Step 1: Find out your flue conditions-why look at temperature and corrosiveness first
Hey, don't think about buying materials and drawing drawings as soon as you come up. Before doing the flue expansion joint, you have to ask yourself first: What kind of fairy gas is running in this flue? What's the temperature? Is it corrosive?
Two days ago, I met a customer, and excitedly said that I wanted to make a general metal expansion joint. As a result, I checked the working conditions-the flue gas temperature was above 600℃, and it also contained a lot of sulfide. Tsk, ordinary stainless steel bellows go up, and it will pierce in a month. What about that? It can only be replaced with high temperature resistant alloy and lined with anti-corrosion layer.
So the first step is to write down these items honestly:
- Medium temperature: Is it normal temperature, medium temperature (below 300℃) or high temperature (above 600℃)?
- Corrosive composition: Are there acidic gases such as SO₂ and HCl? Or is it just clean hot air?
- Pressure range: Positive or negative pressure? Does the pressure fluctuate greatly in the pipe?
- displacement amount: How much do you need to compensate in axial, transverse and angular directions respectively?
These data are slapped on the table, and there is a basis for selecting materials and structures later. Don't guess by your feeling, otherwise it will leak every three days after installation, and the rework cost will be much more expensive than the material cost.
Step 2: Pick the right type – non-metallic or metallic rectangle? Don't make the wrong choice
After getting the working data, the next step is to choose "skeleton". Flue expansion joints are mainly divided into two categories:Non-metallic expansion joints (fabric fiber expansion joints)AndMetal rectangular expansion joint。 How do you choose? Remember a mantra:
High temperature, large displacement, low pressure, fear of vibration, choose non-metal.
Medium and low temperature, small displacement, high pressure, load-bearing, choose metal.
The desulfurization flue of the power plant, with a temperature of about 80℃, contains acidic condensate, and also absorbs the thermal expansion and contraction of the pipeline-in this case, use decisivelyNon-metallic expansion jointIt is corrosion resistant and easy to install. The kiln tail flue in the cement industry, with high temperature but low pressure, is also the world of non-metals.
Conversely, if the flue has a rectangular section, the pressure is above 0.5MPa, and the displacement is not large, then go upMetal rectangular expansion joint, pressed with corrugated plate, high strength and long life. From our stationUniversal corrugated expansion jointAndHigh temperature axial expansion jointIt is also a similar idea, and the number of corrugated layers is selected according to the temperature.
Don't listen to fools. You can do anything. Working condition matching is the first, and if the model is wrong, no matter how cheap it is, it will be scrap iron.
Step 3: Prepare materials-skin, insulation cotton, stainless steel mesh…Don't be careless with these details
After determining whether it is non-metal or metal, you start preparing materials. Take non-metallic expansion joints as an example (everyone loves to use them), you need to prepare:
- skin: Usually use fluororubber cloth, silicone rubber cloth or PTFE cloth, which is temperature-resistant and corrosion-resistant. The thickness is 0.5-2mm, selected according to the temperature.
- Insulating cotton: Aluminum silicate fiber cotton or ceramic fiber cotton, filled inside to prevent heat conduction to the skin. The higher the temperature, the thicker the cotton layer.
- Stainless steel mesh: 304 or 316L is commonly used as a skeleton support to prevent the skin from collapsing under negative pressure. Tsk, this one is easy to overlook.
- Strips and bolts: Generally, galvanized steel or stainless steel is used for pressing strips, and the bolts must be corrosion-resistant, otherwise they will be rusted after one year.
- Sealing gasket: fluororubber or polytetrafluoroethylene board, attached to the flange surface to prevent air leakage.
Stainless steel plate (304/316L/321, etc.), guide tube, end plate, tie rod nut. Note that the function of the guide tube is to protect the bellows from being washed by high-speed airflow, which is specifically mentioned in our Q&A, so don't save it.
All materials should be re-inspected before entering the site, especially the temperature resistance test of the skin and the bulk density of thermal insulation cotton. Take a test yourself to rest assured.
