Everything you wanted to know about cylinders, but never asked – Part one of Dr Irani’s new series covers the development of pressure vessels down the years.

Pressure is a concept scholars learn about early on in their careers, as the force exerted on a surface per unit area.

In the gases industry often the non-SI unit of either bar or pounds per unit area (pounds per square inch, psi) is used to quantify pressure.

But regardless of whether we are referring to working pressure (WP), test pressure (TP), or even the ultimate strength of a gas container, it’s burst pressure (BP), the word PRESSURE immediately tells a gas industry worker that he is handling a potentially dangerous piece of equipment.

The exact definitions of all these pressure related terms can be found in a terminology standard ISO 10286.

As early as 1662, an Irishman, Robert Boyle, had linked pressure and volume in a container to be inversely proportional to one another, at constant temperature.

Then in 1787 Charles showed a direct correlation between volume and temperature, this time the pressure being held constant. Hence all three, pressure, volume and temperature were finally connected and form the backbone of the current compressed gas industry worldwide.

First Weldeds, then Monolithics (Seamless)
In our industry it was not for another half a century after Charles’s Law was formulated that the concept of pressure became relevant. At that time in 1852, a brazed copper sphere at a pressure of around 280psi (~ 19bar) was an innovation.

It took a further three decades for the pressure in the spheres to progress to a modest 400psi (~ 28bar). But with the ‘wonder material’ of steel, now in production by the end of the 19th Century, welded copper spheres were replaced and gave way to seamless gas cylinders in the shape and size as we know them to be today.

When carbon steel was substituted by carbon-manganese (C-Mn) steel, pressures inside monolithic cylinders marched up to 1800psi (~ 124bar), and then to the dizzy heights of 2000psi (~ 137bar) during the second world war.

In came the family of alloy steels in the 1960s with chromium/molybdenum (Cr-Mo) additions, which still today forms the workhorse of the industrial gases business.

The higher strength of these Cr-Mo steels meant that for the same weight (same wall thickness) as the earlier C-Mn cylinders, the pressure in the cylinders could be increased.

Since gas cylinders were considered from the very early days as an explosive material (even today they are classified as ‘Dangerous Goods’ in most regulations), the contained pressure was only increased in gradual steps from 137bar in the 1940s; to 150bar; to 175bar in the 1970s.

Finally in the 1980s the 200bar cylinder had emerged. The same Cr-Mo alloy cylinder was taken to 230bar in the late 1980s by the UK’s leading gas company, an action promptly copied by other players, particularly in Western Europe.

Meanwhile, aluminium alloy cylinders, owing to their late entry in the 1960s into the gases industry, went straight to 150bar WP for industrial gases and then matched their steel counterparts with the passage of time.

However, the quest for ever-increasing efficient cylinders (whereby a higher gas weight can be transported per unit weight of cylinder) meant that while safe steel cylinders operating at a WP of 300bar were developed, the humble aluminium alloy (AA) cylinder struggled to catch up.

At these 300bar type pressures the AA cylinder could not demonstrate a leak-before-burst characteristic, which was proving to be a vital performance parameter users of gas cylinders insisted on achieving.

So whilst a modified Cr-Mo alloy with transition element additions such as vanadium, niobium improved the cylinder’s pressure bearing capacity with some cylinder manufacturers, others used an ultra-low sulphur phosphorous Cr-Mo alloy to achieve similar results.

Apart from increasing the Cr-Mo alloy’s strength, these new steels simultaneously maintained the excellent impact toughness properties of the finished component.

Enter the era of 300bar WP technology. Such was this advance, that the 300bar WP cylinders were almost the same weight as the earlier 230bar WP counterparts for the same water capacity.

Thus a 70bar pressure increase (over 30%) was enjoyed and virtually no weight increase meant a 30% improvement in the cylinder’s efficiency overnight.

Not wishing to be left behind, the AA industry developed an alloy in the 7000 series in France (7060 initially and 7032 almost two decades later in USA) which rivalled the 300bar steel cylinder, albeit at a considerably higher price.

Goodbye Monolithics, Hello Composites
Invention is the mother of necessity – a common statement spoken throughout the laboratories at NASA.

