1. Why is the insulation of flue expansion joint a "big problem"? — — Start with the actual working conditions of the power plant
Brothers who engage in power plants know that the most troublesome thing in the flue system is often not the main flue itself, but the position of those expansion joints. The flue gas temperature is often three to four hundred degrees, and the humidity after desulfurization is high. Coupled with the thermal expansion and contraction caused by the start and stop of the unit, the expansion joint body has to bear displacement and be resistant to temperature and corrosion. Once the insulation layer is not done well, the surface temperature can soar to 70 or 80 degrees, which not only soars the energy consumption, but also may burn people. What's even more annoying is that expansion joints are usually flexible structures-non-metallic expansion joints use fabric fiber layers, while metal expansion joints have bellows-how to fix and seal the insulation material is completely different from that of straight pipe sections. During the maintenance of many power plants, it is found that the insulation layer of the expansion joint is either cracked and fallen off, or the internal water is corroded, or even the expansion joint body is compressively deformed. You say, can this not be called "old trouble"?
2. Material selection logic: non-metallic expansion joint vs metal expansion joint, what "play" should the insulation layer play?
Let's talk about non-metallic expansion joints (that is, we often call fabric fiber expansion joints). The body of this type of expansion joint is a flexible skin with multiple layers of insulating cotton and fluororubber sealing layer inside. The insulation layer cannot be directly welded on the skin, and it must rely on the external coating insulation structure. The common scheme is to wrap a layer of aluminum silicate fiber blanket outside the skin, then fix it with stainless steel mesh and hoop, and the outermost layer of galvanized iron sheet or aluminum sheet is used as protection. Here is a key: the thickness of the insulation layer must be calculated according to the smoke temperature. For smoke below 300℃, 100mm thick aluminum silicate fiber is basically enough; If it exceeds 400℃, it has to be thickened to more than 150mm, and a heat insulation layer has to be added on the inside of the skin to prevent heat from directly baking the skin.
Let's talk about metal expansion joints, such as corrugated expansion joints or metal rectangular expansion joints used in power station industries. The bellows is thin-walled stainless steel, which is not resistant to high temperature and direct baking-the flue gas temperature exceeds the design value, and the bellows will oxidize and become brittle. Therefore, the metal expansion joint usually has a guide tube when blanking (the guide tube is mentioned in FAQ 7), and a cavity is formed between the guide tube and the bellows, which not only reduces the erosion of the flue gas, but also naturally separates some heat. How do you put the insulation layer? Generally, it is divided into two steps: the outside of the bellows is wrapped with thermal insulation cotton, and then covered with iron sheet; If the flue gas temperature on the inside of the guide tube is particularly high, high-temperature anti-oxidation coating must be painted on the guide tube. Don't stuff the insulation cotton directly into the gap between the guide tube and the bellows, otherwise it will hinder the displacement and easily jam the bellows.
Non-metallic expansion joints rely on external heat insulation, focusing on anti-condensation; The metal expansion joint relies on the combination of the guide tube and the external heat insulation, and the emphasis is on preventing the over-temperature oxidation of the bellows. If you don't understand this logic, the insulation layer is equal to doing it for nothing.
3. Construction details determine success or failure: practical experience of insulation layer thickness, sealing and rainwater infiltration prevention
Two days ago, I met a customer. The non-metallic expansion joint next to the desulfurization flue gas baffle door of their factory collapsed after two years of insulation. When I took it apart, it was full of water — rain seeped through the seams of the iron sheet and soaked the aluminum silicate fibers into mud. Alas, this kind of problem has been seen too much.
- Only half the thickness: In order to save materials, many construction teams make the thickness of insulation layer 60% of the design value. Results the surface temperature exceeded the standard, and the skin of the expansion joint aged prematurely. What to do? Before construction, measure the design thickness, lay it in layers, and overlap each layer with staggered seams to avoid straight seams. The bulk density of aluminum silicate fiber blanket is not less than 128kg/m³, which is too loose and does not insulate.
- Sealing like not doing it: The metal shell outside the thermal insulation cotton, the overlap must be in the direction of water flow (up and down), and the overlap length shall not be less than 50mm. Apply high-temperature-resistant sealant at the joints (silicone glue can't work, so you have to use one that is resistant to more than 300℃). At the connection between the two ends of the expansion joint and the flue, an expansion allowance should be left, but it should be sealed with a flexible sealing strip to prevent cold air from inhaling.
- Rainproofing is all about luck: The top of the outdoor flue expansion joint should be made into a slope surface, or a waterproof cover should be added. The screws of the side shell should be made of stainless steel self-tapping nails, and the nail cap should be coated with sealant. The easiest thing to overlook is the bottom-if the expansion joint is at a low position, rainwater may flow into the insulation layer along the guide tube. At this time, a drainage hole (about 10mm in diameter) must be opened at the lowest point of the guide tube, otherwise it will be corroded by accumulated water.
