Let's talk about the construction site first
Last month, a customer of a power plant reported that the corrugated expansion joint on their flue cracked in less than half a year. When removed, the bellows was pulled and deformed like a spring-not enough of the typical expansion and contraction. On the other hand, the amount of expansion is too large, the cost is doubled, and the installation space is not allowed. Therefore, the "expansion and contraction amount of flue expansion joint" is not determined by slapping the head, but has to be calculated from the thermal displacement of the pipe.
What is the amount of expansion?
To put it bluntly, it is how much thermal expansion and contraction displacement is absorbed by expansion energy saving. High-temperature flue gas runs in the flue, and the temperature ranges from tens of degrees to seven or eight hundred degrees. When the thermal expansion of pipe materials (carbon steel, stainless steel) per meter is calculated, the total displacement of the whole pipe comes out. But the amount of axial expansion and contraction alone is not enough-the flue also has lateral displacement (such as lateral deformation at the bend of the pipe) and angular displacement (such as deflection caused by the settlement of the support). For example, we commonly useHigh temperature axial expansion jointIt mainly eats axial direction, while the transverse expansion joint of compound hinge is specially designed to deal with transverse swing.
The formula for calculating the stretching amount is not complicated, but the difficulty lies in Δ T
Δ L = α × L × Δ T, this formula is understood by engineers. But the difficulty is how to take Δ T. The temperature difference of flue gas in the flue may be 300℃, but the temperature of the outer wall of the pipe is often much lower, especially for outdoor pipes. In addition, the ambient temperature at the time of installation has to be included-the pre-stretching amount can be different between summer and winter. There was a case in a cement factory two days ago, they usedMetal Corrugated Expansion Joints in Cement IndustryAccording to the pipeline design temperature of 550℃, the actual operating temperature is only 400℃. As a result, the bellows is in a compressed state all the time, which leads to local instability. Therefore, it is best to get the actual temperature curve of flue gas before selection, and don't just look at the design drawings.
Different types of flue expansion joints have great difference in expansion and contraction capacity
Metal corrugated expansion joints (e.g.Corrugated expansion joint for power station industry) The single wave compensation amount is generally 8-20mm, and the total compensation amount is increased by increasing the wave number, but more wave number will reduce the withstand voltage capacity.Non-metallic expansion joints (fabric fiber expansion joints)It is different. It relies on the deformation of flexible materials, and the one-way compensation can reach several hundred millimeters. It is also corrosion resistant and vibration isolated, so it is especially suitable for the wet flue gas environment of desulfurization flue. However, non-metals have weak pressure bearing capacity, so they can only be used in low-pressure flues. Rectangular flues (like those in coal-fired power plants) are commonly usedRectangular non-metallic expansion jointOrMetal rectangular expansion jointBecause the square section is not easy to install the circular bellows.
Scaling and scaling reservation during installation is the key
Choosing a good model is only the first step, and the reservation of expansion and contraction during installation is the key to success or failure. Such asHigh temperature axial expansion jointPre-stretching is required before installation, and the stretching amount is generally adjusted according to about 50% of the calculated expansion and contraction amount, so that the bellows is tensioned in the cold state and returns to the neutral position in the hot state. If it is a flue supported by a constant force spring, the displacement compensation of the bracket has to be considered. In addition, how to adjust the tie rod nut on the expansion joint? Refer to what we have answered before: the tie rod is used to limit the excessive stretching or compression of the bellows. When installing, it is loosened first, and then locked after adjusting to the designed length. If you forget to loosen the tie rod, the bellows will be welded to death, without any compensation.
Let's talk about the power plant case at the beginning
Later, we helped them recalculate the expansion and contraction amount-using the actual flue gas temperature (average 420℃) instead of the design value of 550℃, and considering the pipe length of 30 meters between the two fixed brackets, plus the lateral displacement of the two elbows, finally selected oneCompound straight pipe bypass pressure balanced expansion joint。 This model can absorb both axial and lateral displacement, and the internal pressure thrust is balanced by itself, eliminating the need for additional main fixation brackets. I have been running for a year after I installed it, and there was no problem. So you see, the larger the expansion and contraction of the flue expansion joint is not the better, nor is it the smaller the less economical-only when calculated accurately can the pipeline system really "breathe" smoothly.