Why are composites the future for so many pressure vessels?

Composite pressure vessels can offer such enormous benefits over metallic pressure vessels. There is some serious engineering in them.

Why are composites so successful for the right application?

Think composite and focus on strength, low mass and freedom from corrosion. Composites have brought to life some incredible improvements to even mainstream consumer products. High performance golf clubs and tennis racquets have enhanced performance. We have seen some incredible advanced designs that move “what is possible” on where previous “conventional” materials couldn’t go. For example, the “tub” monocoque concept used in motorsport and on show in a McLaren sports car or the imagination in using a very high proportion of composites in a Boeing Dreamliner airliner to reduce mass.

Engineers might spoil the celebrations by worrying about manufacturing burdens, the serious implications of damage in use, complicated failure modes and the need for protection from UV. That’s just engineers. Even with the extra costs involved, the achievements already with composites have been staggering. The future also holds so much promise.

Two McLaren monocoque “tubs” in composites for their performance cars. Read more at Composites in Manufacturing website here: https://www.composites.media/mclaren-celebrates-carbon-fibre-heritage

Why would you use a composite pressure vessel?

Using composites to produce lightweight pressure vessels seemed such a strong application from day one. If high mass is the really big issue with metallic pressure vessels in certain applications, the prospect of using the strength of composites to form or reinforce a pressure vessel and achieve light weight is massive win. Think diving air bottles, fire extinguishers and any pressure vessel used on aircraft or in space. Mass is a real key issue in so many applications. A very simplistic starting point might have been “if my metallic pressure vessel is already cylindrical, why not make the metal lighter and reinforce it with composite?” Things are never quite that easy, but that was one starting point “let’s strengthen our existing cylindrical vessel.” Here we are homing in directly on Composite Overlaid Pressure Vessels or COPV’s. That’s a cylindrical pressure vessel like a gas bottle but could easily include spherical vessels or larger diameter short length vessels like a hydraulic accumulator.

Then the means of manufacturing composite vessels also makes the application attractive. One widely used composite manufacturing process is winding filaments around a former using a winding machine. If you are winding around a former that is revolving on an axis, you have amazing control over the angles at which fibres are laid down as layers are built up. That controls the strength of the composite. There is a lot of material on the web. Look at Add Composites for a proper description of composite structure production using a filament winder.

If strength and low mass aren’t in themselves massive drivers to develop this technology, the application for corrosion free pressure vessels is another whole area. For example, for reverse osmosis system pressure vessels, composite construction is so attractive in addition to low mass because of the negligible corrosion. So many applications including reverse osmosis systems now use composite pressure vessels as their corrosion resistant solution.

Who invented composite pressure vessels?

Keep in mind that in the mid 1850’s both the French and British reinforced their gun barrels with wound wire. If the barrel is a “pressure vessel” and winding the wire round in tension meets the basic definition of a composite, then we would say these were early composite pressure vessels and highly effective. The web has more information.

Why might you be less enthusiastic about using a composite pressure vessel?

We are Engineers, so always see problems.

Pick the right application and you have an amazing product. It won’t be cheap. That seems to be inherent in the product, even if you could drive down costs. Feel free to correct us on that. The manufacturing process is quite laborious and demanding. Could they ever be incredibly cheap? Tell us. So, we are talking comparing metallic and composite vessels like with like. As we have already observed, composite design on other products can allow you to achieve what previously was unthinkable, so cost then just becomes one of many inputs.

Metallic pressure vessels are relatively easily produced compared with the difficulty in controlling the manufacture of composites and addressing assorted failure modes on composite. We will briefly cover these issues later and talk about testing. If you are involved in testing composite vessels or simply interested in the engineering, look at our white paper How do I pressure test my composite overwrapped pressure vessels or COPV? There we discuss in more detail failure modes, design and manufacturing standards plus the complex testing involved. We have kept things simple here in this more general discussion.

It is also worth noting that damage to a composite pressure vessel isn’t to be taken lightly. It can cause premature and dramatic failure if repair is attempted and it isn’t effective. Maybe there are some applications where you simply wouldn’t go composite as damage is too risky.

