Whilst pressure retention was the initial driving force behind safety considerations, the compatibility of the gas with the cylinder's internals is equally relevant in today's industry.

When only oxygen and nitrogen were the contained gases, steel cylinders were adequate; but as hydrogen, carbon monoxide (laser gases), and a host of special gases (used in the semiconductor industry) became widely used, steel has been in competition with aluminium alloys and a host of special materials (metallic and even non-metallic).

Since the Debor cylinder (Fig.1) made its first appearance in the early 20th century, gas containment has largely been in vessels resembling cylinders, viz. a long parallel length of material closed at one end, with a neck and shoulder at the other end capable of taking a valve. Cylinder range in size from tiny capsules of around 10 cubic centimetres water capacity, used in specialist breathing apparatus equipment, to huge double ended vessels on the backs of trailers, known as $quot;jumbo tubes$quot;, with water capacities up to 3000L, used for transporting hydrogen and helium in bulk quantities.
These are extreme examples and the majority of cylinders have between 10 and 50 litre water capacity.

In recent years, with increasing legislation concerning manual handling, the larger 50 litre cylinders are gradually being replaced with 10 to 20 litre counterparts with higher working pressures. Miniaturisation has also hit the gas cylinder industry.

Seamless steel cylinders
presence felt with the advent of a $quot;wonder material$quot;, which made its debut on the industrial horizon in the 1880's. This new development was steel!

Lane and Taunton, based in Birmingham, UK, developed one of the first steel cylinders using their patented process, from a plate route. Here circular plates of steel are initially cold-pressed to form a thick cup, which is cold-drawn in a number of passes to form the hollow shell and finally hot forged to produce the neck/shoulder profile.
It is vital in this manufacturing process to control the amount of cold drawing per pass in order not to introduce an excessive amount of cold work, which could result in defects or cracks. Hence the cup is gradually drawn to the final length in a series of steps involving an annealing treatment to remove the cold work from the previous pass, re-lubricating the shell, followed by a further cold drawing operation and so on.

A few years later, in 1891, Ehrhardt simplified the plate route by piercing a red-hot block - or billet - of steel once again to form the thick walled cup. Utilising the retained heat, the cup is then immediately drawn in a single operation, to the required length.
The customary shoulder and neck profile was once again forged to shape, though now most manufacturers, have replaced this ancient form of neck formation by using a hot spinning technique (see Fig.2). Here, like on a potter's wheel, the shoulder and neck of the cylinder are formed by utilising the soft nature of the steel within a controlled temperature range. The temperature must be maintained, because if the temperature is lower than the required range, cracks will appear, and if higher, the steel rapidly oxidises resulting in unsound material. Both of the latter extremes must be avoided by the use of pyrometers controlling the temperatures of the critical zones in the shoulder and neck area during its formation.

Finally, in 1892, a third key method for seamless steel cylinders was established by Max and Reinhardt Mannesmann. This utilised a seamless tube of steel, which was cut to the desired length and hot worked to close one end to form the base. The top end, which receives the valve, was forged as in the above cases. Such cylinders are not seamless in the true sense of the word since the base contains remnants of welds, albeit often only of the parent material. Occasionally, the manufacturer actually cuts out this non-homogeneous central portion of the base, and proceeds to weld in a solid plug in the cavity, to ensure soundness of the base.

Aluminium alloy cylinders
Aluminium alloy (AA) cylinders have been around since the 1920s but their level of production remained low until the introduction of a 6000 series alloy in the 1960s. AA cylinders can be manufactured either by adopting a cold backward extrusion technique or by hot extrusion, like a steel cylinder, to produce an open ended shell.
In both cases, the starting material is a predetermined billet which needs a critical application of a lubricant. In the case of the backward extrusion process, not only is the type of the lubricant critical but so is the thickness of the lubricant film.

The shell is finally hot-headed by uniformly pre-heating the top end, then introducing it into a pre-formed die with the shape of the required shoulder and neck.

Heat treatment processes
To achieve the desired mechanical properties, cylinders experience an exacting heat-treatment cycle. Steel cylinders are quenched and tempered (Q&T) whenever a chromium-molybdenum alloy is used. These often have high-tensile properties and have thinner walls and are lighter in weight than their normalised carbon-manganese counterparts. The latter are always the preferred option whenever the required gas contents are potentially corrosive gases, since normalised, low strength cylinders do offer greater protection in case of ingress of moisture into the cylinder.

Close attention has to be paid to the characteristics of the quench tank and its contents, especially since in the past couple of decades nearly all manufacturers have changed over from using oil as the quench medium to a variety of aqueous quenchants. The changeover was made on Health and Safety grounds, as the earlier oil baths were often set alight by the red hot cylinders entering the oil and the presence of plumes of smoke in the work place. It is estimated that more than 97 percent of today's steel cylinders are supplied in the Q&T condition.

Similarly the vast majority of AA cylinders are supplied in a solution treated and aged condition which results in high mechanical properties.

Final operations
The neck of the cylinder is pierced and internally machined to accept a valve. The heat treated shells are subject to external and internal cleaning operations as appropriate. In the case of steel cylinders, shot blasting techniques are used, while softer media are employed for AA cylinders. Finally the required paint coatings are applied on the externals.

Inspection methodology
None of the above descriptions have attempted to cover the stringent inspection and quality control techniques which a manufacturer has to employ. These techniques not only ensure that the requirements stipulated in standards and legislation are satisfied, but also assist the manufacturer to improve his productivity and throughput. Nowadays ultrasonic in-line inspection - usually for steel cylinders - and hardness testing - for all materials - are extensively employed.
Additionally, all cylinders receive a hydraulic pressure test at their design pressure. Steel cylinders made via the tube route require an additional pneumatic leak test, due to a potential leakage path in the base.

However, prior to starting production of any design, the design must pass a specified prototype test regime.

Concluding remarks
This has been a mere glimpse into cylinder production. A future article will cover detailed aspects of a cylinder's performance, which varies depending on the applicable construction standard.
Which type of cylinder is eventually specified by the user, not necessarily based simply on day one price, will be covered. Price apart, features such as safety, portability and - increasingly - aesthetic appeal all play a role in the final choice.