1. Why is the flue system inseparable from the expansion joint? Let's talk about the culprits who made the pipeline "burst" first
The flue of power plants and cement plants is, to put it bluntly, a huge heat exchanger-high-temperature flue gas rushes out of the combustion chamber, and the temperature can soar to 500 to 600 degrees Celsius, but the outer wall of the pipe is room-temperature air. No one can avoid the physical law of thermal expansion and contraction. A flue dozens of meters long can expand by more than ten millimeters every time the temperature rises by 100℃. You calculate that under a temperature difference of 300℃, a 50-meter pipe can eject a displacement of nearly 200mm.
But the problem goes far beyond thermal expansion. When the fan turned on, the entire flue shook — it was low-frequency vibration and airflow pulsation messing with. There are also foundation settlement, installation deviation, and impact when equipment starts and stops... These forces are superimposed together, and the stress at the pipeline interface is comparable to that of twisting steel bars. Hard carry? Cracked welds, pulled flanges, or even overall twisted pipes are nothing new. Because some power plants have no expansion joints, the flue cracks within half a year after they are put into operation, and the maintenance cost is enough to buy several expansion joints.
Therefore, the flue system must have something like this: it can "eat" displacement, resist vibration, and seal the smoke tightly in the pipe-this is what the metal flue expansion joint does. To put it bluntly, it is an elastic joint, which makes the rigid pipe system flexible.
2. Three hard skills of metal flue expansion joint: absorbing heat displacement, withstanding vibration and sticking to the seal
The first is to absorb heat displacement. The flue expands and contracts in both axial and radial directions, and the metal bellows digest these deformations through the compression and stretching of the corrugations. For example, for corrugated expansion joints used in power station industry, the single wave compensation amount can usually reach 8-10mm, and the multi-wave combination can easily cope with the displacement of hundreds of millimeters. Some rectangular flues will also use metal rectangular expansion joints, and the four corners will be rounded corrugated structure to adapt to multi-dimensional displacement.
The second is to withstand the shock. The continuous vibration generated by the fan and the flue gas flow can cause fatigue fracture of the rigid connection. The bellows itself has damping characteristics, which can filter out part of high-frequency vibration. If you have used the universal corrugated expansion joint, you will notice that its wall thickness is usually only 0.5~2mm, but it increases the fatigue life through the multi-layer structure (even with steel mesh sleeve). That kind of "head-to-head" sleeve-type expansion joint, the shock resistance is just a bit worse.
The third is to stick to the seal. The flue gas contains SO₂, NOx and dust. Leaking out is not as simple as an environmental fine, and it will also corrode the peripheral equipment. The seal of the metal expansion joint depends on the bellows itself and the connection pipes at both ends, and the air flow can be introduced smoothly with the guide tube to avoid vortex erosion. Although non-metallic expansion joints can be sealed, they are not as resistant to pressure and temperature as metal materials. In many high pressure differential or high temperature scenarios (such as the inlet of desulfurization tower), metal flue expansion joints are the only option.
Incidentally, "sticking to the seal" is not absolutely leak-free-any expansion joint has micro-leakage, but good metal flue expansion energy saving keeps the leakage rate below 0.1%. The orientation and quality of the deflector is key during installation (detailed next).
3. Don't pat your head when selecting: which metal expansion joint should be used in power station, cement and desulfurization scenarios
It is also a metal expansion joint, and different working conditions are hundreds of thousands of miles different.
Flue of power station boiler: High flue gas temperature (above 400℃), low pressure (slightly positive pressure or negative pressure), long pipeline and large displacement. High-temperature axial expansion joint is the first choice, stainless steel 321 or 316L is the temperature resistance grade, and multi-wave U-shape or ω-shape is used for corrugated structure, which has a large compensation amount. If space is constrained to absorb lateral displacement, use a double hinge transverse expansion joint or a straight pipe pressure balanced expansion joint-the latter also balances pressure thrust and avoids additional loads on the pipe support.
Cement production line: The working conditions are rougher, the dust concentration is high, and the temperature fluctuates greatly (the outlet of the preheater can reach 600℃, and the inlet of the cooler drops sharply to 300℃). At this time, the metal corrugated expansion joint of the candidate cement industry is lined with wear-resistant guide tube, the wall thickness is thickened, and the corrugated pitch is enlarged to prevent dust accumulation. Some shaft furnace flues also use metal rectangular expansion joints to accommodate rectangular cross sections.
Desulfurization system: The temperature is not high (generally lower than 200℃), but the medium is wet and corrosive (including sulfuric acid and sulfurous acid). Stainless steel 316L is the basic plate, more rigorous with duplex steel or PTFE-lined hoses. However, note that the desulfurization flue is often equipped with flue gas baffle door (such as desulfurization flue gas baffle door), and the expansion joint cannot replace the cutting function of the baffle door. They each perform their own duties (the last one will be talked about). The discharge of condensed water must be considered when selecting the model, and a drainage hole should be opened at the bottom of the guide tube.
