Thoughts triggered by a photo of failure-what can the metal expansion joint make?
Two days ago, I sorted out the on-site cases and turned to a picture: the bellows seemed to have been cut by a knife, the crack extended straight from the trough to the peak, and there were still condensed water droplets hanging at the weld. The sign next to it reads "Just three months into operation". To be honest, I have seen this scene many times, but I still feel distressed every time I see it-not only the equipment money, but also the loss of shutdown is enough to buy dozens of expansion joints. The metal expansion joint looked simple, it was just a telescopic pipe, but if something went wrong, it could leak at the slightest, or even burst at the worst. So what's the problem? Today, let's break and break, from design to installation, and explain the easiest pits one by one.
Six typical flaws: How many have you encountered?
After working in this business for more than ten years, I summed up that the defects of metal expansion joints are just six:
- Bellows cracking— The most common, and the most deadly. Fatigue cracks, stress corrosion, or micro-cracks in the material itself will crack for you in a few days as soon as it is hot and cold.
- Instability— When the crest of the wave falls, the whole expansion joint is like a crushed can. It is usually because the design pressure or compensation amount is too small, or the pre-stretch is not pulled in place during installation.
- Corrosion perforation— — There are chloride ions, sulfide in the medium, or water accumulated in the insulation layer, and 304 and 316L can't bear it. Especially chloride ion stress corrosion, that is called a fast.
- Guide tube falling off-This small part is easily overlooked, but once it falls off, high-speed airflow or liquid directly washes the bellows, and the life is directly folded in half. Many expansion joint guide tubes are fixed by a few screws, and the vibration is loosened.
- tie rod fracture-Don't think that the tie rod is just a decoration, it is subjected to huge stress while limiting displacement. If the material is wrong, the thread processing is defective, or the nut is not locked tightly, it is a matter of time before it breaks.
- Weld leakage— — Whether it is the longitudinal seam of the corrugated pipe or the annular seam of the connection pipe, as long as the welder's craftsmanship is trendy and the flaw detection is not completed, pinholes or slag inclusions will appear. This belongs to the factory with "internal injury".
Where is the source? From material to design to installation, step-by-step dismantling the reasons
You say these defects are all the work of the donkeys of the production team? Not all of them. Let's go through the source.
Let's talk about the materials first. Many customers only ask "Can you use 304", but when the medium temperature exceeds 400°C or contains a trace amount of chloride ions, 304 is equivalent to paper paste. At this time, it is time to use 316L or higher corrosion-resistant alloy. There is also the thickness of the bellows sheet-some people use thin sheets in order to save costs, but as a result, the stiffness is not enough, and instability is a matter of time.
Let's talk about design. Do you think choosing aUniversal corrugated expansion jointCan you conquer the world? Tsk, if the working conditions change, you have to change the model. For example, high-temperature steam pipes should be usedHigh temperature axial expansion jointThe number of bellows layers and the design of reinforcing rings are different. If it is a buried pipeline, you have to chooseDirect buried (fully buried) type expansion joint, with outer protective sleeve and anti-corrosion coating. The most common mistakes made in the design phase were miscalculating the axial compensation amount or ignoring the lateral displacement of the pipe-as a result, the bellows was found twisted into twists as soon as it was run.
Finally, talk about installation. This link is the most pitted and the easiest to be overlooked. I have seen installers treat the transport screw on the expansion joint as a permanent fastener, and forget to remove it before running. As a result, the expansion joint can't expand or contract at all, and it just cracks the pipe. And the direction of the arrow- -The direction of the arrow of the expansion joint refers toMedia flow direction, you install it backwards, the deflector becomes a baffle, and the scour corrosion doubles. Alas, the installation instructions are clearly written, but few people can read them on the spot.
How to prevent it? Critical details before factory and at installation
If you want to prevent defects, you must extinguish the signs before you leave the factory. Pressure test is necessary-the hydraulic test should be reached 1.5 times the design pressure, and the pressure should be held for 10 minutes without leakage. However, many manufacturers only do air tightness, and water pressure is too lazy to do it. As a result, the internal stress is not released, and cracks come to the door after production. In addition,stiffness detectionEspecially for multi-wave expansion joints, the uniformity of stiffness of each wave directly affects the uniformity of force.
What about the installation phase? Remember three things: First, pre-stretching or pre-compression must be according to the drawing, and the error should not exceed 5mm; Second, the fixed bracket and the guide bracket should be in place, and the weight of the pipe should not be pressed on the expansion joint; Third, take precautions when welding, and the splash hitting the bellows is the stress concentration point. And one more detail:Do you need to remove the screw of the expansion joint— — The answer is to dismantle it, but don't dismantle it before installation. You have to wait until the pipeline is installed in place and the welding joint inspection is completed before dismantling the transportation screw, otherwise the expansion joint will be deformed during hoisting.
Selection without rollover: which mines should be avoided in different scenarios
As mentioned earlier, the general-purpose type, the high-temperature axial type and the direct burial type actually have a bunch of special models. For example, the desulfurization system should not use ordinary metal expansion joints, but one with anti-corrosion liningDesulfurization flue gas baffle doorMatching expansion joints, or directly onRubber PTFE compensator？ No, if the smoke temperature is high, the rubber can't handle it, so you have to use itNon-metallic expansion joint (fabric fiber expansion joint)。 You see, if the selection is slightly sloppy, the back is full of pits.
For power stationsCorrugated expansion joint for power plant industryGenerally require high temperature resistance and fatigue resistance, while the cement industry usesMetal Corrugated Expansion Joints in Cement IndustryIt is more concerned about wear resistance and dust resistance. Someone else asked:What is the difference between metal expansion joint and corrugated expansion joint？ In fact, one thing, corrugated expansion joint is a kind of metal expansion joint, but we call it mixed daily. Look right when selecting a modelThe role of the expansion joint— — Whether to absorb axial displacement, lateral displacement or angular displacement, select accordinglyCompound hinge transverse expansion joint、Curved tube pressure balance expansion jointThese dedicated structures.
Don't buy non-standard products cheaply. Products from regular manufacturers are availableModel and size of expansion jointStandard, the interface flange should be worthy of the pipeline pressure class.
Don't wait for the pipe to burst before you remember to check defects, the four-character formula of daily inspection
Having said so much, in fact, what I want to express most is that the defect of the metal expansion joint is not to wait for it to fail before replacing it, but to stare at it at ordinary times. There is a four-character formula circulating within us —See, hear, touch, measure。 See if there is any bulging or rust in the appearance; Listen to whether there is any abnormal whistling sound during operation (possibly the vibration of the guide tube); Feel whether the surface temperature is abnormal (local overheating indicates that the bellows is blocked or worn); Measure whether the displacement and preload force are within the range. It takes less than 5 minutes to inspect once a week, but it can avoid tens of thousands of shutdown losses. Do you think that's the truth?
(The product types involved in this article are all from the existing data of this site, please refer to the specific selectionProduct Center。）