Composite cylinders have been with us as pressure vessels for certain types of applications, for only about half a century.
However, their foundation was laid almost two centuries ago, way back in 1815. Here it is claimed that Napoleon, unable to reach enemy lines with his cannons, strengthened their barrels by wrapping strong iron wire around them.
This additional reinforcement then gave them a capacity of accepting more highly charged cannon balls, capable of travelling further, thus breaching enemy lines. Although unknown to him at that time, he unwittingly became the founder of a thriving composite industry in the 21st century!
Nothing really happened after this ‘Eureka Moment’ for almost another hundred years, until in 1919 a spirally-wound, double-ended pressure vessel was developed. It was wound with two layers of high tensile strength steel wire to prevent sidewall rupture. With a 150mm diameter and a length of 500mm, it had a working pressure of just 69 bar (1000 psi).
This latter development then sparked off a new wave of innovations all enlightened by the enormous advantages of a lightweight (thus efficient) cylinders, which were primarily exploited for aviation applications. So much so that during the Second World War, airmen used these composite cylinders as part of their breathing apparatus.
Here the liner was made via the Lane and Taunton route using a steel plate, onto which high tensile steel wire was carefully hand wound and the whole held in place using a solder. Such is the popularity of these efficient packages that they are still being made today using a 70 year old manufacturing recipe.
In today’s terminology (emanating from the standards in the series of ISO 11119 which deal with composite cylinders) such cylinders, which are only reinforced around the parallel section of the liner, are known as Type 2 composites.
The last 50 years of the 20th century
It did not take long for these Type 2 cylinders to get even more efficient (capable of having a higher weight of gas carried per unit weight of cylinder).
By 1972 NASA had developed a Type 2 cylinder, using an aluminium alloy liner with a glass fibre composite hoop-wrapping. There then flowed a multitude of developments with different combinations of liner and overwrap materials.
Though various grades of glass fibre (E,S) remained popular on account of their low price, carbon fibre and synthetic fibres such as aramid, were also gaining popularity due to their better mechanical characteristics when combating environmental degradation. A few years, later in 1975, fully-wrapped composites emerged.
Known as Type 3 they command a high price premium on account of their even greater efficiency than the Type 2 version. Due to their high cost there is a need to protect the external carbon fibre overwrap from external mechanical damage. One way of achieving this is to wrap a sacrificial layer of glass fibre.
Another feature which needs to be safeguarded is the outer surface of the liner. Since from the moment it is wrapped, it will not be possible to visually examine the liner’s external, as it will be covered by the composite. Hence this external surface of the liner is often coated with a corrosion resistant paint, especially when the liner is made from steel.
Enter the 21st century
Now cylinder manufacturers, driven essentially by the motor vehicle industry and their use of compressed natural gas as a fuel, sought even lighter cylinders.
This quest, followed by much R&D work, led to the development of a composite cylinder with a polymeric (HDPE)* liner, thus abandoning the metal liner and yet again increasing the efficiency of the cylinder.
Known as Type 4, the high pressure version was rapidly displaced by some manufacturers, for low pressure gases such as liquid petroleum gas (LPG). However, recent work to develop Type 4 cylinders at a working pressure of 1000 bar for hydrogen storage applications has already resulted in small quantities to be placed on the market in five different countries (some with PED* approval), while 8400 litre water capacity tanks at a working pressure of 250 bar are also in use in several locations globally.
Returning to the low pressure gases application, pioneering work has been undertaken by Ragasco in Norway. From humble beginnings in 2000 when only 90,000 cylinders with a test pressure of 30 bar were made, well over 1.5 million similar cylinders were placed onto the market a decade later.
Using a fully automated plant, the technology is straightforward. At the outset the HDPE liner (which is seamless) is blow moulded with the boss equipped with a brass insert, which is capable of accepting a valve. This acts as the barrier for the contained LPG. This liner/boss assembly is next wrapped with a glass fibre composite which prevents the vessel from bursting, when it is subjected to extreme conditions during its use.
The whole is then cured in an oven. It is the know-how of constructing the latter junction where a metallic material, a composite and a polymer liner meet, that governs the success and safety of these vessels.
Three dissimilar materials all expanding and shrinking at the same time, as the pressure in the cylinder fluctuates, set-up a complex stress pattern. Designing this to work successfully as one, over the cylinder’s entire lifetime, is truly an engineering feat!
Each cylinder is subjected to a series of pneumatic and tightness pressure tests, together with a host of other stringent testing both at the prototype and batch stages. As in the case of Types 2 and 3, the composite’s externals are protected from mechanical, environmental and chemical damage, by a sturdy polymer case.
Just to give an idea of how efficient these Type 4 cylinders are, a 33 litre water capacity cylinder, is capable of handling 14 kg of LPG and weighs, when empty, less than 8 kg.
Such an enormous leap in the efficiency of the cylinder has opened up a number of rather novel applications such as for use with propelling fork-lift trucks, as a source of cooking for beach barbecues, and for patio heaters. Their shape lends them to convenient stacking during transport, while storing them at distributors is equally easy and aesthetically pleasing too.
Another big advantage is that because the liner/shell are translucent, the LPG contents are visible and so cylinders may be changed over without ever running out of gas.
Apart from enjoying corrosion resistance, another huge plus for these Type 4 cylinders is their behavior in a fire. Regardless of whether the cylinder is fitted with a pressure relief device or not, the casing and the liner will start to burn after a couple of minutes, followed by the controlled burning of the escaping LPG through fissures in the cylinder wall. There is no explosion and no fragmentation of the vessel.
Huge strides have been made by both manufacturers and users of composite cylinders over the past 50 years.
Although by far and away seamless cylinders (Type 1) still dominate the marketplace with almost 200 million cylinders in circulation, composites (with around 10 million) are gaining market share. In some fields like transport, domestic markets and medicine, where lightness is a premium, composite cylinders continue to make in-roads.
Also, as our society gets more and more affluent and users continue to gain trust in the behaviour of composite cylinders, the latter will continue to gain market share. For example, Type 4 cylinders are now approved by Competent Authorities (government bodies) all over the world.
With increasing trust in their safety, the current 15 year life that composites are given for UN journeys will be extended, making them even more popular than they are today.
*HDPE (High Density PolyEthylene)
*PED (Pressure Equipment Directive)