1. Find out first: What exactly is a large-diameter high-temperature resistant non-metallic compensator?
When many people heard the term "large-mouth high-temperature resistant non-metallic compensator" for the first time, their first reaction was-what is this? In fact, it is a subdivision type of what we often call non-metallic expansion joints (fabric fiber expansion joints). There are three core features: large diameter (DN500 or above is considered as the starting point), high temperature resistance (can carry 600℃ or even higher), and non-metal material (composite layer such as silicone cloth, fluororubber and ceramic fiber). It is specially used to absorb the axial and lateral displacements caused by thermal expansion and contraction in the pipeline system, and at the same time can reduce vibration and noise. Compared with the metal corrugated expansion joint, it has greater compensation amount, less thrust, and is not afraid of corrosive media. "Non-metallic expansion joint (fabric fiber expansion joint)" and "rectangular non-metallic expansion joint" in the product catalog of this site both belong to this family, but the large-diameter round version is more common in flue gas and hot air ducts.
Then why does everyone call it "compensator" instead of "expansion joint"? Refer to Article 15 of Q&A on this site-expansion joint and compensator are actually the same thing, and different industries have different names. However, the prefix of "large mouth high temperature resistance diameter" directly frames its most typical application scenarios: desulfurization flue of thermal power plant, preheater outlet of cement plant and hot blast furnace pipeline of steel plant. In these places, the pipe diameter is always one or two meters, and the temperature starts from three to four hundred degrees Celsius. Metal parts can't hold it, but non-metals become the optimal solution.
2. Why can it withstand the high temperature? -Material and Structural Design Disassembly
High temperature resistance is not by blowing. The core of the large-diameter non-metallic compensator is the "loop belt"-a composite of multiple functional layers. The innermost layer is a temperature-resistant layer, commonly used ceramic fiber cloth or high silicon oxide fiber, directly contacting high-temperature medium; In the middle is an insulation layer, such as aluminum silicate cotton, which blocks the heat outside; The outer layer is the sealing layer and protective layer, such as fluororubber coated cloth and stainless steel wire mesh. Each level performs its job. And guess what? This structure also allows the loop belt itself to have a certain elasticity and easily absorb the thermal displacement of the pipe. Moreover, non-metallic materials are not fatigued and cracked like metals. As long as the band is not scratched or aged, its life can reach 3-5 years or even longer. There is another key point in the design: the guide tube. Refer to the question and answer of this site "Specific function of expansion joint guide tube"-in high-temperature flue gas pipeline, the guide tube can protect the loop from being washed by high-speed particles, and at the same time reduce the flow resistance. In addition, the flange connection surface of large-diameter compensator usually needs high-temperature sealing gasket to prevent air leakage.
However, material selection can't only look at temperature. Some customers ask for "resistance to 1000℃" when they come up. As a result, the actual working temperature of the medium is only 400℃, so the cost is wasted. What's more, sulfur dioxide and moisture are mixed in the flue gas, and sulfurous acid corrosion zones will be formed at high temperatures. In this working condition, ceramic fiber cloth alone cannot be used, and fluororubber coating or polytetrafluoroethylene layer must be added (refer to "Rubber PTFE Compensator" on this site). Selection of materials is a technical job, which will be detailed below.
3. Design challenges brought by large diameter: stiffness, guide tube and installation space
As soon as the caliber is large, the problem comes. The first is stiffness: the non-metallic ring belt itself is very soft, but when the diameter is large, the force acting on the ring belt by internal pressure will increase sharply (pressure × area). Therefore, the large-diameter non-metallic compensator must be equipped with a "pressure ring" or "retaining ring" on the outside of the ring belt to restrict the expansion and deformation, otherwise the ring belt will bulge into a balloon. The second is the design of the guide tube: when the diameter exceeds 1 meter, the guide tube itself weighs a lot. If it is supported by only a few ribs, it will easily deform and collapse at high temperature. The correct approach is to use a "segmented guide tube" plus a "reinforcing ring", and the inner wall of the guide tube should be sprayed with wear-resistant coating. The third is the installation space: the axial length of large-diameter compensator is often limited, but the compensation amount is required to be large, which requires the design of a "double" structure (refer to the "double hinge transverse expansion joint" or "double straight pipe bypass pressure balance expansion joint" of this station), and two groups of ring belts are connected in series to increase the compensation amount.
Speaking of this, I think of a negative case: DN1800, a flue gas pipeline of a chemical plant, only gives 500mm installation space on the drawing, but requires 80mm compensation. Single-loop belt couldn't be done at all. Later, it was changed to a duplex structure, and a set of intermediate pipes were added to meet the requirements. Therefore, when asking for demand, don't just give the pipe diameter and temperature, but write the displacement, space size and pressure clearly, so that the factory can make targeted design.
