An overview
From its inception in 1852, the 'envelope' that
contains gas under pressure, a gas cylinder as we know it today has evolved from a brazed copper sphere, to a complex shaped composite vessel. In the past 150 years, gas cylinders have populated our planet, from applications deep on the ocean beds to those in space, such that there are well over 200 million high pressure gas cylinders in use today.

This is the first in a series of articles which will demonstrate how gas cylinders have dominated our lives and will continue to play an ever increasing role. Future articles will develop on the following themes:

•Cylinder manufacturing
•Re-testing
•Regulatory/standards and use
•Aspects of gas cylinders
•Globalisation.

In particular, the globalisation of the gas cylinder market will be covered by demonstrating the powerful impact of regulations supported by relevant ISO (International Standards Organisation) standards for both their construction and subsequent use across international boundaries.

The vast majority (~75%) of the above mentioned 200 million cylinders in service today are made from steel, while about 50 million aluminium alloy (AA) cylinders are also in use. A few million (~10m)composite cylinders are also on the market and their numbers are increasing rapidly as users seek packages which are ever efficient (as measured by the volume of gas contained per unit weight of cylinder).

Seamless steel cylinders
Most of the steel cylinders in current service have used a chromium-molybdenum based alloy, which was developed in the 1960's. Today, seamless steel cylinders are manufactured from one of three
processing routes:-

•Hot extruded from a billet
•Cold pressed from and deep drawn from a plate
•Hot spinning from a tube

Each processing method has its own set of merits and drawbacks, depending on the end-user's requirements. The details of these various methods will be described in a later article in this series. However,
suffice to say that if, as is the current trend, a lightweight package is the goal, it is clear that plate cylinders are rapidly gaining in popularity on technical efficiency grounds. This is exemplified by their widespread use as on-board fuel cylinders for compressed natural gas (CNG) and possibly also for gaseous hydrogen applications in the future. They are popular in niche markets too, where portability is a requisite e.g. small medical cylinders.

Seamless aluminium alloy cylinders
AA cylinders tend to gravitate around the 6000 series alloys. In recent times only alloy 6061 is used by reputable manufacturers, though AA cylinders from other 6000 alloys still do exist in various parts of the world. In the past fifteen years alloys pertaining to the 7000 series are gaining momentum due to their higher strength (compared to the 6000 series) resulting in thinner walls and hence lighter cylinders. The 7000 series alloys produce cylinders about 30% lighter than those from other AAs.

Additionally, cylinders from alloys 2001 and 5283 have also been manufactured in significant quantities for specific
applications.

AA cylinders are manufactured using a pre-determined billet either by cold backward extrusion or hot extrusion, depending on the alloy being extruded. A vital parameter which needs to be controlled during AA cylinder manufacture is the nature and thickness of the lubricant applied to the billet prior to the extrusion process.

Composite cylinders
By wrapping steel wire around the barrels of his cannons, to provide a compressive stress, Napoleon began to use the principle of composite materials in the 19th century.

Industrial gas cylinder applications first used a wire-wound over-wrap approach on steel shells, over 50 years ago. $quot;Composites$quot; as we know them today, really took off about 25 years ago. In that period a number of fibre/resin combinations (FRP) have been employed. The FRP has been wound either along the parallel section of the cylinder (hoop-wrapped), as in Fig. 1, or over the entire external surface (fully-wrapped), see Fig. 2.

A variety of liner materials have been used though steel and AA are by far the most common ones, in combination with wrappings of glass or aramid or carbon fibres. Each combination has its own merits and disadvantages but they all share a common attribute of high technical efficiency.

Even higher efficiencies can be achieved if the
metallic liner is replaced by a plastic one or even, in some cases, a totally linerless configuration. The pros and cons of all these different approaches will be developed in a future article.

Overall considerations
Working pressures in gas cylinders are ever increasing. From the modest 400psi (28bar) copper brazed vessels used by the Brins Brothers (founders of the BOC Group) in 1885, cylinder pressures increased to 2000psi (138bar) in the 1940s to 200bar in the 1980s and to 300bar in the twenty-first century. Some specialist applications are even using 700 bar plus technology.

At all times safety considerations have been at the forefront of the developments. Due to their proximity to people, gas cylinders are tightly governed by exacting regulations and standards. With a stored energy of 3.0MJ (equivalent to an explosive power of 0.5kg of TNT) in a 50 litre, 300bar vessel, gas cylinders will always remain a topic of close scrutiny, especially now that their mobility has been enhanced by ISO standards.