Let’s step back to a couple of basics before going too far.

What is a composite? 

This is a basic question. The definition would be a structure formed from two or more different materials and providing one assembly with properties superior to the constituents. Our interest is fibre composites. In this discussion, a composite is fibres of one material in a matrix of another material. Medieval builders might have used horsehair in render to enhance properties. Today look at glass or carbon fibres in an epoxy or polyester matrix. The web has very comprehensive resources. Composites are just such an important technology and have already found massive application, bringing about transformation on lots of products. The big benefits are low mass and high strength. Low mass is effectively cutting the energy used in moving things around. A very specialist benefit is a low radar signature compared with metallic components, so “stealth technology” has big implications for military applications. Open up the prospect of manufacturing the impossible is truly exciting. Things that were beyond conventional technologies. For example, manufacturing drones that are light enough to be powered by electric motors have been brought to fruition using composites. Battery technology has been big as well, but composites are there. Composites have already made their mark and will continue to be a major area of growth.

What is a pressure vessel? 

We are happy to provide our definition. A container for fluid that is pressurised. Sometimes one homogenous material that forms a leak tight receptacle or you might achieve the same result with a flexible bag within rigid walls to constrain the volume. The web is good on types of vessel. Let’s move on to TYPES of pressure vessel, as this is our starting point.

What are the different types of composite pressure vessels?

We have a second white paper that digs into the testing of composite pressure vessels and what kit we supply to manufacturers. Vessels are classified as different types, as a starting point.

Metallic pressure vessels are a well-understood legacy product that has been around since the Industrial Revolution. A metallic vessel is classified as type 1. Composite pressure vessels can be manufactured in different ways and are classified by different numbers, these being 2,3,4 or 5. These four types would be described as Composite Overwrapped Pressure Vessels or COPV’s. The starting point is that to produce a closed pressure vessel, filaments are wound onto some sort of former on a machine. You could use a metallic pressure vessel and reinforce it by winding composite filaments around it. The metallic vessel has its own strength. Or you use a non metallic former such as a plastic moulding and the finished vessel gains all its strength from the composite material. Type 2 and 3 vessels are composite wound over a metallic former and type 4 is composite over a non metallic former from HDPE or similar. Type 5 is a very advanced vessel that uses no former left in the vessel and forming part of it after production. The vessel fluid is in contact with the composite rather than there being a barrier of metal or polymer. Type 2 would be a basic overwrapping on the diameter of the metallic former whereas type 3 is a complete overwrapping. Advanced Structural Technologies have an excellent explanation of these types and a tabular presentation on features and benefits.

Why take pressure vessels very seriously?

The simple answer is danger to life. You are storing energy. In some systems, the issue of the implications of catastrophic failure arises if the energy stored is lost. If a pressure vessel leaks or bursts the results can range between annoying and inconvenient through to a massive danger to life. Add in heat, pressure and large volumes such as steam boilers and failure is very high risk. Or massive numbers of one design that has high inherent risk should it fail. Think vehicle fuel tanks working on very low pressures but with lots of pressure cycles and highly inflammable vapour. And probably lots of weird and complicated failure modes if a vehicle is in use for 15 years in the massive range of environments on earth.  If there is a single point of failure and high risk to life, that is a significant risk that should be tested. Think of the oxygen pressure vessel bursting on Apollo 13, although we are told that was an explosion in the reservoir. New technology or a new application for materials such as composites brings new risks to be assessed. There are always uncertainties in new designs and materials. In our more detailed testing paper, we do consider the various manufacturing and testing standards for composite vessels.

On the most basic level, the forces involved in pressure vessels can be enormous. If you are looking at a manufactured metallic reservoir that is pressurised, you can quickly see the challenges. If Force = Pressure x Area, even a relatively small pressure over a large area translates to high forces. For example, 10 bar/145psi acting over a 100mm x 100mm (4×4 inch) panel generates 10KN or 2243lbf/1 ton. Hence, a pressure vessel fabricated from flat panels has very limited pressure capacity. Any vessel like this could potentially burst very easily if suddenly over pressurised. Translate this to a spherical or cylindrical vessel and you have the much greater inherent strength in the design as you can benefit from hoop tension, but those high forces are still there.

