1. The expansion joint is not as "solid" as you think-what do common defects look like?
Many people think that the metal expansion joint is a corrugated pipe, which can bend and stretch, and is quite built. You don't know until you really enter the scene. It's so delicate that it can kill people. Take the high-temperature axial expansion joint commonly used in power station industry as an example, it is most likely to have problems at the peak position-stress concentration crack. The crack was as long as a hair. If you didn't look carefully, you couldn't find it at all. After a cycle or two of maintenance, the crest of the wave would directly crack through, and steam would blow out, causing the whole system to stop. There is also an expansion joint matched with the desulfurization flue gas baffle door. There is acid dew point corrosion in the flue gas, and the bellows is partially thinned, which looks like leaves gnawed by insects, stabbing a hole. Don't think that only the bellows will break, and the guide tube falls off and the tie rod breaks are also old faces. Two days ago, a customer said that the guide tube of their cement line corrugated expansion joint fell off, and the broken iron sheet was directly stuck in the corrugation. The next time the corrugated pipe was started, it would be scrapped on the spot. Tsk, if you see these things once, you will never dare to underestimate the expansion joint again.
2. Thunder buried by design selection: choose the wrong model, calculate the wrong displacement, wait and cry later
Many defects of metal expansion joints were buried in the early design. The most typical is that the displacement direction is wrong-the pipeline obviously has lateral displacement, and you give it a general-purpose corrugated expansion joint, which can only compensate in the axial direction. As a result, the wave shell is hard-broken during operation, and it leaks within a few months. There are also frequent starts and stops of steam pipes. Have you calculated low cycle fatigue? Some people installed a large-diameter thick-walled expansion joint, thinking that the wall thickness was fine. As a result, the alternating stress accumulated, and cracks appeared in 10 cycles. What is even more outrageous is the transverse expansion joint of the double hinge. When installing, the distance between the two hinges is not correct, and it is twisted into a twist. These pits are not marked on the drawings, and something will happen at the scene sooner or later. Design selection is really not a matter of patting the head. Temperature, pressure, displacement direction, media corrosiveness and fatigue life can't be missed. Otherwise, just wait for the maintenance team to curse.
3. "Man-made disasters" in installation and operation: brutal construction, medium corrosion, over-temperature and over-pressure
There are too many moths installed on site. The tie rod nut is not leveled – guess what? What can be solved with a wrench and a torque wrench, no one does it. The deflector is installed in the reverse direction, and the welding splash hits the bellows. This low-level error scraps many expansion joints every year. More covert is the media issue. The metal corrugated expansion joint in the cement industry encounters chlorine-containing flue gas, and the stress corrosion cracking of chloride ions quietly develops. It looks nothing on the surface, but it has leaked when it is discovered. The double hinge expansion joint for air-cooled island vacuum pipeline has a design temperature of 200℃. As a result, the operating condition fluctuates to 350℃, and the bellows is directly unstable and deformed, and the whole pipe is twisted into a twist. Overtemperature and overpressure are killers, but many factories don't monitor them in real time at all. I don't think of changing it until I leak. How much is a set of straight tube pressure balance expansion joints? The little inspection cost saved is enough to buy several sets.
4. How to find defects in advance? Don't wait for a leak before replacing it. It's too expensive
A little effort in daily inspections can save a lot of money. Touch what? Feel the bellows peak and trough to see if there is any abnormal temperature-local overtemperature indicates that the medium may go to places it shouldn't go. What's to watch? See if the tie rod nut is loose and if there is any abnormal noise in the guide tube. Infrared thermography can find local overtemperature points, but don't expect it to see through cracks. Acoustic emission detection is useful for developing cracks, but the equipment is expensive and the field operation is troublesome, so it is generally used in critical lines. The most practical way: Use a wrench and a torque wrench to tighten the tie rod nut according to the standard torque, and most of the loose problems can be solved. What should I do if the directly buried expansion joint is buried? It can only rely on pressure test and regular excavation spot check. Every random inspection is like opening a blind box, but it has to be done. After all, changing a directly buried expansion joint, earthwork excavation and equipment fee is enough to buy a trolley.
5. What to do after the defect occurs: repair or replace? Money-saving schemes may not be cost-effective
Can small cracks be repaired? Theoretically, it is possible, but the fatigue life after repair welding is difficult to guarantee, so it is recommended not to take risks under high temperature and high pressure conditions. Can you only change the lining if the PTFE hose is leaking? Most manufacturers do not recommend it because there is a gap between the liner and the metal layer, and the probability of secondary damage is high. Can the rubber compensator be patched if it is bulging? Temporary emergency is ok, but long-term use is risky. The cost of replacing the sleeve-type pipe expansion joint is much lower, and it can be restored by replacing the sealing ring. The key is to assess remaining life and risk. If it is the main steam line of the power plant, it is recommended to directly replace the new straight pipe pressure balance expansion joint, safety first. Non-critical auxiliary systems, such as the expansion joint matched by the flue gas baffle door, can also be temporarily clamped for a maintenance cycle. One thing to remember: Money-saving options are not necessarily cost-effective, and the trade-off is always safety costs and downtime losses.
6. Choose the right product and use the right method, and half of the defects will not happen at all
After understanding the defects, we have to finally fall to the selection. Different working conditions match different products: high temperature and high pressure select external pressure single axial type expansion joint, large transverse displacement select double hinge transverse type, desulfurization system with non-metallic expansion joint or PTFE-lined, vacuum special hose for vacuum environment. As long as the temperature, pressure, displacement and medium corrosiveness are clearly listed in the design, and then let the manufacturer help you review the fatigue life, 90% of accidental defects can be avoided. Don't forget these details about how to adjust the tie rod nut-we have talked about how to adjust the tie rod nut of the expansion joint before, so you can flip through the previous content when you have time. There is also the function of the guide tube, which is to protect the bellows from being washed by the medium. If the direction is reversed, it is equivalent to white. Finally, let me say a word: the expansion joint depends on the product for three points and the selection and installation for seven points. If you make the first seven points solid, the last three points will naturally be worry-free.