Rectangular metal expansion joint machining, these 4 key processes determine how long the equipment can last
When it comes to rectangular metal expansion joints, the first reaction of many engineering brothers is that this thing is much harder than a round one. Why? Because the circular bellows has uniform force and simple forming, the grinding tool comes out as soon as it turns; The rectangle is different. The stress of the four angles is concentrated, and the deformation of the long side and the short side is different. There is a slight deviation, and it will crack within two months of installation.
Two days ago, a customer who was doing a power station project called and complained, saying that a batch of rectangular metal expansion joints they purchased took less than three months, and the corners leaked. When disassembled, the weld was cracked and the ripples were deformed. In the final analysis, the processing link is not controlled. Today, let's break it up and break it up to talk about which processes are the most fatal in the processing of rectangular metal expansion joints.
Why is it harder to machine rectangular expansion joints than round ones? Structural characteristics determine the process threshold
Let's look at the structural differences first. The circular bellows is axisymmetric, and the stress distribution is uniform regardless of which direction it is compressed. What about rectangular sections? The four corners are right angles, and the transition zone has a sudden change in curvature – a natural hotbed for stress concentration. In addition, the length-to-width ratio of rectangular section is different, and the material flow in the long side direction is completely different from that in the short side direction during corrugation molding, and the springback amount is also inconsistent.
Therefore, we can't simply apply the circular process parameters when we engage in rectangular metal expansion joint processing. Mold design, hydraulic pressure, weld position all have to be recalculated. And guess what? Many small factories try to save trouble, and directly use the experience of circles to make rectangles. As a result, either the corrugation height is not enough, or the corners are wrinkled-it is strange that this kind of semi-finished product can have a long life when installed on pipelines.
Material selection and blanking: Plate thickness, material quality and splicing mode directly affect the qualified rate of corrugation forming
The flue gas pipeline of power station has high temperature and sulfur content, so stainless steel 304 or 316L is generally used; Cement desulfurization line is highly corrosive and must be duplex stainless steel or even nickel-based alloy. What about plate thickness? Too thin corrugation easy instability, too thick molding pressure, mold wear fast. Usually, it is selected according to the nominal diameter and pressure grade. For example, 1.0mm is used for those below DN1000, and the larger diameter has to be thickened to more than 1.5mm.
Let's talk about cutting. Bellows with rectangular expansion joints often require multiple plates to be welded-this is different from circular shapes, which can be spun as a whole. The position of the splice weld is very particular: the four corner areas with the highest stress must be avoided, and the weld is usually placed in the middle of the side. If the weld is placed at the corner, the weld will crack as soon as it is stretched during molding. In the projects we have done for customers, more than 60% of early failures are related to unreasonable weld position during blanking.
Corrugation: Mold design, hydraulic parameters and rebound control-if this step is not done properly, there will be scrap
Forming is the core process of rectangular metal expansion joint processing, and there is no one. In the mould design, the mould of rectangular bellows is not a simple rectangular groove, the four corners must be designed as circular arc transition, and the size of R angle directly affects the stress distribution. Generally, the R angle is 3~5 times the plate thickness. If it is too small, the material flow will be blocked, and if it is too large, the compensation amount will not be enough.
How to determine the pressure parameters during hydroforming? If the pressure is too small, the corrugation will not be full, and if the pressure is too large, the thinning rate will exceed the standard. The standard requires that the thinning rate should not exceed 15%. What is even more headache is the rebound-the rebound amount of rectangular section is not constant along the edge length, and the rebound at the corner is small and the rebound in the middle is large. What about that? The experience is: first make a set of specimens, measure the rebound values at different positions, and then make negative compensation on the mold, that is, reduce the mold size by a rebound amount. Large factories generally use finite element simulation to predict the rebound distribution, while small factories can only rely on trial and error.
The rectangular expansion joint used in a desulfurization project is designed to have a corrugated height of 30mm, but the actual measurement after molding is only 26mm, and the compensation amount is not enough. When the pipeline is thermally expanded, the corrugated height is directly straightened, and the guide tube is bent. Later, the investigation found that the hydraulic pressure was only set at 6MPa, and the actual need was 8.5MPa. So, well, the parameters are not determined by patting the head.
