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  • Writer's pictureNed Patton

Fabrication of a COPV and the Machines that Make Them


For this post I’m going to talk about how to make Composite Overwrapped Pressure Vessels – COPVs – possibly one of the most ubiquitous uses of carbon fiber / epoxy composites today. As I talked about in the first post in this series, COPVs are replacing metal pressure vessels at an accelerating pace. This is not only because they are stronger, lighter, and safer to operate than metal pressure vessels, it is also because the cost of carbon fiber has dropped precipitously in the last decade making them cost competitive with all metal pressure vessels.

COPVs are also in some respects a bit easier to make than a welded or seamless metal pressure vessel – especially for high pressure. This is because the liner, since it is just really a gas boundary, is much thinner than the wall thickness of an all metal pressure vessel. This makes the liner fairly easy to fabricate, and the equipment to make the liner in general is relatively inexpensive in comparison to the equipment required to spin a thick metal tube down to make a domed end.

The composite outer wrap is nearly always filament wound, and filament winding machines are largely automated devices which only need to be watched by an operator. And of course, this is where the fun is for a Mechanical Engineer like myself.

Filament winding machines come in all sizes and shapes, some large, some small, some that only make spherical COPVs, some that make long thin COPVs, and some that are exclusively used for making wound pipe and composite drill stem for the offshore oil industry. The pic above, and the one below show COPVs (the one below is actually a rocket motor casing) being made on filament winding machines.

The average size filament winding machine for making high pressure COPVs (3000-10,000 psi) are usually 10 to 12 feet long. These machines are in pretty much every composite shop that makes COPVs. The machines that make drill stem can be as long as a little over 100’, and have a diameter capacity of as much as 10 feet.

But how does a filament winding machine work? There is a simple answer to this and one that is slightly more complex. First, most COPVs are lined with either a metal – usually aluminum – or plastic. In the case of the plastic liner the plastic acts as mostly as a gas barrier since the plastic is usually something fairly soft like nylon or polyethylene. In the case of a metal liner – specifically aluminum – the metal liner material carries load in the axial direction of the pressure vessel, and the composite carries some load in that direction, but nearly all of the hoop load.

A little very basic pressure vessel mechanics is in order for non-engineers. The stress in the hoop direction of a pressure vessel is twice the stress in the long direction of the pressure vessel. So, if the liner carries most of the long direction stress, and the composite carries most of the hoop direction stress, this makes for a very efficient pressure vessel. This is one reason that COPVs are popular.

But, back to making one of these things. What is usually done is to make the liner itself, and put it on the mandrel of the filament winding machine. Usually, a little pressure is put in the liner just to keep it from bending as the fibers are being wound onto it. The fiber and resin get applied by a moving head that has a big spool of bundled fiber wound onto it. The fiber is pulled through the head, down into a bath of resin where it gets completely wet with resin, and then laid down on the liner through a nozzle-like fiber guide using quite a bit of tension to make sure that there is good consolidation of the composite. The angle that the fiber gets laid down on the liner is carefully controlled and is based on the speed of the sideways motion of the fiber head compared to the speed of rotation of the pressurized liner.

All of this angle control is fully automated in modern filament winding machines, and these angles can be held to extremely tight tolerances.

For each COPV, there is what is called a “winding schedule” which comes from the requirements of the design (you can see this in the steps in the pic above). This winding schedule dictates what angle the fiber will be with relation to the liner for each wrap of fiber on the pressure vessel. And, most COPVs use a mix of what are called “helical” wraps which are at some angle off of perpendicular to the axis of the liner and "hoop" wraps which wrap the fiber around the cylindrical section of the COPV.

Once the liner is wrapped, the COPV is then put in a vacuum bag and cured in an autoclave to make the resin solid. Then you have your completed COPV. During this process the liner is kept under enough pressure that the COPV remains straight, and also to counteract the fact that carbon fiber shrinks as you heat it. The last step in all of this - at least for metallic lined COPVs - is to perform what is called an “autofrettage” which basically is to pressurize the finished COPV to a pressure that will yield the liner material slightly so that when the pressure vessel comes back down in pressure, the liner is in compression and the fiber overwrap is in tension. This prevents crack growth in the liner material. This is especially important for aluminum lined COPVs - the most common type.

In the next post in this series I’m going to talk a little about design of a COPV, and what decisions need to be made when the COPV designer first thinks about using a COPV. And, I think I can’t stress this enough, you need a well established set of requirements to be successful.

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