In today's industry there are enormous quantities of welded gas cylinders in use. The vast majority are steel based, (though aluminium alloys are also manufactured for specific applications) with the output from some of the larger steel cylinder manufacturers exceeding six million vessels per annum. Guesstimates of total numbers vary, but most agree that more than a billion $quot;useful$quot; cylinders are currently in circulation. $quot;Useful$quot; because in many communities, they are essential for survival. Numerous city dwellers around the world take piped gas (natural gas/methane) for granted for cooking and heating. Where such a luxury is absent, $quot;bottled$quot; gas in the form of LPG (Liquefied Petroleum Gas) is the substitute. As a result cylinders used for LPG account for, by far, the greatest number of welded cylinders in circulation.

Following a rise in popularity in the last decade, welded steel cylinders are also commonly used for refrigerant gases. However, many cylinders for such gases are of the non-refillable type i.e. filled only once and then scrapped. Welded & refillable cylinders are also used for acetylene, chlorine, ammonia and a host of other low pressure liquefied gases. Great care is needed in the manufacture and testing of these products (often non-destructive tests in varying quantity for batch tests) due to the toxic/flammable nature of some of these gases.

The refillable welded cylinder
Welded cylinders are either two-piece (this depends on the water capacity, for low water capacities, say less than 50 litres) while larger cylinders are often three-piece. The two-piece geometry consists of two cold, deep drawn plates that are circumferentially welded in a $quot;joggle$quot; geometry, whereby the one plate slightly overlaps the other resulting in a small gap on the internal surface.

Such two-piece cylinders are also occasionally $quot;butt$quot; welded, (e.g. when used for the storage of acetylene and its associated $quot;porous material$quot;, so that there are no $quot;gaps$quot;) to eliminate the possibility of the porous material from developing cracks.

Looking to the larger cylinders, the three-piece geometry consists of two deep drawn plates that form the base and the dome sections. These two sections are then welded circumferentially to a longitudinally welded parallel section, also formed from a steel plate.

When manufacturing both the two and three-piece geometries, a disc of steel is cut away at the apex of the dome and a threaded boss welded in its place to accommodate a valve. Finally a shroud and a footing are welded onto the top dome and base sections respectively. Post-welding, the finished cylinder needs to be heat-treated to relieve all the stresses in the cylinder introduced by the welding processes (see Fig.1) and the cold work.

However, this practice varies between manufacturers and according to the requirements of the cylinder specification. Often the cylinders are normalised, but a smaller number are merely stressed relieved. Some cylinders need not be heat-treated at all, as exemplified by the three-piece geometry made to EN1442. However, from a long-term viewpoint it remains to be seen if the latter is a satisfactory approach.

From here the cylinders are hydraulically tested at the cylinder's test pressure, externally shotblasted, coated with layers of surface protection, see Fig.2, (customer preferences here are nowadays overridden by the cylinder manufacturer's need to follows local environmental regulations based on solvent emission limits and economic considerations), valved and finally pneumatically tested to ensure that there are no potential gas leakages especially from the valve end.

Design is key, and to avoid the problem of blockage of fine tubing (sometimes found in expensive refrigeration systems) refrigerant gas cylinders need to be free of all loose scale and debris on the internals, and receive additional cleaning treatment on this area.

Standards are high in this industry and refillable welded cylinders are designed and manufactured in line with regulations for transportable gas cylinders. This entails strict requirements consistent with the cylinder being refilled perhaps hundreds of times during its lifetime.

The non-refillable welded cylinder
This category of cylinders essentially consists of two halves of cold-drawn steel plates with a single circumferential weld. Its specification is not as demanding as the refillable counterpart (e.g. there is no post-weld heat treatment and of course it has no fatigue characteristics.) As a result these products are generally conveyed within specially designed cardboard boxes that are meant to offer some protection against mechanical damage.

Some specifications require the fitting of a non-refillable valve, which attempts to limit the use of these cylinders to their intended single fill. This requirement is strictly enforced and offenders attempting to refill such cylinders in some countries face huge penalties, of up to five years imprisonment and / or $500,000 fines. Non-refillable cylinders are very widely used with certain refrigerant gases, though in recent years they have found other prominent applications such as utilising helium that is unfit for sophisticated MRI machines to inflate party balloons.

The future
As the standard of living increases worldwide, there is a greater need for $quot;bottled$quot; gas. Hence a greater number of refillable welded cylinders will be manufactured to cater for this demand. However, even here the manufacturers are looking for ways to conserve valuable resources. So new technological specifications are being formulated, whereby cylinders are no longer designed but instead $quot;performance tested$quot;. In this approach cylinders are required to meet a set of prescribed tests which attempt to reproduce in-service damage at their critical, high-stressed locations. Conclusions are then drawn from analysis of the required $quot;cylinder$quot;. Invariably, the resulting cylinder, which meets these performance criteria, is thinner in cross-section than the conventionally designed product.

Such a concept is already used successfully, putting tens of thousands of welded stainless steel cylinders into the market. A similar approach is now being assessed for the common welded carbon steel cylinder, where there are huge environmental related savings to be enjoyed if a safe and acceptable specification can be drawn up.

Markets are also growing, and the modern suite of refrigerant gases (which are less damaging to the ozone layer) are creating a new demand for welded cylinders as these products have a much higher developed pressure than previously.

To conclude, looking at the range of cylinder usage and current demand it's fair to say that welded steel cylinders have been with us for a long time and although the future will require innovation, they are going to be with us for many years to come.