Welding and Assembly: The quality of connection of end tubes, guide tubes and bellows determines the risk of leakage
The formed bellows should be welded together with the end pipe and the guide tube. The most problematic points here are two:
- Connecting weld between end pipe and bellows: The rectangular end tube is also rectangular and the weld is a continuous fillet weld. The key is the welding sequence-you can't weld it all in one turn, because thermal deformation will cause the rectangle to twist. You must jump welding in sections, for example, first welding the middle section of the long side, then welding the short side, and finally welding the corner. The length of each section shall not exceed 200mm, and the next section shall be connected after the temperature drops to room temperature.
- Installation of guide tube: The function of the guide tube is to guide the medium and protect the inner wall of the bellows. There must be a gap between it and the bellows, generally 3~5mm. If it is too small, it will get stuck, and if it is too big, it will lose its protection. Many leakage accidents are because the guide tube and bellows are welded to death, and they pull on each other when they expand heat, resulting in tearing.
Besides, our commonMetal rectangular expansion jointIn the product, the direction arrow of the guide tube must be facing the flow direction of the medium. Once the customer installed it backwards on the spot, the media directly flushed the bellows, and it was perforated in three months. So the arrow direction must be checked before leaving the factory.
Factory inspection: water pressure, air tightness, stiffness tests, and common failure points in practical applications
Hydraulic test: Hold the pressure 1.5 times the design pressure for 10 minutes to see if there is any leakage in the bellows body and weld. Pay attention to the use of clean water when holding pressure, and the water temperature should not be lower than 5℃ to prevent low-temperature brittleness. After testing the water, blow dry in time, otherwise the internal rust will affect the subsequent use.
Airtightness test: For the pipeline transporting flammable and explosive media, the water pressure is not enough, and the airtightness test has to be done. Usually use compressed air or nitrogen, the pressure is 1.05 times the design pressure, immersed in water or coated with soapy water for leak detection. This step is life-saving and cannot be omitted.
stiffness test: Measure the deformation curve of the expansion joint under axial force to verify whether the actual stiffness reaches the design value. Too much stiffness will cause excessive pipeline thrust and damage the bracket; If it is too small, it will not achieve the compensation effect. We have encountered a project in which the on-site pipe gallery support was broken, and it was found that the stiffness of the expansion joint exceeded the standard by 30%.
In practical application, the most common sequence of failure points of rectangular expansion joints is: corner weld crack> corrugated root fatigue crack> guide tube fall off> flange surface leakage. Among them, corner weld cracking accounts for nearly half-this goes back to what was mentioned earlier, the R angle of the mold, the welding sequence and the rebound control. If one of these three links is not done well, the corner is a short board.
Different industries have special requirements for processing: from power station flue gas to cement desulfurization, the working condition changes and the process has to be adjusted
Don't expect one craft to eat all over the world. of the power station industryCorrugated expansion joint for power station industryIt is required to be resistant to high temperature and creep, and the material must be heat-resistant stainless steel, such as 310S, with less corrugation wave number and large wave height, and enough compensation ability. Of the cement industryMetal Corrugated Expansion Joints in Cement IndustryWhat? There is much dust and strong corrosion, so the guide tube should be wear-resistant surfacing, and it is best to add a dust cover to the outer surface of the bellows.
For desulfurization systemsDesulfurization flue gas baffle doorIt is used in conjunction with an expansion joint and requires extremely high air tightness, and the rectangular expansion joint must achieve zero leakage. At this time, the accuracy of forming and welding has to be raised to a new level-welding splashes should be cleaned up, and there should be no scratches on the bellows surface, because any minor defects may become the starting point of corrosion.
After all, the processing of rectangular metal expansion joints is a systematic project. From material selection, blanking, forming, welding, testing to industry-specific adjustment, no step can be made do. The expansion joint you bought was thousands of dollars cheaper, but it was scrapped after one year of use. The loss of shutdown was included. Which one is more cost-effective?