What exactly is the crest? — — Start with the structure of the bellows
People who engage in pipe compensation know that the core of metal expansion joints is corrugated pipes. This bellows is not an ordinary pipe, it is stacked with corrugations in circles. When you touch it with your hand, those raised "waves" are crests, and the concave ones are called troughs. To put it bluntly, the crest is the ring at the highest point on the bellows. Don't underestimate this geometric feature. More than 80% of the displacement compensated by the whole expansion energy saving and the pressure it can carry are directly related to the shape, size and quantity of the wave peaks.
Why can some expansion joints of the same caliber absorb tens of millimeters of axial displacement, while others can only absorb ten millimeters? The answer is hidden in the crest. The first thing we do in this business is to recognize that the crest is not a decoration, it is a functional piece. From the structural point of view, the crest is equivalent to a "spring ring", and multiple crests are connected in series to form an elastic body that can be repeatedly stretched and compressed. So, discussing a metal expansion joint crest is essentially discussing its "bones" and "joints".
Wave height, wave pitch, wave number: How three parameters affect the compensation amount and stiffness of expansion joints
Let's talk about wave height first. Wave height is the vertical distance from the top of the crest to the bottom of the trough. The greater the wave height, the greater the amount of displacement that a single wave can absorb-this is called "compensation ability". However, the wave height has a price: if the wall thickness remains unchanged, the stress at the root of the wave peak will soar, and the fatigue life will plummet. Usually we will use the ratio of "wave height/wall thickness" to evaluate, which is generally between 5 and 15. For example, for general-purpose corrugated expansion joints, the wave height is 8~12mm, which is the common range. However, for high-temperature axial expansion joints, in order to cope with thermal stress, the wave height will be appropriately reduced to ensure the strength.
Let's talk about wave length. Wave distance is the distance between the centers of two adjacent crests. The smaller the wave pitch, the more waves per unit length, the greater the overall stiffness, but the amount of deformation per wave will also be more uniform. On the contrary, when the wave pitch is large, the bellows looks "loose", with low stiffness but large compensation. Here is a matching logic: if the pipeline displacement is large but the space is cramped, you have to "squeeze" out the compensation amount with large wave height + wavelet distance. For example, the wave pitch of the corrugated expansion joint used in the power station industry is generally controlled between 0.8 and 1.2 times of the wave height, which not only ensures the deformation ability but also prevents instability.
Single wave compensation × wave number = total axial compensation. But this product does not expand infinitely. As soon as the number of waves is large, the bellows as a whole becomes longer, which is prone to lateral instability or "bulging". For products such as compound hinge transverse expansion joints that need to bear transverse displacement, the wave number must be strictly controlled, generally no more than 6 waves. Because of the pressure balance structure, the wave number of straight pipe pressure balance expansion joint can reach 8~12 waves.
These three parameters are linked-adjust one to move three. When selecting, it is necessary to determine the target compensation amount and allowable axial stiffness first, and then inversely deduce the wave height, wave distance and wave number. For the specific calculation formula, please refer to the content of this site "Stiffness and Calculation Formula of Bellows", which gives the derivation of single wave stiffness and axial stiffness. Simply put, the stiffness is inversely proportional to the cubic power of the wave height and directly proportional to the first power of the wave distance, so changing the wave height is most effective.
Differences in wave crest design under different working conditions: How to choose high temperature, high pressure and large diameter
In high temperature conditions, such as steam pipelines, when the temperature exceeds 400℃, material creep and stress relaxation are serious. At this time, the wave peak should not be too deep (the wave height is 20% ~30% smaller than the normal temperature), and multi-layer thin-wall structure should be adopted instead of single-layer thick wall. The high temperature axial type expansion joint we recommend to customers is usually made of 304 or 316L stainless steel, with wave height controlled at 8~10mm, wave pitch about 20mm and wall thickness 0.8~1.2mm in two layers. Why not use thick walls? Because the thick wall causes the stress concentration factor at the root of the wave crest to increase, the fatigue life is shorter instead.
In high-pressure conditions, such as the design pressure of chemical pipelines above 10MPa, the peak shape should be changed to "U" shape or "Ω" shape instead of the ordinary "S" shape. The root arc radius of the U-shaped crest is larger and the stress dispersion is better. At this time, the wave height generally does not exceed 6mm, the wave distance is about 15mm, and a reinforcing ring (also called a support ring) is added to prevent the wave crest from being squashed. Our large-diameter thick-walled expansion joint is specially designed for this kind of scene. The wall thickness can reach 3~6mm, but the wave height is very shallow.
