Two days ago, I received a phone call from a heating network project. I asked, "Teacher, are single-wave metal expansion joints cheaper than multi-wave ones? We can just use single-wave ones, and the displacement is not large anyway." I didn't rush to reply after listening to it. This kind of problem is too typical in the selection stage-single-wave metal expansion joints do have their places, but the conclusion that "one wave is enough" is really not as simple as looking at the displacement.
What exactly is a single-wave metal expansion joint: the essential difference between one-wave and ordinary bellows
Figure out the concepts first. A single-wave metal expansion joint does not mean that the bellows has only one layer, but that the bellows has only one complete wave. The common general-purpose corrugated expansion joint on the market, the corrugated pipe is usually composed of several waves as a whole, and the displacement is absorbed by the coordinated deformation of multiple waves. The single-wave structure is equivalent to pressing all the displacement onto this one wave and letting it carry it alone.
What is the essential difference between this one wave and multiple waves? In stiffness and displacement distribution. The axial stiffness of a single wave is one cut higher than that of a multi-wave structure with the same diameter and wall thickness, because each unit displacement corresponds to a larger strain on a single wave. In other words, also absorbing 10 mm axial displacement, the degree of deformation between peaks and valleys of the single-wave bellows is much more intense than that of the multi-wave structure. Pressure thrust is the same logic. Under the premise of the same effective area, the internal pressure thrust beared by single wave and multi-wave is the same.
Bellows originally need thicker wall thickness due to high pressure. At this time, even if the displacement requirement is small, the stiffness and cost of multi-wave structure become a burden. Single wave is more suitable in this kind of scenario-solving problems with one wave has the simplest structure, the least welds and the highest reliability.
Which working conditions are suitable for single-wave structure: How to stick the boundary conditions of displacement, pressure and caliber
When it comes to boundary conditions, many selection personnel are used to only focusing on the displacement amount, which is not enough. There are three hard parameters that determine whether a single-wave metal expansion joint should be used:Displacement, working pressure, caliber。
In terms of displacement, the axial displacement that a single wave structure can absorb is usually within the maximum allowable displacement range of a wave. According to the allowable displacement value corresponding to the fatigue life in GB/T 12777 standard, the single wave structure is generally suitable for absorbing the axial displacement within 10mm. If it exceeds this amount, it will be calculated whether the bellows will enter the plastic instability state. At this time, don't choose single wave hard.
Stress is another dimension. The advantage of single wave in high-pressure environment is more obvious-the wall thickness of bellows increases with the increase of pressure. Because of the small wave number and the high spring stiffness of single wave structure, the material utilization rate is better than that of multi-wave under high-pressure conditions (such as 2.5MPa and above). Low-pressure and large-diameter scenes should be cautious: large diameter means that the effective area of the bellows is large, and the blind plate force is large under the same pressure. The single-wave structure itself does not change the blind plate force, but it has to be borne by the main fixing bracket.
There is no absolute red line in this piece of caliber, but the fatigue life of single-wave metal expansion joints above DN300 will soon drop if the displacement requirement is slightly more. Large-diameter pipeline itself has large thermal expansion, and single wave is often insufficient. At this time, it is better to change to multi-wave structure or consider external pressure single axial expansion joint.
Three points that are most prone to rollover during model selection: fatigue life, deflector configuration, and tube stiffness
Let's start with fatigue life. Every time a single-wave structure absorbs a displacement, the amplitude of stress change in the peak and trough of the bellows is full, and no other wave can share it. As a result, the fatigue life of single-wave bellows is significantly lower than that of multi-wave structure under the same displacement condition. For example, the same DN150 absorbs 10mm displacement, and the single-wave displacement of the three-wave bellows is about 3.3mm. The single-wave structure has to carry 10mm alone, and the fatigue life may be one or two orders of magnitude worse. When selecting the model, be sure to let the manufacturer calculate the fatigue life according to the actual displacement, and don't use the allowable displacement directly as the design displacement.
