Metal corrugated expansion joint modification is not as simple as replacing a new one
Many owners feel that the expansion joint is broken, so measure the size, buy a new one according to the original model and install it. This idea is fine when it comes to the replacement of ordinary spare parts, but when it comes to the transformation of metal corrugated expansion joints, it will probably stumble. Two days ago, I met the equipment supervisor of a cement plant, saying that the expansion joint of their kiln head flue leaked again after less than three months of replacement. When I removed it, I saw that the bellows was full of worn holes-it was not fatigue crack at all, but dust erosion and wear. You said to change it to the exact same one. Can it not break?
Find out first: How is the original expansion joint broken?
The first thing in retrofitting is not drawing a picture, but a failure analysis. Fatigue cracking of bellows, wear-out of guide tube and jamming of tie rod hinge are completely different solutions for these three faults. Take the most common working conditions of power stations and cement plants: the expansion joints in high-temperature flues are most likely the superposition of corrosion and wear. SO₂ and dust particles in the flue gas can damage the bellows far faster than imagination at high temperature. At this time, just changing a bellows of the same material is equivalent to no cure. You have to consider adding wear-resistant liners and upgrading materials, such as weather-resistant steel or Inconel alloy. But the steam pipeline is different. There are mainly fatigue cracks caused by thermal cycling. We have to re-check the fatigue life to see whether the wave number, layer number and single wave displacement of the bellows still match the current working conditions. Two working conditions, two playing styles, if you mix it up, something will definitely happen.
Surveying and mapping and working condition checking: the most fooled step in renovation
You take a five-year-old structure diagram and ask someone to redo it? After several overhauls and changes, the actual thermal displacement is no longer the same as before. During the transformation, we must go to the site to measure the cold and hot displacement, pressure and temperature of the pipeline, and check whether there are corrosive components in the medium. Especially for expansion joints in desulfurization systems, acid dew point corrosion is the number one killer. As soon as the flue gas temperature is lower than the acid dew point, the sulfuric acid condensate directly corrodes the bellows, and the ordinary general-purpose corrugated expansion joint can't bear it at all. At this time, either choose a metal expansion joint with corrosion-resistant lining, or simply replace it with a non-metallic expansion joint (fabric fiber expansion joint), and use flexible materials to isolate the corrosive medium. This choice directly affects how long it can be used after the transformation, and can't be decided by feeling.
The Core of Type Selection and Transformation: Constraint Form Must Match with Pipeline System
It doesn't mean that if you used the axial type, you can only continue to use the axial type. You have to count blind power. If the fixed bracket is not firmly laid in the original design, and the unconstrained general type of the expansion joint is selected, the thrust generated by internal pressure will be fully pressed on the pipe frame. Over time, the pipe frame will deform and the weld will crack, and the problem will be more serious than air leakage. In this case, it is better to choose the external pressure single axial expansion joint, so that it can bear part of the reaction force while absorbing the displacement, so as to reduce the dependence on the bracket. If the pipe needs to absorb the lateral displacement, the double hinge transverse expansion joint may be more suitable-it can be used in pairs to break the lateral displacement into the angular displacement of the hinge, and the reaction force is balanced by the expansion joint itself. The calculation of pressure thrust and elastic reaction force here requires the manufacturer to produce a detailed calculation book, which is not as simple as selecting the model by looking at the sample.
Small details in installation and renovation determine success or failure
The direction of the deflector is facing the direction of the medium flow, which is the most basic, but there are examples of reverse installation in the field every year. Install backwards, the medium directly flushes the bellows, and the wear rate doubles. The pre-displacement of the tie rod nut is not casually screwed, and it must be adjusted according to the actual cold tightening amount. Cold tightness is the displacement corresponding to the difference between the installation temperature and the working temperature of the pipeline. If it is twisted too much or less, the actual working displacement of the bellows will be biased, and the fatigue life will be directly discounted. In addition, don't forget to check whether there is an insulation layer under high temperature conditions, or whether it is necessary to install an insulation sleeve outside the bellows. These details must be listed one by one in the transformation list, checked with the manufacturer item by item, and signed for confirmation.
How to verify the effect after transformation? Don't just leak or not
Does the bellows have any abnormal deformation? Is the tie rod loose? Is there any abnormal vibration in the flue system where the baffle door and the expansion joint cooperate? If possible, do a cold displacement compensation test to see if the bellows can return to the designed position. Or scan it with an infrared thermometer. Places with large temperature differences are often signs of insulation failure or abnormal internal structure. And guess what? Many on-site problems are actually caused by welding splash and burning bellows. This hidden danger can't be detected with the naked eye at all, and it takes two or three months to be exposed. Therefore, the transformation is not a one-time sale, and the manufacturer's debugging records and quality assurance commitments must be written into the contract. If something goes wrong, there must be a basis for tracing it back.
The modification of metal corrugated expansion joint is a systematic project in the final analysis. Failure analysis, surveying and mapping verification, constraint matching, installation details, acceptance verification, missing one link will not work. Don't treat it as a replacement spare part, but as a small engineering project to manage it, so it can be changed once for ten years.