How harsh are the high-temperature flue conditions? At first, many project parties tried to save trouble and put on metal expansion joints. As a result, they cracked, corroded and stuck within a few months. After all, there is a high temperature in the flue (five or six hundred degrees at every turn), sulfur oxide corrosion, and a lot of dust washing. The reason why nonmetallic compensators have become mainstream is the flexibility of fabric fibers and composites-they can absorb three-dimensional displacement, withstand corrosion, and block thermal bridges. And guess what? Many power plants and cement plants now designate non-metallic, especially for rectangular flues. The "rectangular non-metallic expansion joint" in our station is specially designed for this kind of scenario.
Let's start with a real case. Last year, there was a cement factory customer, and the flue gas temperature was around 550℃ all the year round, and the sulfur content was not low. At first, they wanted to be cheap and put on metal corrugated expansion joints. As a result, within three months, the welds cracked and the corrugated pipes were corroded by sulfur like honeycomb briquettes. Finally, I had no choice but to replace it with our non-metallic compensator. It has been more than two years now, and there is no problem at all. To put it bluntly, in the environment of high-temperature flue, metal materials are born to suffer-it is rigid, can't absorb multi-dimensional displacement, and the thermal stress will crack as soon as it accumulates; When it hits acidic media, the corrosion rate doubles. Non-metal is different. It relies on multiple layers of fabrics and coatings, soft connections, thermal expansion and contraction, and can also insulate heat from being transmitted to the pipe support.
Then why is rectangular flue especially suitable for non-metal? Because the rectangular section itself is prone to angular displacement and transverse displacement, it is difficult for the metal expansion joint to handle the deformation in these directions, while the multi-wave wrinkle structure of "rectangular non-metallic expansion joint" can easily cope with it. Think about it, which is not rectangular, the import and export of desulfurization tower in power plant and the flue pipe in the tail of cement kiln? Therefore, now the design institute draws, as long as the flue temperature exceeds 400℃ or there is corrosive medium, the non-metallic compensator is basically directly marked.
Structurally, how do non-metallic compensators carry high temperatures? The core lies in multi-layer composite: the outer layer is a glass fiber cloth coated with temperature-resistant silicone or fluororubber, with a ceramic fiber insulation layer sandwiched in the middle, and the inner layer is lined with corrosion-resistant polytetrafluoroethylene or teflon. Like the "non-metallic expansion joint (fabric fiber expansion joint)" of our station, the working temperature can be above 1000℃. Note that this temperature refers to the instantaneous peak inside the flue, not the continuous working condition. The continuous operating temperature is generally controlled within 600℃, which is relatively safe. In addition, there is a big difference between rectangular and circular structures-rectangular flues are usually equipped with rectangular non-metallic expansion joints, which rely on multi-wave wrinkles to absorb thermal expansion and contraction.
"Can you handle 1000 degrees?" I have to explain clearly: 1000 degrees is an instantaneous peak, such as the few seconds when the boiler blows soot or the working conditions fluctuate, so it is fine to endure it. If it runs above 800℃ for a long time, the coating will accelerate aging, and the life will be directly discounted in half. Therefore, when selecting a model, we generally recommend that customers provide an operating temperature curve, rather than an isolated maximum value. How to choose coating materials? The temperature resistance of silicone coating is about 250℃, and the temperature resistance of fluororubber can reach 350℃. Further up, Teflon or all-ceramic fiber bands should be used. Don't underestimate this lining-PTFE lining is not only temperature resistant, but also can withstand the corrosion of sulfur oxides and chloride ions. Many wet flues after flue gas desulfurization rely on it to save their lives.
Let's talk about the difference between rectangle and circle. Circular flues may be made of "universal corrugated expansion joints" or metal, but rectangular flues must be made of non-metal. Why? Because the rectangular section will generate a large bending moment under the action of pressure, the metal plate is easily unstable. The rectangular non-metallic expansion joint disperses and absorbs the heat displacement through multi-layer fabric folds, and each wave bears a little, and the whole is stable. What we often call "multi-wave folds" is to make a fuss about circles of ripples. The more waves, the greater the compensation.
Three parameters are selected: temperature, pressure and displacement. Temperature determines whether you choose silicone or fluororubber coating, pressure determines the number of belt layers and stiffener design, and the amount of displacement determines compensator length and wave number. Two days ago, I met a customer, and the axial displacement and lateral displacement were reversed. As a result, the tie rod nut didn't die after installation, and the compensator was directly torn (refer to how to adjust the tie rod nut of the expansion joint in our FAQ). In terms of size, there is no unified national standard model for non-metallic expansion joints, which are generally customized according to the pipeline diameter and design parameters-but the national standard JB/T 12235-2015 has clear provisions on technical requirements and test methods, so don't buy three-no products.
