Circular belt is not just cutting pieces of cloth-only by knowing the structure can you calculate correctly
The ring belt of non-metallic compensator, to put it bluntly, is a flexible section stacked with multilayer composite materials. Common ones like the ones we have on our stationNon-metallic expansion joint (fabric fiber expansion joint)、Rectangular non-metallic expansion jointSilicone cloth, fluororubber, fiberglass wool and PTFE film are sandwiched in the band. Each layer has different functions: some are temperature-resistant, some are pressure-resistant, and some are sealed. You just look at the thickness to calculate? There is a high probability that the car will flip.
For example, silicone cloth is mainly responsible for sealing and elastic deformation, glass fiber wool is responsible for heat insulation, and fluororubber layer is responsible for corrosion. What adhesive is used between layers and how to arrange the lamination sequence directly affects the overall performance of the loop belt. Many customers took the sample block and said to me, "My ring belt is 5mm thick, is it enough?"-I asked him what the temperature, pressure and displacement of the medium was, but he didn't know. Do you think this algorithm can be reliable?
Before calculating the circular zone, figure out the displacement
Many engineers ask "what is the band width" when they come up, but the most important thing to determine is the thermal displacement of the pipe in all directions-axial compression/tension, lateral offset and angular displacement. For example, the power station industryCorrugated expansion joint for power station industryThe matching non-metallic ring belt often absorbs three-way displacement at the same time. The width of the band (also called the effective length) is directly linked to the displacement: the larger the displacement, the wider or more layers the band must be, otherwise the stress will concentrate and it will tear in a few months.
I have seen a case of a coal-fired power plant. The exhaust duct used a universal corrugated expansion joint with non-metallic ring belt. Only the axial compression was calculated in the design, and the lateral offset was not considered. As a result, after three months of operation, a hole was directly torn on the side of the loop belt. Later, the problem was solved by recalculating, adding the lateral displacement, and changing the effective width from 200mm to 350mm. So remember: the amount of displacement is not accurate, and the rest is all for nothing.
Temperature and pressure are hard boundaries, don't guess
Refer to JB/T 12235-2015 National Standard for Non-Metallic Expansion Joints, the temperature resistance grade of the ring belt material covers the maximum temperature (plus safety margin) that the medium can reach. TakeMetal Corrugated Expansion Joints in Cement IndustryIn terms of scenario, the flue gas temperature is often above 400℃, so the glass fiber base cloth in the ring belt has to be made of high silicon oxygen, and the outer coating has to be made of temperature-resistant fluororubber. The pressure is more direct. The blasting pressure of the ring belt must be 3-5 times the design pressure, which depends on the number of layers and suture strength.
Two days ago, I met a customer who used the room temperature ring belt on the steam pipe, and it bulged in a week-the wrong material was chosen, and it was useless to count more. The design pressure of his working condition is 0.8MPa, but the blasting pressure of the ring belt is only 1.2MPa, and the safety factor is less than twice. Normally, it should be used 3-5 times, that is, at least 2.4MPa. The number of layers was added by two layers, and the sutures were replaced with Kevlar, so the problem was solved. So before you calculate the loop belt, weld the two boundary conditions of temperature and pressure to death, otherwise the rest is full of bullshit.
The "effective width" and the "total width" of the band are two different things
A lot of people are planted here. The effective width refers to the part of the ring belt that can be freely deformed, and the total width should be added with the margin of compression of flanges at both ends. How to calculate margin? Generally, leave 50-80mm according to the distance from the center of the flange bolt hole to the edge of the ring belt, depending on the media erosivity. The total length (perimeter) of the ring belt is multiplied by a coefficient according to the cross-sectional size of the pipe plus 2 times the effective width, and the wrinkle margin at the fillet corner of the rectangular pipe must be considered. Calculate a wrong step. Either it can't be pressed tightly and leaks, or the length is not hard enough to pull.
