Pressure vessel

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Steel Pressure Vessel
Steel Pressure Vessel

A pressure vessel is a closed, rigid container designed to hold gases or liquids at a pressure different from the ambient pressure. The end caps fitted to the cylindrical body are called heads.

In addition to industrial compressed air receivers and domestic hot water storage tanks, other 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 under pressure, 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 like Lloyd's, Germanischer Lloyd, Det Norske Veritas, Stoomwezen etc.

Contents

[edit] Shape of a pressure vessel

Theoretically a sphere would be the optimal shape of a pressure vessel. Most pressure vessels are made of steel. To manufacture a spherical pressure vessel, forged parts would have to be welded together. Some mechanical properties of steel are increased by forging, but welding can sometimes reduce these desirable properties. In case of welding, in order to make the pressure vessel meet international safety standards, carefully selected steel with a high impact resistance should be used. Most pressure vessels are arranged from a pipe and two covers. Disadvantage of these vessels is the fact that larger diameters make them relatively more expensive, so that for example the most economic shape of a 1000 litres, 250 bar pressure vessel might be a diameter of 450 mm and a length of 6500 mm.

[edit] 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

M = {3 \over 2} p V {\rho \over \sigma}

Where M is mass, p is pressure, V is volume, ρ is the density of the pressure vessel material, and σ 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 ρ / σ, and very cold helium for best possible M / pV.

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.[citation needed] Thus if the walls are made of the same material, the spherical tank can hold twice the pressure of the cylindrical tank, or at the same pressure, the spherical tank wall can be half the thickness.[citation needed]

[edit] Stress in thin-walled pressure vessels

The stress in a thin-walled pressure vessel in the shape of a sphere is:
\sigma_\theta = \frac{pr}{2t}
Where σθ 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 20 times larger than the wall thickness.[1]

The stress in a thin-walled pressure vessel in the shape of a cylinder is:
\sigma_\theta = \frac{pr}{t}
\sigma_{long} = \frac{pr}{2t}
Where σθ is the hoop stress, or stress in the radial direction, σlong 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.

[edit] Design Standards

  • BS 4994
  • ASME Code Section VIII Division 1
  • ASME Code Section VIII Division 2 Alternative Rule
  • ASME Code Section VIII Division 3 Alternative Rule for Construction of High Pressure Vessel
  • BS 5500
  • Stoomwezen
  • AD Merkblätter
  • CODAP
  • AS 1210

[edit] See also

[edit] External links

[edit] Further reading

  • Megyesy, Eugene F. (2004, 13th ed.) Pressure Vessel Handbook. Pressure Vessel Publishing, Inc.: Tulsa, Oklahoma, USA. Design handbook for pressure vessels based on the ASME code.

[edit] References

  • A.C. Ugural, S.K. Fenster, Advanced Strength and Applied Elasticity, 4th ed.
  • E.P. Popov, Engineering Mechanics of Solids, 1st ed.
  1. ^ Richard Budynas, J. Nisbett, Shigley's Mechanical Engineering Design, 8th ed., New York:McGraw-Hill, ISBN 978-0-07-312193-2, pg 108
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