How does a composite pressure vessel fail? What is the difference between a metal and composite vessel bursting?

A metallic vessel relies on the properties of metallic materials. The material deforms elastically up to a point. Add pressure and it expands. Remove the pressure and it contracts to its starting point. That is elastic deformation. At a certain pressure, material will somewhere start to plastically deform, where it is permanent deformation and the material won’t spring back. Pressure vessel people won’t tolerate this, as at some level of plastic deformation, metallic compounds simply fail. Material somewhere will tear or blow out and the pressure is dramatically released. On a new metallic pressure vessel, the prime concern is excessive pressure that causes leakage, bursting or permanent deformation. In addition, the vessel will be designed carefully so that it isn’t prone to “fatigue,” where the material can fail at lower stresses having been deformed and released a massive number of times.

Add in composites and failure is very much more complicated. The implications of pressurising over lots of cycles is very much more important. We discuss this in more depth in our other white paper on testing and manufacturing standards.

The process of laying up composites is inherently a difficult process with a lot of parameters that need controlling. Even the mode of failure is complex.  On the most basic level, individual fibres of the reinforcement material can break. You would lose your strength. Another mode of failure at this micro level is fibres breaking their bond with the matrix that they are supported in. If you are relying upon two different materials to be stronger than the sum of the individual constituents, your mixture is no longer homogenous. You will lose your strength. At a larger level, the matrix itself can crack due to stress and start a failure event. Finally, another failure mode is delamination where a small area of cracking between one layer of fibres and another can lead to whole layers of fibre lamination cracks relative to the next layer and a peeling effect. If you are winding onto a former, there is a bond between the composite matrix and this former material. There are good web resources looking at modes of failure for composites. It is just much more complicated than metallic vessels. There, welding is a “special process” in quality terms where you cannot simply inspect the finished product to verify the welding. You will carry out non-destructive testing. Move to composites and the process is a massive “special process.”

The effect of a large number of pressure cycles causing a metallic vessel to fatigue is an issue but limited by careful design. Move to composites and the combination of low or high temperatures plus large numbers of cycles is absolutely key to performance. Temperature range is added into the mix. This is a risk on these vessels. As we have already observed, a vessel bursting is potentially so dangerous. If you have mass applications for these vessels like gas bottles or liquified natural gas tanks in vehicles, you can’t tolerate a high number of accidents. Hence, the tight regulation on composite pressure vessels.

We have talked about a relatively costly manufacturing process. The requirements for approval under various standards that involves a lot of complex testing will add yet more to cost. Then on top of that, a manufacturer would use something like our own Micropac® PTR-A programmable hydraulic cycle tester to test individual units after manufacturer then a sample of production to destruction. That’s a cost effective way of basic cycle testing without the extra capabilities on ramping and temperature control. But the basic cycle test is invariably a valid production test process. Testing is a complex subject.

Why is the future for composites so massive?

In so many products, reducing the mass and often the space envelope saves large amounts of energy in moving them around. A lorry full of composite gas bottles will weigh a lot less than metallic vessels. Or composites used in the automotive sector should reduce fuel consumption. Add in our previous point that composite construction may allow you to design solutions to problems that were insoluble previously. Or make something massively superior.

We’ve picked composite pressure vessels as our particular interest. We make the kit for hydrostatic pressure testing and programmable cycle testing for manufacturers of composite pressure vessels. We have briefly highlighted the burdens in the manufacturing process for composites and specifically the extra complication over metallic vessels in testing. Invariably, these same issues will feed through to other composite products. Pick the application and composites are still the future.

Sarum Hydraulics are the experts on pressure testing for over four decades.

Talk to us about your pressure testing application. We can tell you how our Micropac® Hydraulics can meet your needs. After over four decades, we are the experts.