steel and aluminium alloys have been withdrawn from service. Fibre composite cylinders were originally specified for a limited life span of 15, 20 or 30 years, while steel cylinders are nowadays typically withdrawn after 70 years, or may continue to be used indefinitely providing they pass periodic inspection and testing. Since some years there exist composite cylinders that are nominated for a non-limited-life (NLL), as long as no damage is to be seen. Types Since glass-fibre-composite materials were used to reinforce cylinders, there are various types of construction of high-pressure vessels: Metal only. Mostly seamless forged metal. But for lower working pressure, e.g., liquefied butane, there are welded steel vessels, too. Metal vessel, hoop wrapped with a fibre composite only around the cylindrical part of the "cylinder". (Geometrically there is a need for twice the tensile strength on the cylindrical region in comparison to the spherical caps of the cylinder.) Thin metal liner (that keeps the vessel tight, but does not contribute to the working pressure) fully wrapped with fibre in the matrix material. Metal-free liner from plastics, fully wrapped with fibre material. The boss, the centre of the head(s) of the cylinder is still from metal and includes the thread for the valve. Pressure vessels for gas storage may also be classified by volume. In South Africa, a gas storage cylinder implies a refillable transportable container with a water capacity volume of up to 150 litres. Refillable transportable cylindrical containers from 150 to 3,000 litres water capacity are referred to as tubes. Regulations and testing The transportation of high-pressure cylinders is regulated by many governments throughout the world. Various levels of testing are generally required by the governing authority for the country in which it is to be transported. In the United States, this authority is the United States Department of Transportation (DOT). Similarly in the UK, the European transport regulations (ADR) are implemented by the Department for Transport (DfT). For Canada, this authority is Transport Canada (TC). Cylinders may have additional requirements placed on design and or performance from independent testing agencies such as Underwriters Laboratories (UL). Each manufacturer of high-pressure cylinders is required to have an independent quality agent that will inspect the product for quality and safety. Within the UK the "competent authority" — the Department for Transport (DfT) — implements the regulations and appointment of authorised cylinder testers is conducted by United Kingdom Accreditation Service