Carbon Fiber Gas Cylinder
Composite Carbon Fiber Gas Cylinder made of continuous fiber-reinforced plastics do have weight advantages over those made of metal. This makes them suitable for mobile use such as storage units in breathing apparatus for emergency services, fuel storage for gas-powered vehicles, or hydrogen transportation in battery vehicles or tube trailers even as PCP air guns. Weight advantages are most significant when carbon fiber material is used. Unfortunately, these gas cylinders also suffer from high production costs, particularly if they are made from Carbon fiber.
Compared to metal cylinders the determination of safety and reliability of composite cylinders is more complex, which increases the effort during the approval process and for manufacturing quality control. The strength properties of composite gas cylinders are known to depend on a larger variety of design and production influences than the properties of cylinders made of metal. Also, composite material degradation is more complex, because of different failure mechanisms, which are mainly dependent on time and temperature.
Gas Cylinder Testing
Gas cylinders have large amounts of stored energy. They must be made to appropriate standards and are regularly maintained to ensure safety. All gas cylinders are subject to Statutory regulation. It is required that gas cylinders are inspected and tested on a routine basis. This assures that each cylinder is fit for purpose. The periodic inspection and testing of a gas cylinder is an essential requirement for its continued and safe use. There are few items of industrial equipment that last for so many years. The safety record of these cylinders is excellent and reflects the integrity of the design, manufacturing, and subsequent maintenance processes.
Requirements for different standards and certification
There is a multitude of standards and regulations for pressure receptacles that are aiming at various applications and valid in various geographical limits. The gas cylinder undergoes different tests and reviews such as burst pressure testing, leak tests, and pressure load change tests are parts of suitability testing and quality monitoring in ongoing production. All those regulations have more or less varying safety factors. ISO11119-2 is applicable worldwide for the transport of dangerous goods (TDG) in fully wrapped gas cylinders with load-bearing liner, while ECE 406/2010 is the current code for fuel storage in hydrogen vehicles in the EU.
Besides materials testing and initial pressure testing of each gas cylinder, samples from every production batch will be employed for destructive testing. EC 406/2010 demands load cycle testing of one cylinder out of every production batch. Only if 10 production batches have passed batch testing successfully, load cycle testing can be reduced to one cylinder per 5 production batches. The size of production batches is limited to 200 cylinders plus cylinders for testing in EC-Regulation 406/2010. If a tested cylinder does not fulfill the requirements, the reason for this failure has to be investigated. In a simplified way, the relevant regulations permit repetition of the test with a different cylinder, if a problem can be traced back to the individual failed cylinder. If the repeated test is a success, the production batch can be admitted to the market. If a systematic problem can be found for the whole production batch, the whole batch has to be discarded.
Overall, the probability to pass an approval or batch test depends on the mean value and scatter of a population. Another influence is the “lucky hand” in picking the test specimens.
Use of Composite Gas Cylinder
- Compressed Air
- Oxygen
- Valve Removal and Insertion
- PCP Air Gun
- Paintball
Damages To The Composite Gas Cylinder
Abrasion Damage:
The cylinder rubbing against a harder object or surface or in extreme cases by grinding causes this type of damage. This is typified by the removal of material from the surface. Scuffs, removing paint from the surface of the cylinder, would be considered minor abrasion damage. Abrasions would involve greater wearing away of the surface of the cylinders and typically numerous fibers would be visible. A flat spot on the surface of the cylinder could indicate excessive loss of the composite layer.
Cut Damage:
Cuts are caused by contact with sharp objects, surface edges, or corners in such a way as to cut into the composite, effectively reducing its thickness at that point.
Impact Damage:
Impact damage is caused by the cylinder coming into contact with edges or corners of objects. This can come about from dropping the cylinder or the cylinder being involved in some kind of collision. Impact damage can be observed in the form of dents, as small hairline cracks in the epoxy resin, or by delamination of the composite overwrap.
Heat / Fire Damage:
Heat or fire damage is shown by discoloration, charring, burning, or melting of the cylinder, paint labels, or valve materials.
Chemical Damage:
A chemical attack would appear as deterioration of the paint coating or dissolution of the epoxy resin surrounding the fibers. In other instances where solvents are involved the cylinder surface may become sticky when touched. Some acids e.g. sulphuric and hydrofluoric acid are known to attack glass fiber and so where contact with acids is known, the cylinder should be de-pressurized and Worthington contacted for guidance.
Crack Damage:
The Crack is situated under the final layer of glass fiber and as a result, there is a localized area, which is slightly raised from the rest of the cylinder. Sometimes a circumferential crack can be observed at the actual edge of the label, which is typically 5-10mm into the painted region above or below the cylinder label. This has no impact on the integrity of the cylinder and repair is not necessary.
Neck Damage:
A small circumferential crack may appear in the composite material between the cylinder body and the neck, which in some circumstances can be seen to open up during filling. This crack is the boundary between the neck wrap and the cylinder overwrap and is not structurally critical. Repair is not necessary, but the crack may be repaired by filling with a commercial room temperature cure two-component epoxy resin system. This can be carried out more easily when the cylinder is in the filled condition.
