The root cause of cracking: real-world data on stress corrosion versus fatigue life
I have been in this business for more than ten years. In the past two years, the most common complaint from customers is that "the expansion joint is cracked again, and the quality of the manufacturer is not good". But every time I go to the scene, tsk, 70% to 80% of them are working conditions. Don't rush to give the material to the cracking of the metal expansion joint-you have to ask yourself three questions first: Are there chloride ions in the medium? How much has the temperature fluctuated? Did the actual number of fatigue times exceed the design value?
Taking stress corrosion cracking as an example, the crack propagation speed of 304 stainless steel can soar to 0.1mm/h when the temperature exceeds 60℃ in chlorine-containing environment. This is not a theoretical value, but the data measured on site in the desulfurization system of the power station. My friend is in a power plant in northwest China. The corrugated expansion joint behind the flue gas baffle door cracked after less than half a year. When I removed it, there were dense intergranular corrosion marks. After changing to 316L, it lasted another two years before micro-cracks began to appear. So don't be superstitious that stainless steel is universal. If the chloride ion concentration exceeds 100ppm, you have to consider duplex steel or PTFE-lined scheme.
Let's talk about fatigue life. The fatigue design life of corrugated expansion joints in national standard is usually 1000 to 5000 times, but this is the data under normal temperature, no corrosion and pure displacement conditions. Once high temperature (> 400℃) or pressure fluctuation is added, the actual life is directly discounted by 30%. Two days ago, I met a customer in the cement industry. The design displacement of the metal corrugated expansion joint on their kiln head pipeline was 30mm, and the actual operation started and stopped twice a day. As a result, it cracked in one year. Calculate: 730 starts and stops ×2 thermal expansion and contraction =1460 times, which has exceeded the design 1000 times. This is not a quality problem, it is the working condition that overdraws the life.
Mistaken selection: Choosing the wrong type is more dangerous than using the wrong material
If the material is wrong, it can be remedied, but if the type is wrong, it can only be scrapped. The most outrageous case I have ever seen: a steam pipe system with a displacement requiring an axial compression of 80mmUniversal corrugated expansion joint-It can only absorb 30mm. When the pipeline is hot, the expansion joint is directly killed, the stress at the wave root is concentrated, and cracks appear on the spot.
Then how to choose? You have to first figure out whether the pipe displacement is axial, lateral, or angular. For example, air-cooled island pipelines have large temperature differences and complicated directions, so they have to be used at this timeDouble hinge expansion joint for air-cooled island vacuum pipelineOrDuplex hinge transverse type expansion joints, they can absorb displacement in multiple directions instead of hard carrying. Another example is buried pipes. Don't use ordinary goods blindly,Direct buried (fully buried) type expansion jointWith its own anti-corrosion and thermal insulation layer, it can be buried directly in the soil. Do you want to try a different model? Three years to wear rust.
In addition, the problem of pressure balance is often overlooked. For large-diameter high-pressure pipes, if the axial force is not balanced, the expansion joint will be pulled off. At this timeStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion jointThat's the right answer. Selection saves that little money, and the maintenance cost of late cracking can make you hurt.
Installation and operation and maintenance: Have you handled the details of deflector and tie rod correctly?
Many on-site masters think that the deflector is a baffle, and it doesn't matter whether it is installed or not. Wrong! The role of the guide tube is to direct the flow of media and prevent high-speed fluid from directly washing the bellows-especially particle media. In the pneumatic conveying pipeline of cement industry, if there is no guide tube, the wall thickness of corrugated pipe can be worn off by 1mm in one month, and then crack. So buyWhen the metal corrugated expansion joint in the cement industry is used, it is necessary to mark whether the built-in guide tube is needed.
The pull rod is another minefield. Expansion joints usually come with tie rod bolts when they leave the factory. That thing is for transportation and fixation, not for you as permanent support. After installation, the tie rod nut must be loosened, so that the expansion energy saving can expand and contract freely. But how many times have I seen it live? The workers don't dismantle it because it is troublesome, or the expansion joint can't move at all because it is too tight. As a result, the displacement of the pipeline is all stuck on the bellows, so it is strange that it doesn't crack.How to adjust the tie rod nut of expansion joint？ The standard practice is: before installation, screw the nut to the limit mark, and after the system is pressed, loosen the 2-3mm gap to ensure free displacement. If you forget, go back and flip through the question and answer on our station, which is very thorough.
Typical cases under different working conditions: lessons from power stations to the cement industry
Let's talk about the power station first. For a 600MW unit, boiler outlet flueCorrugated expansion joint for power station industryThe medium temperature is 450 °C and the pressure is 0.1 MPa. After two years of operation, circular cracks appeared at the bellows crest. Laboratory analysis: It is mainly due to frequent peak shaving of boilers, low cycle fatigue caused by temperature cycle, and the formation of acidic condensate by SO₂ in flue gas, which accelerates corrosion. What's the lesson? Don't just look at the design temperature, but evaluate the start-stop frequency; At the same time, the material selection should be 317L or super austenitic stainless steel.
The cement industry has more headaches. The exhaust gas pipeline at the end of the kiln has high temperature, large dust content and alkali metal vapor. A previous customer used normalUniversal corrugated expansion jointIt will crack in half a year. changedHigh temperature axial expansion joint, the wall thickness is thickened to 3mm, and the wear-resistant guide tube is installed. Now it's okay after running for two years. What's the difference? High-temperature type corrugated pipe multi-layer structure, stress dispersion, plus the guide tube blocks dust, naturally less cracks.
There are also pits in the petrochemical industry. A chemical plant steam pipeline, usedExternal pressure single axial expansion jointOriginally, it was no problem, but because of the displacement of the pipeline support, the expansion joint actually suffered additional lateral displacement, and the stress at the trough exceeded the standard, and it cracked for half a year. Therefore, when selecting the type, we must complete the pipeline flexibility analysis, and don't just look at the axial working condition.
Prevention and improvement: How to avoid cracking from the design side
Having said so many questions, how can we kill the cracking in the cradle at the design stage? First, do the stress analysis of pipe system. Nowadays, many design institutes are lazy and select models directly based on experience. But what you really should do is run it with CAESAR II or AutoPIPE, input the actual displacement, temperature and pressure of each expansion joint, and verify the safety factor. Second, choose the right structural form. Where space is constrained and large compensation is required, useCompound straight pipe bypass pressure balanced expansion jointOrSleeve type pipe expansion jointThey provide greater axial compensation and do not produce blind plate forces.
Third, anti-corrosion and wear-resistant measures are one-stop. Lined with PTFE or rubber lining for corrosion conditions-on our stationPTFE-lined hose、PTFE compensatorThat's what it does. For high-temperature dust-containing working conditionsNon-metallic expansion joint (fabric fiber expansion joint)Or metal bellows wrapped with thermal insulation. Fourth, don't forget attachment detection. The adjustment values and removal requirements for tie rods, guide tubes and limit bolts are clearly marked on the installation drawings.
The cracking of metal expansion joint is 90% caused by working condition and 10% by design selection error. Don't attack the supplier when something goes wrong. Pull out the field data first. Media composition, temperature fluctuation curve, actual displacement times-these are clear, and then go to the expansion joint fault. Otherwise, replace it with a new one, and it will crack correctly.