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.
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.
Pipe alignment guide
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.
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.
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.)
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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