What is radial force? Why do so many people fall on this
Anyone who designs pipelines knows that the biggest function of metal expansion joints is to absorb heat displacement. However, when the radial force was mentioned, many people began to be confused. Radial force, to put it bluntly, is the force that pushes the bellows perpendicular to the axis of the pipe, from the center outward or from the outside inward. Wrong direction, light bulge, worse bracket collapse, pipeline twist. Two days ago, I met a customer. A DN600 steam pipe was equipped with a universal corrugated expansion joint. As a result, the radial force was not calculated accurately. After three days of production, the corrugated pipe bulged like a toad's belly. Alas, this kind of thing is not rare.
Radial forces under internal pressure: Outward expansion is mainstream, but not so simple
As soon as the pressure inside the pipe comes up, the bellows expands outward like a balloon-this is the most intuitive direction of radial force, pointing from the central axis to the outer wall. But don't think it's just evenly outward. withUniversal corrugated expansion jointFor example, the radial force generated by internal pressure will make the wave peak expand outward and the wave trough contract inward. The magnitude of this force is directly related to the pressure and wave diameter. If the medium is high temperature and high pressure steam, the radial force has to be doubled instantly. What's more troublesome is that this force will be transmitted to the whole pipeline system, and if the constraint is not good, it can push the fixing bracket askew.
Don't ignore end effects
Where the ends of the bellows are close to the end pipe, the radial force distribution will change abruptly. Stress is concentrated there, and the ripples are more likely to crack. So a lotHigh temperature axial expansion jointThickened walls or reinforcing rings may be provided at the ends in order to hold the non-uniform radial forces of this piece.
Radial force reversal under displacement condition: axial compression, transverse tension, direction change accordingly
Internal pressure is only the basic working condition, and what really makes the radial force "discolor" is displacement. When the pipe is heated and elongated, and the expansion joint is axially compressed, the relative positions of the peaks and valleys of the bellows change, and the direction of the radial force will be partially reversed-the place that originally wanted to be pushed out may be retracted inward at this time. Conversely, if it is a lateral displacement (such as a pipe route turning), the radial force will become a complicated state of "pulling and pressing at the same time". At this time, useCompound hinge transverse expansion jointOrLarge tie rod expansion jointCan effectively restrain and avoid the radial force runaway.
I've seen a case in the cement industry, usingMetal Corrugated Expansion Joints in Cement IndustryOriginally, the axial displacement was handled well, but the radial force reversal was not considered after the lateral displacement was added, and the spacing between the guide brackets was too large, so the bellows was directly twisted into a twist. Tsk, it's more expensive to fix than to buy a new one.
Differences in Radial Force of Different Types of Expansion Joints: From Universal Type to Pressure Balance Type
Different structural designs, the performance of radial force is very different.
- Universal corrugated expansion joint: The radial force caused by internal pressure is the largest. If there is no guide tube, the bellows will easily become unstable under high pressure.
- Straight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion joint: Most of the internal pressure thrust is offset by balancing the bellows, and the radial force mainly comes from media flow disturbance and installation deviation, which is relatively easy to control.
- Compound hinge transverse expansion joint: The radial force is concentrated on the hinge structure, and the bellows itself is more stressed, but special attention should be paid to the fatigue life of the hinge.
- External pressure single axial expansion joint: On the contrary, its bellows is compressed on the outside, and the radial force generated by the internal pressure is squeezed inward, in the entire reverse direction. Don't get confused when designing.
You see, it is also called an expansion joint, and the direction of radial force varies widely. When you don't see clearly when you select the model, you have to cry at the scene as soon as you sign the drawings.
Common consequences of misdesigning radial forces: bulging, twisting, stent collapse
The radial force is not counted correctly, how serious are the consequences? Tell me a few real things:
- bulge: The internal pressure radial force is too large, the bellows peak excessively expands outward, and the material forms permanent bulge after yielding. Common inLarge diameter thick wall expansion jointAlthough the wall thickness is large, the stress is concentrated in the zone of abrupt curvature change.
- Twisted: The direction of the radial force is inconsistent during lateral displacement, causing the bellows to twist like twisting a towel. Generally occurs in the middle of long pipelines, when the spacing between guide brackets exceeds the standard.
- Stent collapsed: The radial force is transmitted to the fixed bracket through the end tube, and if the bracket is not designed according to the thrust, it will tear the base directly. There was a desulfurization flue project last year, usingDesulfurization flue gas baffle doorAndNon-metallic expansion jointCombination, just because the radial force is not counted, the fillet weld of the steel support is broken.
These problems can actually be avoided by preliminary calculation. The point is, you have to know exactly where the radial force is pushing.
How to judge the direction of radial force during installation? Look at arrows, calculate thrust, set guidance
Don't have time to do finite elements for on-site installation? There are shortcuts, too.
First, look at the arrows.Products in the station (such asUniversal corrugated expansion joint) Before leaving the factory, arrows are basically made, and the direction of the arrows is the flow direction of the medium, which also corresponds to the main direction of the radial force after installation-the flow direction side is the peak expansion side, and the reverse side is the trough contraction side. Don't act backwards.
Second, count thrust.The internal pressure thrust formula is not complicated: F = p × A (A is the effective area of the bellows). However, the radial force has to be multiplied by a coefficient, which is related to the shape and height of the wave. In the stationStiffness and Calculation Formula of BellowsThe detailed algorithm is listed in the question and answer, so just apply it directly.
Third, set the guidance.Whether the radial force can be controlled or not, the guide bracket is the key. normalUniversal expansion jointThe first set of guide brackets is required to be Direct buried (fully buried) type expansion jointIt is another set of calculation logic to consider the influence of soil lateral constraint on radial force.