Two days ago, I met a customer who asked me with a selection table, "My medium temperature is 650℃, is 304 stainless steel enough?" When I looked at his pipeline layout, there was a high-temperature furnace wall next to it, and there was no insulation layer for direct burial installation. I said if you use 304, it will burn through in three months. He didn't believe it. As a result, when he went back and calculated, the wall temperature of the bellows had already soared above 850℃-this was not a problem of material, but the whole temperature standard was not thoroughly understood.
Why are temperature standards important? To put it bluntly, burn-through is just the most conspicuous accident, and more concealed is stress relaxation and plummeting fatigue life. Once the temperature standard of the metal expansion joint is wrong, the high-temperature creep of the bellows will be accelerated, and the elastic compensation ability of the corrugation will be directly folded in half. The pipeline can't absorb it because of the thermal displacement, ranging from flange leakage to pulling off the whole bracket. I have seen a steam pipe. Just because the temperature is 50℃ higher than the design value, the bellows cracks in fatigue within two years, and it costs more than 100,000 yuan to replace it once.
Temperature limit of common materials-don't just look at the grade, look at the "real temperature" under working conditions
304 can withstand 800℃? No, that's the oxidation limit, not the structural strength limit. 304 Under the actual pressure condition, it is recommended that the long-term use should not exceed 425℃; 316L can reach about 450℃; The Incoloy 800 can carry up to 650°C; But above 750℃, you have to ask superalloys to play, such as Inconel 625.
Take usHigh temperature axial expansion jointFor example: the medium temperature of the flue gas pipeline is 680℃. We chose Incoloy 800 as the bellows material, and at the same time, we added the guide tube and the lining heat insulation layer to control the wall temperature of the bellows below 550℃. If you take 316L directly, the wall temperature is the same as 680℃, and the rigidity of the bellows drops badly, and it will leak after two heat cycles.
Temperature depends not only on the medium, but also on the environmental heat radiation and installation method
The medium temperature is not equal to the bellows wall temperature. Ambient heat radiation, wind speed and insulation layer thickness will all affect the actual wall temperature. And guess what? The temperature of the same medium is 600℃, and the wall temperature difference between open air installation and direct burial installation is 150℃.
EspeciallyDirect buried (fully buried) type expansion jointIf the pipeline is not externally insulated, the heat dissipation capacity of the soil is limited, and the wall temperature of the bellows may not be much lower than the medium temperature. In turn, the wall temperature can be lowered by 50~80℃ by making an insulation layer. Therefore, when selecting the model, you must ask clearly: Is the pipeline insulated? Does the environment have a high-temperature radiation source? How many meters away? These data are more critical than the material grade itself.
Where do the standards come from? How to understand "safety margin" in GB/T 12777 and EJMA
The domestic standard GB/T 12777 "General Technical Conditions for Expansion Joints of Metal Bellows" clearly classifies temperature grades, such as Grade I (-20℃ ~100℃), Grade II (100℃ ~350℃), Grade III (350℃ ~550℃) and Grade IV (550℃ ~700℃). Each level corresponds to a different allowable stress reduction factor. However, note that this temperature refers to the design wall temperature of the bellows, not the medium temperature. The American EJMA standard is finer and gives the fatigue life correction coefficient at different temperatures. For every 50℃ increase at high temperature, the fatigue life may be reduced by half.
How to keep security allowance? My experience is that the design wall temperature should be at least 30~50℃ higher than the worst case of actual working conditions. Especially when the medium contains corrosive components (such as sulfides), the high temperature can aggravate the corrosion, and the balance has to be amplified. Don't get stuck in the limit choice to save money. It's a gamble on the life of the equipment.
Type selection practice: power station vs cement industry, temperature difference determines completely different structures
Used in power station industryCorrugated expansion jointThe typical working condition is the main steam pipeline, the temperature is 540~570 ℃, the pressure is high (10~20MPa), and the displacement is large. At this time, the bellows must be made of multi-layer thin-walled structure (for example, three layers of 0.5mm thick), and the material is Incoloy 800 or similar superalloy. At the same time, a guide tube needs to be set to reduce the impact of high-speed steam on the corrugation. Structurally, duplex or pressure-balanced types are usually adopted, such asStraight pipe pressure balanced expansion jointTo absorb the axial displacement of the large diameter pipe.
The cement industry is completely different. The temperature of flue gas at the end of cement kiln is usually 350~450 ℃, but it contains a lot of dust and corrosive gases (SO₂, NO₂). Used at this timeMetal Corrugated Expansion Joints in Cement IndustryInstead, stainless steel 304 or 316L is commonly used as a material, because the temperature is not too high, but special emphasis is placed on wear-resistant lining and ash discharge structure. The wall thickness of the bellows will be thickened to 1~1.5mm, and is often used with non-metallic compensators, such as those used at the outlet of the preheaterRectangular non-metallic expansion jointTo absorb the three-dimensional displacement.
You see, the same is a metal expansion joint, one is high temperature and high pressure, and the other is wear-resistant at medium temperature. The material selection and structure are very different. So stop "one meter is universal". The temperature standard is never the temperature-resistant meter hanging on the wall, but it is calculated everywhere at the project site.