The first step of design: if the working conditions are not thoroughly understood, everything is useless
When designing metal corrugated expansion joints, the most common mistake is to turn over the formula and check the sample. Pressure, temperature and medium are not clear, and there are all castles in the air behind them.
The pressure determines the pressure resistance grade and the number of layers of the bellows, the temperature determines the material selection, and the medium determines the corrosion resistance scheme and whether to add a guide tube. For example: in the air duct of the cement industry, the medium has a large dust content and a high flow rate. Without the guide tube, the trough of the bellows will soon be worn out. In the flue gas pipeline of the power station industry, the medium contains sulfur and the temperature is high. Ordinary 304 can't bear it at all, and it has 316L or even higher grade corrosion-resistant alloy.
The deflector is not just added if you want to. Adding a guide tube can protect the bellows from being washed by high-speed media, but it will also increase the stiffness of the whole expansion joint and affect the compensation amount. It is necessary to calculate clearly when designing: the flow rate of the medium, the dust content and the erosion resistance of the bellows material. Generally, when the flow rate exceeds 15m/s or the medium contains hard particles, the guide tube is standard, not optional.
In addition, note that the installation direction of the guide tube is particular, which must be consistent with the flow direction of the medium. If it is installed backwards, the medium is directly poured into the trough, which is equivalent to not being installed. More on this later.
Geometric Parameter Design: Quadrangle Game of Wave Height, Wave Pitch, Layer Number and Wall Thickness
The geometric parameters of bellows, to put it bluntly, are a balance game of one and the other. When the wave height is large, the compensation amount is large, but the pressure withstand capacity decreases, and it is easy to become unstable; When the wave distance is small, the wave number per unit length is large, and the compensation amount goes up, but the stiffness also goes up, and the pipeline thrust becomes larger.
The relationship between the number of layers and the wall thickness of a single layer is more subtle. With the same total wall thickness, the compression and fatigue performance of single layer is better than that of multi-layer, but the multi-layer has better flexibility and large compensation. The "multi-layer thin wall" often said in design is to take into account both pressure resistance and flexibility. For example, high-pressure steam pipelines, multi-layer bellows are commonly used, and the wall thickness of each layer ranges from 0.5mm to 1.0mm. The number of layers is calculated according to pressure and diameter.
The general-purpose corrugated expansion joint of DN500 has a working pressure of 1.6MPa, usually needs 3 to 4 layers, and the wall thickness of a single layer is about 0.8mm. If the wall thickness of a single layer is increased to 1.5mm and the number of layers is reduced to 2, the pressure resistance can be increased, but the compensation amount will be directly reduced by a section, and the stiffness will be doubled. Therefore, when designing, ask yourself: Is this working condition lacking compensation amount or pressure resistance? Prioritize the satisfaction of the main contradiction.
Structural form: the displacement type determines the structure, and the constraint condition determines the selection
When it comes to structure selection, many people directly follow the picture on the sample, and the result does not match when installed. The key to the design of metal corrugated expansion joint lies in: first find out what displacement the pipeline should absorb, axial? Horizontal? Angular displacement Or combined displacement?
Only the axial displacement is absorbed, and the universal corrugated expansion joint is sufficient. Lateral displacement is the main one, so you can choose a compound hinge transverse expansion joint or a compound tie rod type, and use two bellows to share the rotation angle. When the pipeline pressure is high and the lateral displacement needs to be absorbed, the straight pipe pressure balance expansion joint is more suitable-the thrust generated by the internal pressure of the bellows is balanced, and the bracket does not have to bear the huge blind plate force.
External pressure single axial expansion joint is a good choice for high pressure and large diameter situations such as steam pipeline and hot air pipeline. The external pressure structure makes the bellows under pressure, which has good stability and large compensation. Moreover, the guide tube is inside the bellows, so the medium does not directly flush the trough, and the service life is longer. For air-cooled island vacuum pipelines, double hinge expansion joints are commonly used to absorb large lateral displacements.