Step 4: Hands-on production-blanking, assembling, pressing, sealing, every step is exquisite
The materials are ready, go dry! Take a non-metallic expansion joint as an example:
① Blanking: According to the drawing size, cut the skin into rectangle or round shape, leaving 10~15mm overlap amount. The insulation cotton is cut by the number of layers, and the stainless steel mesh is the same size.
② Layup: Start from the innermost layer: stainless steel mesh → insulation cotton → skin. The order should not be chaotic, otherwise the outer layer will burn out first at high temperature. When laying, brush a layer of temperature-resistant glue (silicone glue) between each layer to prevent slipping.
③ Assemble the press strip: Put the stacked layer into the flange groove, cover with the press bar, and tighten with bolts. Note: The bolts should be twisted diagonally and gradually tightened 2~3 times to ensure even pressure. Pad a sealing gasket between the press bar and the flange.
④ Sealing treatment: All seams are applied with temperature-resistant sealant, especially the four corners, which are most prone to leakage. Finally, apply another waterproof coating to the outer surface of the skin (if there is rain contact).
What about metal rectangular expansion joints? The difficulty lies in bellows forming and welding. The bellows is pressed out by hydraulic press or rolling press, and the welding should ensure full penetration. After welding, solution treatment should be done to relieve stress. From our stationLarge diameter thick wall expansion jointThe process is similar, but the wall thickness should be strictly controlled.
Hey, these steps just mentioned sound simple, but when you actually do it, every link may go wrong. For example, the insulation cotton was not paved and bulged; Or the beading bolts were tightened too tightly and the skin was fractured. Therefore, it is recommended to take the scraps and try a sample first when you do it for the first time.
Step 5: Inspection and factory delivery-pressure, leak detection, marking, don't wait for installation before reworking
You can't send it away directly after you finish it, you have to inspect the goods. The inspection is divided into three steps:
1. Appearance inspection: The skin is free of damage, wrinkles and bubbles, the pressing strips are flat without warping edges, and the bolts are tightened in place.
2. Airtightness test: For non-metallic expansion joints, apply soapy water to all joints and flange surfaces, fill 0.05~0.1MPa compressed air inside, and observe whether there are bubbles. For metal expansion joints, hydraulic tests are carried out according to standards, and the pressure is usually 1.5 times the design pressure. Note: Both ends of the expansion joint should be fixed during pressure test to prevent ejection from injuring people.
3. Marking and recording: Stamp the side of the flange: model, batch, date of manufacture, applicable temperature/pressure. At the same time, attach the factory certificate and test report. Many customers can't find the model after installation, and it is troublesome to replace it later.
And guess what? Some small factories skip this step, and as a result, it leaks when installed. When removed, it is found that the strips are not pressed tightly. The rework labor cost is more expensive than the expansion joint itself. Therefore, the money that should be saved can't be saved, and what should be measured must be measured.
Step 6: Installation Precautions-The expansion joint is not just installed and done
The last step, and it is also where a lot of people roll over. Keep these in mind when installing:
- Lifting should be smooth: The non-metallic expansion joint is relatively soft, so it is supported by a pallet when lifting, and the skin is not subjected to tension.
- Flange alignment: The pipe flange and the expansion joint flange should be naturally centered, and do not tighten it strongly with bolts, otherwise the internal gasket will be misaligned.
- tie rod adjustment: The tie rod nut on the metal expansion joint should be loosened before installation, and then tightened according to the designed displacement after the pipe is in place. For specific adjustments, refer to ourHow to adjust the tie rod nut of expansion jointThe Q&A.
- Thermal insulation protection: If the outer surface of the flue needs to be insulated, the expansion joint should also be insulated, but do not press it too tightly, which will affect the expansion and contraction of the expansion joint.
- Leave room for overhaul: Leave at least 200mm space around the expansion joint to facilitate later inspection or replacement of bolts.
The production of flue expansion joint is not a black technology, but every detail affects the service life. From working condition analysis to material selection, from manufacturing process to installation acceptance, one wrong step may cause the whole line to stop. You can practice it according to the steps in this article, or you can directly find reliable manufacturers to customize it-all types of expansion joints (non-metal expansion joints, metal rectangular expansion joints, general-purpose corrugated expansion joints, etc.) in our station have mature products or technical support, so feel free to contact us.