What is the difference between transverse metal expansion joint and general expansion joint?
The biggest difference between the transverse metal expansion joint and the general corrugated expansion joint is not that there is one more bellows. You go to the structure of the double hinge transverse expansion joint. A section of receiver is sandwiched between two bellows, and both sides are connected with hinge plates-the hinge plate is the key, and it eats the blind plate force generated by internal pressure. That is to say, the medium pressure pushes the pipe to run out, and this force is digested by the hinge structure itself, and is not transmitted to the fixed bracket.
What about universal expansion joints? The bellows not only absorbs the displacement, but also has to carry the internal pressure thrust. Therefore, both ends of the universal expansion joint must be equipped with main fixing brackets, otherwise the pipeline will be directly expelled. This is the fundamental difference in principle. If you don't figure this out before choosing the model, it will be all in vain later.
Take the double hinge expansion joint of air-cooled island vacuum pipeline as an example. It is essentially a transverse structure of double hinge. When used in vacuum conditions, the hinge not only has to bear the pressure thrust, but also the reverse force caused by the vacuum negative pressure. You see, the same structural logic, changing the working conditions, the design details are completely different.
Lateral displacement is not estimated: How to calculate angular displacement, force and moment
Many people think that lateral displacement is almost the same as taking a ruler to gesture at the scene. That's not what happened. The lateral displacement of the transverse expansion joint of the compound hinge is realized by the superposition of the angular displacement of the bellows. Each bellows is turned by an angle θ, and the two bellows plus the intermediate connector form a lateral displacement Y. The geometric relationship is: Y =2L · sin (θ/2), and L is the distance from the center of the hinge to the center of the bellows.
When you calculate the lateral displacement, you have to calculate the moment at the same time. When the bellows is bent, a bending moment will be generated, which reacts to the hinge pin. The strength of the pin and the hinge plate must be checked. How does the force come from? The lateral stiffness of the two bellows is different, and the moment distributed is different. When designing, finite element or stiffness matrix provided by the manufacturer should be used to calculate the actual angle and moment of each bellows.
By the way, the design of the hinge structure is not as casual as finding a steel bar and turning a pin. The thickness of the hinge plate, the diameter of the pin shaft and the length of the weld of the ear plate should be calculated according to the pipeline thrust. This thrust is the effective area of internal pressure bearing by the bellows multiplied by the pressure, which is usually an order of magnitude greater than the transverse force.
Three-step selection: take-over parameters, bellows material, and whether to add guide tube
The first step is to take all the takeover parameters. The nominal diameter, design pressure, design temperature and medium of the pipeline must not be less. Temperature determines the material of the bellows, pressure determines the number of layers and thickness of the bellows, and diameter determines the wavenumber of the bellows.
The second step is to select the material of the bellows. 304 is fine in general working conditions, but for corrosive media such as flue gas and desulfurization, you have to consider 316L or even higher alloys. High-temperature working conditions, such as the smoke duct of power station, have to use high-temperature resistant alloy when the temperature exceeds 400℃. This refers to the high-temperature axial expansion joint of this station. Although the structure is different, the material selection logic is the same.
The third step is whether the guide tube is added or not. The function of the guide tube is to reduce the direct scouring of the medium to the inner wall of the bellows, and at the same time reduce the flow resistance. However, with the addition of the guide tube, the lateral displacement ability will be discounted-the guide tube occupies a part of the deformation space, and the bellows has not reached the limit yet, so the guide tube may have met it first. Therefore, if the medium has a high flow rate and contains dust particles, add; If the medium is clean and the flow rate is low, don't add it if you can.
Installation and pre-displacement: How to adjust the tie rod nut and swing the hinge direction
The hinge direction of a transverse expansion joint of a compound hinge determines which direction of transverse displacement it can absorb. If the pipe bends in the horizontal plane, the pin axis of the hinge must be perpendicular to the direction of displacement. The hinge is fixed when the manufacturer leaves the factory, and the angle cannot be changed casually on the spot. Some customers bought it back and found that the direction was wrong, so they cut it with gas cutting and re-welded it-this operation was directly scrapped, and the hinge was a force-bearing part, so on-site welding couldn't guarantee the quality of the weld.
The adjustment of tie rod nut is the key to pre-displacement. When leaving the factory, the nut on the tie rod is usually locked tightly to hold the bellows down and prevent transportation bumps. After installation, you need to loosen the locking nut to the predetermined position. How much pre-displacement? This is equivalent to giving the expansion joint a reverse initial displacement, allowing it to return to the intermediate position at operating temperature. The calculation formula is: pre-displacement amount = actual displacement × (installation temperature-minimum temperature/ (maximum temperature-minimum temperature)). For example, when the pipe rises from normal temperature of 20 °C to 200 °C, the actual lateral displacement is 40 mm, and the pre-displacement amount is 40× (20- (-10)) / (200- (-10)) ≈ 5.7 mm. Calculate this value, just adjust the nut in the opposite direction to correspond to the number of turns.
But remember, the tie rod is not a force rod. Many customers think that the tie rod is fixed. In fact, the tie rod of the compound hinge is only used for installation assistance and pre-displacement adjustment, and the hinge and pin are really stressed. If the tie rod nut is not loosened after installation, it is equivalent to locking the lateral displacement, the bellows can't move, and the compensation function is lost.
Transverse type, general type, pressure balance type... Where is the boundary of working conditions
How to draw the boundary? Look at two things: the form of displacement and the way the pipe is fixed.
General-purpose corrugated expansion joint, suitable for axial displacement-based pipe sections, such as straight pipe sections for thermal elongation. However, the expansion joint itself does not bear the internal pressure thrust, and the rear fixing bracket must be strong enough. In some projects, the general-purpose type is installed between two guide frames. As a result, the spacing between the guide frames is too large, and the column instability of the bellows occurs-this is a typical selection error.
The transverse metal expansion joint is suitable for the occasion where the pipeline has an elbow and needs to absorb the transverse displacement but cannot expand and contract axially. For example, in the air-cooled island pipe system, the steam pipe spans tens of meters from the exhaust device, and the lateral displacement caused by temperature change is very large, and the thrust cannot be transmitted to the equipment interface-at this time, the double hinge transverse expansion joint is the correct solution.
Pressure balance type expansion joint, including straight pipe pressure balance type and curved pipe pressure balance type, suitable for high pressure large diameter pipeline. The internal pressure thrust is cancelled by the balanced bellows itself, and the fixed bracket can be made much smaller, but the structure is complicated and the cost is high. If it's just to save a concrete foundation with fixed brackets, it's not necessarily cost-effective.
When you are unsure, send the working condition parameters to the manufacturer for calculation, and don't pat your head. The displacement calculation book and hinge strength verification report provided by the manufacturer are the final basis for whether the selection can be established. What you need is not a team that can estimate displacement, but a selection plan that can be calculated.