Flue Expansion Joint Liner: Why It's More Important Than You Think
Two days ago, I met a buddy who was engaged in power plant maintenance. I talked about it and said that there were always problems with their flue expansion joints, and the bellows wore out in a few months. I asked him if the liner was installed? He was stunned for a moment and said, "Isn't that thing just a piece of iron? How big could it be?" Alas, this kind of thinking is dangerous.
The lining plate of the flue expansion joint, to put it bluntly, is the protective cover installed inside the expansion joint and close to the bellows. In our business, it is also often called a guide tube. Its own work is two: first, guide the high-temperature flue gas smoothly, so as to prevent the airflow from directly impacting the bellows; The second is to reduce eddy current and reduce local wear. Think about it, there are hundreds of degrees of smoke in the flue, sandwiched with ash particles and sulfide. Without this layer of lining, the bellows would be like standing shirtless in the wind and sand. How long could it last?
Therefore, don't look at it as inconspicuous. If the lining plate is not selected and installed correctly, the whole expansion joint will have to be scrapped.
From deflectors to liners: different names, same lives
Regarding the specific function of the expansion joint guide tube, we have a special article before our station-it was first used to divert the flow to avoid the formation of a dead zone in the corrugated valley bottom. However, as the working conditions become more and more complicated, especially in desulfurization, dust removal and cement industries, there is not only high temperature, but also strong corrosion and high particle concentration in flue gas. At this time, light flow guidance is not enough, and it must be able to withstand erosion and corrosion. Therefore, the guide tube has evolved into an inner lining plate, and the material, thickness and structure have been upgraded.
For example, corrugated expansion joints in the power station industry (such as oursCorrugated expansion joint for power station industry) Usually stainless steel 304 or 316L as the inner lining plate, because the flue gas temperature is within 400 ℃, and the corrosiveness is medium. However, in the cement industry, the flue gas is full of clinker dust, and the temperature can soar above 600℃. At this time, the lining plate has to be replaced with heat-resistant steel, or even a ceramic coating. OurMetal Corrugated Expansion Joints in Cement IndustryIt is specially designed for this kind of working condition.
Metal? Non-metallic? Compound? — — Selecting materials under different working conditions is not a slap on the head
Many people think that the lining plate is just a steel plate, and it can be welded casually. Tsk, that's not true. The material selection directly determines the service life.
Metal liner: Most common. Ordinary stainless steel (304, 316L) is used in low sulfur and low temperature environments; Heat resistant steels (e.g. 310S, Inconel) are used in high temperature, oxidizing atmospheres. However, the weakness of metal is also obvious-acidic condensate that is afraid of chloride ions and wet desulfurization. Once pitting occurs, the perforation is quick.
Non-metallic linerLike oursNon-metallic expansion joint (fabric fiber expansion joint)Andrubber compensatorWith emphasis on sealing and vibration damping, the liner is usually made of multilayer composite fabric or rubber PTFE. This type of liner has good flexibility and can absorb a small amount of displacement, but its temperature resistance is limited (generally below 200℃) and its wear resistance is average. Therefore, it is suitable for scenes with low flue gas temperature and less dust.
Composite liner: This is the trend in recent years. For example, metal matrix + ceramic liner, or metal + PTFE plate. OurPTFE-lined hoseAndPTFE compensatorThis is the line of thinking – metal provides strength, PTFE provides corrosion resistance. Used in a desulfurization system, that's just right. AndLarge diameter thick wall expansion jointThe inner lining plate is sometimes made into a double layer: the outer layer is heat-resistant steel, and the inner layer is wear-resistant coating, which is specially used to deal with the high wear areas of the inlet and outlet of the flue gas desulfurization tower of the power plant.
How many culprits of the failure of the inner liner, how many have you been tricked?
Say a thousand words, but in the end, you have to watch the scene. The failure cases of liners I have seen over the years can be summarized as follows:
- wear-out— — The particles carried by the airflow directly hit the lining plate, especially the expansion joint near the elbow and baffle door, and the local wear is particularly fast. Solution: Thicken the inner lining plate (such as 6mm starting), or weld a wear-resistant surfacing layer on the inner wall.
- Corrosion perforation— — The humidity of the flue gas after wet desulfurization is high, and the acidic condensate accumulates at the bottom of the lining plate, which will rot over time. At this time, corrosion-resistant alloy or PTFE lining must be used, and drainage holes should be made at the bottom of the lining plate.
