Doing fan duct design or equipment maintenance, who hasn't been pitted by expansion joints a few times? You choose one according to the standard sample and install it. As a result, as soon as the unit runs, the bellows clicks, the flange leaks, and even the weld seam directly cracks. This problem does not necessarily lie in the product quality, but most likely because you think about the customization of the metal expansion joint of the fan simply. It's not like buying standard parts, just report the diameter and pressure. The vibration, thermal displacement and dusty airflow in the fan duct are all testing the design margin. To find out how to customize it, you have to break up the working conditions of the fan duct and crumble them into pieces.
Why can't the fan duct bypass the metal expansion joint? Understand the working conditions first and then talk about customization
What is the biggest difference between the fan inlet and outlet pipeline and the general process pipeline? It is the simultaneous existence of vibration and thermal expansion. When the fan body runs, it produces continuous low-frequency vibration, and the medium in the pipeline is high-temperature flue gas or hot air. As soon as the temperature of the pipeline wall goes up, the pipeline will extend along the axial direction and deform in the radial direction. At this time, you hard weld the pipe to death, and the stress is all held on the fan interface. In the least, the bearing wear is aggravated, and in the worst case, the shell is torn.
It is necessary to absorb thermal displacement, resist vibration fatigue, and at the same time, it is necessary to withstand airflow erosion. When these three things are put together, it is not something that a standard sample can cover.
Four hard indicators that must be confirmed before customization: diameter, pressure, temperature, displacement
Don't rush to draw a picture and quote, first set the four numbers to death:
- Diameter:Not only the size of the flange interface, but also the wall thickness of the pipe. If the inner diameter is inconsistent with the inner diameter of the pipe, steps will be produced, and the airflow will form a vortex here, which will be washed down for a long time, and the guide tube will rot first.
- Pressure:There is a common mistake here-use the full pressure of the fan as the design pressure. In fact, the expansion joint is subjected to the operating pressure of the duct system, plus the instantaneous fluctuation when the fan starts and surges. If you report 0.05MPa, the actual operation rushes to 0.1MPa, and the bellows will bulge.
- Temperature:Temperature directly affects the material selection and fatigue life of bellows. Below 100℃ and over 400℃ are completely two design ideas.
- Displacement Amount:Many people only report axial displacement and ignore lateral displacement. The fan duct often has turns, and the thermal expansion will be converted into lateral displacement at the elbow, so axial absorption alone is not enough.
Among these four items, the displacement is the easiest to report errors, and it is also the most fatal. When I encountered a project before, the customer said, "The displacement is not big, so you can just leave some margin". As a result, after installation and operation, the thermal expansion of the pipe twisted the expansion joint into a twist. So don't be afraid of trouble. Provide the position, direction and medium temperature of the pipe fixing bracket to the manufacturer, which is much more reliable than you pat your head and report a number.
Structural Selection: Single, Duplex or Hinged? Different fan pipe systems have different solutions
After setting the parameters, the next step is the structural form. There are three common scenarios on fan ducts:
A fan inlet and outlet straight pipe section,Mainly absorbs axial displacement, withUniversal corrugated expansion jointThat's right, you can find it in the standard parts. But if you are worried about excessive pressure on the bellows or axial displacement exceeding the single-wave limit, considerExternal pressure single axial expansion joint— — The external pressure structure allows the bellows to bear external pressure instead of internal pressure, which has good stability and is suitable for large displacement occasions.
Space constrained or need to absorb lateral displacement,For example, at the corner of the fan outlet, selectCompound hinge transverse expansion jointSmarter than hard-carrying lateral displacement. It converts lateral displacement into angular displacement through two bellows and hinge structure, and the bellows bear uniform force. In another case, the pipeline is complicated and needs to absorb displacements in multiple directions at the same time, so it is necessary to useCurved tube pressure balance expansion jointIt can balance the internal pressure thrust off and reduce the thrust on the fixed bracket.
