1. What exactly is the metal circular corrugated expansion joint? First distinguish the relationship between it and metal hose and general corrugated expansion joint
Are metal round corrugated expansion joints and metal hoses the same thing? Don't worry yet. Both of them have ripples, but they do completely different jobs. Metal hoses are mainly used to absorb pipe vibrations, compensate for installation deviations, and connect devices with relative displacement. To put it bluntly, he is more like a pipe that can be bent. The core task of the metal circular corrugated expansion joint is to absorb the axial displacement, transverse displacement and angular displacement caused by thermal expansion and contraction of the pipeline, and at the same time bear the internal pressure and temperature of the pipeline. If you use a metal hose as an expansion joint, the corrugated section can easily become unstable because it can't withstand the axial compression force, which has caused many accidents in engineering.
Then what is the relationship between it and the general corrugated expansion joint? To put it bluntly, the general corrugated expansion joint is the category, and the metal circular corrugated expansion joint is the most widely used form among them. The universal corrugated expansion joint absorbs the displacement through the deformation of the corrugated flexible section, and the two ends are generally matched with connecting pipes or flanges. Many of the corrugated expansion joints used in the power station industry and the metal corrugated expansion joints used in the cement industry are circular cross-sections-because the circular pressure is the most uniform and the manufacturing is the most mature. Therefore, you see the term "metal circular corrugated expansion joint" when selecting the model, which probably refers to the landing version of the general-purpose corrugated expansion joint under specific working conditions.
Second, how to design the ripples to withstand the pressure? Talk about the key details of wavenumber, wall thickness and deflector
Ripple is the heart of the expansion joint. Its design directly determines the life of the equipment, and the core here is the wavenumber and wall thickness. The more the wave number, the smaller the single wave displacement, and the longer the fatigue life theoretically, but at the cost of axial stiffness decrease and easy instability. Wall thickness is the key to pressure bearing capacity: if the pressure is high, the wall thickness has to be increased, but guess what? As the wall thickness increases, the stiffness also increases, and the ability to absorb displacement becomes worse. Therefore, the design is always a balance problem-on the premise of meeting the pressure requirements, try to use multi-layer thin wall instead of single-layer thick wall, which is the mainstream solution for high temperature and high pressure working conditions at present.
Deflector tube. The function of the guide tube has three layers: first, it reduces the erosion of the inner wall of the corrugation when the medium flows, and avoids the corrugation from being worn down; The second is to reduce the fluid resistance and make the medium flow smoother; Third, in the flue gas and powder pipeline containing solid particles, the particles are blocked outside the corrugation. Our supporting metal corrugated expansion joints for the cement industry, the material and wall thickness of the guide tube should be calculated separately, and it is not just to weld a section of pipe.
3. What working conditions must it be used? Real application scenarios in power stations, cement and desulfurization flues
In the smoke duct system of power station boiler, the medium temperature is often three to four hundred degrees, and the thermal expansion of pipeline is very considerable. The metal circular corrugated expansion joint here has to cope with high temperature, dust, positive pressure pulsation and frequent start and stop at the same time. The corrugated material has to be stainless steel, and sometimes an external insulation layer has to be added. The working conditions of the cement industry are even more ruthless. The flue gas at the head and tail of the kiln is full of cement dust. If the expansion joint is not tightly sealed, the dust will drill into the ripple gap and jam the ripple. When it is discovered, the ripple will have cracked by fatigue. Therefore, in the cement industry, the expansion joint should not only choose the right structure, but also design the purge and ash discharge device.
What about the desulfurization flue? That's the other extreme entirely – low temperature, high humidity, strong corrosion. The flue expansion joint at the inlet and outlet of desulfurization tower contains a large amount of SO₂ and chloride ions in the medium, which ordinary austenitic stainless steel can't bear. At this time, the metal circular corrugated expansion joint has to be made of corrosion-resistant alloy, or anticorrosion coating on the corrugated surface, and then linkage control with desulfurization flue gas baffle door next to it. So you see, the design logic of the three scenarios of power station, cement and desulfurization is completely different for the same metal circular corrugated expansion joint.
4. Don't only look at the caliber when selecting the type. Displacement, stiffness and fatigue life are the core parameters
When many people choose a model, they ask "How much is DN500" in the first sentence, which is completely understandable, but the caliber is really only the threshold condition. What really determines the expansion energy saving can not be used and how long it can be used are the three hard indicators of displacement, stiffness and fatigue life. Displacement refers to the maximum axial/lateral/angular displacement absorbed by expansion energy saving. When you report this value to the manufacturer, you should leave a safety margin-for example, the actual thermal expansion is 20mm, and it is recommended to require it according to 25mm or even 30mm.
The stiffness is directly related to the force of the pipe support. If it is selected too hard, the thrust of the bracket and nozzle is large; Choose too soft, and the ripples may not be able to withstand the internal pressure thrust. Fatigue life is the underlying logic of design life. Generally, the design fatigue life of corrugated expansion joint is between 1000 and 5000 times. The power station units start and stop once a day, more than 300 times a year. If the design life is only 1,000 times, it will expire in three years. Therefore, when selecting the model, you must ask the manufacturer for the fatigue life curve, and don't just focus on "how many kilograms of pressure".
V. The easiest pits to step on and corresponding avoidance methods in installation and daily maintenance
The most common pit in the installation process is forgetting to remove the transport protection tie rod. In order to protect the corrugation, the manufacturer will use a tie rod or screw to fix the expansion joint. These constraints must be loosened after the installation is completed. We have encountered it more than once. After the customer installed it, he tested the pressure without removing the tie rod, and the corrugations could not be deformed at all. As a result, the thermal expansion of the pipeline broke all the flange bolts. So first thing after installation: check and remove all shipping fixings.
There is also a pit where the guide tube is installed in the reverse direction. The deflector is straight at one end and a bell mouth at the other, and it must be installed along the direction of media flow. The direction is reversed, and the medium directly washes the inner wall of the corrugation, which can be worn out in a few months. How do you see the direction? Manufacturers generally mark the flow direction arrow on the takeover, and just check it before on-site installation.
In terms of daily maintenance, the expansion joint body does not need to be lubricated, but if an insulation layer is set on the outside, it is necessary to regularly check whether the insulation layer is damaged and whether there is rainwater infiltration. Rainwater enters between the corrugation and insulation, which can form crevice corrosion, which is the number one cause of expansion joint failure in coastal areas. During the inspection, take a flashlight to shine on the corrugated surface, and find obvious corrosion pits or cracks. Don't hesitate to contact the manufacturer directly to evaluate the replacement cycle.
In the final analysis, the metal circular corrugated expansion joint is not a standard part that can be bought casually. It is an engineering component deeply bound to the working condition. Find out the three core parameters of displacement, stiffness and fatigue life, and cooperate with correct installation and maintenance. It is no problem for an expansion joint to last for ten years. On the other hand, the basic concepts were not sorted out before the order was placed. In the later stage, either the displacement was not enough to break the bracket, or the stiffness was wrong to tear the nozzle-this account was much more expensive than the original careful selection.