For large-diameter working conditions, such as the diameter of flue gas pipeline exceeding 2 meters, the peak design should focus on its own weight and local instability. Because the large-diameter corrugated pipe has thin walls, the wave crest is easy to "fall down". The solution is to set a guide tube at the wave peak (refer to the question and answer of this site "Specific Function of Expansion Joint Guide Tube"), which can not only guide the flow to reduce vortex, but also provide radial support for the wave peak. In addition, the wave number should be reduced to avoid the instability of the long column. The metal corrugated expansion joints used in our cement industry usually only do 3~4 waves above DN2000, the wave height is controlled within 20mm, and the wave pitch is enlarged to more than 30mm.
Common causes of crest failure: relationship between fatigue cracks, stress corrosion, and deflector tubes
If you look at more failure cases, you will find that 80% of the damage occurs at the root of the crest-the arc area where the crest and trough transition. The reason is simple: there's the most stress there. Fatigue cracks are the most common. Pipelines expand and contract thermally once a day, which is 365 cycles a year and 3,650 times a decade. The roots of wave peaks have long accumulated to the limit in micro-cracks. Therefore, we must calculate the fatigue life when designing, which generally requires at least 2000 cycles.
Another killer is stress corrosion. Especially for desulfurization flue gas pipelines, the medium contains Cl⁻¹ and S²⁻¹, and the corrosion rate can be multiplied by ten times in the high stress area at the root of the wave peak. At this time, the material should be corrosion-resistant, such as duplex stainless steel or nickel-based alloy. In the desulfurization flue gas baffle door project, we saw that the peak was corroded and worn into a honeycomb shape. Later, we added a layer of PTFE or rubber coating on the inside of the peak, and the life span came up.
What about the deflector? Not only is it an anti-scour tool, but it also improves the flow field at the crest of the wave. When there is no guide tube, the high-speed medium directly washes the front surface of the wave peak, resulting in erosion wear; With the guide tube, the medium travels from the inside of the guide tube, and the crest bears only static pressure. At the same time, the guide tube also plays the role of "temperature isolation", reducing the peak temperature gradient and thermal stress. Therefore, if you find frequent cracks in the crest, first check whether the guide tube is installed in place and short.
How to inversely deduce the peak parameters according to the displacement of pipeline during type selection- -a practical case
Let's go straight to the case. A steam pipeline of a thermal company, DN500, has a working temperature of 350℃, a design pressure of 1.0MPa, a total length of 100 meters, and a calculated thermal expansion of 120mm. Customer requires universal corrugated expansion joints, and only the length of expansion joints is allowed to exceed 400mm in the installation space.
The first step is to determine the total compensation amount. An axial displacement of 120mm requires the expansion joint to operate safely while absorbing all the displacement. Considering the installation deviation and dynamic load, we take 1.2 times the safety factor, that is, the target compensation amount of 144mm.
The second step is to inversely deduce the wave height, wave distance and wave number. The conventional single wave compensation amount is between 15~25mm (when the wave height is 10~14mm). We take the single wave compensation amount of 20mm, then we need the wave number =144/20=7.2, and round 8 waves. 8 wave × single wave length (wave pitch + wall thickness) ≈8×20mm =160mm, plus straight edge segments at both ends 80mm each, the total length is 320mm, which meets the 400mm limit. Okay, set the wave number to 8.
The third step is to check the stiffness. The axial stiffness of bellows is inversely proportional to the wave number, and the single stiffness at 8 waves is about 600N/mm. For DN500 pipe, the thrust force transmitted to the fixed bracket by this stiffness is about 86kN, which is within the allowable range of pipe design. If the stiffness is too large, the wave height can be appropriately increased (for example, from 10mm to 12mm), and the stiffness is inversely proportional to the cubic power of the wave height. The stiffness of the 12mm wave height will drop to the original (10/12) ^3≈0.58, and the thrust will drop to about 50kN. However, the peak stress should be re-checked after the wave height increases. We found that the maximum stress at 12mm wave height is 320MPa, which is lower than the allowable stress of material 304 at 350℃, OK.
The last recommended product model is the universal corrugated expansion joint, DN500, 8 waves, wave height 12mm, wave pitch 20mm, wall thickness 1.2mm single layer, guide tube extension length 50mm. After installation, it runs for two years, and the wave crest is checked regularly for no crack, and the customer is satisfied.
Therefore, selection is a balance game of "wave height-wave distance-wave number". The pipe displacement, pressure, temperature and installation space you get in your hand are the boundary conditions of the game. The metal expansion joint crest is the core pawn that determines whether you can clear the level.