I also encountered on-site workers who directly cut the ring belt into the circumference of the pipe section. As a result, when they installed it, they found that the flange could not be pressed because the pressing allowance was not counted. Later, the factory did it again, and it took three more days to build it. If you use the one from our stationRubber PTFE compensatorOrRectangular non-metallic expansion jointWhen you leave the factory, the effective width and total width are generally marked, but once the size of the pipeline on site changes, you have to recalculate it yourself. Remember the formula: total width = effective width +2× compression allowance; Perimeter =2× (length + width) +2× effective width + fillet compensation. This fillet compensation is generally multiplied by the sum of side lengths by 0.1-0.2, depending on the actual curvature.
Let's talk about the calculation of the number of circle layers-the more the better
Each layer of material has its own elastic modulus and fatigue resistance characteristics. With more layers, the overall stiffness will increase sharply, and the compensator will not be able to absorb the displacement. The common practice is: first calculate the required total tensile/compressive deformation according to the displacement, then check the allowable deformation rate of each layer of each material (for example, silicone cloth single layer can be deformed by ±15%, fluororubber ±10%), divide the total deformation by the allowable rate of single layer, and round up to get the minimum number of layers. Then hit 1.2 times the fatigue safety factor.
For example, if the axial displacement is 20 mm and the effective width is 300 mm, the tensile deformation rate =20/300 ≈ 6.7%. If the allowable rate of single-layer silicone cloth is 15%, one layer is enough in theory, but considering the fatigue life, two layers plus a layer of fluororubber are generally used as the protective layer. That 2 layers of silicone cloth plus 1 layer of fluororubber, will the total deformation rate decrease? Yes, because the elastic modulus of fluororubber is high, it will pull up the overall stiffness, so the actual deformation ability should be re-checked. It is best to use the stacked deformation model to calculate, and the simple point can be weighted average according to the deformation rate of each layer. If you use our site'sRectangular non-metallic expansion jointOrrubber compensatorWhen you leave the factory, there is usually a recommended layer table, but you still have to recalculate it yourself if the working conditions on the site change-don't be lazy.
Finally, let's talk about a point that is easily overlooked: the amount of pre-compression after the loop belt is installed
When the non-metallic compensator is installed in the cold state, it is usually necessary to pre-compress 10%-20% of the belt length, so that the belt will not be tensioned during thermal expansion. How to calculate the amount of pre-compression? Refer to half of the design thermal elongation of the pipe, combined with the rebound rate of the ring belt material. In the example, there is a project for desulfurizing flue gas, which usesDesulfurization flue gas baffle doorWith the non-metallic expansion joint, the pre-compression amount was not calculated correctly, and the edge of the loop belt was wrinkled after two weeks of trial operation.
The thermal elongation of that project is designed to be 40mm, so it stands to reason that pre-compression of 20mm is enough. However, the operator saved trouble when installing it, and only pressed 5mm. As a result, as soon as the pipeline heated up, the ring belt was tensioned and pulled out the folds. Later, it was removed and reinstalled, pre-compressed and adjusted to 18mm, with 1.2 times rebound coefficient, and there was no problem after half a year's operation. So don't bother, this step can't be saved. The calculation formula of the pre-compression amount is: P = (Δ L/2) × K, where K is the material rebound coefficient (1.1-1.3 for silica gel cloth and 1.0-1.2 for fluororubber), and Δ L is the total thermal elongation of the pipe. When installing, use hydraulic pressure or screw to press the ring belt to the design value, and then lock the flange bolt. If you use our station'sNon-metallic expansion joint (fabric fiber expansion joint)The recommended value of pre-compression will be marked in the installation manual, but the direction of the pipeline on site and the position of the fixed bracket have changed, so you have to calculate it yourself.
Belt calculation is not something that can be done by a set of formulas. Displacement, temperature, pressure, effective width, number of layers and pre-compression amount, if any of these five parameters go wrong, the life will be discounted. Next time someone asks you "How to calculate the non-metallic compensator ring belt", dump this article to him and save you the words.