- Weld cracking— — The welding place between the inner lining plate and the cylinder body is prone to fatigue cracking due to repeated stress of thermal expansion and contraction. Note when designing: the weld should not be fully welded, and space should be left to release stress, such as spot welding every other section or flanged structure.
- Thermal deformation-The temperature exceeds the creep limit of the material, and the lining plate bulges, twists, or even falls off. This requires material selection strictly according to the working condition and temperature, and sufficient expansion gap is left during installation.
How to avoid it? Simply say three things: do a good job in material matching and thickness calculation during design, ensure the correct direction and reasonable clearance during installation, and regularly stop the machine to check the status of the lining plate during inspection. Don't wait for a leak to replace it, the cost doubles.
Those easy-to-miss details of installation and maintenance
And guess what? Many inner liner failures are simply buried mines during installation.
First, the direction arrow.The expansion joint cylinder and lining plate are often marked with flow direction arrows, which tell you from which side the smoke comes in and out. The inner liner is usually designed as a bell mouth with a slightly larger diameter at the inlet end to form a flow diversion. However, some people ignored the arrow and pretended to be reversed. As a result, the smoke directly hit the back of the lining plate, and the bellows was washed out-that was a misery. So remember: the direction of the arrow of the expansion joint is pointing to the direction of the media flow, and the large mouth of the liner plate is facing upstream.
Second, clearance control.Sufficient clearance (generally 5-10mm) should be left between the inner liner and the bellows to compensate for the axial and transverse displacement of the bellows. If the gap is too small, the corrugated tube will rub against the lining plate as soon as it is compressed, resulting in abnormal noise and even wear. If the gap is too large, the airflow will form a step at the end of the liner plate, which instead intensifies the vortex.
Third, bolt tightening.Some liners are bolted to the inside of the flange. When it was disassembled and reinstalled during maintenance, the bolts were not tightened or anti-loosening gaskets were not added, and they were loose after running for a while. If the liner plate falls off and gets stuck in the expansion joint, that's a big problem. We have an article dedicated toHow to adjust the tie rod nut of expansion jointThe same applies to the fixing bolts of the inner lining plate: symmetrical tightening and consistent torque.
How to use it with smoke baffle door and expansion joint?
A complete expansion joint system is often not isolated. In the flue, the expansion joints are responsible for absorbing heat displacement, the flue baffle doors are responsible for cutting off or regulating the airflow, and the lining plates are responsible for protecting both — they are a set of combination punches.
Take the desulfurization system. The flue gas comes out of the boiler and passes throughDouble-sealed single-axis circular baffle door(That is, ourDesulfurization flue gas baffle door), and then enter the absorption tower through the expansion joint. Zero leakage is required when the flapper door is closed, so the sealing surface is precise. However, when the high-temperature flue gas passes through the baffle door, if there is no lining plate to guide the flow at the inlet of the equipment, the scouring of the high-speed airflow will destroy the sealing blades of the baffle door, resulting in leakage. Therefore, many designs will add an expansion joint with thick lining plate upstream of the baffle door, which not only absorbs the thermal expansion of the pipeline, but also protects the baffle door.
Another example is the inlet and outlet of induced draft fans in power plants, which useMetal rectangular expansion joint, with insideRectangular non-metallic expansion jointThe inner liner? Don't get confused. The liner plate of the rectangular expansion joint is usually metal because of the high flue gas temperature. For square flue, the inner lining plate should be made into a square-to-round or square-to-square flow diversion structure, and the four corners should be smooth transition to avoid stress concentration. OurRectangular non-metallic expansion jointIt is more suitable for low pressure and low temperature occasions.
AndHigh temperature axial expansion jointSuch products are often used in high-temperature main pipelines, and the lining plate must be resistant to high temperatures and wear. If cooperateElectric plug-in insulation doorOrManual plug-in insulation doorWhen designing the liner, we should consider the protection of the sealing surface after the plug-in board is inserted-not allow the plug-in board to rub hard with the liner.
Back to the beginning
The lining plate of the flue expansion joint is really not an ordinary "iron sheet". It determines the expansion energy-saving run for several years and also indirectly affects the safety and maintenance costs of the flue gas system. Choosing the right materials, installing in the right direction, and regularly checking the wear points can make the whole set of equipment use for two or three years. In turn, saving hundreds of dollars of lining plates is exchanged for tens of thousands of expansion joint replacements-is this account a good deal?