Two days ago, I chatted with a friend who was doing a power station flue project. He said that when purchasing rectangular metal expansion joints, he was asked by the manufacturer "How much is the length and width? How much is the compensation?", and he was stunned at that time. Isn't this scene quite familiar? In fact, the size of rectangular metal expansion joint is far more than the length and width, but most people easily simplify it in the selection stage. This article will talk about the size from parameter definition, working condition selection, to installation and docking.
What are the core parameters of rectangular metal expansion joint dimensions
What exactly is quoted when quoting dimensions? In the size system of rectangular metal expansion joint, the flow area is the first place. This value directly determines whether the smoke can pass through smoothly. If the calculation is wrong, the flow rate will increase, the resistance will increase, and the pressure head of the entire air duct system will have to be re-checked. The flow area is usually calculated according to the width × height of the inner wall of the pipe, and the unit is mm². It is not a simple opening size, but the space occupied by the internal guide tube and heat insulation layer should be deducted.
Then there's wave height and wave distance. Wave height is the vertical height of the ripple from peak to trough, which determines the displacement absorption capacity and pressure level-the wave height is large, the flexibility is good, but the pressure tolerance will decrease. Wave pitch is the distance between two adjacent crests, which affects the axial stiffness and fatigue life of the expansion joint. The effect of these two parameters on the size is that the total length of the expansion joint has to be adjusted as soon as the wave height and wave pitch change for the same flow area.
The number of layers is also one of the least asked parameters. Many people think that the more layers, the stronger they are. In fact, the more layers are mainly to reduce the stress of single-layer corrugations, which is suitable for scenarios with high pressure or greater compensation. However, as the number of layers increases, the overall thickness increases, the flange spacing will be enlarged, and the installation length will also change. The flange size is more intuitive here: the outer dimensions of the rectangular flange, the center distance of the bolt hole, the number of bolts and the hole diameter, these must be completely aligned with the pipe flange, and it can't be installed by one millimeter.
How to choose the size under different working conditions
The flue gas pipeline of power station takes the road of large flow, low pressure and high temperature. The flue section is always two or three meters wide, and the dimensional design of rectangular metal expansion joint focuses on absorbing heat displacement and preventing flue gas leakage. Under this condition, the flow area is the first constraint condition, and the flow velocity is generally controlled at 15~20m/s. If the area is small, the vibration and noise of the pipeline will appear immediately. The corrugated part is often single-layer or double-layer, and the inner wall of the corrugation is protected by the guide tube. The length of the guide tube should be counted into the total installation size when designing the size.
The situation in the cement industry is different. The dust content of the kiln tail flue gas is large, and the temperature fluctuates frequently, so the internal wear of the expansion joint is a big problem. When selecting the model, the thickness of the wear-resistant lining should be considered in size, and the flow area will be larger than the theoretical calculation value. Otherwise, the effective flow area will be reduced after dust accumulation, and the system resistance will soar. In addition, the replacement window period of expansion joint of cement line is short, and the size of flange bolt hole should be conventional specifications as much as possible, which is convenient for quick replacement on site.
The size of rectangular metal expansion joint of desulfurization system should focus on corrosion margin and slurry deposition risk. The rectangular expansion joints at the inlet and outlet of the desulfurization tower are usually larger in size because they are required to accommodate FRP or alloy liners. Under this working condition, the wave height of the corrugation should be increased appropriately to prevent the slurry from depositing and fouling in the trough. It is recommended that the flange surface be processed as a whole sealing surface, and the size of bolt holes should be implemented according to HG/T 20592 standard, which is convenient for docking with the desulfurization flue gas baffle door.
Conversion Logic of Size, Compensation Amount and Pressure
The relationship between size and compensation amount is not simply "the larger the length and width, the larger the compensation amount". The amount of compensation depends on the wave number, wave height and the number of layers of the ripples, and has no direct linear relationship with the flow area. Two examples are given for comparison: it is also a rectangular section of 3000×3000mm, one with 4 waves and a wave height of 120mm, and the other with 6 waves and a wave height of 90mm. The axial compensation amount of the former may be 1.5 times that of the latter, but the pressure capacity is 20% lower. This is why you can't just look at the length and width to place an order.
