1. What does the expansion joint at the end of the flue do?
Many people think that the expansion joint at the end of the flue is a "telescopic tube", which is used to offset thermal expansion and contraction. Actually, what it does is more complicated than you think. Especially in power stations, cement and chemical industries, the end of the flue is connected to a dust collector, desulfurization tower or induced draft fan, and the pipeline is full of high-temperature flue gas, as well as dust and corrosive gases. The expansion joint should not only absorb axial displacement, but also carry radial offset and angular displacement. Think about it, the flue is hundreds of meters long. As soon as the temperature rises, the pipe will not only become longer, but also be distorted due to uneven settlement of the support and wind load. If the expansion joint is not selected correctly, it will either get stuck or leak, and in serious cases, the equipment will be directly pulled out.
Many on-site faults are not the poor quality of the expansion joint at all, but the parameters are not calculated correctly. Therefore, don't turn over the sample to select the model as soon as you come up, and honestly find out the three basic parameters: temperature, pressure and displacement.
2. Three bottoms that must be found out before calculation: temperature, pressure and displacement
These three parameters are like oil, salt, sauce and vinegar for cooking. If one dish is missing, it will be wasted.
- Design temperature: It is not the operating temperature, but the limit temperature. For example, the smoke is normal at 250℃, but it may rush to 350℃ when it starts and stops. You have to calculate it according to 350℃. In addition, consider the temperature gradient-the temperature difference between the inner and outer walls of the flue will lead to uneven expansion, especially the fiber layer of non-metallic expansion joints, which will burn up when the temperature is too high.
- Design pressure: Most flue systems are slightly positive or negative pressure, but don't be careless. The entrance area of the desulfurization tower may have an instantaneous positive pressure impact due to the fan surge. One project once selected an ordinary non-metallic expansion joint, and as a result, the pressure fluctuation directly tore the skin. It is an industry rule that the pressure parameter should be 1.5 times the maximum working pressure as the safety margin.
- displacement amount: Here are the most pits. The displacement includes thermal expansion displacement, installation error displacement and foundation settlement displacement. Thermal expansion can be calculated by formula (detailed in the next section), but the installation error and settlement are estimated by experience, and it is generally recommended to reserve a margin of ±10mm. For example, the horizontal section of the flue is 20 meters long, and the thermal expansion is calculated to be 45mm, so the displacement capacity should be at least 60mm when you select the model.
3. How to calculate thermal expansion? The formula is simple, but the pits lie in the details
Δ L = α × L × Δ T. α is the linear expansion coefficient, about 12×10⁻⁶/°C for carbon steel and about 17×10⁻⁶/°C for stainless steel; L is the length of the pipe in mm; Δ T is the temperature difference in °C. For example: a section of carbon steel flue with a length of 15 meters, rising from the installation temperature of 20°C to the operating temperature of 300°C, Δ L =12×10⁻⁶ ×15000×280=50.4mm. ok, the numbers are out, and then what? Here comes the pit.
The pipe is not a whole one, there are elbows, tees, and baffle doors in the middle. The elbow itself will absorb part of the displacement, but in actual calculation, it is usually conservatively calculated according to the length of the straight pipe section, and safety comes first. The second pit: multiple pipes in parallel flue, such as double flue, each pipe has different expansion amount, so the expansion joint can't be selected independently, so the mutual constraints between pipe bundles have to be considered. Third Pit: If the end of the flue is connected toFlue gas baffle door(For example, the desulfurization flue gas baffle door or electric plug-in insulation door of this station), the frame stiffness of the baffle door is limited, and the thrust generated by the expansion joint will deform the door body. At this time, it is necessary to select the expansion joint with a tie rod to offset the pressure thrust.
So don't worry and be happy after calculating the values, ask yourself: Do I have any of these "invisible killers" in my plumbing system?
Fourth, the key to selection: non-metallic expansion joint or metal rectangular expansion joint, which should you use?
Non-metallic expansion joint (fabric fiber expansion joint)AndMetal rectangular expansion joint。 How to choose? In one sentence: large displacement, low pressure, high temperature flue gas, with non-metal; For small displacement, medium and high pressure, wear resistance and erosion resistance are needed, and metal rectangles are used.
The advantages of non-metallic expansion joints are large compensation (a single layer of skin can absorb 100mm axial displacement), high temperature resistance (silica gel cloth + ceramic fiber layer can reach 1000℃), and no thrust-because the fiber layer has basically no stiffness. However, its weakness is poor compression resistance. If the positive pressure exceeds 0.05MPa, it should be cautious, and there should be no sharp particles to scour, otherwise the skin will wear out quickly. Therefore, the end of the dusty flue, such as the electrostatic precipitator outlet, often needs to be addedguide tubeProtection (the guide tube can prevent the flue gas from directly eroding the internal parts of the expansion joint, which is specifically mentioned in the FAQ of this site).
