First, why do you have to use special corrugated expansion joints for rectangular pipes? — — Talk about the pain points of thermal displacement of high-temperature rectangular pipes
Think about it, the smoke duct of the boiler of the power plant, the preheater outlet of the cement plant, and the sintering machine bellows of the steel plant are all large-section rectangular pipes, which are several meters wide at every turn. As soon as the temperature goes up, the thermal expansion of steel is amazing-the linear expansion coefficient of carbon steel is about 12×10⁻⁶/℃, and the pipe elongates 7.2mm per meter under the temperature difference of 600℃. For a 10-meter-long flue, the optical axial displacement is 72mm. Coupled with the lateral and angular displacement, ordinary circular expansion joints can't be stuffed into it at all.
The premise of circular expansion joint design is that the pipe section is round. If you insist on mounting it on a rectangular pipe, either add reducer pipe (extra resistance and extra air leakage point) or combine multiple small circular expansion joints (complicated stress and large space occupation). Therefore, the metal rectangular high-temperature corrugated expansion joint is tailor-made for this scene: it can directly connect with the rectangular flange, absorb the thermal displacement clearly, and at the same time maintain the system sealing and structural stability.
Moreover, the high-temperature smoke duct operating condition is not only high temperature. The flue gas carries dust (the concentration of cement kiln tailing ash can reach 100g/Nm³), corrosive gases (SO₂, HCl), and even pressure fluctuations. Ordinary expansion joints are used, either corrugated wear or corroded perforation, and they have to be replaced in three months. Do you think this job has to be a special part?
Second, what is the difference between it and ordinary circular corrugated expansion joint? — Core differences in structure, materials, and design
The wave pattern of rectangular bellows is not annular, but is continuously shaped along four sides, with straight corrugations at the top and bottom, and arc transitions on both sides. This structure determines that its stress distribution is much more complicated than that of a circular shape-the four corner areas are stress concentration areas, which must be thickened or stiffened in design, otherwise the fatigue life will be halved directly.
In terms of material, ordinary circular expansion joints can cope with most working conditions with 304 stainless steel, but metal rectangular high-temperature corrugated expansion joints can't. For the high temperature section (above 450℃), you have to use 321 or 316Ti (stabilized with titanium), and for the above 800℃, you have to pay for Inconel 625 or Haynes 282. Don't forget the guide tube-many high-temperature rectangular expansion joints have guide tubes welded inside, and the function is the same as what we said in our Q&A: protect the bellows from direct scour of dusty air flow and reduce flow resistance. The material of the guide tube is usually the same grade or slightly lower than the bellows, but the thickness should be sufficient (starting at 6mm), otherwise it will be worn out in a few years.
Such expansion joints typically come with tie rods. To put it bluntly, the function of the tie rod is to limit the excessive stretching or compression of the bellows, and at the same time transmit the blind plate force of the pipe. When selecting the type, the number, diameter and material of the tie rod must be calculated according to the pressure and thrust, otherwise the tie rod breaks during operation, which will cause a big disaster.
3. How to choose the material of metal rectangular corrugated expansion joint under high temperature conditions? — — The trade-off from austenitic stainless steel to heat-resistant alloy
- Below 400℃: Carbon steel or 304 stainless steel can dry, but pay attention to the surface treatment of carbon steel (such as coating high-temperature resistant paint), otherwise flue gas condensation will lead to corrosion.
- 400~650℃: This is a common section of the smoke air duct of a power station boiler. With 321 or 316Ti, the titanium content of 321 (≥5× C%) can fix carbon atoms and prevent intergranular corrosion. 316Ti is slightly better resistant to chloride ion corrosion than 321, but it is also 15% ~20% more expensive.
- 650~850℃: cement kiln preheater, steel heating furnace flue gas section. At this time, the oxidation resistance of ordinary stainless steel can't keep up, so nickel-based alloys, such as Incoloy 800H (heat resistant but not necessarily corrosion resistant) or Inconel 625 (also corrosion resistant and heat resistant), have to be used. Brothers who choose 625 note that its creep strength is much higher than 800H, but the price is impressive (more than 10 times that of 304).
- Above 850℃: Almost only Haynes 282 or GH series superalloys can hold up. In this case, an insulation layer (internal ceramic fiber blanket) is generally added to reduce the actual working temperature of the bellows by 100~200℃, which is equivalent to loosening the material.
Don't think that everything will be fine if you choose expensive materials. Two days ago, I met a customer. The cement line preheater used Inconel 625 expansion joint. As a result, the weld cracked half a year later-it was found that the welding process was wrong, and the welding heat input control requirements of 625 were much more stringent than those of 304. Choosing the right material is only the first step, and the manufacturing process is the real skill.
