Flexible metallic elements

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Basic forms moved with line elements.jpg

Rigid metallic piping systems usually consist of pipes , fittings and fittings . Flexibility is provided by flexible metallic line elements that are able to compensate for operational relative movements between the rigid line components.

Their main forms are:

A total of nine basic types of movable metallic pipe elements can be defined by further subdivision.

Complete systems with special functions

Metal hoses, metal bellows and expansion joints are used as flexible line elements in line systems, but can also serve as complete lines to connect devices and machines. They not only take on the general management functions of rigid systems, such as B. the guiding, conveying or suction of gaseous, liquid or solid substances, but also additional special functions based on their mobility:

  • Compensation of relative movements in the pipe system
  • Compensation of assembly inaccuracies
  • Compensation of forced movements, such as B. thermal expansion or foundation settlements
  • Damping of mechanical and acoustic line vibrations
  • Absorption of pressure surges in the pipe
  • Volume compensation of hermetically sealed systems
  • Heat exchange between internal and external medium
  • Measurement of pressure and temperature
  • Protection and shielding of electrical cables and light guides

Basic characteristics

Flexible metallic cables are in constant competition with the mostly cheaper flexible cables made of plastics or elastomer materials . However, only hoses, bellows and expansion joints made of metal are suitable for many industrial applications, especially where there are high requirements for resistance to temperature and thermal shock, tightness and pressure resistance. They are assigned to the terminology according to the machine elements and are characterized by a large variety of shapes and functions. In many areas of application they are therefore used as indispensable and mostly inconspicuous functional elements.

The following overview shows their common, but mostly different, basic properties:

  • Flexibility or mobility is the property of absorbing external loads due to changes in length or bending with an unrestricted management function. The three basic types of movement that metal bellows and expansion joints can perform, for example, are shown schematically in the bottom right.
  • Fatigue strength is the ability to reliably withstand repeated bending, changes in length and changes in pressure.
  • Compressive strength is the resistance to internal and external overpressure.
  • Leak-tightness is a relative property, because absolutely leak-proof parts do not exist. The requirement for tightness of a component always refers to the conditions at the later place of use. The leak test procedure, the amount of pressurization and the permissible leak rate must be determined accordingly. A distinction is made between different degrees of tightness, e.g. B. waterproof, gas-tight, vacuum-tight, diffusion-tight, etc.
  • Mechanical strength is the deformation resistance against external forces and moments. A distinction is made between tensile strength , shear strength , bending strength and torsional strength .
  • Temperature resistance is the resistance to high and low operating temperatures, which can range from absolute zero to over 1000 ° C. The significantly higher temperature resistance compared to non-metallic cables also includes high temperature shock resistance and fire or flame resistance.
  • Corrosion resistance is the resistance to chemical attack by the media on the inside and outside.
  • Maintenance-free means that, unlike other moving machine elements, neither control nor maintenance work is necessary during operation, in particular no lubrication to maintain mobility.

Metal hoses, metal bellows and expansion joints are graded according to nominal size (DN) and nominal pressure (PN). These standardized parameters ensure that different line elements fit together due to the same nominal widths and the same nominal pressure. The nominal width corresponds approximately to the clear inner diameter in mm.

Many manufacturers also offer special designs with special diameters and compressive strengths. The nominal pressure of flexible metallic elements always corresponds to the permissible operating pressure in bar at 20 ° C, whereby the nominal pressure decreases with increasing diameter.

Standards and regulations

International standards and regulations are available for the design, manufacture and testing of flexible metallic cables. The most important are:

  • ISO 15 465 for metallic wrapped hoses
  • EN ISO 10 380 for metallic corrugated hoses
  • EN 14 917 for metal expansion joints

as well as the EU directive 97/23 / EG (pressure equipment directive) with the associated product standard DIN EN 14 585-1. This guideline has legal status and must therefore be observed for deliveries within or to the European Union.

swell

  • Reinhard Gropp: Flexible metallic cables. In: The library of technology 188. Verlag modern industry, Landsberg Lech 1999, ISBN 3-478-93216-5 .
  • Carlo Burkhardt, Bert Balmer: Decoupling elements in vehicle technology. In: Die Bibliothek der Technik 237. Süddeutscher Verlag onpact, Munich 2008, ISBN 978-3-937889-73-3 .
  • The manual of metal hoses. Witzenmann, Pforzheim 2007, ISBN 978-3-926937-26-1 .