How to handle these three parameters? For example, if the flue gas temperature is 450 °C, the pressure is 0.05 MPa, the axial displacement is 30 mm, and the lateral displacement is 10 mm. That coating is enough with fluororubber, and the loop belt is made in three layers (one layer of fluororubber cloth + one layer of ceramic fiber + one layer of polytetrafluoroethylene lining), and the reinforcing ribs are sandwiched in the middle with stainless steel wire mesh. How to calculate wave numbers? Each wave can absorb about 5-8mm axial displacement, and 30mm requires 4-6 waves. The length will be set. However, many people only give a diameter and temperature, and don't mention the displacement at all-we dare not accept this kind of list, because what nonmetallic compensators are most afraid of is that the displacement exceeds the design range, and it will explode when pulled.
When it comes to national standards, JB/T 12235-2015 is a hard lever. Regular manufacturers will do air tightness test, pressure test and displacement fatigue test before leaving the factory. Some small workshops dare to sell it by sewing a cloth in a circle. There is no test data or material report. Then how to identify? See whether there are any implementation standards on the product label and whether there are third-party testing reports provided. Don't wait until the smoke leaks from the site to find out that it is a fake. At that time, the construction period can be delayed much more than the money saved.
The installation link has the most pits. Many people think that non-metallic compensators are soft and can be installed casually-a big mistake. It is necessary to ensure that the direction of the guide tube is consistent with the air flow, otherwise the high-temperature flue gas directly washes the ring belt, and the service life will be suddenly reduced. In addition, it is necessary to unlock the transport tie rod during installation (refer to the screw of the expansion joint in our FAQ), but do not loosen it all at once, and adjust it to the design displacement step by step. We have a question and answer on "The Specific Function of the Expansion Joint Guide Tube", which makes it very clear that the compensator is a disposable consumable if the guide tube is not installed or installed backwards.
The flow guide tube looked inconspicuous, but it was actually a life-saving piece. It is installed inside the flue, adhered to the inside of the compensator ring belt, and guides the high-temperature flue gas away, preventing it from directly washing the ring belt fabric. If there is no guide tube, or it is installed backwards (the arrow is in the wrong direction), the flue gas will blow directly into the ring belt, and the 800℃ airflow plus dust particles will break through the ring belt in a few hours. Therefore, we repeatedly emphasize in the FAQ that the direction of the arrow of the expansion joint refers to the flow direction of the medium, which must be consistent with the flue airflow.
Let's talk about the transport rod. When the non-metallic compensator leaves the factory, for the convenience of transportation, several pull rods will be installed to hold down the ring belt to prevent deformation. However, when installing, these tie rods must be removed or loosened, otherwise the compensator cannot be freely expanded and contracted, which is useless. How to dismantle? Don't loosen it all at once-loosen half of it first, and then gradually loosen it to the design position after the pipe butt welding is completed. There are detailed steps in our FAQ "Does the screw of the expansion joint need to be removed?" There is also the adjustment of the tie rod nut. Many people tighten it too tightly, resulting in the ring belt being crushed to death and the compensation amount returning to zero. As a result, it is directly torn when it is heated.
Life problem: Non-metallic compensators are usually designed to have a service life of 3~5 years, but the practice depends on the working conditions. In the high-temperature flue, if the dust concentration is high and the start and stop are frequent, it may have to be changed in two years. How do you judge that it's time to change? Look at the appearance: the surface of the ring belt is cracked, delaminated, bulging, or abnormal vibration. Don't wait until there is a smoke leak before replacing it-by then the pipe stress may have been transmitted to the equipment foundation. There are detailed data in the Q&A of "Service Life of Expansion Joints" on our site, which can be referred to. Finally, it can be used together with flue gas baffle doors (such as "desulfurization flue gas baffle doors" and "circular baffle doors (double seals)") to isolate the flue during shutdown for maintenance, so there is no need to stop the whole line when changing the compensator.
The same set of non-metallic compensator, used at the outlet of the induced draft fan of the boiler (less dust and stable temperature), can be easily used for five years. However, if it is used at the tail of cement kiln (the dust concentration is as high as 50g/Nm³, and the temperature is still high and low), it may be wasted in two years. Judge when to change, don't rely on guessing. The daily inspection depends on three points: First, whether there are fine turtle cracks on the surface of the ring belt, and there are precursors of aging; Second, touch with your hand, whether there is any layering or bulging, that is, the inner fabric has been debonded; 3. Is there any abnormal jitter during operation? It may be that the loop belt is loose or the guide tube falls off. Once you find it, stop and change it quickly. Don't delay it. Because once the compensator is damaged, the high-temperature smoke is sprayed, which not only burns people, but also transmits the pipeline stress to the equipment such as fans and dust collectors, causing more serious damage.
So how do you extend your life? In addition to choosing the right product and installing it in the right direction, there is another good way-match the smoke baffle door. For example, when the machine is shut down for maintenance, the desulfurization flue gas baffle door is closed, and the flue is isolated, so that the compensator can be safely replaced without stopping the whole production line. The "desulfurization flue gas baffle door" and "double seal single axis circular baffle door" of our station are specially designed for this kind of scenario, with good sealing performance and low leakage rate. Therefore, equipment selection is a systematic project, and the compensator should not fight alone. Only by considering the baffle door and pipe support can the overall reliability and maintenance convenience be in place.