CO_2 gas cylinders have traditionally had a 'design pressure to test pressure ratio' of about 15% for over 30 years and the chief mechanism for failure has been gross corrosion. During the past decade or so there has been a move to use mixed gas cylinders (Nitrogen and carbon dioxide in 70/30, 50/50 and 40/60 mixes respectively) with a 'design pressure to test pressure ratio' of about 60%. This increase in design to test pressure has resulted in an increase in failures, chiefly due to Stress Corrosion Cracking (SCC), and a number of these have been well documented in the UK. Fortunately, in these recent failures there have been no serious injuries or fatalities. It is worth emphasising that such failures can cause very serious injuries such as burst ear drums and lung collapse at 10m away, and fatalities at 1m. There are also secondary projectiles that can lead to fatalities, as well as serious injury from flying debris. The main aim of this paper is to provide a review of the current state of NDT at the time of manufacture, at the time of fill and at the time of periodic inspection for transportable seamless steel gas cylinders.
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