Compression is not metaphysics- -let's start with definitions
To put it bluntly, the amount of compression of a metal expansion joint refers to the length at which it is compressed in the axial direction, usually in millimeters. This parameter determines how much thermal displacement of the pipe is absorbed by the expansion energy saving. For example, universal corrugated expansion joint and high-temperature axial expansion joint, the compression amount is directly linked to the number of corrugations, wave height and wall thickness. You may ask: Is the expansion joint the more pressed the better? Of course not. The compression amount exceeds the limit, the bellows is directly plastically deformed, and the life of the bellows drops by a cliff. Last year, there was a case of a chemical plant. Because the compression amount was 20% more during the selection, cracks appeared in the bellows after half a year's operation-high-pressure steam was repeatedly pushed and pulled, and the corrugated roots couldn't bear it first.
When selecting, the compression amount is the first parameter to be killed
The steam pipeline has high temperature and large displacement, so it is necessary to choose the model with large compression capacity, such as external pressure single axial expansion joint or general-purpose type with guide tube. However, if it is a metal corrugated expansion joint used in the cement industry, there is much dust and frequent temperature fluctuation, so the compression amount should not only be enough, but also a safety margin should be left. Two days ago, I met a customer, and the compression amount was 10% less during the selection. As a result, the pipeline pushed the expansion joint and ran, and the joints collapsed. If you think the compression amount depends on guessing, then wait for rework. Another typical scene is the corrugated expansion joint used in the power station industry-the high-temperature and high-pressure steam pipeline is often tens of meters long, the compression amount is miscalculated by an order of magnitude, the weld joint is directly torn, and the maintenance cost is enough to buy a dozen expansion joints.
The calculation of the compression amount is not complicated, but it is easy to be neglected
Δ L = α × L × Δ T, α is the coefficient of linear expansion, L is the length of the pipe, and Δ T is the temperature difference. It is calculated as a theoretical value, and the actual selection is multiplied by a coefficient of 1.2-1.5. Why? Because installation errors, cold-tight pre-stretching, and even foundation settlement can eat up a portion of the compression. For example: a power station project used a corrugated expansion joint for power station industry. The pipeline was 50 meters long, the temperature difference was 200℃, and the theoretical compression amount was 120mm. The model with a compression amount of 150mm was selected. As a result, it was found that the tie rod nut was not adjusted in place during installation, and the margin of 20mm was abruptly lost. So, don't just look at the numbers in the product manual, but also keep an eye on the installation process. What's more interesting is that some design institutes like to hit the safety factor to 1.8-euphemistically called "conservative", which actually leads to overweight expansion joints, and the fatigue life of bellows decreases. This is the reason why too much is not enough.
The compression amount of different types of expansion joints has its own temper
The axial type is the most direct, such as high-temperature axial expansion joint and directly buried (fully buried) expansion joint, and the compression amount is the axial displacement. But what about the double hinge transverse expansion joint? It mainly absorbs lateral displacement, and the axial compression amount is almost zero. If you choose the wrong one, you will spend money for nothing. There are also pressure balance types, such as straight pipe pressure balance expansion joints and curved pipe pressure balance types, which rely on the internal structure to offset the pressure thrust, and the compression amount is designed more delicately, and the balance effect is affected by more and less points. By the way, the metal rectangular expansion joint is used on the flue, and the compression amount is usually not large, but when used with the baffle door, it has to match the stroke of the flue gas baffle door-otherwise, the baffle door is halfway opened and held by the expansion joint, and the scene will not look good.
In installation and operation and maintenance, compression management is a technical task
The expansion joint you get in your hand usually comes out of the factory with an installation length mark, which is the starting point of the compression amount. Pre-compression or pre-stretching is necessary on site-for example, when the steam pipe is cold and tight, it will be pre-stretched to bring the expansion joint closer to the neutral position when it is hot. How to adjust the tie rod nut? As mentioned in the FAQ, adjusting the nut actually controls the amount of compression. The locking nut is equivalent to limiting the displacement, the expansion joint becomes a rigid part, and the pipeline stress is all on the weld joint. I have seen a case where the operator screwed the tie rod nut to death for fear of leakage, and as a result, the pipe elbow directly cracked. Does the screw of the expansion joint have to be removed after installation? Sub-situation: For those with transport screws, they must be loosened after installation, otherwise the compression amount will be locked-this problem is also FAQ, so don't make low-level errors. What is even more headache is that some projects use the expansion joint as a fixed bracket-once the tie rod is locked, there is no place for the thermal expansion and contraction of the pipeline, and it directly holds back a big bend.
The amount of compression is not an isolated number after all. The selection is correct, the installation is correct, and there is regular inspection during operation-is there any bulge in the bellows? Is the tie rod nut loose? These details determine whether an expansion joint will last 5 or 15 years. Don't expect to buy it and install it once and for all. There is no word "worry-free" in the project.