What is the relationship between axial metal corrugated expansion joint and other expansion joint?
Axial metal corrugated expansion joint is not three unrelated things with general corrugated expansion joint and external pressure single axial expansion joint. The axial type relies on the axial deformation of the bellows to absorb the thermal displacement of the pipe, which is the most common type. The general corrugated expansion joint is actually a typical representative of the axial type, while the external pressure single axial type is a variant of the axial type-the bellows bears external pressure, has better stability, and is suitable for high pressure and large displacement scenarios.
Therefore, the first step in selection is not to look at the price, but to set four parameters: thermal elongation of the pipeline system, working pressure, medium temperature and fatigue life requirements. These four numbers can't be determined, and the rest is all busy. How to calculate thermal elongation? The length of the pipeline multiplied by the linear expansion coefficient and then multiplied by the temperature difference is available in junior high school physics, but in engineering, it is often stuck in the value of the temperature difference-is it the highest operating temperature or the design temperature? It is recommended to calculate according to the design temperature, leaving sufficient margin.
Material and number of layers, don't pat your head
The material of corrugated pipe is simple to say. 304, 316L, and Inconel 625 each have their own uses. But if you choose the wrong material, no matter how thick it is, it will be useless.
In the case of flue gas desulfurization, the medium contains chloride ions, which can't be carried by 316L, so you have to use Inconel 625 or more ruthless C276. Power station steam pipeline, high temperature and high pressure, 304 enough but short life, 316L comprehensive better. In the cement industry, dust wear is severe, and wear-resistant lining has to be considered in addition to the material.
What about the number of layers? The number of layers is more flexible, but the pressure bearing capacity decreases; When the wall thickness of the single layer goes up, the bellows becomes hard, and the compensation amount is not enough. This balance point can't be calculated hard by formula, but it has to be supported by experimental data and actual cases. When the manufacturer gives the plan, you have to ask one more question: How long have you been running in similar working conditions with this layer? Do you have any case data? Better than looking at samples.
Deflector, add or not add?
The function of the guide tube is to reduce the erosion of the medium to the inner wall of the bellows, and to reduce the flow resistance. However, with the addition of the guide tube, the circulation area becomes smaller, the weight and cost increase, and vibration problems may also occur.
High-temperature flue gas or dusty gas flow, the guide tube is almost necessary. Look at the corrugated expansion joints used in the power station industry and the metal corrugated expansion joints in the cement industry in this station. The configuration of the guide tube is completely distinguished according to the working conditions-the flue gas speed of the power station is fast, and the guide tube should be temperature-resistant and erosion-resistant; The dust concentration of cement is high, the guide tube should be wear-resistant, and the material and thickness are different.
Direction of the guide tube. The arrow is clearly marked, and when it is installed backwards, the medium directly washes the bellows, and the life of the corrugated pipe decreases by a cliff. Such low-level mistakes are not uncommon on the scene.
Stiffness, a hard index, is the most easily overlooked
The stiffness of axial expansion joint directly determines the force of pipe support. The stiffness is too large, the thermal displacement can't be compensated, and the reaction force is pushed to the fixed bracket; If the stiffness is too small, the bellows is prone to instability, especially the column instability caused by internal pressure.
The stiffness value given by the manufacturer must be distinguished whether it is single wave stiffness or the stiffness of the whole product. The unit is N/mm, but the value is far from the same. The single wave stiffness is the stiffness of a single waveform, and the whole stiffness is the total stiffness after all waves are connected in series. The relationship between the two is similar to the spring series. When you calculate the pipe thrust, you must use the whole root stiffness, don't get confused.
Plus, tie rods and stiffness calculations are two different things. Two days ago, someone asked me how to adjust the tie rod nut of the expansion joint. The tie rod is used to limit displacement, not to adjust stiffness. The tie rod was not adjusted correctly, and the displacement that the bellows should absorb was not absorbed, and all ran to the pipe rack.
In installation, pre-stretching is the hardest hit area
The cold tightness of the axial expansion joint when it leaves the factory is calculated according to the design temperature. If you do not pre-stretch according to the drawings on the spot, or forget to dismantle the tie rod for transportation, the product performance will not be brought into play at all.
Let the bellows be in a pre-compressed state when installed at room temperature. When the pipeline heats up and elongates, the bellows just returns to the neutral position, so that there is compensation margin in both positive and negative directions. If it is not pre-stretched, the bellows can only go in one direction, and the compensation amount is directly halved.
Also, be sure to check that the direction of the liner sleeve is consistent with the flow direction of the medium after installation. The direction of the arrow is clearly marked, but the construction team is busy, and someone really installs it backwards. Consequences? The guide tube loses its function, the inner wall of the bellows is washed and thinned, and it is only a matter of time before it leaks.
How exactly is lifespan calculated?
Axial metal corrugated expansion joints are not disposable consumables. The design fatigue life is usually 1000 times, but how many times it can actually be achieved depends on the operating temperature and pressure fluctuations.
As for fatigue life, the curves on the sample are all under ideal working conditions. In actual use, every 50 degrees of temperature increase, the life may be discounted in half; Pressure fluctuates frequently, and micro-cracks initiate early. Therefore, when selecting the model, put the fatigue life requirements directly to the manufacturer, and let them re-check according to your working conditions. Don't pat your head to decide.
If you are not sure, simply ask the manufacturer for case data of similar working conditions. The actual feedback of the same factory, which has been running in power stations, cement and chemical industries for several years, is more reliable than any theoretical calculation. In this trade, cases are the best samples.