Catastrophic Failure of Older Aluminum Tanks Prompts HSE Alert
The British Health and Safety Executive (HSE) has released a safety bulletin regarding older aluminium tanks, after a catastrophic failure of such a tank which caused 'great harm' to a person in the UK in 2017, following similar incidents in Indonesia and Australia in 2016.
The bulletin specifically refers to tanks manufactured using aluminium alloys designated as HE30/AA6082 (the type that failed in England) and AA6351 (the alloy that failed in Indonesia and Australia). These cylinders were manufactured by several different companies in several different countries between 1963 and 1995, meaning the cylinders are between 23 and 55 years old.
There is – at least in principle – no definitive 'lifespan' for a scuba cylinder, as long as they are regularly inspected and serviced. A visual inspection is almost universally accepted as being required every year, regardless of legislation that might extend the test period. International standards for hydrostatic testing vary around the word between three and five years. Eddy-current testing – where electromagnetic pulses are delivered through the tank's metal to test for cracks and fatigue that are not visible, is not always performed as part of the testing process.
Problems associated with cylinders manufactured using the AA6351 alloy are not new. The US Department of Transportation (DOT) and National Institute for Industrial Safety and Health (NIOSH) issued advice for cylinders made from this particular alloy as early as 1999, after a series of failures of both SCUBA tanks and the cylinders used by firefighters. Some of these failures were attributed to mechanical damage and over-pressurisation, but at least six were determined to have failed as a result of 'sustainable load cracking' (SLC), a phenomenon where cracks grow slowly over a long period of time through repeated subjection of the tank to high pressure, but remain undetected until they are almost at the point of failure.
New Zealand's HSE equivalent – WorkSafe – banned the use of AA6351 alloy cylinders outright in October 2017.
The HSE's advice regarding tanks made from both HE30/AA6082 and AA6351 alloys is to check the tank's age, service and usage history and determine whether it is still suitable for use, before ensuring it undergoes a thorough inspection, including an eddy-current test. If it is not possible to identify the alloy from the tank's markings, or there is any concern over prior use, it should be immediately withdrawn from service and condemned.
The general advice from the professional dive community is to simply get rid of these tanks and buy new ones. Some dive centres will refuse to fill cylinders constructed from these alloys, regardless of the history and current testing status of the tanks.
Manufacturers highlighted by the HSE as having used the HE30/AA6082 and AA6351 alloys in SCUBA tank construction are as follows:
- Luxfer Gas Cylinders HE30/AA6082 and AA6351 cylinders in the UK / AA6351 cylinders in the USA and Australia
- Walter Kidde Company in the USA
- CIG Gas Cylinders in Australia (acquired by Luxfer Gas Cylinders in 1997)
- Reynolds Tube Company Ltd (later known as TI Hollow Extrusions) in England
Tank markings indication construction from HE30/AA6082 and AA6351 alloys are:
- HOAL 1
- HOAL 2
- HOAL 3
- HOAL 4
- P****X (as part of the serial number)
- P****P (as part of the serial number)
The full HSE report can be found here: http://www.hse.gov.uk/safetybulletins/aluminium-cylinders.htm