If the expansion joint on the high-temperature pipeline leaks, the first reaction at the site is often to stop, relieve pressure and replace it with a new one. However, in many cases, the leakage point may be just a stress crack on the bellows or a needle hole on the weld, which can be plugged under pressure without stopping the machine, or even until the next maintenance cycle. The problem is-you have to figure out where it is leaking first.
Three typical leakage points: bellows cracking, weld leakage, guide tube wear-out
These three situations are the most common in our actual maintenance.
Cracked bellows.This leakage point is usually near the crest or trough of the wave, and the crack direction is mostly along the axial direction. The reason is nothing more than the fatigue life, or the system pressure and temperature exceed the design value. The judgment method is also simple: see if there is obvious oxidation color on the edge of the crack. If so, it means that it has been cracked for a while; Shine the back with a flashlight again to see if there are any blowing marks.
Weld leakage.The circumferential welding seam connecting the bellows with the end pipe and flange is the most prone to problems. This leakage point is often not an obvious crack, but a dense pinhole, or a fine crack along the heat-affected zone of the weld. You may not be able to see it clearly even if you look at it with a magnifying glass, but when the medium seeps out, it means that the weld has failed.
The deflector is worn out.This is most likely to happen on high-temperature pipes containing dusty media-such as flue gas pipes in power stations and hot air pipes in cement industries. The role of the guide tube is to protect the inner wall of the bellows from direct washing by the medium, but it is also a consumable part itself. Once worn out, the high-speed dusty airflow hits the bellows directly, quickly breaking through the trough. This leakage point is often accompanied by obvious abnormal noise and vibration, which can be heard by sticking the needle bar to it.
What about that? Don't be in a hurry to dismantle it. Use a small mirror and a bright flashlight to check it carefully from the outside of the expansion joint. The leakage location is accurate, and the rest of the things will be much easier to handle.
High temperature leak plugging process: pressure leak plugging, welding and repair welding, local replacement, with much difference in boundary
The wrong plugging process is chosen, just like not plugging. Let's go through the three commonly used ones.
Pressure plugging
It is suitable for the occasions where the pressure is not high, the medium is gas or steam, and the leakage is not large. A new sealed cavity is established around the leakage point with a special fixture and sealing injection. The advantage is that there is no downtime, but its essence is to "put an armor on the expansion joint" and does not restore the structural strength of the bellows. For high-temperature and high-pressure pipelines, it can only be used as an emergency, not as a permanent repair means.
Patch welding and repair welding
If the leakage point is on the bellows, and the material of the bellows is 304 or 316L, it can be polished clean first, and then welded with the same material electrode. However, the wall thickness of bellows is usually only 0.5mm to 2mm, and it will burn through if the welding heat input is slightly larger. Therefore, we have to use argon arc welding, small current, fast welding, and coloring for flaw detection after welding. In addition, the repair welding area will change the flexibility of the bellows, and there is a tendency to generate stress concentration around the weld-don't expect it to expand and contract as freely as it used to.
Local replacement
If the part where the leakage is located has been severely thinned, or there are multiple cracks in the bellows, then don't repair it, just cut this section and replace it with a new oneHigh temperature axial expansion jointOr metal corrugated expansion joint of the same specification. Local replacement needs attention: before cutting, the pipeline must be suspended to release the displacement, so as to avoid the pipeline bouncing open and injuring people after cutting; When welding, it is also necessary to protect the back of argon gas to prevent inner oxidation.
Is it done when it's finished?
Of course not. Repair welding can only solve the immediate leak, but can't solve why it leaks. If the original selection is wrong, for example, if the general-purpose corrugated expansion joint is used in high-temperature and high-pressure applications, and the material is not treated with solid solution, or there is no guide tube, it will still leak after plugging, and it may be faster.
