What is an Elastic Metal Expansion Joint? What is the difference between it and regular metal expansion joints?
Simply put, the elastic metal expansion joint is the kind of bellows compensator with its own recovery ability. Ordinary metal expansion joints (such as some old-fashioned sleeve-type or non-metallic structures) mainly absorb displacement by sliding or deforming, but the core of elastic metal expansion joints is bellows-it absorbs thermal expansion and contraction by elastic deformation of corrugations, and can automatically rebound after pressure disappears. And guess what? This rebound characteristic is particularly useful in frequent start-and-stop pipeline systems, which can avoid residual stress accumulation.
With the guys in our stationUniversal corrugated expansion jointSimilarly, bellows of elastic metal expansion joints are typically made of 304 or 316L stainless steel. But the difference is that elastic metal expansion joints are often designed with more precise restraint structures, such as tie rods or hinges, to control the bellows to absorb only axial or lateral displacement, rather than twisting randomly. To put it bluntly, the ordinary expansion joint may be a "soft persimmon", while the elastic metal expansion joint is a "soft persimmon with backbone"-good elasticity, but the direction is also tightly limited.
Main types and application scenarios of elastic metal expansion joints
There are many types, but don't get confused. Let's pick a few typical ones and say:
- Universal corrugated expansion joint: Just like a jack of all balm, you can make do with any working condition. The temperature does not exceed 300℃ and the pressure is within 1.0MPa, which is commonly found in HVAC and hot water pipes.
- High temperature axial expansion joint: Specifically dealing with high temperatures-600℃ and even higher. The inside of the bellows is usually insulated, and the outside is covered with heat insulation. Power plant steam pipeline, boiler outlet, this thing is standard.
- Corrugated expansion joint for power station industry: The working conditions of the power plant are complicated, with large flow and frequent start-and-stop in addition to high temperature. Such expansion joints are often made into duplex structures (e.g.Compound hinge transverse expansion joint), or with bypass (Compound straight pipe bypass pressure balanced expansion joint), which is used to absorb displacement in multiple directions.
- Metal Corrugated Expansion Joints in Cement Industry: Cement factory, dust, high temperature, thick air duct. Using theLarge diameter thick wall expansion jointThe inner diameter is easily two or three meters, and the wall thickness is also thickened.guide tubeAbrasion resistant.
- And there's alsoStraight pipe pressure balance type、Curved tube pressure balance type、External pressure single type axial typeand so on, each corresponding to a particular piping arrangement. The wrong choice can lead to air leakage or pipeline collapse. Is that the truth?
The three most critical parameters in model selection: pressure, temperature and displacement
It determines the wall thickness and number of layers of the bellows. In high-pressure applications (such as above 2.5MPa), multi-layer bellows must be used, orSleeve type pipe expansion jointDo strengthening. What about the temperature? Affect material. The heat-resistant alloy (such as Inconel 625) is replaced with 304 below 300 °C, and the heat-resistant alloy (such as Inconel 625) above 500 °C. The amount of displacement — including axial, lateral, and angular displacement — determines the wavenumber and length of the bellows. Don't think it's almost enough. Two days ago, I met a customer, and the temperature was incorrectly calculated at 50℃. As a result, the expansion joint directly cracked in three months.
How to calculate? You must look at the thermal expansion formula of the pipe: Δ L = α · L · Δ T. α is the coefficient of linear expansion (about 12×10⁻⁶/℃ for carbon steel), L is the length of the pipe, and Δ T is the temperature difference. After calculation, the wave number is inversely inferred according to the single wave compensation ability of the bellows. Note that elastic metal expansion joints have a characteristic: the larger the displacement, the shorter the fatigue life. So don't be cheap and choose the one with less wave number, otherwise your life will be discounted.
Installation and maintenance: Frequently asked problems such as pull rod adjustment, guide tube action, screw removal, etc.
Installing this piece, there are many pits. The first one is the pull rod. Many elastic metal expansion joints come out of the factory with tie rods-used to fix the bellows and prevent deformation during transportation. Many people think that the tie rod is permanently fixed and directly welded to death. As a result, when the pipeline is hot, the expansion joint can't move, and the whole system is broken. Correct practice: Before installation, loosen the tie rod nut to the designed pre-tension value (usually the manufacturer's mark). If it is axial type, the tie rod is only temporarily positioned, and the nut must be loosened after installation. thatHow to adjust the tie rod nut of expansion joint？ Look at the pre-stretching amount in the instructions, adjust it with a wrench, and confirm that the bellows can expand and contract freely after adjustment.
Another guide tube. The guide tube is a sleeve installed on the inside of the bellows, and its function is to guide the flow direction of the medium and avoid the direct erosion of the corrugations by high-speed fluid. The one in your stationSpecific Function of Expansion Joint Guide TubeThe question and answer is quite clear-reduce turbulence, reduce vibration. Especially for media with particles (such as cement plant dust), if there is no guide tube, the bellows will wear out in half a year. However, the guide tube also has a price: it increases the flow resistance of the medium, and the pressure drop should be paid attention to when selecting the type.
Do you need to remove the screw of the expansion joint？ It depends on the type to dismantle or not. If it isLarge tie rod expansion joint(e.g.Compound hinge transverse type), the tie rod is a working element, provides binding force and cannot be disassembled. If it is a screw for transportation and fixation, of course, it has to be disassembled. If it is not disassembled, it is equivalent to locking the expansion joint. How to distinguish? Manufacturers will generally paint red paint on the fixed screw, or mark it in the drawings. You do it without figuring it out at your own peril.
Common faults and factors affecting life
- corrosion: Chloride ions, acidic media are killers. If the material is not selected correctly, stainless steel can't hold it. For example, the flue gas desulfurization system has to be usedDesulfurization flue gas baffle doorWith corrosion-resistant expansion joints, or simplyPTFE compensator。
- Fatigue fracture: This is directly linked to the displacement amount and the number of cycles. According to the standard JB/T 12235-2015, the design fatigue life of expansion joints is generally 1000 to 10000 times. The start and stop of your pipeline is counted as a cycle at a time, and the life of frequent start and stop is definitely short.
- Instability: If the pressure is too high or the bellows is too long, plane instability or columnar instability will occur. The solution is to set more constraints, such as usingStraight pipe pressure balanced expansion jointTo counteract the pressure thrust.
- abrasion: The high-speed particle flow flushes the guide tube or bellows. As mentioned earlier, adding the guide tube can improve it, but the guide tube itself will also be worn, so check and replace it regularly.
How does life span affect? In addition to working conditions, installation quality is also critical. Such asThe direction of the arrow of the expansion joint refers toThe medium flow direction, if you install it backwards, the guide tube will play a role in blocking the flow, but accelerate the abrasion. Another exampleMetal hose pressure standardThe expansion joint is different from the hose, so don't mix it. In short, elastic metal expansion joints are not disposable consumables, but structural parts that require careful calculation-if you choose them correctly, install them correctly, and maintain them, you can use them for ten or eight years.