Phenomenon Direct Strike: Expansion Joint Burning is not an incident, but the system is asking you for help
When most on-site personnel see that the expansion joint is red, their first reaction is "the material is not good" or "stop the machine and change parts quickly". But to be honest, 80% of the root of red burning is not in the material itself, but in the details of the working conditions under your nose. The metal oxide scale begins to turn red above 600℃, and enters the rapid oxidation stage when it exceeds 700℃-you feel it closer with the back of your hand (don't really touch it!). If the surface temperature has exceeded the design value (non-metal expansion joints usually withstand 300℃, and metals withstand 450℃), then the problem is not solved by changing the expansion energy saving. Last year, I was in a waste-to-energy plant in southwest China. The on-site master pointed to the red expansion joint and scolded the manufacturer. As a result, I swept it with thermal imaging, and the smoke temperature soared to 620℃, and the design only reached 450℃. Who do you think should bear this pot?
Three culprits dissected one by one: air leakage, coking, and design to back the pot
The first culprit: the seal wind pressure is lower than the flue pressure.This is a stupid but very common mistake. The function of sealing wind is to block the high-temperature flue gas from drilling into the expansion joint. Once the wind pressure is lower than the negative pressure of the flue, the high-temperature flue gas will directly flow backward and wash the metal bellows. And guess what? The windward side of the bellows is instantly locally overheated, and the surface oxide scale peels off, followed by perforation. I have seen a project in which the selection of sealed fans was 30% smaller, and the expansion joint burned red after three months of use. After replacing it with a big fan, the problem never appeared again.
The second murderer: coking and dust.Fly ash accumulates inside the expansion joint and has an extremely low thermal conductivity-the thermal conductivity of ash is only about 1/50 of that of steel. The heat under the ash layer cannot be dissipated, forming local hot spots, and the surface temperature directly rushes from 400℃ to 650℃. Moreover, dust accumulation will also cause the expansion joint to not expand and contract freely, the bellows will be hard-pulled, and the fatigue life will collapse from 100,000 times to thousands of times. Some time ago, a biomass power plant customer burned an expansion joint red every week. It turned out that the soot blower program was set incorrectly, and it was not put into use for a month, and the flue ash was blocked by 2 meters. After cleaning up the dust, the temperature dropped immediately.
The third guy: the actual operating temperature was underestimated when designing and selecting.Many design institutes directly copy the "theoretical smoke temperature" given by boiler suppliers, but in actual operation, boiler overload, coal type change, and even burner adjustment deviation can raise the smoke temperature by 50-80℃. In addition, the installation position of the expansion joint is too close to the elbow, which is also very fatal-the flue gas produces bias flow at the elbow, the flow rate on one side is high, the scour is severe, and the bellows is blown into a "crescent shape" and ablated. You have to know that with the same material, the temperature difference between the windward side and the leeward side can reach more than 100℃.
What happens after a red burn? It's not as simple as replacing parts
After the local thinning of corrugated pipe, the fatigue life of corrugated pipe decreases by a cliff. The normal design life is more than 100,000 times, and there are thousands of times left when it is red. In case of tearing, the high-temperature flue gas leaks out, and the furnace will be shut down for 48 hours for emergency repair, or the surrounding insulation materials will be ignited, and then the cable will be burned, which is a fire. A biomass power plant in northwest China was planted in this pit last year-the expansion joint was burned through, the flue gas was poured back into the fan bearing seat, the grease was carbonized, the rotor was stuck, the fan was scrapped, and the loss of furnace shutdown exceeded 2 million. Calculate this account, how much is an expansion joint?
Practical investigation steps: Don't guess, follow the process
The first step is to scan the infrared thermal imaging camera. Instead of measuring just one point, sweep the entire surface of the expansion joint to find the position where the temperature is the highest. If the temperature difference between the highest point and the lowest point exceeds 80℃, it means that there is a problem with the internal flow field or ash accumulation. The second step is to measure the difference between flue negative pressure and sealing air pressure. The sealing air pressure must be at least 500Pa higher than the flue pressure. If it is low, adjust the valve or change the fan. The third step is to disassemble the expansion joint to check the internal dust accumulation-don't forget to wear anti-scalding gloves, the surface temperature of ash may be 30℃ higher than that of metal surface. The fourth step is to turn the design drawings to see if the calculation of thermal expansion covers the actual displacement. Two days ago, I met a paper mill customer, and the expansion joint burned red after two years. As a result, it was found that the boiler was overloaded, and the smoke temperature was dried from the designed 450℃ to 580℃. The expansion amount directly exceeded 30%, and the bellows was deformed by hard pulling.
How to avoid pits in material selection and structure? Don't blindly chase high
Do you use 316L or Inconel625 for metal expansion joints? Many people think it's expensive, but 316L falls down at 600℃. Inconel625 can carry 900℃, but the price is five times that of 316L. If your actual working condition is only 550℃, 316L is completely enough, so there is no need to waste money. The upper temperature resistance limit of non-metallic expansion joints is usually 350℃, silica gel is suitable for low-sulfur flue gas, and fluororubber has strong sulfur corrosion resistance but doubles its price. There is an iron rule in structure:The guide tube must be added with。 The guide tube can guide the high-temperature smoke away and avoid directly shooting to the bellows. In addition, the length of the expansion joint should not exceed 1 meter. If it exceeds this number, the thermal displacement will be uneven, and the bellows will easily be twisted into twists-the longest one I have ever seen is 1.8 meters, and it took three months to twist it into an "8" shape.
Expansion joints are not the end of the day, but you have to prescribe the right medicine. Check the working conditions first, then consider changing materials, and don't reverse the order.