Why do standard parts always roll over in the flue? — — There are so many working condition variables that manufacturers dare not promise
Two days ago, I met a customer, saying that their factory used universal expansion joints from three suppliers, but the longest one only lasted for eight months. You say standard parts manufacturers are not professional? No, they're really afraid to promise. In this place, the temperature of the boiler flue jumps back and forth from 200℃ to 800℃, and there are sulfur, chlorine and water vapor in the flue gas. When the fan is turned on, the pressure fluctuates high and low, and the thermal expansion and contraction of the pipe is still pulling in the three-dimensional direction. The standard parts are designed according to the hypothetical working conditions of "normal temperature, no corrosion and one-way displacement". Can there be no accidents if they are thrown into this environment?
Why don't the manufacturers insure you? Because there are too many variables, he can't override them with one or two arguments. You ask him to pat his chest and say that in three years, he will have to prepare to lose money the next day. So don't scold standard parts as bad as customization-this is determined by the nature of working conditions. The flue expansion joint is not mineral water in the supermarket. If the bottle type is wrong, at most some water will leak. If a parameter is wrong here, it may be interlocked to the boiler shutdown and the safety valve exploding.
The first step of customization: calculate the three numbers of temperature, displacement and corrosivity accurately
Customization is not a slap on the head. These three numbers are a little different, but the result is a hundred and eight thousand miles.
Temperature.Not the average temperature, but the highest continuous operating temperature and instantaneous peak. I have seen a project, which was designed to be 450℃. During actual commissioning, it instantly rushed to 680℃, and the 304 bellows was directly oxidized and peeled off. When calculating the temperature, take into account the full load of the boiler, abnormal flue gas reflux, and even the oven stage. Usually leave a margin of 20%, but do not exceed the upper allowable material limit.
Displacement.Including thermal expansion, equipment vibration, pipeline foundation settlement. Many people only count axial elongation and ignore lateral offset and angular displacement. For example, a 20-meter-long flue rises from 30℃ to 600℃, and the expansion amount is close to 200mm. This 200mm is not a straight stretch, but will also be twisted due to uneven pipe support. And guess what? In some scenes, the lateral displacement was not counted, and as a result, the bellows was forcibly twisted into a twist.
Corrosive.The smoke composition analysis report must be obtained. SO2, H2S, HCl, water vapor dew point temperature, these directly determine the material selection. Don't say "probably a little corrosive", specific to the ppm value per cubic meter. Otherwise, you used 316L, thinking that you could bear it, but as a result, the chloride ion exceeded the standard, and the pitting corrosion perforated.
If these three data are calculated accurately, the customized plan will not be in vain. Otherwise, if you fill in the wrong parameters, you will work for three years in vain-this is not an exaggeration.
If the material is not selected correctly, the expansion joint is a time bomb: what is the difference between 304, 316L and Inconel 625
The matter of materials was very boring to talk about, but the price of choosing the wrong one was bomb-level.
- 304 stainless steel: The cheapest, the upper temperature resistance limit is about 870℃ (intermittent), but the resistance to chloride stress corrosion is extremely poor. It is suitable for clean flue gas, temperature below 400℃ and chlorine-free working conditions. If you have a bit of wet chlorine or chlorine-containing organic compounds in your condition, 304 will experience intergranular corrosion within a few weeks. Don't ask me why I know, I've seen it twice.
- 316L: Molybdenum is added, the pitting corrosion and crevice corrosion resistance is stronger than that of 304, and the chloride ion tolerance is slightly higher. However, note that the strength of 316L decreases significantly above 600℃, and it cannot be used for flue gas containing hydrofluoric acid. Many power plants use it as the expansion joint of the flue after desulfurization, but when the desulfurization system starts and stops, the pH value of the condensate is as low as 2, and 316L still can't bear it.
- Inconel 625: Nickel-based superalloy, temperature resistance can reach more than 1000℃, chloride, sulfide, fluoride corrosion resistance is very strong. The price is 5-8 times the price of the 304. But don't do it blindly-it is difficult to process and requires high welding technology. If the welding electrode is selected wrong or the heat treatment is not in place, the weld will still crack.
Then which one to choose? There is no universal answer. But remember one principle: choose the cheapest one while meeting temperature resistance and corrosion resistance. Don't be superstitious about "what is expensive is good", otherwise the material cost alone can make the project exceed the budget.
Structural design pits: single-layer bellows, multi-layer composite, with guide tube, each pipe has its own work
The more complex the structure is, the better. Every structure has its scenes of existence and its pitfalls.