4. The easiest pit to step on when selecting a model-don't wait until you install it to find out that it's wrong
Two days ago, I met a customer who insisted on equipping the flue gas pipe with a metal corrugated expansion joint, saying that "the metal is strong". As a result, it cracked in less than half a year-because of the sulfide in the flue gas, the metal parts were stress corroded. In fact, in his working condition, using a non-metallic compensator is more worry-free. When choosing a large-diameter high-temperature resistant non-metallic compensator, we should focus on three things: temperature, pressure and medium composition. In terms of temperature, short-term overtemperature (such as the oven when driving) and long-term operating temperature should be marked separately. In terms of pressure, most non-metallic compensators can only be used at low pressure (within 0.1MPa), and don't think about using them in steam pipelines. The composition of the medium is more critical: if it contains oil, acid and alkali, choose the corresponding fluororubber or polytetrafluoroethylene coating (refer to the rubber PTFE compensator of this site). In addition, the installation direction is also easy to be mistaken-does the arrow direction of the expansion joint refer to the medium flow direction or the displacement direction? Refer to Article 10 of the question and answer of this site. The arrow usually points to the direction of medium flow, and the bell mouth of the guide tube should face the medium. If it is installed backwards, the deflector will directly become a "jacket board", which will be torn apart after a few vibrations.
The design standard of non-metal compensator is JB/T 12235-2015 (refer to Article 9 of Q&A on this site). When selecting, ask the manufacturer to take the test report of this standard to see whether the pressure resistance, temperature resistance and leakage are up to standard. Don't listen to the sales fool "Our product is resistant to 800℃". In fact, there may only be 500℃ test data.
5. Actual case: How to match the outlet pipe of high-temperature fan in cement plant?
The cement industry is the "hardest hit area" of high-temperature non-metallic compensators. The flue gas temperature at the outlet of the preheater of the clinker production line is usually 350-450℃, and the diameter of the outlet pipe of the fan often reaches above DN2000. A few years ago, I helped a cement factory select a model. Their original metal corrugated expansion joint leaked once every six months, and it had to be shut down for two days once it was replaced. The loss of production reduction was ten times more expensive than that of the compensator itself. Later, it was replaced with a large-diameter non-metallic compensator (refer to the metal corrugated expansion joint in the cement industry of this station, but the non-metallic model is used here), and the ring belt used a 4-layer structure: ceramic fiber cloth + aluminum silicate cotton + fluororubber coated cloth + stainless steel wire mesh. The guide tube is made of SUS310S stainless steel with a thickness of 6mm and is lined with wear-resistant castable. After installing it, it took two and a half years to replace the ring belt for the first time, and when replacing it, you only need to loosen the flange bolts, pull out the old ring belt and plug in the new ring belt, which is done in half a day. This is the rolling advantage of non-metallic compensators in high temperature and large caliber scenarios.
Then why did they use metal before? Because the purchase feels "metal is stronger than cloth". In fact, the non-metallic band is misunderstood-its strength comes from multi-layer composite and pressure ring constraints, not from a single material. Moreover, the metal expansion joint is prone to fatigue fracture at high temperature, especially at the weld. On the other hand, non-metals, as long as the right material is selected, the aging rate is much slower than the stress fatigue of metals.
6. Installation and routine maintenance: A few details that must not be ignored
Check the loop for transport damage before installation, especially for tears at the edges. The connecting surface of the flange must be flat, and if it is uneven, level it with a gasket, otherwise the ring belt will be pressed out of pits. For the treatment of tie rods and screws, refer to the question and answer of this site "Do the tie rods of expansion joints need to be removed"-the tie rods for transportation and fixation must be removed after installation, otherwise the compensator cannot be freely expanded and contracted. In addition, how to adjust the expansion joint tie rod nut? After installation, it should be adjusted to the design length to ensure that the compensator is in the middle position when cold, leaving a thermal expansion margin. The daily inspection mainly looks at whether there are cracks, blisters and corrosion spots on the surface of the ring belt, especially the root of the flange. If a minor leak is found, it can be temporarily repaired with high temperature resistant tape, but it is recommended to replace it as soon as possible. Also, non-metallic compensators are strictly prohibited from touching the ring belt with sharp objects during operation-don't ask me how I know, anyway, a customer used a steel brush to clean up the dust and directly brushed through the sealing layer.
Proper maintenance, large-diameter non-metallic compensator can be used for more than five years; Improper maintenance, scrapped in three months. It is also crucial to regularly check the wear of the guide tube-if the guide tube is broken, the high-temperature flue gas directly washes the ring belt, and the ring belt can't hold it no matter how thick it is. When dealing with equipment, it is true that the details determine success or failure.