Why do you use square non-metallic compensators for smoke and air ducts?
You may think that the smoke duct is full of high-temperature smoke and dusty air flow. The pressure is not high, but the temperature always moves by two or three hundred degrees. Why don't you use metal bellows, but choose a square non-metallic compensator? In fact, the reason is very simple: the cross-section of smoke and air ducts is usually square, and the size is large, and it is always one meter by one meter or even larger. The metal bellows should be made into a square shape, which is difficult and costly, and has limited ability to compensate axial displacement. Non-metallic compensators (that is, what we often call fabric fiber expansion joints) are different-the skin material is flexible fabric, which can naturally absorb multi-directional displacement, and it is corrosion-resistant, high temperature-resistant, light in weight, and hassle-free to install. To put it bluntly, this is the most cost-effective scheme.
But don't think that just buying a square non-metallic compensator will work. It's no joke to make a mistake in the selection, and it leaks after three months of installation. Let me break it down and talk about it step by step, and there is a way for each step.
Three data that must be understood before selection: temperature, pressure and displacement
I met a customer two days ago, and he asked, "What is the maximum degree that your square non-metallic compensator can withstand?" I said you should send the working condition parameters first. He threw a temperature: 350℃. What about the stress? Micro positive pressure. What about displacement? He said he didn't know. That's the typical rollover precursor.
The first hard indicator of model selection isTemperature。 Skin materials are divided into silicone cloth, fluorine cloth, polytetrafluoroethylene, etc., and the upper limit of temperature resistance is different. It is no problem to use silicone cloth within 350℃, but ceramic fiber insulation layer must be added if it is higher. The second ispressureDon't just look at the design pressure, but also whether there is alternating positive and negative pressure in actual operation. Under negative pressure, the skin is easy to deflate, so it has to be supported by a skeleton. And the third isdisplacement— — Axial, transverse and angular displacements should be clarified. The square non-metallic compensator is good at absorbing lateral displacement, but when the axial displacement is too large, the corrugation depth or the number of layers must be increased in the structural design. If one of these three sets of data is missing, the manufacturer can't match the thickness of insulation layer, frame stiffness and number of skin layers for you, and the final product is waste.
Structural disassembly: skin, frame, insulation layer, any problem in which link is useless
Although the square non-metal compensator is simple in appearance, the three-layer structure inside can't be sloppy.
skinIs directly in contact with smoke, generally made of multi-layer composite materials superimposed: the outermost layer is fluororubber or silicone rubber coated cloth, anti-corrosion; The middle is glass fiber cloth or polytetrafluoroethylene film to enhance airtightness; The inner layer is exposed to high-temperature flue gas, and ceramic fiber cloth or high silicon oxide cloth is used. The lap width of each layer, the sewing process will affect the service life. Some small factories only make two layers of skin, which saves costs, and as a result, it cracks in three months.
frameIt is a steel skeleton, usually with Q235 or weathering steel, and the surface anti-corrosion coating should be in place. The corner weld of the frame is the weakest, so it is necessary to be grooved and fully welded, not spot welded. AgainThermal insulation— — Many non-metallic compensators fail early because of unreasonable insulation design. When the flue gas temperature exceeds 200℃, the insulation layer must be filled densely with a ceramic fiber blanket with a thickness of at least 50mm. If the thickness of the insulation layer is not enough or the filling is uneven, the heat is directly transferred to the skin, and the skin will accelerate the aging. Can it not break?
In addition, some working conditions need to install guide tubes (refer to the function of expansion joint guide tubes in this station) to avoid high-speed dusty air flow directly washing the skin. This detail is often overlooked, but the actual effect varies greatly.
Several details of the easiest rollover at the installation site
Half of the equipment was chosen right, and all the wrong equipment was wasted. The most outrageous thing I've ever seen is that installers welded square non-metallic compensators directly to the pipe as ordinary flanges-the compensators couldn't move freely at all, and they pulled hard and tore in a month.
First, before installationThe fixing screw for transportation must be removed(Refer to the question and answer of this site "Does the screw of the expansion joint need to be disassembled"). Many manufacturers will add screws between flanges to tighten them for transportation safety when they leave the factory. If they are not disassembled, the compensator is equal to a rigid connection and has no compensation effect.
Second, when installingThe flange surfaces should be aligned and the bolts should be tightened symmetricallyThe moment is uniform. Don't tighten one side before the other, because the skin will twist and deform. It is recommended to tighten diagonally in 2-3 times.
Third,Pre-tension/pre-compression of compensatorBe sure to adjust according to the installation length given by the manufacturer. Smoke and air ducts usually have cold and hot states. If the ambient temperature is low during installation, it should be pre-pulled for a certain displacement; If the temperature is high, it is pre-pressed. Specific numerical design drawings will be marked, don't rely on feeling.
Fourth, the pipeline support close to the compensator should have guiding and limiting functions to prevent excessive lateral displacement of the pipeline. The compensator is not a panacea, it can only absorb the displacement within the design range, and it will break if it exceeds.
How can routine maintenance be done to make it last to its designed life?
The design life of non-metallic compensators is generally 3-5 years, but many leak after only one year of use. There are no more than two reasons: media corrosion and mechanical damage.
One isInspect skin appearance。 Focus on whether the skin has bulging, cracking and delamination. Especially at the root of the flange, where stress is concentrated. If small cracks are found to be repaired in time, they can be temporarily treated with special tape, but it is best to change the skin.
Second,Check insulation condition。 By observing the holes or removing the local skin, see if the insulation cotton has collapsed and coked. If the temperature of the shell is found to rise abnormally (exceeding 60℃), it means that the insulation layer has failed, so replace it directly.
Third,Cleaning up dust。 There is a lot of dust in the smoke and air duct, and the trough of the square non-metallic compensator is easy to accumulate dust, which agglomerates and hardens over time, which affects the displacement performance. Clean it with compressed air every year during shutdown for maintenance.
In addition, it is recommended to install temperature monitoring points near the compensator if conditions permit. Once the operating temperature is found to exceed the design upper limit by more than 10%, the machine has to be stopped for troubleshooting-the skin may have aged. Don't wait until there is an air leak before rushing for spare parts. That kind of passive repair is much more costly than regular replacement.
Square non-metallic compensators are not the more expensive the better, nor is it the cheaper the better. The key is working condition matching and installation specification. When selecting the model, give sufficient temperature, pressure and displacement data, so that the manufacturer can make customized plans according to your working conditions. Saving trouble is saving money. Only by understanding this truth can the equipment be used.