Causes of Metal Expansion Joint Deformation? Don't wait for the pipeline to tear to find an answer
Two days ago, I met an old customer, who called and said that there was an abnormal noise in the expansion joint bellows in the power plant, like something scraping in the pipe. The inspector dared not get too close for fear of something going wrong. When the machine was stopped and disassembled, the bellows had already bulged a bag, and there were still a few fine cracks at the folds. If you delay it for another two days, the whole pipeline will have to be changed.
The deformation of the metal expansion joint is really not a small problem. Many people think that it is just a soft connection, which can be bent and stretched. When they see that something is wrong, it is often too late. Let's get it straight. Why does it deform? What should I do if I see the deformation?
Find out first: What exactly does the expansion joint deformation look like, is it bulging, twisting or flattening?
Deformation is not a single kind, there are three common types:bulge(Local bulge, like a tumor),Twisted(The axial twisting force is crooked),flattening(radially recessed). Bulging mostly occurs at the peak of the bellows, which happens when the medium pressure exceeds or the wall thickness of the bellows is corroded and thinned. The distortion is mostly due to the lateral displacement exceeding the design value, or the pipeline system does not make a reasonable guide bracket. Flattening, basically, the external pressure is regarded as the internal pressure in vacuum conditions, or it is smashed by external force during installation.
There is also a hidden deformation called "instability"-the bellows looks like there is no obvious bulge, but the wave distance is uneven. Some waves are close to each other, and some waves can be pulled away. This is the most insidious, because it can't be seen on the outside, but the fatigue life has been greatly reduced.
One of the culprits: the hidden danger left during installation-the cold tightening was not done correctly and the tie rod was not removed
Let's play back the most common metamorphosis cases. A steam pipeline in a chemical plant is equipped with a general-purpose corrugated expansion joint. Less than two months after operation, the corrugated pipe is twisted. The manufacturer sent someone to take a look, and the problem was there: the cold tightening amount was simply not enough during installation, and the pre-stretching was not pulled enough. As soon as the pipeline heated up, the expansion joint directly received the compression limit.
Another high-frequency operation error is that the tie rod is not disassembled. Some expansion joints with tie rods, such as double hinge transverse expansion joints, the tie rods are used for transportation protection when they leave the factory, and must be loosened or adjusted according to design after being installed in place. Some construction teams find it troublesome, or don't understand it at all, so they directly fix the tie rod with nuts in place. As a result, the displacement of the pipeline was not absorbed, and it was all on the bellows, so it was strange that it did not deform.
To go a little further, many scenes are now usedExternal pressure single axial expansion jointTo absorb large axial displacement, the stability of this structure itself is good, but if you don't pay attention to the distance between the flow sign and the guide bracket when installing, it will still give you a look.
The second culprit: the invisible killer of media and working conditions-overtemperature, corrosion and water hammer can't escape
Changes in operating conditions are the ones that are most often overlooked. You think the design temperature is 300℃, but it soars to 350℃ in actual operation. The allowable stress of bellows material falls straight down, so it's weird that it doesn't bulge. There is also corrosion. The sulfide in the flue gas pipe is dilute sulfuric acid when it meets condensed water. The corrosion rate of stainless steel bellows in this environment is more than ten times higher than that in the air. To withstand this environment, you have to rely on non-metallic expansion joints or PTFE-lined metal hoses to support it.
Water hammer This problem is especially common on steam pipes. Condensed water accumulated in the pipes, and when the steam came, the water was pushed away and slammed into the elbow or expansion joint. This impact load is not constant, but the instantaneous pressure can reach several times the normal operating pressure. Once or twice is fine, but more times, the trough position of the bellows will produce micro-cracks.
By the way, if the medium is powder or slurry with particles, the role of the guide tube is very critical. The guide tube can prevent the particles from directly washing the bellows, but many projects save the guide tube in order to save costs. As a result, the bellows is worn out, and then the whole becomes unstable and deformed. This account is really not cost-effective after calculation.
The third culprit: the design selection is not well matched-the rigidity and fatigue life of corrugated pipe are not determined by patting the head
The selection is the most controversial. Some people think that the expansion joint can be connected, and the more bellows layers, the better. That's really not the case. The stiffness and fatigue life of bellows are a pair of contradictions. If the stiffness is small, it will easily deform and become unstable, while if the stiffness is large, it will not absorb the displacement. The design of each expansion joint has a strict calculation basis, including compensation amount, pressure level, temperature, media characteristics and number of cycles. You take a general-purpose corrugated expansion joint with a design life of 1,000 times and use it in a system that starts and stops once an hour, and it will be scrapped for you in three months.
Let's talk about the choice of pressure balanced expansion joint. If the pipeline pressure is high and the axial displacement is large, the straight pipe pressure balance expansion joint is a safe choice, which can balance the internal pressure thrust and reduce the thrust on the fixed bracket. However, some people try to save money and use ordinary axial expansion joints to carry the pressure and thrust, and finally the bellows and bracket suffer together.
What to do after transformation? Can it be fixed? Or just change? How to prevent it next time?
Bulge, crack, bellows perforation, no repair, new one。 If the twist is not serious, if it is just the wrong adjustment of the tie rod, you can re-adjust the tie rod nut to see if the bellows can be restored. But to be honest, once the permanent plastic deformation occurs, the fatigue life of the bellows has been greatly reduced, and even if the shape is coming back, the heart is not at ease.
Then how to prevent repeat offense when changing? Three words: depending on the working conditions. Check the cold tightness before installation, find out whether the tie rod is for transportation or function, and check the guide bracket spacing. During operation, attention should be paid to the abnormal working conditions such as over-temperature, over-pressure and water hammer. Don't just look at the appearance during regular inspections. Use a flashlight to check the bellows trough for salt, rust or cracks-these things look inconspicuous from the outside.
In the procurement process, make the working condition parameters clear, and let the manufacturer help you choose the model. For example, the corrugated expansion joint in the power station industry and the corrugated expansion joint in the cement industry have completely different working conditions. The former should carry the creep temperature and pressure cycle of the main steam, while the latter should prevent the erosion and erosion of high-temperature dust. Choosing the right type is much more important than remediation after the fact.
The metal expansion joint is not a display on the pipe, it is the breathing joint of the pipe. Its deformation is that the pipeline system is sending a distress signal with its body. Don't ignore it, let alone wait for the pipeline to rip to find an answer.