As for the high-temperature flue gas section,For example, the material and structure of the flue in front of the fan have to be changed. Here, I don't recommend using ordinary metal bellows to carry it hard. When the temperature exceeds 600℃, the fatigue life of metal bellows will drop from a cliff. Can considerNon-metallic expansion joint (fabric fiber expansion joint)For flexible connection, its temperature resistance is achieved by heat insulation layer, low cost and convenient maintenance. Of course, if the medium is corrosive and abrasive, then useLarge diameter thick wall expansion jointSacrifice a part of the compensation amount to exchange the service life.
Material and details: bellows, guide tube, flange and interface, how to match so that no accident
The structure is fixed, and the material is the line of life and death. Look for several corrugated pipe materials: the medium contains chloride ions, don't use 304, use 316L; When the temperature exceeds 400℃, 321 stainless steel is used, which has better high-temperature creep resistance; If it is an acid and grinding environment of desulfurization flue gas, go directly to 254SMO super austenitic stainless steel, don't hesitate.
Another important point isguide tube。 Many people ignore it and think it is just a liner. Wrong. The function of the guide tube is to let the airflow pass through the bellows smoothly and avoid the high-speed airflow directly washing the corrugated valley. The wind speed at the outlet of the fan is always more than ten meters per second, and the dust content is slightly larger. Without the guide tube, the bellows wall can be worn out in less than a month. However, the guide tube is not casually equipped-it must form a smooth transition with the inner wall of the pipe, and the front end should be made into a bell mouth, otherwise the airflow will separate at the step, which will aggravate local wear. The material should be wear-resistant, and the thickness should not be thinner than the wall of the pipe. It is best to choose the same material or more wear-resistant material as the pipe.
Flanges and interfaces are another pit again. Fan pipelines are usually not standard pipe diameter, especially in the renovation of old projects, and the actual outer diameter of pipelines does not match the standard. It is much better to provide flange bolt hole distribution circle diameter and bore diameter directly when customizing than reporting a DN100 and then not installing it on site.
From drawing to delivery: Pit avoidance guidelines for custom processes, factory inspections, and on-site installations
A real professional fan metal expansion joint customization factory will confirm three nodes with you after placing an order, and you have to pay attention to one less:
First, the drawings are countersigned.This link is not a process. You have to confirm the details of bellows wave number, guide tube length and flange material. At the same time, look clearly at the overall length of the expansion joint and other mounting dimensions-the overall length is the basis of the mounting. Generally, when leaving the factory, the expansion joint will be pre-stretched or pre-compressed, so you should also confirm whether the pre-displacement amount of the bellows conforms to the actual situation at the site.
Second, factory inspection.According to GB/T 12777 standard, hydraulic pressure test and air pressure test must be done before leaving the factory, and displacement sampling inspection must be carried out. You can ask the factory to provide bellows material reports and pressure test records. If the factory says "our quality is stable, there is no need to do testing", change the supplier directly.
Third, on-site installation.What's the easiest mistake to make here? Just don't remove the transport tie rod. The transport tie rod is to prevent the bellows from deforming during transportation and must be loosened or removed after being installed in place. Many people forget this, the expansion joint completely loses its compensation function, and the force of thermal expansion and contraction of the pipeline is all suppressed on the equipment.
In addition, there is usually a corresponding error when installing fan ducts-the misalignment cannot exceed 10% of the wall thickness, otherwise the bellows will be unevenly stressed and the local stress will be too large. After alignment, then weld, check whether there are cracks in the weld after welding, and do another air tightness test to confirm that there is no air leakage.
In the final analysis, the customization of fan metal expansion joint is a matching homework: the working conditions are passed to the design, the design is transformed into structure and material, and the manufacturing is restored to the drawing requirements. Which link is disconnected, and the failure will pay for it in the end. Do you have a recent fan duct project on hand? Compare these five links and go through it. If you are unsure, you can send the drawings to have a look at them at any time.