What exactly is a double wave metal expansion joint?
To put it bluntly, a double-wave metal expansion joint is a metal compensator with only two lobes on the bellows. Compared with single wave, it compensates more-single wave can only absorb a little axial displacement, and double wave can directly double. What about Dobby? Although the compensation amount of multi-wave is greater, the stiffness will become soft, easily unstable, and the overall length is long, so the installation space is tight. The double wave is just stuck in the middle: the compensation is sufficient, the stiffness is not too soft, and the length is moderate.
"Can double wave replace the universal corrugated expansion joint?"-it depends on the working conditions. The universal corrugated expansion joint usually has a wave number of 4~6, which is suitable for large displacement absorption of long-distance pipelines. But if your pipe space is limited and the amount of displacement is not large or small, double wave is a good choice. For example, short-circuit of steam pipeline, pump inlet and outlet, and valve accessories. In these places, the general type may be too long to fit, and the single wave is not enough. Double wave is just right.
Why is double wave design so common in engineering?
It finds a balance between compensation capacity and structural stability. The two waves mean that the axial displacement absorption range is reasonable, while the overall length is short and the installation space is saved. A few days ago, there was a power plant user, and the pipe space was stuck. There were equipment on the left and right of the pipe gallery, and the length was only enough to put a 200mm-long expansion joint. The general-purpose type has many wavenumbers and exceeds the standard in length; The amount of single wave compensation is not enough. Finally, the double-wave metal expansion joint is selected, with an axial compensation amount of 12mm, which just meets the thermal displacement, and the installation position is perfectly fitted.
Fatigue life ratio multiple wavelengths. The more the wave number, the more uneven the stress distribution of each wave, and the most edge wave is easy to crack first. Because the wave number of double wave is small, the stress distribution is relatively uniform, and the life of double wave is longer under the same conditions. But this is not absolute, it depends on the specific design-the cooperation of wave height and wave distance directly affects the life span.
Three pits that are most prone to rollover when selecting a model
First Pit: Pressure Rating.Although the nominal compensation amount of double-wave metal expansion joint looks good, when the pressure is high, the wall thickness of corrugated pipe has to be increased, or a reinforcing ring has to be added. Many people only stare at the amount of compensation, regardless of the pressure. As a result, once it was installed and pressed, the bellows bulged directly. Remember: The pressure level must match the design pressure of the system, with a margin but not too large, otherwise the stiffness is too high and the compensation will be affected.
The second pit: temperature.For high-temperature steam pipes, such as superheated steam above 450℃, the bellows material must be made of heat-resistant stainless steel (such as Incoloy 800H), and the wave pitch should be adjusted. If the wave distance is too small, the stress concentration will increase at high temperature; The wave distance is too large, and the compensation amount shrinks. At this time, don't just look at the wave number, but check the fatigue life-usually in EJMA standard, the fatigue life in high temperature conditions should be more than twice the design times before it is safe.
The third pit: media corrosivity.There are many sulfide and chloride ions in chemical pipelines, and it won't take long for ordinary 304 stainless steel to stress corrosion and crack. Gotta change 316L or even duplex steel. In addition, do you want to add the deflector? If the medium has a high flow rate or contains particles, it must be added. The function of the guide tube is to isolate high-speed fluid and particle erosion, and protect the inner wall of the bellows. Not added? The bellows wore out quickly, and I regretted it when it leaked. To add: The material and wall thickness of the guide tube also have to follow the medium, so you can't just weld an ordinary carbon steel tube.
Two common errors in installation and maintenance
When will the tie rod be removed? In fact, the tie rod is used to limit the position during transportation and installation, so as to prevent the bellows from being over-stretched or compressed. Once installed in place, it must be loosened or removed as required by design. If you forget to dismantle it, the bellows will be locked, and the expansion joint will not move when the pipe expands and contracts, and the force will all be transmitted to the bracket and the nozzle-the consequence will be the deformation of the bracket or the cracking of the weld at the nozzle.
There's also the problem of pre-stretching. Some working conditions require pre-stretching, such as the pipe installation temperature is lower than the operating temperature and the direction of thermal expansion is positive. The amount of tension should be accurately calculated, and an extra millimeter may overload the bolt. In actual operation, use special tools or jacks to stretch evenly, measure the flange spacing at both ends, and ensure that the stretching amount is within ±1mm. Don't twist the tie rod nut by feel, it will only twist the bellows and deform it.
Double-wave structures are also commonly used in combination with other products
Going solo is not the style of a double wave. For example, when encountering the lateral displacement of the pipeline, the double wave can't handle it by itself-it mainly absorbs the axial displacement. At this time, you can match the double hinge transverse expansion joint, and the two products are combined, with double waves in the axial direction and hinge type in the horizontal direction, each performing its own duties. Another example is the underground pipeline. Double wave is matched with directly buried (fully buried) expansion joint, and the outside is wrapped with an anti-corrosion layer, so that it can be directly buried in the soil. The directly buried structure is equipped with a guide sleeve and a protective cylinder to prevent lateral instability.
For more complex conditions, such as the need to absorb axial displacement and eliminate pressure thrust at the same time, don't expect a double wave to do everything. Do not hesitate to use the upper straight pipe pressure balanced expansion joint or the double straight pipe bypass pressure balanced expansion joint. These products have balanced bellows inside, which counteract blind plate forces and make pipes and equipment less stressed. Double wave can be used as one of the links, but the overall scheme has to be calculated by the system.
To put it bluntly, there is no universal product, only the most suitable combination scheme. When selecting the model, list the working condition parameters (pressure, temperature, displacement direction, medium, space size) clearly, and then choose against our product line. This station has general-purpose corrugated expansion joint, high-temperature axial expansion joint, corrugated expansion joint for power station industry, metal corrugated expansion joint for cement industry, etc. Double wave is only one of the subdivision options.