Before selecting the type of telescopic section of high-temperature metal expansion joint, understand these 5 key points first
Two days ago, a customer called and asked, "Can your high-temperature expansion joint withstand 800 degrees?" I said yes, but it depends on whether your pipe is running smoke or steam, whether there is dust, and how many times a day it starts and stops. He was stunned for a moment-many people only choose models based on the temperature, but it cracked after installing it for half a year. High-temperature metal expansion joint expansion joint, to put it bluntly, is a flexible compensation piece, but under high-temperature working conditions, which material fatigue, oxidation corrosion and stress relaxation can't bear it first directly determines how long you can use it.
1. Material temperature resistance grade and structural design are the first life
Under high temperature conditions, the expansion joint faces not only thermal expansion and cold contraction. Take the "High Temperature Axial Expansion Joints" and "Corrugated Expansion Joints for Power Station Industry" in our station, for example, they use heat-resistant alloys, such as Inconel 625 or 321 stainless steel. But don't think that stainless steel is universal-321 starts to precipitate brittle phase above 600℃, while Inconel 625 can last to about 900℃. The difference in life is also large: the former may have fatigue cracks in two or three years under the scenario of frequent start-and-stop of power station boilers; The latter is in the constant-temperature flue gas pipe, and it can be used for ten or eight years. Therefore, the first choice: first find out your medium temperature and long-term operating temperature peak, don't just look at the design temperature. And the structure-the role of the guide tube comes out at this time. High-speed airflow flushes the bellows directly? Without deflector protection, it will pierce in a month. As mentioned in the Q&A of "Specific Functions of Expansion Joint Guide Tube" in our station, it guides the airflow away, avoids the bellows from contacting high-temperature and high-speed media, and reduces the flow resistance at the same time.
2. Different industries have completely different faces
The temperature of the boiler exhaust pipe of the power station and the flue gas pipe of the cement rotary kiln are far different, and the medium is far different. The exhaust temperature of the power station is generally 150-400℃, occasionally rising to 500℃. The medium is relatively clean, but it starts and stops frequently, and the temperature difference has a large impact. At this time, "corrugated expansion joint for power station industry" usually adopts multi-layer bellows and external tie rod structure, which absorbs axial displacement and has to withstand vibration. The cement industry is ruthless: the flue gas temperature is 350-500℃ all the year round, which is full of alkaline dust and particles. After being washed down at high speed, ordinary bellows can't bear it at all. Therefore, the "metal corrugated expansion joint of cement industry" must be thickened with the guide tube and even lined with wear-resistant ceramic layer. Selection Step 2: Look for the working condition label-is it high dust? Is it a corrosive medium? Is it frequent start and stop? Don't install the expansion joint of the power station on the cement pipe, that's called a mess of mandarin duck spectrum.
3. Pressure, displacement, installation space-how to match the three parameters?
Don't just stare at the temperature. Under high temperature, the rigidity of bellows will decrease, and the displacement compensation ability will be discounted a lot. For example: a bellows with an outer diameter of 200mm can be pulled by 20mm at room temperature, but may only be pulled by 15mm at 500℃, because the yield strength of the material is reduced. You have to leave a margin for the amount of displacement you want to compensate. What about that? Select different products for different displacement modes: for large axial displacement, use "External Pressure Single Axial Expansion Joint" or "Straight Pipe Pressure Balance Expansion Joint"; If the lateral displacement is large, use "double hinge transverse expansion joint" or "curved tube pressure balance expansion joint". The installation space is also stuck in the neck-is there enough space for the deflector? Is it enough to leave maintenance distance? The guide tube is not optional, but standard under high temperature conditions. It protects the bellows from erosion by high-speed airflow while also reducing localized overheating. The question and answer in our station clearly said that the expansion joint without the guide tube is easy to crack on the inside of the bellows when used in high-temperature and high-speed airways.
4. Should the tie rod nut be removed during installation? Where does the arrow go?
What happens when the expansion joint is installed backwards? The answer is simple: compensation fails in the slightest and tears in the worst. The direction of the arrow on the superscript of the expansion joint refers to the direction of the medium flow-the guide tube can only run along the medium, and when the guide tube is installed backwards, it becomes a dustpan for a ride, and the airflow is directly poured into the gap between the bellows and the guide tube. It is only a matter of time before local overtemperature and wear out. As for the tie rod nut, be sure to look at the drawings carefully before installation. Like the core one mentioned in "Proper Installation Method of Large Tie Rod Expansion Joint": The amount of pre-tension or pre-compression must be according to the design, and do not rely on hand strength. For example, the design reserves a pre-stretching amount of 10mm for you, and you directly screw the nut. As soon as the pipeline heats up, the expansion joint has no room to compensate, and the bellows is directly deadlifted and scrapped. In addition, the positioning screws for transportation (that is, those screws that compress and fix the expansion joint) must be dismantled before the pipeline is installed and the pressure test is completed, otherwise it is equivalent to using the expansion joint as a dead part, and there is no compensation effect at all. In our Q&A, we specifically talked about "Does the screw of the expansion joint need to be disassembled?"-It must be disassembled, and if it is not disassembled, it will be a time bomb.
5. How many years will it take for high-temperature expansion to save energy? There is no fixed answer, but there is a simple judgment
The most customers ask about life expectancy, but I can't really give an accurate number. The same product can be used for 8 years in the steam pipeline of the power station, but it may leak for 3 years when it is changed to the cement pipeline. How to judge whether you should change? Look at the bellows surface: are there any cracks? Is there any oxide scale shedding? Is there any noticeable distortion? If the guide tube falls off or the bellows leaks, don't repair it, just change it. Repairing the heat expansion joint is different from repairing the tire. After welding, the stress concentration is more serious, and it will break faster next time. In addition, pay attention to the temperature range matching: the "metal hose" in our station is suitable for-200℃ ~500℃, and the "non-metallic expansion joint (fabric fiber expansion joint)" can reach above 1000℃ but has poor pressure resistance; The "rubber compensator" can only be used within 100℃. Mixing is prone to accidents-someone put a rubber compensator on the steam pipe, and it smokes as soon as it is turned on. When selecting a model, ask more: What is the highest temperature at your site? Is there an instant overheat? Ask clearly, and then take your seat. Don't try to save trouble.
In the final analysis, the telescopic section type of high-temperature metal expansion joint is not as simple as looking at the formula of the parameter table. Materials, structure, working conditions, installation and maintenance, every link may pit you. Figure out these 5 key points to avoid at least 90% of the pits. For the rest, welcome to come directly to our site to flip through the product information, or flip through the Q&A-those cases are lessons learned by real money.