For while in the world of monolithic gas cylinders alloy development moved at a rapid pace and resulted in gas cylinder efficiency increasing from around 5% in the 1930s to around 22% in the 1990s, the appearance of the first cylinder which had been hoop-wound with a glass fibre/resin combination in industrial service in the 1980s, suddenly rocketed this efficiency to almost 45% in one single shot!

Enter the world of composite technology into the gas cylinder domain. The R&D at NASA was gradually stepped up by gas cylinder manufacturers and composite cylinders were rolled out for many non-astronautical applications.

With this tremendous efficiency increase came a concurrent series of pressure rises. Further developments using fully-wound metallic lined cylinders with carbon fibres plus resin, to be followed by plastic liners, meant that 450bar WP was quickly followed by 700bar WP cylinders.

Today there is talk of even a 1000bar WP cylinder in the offing. These latter developments are driven by the automotive industry where cylinders are being increasingly used to store fuel gases on-board vehicles.

From Designing to Performance Testing
Using standard pressure vessel design concepts and the mean-diameter formula, most cylinders were constructed by most manufacturers to tried and tested engineering theory principles.

Then the quest for higher pressures diverted some countries to further research the scientific principles governing the elastic/plastic behaviour of materials with the advent of the Lamé von-Mises formula.

The latter, simultaneously increased the safety prediction of the cylinder and reduced the wall thickness by around 8%. This approach was universally adopted in ISO standards and the ISO 9809 series and ISO 7866 for seamless steel and AA gas cylinders respectively resulted.

But the heterogeneous properties of composite materials with their non-reproducible construction meant a theoretical design calculation approach to predict the wall thickness, in the conventional way was not possible.

Hence to qualify composite cylinders a series of performance tests relevant to service conditions, over a cylinder’s life, were developed. Due to the nature of this somewhat inexact approach, composite cylinders are treated with great respect and at times with suspicion.

For example, some countries expect a composite cylinder to be scrapped after just 15 years of service.

New Cylinders for Old!!
Finally an article on cylinder pressures would not be complete without a brief description of the concept of Re-qualification (or Re-rating) of gas cylinders.

As greater understanding of the behaviour of gas cylinders was acquired, cylinder users began to question if their current population of cylinders can indeed be utilised more efficiently. In other words, can the cylinders be filled to a higher pressure than their original working pressure?

This concept was based on a heavy dependence on a non-destructive examination technique, namely ultrasonic examination (UE) which had been introduced by a UK cylinder manufacturer in the 1960s.

The actual procedure is highly complex and needs to be professionally undertaken by competent staff.

But with the help of Lamé von-Mises, UE, test certificates from the time of the cylinder’s manufacture and the fact that the cylinders were probably over-designed, gas companies around the world embarked on a series of Re-rating programmes.

This resulted in some cases in the same gas cylinder being pressurised by up to 30% more than the stamped working pressure.

Although this Re-rating exercise has to be performed with the utmost care, using a documented set of approved instructions and often under the control of a body responsible to the National Authority (sometimes called the Competent Authority or simply the Government) some gas companies were able to utilise their cylinder fleets with much greater efficiency (~ 30%) than before.

The author of this article will be pleased to advise readers further, with precise details of such Re-rating exercises.

Where Next??
Well, from humble beginnings which took thirty years to progress 9 bar (from 19 bar to 28 bar), we have in the past decade gone from 230 bar to almost 700 bar with ease! Not only pressure increases have taken place but the safety record now is even better than in the past.

Helped by NDT methods, cleaner materials, finite element techniques for estimating critical stress areas, SPC and I can go on, there seems to be no limit on how high in pressure we can progress for gas cylinders.

There is however the compressibility factor for compressed gases which we must not overlook. After a time, pressure alone will not help.

Work on advanced materials must be the answer to maintaining the high pressures we currently enjoy, since gas cylinder users will always want lighter packages now that they know what can be achieved.

Extremely light, efficient, and linerless, composite cylinders currently abound but only for cylinders designed for low pressure liquefied gases such as propane or butane. But with man’s prowess for developing new products and even entire industries, who knows where we will be in the next 20 years?

A linerless composite at Pressures of 300 bar or above is not beyond our imagination.

Coming up!
Part two in Dr Irani’s new series will focus on gas cylinders identification, including colour coding, labelling, and stamp marking.