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bellows
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The flexible element of an expansion joint consisting of one or
more convolutions.
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bellows
expansion joint
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Any device containing one or more bellows used to absorb
directional changes, such as those caused by thermal expansion or
contraction of a pipeline, duct, or vessel
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bellows
side effects
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We have noted how placing a flexible metal bellows at selected locations in a
piping system can accomodate its thermal growth. There are some side effects which
occur when a pipe is cut in two and a bellows (flexible seal) is inserted to take up the
movement of the pipe:
Pressure Thrust: By its very nature of being flexible (to absorb
movement) a bellows will extend (straighten out) due to the line pressure. This
pressure thrust must then be absorbed by some means or the line will tear itself
apart. This force may be accommodated by anchoring the pipe or by using an expansion
joint which incorporates tie rods, hinges, gimbals, or pressure balancing bellows.
Spring Rate: In very low pressure applications the more
significant force may be the spring rate which is expressed in pounds per inch of
motion. Thus, as the pipe grows due to increasing temperature, the bellows will
resist compression by the force noted as the spring rate.
A comparison of pressure and force data to spring rate will show that it does
not require very much line pressure for pressure thrust to be the dominant factor of the
two in expansion joint applications.
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control rods
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Control rods are devices attached to the expansion joint with the
primary function of distributing movement between the two bellows of a universal
expansion joint. Control rods are not designed to restrain bellows pressure
thrust. (These devices do not serve the purposes defined by the Fluid Sealing
Association as applied to rubber expansion joints.)
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convolution/corrugated
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The smallest flexible unit of a bellows with total movement of a bellows
being proportional to the number of convolutions.
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covers
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Expansion joints require careful handling and must be protected from any
impact, weld spatter, etc. Before insulating an expansion joint, care must be
taken that foreign material is not trapped in the convolutions. It is suitable
to install a metal cover over the flanges and then wrap the insulation around it.
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directional anchor
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A directional anchor, or sliding anchor, is one which is designed to
absorb loadings in one direction while permitting motion in another. It may be
either a main or intermediate anchor, depending upon the application
considered. When designing a directional anchor, an effort should be made to
minimize the friction between its moving or sliding parts, since this will
reduce the loading on the pipe and equipment, and will ensure proper function of
the anchor.
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installation guidelines
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Installation of expansion joints require special attention to ensure
proper function.
Intermediate anchors are designed to withstand differences in spring
forces and friction forces. In the case of different pipe diameters, the anchor
must be designed to withstand the difference in pressure thrust of the two pipe
sections. Anchors at the ends of pipe sections are usually called main anchors
and must be designed to withstand the full pressure thrust of the bellows in
addition to other forces.
*A pipe section in which an expansion joint is installed must be
firmly fixed with anchors at both ends.
*Expansion joints should be installed adjacent to an anchor;
otherwise, a pipe guide must be installed on each side of the extension
joint.
*Pressure Thrust is the result of the bellows effective area
multiplied by the medium pressure. If the pressure changes, pressure thrust
changes.
*Forces transmitted to the pipe anchors are the sum of the spring
rate, friction forces of pipe guides, and pressure thrust of the bellows.
(Provisions for pressure thrust are not required if the expansion joint has tie
rods.)
When pipe movement requires the use of several axial
expansion joints, intermediate anchors are necessary. Only one expansion joint
can be installed between two anchors.
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intermediate anchor
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An intermediate anchor is one which divides a pipeline into individual
expanding pipe sections. Such an anchor must be designed to withstand the
forces and moments imposed upon it by each of the pipe sections to which it is
attached. In the case of a pipe section containing one or more bellows units,
these forces will consist of forces and/or moments required to deflect the
bellows unit plus the frictional forces due to the pipe moving over its guides.
The pressure thrust is absorbed by the other anchors or devices on the bellows
unit such as limit rods, tie rods, hinged restraints, etc.
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limit rods
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Limit rods are devices with the primary function of restricting the
bellows movement range. The limit rods are designed to prevent bellows
over-extension or over-compression while restraining the full pressure thrust in
the event of a main anchor failure.
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main anchor
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A main anchor is one installed at any of the following locations in a
pipe system containing one or more bellows:
1.) At a change in direction of
flow.
2.) Between two bellows units of different size installed in the same
straight run.
3.) At the entrance of a side branch from a main line.
4.) Where a shut-off or pressure-reducing valve is installed in a pipe run between
two bellows units.
5.) At a capped end of pipe.
A main anchor must be designed to withstand the forces and moments
imposed upon it by each of the pipe sections to which it is attached. In the
case of a pipe section containing an unrestrained bellows, these will consist of
the thrust due to pressure, the force required to deflect the bellows unit, and
the frictional force due to the pipe moving over its guides.
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pipe alignment guide
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A pipe alignment guide is a form of sleeve fastened to some rigid
part of the installation which permits the pipeline to move freely in only one
direction; i.e., along the axis of the pipe. Pipe alignment guides are designed
primarily for use in applications involving axial movement only.
Pipe Alignment Guides:
* An expansion joint should not support the pipe weight.
* Pipe sections must be properly guided in an axial direction by guides adjacent to the
expansion joint.
In installations with axial expansion joints, the pipe growth, or
expansion, should push the expansion joint axially. To assure this, the pipe
must be guided directly before the expansion joint. Clearance in pipe guides
should only be sufficient to allow the pipe tp glide easily. The distance
between the expansion joint and the first pipe guide should not exceed four pipe
diameters. The distance between the first and second pipe guides should not
exceed 14 pipe diameters, and the distance between any remaining guides should
be based on equations from the standards of the Expansion Joint manufacturers
Association.
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planar guide
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A directional planar guide is a pipe alignment guide modified to permit
limited movement and/or bending of the pipe in one plane. It is used only in
applications involving lateral deflection or angular rotation resulting 2- or
3-hinge piping configurations.
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tie rods
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Tie rods are devices with the primary function to restrain the bellows
pressure thrust. It should be pointed out that when tie rods are furnished on
expansion joints subject to external axial movement, they will only restrain the
pressure thrust in the event of an anchor failure; during normal operation, the
adjacent equipment will be subject to the pressure thrust forces. (These
devices are similar to, but do not serve the same function as "control rods" as
defined by the Fluid Sealing Association applicable to rubber expansion
joints.)
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torsion
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A metal bellows expansion joint should never be subjected to torsion.
This issue must be considered when welding the companion flange on the pipe
end.
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