First, find out what single wave and double wave are: the structural differences of bellows
Don't be fooled by the name. Single-wave expansion joint means that the bellows has only one wave shell, and double-wave is two layers. But by "layer" here we don't mean thickness-it is independent corrugated cavities. You take apart a metal expansion joint and see that the bellows consists of multiple consecutive peaks and troughs, but single wave means that the whole bellows has only one complete node? No, it's biased to understand it that way.
Single wave and double wave in the industry usually refer to expansion jointsNumber of effective nodes of bellows。 But the more accurate names are "single-wave nodes" and "double-wave nodes" — that is, a set of independent corrugated structures. For example, in the general-purpose corrugated expansion joint we often say, the single wave may be a single-layer corrugated pipe, while the double wave is two sections connected in series, with end plates connected in the middle. You see, in our product line, there are both general-purpose corrugated expansion joints and compound hinge transverse expansion joints, which are typical multi-wave structures.
A single wave has only one corrugated cavity to absorb displacement; The double wave has two cavities, separated by an intermediate tube or an end plate, and can be deformed independently. This directly determines that the compensation ability and stiffness of the two are completely not in the same order of magnitude.
2. Core difference: the fluctuation of compensation ability and stiffness
Let's start with compensation. Can a double-wave expansion joint theoretically provide twice the amount of axial compensation as a single wave? Beautiful thought. In fact, after two waves are connected in series, the displacement shared by each node is smaller, but the total compensation amount is indeed larger than that of a single wave-only it is not linear superposition and limited by the stability of the bellows. For example, the general-purpose corrugated expansion joint of DN200 has an axial compensation of about 40mm for single wave and 70-80mm for double wave. But at what cost? Stiffness.
Bellows stiffness is inversely proportional to the number of nodes (approximate). The overall axial stiffness of a double wave is about half that of a single wave. And guess what? Many engineers think that the low stiffness of double waves is a good thing-soft. But it is too soft. If the pipeline system produces blind plate force under pressure, the double-wave expansion joint may be directly pressed into a "twist". Our common high-temperature axial expansion joints must use single-wave or reinforced structure in high-temperature working conditions, because the double-wave stiffness is not enough and it is easy to become unstable.
Fatigue life. The deformation of each node of double wave is small, and the fatigue life is longer in theory, but the premise is that the design is reasonable. If the wave pitch and wall thickness are not well matched, the double wave will be broken first because of local stress concentration.
3. Practical application scenarios: Where are single waves used and where are double waves required?
Two days ago, I met a purchaser in the cement industry and asked me whether to choose single wave or double wave for metal corrugated expansion joints in the cement industry. I asked directly: Is your pipe a straight section or a bend? What is the medium temperature? Is there any vibration?
- High pressure pipeline: For example, the main steam pipeline of the power station has high pressure, high temperature (> 500℃), high stiffness requirements, and single wave cooperation with reinforcing ring is more reliable. Many corrugated expansion joints used in our power station industry are single-wave reinforced types.
- Occasions where precise guidance is required: For example, straight pipe pressure balance expansion joint, single wave structure is easier to control deflection.
- Space-constrained short tube: The length of double wave is more than twice that of single wave, and only single wave can be used when the installation space is insufficient.
- Large displacement compensationFor example, for long-distance thermal pipelines, the single-wave compensation is not enough, so double or even multiple waves must be used. At this time, it is necessary to match a tie rod or hinge to limit the blind plate force-such as the transverse expansion joint of compound hinge, which is a typical example of double wave plus hinge.
- Low pressure large diameter pipeline: For example, flue gas pipes, low pressure and low double wave stiffness are not a problem, but can better absorb axial and lateral displacements. Our metal rectangular expansion joints and non-metal expansion joints are commonly used in this type of scenario.
- Occasions requiring vibration damping: The double wave has great flexibility and better absorption effect on mechanical vibration. For example, the air-cooled island vacuum pipeline will use double hinge expansion joints.
4. Record of type selection and pit stepping: I thought double wave insurance, but the result was that the stiffness was not enough to directly deform
There's a real case. Steam pipeline of a chemical plant, design temperature 350℃, pressure 1.6MPa, pipe diameter DN300. The original single-wave expansion joint was leaked after several years of use, and the maintenance worker said, "Change to a double-wave one, and the compensation amount is larger and safer". As a result, less than three months after installation, the pipe began to click. After a closer look, the middle of the double-wave expansion joint was bulged-without adding a tie rod, the blind plate force directly pushed the bellows into a "beer belly".
Double-wave expansion joints without tie rods or hinge constraints, the axial thrust generated under pressure can elongate or compress the bellows beyond the design value. In our product information, if the universal corrugated expansion joint is made into double wave, it will definitely be equipped with a tie rod or limit device as standard. You go to the question and answer about the function of the expansion joint tie rod-the tie rod is used to bear the pressure thrust and not let the bellows overload.
Another pit is an occasion with a high frequency of thermal cycling. Because of the low stiffness of double wave, the deformation of each node is large, and fatigue cracks are easy to occur under high frequency vibration. Therefore, the selection does not mean that "the more wavenumber, the better", but must be calculated in combination with the displacement, pressure, temperature and number of cycles of the system. If you are not sure, go directly to the double straight pipe bypass pressure balanced expansion joint or the curved pipe pressure balanced expansion joint-these structures come with their own balancing function and do not transmit pressure thrust to the pipe.
5. Details to be paid attention to during installation: different wave numbers, the configuration of guide tube and tie rod changes accordingly
Install single-wave expansion joints, and the guide tube is usually made on the inside to protect only one wave node. But the double wave is different-the guide tube length must cover the gap between the two nodes, otherwise the medium directly flushes the intermediate tube and quickly perforates. We often encounter customers who buy double-wave expansion joints by themselves and forget to match the guide tube. As a result, it leaks after half a year. The specific function of the expansion joint guide tube is mentioned in the question and answer. It can also reduce flow resistance and prevent vortex.
If a single-wave expansion joint needs a tie rod, it is generally symmetrical 4 or 6. Because of the long length of double wave, the number of tie rods should be large, and the preloading force of each tie rod should be uniform. How to adjust the tie rod nut when installing? It must be adjusted to the design length when cold, leaving the amount of pre-compression. Does the screw of the expansion joint need to be removed? Remember: the transport screw is to prevent deformation during transport and must be removed after installation, otherwise the expansion joint will not work.
Different wave numbers have different weights. In the weight table of metal expansion joint, the weight of double wave under the same diameter is 1.6-1.8 times that of single wave-not twice, because the proportion of structural parts such as intermediate tube and end plate. If you design your pipe support without considering extra weight, it may not hold up. Therefore, before selecting the model, you must get the size, weight and stiffness data, and don't pat your head.
Alas, in the final analysis, the difference between single wave and double wave of metal expansion joint? The core is to weigh the amount of compensation and stiffness. There is no universal model, only the one that is best suited to the working conditions. Before the next selection, calculate the thermal expansion of the pipeline first, then check the corrosion margin, and finally look at the pocket budget-double wave is usually 30%-50% more expensive than single wave. Don't choose a single wave to save money, which leads to insufficient compensation, and don't blindly go on a double wave for "safety", which results in stiffness failure. If you are really unsure, look through the product information on our website.