Alas, two days ago, I met a cement factory customer, who bought a general-purpose corrugated expansion joint for cheap use at the preheater outlet. As a result, the corrugated pipe was worn out by dust in three months. Regardless of the media characteristics when selecting the model, the later operation and maintenance cost will double. Remember one thing:Working parameters (temperature, pressure, medium, displacement) must be clear, don't pat your head.
4. Pits in installation and operation and maintenance: Should the tie rod be disassembled? Which way does the deflector face?
Expansion joints usually leave the factory with tie rods (also called transport tie rods or limit tie rods), which function to prevent accidental stretching or compression of bellows during transportation and installation. Many people forget to dismantle it after installation-the consequence is that the expansion joint becomes a "dead joint", can't deform freely, and the pipeline is still stressed. The correct way to do this:After installation is in place and the pipe is fixed, the tie rod nut must be loosened and the tie rod removed(For details, see the question and answer of this site "Does the screw of the expansion joint need to be disassembled"). Of course, if it is a large tie rod expansion joint, the tie rod itself participates in the force (such as absorbing lateral displacement), then it will not be disassembled, but the nut position needs to be adjusted according to the design.
The orientation of the guide tube is even more a "metaphysical" problem. The guide tube is installed inside the expansion joint, and its function is to guide the airflow smoothly through the bellows, preventing high-speed flue gas from directly washing the corrugations (which can cause local wear and acoustic vibration). There must be and only one direction:The taper of the guide tube is directed toward the airflow。 You read that right, toward the incoming stream. If it is installed backwards, the airflow will impact the bottom surface of the guide tube to form vortex, which not only makes loud noise, but also accelerates ripple fatigue. There are generally arrows on the expansion joint to indicate the flow direction, so don't make low-level mistakes.
In addition, the bracket should also be well matched when installing. The spacing between the fixed bracket and the guide bracket cannot exceed the design value, otherwise the expansion joint will bear additional pipe dead weight and thrust. Especially for directly buried or fully buried expansion joints, they must be tamped in layers when backfilling soil to prevent the bellows from skewing caused by uneven settlement.
Regularly check the corrugated surface for cracks, corrosion pits, and wear marks. Is the deflector falling off? Is the tie rod loose? If the expansion joint is found to have abnormal deformation (such as uneven corrugation spacing), it means that the pipeline system may have unexpected displacement, so it must be checked quickly.
5. Don't use the expansion joint as a baffle door: common misunderstandings and matching relationships
This pit I've seen the most. Expansion joints are "flexible connectors" and baffle doors are "cutting devices", which function completely differently. In order to save costs, some sites use expansion joints as cut-off valves-flattening the bellows to block the smoke? That's not a joke. Once the bellows is forcibly compressed to the limit, it will immediately crack, and the smoke will leak. Maybe the whole system will have to stop.
The baffle door is responsible for opening and closing and isolation, and the expansion joint is responsible for compensating for displacement.In the desulfurization island or power plant flue, the usual arrangement order is: fan outlet → expansion joint → baffle door → next section of pipe. The expansion joint absorbs the vibration and thermal displacement of the fan, and the baffle door isolates the back. This site has a product called double-seal single-axis circular baffle door, with electric or manual actuators. When the double seal is closed, there is almost no leakage-but it can't compensate for displacement. And the expansion joint can not replace the cutting action of the baffle door. Do your own thing, don't mix it.
Expansion joints are considered to "correct" mounting deviations. The expansion joint does absorb a certain amount of mounting deviation (such as a mismouth when the pipe is butted), but that's as part of the compensation function, not for you to use it as a "deviation corrector". If the flange is forcibly aligned by pulling the expansion joint, the result is that the bellows is subjected to torsional stress, and the life of the bellows is sharply reduced. Correct practice: When installing the pipeline, align the flange first, adjust the gap well, then install the expansion joint, and finally tighten the bolts.
When it comes to bolts, professional metal-wound gaskets or graphite gaskets should be used at the joint between expansion joints and flanges. Don't use ordinary asbestos gaskets for cheap. At high temperature, the asbestos pad is quickly powdered, and the leakage rate rises straight.
Write at the end
The function of metal flue expansion joint, in the final analysis, is one sentence: make the rigid flue system still operate safely under the multiple torture of heat, vibration and pressure. It's not a universal part, but without it, flue life in power plants and cement plants is at least 50% off. Choose the right model, install it in the right direction, and maintain it regularly. This thing can last for more than ten years. If you only install it as an "iron knot" at will, then wait for the maintenance order to be slapped in the face repeatedly.