Composite Material in Pressure Vessels

Pressure Vessels

Pressure vessels are used to contain pressurized fluids or gases. Most fluid-containing vessels are maintained at fairly low internal pressure, which means that tank walls can be relatively thin. Steel is the material of choice for most low-pressure applications unless the vessel will be in contact with corrosive media, either contained within it or in its external service environment. For such corrosion applications, composite materials are preferred.

For high-pressure storage of compressed gases, composite materials offer the most competitive strength-to-weight ratios. Applications include compressed natural gas (CNG), hydrogen fuel, paintball, and breathing air for personal use and emergency rescue. Within CNG, which this discussion will focus on, is a wide range of sub-segments and pressure vessel sizes. At the low end, passenger vehicles will have tanks in the 50 L range, whereas bulk hauling modules will have pressure vessels the size of shipping containers. Although metal is dominant with more than 90% share, with the current number of CNG pressure vessels on the market close to 30 million and an annual growth rate of greater than 25% in select regions, the 10% share belonging to composites represents an attractive opportunity.

Vessel Types

There are four general pressure vessel types, described in Table II, that are recognized by domestic and international design and qualification standards.

Type 1 vessels, which are all steel (typical) or aluminum, are generally the least expensive to manufacture, but are the heaviest. Types 2–4 are composite vessels. Type 2 vessels have a heavy liner and utilize composite reinforcement only in the hoop or circumferential orientation. Types 3 and 4 have a full composite overwrap, providing both circumferential and axial reinforcement. The difference between Type 3 and 4 is in the liner. Type 3 cylinders have a metal (typically aluminum) liner that may share a small fraction of the internal pressure load. Type 4 cylinders have a plastic ([HDPE]) liner that serves only as a substrate for the load-bearing composite. In general, in order from Type 1 to Type 4, as Type number increases, price increases, and weight decreases. The indicative cost data in the Table II will change with raw material prices, but the relative cost ratios between the four vessel types should remain fairly constant.

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