the vessel will fail mechanically. If the vessel contents are flammable, this event may result in a "fireball". Oxidisers such as oxygen and fluorine will produce a similar effect by accelerating combustion in the area affected. If the cylinder's contents are liquid, but become a gas at ambient conditions, this is commonly referred to as a boiling liquid expanding vapour explosion (BLEVE). Medical gas cylinders in the UK and some other countries have a fusible plug of Wood's metal in the valve block between the valve seat and the cylinder. This plug melts at a comparatively low temperature (70 °C) and allows the contents of the cylinder to escape to the surroundings before the cylinder is significantly weakened by the heat, lessening the risk of explosion. Gas cylinders found in hot air balloons have to be checked once a year as part of an annual inspection by a CAA approved inspector ensuring no dents or scratches pose a risk to the cylinder. Pressure relief valves on balloon cylinders are replaced after 10 years or sooner if there are signs of damage to them along with an internal inspection of the cylinders to check for corrosion and foreign elements within the cylinder. More common pressure relief devices are a simple burst disc installed in the base of the valve between the cylinder and the valve seat. A burst disc is a small metal gasket engineered to rupture at a pre-determined pressure. Some burst discs are backed with a low-melting-point metal, so that the valve must be exposed to excessive heat before the burst disc can rupture. The Compressed Gas Association publishes a number of booklets and pamphlets on safe handling and use of bottled gases. International and national standards There is a wide range of standards relating to the manufacture, use and testing of pressurised gas cylinders and related components. Some examples are listed here. ISO 11439: Gas cylinders — High-pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles ISO 15500-5: Road vehicles — Compressed natural gas (CNG) fuel system components — Part 5: Manual cylinder valve US DOT CFR Title 49, part 178, Subpart C — Specification for Cylinders US DOT Aluminum Tank Alloy 6351-T6 amendment for SCUBA, SCBA, Oxygen Service — Visual Eddy inspection AS 2896-2011:Medical gas systems—Installation and testing of non-flammable medical gas pipeline systems pipeline systems (Australian Standards). Color coding Gas