How "deadly" is a high-temperature flue? — — Why can't ordinary expansion joints bear the three mountains of corrosion, high temperature and dust
Our colleagues in power stations, cement and steel all know that high-temperature flue is not as simple as "passing hot air". The flue gas temperature is always six to seven hundred degrees Celsius. When it comes to a coal-fired boiler, sulfur-containing gas plus fly ash particles is simply a micro sandblasting machine + chemical corrosion bath. When ordinary corrugated expansion joints are used, the thin-walled stainless steel segments are worn out in less than two months, and the welds are cracked, and then there is air leakage, shutdown and replacement-a set will take at least half a month, and the overhaul will take half a year.
Two days ago, I met a customer. A general-purpose corrugated expansion joint was used in the desulfurization flue in the factory, which was perforated in less than three months, which caused the whole production line to stop production for maintenance. The boss slaps the table: If this thing is chosen wrong once, the production will be stopped for half a year. You say you want your life or not?
The high-temperature flue thick-wall expansion joint is specially designed to do this dirty work. It is not as simple as making the bellows thick, but is designed for high temperature, corrosion and dust in terms of material, waveform and structure. The core is: How to add the wall thickness? Where to add? How does the wave height and pitch match the arrangement of the guide tube? These methods, each sum is a lesson gained by money and time.
The core parameters of thick-walled expansion joints: the choice of wall thickness, material and wave design, don't think that thickness is enough
Many people fiercely add the thickness of steel plates when they hear "thick walls". Tsk, if you really follow this idea, the stress will be greater when you install it, and the fatigue life will drop from a cliff. The "thickness" of thick-walled expansion joints is premised.
Let's start with wall thickness.The large-diameter thick-walled expansion joints in our station generally range from 2.5mm to 6mm, or even higher. However, with every increase of 1mm in thickness, the rigidity of bellows jumps a step, and the ability to absorb displacement decreases instead. So the real key is not the absolute thickness, but the matching of wall thickness with waveform parameters. For example, design wave height, wave pitch and number of layers-single-layer thick-wall and multi-layer thin-wall combinations, each with its own applicable scenarios. Austenitic stainless steel (e.g. 316L, 321) or heat resistant alloy (e.g. Inconel 625) is recommended when the flue gas temperature exceeds 550°C. If the material is not selected correctly, thick walls are useless.
Let's talk about waveforms.U-shaped corrugation, ω-shaped corrugation, multi-layer corrugation... U-shaped flue is the most common in high-temperature flue, but it needs to be matched with large wave height to reduce stress concentration. If the dust concentration in the smoke exhaust system is high, the guide tube must be put on-as specifically mentioned in the Q&A of this site, the function of the guide tube is to block high-temperature particles from directly washing the bellows, and by the way, channel the airflow. Not installing a deflector? The wall thickness of 5mm can't hold for three months.
So don't just stare at the word "thick". A qualified thick-wall expansion joint is the result of the synergy of wall thickness, material, waveform, guide tube and tie rod system.
Guide to selection and pit avoidance: Which one ranks first, pressure, temperature, displacement and fatigue life?
At every technical exchange meeting, someone always asks: Which of these parameters is the most important? My answer may surprise you-the order is: temperature → amount of displacement → fatigue life → withstand pressure. Why?
The most rigid limit of flue operating conditions is temperature. As soon as the temperature exceeds the standard, the material strength plunges straight. For example, the allowable stress of 304 stainless steel at 600℃ is only about 1/3 of that of normal temperature. Therefore, first choose the material according to the highest temperature of flue gas: if it is lower than 400℃, use 304 enough for 550℃ to go up to 316L or 321. Only when the temperature is fixed can the displacement be calculated. The compensation ability of bellows depends on the wave number, wave height and wall thickness-if the displacement is not correct, it will either be pulled off or crushed to death.
What about fatigue life? Many people use it as decoration. In fact, most high-temperature flues belong to low-frequency and large displacement conditions, and a start-stop cycle is a fatigue cycle. The design is calculated according to standards (such as EJMA), and guarantees at least 1000 cycles-enough for 5 to 10 years for equipment with an annual maintenance cycle. Don't blindly chase "ten thousand times of life", which is for vibration conditions.
