1. Where does the radial force come from? Don't confuse axial with radial
Metal expansion joints are mainly designed to absorb axial displacement-the pipeline is elongated or shortened when it expands and contracts thermally, and the bellows is stretched and compressed in the axial direction, which is called axial displacement. However, once the pipeline system is transversely displaced (such as thermal expansion pushing at the corner of the pipeline), the equipment vibrates, or the installation is misaligned in the first place, radial forces can be generated on the expansion joint. The direction of the radial force is perpendicular to the centerline of the pipe, which, to put it bluntly, is to push or pull the bellows sideways. If this force is ignored, it can lead to destabilization of the bellows (bulging, lateral buckling), or directly pull the flange or the interface of adjacent equipment. In the actual case, a chemical pipeline uses an ordinary axial expansion joint, which ignores the radial force generated by the sinking of the pipe frame. Within three months, a big bulge bulged out on the side of the bellows-the equipment was lost and the construction period was missed.
2. To judge the direction of radial force, look at the pipeline direction and constraint mode
The direction of the radial force is no mystery – it always points in the direction of the lateral offset of the pipe due to thermal expansion or mechanical displacement. For example: a horizontal pipe has thermal displacement in the vertical direction (such as vertical upward thermal expansion), and the radial force on the expansion joint is vertical upward. However, in actual engineering, pipelines often have thermal displacements in multiple directions, which need to be accurately calculated with the help of stress analysis software (such as CAESAR II), or predicted by the tie rod and hinge structure on the expansion joint. The compound hinge transverse expansion joint (product number 24) in this station's product is a typical example of specially using hinges to bear radial force and only allowing transverse displacement-the hinge axis is perpendicular to the center line of the pipeline, the radial force is directly borne by the hinge bearing, and the bellows is only responsible for deformation and absorption of displacement. When you install this expansion joint, you must ensure that the hinge turns in the same direction as the expected displacement of the pipe.
3. Different structural expansion joints have completely different ideas of resisting radial force
- Universal corrugated expansion joint(Product No. 13): Its ability to resist radial force is very weak, so it must rely on the external guide bracket to bear radial force, and the radial force cannot be directly handed over to the bellows. Therefore, when choosing it, be sure to install guide brackets on both sides adjacent to the expansion joint, and the spacing is according to the design drawings.
- Large tie rod expansion joint(For example, straight pipe pressure balance type-Product No. 26, double straight pipe bypass pressure balance type-Product No. 28): The radial force is transmitted to the pipe support through the tie rod, and the bellows is only responsible for deformation. The function of the tie rod is to "block the knife"-refer to the common question and answer of this site "The function of the tie rod of the expansion joint". The tie rod itself does not absorb displacement, but it can limit the bellows from bearing excessive radial force.
- hinged expansion joint(e.g. double hinge transverse type-Product No. 24, single hinge): The hinge bearing is directly subjected to radial force, and the direction must be perpendicular to the hinge axis. When installing, the direction of the arrow on the expansion joint should be consistent with the expected direction of displacement-this question is clearly stated in the FAQ "What is the direction of the arrow on the expansion joint": the arrow represents the allowable main direction of displacement (axial or transverse), and the direction of the radial force is perpendicular to it.
Figure out these differences so that the selection will not overturn.
4. How serious is the consequence of ignoring the radial force during model selection?
Two days ago, I met a customer who chose an ordinary axial expansion joint on the pipeline in the cement industry. As a result, the lateral bulging of the bellows appeared after three months of operation. The reason is simple: the thermal displacement of the pipe produces a large radial force, and the expansion joint does not have a matching limit tie rod or guide bracket. The correct approach is: according to the stress analysis report, either install the guide bracket or replace it with a double hinge transverse expansion joint or a straight tube pressure balance expansion joint. The product page of metal corrugated expansion joint in cement industry of this station (product number 6) has strengthened design for such working conditions-such as thickening corrugated pipe, adding guide sleeve, and even built-in limit structure. Don't save that little selection effort, otherwise it will be a lesson of real money in the end.
5. How to ensure that the direction of radial force is not wrong during installation? Three Practical Key Points
- First, look at the arrow logo on the expansion joint.The direction of the arrow usually indicates the direction of media flow or displacement? In fact, the following question and answer have already given the answer: The arrows represent the allowable main displacement direction (axial or transverse) at the time of installation, and the radial force direction should be perpendicular to this direction. For example, if the arrow points in the axial direction, the radial force is the direction perpendicular to the pipeline.
- Second, how to adjust the tie rod nut?Refer to "How to adjust the expansion joint tie rod nut"-preloading or loosening the tie rod will change the distribution of radial force, which must be adjusted according to the drawing data, not by feeling. If the tie rod is too tight, it will limit the displacement absorption, and if it is too loose, it will not play a protective role.
- Third, the position of the guide bracket.A firm guide bracket must be set on the side corresponding to the radial force direction, and the spacing must meet the design requirements. If it goes off, no matter how good an expansion joint is, it won't hold up-because the bellows themselves are not designed to withstand radial forces, unless you choose a hinged type.
Tsk, I said all this, just to make you understand:Direction of radial force of metal expansion jointNot metaphysical, it is determined by pipe displacement and balanced by structure or brackets. Only by selecting the right model, installing the right direction and adjusting the tie rod can the pipeline system operate safely.