposures. Perhaps the most important development in compressed air working practice currently is so-called 'high pressure compressed air (HPCA) work'. This involves the use of higher exposure pressures than currently permitted by the legislation in most countries. At such pressures whilst pressurisation of the working chamber and manlocks is by compressed air, it is not desirable to breathe that air due to the adverse response by the body to high pressure nitrogen and oxygen. Consequently a major difference between HPCA work and conventional compressed air work is that HPCA requires the use of non-air breathing mixtures, and in some circumstances the use of 'saturation' techniques. Typical breathing mixtures are oxygen and helium blends (heliox) or oxygen, nitrogen and helium mixtures usually known as 'trimix'. Breathing mixtures are supplied by line-fed mask. Exposure to excessively high oxygen pressures leads to lung degeneration and other adverse health effects. High pressure nitrogen is both more difficult to breathe due to its density and is narcotic. Helium, whilst expensive, acts as an inert and low density diluent.