4. Three "culprits" of thermal insulation failure: condensation, ablation and mechanical damage-with solutions
Dew condensation mainly occurs in the desulfurization flue, the flue gas temperature is low (50-70℃), and the humidity content is large. If the thickness of the insulation layer is not enough or the seal is not tight, the temperature of the flue wall is lower than the dew point, and the water drops will come out. Water seeps into the thermal insulation cotton, the thermal conductivity rises straight, and the thermal insulation effect deteriorates. More troublesome, water can corrode the metal flanges and beads of non-metallic expansion joints. Solution: Increase the thickness of the insulation layer, add a vapor barrier layer (such as aluminum foil veneer) in the insulation layer, and keep the flue gas flow rate stable to avoid local low temperature.
Ablation mostly occurs in the parts where the flue gas temperature fluctuates greatly-for example, in the early stage of boiler start-up, the flue temperature instantly rushes to more than 600 degrees, and ordinary aluminum silicate fiber will sintered and become brittle. At this time, it is time to change the high-temperature polycrystalline mullite fiber or ceramic fiber blanket. Although it is more expensive, it can withstand 1300℃. In addition, if the guide tube of the metal expansion joint is worn and perforated, and the high-temperature flue gas directly washes the bellows, the expansion joint body will be finished-the thickness of the guide tube must be checked regularly with an endoscope.
The insulation layer is too heavy, and the expansion joint is deformed by compression. Especially for non-metallic expansion joints, its skin bearing capacity is limited, and the outer surface is covered with a thick layer of thermal insulation cotton and iron sheet, and the weight may exceed 50kg/m². What to do? Add an independent bracket at the top of the expansion joint, so that the weight of the insulation layer can be transmitted to the flue from the bracket, and don't let the skin carry it.
5. Accessories should not be ignored: key points of cooperative design of flue gas baffle door and expansion joint insulation
Flue gas baffle doors (such as double-sealed single-axis circular baffle doors or single-axis double baffle doors) are usually installed near the expansion joint to switch flue or isolate maintenance. The baffle door body is also a blind spot for heat insulation-there are sealing sheets and frames inside the door panel, and the insulation layer cannot hinder opening and closing. Common mistakes are: insulation cotton wrapped the transmission connecting rod of the baffle door, causing the thermal expansion of the connecting rod to be blocked and the switch to be stuck. The correct way is to keep the baffle door housing warm up to the flange surface, leave enough clearance between the actuator and the transmission part, and wrap it separately with a removable heat insulation cover.
This section of flue between the expansion joint and the baffle door is often the location where the thermal stress is most concentrated. The expansion joint absorbs displacement and the baffle door provides a zero-leak seal, but the insulation between the two must be continuous and free of cold bridges. If the heat insulation at the flange of the baffle door is leaked, the temperature of the outer wall of that section of the flue will be obviously higher, resulting in a thermal expansion difference, but pulling the expansion joint instead. The core of collaborative design is one sentence: the insulation layer covers all the outer surfaces of flues, including flanges, manhole doors, expansion joints and baffle door shells, leaving only the parts that must dissipate heat or move.
6. From installation to operation and maintenance: How to make a set of insulation schemes that can last an overhaul cycle
The overhaul period is generally 4-6 years, and whether the insulation layer can last until that time depends on the early base.
When installing, you have to consider the convenience of later inspection. For example, reserve an observation hole (about 100mm in diameter, with a quick-opening cover) on the thermal insulation shell of non-metallic expansion joint, and open it regularly to see if the skin is bulging or discolored. The insulation layer of the corrugated pipe part of the metal expansion joint is best made detachable-fixed with stainless steel cable ties, and directly removed without destroying the insulation during maintenance. Don't weld the insulation layer to the flue in order to save money, otherwise it will have to be dismantled during overhaul, which is labor-consuming and time-consuming.
Look at the surface temperature of the shell-measure with an infrared temperature measuring gun. If the local temperature is more than 15℃ higher than that of the adjacent area, it means that the insulation layer is hollow or damp; Second, listen to abnormal sounds-if there is a "hissing" sound at the expansion joint, it may be that the seal is damaged and leaked, and the smoke short circuit will burn through the insulation layer; Three touch fasteners-are the hoops and self-tapping nails loose? After the rainy season, focus on checking whether the bottom waterproofing fails.
Don't be superstitious about imported materials. Domestic aluminum silicate fiber and stainless steel shell, as long as the parameters meet the standards and the construction specifications, are completely fine to support an overhaul cycle. What really falls off the chain is often that the construction team saves trouble and Party A's supervision is not in place. You asked how to survive a cycle? In a word: For design selection, find the manufacturer to check the working condition parameters (temperature, displacement, medium corrosiveness), check and accept the construction according to the specifications, and inspect the operation and maintenance once a quarter. Do these three points, and the insulation layer can't run.