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A pressure vessel is a closed, rigid, container designed to hold gases or liquids at a pressure different from the ambient pressure. Some examples of pressure vessels are: diving cylinder, recompression chamber, distillation towers and many other vessels in oil refineries and petrochemical plants, nuclear reactor vessel, habitat of a space ship, habitat of a submarine, pneumatic reservoir, hydraulic reservoir, rail vehicle airbrake reservoir, road vehicle airbrake reservoir and storage vessels for liquified gases such as ammonia, chlorine, propane, butane and LPG. In the industrial sector, pressure vessels are designed to operate safely at a specific pressure and temperature, technically referred to as the "Design Pressure" and "Design Temperature". A vessel that is inadequately designed to handle a high pressure constitutes a very significant safety hazard. Because of that, the design and certification of pressure vessels is governed by design codes such as the ASME Boiler and Pressure Vessel Code in North America, the Pressure Equipment Directive of the EU (PED), Japanese Industrial Standard (JIS), CSA B51 in Canada and other international standards.
Scaling No matter what shape it takes, the minimum mass of a pressure vessel scales with the pressure and volume it contains. For a sphere, the mass of a pressure vessel is Where is mass, is pressure, is volume, is the density of the pressure vessel material, and is the maximum working stress that material can tolerate. Other shapes besides a sphere have constants larger than 3/2, although some tanks, such as non spherical wound composite tanks can approach this. As can be seen from the equation, there is no theoretical efficiency of scale to be had in a pressure vessel; and further, for storing gases, tankage efficiency can be easily shown to be independent of pressure. So, for example, a typical design for a minimum mass tank to hold helium (as a pressurant gas) on a rocket would use a spherical chamber for a minimum shape constant, carbon fiber for best possible , and very cold helium for best possible . A spherical tank has less surface area for a given volume than any other tank shape. Also, the hoop stress in the wall of a sphere is half that of a cylinder at the same pressure. Thus if the walls are made of the same material, the spherical tank can be used to twice the pressure of the cylindrical tank or, at the same pressure, the spherical tank wall can be half the thickness. Stress in Thin-walled Pressure Vessels The stress in a thin-walled pressure vessel in the shape of a sphere is: Where is the hoop stress, or stress in the radial direction, p is the internal gage pressure, r is the radius of the sphere, and t is the thickness. A vessel can be considered "thin-walled" if the radius is at least 5 times larger than the wall thickness. The stress in a thin-walled pressure vessel in the shape of a cylinder is: Where is the hoop stress, or stress in the radial direction, is the stress in the longitudinal direction, p is the internal gage pressure, r is the radius of the cylinder, and t is the wall thickness. See also Further reading | ||||||||
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