What exactly is a rectangular rounded corner metal expansion joint? First distinguish the difference between it and the circular expansion joint
On large-section pipelines such as flue of power plants and air ducts of cement industry, circular expansion joints often can't be installed-either there is not enough space or the cost is ridiculously high. At this time, you have to use a rectangular structure. The metal rectangular expansion joint of this station is specialized in dealing with this kind of working conditions.
What is that "rounded corner" for? Not to look good. The biggest difference between a rectangular bellows and a circular bellows is in the four corners. The fillet is directly related to the forming quality and stress distribution of the corrugation, and this detail determines whether you can safely run a maintenance cycle.
Why do you want rounded corners?
The radius of the fillet is small, and the root of the corrugation is easy to crack; The radius is larger, and the effective compensation area is eaten again. In actual design, the fillet radius is usually 0.2 to 0.5 times of the wave height, and the specific value depends on the thickness of the plate and the forming process. There's no room for patting the head here.
With rectangular expansion joints without rounded corners, the stress concentration on the four corners is particularly serious, and problems occur in less than a few cycles. This is the same as sheet metal bending-you fold a right-angled iron sheet, and the corners will definitely crack first. When the bellows bears repeated displacement, the corner is the weakest link, and the rounded corner is equivalent to smoothing out this stress peak.
Three Parameters Most Easily Overlooked in Type Selection
Many people only focus on the compensation amount, and forget that when the rectangular expansion joint is subjected to internal pressure, the film stress at the four corners is much higher than that at the straight edge section. The pressure parameter is much more sensitive to rectangular structure than circular structure, so the corner stress must be checked separately when selecting the type.
Temperature. When the temperature is high, the allowable stress of austenitic stainless steel falls down, and the number of corrugations has to increase. For the same rectangular expansion joint of 3000×2000, the corrugation number may be twice different under the working conditions of 150 ℃ and 350 ℃. If you take the normal temperature parameters to set the high temperature working conditions, the fatigue life of bellows will drop like a cliff.
Not to mention fatigue life. When designing, it should be calculated according to the actual number of cycles. Don't take the general parameters hard. This station has corrugated expansion joints for power station industry and metal corrugated expansion joints for cement industry. The cycle times of the two industries are required to be several orders of magnitude-the desulfurization flue of power station may be cycled several times a day, and the kiln fan pipe of cement industry can give you dozens of times an hour.
Common pits at installation sites
The deflector is set in the opposite direction? The medium directly washes the corrugation, and it is a matter of time before it wears out. The function of the guide tube is to let the medium go through a smooth channel and prevent the airflow from directly hitting the corrugated surface of stainless steel. Take a look at the arrow sign before installing it. This step takes less than two minutes.
Many people simply skip the step of pre-displacement. For pipelines with large thermal displacement, no pre-displacement is equivalent to letting the expansion joint do half the work extra, and the life is directly discounted in half. The direction of pre-displacement is opposite to the direction of thermal displacement. To put it bluntly, it is to "break" the expansion joint to the middle position in advance, so that its deformation in actual operation falls within the design range.
The brace can't be sloppy. The distance between the main fixed bracket and the guide bracket is not calculated correctly, and the actual displacement of the expansion joint is completely different from the design value. As soon as the pipe expands and contracts by heat, the force does not take the design route, and it is all stuck on the bellows. As a result, the corner is the first to have problems.
Maintenance and overhaul Don't wait until you leak to remember
The daily inspection focuses on whether there are rust spots, scratches and dust accumulation on the corrugated surface, especially the rounded transition area. This position has a poor view and is easily overlooked, but it is where the stress is concentrated. Once the corroded spot penetrates the corrugated wall thickness, the entire expansion joint is scrapped.
Check if the guide tube is loose every six months, and confirm the locking state of the tie rod and nut. This site has questions and answers on how to adjust the tie rod nut of the expansion joint. The general idea is that the nut should be re-locked after the displacement reset, otherwise the expansion joint will lose its constraint and allow the pipeline displacement to pull the bellows hard.
Tell me a practical case
A rectangular rounded metal expansion joint is used in the desulfurization flue of a power plant, with a specification of about 3000×2000. The design temperature is 150℃, the pressure is 5kPa, and the compensation amount is 30mm in the axial direction and 15mm in the transverse direction. At the beginning, the ordinary rectangular corrugated pipe was not optimized for rounded corners, and cracks appeared in the corners after three months.
Later, it was changed to a fillet structure, the fillet radius was made according to 0.35 times of the wave height, and the plate was replaced with 316L, which took more than two years without any problems. The same working condition, the same set of design parameters, the difference lies in this rounded corner. Therefore, the details of rounded corners can't be saved.