Find out the difference between single wave and three wave first-structure determines performance
Two days ago, a customer who made steam pipelines called and asked, "Which one is better, single wave or three wave?" I asked him back, "What's the pressure of your pipe? What is the displacement?" He was stunned for a moment and said, "I see that the compensation amount of three waves on the parameter table is greater. Is it right to choose three waves?" Alas, this kind of thinking is horrible. Which is better, single wave or triple wave of metal expansion joint? It's not a question of "who's stronger" at all, but what kind of structure you need for your working conditions.
Single wave, as the name implies, is a ripple; Three waves are three ripples connected in series. The structural differences directly determine their mechanical properties. There are few single wave corrugations, the wall thickness can be made thicker (of course, it is also related to the material and molding process), and the overall stiffness is large. Three waves are equivalent to three springs strung together, and each ripple shares a part of the deformation. The overall flexibility is large, and the compensation amount will naturally go up. However, large flexibility does not mean that everything will be fine-the force between the three ripples is uneven, and the ripples near the ends tend to bear greater stress, which is a flaw of fatigue life.
Therefore, don't just look at the "compensation amount of 100mm" written on the sample and feel that the three waves have won. You have to break it apart and crumble it to see: How many cycles of life is this compensation measured? How much is the design pressure? How hot is the temperature? The combination of these parameters is called the real working condition.
Single wave: Large stiffness and small displacement, suitable for high pressure and small displacement scenarios
The typical application scenario of single-wave expansion joint, let me make an analogy: like the main steam pipeline of a power station boiler, the pressure is always more than ten MPa or even higher, and the temperature is five or six hundred degrees, but the thermal displacement of the pipeline is actually not large-maybe only a few millimeters to ten millimeters. With a single wave at this time, the advantage comes out. The single wave has high stiffness and can resist high pressure without instability; At the same time, because the wave number is small, the displacement of each wave is relatively concentrated, so as long as the design stress is within the allowable range, the fatigue life is easy to guarantee.
We have one on our siteHigh temperature axial expansion jointIt is used a lot in the petrochemical industry, which is a typical single-wave or multi-wave customization. However, in high-pressure scenarios, engineers generally prefer single-wave-with-thick-wall design, which can reduce the overall stress level. In addition,Universal corrugated expansion jointThere are also single-wave models in, which are suitable for the working conditions of "high pressure, small displacement and thick pipeline". It's easy to say, but you have to calculate when you actually choose the modelStiffness and Calculation Formula of BellowsIn that formula, the wave number n is placed directly on the denominator-the fewer the wave number, the greater the stiffness. You taste, you taste.
The structure is simple and there are few leakage points. Because of the many welding parts in three waves (there are usually circumferential welds between waves or formed integrally, but the crest and trough stresses are concentrated), the quality control points in the manufacturing process are also denser. For scenarios such as nuclear power and chemical industry that pursue the ultimate reliability, single wave is a safe choice.
Three waves: good flexibility and large compensation, but fatigue life and stability have a price
Three more waves. Its core selling point is the large amount of compensation. For example, some long-distance thermal pipelines, or scenarios with large installation errors, require the expansion joint to absorb the axial displacement of tens or even hundreds of millimeters. That is when three or more waves (such as four or five waves) come in handy. I've seen aCorrugated expansion joint for power station industryIn the case, the pipe diameter is DN1200, the temperature difference is 200℃, the calculated thermal elongation is 80mm, and the four-wave structure is finally selected. If a single wave is forcibly used, the wall thickness will go up to the sky, and the displacement of a single wave is too large, and the stress will long exceed the limit.
But the cost is also obvious. First of all, the axial stiffness of the three waves is low, and it is prone to columnar instability under high pressure-that is, the ripples bulge like balloons and lose their load-bearing capacity. So the three waves usually need to cooperatetie rodOrguide tubeTo enhance stability. We specifically explained it in the FAQ on our siteFunction of expansion joint tie rod: The tie rod is not used to compensate for displacement, but to restrain the pressure thrust and prevent the expansion joint from excessively elongating or twisting. Nine times out of ten, the three-wave products should be equipped with tie rods, or even designed toStraight pipe pressure balanced expansion jointOrCompound hinge transverse expansion jointTo balance the blind force.
Secondly, fatigue life. With the same displacement of 10mm, a single wave may experience 100,000 cycles without any problem. Because the displacement of each wave division of the three waves is only 3.3mm, it stands to reason that the life is longer-but in fact, because of uneven stress distribution (edge wave stress concentration), the test data often show that the fatigue safety factor of the three waves is lower than that of the single wave. Therefore, in occasions with frequent pulsating pressure (such as compressor inlet and outlet pipelines), choosing three waves should be extra cautious, and it is best to do a fatigue life check.