The metal rectangular expansion joint has high stiffness, high pressure and long life, but the manufacturing cost is high, and the compensation per wave is limited (generally, a single wave can only absorb 10~20mm). If your flue size is very large, such as a rectangle of 2 meters by 3 meters, the metal rectangular expansion joint is usually made of a multi-wave structure, and it should be equipped withtie rodTo withstand internal pressure thrust. The function of the tie rod is to limit the excessive stretching of the expansion joint and prevent the instability of the bellows (refer to the question and answer of this site "The function of the expansion joint tie rod").
In addition, don't forget that some projects will also use the two in combination: metal rectangular expansion joints for the axial direction of the flue, and non-metallic ones for angular displacement.
5. Don't be in a hurry to place an order after calculation: do you want to count the guide tube, tie rod and fatigue life?
The parameters have been calculated and the model has been selected, but there are still a few things to confirm before placing an order, otherwise it may break after three days of installation.
- guide tube: High-frequency wear conditions must be matched. The thickness and material of the guide tube should also be selected according to the flue gas temperature. Generally, the temperature resistance of carbon steel guide tube is ≤500℃, which exceeds that of stainless steel.
- Tie rod and limit structure: When the metal rectangular expansion joint is installed, how to adjust the tie rod nut is a technical job (this site has special questions and answers). Remember: After installation, the tie rod nut should be loosened to half of the designed displacement, otherwise the expansion joint will not expand or contract normally.
- Fatigue life: Flues that start and stop frequently, such as peak shaving units in thermal power plants, expansion joints may experience multiple temperature cycles every day. The fatigue life of metal bellows is inversely proportional to the displacement, and generally requires at least 1000 cycles. Although the non-metallic expansion joint has no fatigue problem, the aging life of the skin is usually 3~5 years, so it needs to be replaced regularly.
- Medium corrosivity: The wet flue gas after desulfurization contains sulfuric acid condensate, which has strong corrosion to metals. That's when you have to goPTFE-lined hoseOrPTFE compensatorOr choose a corrosion-resistant non-metallic skin.
Oh yeah, there's another one that's easy to overlook-pressure thrust. The internal pressure of the pipeline acting on the effective area of the expansion joint will generate a large axial thrust. If the tie rod or fixed bracket is not installed, this force will destroy the adjacent equipment. Especially for large-size flues, the pressure thrust can easily reach tens of tons, so don't take it seriously.
VI. A practical case: the complete calculation process from parameters to selection
Two days ago, I met a customer, the kiln tail flue of a cement plant, with a rectangular cross-section of 1.8m ×1.2m, a length of 25 meters, and a material of Q235. Working conditions: Flue gas temperature 350℃, maximum 400℃ at start-up, pressure-0.03MPa (negative pressure), horizontal installation, two ends connected respectivelyRound Flap Door (Double Seal)AndFlue gas baffle door。 Displacement requirements: axial thermal expansion + installation allowance total 80mm, transverse displacement estimated ±5mm.
Step 1: Calculate thermal expansionΔ L =12 x 10⁻⁶ x 25000 x (400-20) =114 mm. With a reserved margin of 10mm, the total displacement is 124mm. However, note that there are baffle doors at both ends of the flue, which are rigid, and the actual absorption of the expansion joint is 120mm.
Step 2: Judge positive and negative pressuresUnder negative pressure conditions, the internal pressure thrust is very small, and the pull rod is not needed. However, negative pressure may cause the skin to be concave, and a support ring needs to be added inside the non-metallic expansion joint.
Step 3: Type selectionConsidering the temperature of 400℃, large displacement, negative pressure and rectangular section, the first choiceRectangular non-metallic expansion jointThe skin structure adopts ceramic fiber + silica gel cloth + stainless steel wire mesh, which has a temperature resistance of 450℃, and a guide tube (carbon steel, δ =3mm) is installed to protect the inner wall. Since the lateral displacement is only ±5mm, the non-metallic expansion energy saving is easily absorbed.
Step 4: CheckCheck the product information of non-metallic expansion joint in this site, select the model with a length of 500mm, and the axial compensation amount can reach 150mm, which meets the requirements. The length of the guide tube is designed according to 1.2 times the inner diameter of the pipe to avoid flushing. Finally, confirm the size of the flange of the baffle door interface, and the order can be placed if it matches.
After walking down the whole process, you will find:How to calculate the expansion joint at the end of the flueThis matter was not just a formula. There are living working condition details behind each parameter of temperature, pressure and displacement, and if one is missed, the car may overturn. But as long as you follow this logic step by step, even doing it the first time can reduce your selection mistakes by 90%.