Directly buried (fully buried) expansion joint is used for directly buried pipelines, which is a structure specially designed for direct buried laying. The outer protective pipe and insulation layer are prefabricated in the factory, and they can be welded directly on the spot.
Details determine success or failure: guide tube direction, tie rod allowance, pre-displacement
The question of the direction of the guide tube was mentioned earlier, and then it will be discussed. When installing the guide barrel, the arrows on the barrel must point to the direction of the media flow. This arrow is marked when it leaves the factory, so don't ignore it during construction. Once installed backwards, the medium directly washes the bellows trough, doubling the wear rate. Special attention should be paid to the expansion joint at the inlet and outlet of the flue gas baffle door. The flue gas contains dust, and it will leak after being installed backwards for half a year.
The adjustment margin of tie rods and nuts is also where problems often go wrong on the spot. The function of the tie rod is to restrain the bellows and prevent the pressure thrust from pushing the pipe open. But the tie rod is not locked, it has a certain adjustment margin to control the pre-displacement. The degree to which the nut is tightened depends on the dimensions on the design drawing. Some projects lock the nut, and the bellows lose the ability to compensate; Some are not locked in place, and as soon as the pressure comes up, the structure deforms directly.
Installing pre-displacement is even more a key point that has been overlooked. When the pipeline is installed in cold state, first stretch or compress the expansion joint for a certain distance, so that it is in the middle position at the working temperature, so that the displacement in both positive and negative directions can be absorbed, and the fatigue life can be doubled. For example, a steam pipeline with a working temperature of 200℃ has a total elongation of 50mm, is pre-stretched by 25mm during installation, and returns to the middle from the stretched state during operation, with the smallest cyclic stress amplitude. Without pre-displacement, the bellows is always biased in one direction, and the life is greatly reduced.
Fatigue life calculation: not going through the process, but leaving a trump card for safety
Fatigue life is the last step in the design of metal corrugated expansion joints, and it is also the most easily regarded as formalism. Some designers set a formula, fill in a number for the number of cycles, and then cross the difference. This thing is related to pipeline safety, and if something goes wrong, it is a big deal.
Let's see how to determine the number of cycles first. It is not calculated according to the design life of the pipeline, but according to the frequency of temperature change in actual operation. A thermal power plant starts and stops once a day, 365 times a year, and has a design life of 20 years, that is, 7,300 times. If frequent peak shaving, start and stop multiple times a day, the number of cycles should be estimated according to the actual working conditions, and you can't pat your head according to the theoretical value.
The temperature correction factor must be considered. Under the same strain amplitude, the fatigue life of 304 stainless steel at 400℃ is much lower than that at room temperature. In the design, the temperature correction coefficient should be multiplied, and the equivalent cycle number should be adjusted up.
The invalidation criterion should be made clearer. Bellows fatigue failure is not based on leakage, but penetrating cracks appear. The safety factor is used in design, which is generally 10 to 15 times the life expectancy. What's the meaning? The predicted life is 20,000 times, and the actual design is checked by 200,000 to 300,000 times. This margin is not slapped on the head, but uncertain factors such as dispersion of material properties, manufacturing deviation and calculation error are considered.
In a power station project, the high-temperature axial expansion joint of DN1200 has an operating temperature of 450℃ and a pressure of 0.6MPa, with an expected cycle number of 5000 times. According to the above method, the material of Incoloy 825 was selected, the number of bellows was 4 layers, and the total wall thickness was 4mm. It is calculated that the design life is 80,000 times, leaving 16 times the safety margin. Five years in operation, checked once a year, the bellows is in good condition.
On the other hand, in order to save costs, some projects pressed the safety factor to less than 5 times, and as a result, cracks began to appear in two or three years. The cost of one repair is enough to buy several new expansion joints.
How much is appropriate for design margin? About 10 times in general occasions and more than 15 times in important occasions. It's not waste, it's basic respect for pipeline safety.