How to make a decision on the actual selection? Three dimensions of hard indicators to help you set
Having said all that, how do you choose? I summarize three hard indicators. If you take this to set your working conditions, it will be almost inseparable.
- Pressure dimension: Design pressure> 2.5MPa, preference is given to single wave, especially pipes with larger nominal diameter. The higher the pressure, the more critical the stability. You check itMetal expansion joint weight tableIt will be found that the wall thickness of multi-wave products under the same caliber is often a little thinner than that of single-wave products-that is to reduce the stiffness in exchange for compensation, but it becomes a short board under high pressure.
- Displacement dimension: The required axial compensation amount exceeds the single wave limit compensation amount (usually the maximum single wave can be 20~30mm, depending on the diameter and wall thickness), then multiple waves must be used. However, note that the compensation amount is not greater, the better. If it exceeds 50mm, it is recommended to consider duplex structure or hinge-type expansion joint, such asCompound straight pipe bypass pressure balanced expansion jointInstead of a bunch of wave numbers.
- Temperature/Media Dimension: Under high temperature (> 400℃) environment, the material creep is intensified, the stress relaxation at the peak of multi-wave is faster, and the life decay is obvious. At this time, the stress level of single wave is lower, which is relatively more advantageous. Corrosive media (such as flue gas desulfurization systems) should be consideredPTFE-lined hoseOrPTFE compensatorMore wavenumbers will increase the risk of medium retention, and single waves are easier to clean.
Waste heat power generation pipeline of a cement plant, pressure 1.0MPa, temperature 350℃, displacement 30mm. According to the above three dimensions: the pressure is not high, the displacement is medium, and the temperature is high. At this time, single wave can also be done, but the wall thickness needs to be thickened, which leads to an increase in cost; Three waves can also be done, but it needs to be equipped with a strong tie rod. Eventually we recommended to our clientsMetal Corrugated Expansion Joints in Cement IndustryThe three-wave zone guide tube scheme in China has been used by customers for three years and good feedback. There is no absolute answer, it is all calculated.
Don't ignore the matching: the guide tube, pull rod and installation direction can't be wrong
Choosing the right wave number is only the first step, and the matching accessories are the details that determine success or failure. Guide tube, we have a special article on our stationSpecific Function of Expansion Joint Guide Tube— — It is mainly used in pipelines with high medium flow rate (such as steam and flue gas), so as to prevent ripples from being directly washed by high-speed fluids, and at the same time, it can reduce flow resistance and vibration. Single-wave products are less equipped with guide tubes, because they have great stiffness and are not afraid of erosion; However, three-wave products, especially large-diameter ones, have deflectors that are almost standard. Otherwise, the high-speed airflow will wear through the thin ripples, and it will be impossible to change them.
The adjustment of the tie rod is also critical.How to adjust the tie rod nut of expansion joint？ Remember one sentence: tie rods are used to adjust pre-tension or pre-compression after installation is complete, not to lock up expansion joints. Many on-site workers screw the tie rod to death directly, which is equivalent to turning the expansion joint into a rigid connection, so why do you need it? The correct way to do this is: according to the ambient temperature at the time of installation, calculate the amount of pre-tension required for the pipe, adjust the nut position, and then let the expansion joint expand and contract freely. Because of the low axial stiffness of the three-wave structure, the installation direction is easy to reverse.The direction of the arrow of the expansion joint refers toMedia flow direction, the arrow must be pointed to the fixed bracket or end point, otherwise the deflector will drive and cause vibration.
If the pipeline has large lateral displacement, the three-wave structure is prone to the combined failure mode of "column instability + plane instability". This is the time to considerCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointInstead of simply changing the wave number. Type selection is a systematic project, and wave number is just one of the variables.
Summary: There is no absolute good or bad, only suitable or unsuitable working conditions
Which is better, single wave or triple wave of metal expansion joint? Did you notice that the question itself is not valid. Single wave is an epee without edge, specializing in high pressure and small displacement; Three waves are seven-foot soft swords, which focus on big compensation flexibility. You took the soft sword to cut the iron plate, and the blade collapsed; You use the epee to play tricks, and you break your wrist. So next time someone asks you "which is better" with the parameter table, you will throw him three words: depending on the working conditions.
If you are really unsure, just roughly sieve it according to the three dimensions (pressure, displacement and temperature) I mentioned above, and then find a reliable manufacturer to report the plan. Our product line in our site is very complete, fromMetal hoseToNon-metallic expansion jointToAll kinds of baffle doorsEach has an applicable boundary. Again: there are no perfect expansion joints, only matching designs.