Wrong choice of material and structure, still leakage after plugging
For example,High temperature axial expansion jointIt is generally required that the bellows be made of a higher temperature resistant material, such as Inconel 625 or Haynes 214, instead of the ordinary 304. Many users have cheap pictures and use 304 hard top. As a result, the bellows undergoes intergranular corrosion at high temperature, and the surface cracks. Can you block this? There is also a structural problem-when the medium contains dust, the guide tube must be designed, and the wall thickness of the guide tube should be calculated according to the wear speed. The guide tube given by some manufacturers is only a 1.5mm thin plate, which is worn out in less than half a year. No matter how well you weld, you won't survive a service cycle.
Therefore, when plugging the leakage, write down the material grade, wave number and guide tube of the bellows by the way, and check the original design parameters with the manufacturer. If you find that the original design is unreasonable, change the appropriate model directly while the machine is shut down.
Two practical cases
Talk about a power station. The expansion joint of the flue gas pipeline of a 300MW unit leaked, and the screaming sound was loud at the scene. It was found that after the guide tube was worn out, the flue gas directly washed the bellows, resulting in the perforation of the trough. The operators were so anxious that they lost hundreds of thousands of power generation a day after shutting down. After we arrived at the scene, we first made pressure plugging-using stainless steel clamps and ceramic fiber blankets, temporarily pressing the perforated parts to control the leakage. When the machine was shut down for maintenance, it was disassembled and found that the bellows was almost half pierced, and the one with thickened guide tube was replacedCorrugated expansion joint for power station industryBy the way, the lining of the front and rear pipes was also repaired. It hasn't leaked for three years now.
Talk about the cement factory. Hot air duct of kiln head, medium temperature 850℃, expansion joint weld leakage. The loss of kiln shutdown in cement plant is not small, and it takes at least three to five days to change the expansion joint. We judged that the weld seam was only a local pinhole and the bellows body was intact, so we decided not to stop the repair welding. The operator wore insulated clothes and masks, and the welder leaned over from the scaffolding and made three points with argon arc welding. The whole process took less than forty minutes. After repairing, the adjacent welds were inspected by ultrasonic thickness measurement, and the thicknesses were all within the qualified range. ThisMetal Corrugated Expansion Joints in Cement IndustryUsed it for another year and didn't replace it entirely until planned overhaul.
Alas, to put it bluntly, the most afraid thing about plugging a leak is that when you see a leak, you will panic first, remove it and throw it away for a new one. In fact, many leakage points will not scrap the whole expansion joint at all.
How to make the expansion joint leak less times
Finally, let's talk about something real, how to prevent it.
When designing and selecting,Don't place an order just by quoting caliber and pressure. Tell the manufacturer the medium temperature, dust content, starting and stopping times per hour, axial displacement and even ambient wind speed. In high-temperature dust conditions, the material and wall thickness of the guide tube should be considered when selecting the type, and anti-wear sleeves should be added if necessary.
When installing,Many people ignore this link of pre-stretching. If the high-temperature pipe expansion joint is not pre-stretched, after cold installation, once the temperature rises, the bellows directly bears a huge thermal displacement, and the life may be discounted by 50%. How to calculate the amount of pre-stretch? Multiply the line expansion coefficient of the pipe by the temperature difference between the installation temperature and the operating temperature, and subtract the portion absorbed by the natural bending of the pipe. The value is often on the drawing, so don't skip it to save trouble during construction.
Daily inspection,Focus on three places: whether there is abnormal deformation on the surface of the bellows, whether there is any trace of media seepage near the weld, and whether there is abnormal temperature rise in the outer wall area corresponding to the guide tube. Sweep with an infrared thermal imaging camera, and the part with a temperature difference exceeding 30℃ is basically the leakage point.
The expansion joint is not a thing that can be installed once and for all. If you usually ignore it, it will give you big news when you are at your busiest. Therefore, next time you encounter the work of plugging the leakage of the high-temperature metal expansion joint, remember: position it first, then select the process, and reflect on why it leaks after completing it. Don't rush to shut down, and don't be reluctant to shut down. When it is time to shut down and replace it with a new one, hesitation drags small things big.