The last is the withstand pressure. The flue pressure is generally not large (several kPa to tens of kPa), as long as the thickness of the bellows can withstand internal pressure instability. On the contrary, after installation, pay attention to the gap between the guide tube and the inner wall of the pipeline. If the gap is too small, it will suppress the pressure, and if it is too large, it will not achieve the diversion effect.
Horizontal comparison of similar products: thick-walled expansion joint vs high-temperature axial type vs non-metallic expansion joint, when should we use which?
High temperature axial expansion joint: Simple structure, low cost, suitable for straight pipe sections with uniform temperature and axial displacement. But the disadvantage is that it cannot withstand large lateral displacement and is sensitive to deflection. If there is an elbow or uneven settlement in the direction of the pipe, be careful not to select it.
Non-metallic expansion joints (fabric fiber expansion joints): The upper limit of temperature resistance is generally 350℃ (special can reach 500℃), which has the advantages of large displacement compensation, vibration and noise reduction and good corrosion resistance. But fatal injuries are afraid of pressure and sharp particles. When the dust content of flue gas is large, the fiber cloth layer is quickly worn out, and the pressure bearing capacity is weak (usually less than 0.1MPa). Therefore, it is only suitable for low-pressure and clean flue gas occasions, such as the tail of flue in power plants or dry desulfurization systems.
Large diameter thick wall expansion jointThat's the subject of our article. It can carry three conditions of high temperature, high dust and medium pressure at the same time. The wall thickness is more than 4mm, and with the guide tube and the pull rod system, it can withstand the flue gas above 600℃, and the pressure is 0.1~0.5MPa. Used in "hell-level" working conditions such as cement kiln tail, power station economizer outlet, steel hot blast stove, etc., the other two products are afraid to go on at all.
If the flue gas temperature is> 450℃ and the dust content is high, select the high-temperature flue thick-walled expansion joint without brain; When the temperature is lower than 350℃ and the flue gas is clean, the non-metallic expansion joint has higher cost performance; The axial displacement is dominant and the temperature is uniform, and the high-temperature axial type can be considered. But most of the time, the on-site working conditions are "hot and dirty", so don't save that little money.
Installation and maintenance on-site practical operation: how to adjust the tie rod? Does the deflector fit or not? Is the screw disassembled or not?
Well, if you choose the right equipment, installing the rollover will still be in vain. Talk about a few of the easiest pits to step on on the scene.
How to adjust the tie rod?The role of the tie rod is to restrain the excessive expansion and contraction of the bellows during the transportation and installation phases and prevent permanent deformation. When installing, first loosen the tie rod nut to the pre-tension or pre-compression state (according to the design pre-deformation amount), then fix the pipe, and finally loosen the nut to the working position-NOTE! It is not removed, but loosened to the specified scale. For the specific adjustment method, refer to the question and answer "How to adjust the tie rod nut of the expansion joint" in the station. One step can't be wrong.
Does the deflector fit or not?High-temperature flue thick-wall expansion joints are equipped with guide tubes as standard and must be installed. It not only keeps out dust particles, but also reduces the wear of the bellows caused by the vortex. However, make sure that the direction of the guide tube is consistent with the direction of the medium flow-the expansion joint is generally marked with arrows. If it is installed backwards, the guide tube will be deformed by the high-temperature airflow top, which will aggravate the damage of the bellows.
Is the screw disassembled or not?This question was asked no less than 20 times in the Q&A section. If it is a transport screw (only used to fix the bellows in transport condition), it must be completely removed after it is installed in place; If it is the adjustment screw on the working tie rod, only loosen the nut to the set length, and the screw body remains. Some workers cut it directly to save trouble. As a result, the bellows lost its constraint when the pipeline was running, resulting in over-tension fracture. Remember: Remove the transport screw and loosen the working screw.
After installing the thick-walled expansion joint, remember to do a full-stroke displacement check to confirm that the bellows is not stuck or twisted. Don't be too troublesome. If this work is done at once, there will be no need to stop production for at least three years. Choose the wrong one to stop production for half a year, choose the right one to save half a year-this account, can you calculate it?