Single layer bellows:Suitable for low pressure (≤0.05MPa), small displacement (≤50mm) working conditions. Advantages Cheap and fast to manufacture. The disadvantage is that the single layer wall is thin, and once corroded and perforated, it will be completely wasted. Moreover, the fatigue life is short, generally about 10,000 cycles. However, the number of starts and stops of the boiler flue is small. If the working conditions are stable, a single layer is enough.
Multilayer composite structure:Two layers, strong pressure resistance (up to 0.5MPa), double fatigue life (more than 20,000 times). Disadvantages: Water is easy to enter between layers. If the water vapor in the flue condenses, the water will rust between layers and corrode from the inside. Therefore, the multi-storey structure must design drainage holes, but many people on the scene forgot to open them.
With guide tube:For flue gas with high flue flow rate (> 15 m/s) or containing particulate matter. The guide tube can prevent the solid particles from directly impacting the bellows, and at the same time guide the flue gas to reduce the vortex vibration. Where is the pit? If the gap between the guide tube and the bellows is too small, Karman vortex will be generated, which will induce high-frequency vibration fatigue fracture. The clearance is generally greater than half the height of the bellows. In addition, the guide tube itself should be corrosion-resistant, so don't make it into carbon steel, otherwise it will rust out for half a year.
Have you ever seen a structure called "bypass"? It is designed for flushing flue with water, but most people don't consider the sudden change of flushing water temperature and flue gas temperature, which leads to thermal shock deformation. Alas, ask more about the use at the beginning of the design, and fill in less pits later.
The installation site is the place to see the real chapter-pre-stretching, cold tightening, support, all in vain if one is not done properly
No matter how well designed it is, there are plenty of rollovers installed. I have seen a project with my own eyes. Custom expansion joints were imported from Germany, and the parameters were all correct. As a result, the site installer forgot to pre-stretch them.
Pre-stretchingWhat is it? When installing at room temperature, the bellows must be pre-stretched to half the expansion at operating temperature. For example, the thermal expansion is 200mm, and the pre-pull is 100mm during installation. In this way, the bellows is most uniformly stressed at the middle position. Without pre-stretching, one end of the bellows is crushed to death and the other end is pulled full under hot state, and the fatigue life is halved due to uneven deformation.
Cold tightIt is for low-temperature start-up conditions. If the boiler is started from the cold state (-20℃), the cold shrinkage of the pipe will produce opposite displacement, so the cold should be pre-pulled in the reverse direction. This parameter is usually provided by the designer, but often no one checks it on site.
SupportIs to fix the bellows position. Many engineers design the support to be rigid and fixed. As a result, the pipe can't be released when it expands thermally, and the bellows is directly pulled and deformed. The right thing to do is to use a sliding mount or guide mount that allows axial displacement but limits lateral deflection. Others install the support behind the main fixing point and turn it in the opposite direction completely.
You ask me how to avoid it? Before installation, the designer and the installation team must hold a technical briefing meeting, write the pre-stretching amount, cold tightening direction and bearing type on the drawing, and sign it on the spot for confirmation. Don't think about relying on the construction team to realize it by themselves. They install flanges every day, and they don't necessarily know bellows.
Be clear: Customization is 50% more expensive, but lifecycle costs can be saved three times
When it comes to customization, many people's first reaction is "expensive". That's right, custom expansion joints typically cost 50% or even double the price of standard parts. But let's settle the score.
The unit price is 5,000 yuan, the life span is 1.5 years, and it is replaced once a year. Plus the loss of shutdown (100,000 yuan for one day of shutdown) and manual replacement (including hoisting, welding and flaw detection, about 20,000 yuan), the total cost in three years is ≈ (standard parts 5000*2+ shutdown loss 100,000 * 2+ labor 20,000 * 2) =245,000.
The unit price is 12,000 yuan, and the life span is 6 years (in fact, many of them have been used for 8 years without breaking). There is no need to replace them within three years, and only one installation cost. Total cost for three years =12,000+ downtime loss 0+ installation fee 20,000 ≈ 32,000? Wait, is the downtime loss 0 here? No, the custom installation also requires downtime, but only once. Assume the first day of downtime installation loss of 100,000, plus the custom parts of 12,000 and the installation fee of 20,000, the total is 132,000. The standard parts are replaced twice in three years, and the downtime loss is 200,000, plus procurement and labor, totaling 245,000. The custom plan saved 113,000, nearly half.
Moreover, the custom parts are maintenance-free during the design life, so there is no need to change parts frequently. If you calculate the hidden costs of manpower, management and safety risks, it is no exaggeration to save three times.
Don't just look at the numbers on the quote. Take into account downtime losses, labor replacement frequencies, spare parts inventory costs, and even accident fines, and you'll find out —Customized scheme of boiler flue expansion jointIt's not spending money, it's saving money.