Pressure vessels, just like many other pressure containers, are containers designed to hold gasses or liquids at a pressure that is different from the outside ambient pressure. Sometimes it is necessary to hold different materials at different pressure ratings to keep them from expanding, escaping, or otherwise changing. Pressure vessels are used in many different areas, from storing oxygen to containing steam.
Even though pressure vessels can be made in a variety of shapes, the traditional shape created by pressure vessel manufacturers is a cylindrical or rounded cylinder shape. Other than tradition, there is no real reason for the rounded design. A completely spherical shape has twice the strength of a cylinder, but the expense of creating spherical shapes and the awkwardness of storing a perfectly round container makes spherical pressure vessels nearly unheard of.
It is possible for pressure vessel manufacturers to create a wide variety of container materials for pressure vessels. There are many different approved containers in the national design and application codes. Most commonly, the containers are made from steel. Steel is an ideal material because it can withstand high pressures, will not dent or break easily, and is an inexpensive material to manufacture. Some other approved containers for pressure vessels include:
Composite materials: Composite materials made from a mixture of wood pulp, carbon fiber, and plastic polymers are sometimes used to create pressure containers. The mixture of this material is somewhat new, so there are not a lot of pressure applications as of 2012. However, the strength of the container is comparable with that of steel containers, and the weight of the container is much less, so we may see more composite pressure containers in the future.
PET plastic: PET plastic can hold lower-pressure materials. Even carbonated beverage containers are one type of pressure container that PET plastic is used to contain. The plastic must be strong enough not to burst under the pressure of the carbon dioxide inside the bottle.