The second pit is the guide tube. The function of the guide tube is to protect the inner wall of the bellows and avoid high-speed media erosion and wear. Many engineers think that the guide tube is standard and directly hooked. However, for single-wave structures, the guide tube will significantly reduce the circulation area, increase the flow resistance of the medium, and may also affect the effective deformation space of the bellows. When the medium flow rate is high, contains particles or requires frequent cleaning, the guide tube must have; Clean media, low flow rate scenario, can be omitted. Moreover, the installation direction of the guide tube must face the flow direction of the medium, and if it is installed backwards, it is equivalent to not being installed.
The third is the takeover stiffness. Single-wave metal expansion joints usually have a short connection, and if the connection is insufficiently rigid, the pipe bending moment will be transmitted directly to the bellows. In order to save materials, some manufacturers thin the wall thickness of the pipe, and local bulging of the bellows appears after installation. The wall thickness of the joint shall not be less than the nominal wall thickness of the pipeline, and the length shall ensure that the deformation of the bellows is not restricted.
Frequently asked questions during installation and operation: How to calculate pre-stretching, whether the tie rod nut should be removed, and what will happen if the guide tube is installed in the reverse direction
The purpose of pre-stretching is to leave the bellows at zero stress or compressive stress at operating temperatures. How to calculate the amount of pre-stretching of a single wave structure? Theoretically, the pre-stretching amount is equal to half of the thermal expansion of the pipeline from cold state to hot state, but the allowable stretching amount of single wave structure is limited, so it must be strictly carried out according to the value given by the design. In practice, tie rods or screws are commonly used for cold tightening. This amount is not a big problem within the range of installation deviation of ±2mm, but the bellows will directly deform plastically if it is pulled too far.
Do you want to remove the tie rod nut? Come, write down this sentence:Transportation tie rods and positioning screws must be loosened or removed after installation, unless the product is designed for use with tie rods.If the tie rod nut is not removed, it is equivalent to locking the expansion joint, the bellows cannot be deformed, and the thermal stress of the pipe is all transmitted to the fixed bracket and the equipment interface. When an expansion joint with a tie rod (such as a double hinge transverse expansion joint) is designed to absorb transverse displacement, the tie rod nut can be adjusted according to the design requirements and cannot be locked. The judgment standard is very simple: before installation, test whether the bellows can expand and contract freely. If it can't move, it means that the tie rod is not loose.
What happens if the deflector is installed in the reverse direction? In the light case, the protection effect is lost, and in the worst case, the medium directly washes the trough of the bellows, especially in the dusty airflow, which can wear through the bellows in a few months. There is still a hidden danger after installation: vortex current forms at the gap of the guide tube, which induces the vibration of the bellows. After installation, illuminate the inlet end of the guide tube with a flashlight, and it is right to see that the smooth bell mouth faces the direction of media flow.
Single wave, multi-wave, external pressure single axial type: When should choose single wave, when should change to other structure
Type selection is not a single-choice question, but a true or false question. Single-wave metal expansion joints are most suitable in two kinds of scenarios: one is high pressure, small diameter and small displacement requirement (no more than 10mm), such as pump outlet pipeline and compressor pipeline; The other is a space-constrained scenario that requires the shortest structural length, because a wave has the shortest structural length and can be stuffed into a narrow pipe gallery.
For scenes with large displacement demand or large caliber, don't fight with single wave. The multi-wave structure of the universal corrugated expansion joint can absorb more axial displacement, and the cost is not high. If you need a large displacement but a high pressure, you have to consider the external pressure single axial expansion joint-the bellows is affected by external pressure, and its stability is better than that of internal pressure, and the allowed displacement is larger. Moreover, the external pressure structure has its own guide tube function, so the flow direction doesn't matter.
For pipelines dominated by transverse displacement or angular displacement, single-wave structure is even less robust. At this time, the transverse expansion joint of compound hinge or the pressure balance expansion joint of curved pipe should be selected. In some special scenarios, such as vacuum pipelines or occasions that need to bear alternating loads, single-wave structures may cause pipe system vibration problems because of their high stiffness and high natural vibration frequency, and metal hoses need to be used to isolate vibration.
In the end, the phrase "one wave is enough" only makes sense in specific working conditions. Single wave metal expansion joints are a good tool, but it addresses specific problems and is not a universal option. Next time someone asks you if you can choose a single wave, ask three questions first: What is the displacement? How high is the pressure? How is the pipeline fixing bracket arranged? Once these three questions are answered clearly, the answers will come out by themselves.