Let's talk about what is expansion joint plane instability
To put it bluntly, when the bellows is subjected to internal pressure, the corrugations are not axially compressed or stretched honestly, but like a flattened can, the plane direction of the corrugations is twisted, wrinkled or even bent laterally. This thing is called plane instability in engineering, and it is also called "bellows instability" in the industry. Have you ever seen the kind of ripples that were originally neat in circles suddenly turn into crooked twists? That's the typical symptom. Understanding this can avoid big problems in the pipeline system.
For example, if the pressure of the corrugated expansion joint used in the power station industry is not accurately calculated in the design, it will change from a ring to an ellipse with corrugation, or even partially deflate. That's not a cosmetic problem, it's a precursor to structural failure.
Where did the plane instability come from?
Excessive internal pressure and insufficient design stiffness. For example, when selecting the type, the working pressure is not accurately calculated, or the corrugated plate is too thin, or the ratio of wave height and wave pitch is unreasonable. When you think about it, bellows are inherently dependent on the geometry of corrugations to absorb displacement. Once the pressure exceeds its critical value, it is like a human spine suddenly bends under excessive compression-it can't hold up. In addition, if there are additional lateral forces or torsional stresses during installation, instability can also be induced. Two days ago, I met a customer, saying that the general-purpose corrugated expansion joint used in power stations was crooked after half a year of use. When I checked the drawings, the wave number was large but the wall thickness didn't keep up, and the typical pressure exceeded the limit.
The external pressure single type axial expansion joint was selected, but the actual operating pressure exceeded the design pressure by 1.5 times, and the instability probability increased greatly. The design pressure is 1.0MPa, but when it actually runs to 1.6MPa, 80% of them will have problems-this is not metaphysics, but common sense of mechanics.
The dangers of plane instability are no joke
Failure and air leakage are mild, and pipe burst is severe. Especially in some high-temperature and high-pressure pipelines, such as corrugated expansion joints and high-temperature axial expansion joints used in power station industries, once they become unstable, the corrugation breaks and leads to media leakage, which may directly cause shutdown accidents and even personal safety risks. Do you think the loss is big? Moreover, instability often does not happen suddenly, and there will be deformation accumulation in the early stage, but once it passes the critical point, the collapse speed is extremely fast. So don't wait until you hear an abnormal noise or see a noticeable distortion before dealing with it, it's already too late.
A cement factory used a batch of metal corrugated expansion joints in cement industry. In the third month after installation, it was found that the corrugations were partially depressed, and the operator didn't take it seriously. As a result, it burst directly on the sixth day, and high-temperature gas was ejected. Fortunately, no one came close at that time. This matter was later resumed, that is, the wave height ratio exceeded the standard and the plane stability was not enough.
How to judge whether the expansion joint has plane instability?
The simplest and crudiest way-eyes. If it is found that the corrugations of the bellows have irregular wavy wrinkles and local depressions, or the whole bellows rotates like a twist, it is basically unstable. More professionally, you can measure the change of outer diameter and wave pitch of bellows, or monitor it with a strain gauge. In addition, it is also critical to compare the design parameters: if the actual operating pressure exceeds 1.5 times the design pressure, the probability of instability increases greatly. For example, the external pressure single axial expansion joint, with a design pressure of 1.0MPa, actually runs to 1.6MPa, and 80% of them will have problems.
Scratches or abnormal wear on the inner wall of the guide tube. Because the corrugation deforms after instability, it will rub the guide tube. Therefore, regularly checking the state of the guide tube is also one of the basis for judgment. The product information of this site mentions the specific functions of the expansion joint guide tube, one of which is to protect the corrugation, but it can also reflect the health of the corrugation.
To prevent plane instability, the core is four words: choose the right one and use it well
When selecting the type, don't just look at the pressure grade and diameter, but also check the stability coefficient of the bellows. Like straight pipe pressure balance expansion joint or double hinge transverse expansion joint, because the structure itself has constraints, the risk of instability will be lower. However, if it is a general-purpose corrugated expansion joint or a large-diameter thick-wall expansion joint, special attention should be paid to the matching of wave height and wall thickness. Install a guide tube when necessary-the guide tube can not only protect the inner wall, but also reduce the direct impact of the medium on the corrugation, indirectly reducing the risk of instability. Concentricity should also be ensured during installation, and the pipe should not be pulled and dragged.
In some scenarios, such as the double-hinged expansion joint of air-cooled island vacuum pipeline for air-cooled island vacuum pipeline or the high-temperature axial expansion joint for high-temperature pipeline, the stability check must be done during design, and it can't only rely on experience. In addition, although the materials of metal rectangular expansion joints and non-metal expansion joints are different, the principle of plane instability is the same-the instability of corrugated structure is a geometric problem and does not change due to the material.
The product drawings of many small factories are cheap with thin materials, or they are not designed according to standards. It doesn't take long for users to buy them back and use them before they go wrong. When making products in our industry, such as metal rectangular expansion joints and non-metal expansion joints, stability must be regarded as a hard index. If you find that the expansion joint on the pipe looks wrong one day, don't hesitate to stop it and check it quickly. After all, a pipeline costs millions and an expansion joint costs thousands of pieces. Is it a loss to scrap the whole line because of instability?
For expansion joints that have been installed, strain gauges can be attached to the bellows surface to record the deformation regularly. Once it is found that the strain value of a certain side corrugation continues to increase, it indicates that instability is occurring. This is much more reliable than observing it with the naked eye. Especially for high-risk scenarios such as power stations and chemical industries, early warning saves too much money than remediation afterwards.