A pressure vessel is a closed container that is designed to hold the contents at a certain pressure. Gas or liquids can be kept at pressures different from the ambient temperature. Examples include hot water storage tanks and diving cylinders. Submarines and space ships are basically giant pressure vessels.
Pressure vessels are most commonly made of steel because steel is strong and can resist impacts. The cylinder can also be made of other metals, carbon fibers, or polymers. They are often lined with metal, ceramics, or other polymers. Lining protects the structural integrity of the pressure vessel and gives added protection against leaking.
The most stable design for a pressure vessel is a sphere-shaped tank. The pressure in a pressure vessel naturally bows the walls of the vessel out. A sphere-shaped vessel takes advantage of that natural tendency. Despite this, most pressure vessels are not sphere-shaped because a sphere is difficult and expensive to make. As a result, most pressure vessels are cylinder-shaped with rounded caps on each end.
Common forms of pressure vessels include thin-walled vessels, storage tanks, and transportable containers. Thin-walled vessels are those with a diameter that is 10 times or more the thickness of the wall. Storage tanks are a kind of super thin-walled vessel. Transportation vessels are mass produced thin-walled vessels. The least common type is the thick-walled vessel. This is a vessel with a diameter that is less than 10 times the thickness of the wall.
The stability of a pressure vessel is very important. A pressure vessel that is not structurally sound may leak or even burst under pressure. Depending on the type of liquid or gas inside the tank, leaks can cause poisoning, fire, or suffocation.
Violent burst failures can cause death from shrapnel or explosion. To guard against violent failure, many pressure vessels are designed to leak before the burst. This allows the pressure to equalize slowly and gives time for workers to fix or shut down equipment without injury.
To guard against death or injury by faulty pressure vessels, various countries have created codes of design to make sure that all pressure vessels are safe to use. In the United States, the codes governing pressure vessel design are listed in the American Society of Mechanical Engineers Boiler and Pressure code. The Canadian code is B51 of the Canadian Standards Association. Australia, England, and many other countries also have similar rules.