What is a Cyclic Metal Expansion Joint? How is it different from ordinary expansion joints?
Simply put, circulating metal expansion joints are specially designed to serve those pipeline systems that start and stop frequently and have high and low temperatures-such as circulating water pipes in power plants, heating pipe networks and industrial waste heat recovery devices. Under this working condition, the bellows may crack in half a year, because the fatigue times are not considered in the design. The core index of circulating metal expansion joint is the cycle life, which usually requires ≥2000 times or even higher. Think about it, a set of heating networks starts and stops once a day, 365 times a year and 1,825 times in five years. Ordinary expansion joints simply can't bear it.
Two days ago, I met a customer. Their thermal power plant originally used a universal corrugated expansion joint, but the crack was found in half a year. After changing to the cycle-specific type, it is still running well after five years. What's the difference? Instead of changing the name, it has been re-optimized for fatigue from materials to structures.
The doorway of structural design: multi-layer corrugation + guide tube, the radius of curvature is the soul
Corrugated pipes with circulating metal expansion joints usually adopt a multi-layer structure, and the common material is 304 or 316L stainless steel. Why multi-layered? Because the stress can be shared between layers, it can also improve the corrosion resistance-especially in the face of circulating water containing chloride ions, single-layer bellows are prone to stress corrosion cracking. In addition, the guide tube is standard, and its function is to reduce the direct erosion of the inner wall of the bellows by high-speed medium and avoid local thinning. This site'sCorrugated expansion joint for power plant industryAndHigh temperature axial expansion jointThe design idea is very similar to it, but the water hammer impact and vibration frequency should be additionally considered in the cyclic scene.
What really reflects the level is the radius of curvature of the ripples. The larger the radius of curvature, the smaller the stress concentration and the longer the cycle life. This principle is the same as bending wire-you slowly bend a large arc, and the wire will not break easily; Fold it at a right angle and it will break in two strokes. In order to save materials, many suppliers make the trough very deep and the radius of curvature is small. As a result, the fatigue failure occurs as soon as the pressure is cycled. When selecting, be sure to ask what the radius of curvature is and whether there is a fatigue test report.
Three parameters that must be determined in model selection: cycle number, compensation amount and pressure level
The first, the number of designing cycles. It's not just slapping your head and setting 2000 times. You have to calculate according to the actual start-stop frequency, such as a cold start and a hot stop once a day, plus the slow displacement caused by seasonal temperature differences, which have to be converted into it. If there are still vibration sources (such as water pumps) on site, the vibration frequency should also be included in the calculation of the number of cycles.
The second, the single compensation amount. Axial, lateral and angular displacements should be superimposed and calculated, and cannot only look at one direction. In many accidents, the lateral displacement is not enough, and as a result, the bellows are twisted into twists. If you encounter large lateral displacement, do not plug the circulating metal expansion joint. TryCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint, they are better at absorbing lateral displacement.
Third, the pressure rating. Don't just look at nominal pressure (PN) and place an order. Under cyclic loading, the fatigue strength of materials will be discounted, and the cycle life may only be 60% of that of static conditions for the same pressure class. Therefore, it is best to do fatigue check according to GB/T 12777 standard when selecting the model, or ask the manufacturer to provide S-N curve data. For example, a chemical plant used PN16 circulating metal expansion joint, and the actual pressure was only 1.0MPa. However, because it started and stopped twice a day, it leaked in half a year-because fatigue reduction was not considered.
The easiest pits to step on during installation: cold tightening and brackets
The cold tightness is set wrong, and it is the first big pit. The purpose of cold tightening is to make the expansion joint have a pre-deformation at normal temperature, so that the compensation amount can be fully utilized during hot operation. However, the cold tightness value should be calculated according to the actual pipeline installation temperature, and the drawings cannot be copied. Many construction teams try to save trouble, and the cold tightness can be pulled down for a few turns. As a result, the pre-deformation is insufficient, and the expansion joint is pulled to the limit in the hot state; Or the pre-deformation is too large, and it will be internally injured when it is cold. It is recommended to make pre-stretching or pre-compression records after installation. Refer to the methodAdjustment method of expansion joint tie rod nut— That question-and-answer was clearly written.
The second pit is that the fixing bracket and guide bracket are not designed. The circulating metal expansion joint itself only compensates for displacement and cannot withstand pipe thrust. If the fixing brackets are not locked, or if the guide brackets are too spaced apart, the pipe displacement will run to the weld or elbow beyond the expansion joint, resulting in stress concentration. In the most outrageous case I have ever seen, the installation team used the fixed bracket as a guide bracket, and as a result, the pipe directly broke the adjacent valve when it expanded. Remember: The first bracket near the expansion joint must be the guide bracket, and the distance from the expansion joint must not be greater than 4 times the tube diameter.
In addition, the direction of the guide tube must coincide with the flow direction of the medium. The arrows are clearly marked, but someone always plays backwards. When the guide tube is installed backwards, it becomes a baffle, and the medium directly washes the weld of the bellows, which doubles the corrosion rate. This detail is inSpecific Function of Expansion Joint Guide TubeThere are pictures in that Q&A, so you can take a look.
Circulating Metal Expansion Joints vs. Non-Metal Expansion Joints: Each in its own place
Non-metallic expansion joint (also called fabric fiber expansion joint, this site hasNon-metallic expansion joint (fabric fiber expansion joint)Product) Temperature resistance generally does not exceed 400℃, good vibration absorption effect, low price, suitable for flue gas pipeline and low-pressure air duct. However, the temperature of the circulation scene often exceeds 400℃, such as steam pipeline and high-temperature flue gas. At this time, only metal can be used. The temperature resistance of the metal expansion joint can reach above 800℃, but its disadvantage is that it can't absorb excessive lateral displacement-if the lateral displacement of the pipeline exceeds 30% of the axial displacement, don't use the axial type. It is more reliable to change the lateral type of compound hinge or the pressure balance type of curved pipe.
Another situation is that the medium contains particulate matter, such as gypsum slurry in desulfurization flue gas. At this time, the non-metallic expansion joint is easy to wear out, and the metal wear-resistant lining is more durable. This site'sDesulfurization flue gas baffle doorAndLarge diameter thick wall expansion jointIt is designed for this kind of working condition.
How long can the life span be? Data Speaks
Under the premise of normal selection, installation, operation and maintenance, the design life of circulating metal expansion joints is usually 10-15 years. However, the actual life often fails in the details: the water quality contains chloride ions, which leads to stress corrosion, the damage of insulation layer causes local overheating, or the long-term low-load operation causes intermittent resonance of bellows. These factors add up and the actual lifespan may be only 5 years.
How to break? It is much more reliable to regularly check the bellows surface for micro-cracks and the lining for wear than superstitious "maintenance-free for life". It is recommended to do a visual inspection every year and a stress test every three years. In addition, pay attention to the integrity of the insulation layer-if the insulation is broken, the local temperature drop will cause the bellows to generate additional thermal stress and accelerate fatigue. Also, don't let the pipeline accumulate water for a long time when shutting down, especially stainless steel bellows. The corrosion rate of chloride ions in static water is more than ten times higher than that in flowing state.
Circulating metal expansion joints are not a panacea. In case of ultra-large displacement or ultra-high temperature, other schemes such as sleeve expansion joint and rotary compensator have to be comprehensively considered. This site'sSleeve type pipe expansion jointAndRotary compensatorMight be more economical in some scenarios. When it comes to model selection, don't try to be cheap, and don't be superstitious about "importing is good". The key is to have a bottom of the working conditions.