Pressure affects the combination of wave height and layer number. For flue gas pipelines with pressure below 0.1MPa, single-layer corrugation and large wave height are sufficient; Once the pressure exceeds 0.2MPa, either reduce the wave height and increase the wave number, or increase the number of layers, but the axial stiffness of the expansion joint will rise, the elastic reaction force will become larger, and the force of the fixed bracket should be re-checked. There is a complete calculation process here, which corresponds to the stress analysis report of the design institute. When selecting the model, the four numbers of length, width, temperature, pressure and displacement are directly reported, so that the manufacturer can give you a plan according to EJMA standards, which is much more reliable than patting your own head.
Non-standard rectangular expansion joint size customization process
Most rectangular metal expansion joints are non-standard custom-made.The first step is field measurement— — The actual length and width of the pipe flange interface and the hole spacing of the flange bolts must be measured. There are often deviations between the dimensions on the drawings and the on-site installation, especially in old system renovation projects. The original flange may have been deformed or offset, and the paper data can only be used as reference.
The second step is to confirm the amount of displacement. For the values of axial, transverse and angular displacement of pipelines, it is best that the design unit issue written compensation data or provide a pipeline stress analysis report. Without these data, you can also make an estimate based on the direction of the pipe, the position of the fixed bracket and the maximum operating temperature, but the error will be much larger.
The third step is to connect with the manufacturer's product information with the demand. The cross-section size range, wave height series, layer number options and corresponding compensation ability of the rectangular expansion joint will be marked on the product information of regular manufacturers. You send the field measured dimensions and displacement requirements, and the manufacturer's application engineer will calculate the flow area and flow rate back, and determine the length of the guide tube and the flange structure. In this process, it is best to write down the media characteristics (dust content, corrosive components, maximum instantaneous temperature) clearly at one time, so as to avoid rework due to material change after quotation.
Installation Dimensions Considerations
The most prone to problems during the installation stage is the direction of the guide tube. The orientation of the guide tube must be consistent with the flow direction of the medium in the tube. If it is installed backwards, it will not only fail to protect the corrugation, but also hold the wind and generate vortex. After a long time, the inner wall of the corrugation will be washed away and worn, and air leakage will be a matter of time. Before installation, compare the arrows marked at both ends of the expansion joint with the flow direction of the flue. This kind of low-level mistake should really not be made.
The matching of flange bolt holes is another high-incidence problem. The bolt hole positioning hole spacing and bolt diameter of the rectangular flange must be checked with the pipe flange hole by hole. Especially when it is directly connected with equipment such as desulfurization flue gas baffle door and electric plug-in plate insulation door, the flange hole position of the other equipment is fixed, and the flange of the expansion joint must be processed according to its actual size, and the standard flange hole distance cannot be hard sleeved. The length of the connecting bolt should also be calculated well. After adding gaskets and nuts, it is advisable for the screw to expose 2~3 buckles. If it is too long, it will affect the operating space of adjacent pipelines.
Finally, don't forget to reserve access space. The rectangular metal expansion joint is generally set in the middle section of the flue, and there are often obstacles such as baffle door actuator, measurement and sampling port and insulation layer around it. In addition to its own length, the installation size must also include the space for flange bolt disassembly and the gap between the corrugated surface and the insulation layer, leaving at least 50mm. Otherwise, if you want to change the bolt or check the corrugated surface after running for a period of time, and find that the wrench can't reach in, you will know what it means to be wronged.
The dimension of rectangular metal expansion joint is, in the final analysis, a matter of communication accuracy. Report the parameters and explain the working conditions thoroughly, and the manufacturer can give you a suitable scheme. On the other hand, only throw a length and width dimension in the past and let the manufacturer take care of it. If there is a problem with the installation, the construction period and cost will be included.