That bulging expansion joint in the flue exhaust system of a power plant, you really think it's just an "expansion joint"? Two days ago, a customer spat with me. Their factory changed two expansion joints, and each time it lasted less than three years, it leaked. Ask for details-the general-purpose corrugated expansion joint used, the flue temperature is above 400℃ all the year round, and it is also corroded by sulfide. Isn't this just taking ordinary goods to carry special working conditions? Let's get this straight today.
The core task of exhaust expansion joint is not "telescoping", but life saving
Many people think that the expansion joint is about the thermal expansion and contraction of the pipe. In fact, in the flue of the power plant, it has to withstand the impact of the fan when it starts and stops, the corrosion of acidic condensate in the flue gas, and the local wear caused by dust accumulation. Take the "non-metallic expansion joint (fabric fiber expansion joint)" of this station as an example. Its flexible compensation ability is indeed strong, but if it encounters high-temperature sulfur-containing flue gas, the fiber layer is not selected correctly, and it will be crisp in half a year. On the other hand, the "metal rectangular expansion joint" has high temperature resistance, but the stress at the corner of the rectangular section is concentrated, and it is easier to crack by fatigue if it is not handled well. To put it bluntly, the expansion joint is a "buffer zone" in the flue-it has to withstand the temperature, chemical attack and mechanical vibration. Which link goes wrong, air leakage, tearing and collapse will follow.
Find out three parameters before selection: temperature, pressure and media composition
If the temperature exceeds 350℃, don't worry about the rubber compensator. Go directly to "corrugated expansion joint for power station industry" or "high-temperature axial expansion joint". There are acidic gases such as SO₂ and HCl in the medium? Then it has to be lined with "PTFE-lined hose" or "PTFE compensator", otherwise the metal bellows will pit and perforate in minutes. Another thing that is easy to overlook is that when the flue gas baffle door is used with the expansion joint, such as the "desulfurization flue gas baffle door" of this station, the installation sequence is also particular-the baffle door is in front and the expansion joint is in the back, so as to avoid hot flue gas backflow from damaging the sealing surface. Calculate, if a parameter is selected wrong, the life will be cut in half, and the loss of shutdown for maintenance will be worth it?
The Three Details That Are Most Prone to Rollover When Installing
Direction of guide tube
Some appliance factories installed the guide tube of "metal rectangular expansion joint" backwards, and the flue gas directly washed the corrugated root, which cracked in less than a year. Remember: The arrow of the deflector must point in the direction of the media flow.
Whether tie rod nut is removed
And guess what? Many people keep the tie rod nut used for protection when leaving the factory as a permanent part, which limits the displacement of the expansion joint and deforms the pipeline interface. The function of the expansion joint tie rod is only temporary fixation in the transportation and installation stages, and it must be loosened or removed before putting into production. For the specific adjustment method, please refer to Article 12 of FAQ.
Corner reinforcement of rectangular expansion joint
Many on-site diagrams save trouble. The four corners are directly welded, and there is no rounded transition. When the thermal stress is concentrated, the corners crack first. If the weld is not treated in place, the crack begins to spread from the corner, which eventually leads to the scrapping of the entire expansion joint. If you stepped on any of these three pits, three years of life would be considered a luxury.
Real case: The painful lesson of that white crystal
Last year, "rectangular non-metallic expansion joint" was used in the outlet flue of induced draft fan of a 2×300MW power plant. Air leakage began just two months after it was put into operation, and inspection revealed a large number of white crystals on the surface of the fiber layer-that is the corrosion product of acidic liquid. Although the temperature resistance grade selected at that time was enough, it ignored the high water vapor content in the flue gas, which formed dilute sulfuric acid after condensation. Later, it was replaced with "non-metallic expansion joint (fabric fiber expansion joint) + PTFE anti-corrosion layer", and at the same time, a drainage port was added downstream, which has been running for more than two years without any problems. Therefore, you can't just buy the expansion joint and install it. There are many more invisible killers in the working condition than in the instructions-flue humidity, condensate pH, start-stop frequency, each of which is torture.
Be honest
The flue exhaust expansion joint of power plant is essentially a consumable part, but it is completely possible to extend the service life from two or three years to seven or eight years by choosing the right type and installing the right details. Don't be superstitious about "general-purpose" or "standard products", and don't buy models with parameters to save thousands of dollars. Spend more time checking the working condition parameters, or directly ask the manufacturer for similar successful cases-for example, the "corrugated expansion joint for power station industry" and "desulfurization flue gas baffle door" of this station have been matched, and the accumulated data is more reliable than any brochure. After all, the labor and downtime cost of replacing the expansion joint has long exceeded the price of the expansion joint itself. This account should be settled clearly before starting.