4. Typical battlefield: where is it used in power stations, cement and steel industries? — — Disassembly of three real scenes
Scenario 1: Smoke duct of power station boiler
The air supply, induction, primary air and secondary air ducts of power station boilers are mainly square cross-sections. Especially from the back of the denitrification system to the air preheater, the flue gas temperature is 330~400℃, and it contains a small amount of SO₂ and fly ash. Many of our power station customers install metal rectangular expansion joints with flue gas baffle doors, which can not only compensate for thermal displacement, but also be used as isolation dampers. The most fearful thing here is the ash accumulation in the lower part of the expansion joint-if there is too much ash, the guide tube can't support it, and the bellows will be deformed by the ash pile. The solution is to set a sewage outlet at the bottom or use a continuous guide tube (let the ash slide off on its own).
Scenario 2: Cement Kiln Preheater Outlet
Preheater C1~ C4 outlet flue, temperature 650~850 DEG C, containing a large amount of alkali metal vapor and viscous raw material dust. The first reason for the failure of the expansion joint is that the crust is stuck-the raw material clumps in the gaps of the bellows, resulting in the loss of compensation function and the stress of the pipe tearing the weld. Therefore, many designs need to add a purge device (compressed air or steam) outside the expansion joint, or choose a large wave height (wave height increased by 30%) to prevent dust accumulation and sticking.
Scenario 3: Steel sintering machine bellows
The bellows of the sintering machine are under negative pressure, with a temperature of 150~300℃, but the dust concentration is huge (it can reach 10g/Nm³), and there is tar attached. Rectangular expansion joint is installed here, and the guide tube must have wear-resistant lining plate (such as ceramic patch), otherwise it will wear and perforate in half a year. In addition, the negative pressure system requires extremely high sealing performance-for every 1% increase in air leakage rate, energy consumption increases by 3%. Therefore, the sealing structure of metal rectangular expansion joint often uses multi-layer corrugation (two or three layers) plus outer sealing ring.
5. The "pit" during installation and the "account" during type selection-the draw rod, guide tube and pre-stretching are explained at once
Don't forget to dismantle the transport pad! Some on-site workers directly welded the expansion joint, and when they were transported to the construction site, they found that the pad was not pumped, and the bellows was holding back its strength, and it was directly cracked when the temperature rose. The amount of pre-stretching/pre-compression must be calculated according to the compensation amount given by the manufacturer-for example, the axial compensation is 100mm, and the cold state is pre-stretched 50mm during installation, so that the bellows will work in the neutral position in the hot state and have the longest life.
How to adjust the nut of the tie rod? Look at our question and answer (how to adjust the tie rod nut of the expansion joint), the core is one sentence: first loosen it symmetrically, and then lock it pair by pair according to the design load. Remember to measure whether the wave pitch of the bellows is uniform after adjustment. If the deviation exceeds ±2mm, you have to re-adjust it.
The direction of installation of the deflector must not be wrong! The direction of the arrow of the guide tube is the direction of the airflow. Install backwards, the guide tube becomes a baffle, and the airflow rushes from the side of the bellows, and it will be perforated in one month. That's a cliche, but people step on it every year.
How to settle accounts when selecting a model? Don't just look at the price of the expansion joint itself, but also the full life cycle cost. For example, the scheme of stainless steel waveguide + ordinary tie rod may be half cheaper than that of nickel-based alloy, but if the life is only three years, and nickel-based alloy can be used for eight years, the annual cost will be lower. And considering the loss of shutdown, the difference is not a star and a half.
6. What problems will happen after running for a long time? — — Common failure modes and maintenance inspection points
①Fatigue crack of bellows-It mostly occurs in the transition zone between the trough and the peak, especially at the four corners. Use penetration detection or ultrasonic phased array during inspection, focusing on these areas. If micro-cracks (length
②Guide tube wear down— This one is the most hidden. The guide tube is inside the expansion joint and cannot be seen at all without disassembling it. Regularly use an endoscope to look through the sewage outlet, or install a wear indicator (embedded resistance probe in the wall of the guide tube). Once the deflector is found to be worn out, it must be replaced immediately, otherwise the airflow will blow the bellows directly, and the perforation will explode in three weeks.
③Fatigue fracture of tie rod— — The tie rod bears alternating stress, and fretting fatigue is easy to occur at the bolt connection. During maintenance, check whether the nut is loose and the surface of the tie rod is corroded or cracked. If one is found to be broken, it must be replaced in pairs (same side tie rods). Don't only replace one, otherwise the eccentric load will aggravate the burden on the other tie rods.
④Gasket failure— — High-temperature rectangular expansion joint flange surface with winding gasket or graphite gasket, after half a year of operation, gasket creep relaxation leads to air leakage. Treatment method: Open a glue injection hole at the edge of the flange, and inject high-temperature sealant online. If the air leak is serious, you have to stop the furnace and change the gasket.
If conditions permit, it is recommended to remove the metal rectangular high-temperature corrugated expansion joint for hydraulic test or air tightness test every overhaul cycle (3~5 years), and don't rely on appearance inspection alone. Internal defects can't be seen with the naked eye, so pressure test can find hidden dangers in advance.