Why can the metal expansion joint "carry" the thermal expansion and contraction of the pipe? — — Core Principle and Structural Dismantling
Think about it, how much can a steam pipe several hundred meters long expand when its temperature rises from normal temperature to five or six hundred degrees? Dozens of millimeters are light. Hard carry? The welds and brackets of the pipeline have to be scrapped. At this time, the metal expansion joint plays a decisive role. What does it rely on? By the bellows.
The bellows is the heart of the metal expansion joint, which is, to put it bluntly, a set of annular corrugated thin-walled metal tubes. It can produce elastic deformation in axial, transverse and even angular directions, and "eat" the thermal displacement of the pipe. Bellows alone is not enough, but also accessories such as upper end pipe, guide tube and tie rod. For example, our commonUniversal corrugated expansion jointThe end pipe is responsible for connecting the pipeline, the guide tube protects the bellows from being washed by high-speed media, and the tie rod limits the displacement direction-if this thing is installed backwards, it will suffer a big loss later.
Structurally, according to different compensation directions, it can be divided into axial type, transverse type, hinge type and so on. LikeExternal pressure single axial expansion joint, bellows bear external pressure, better stability, suitable for large-diameter pipes. AndCompound hinge transverse expansion joint, with two sets of bellows and hinge structure, specially absorbs transverse displacement. Therefore, metal expansion energy saving to withstand thermal expansion and contraction relies not on brute force, but on delicate corrugated elastic deformation.
Temperature resistance, pressure resistance and fatigue resistance-three hard indicators determine what harsh applications it can be used in
Q: What are the features of metal expansion joints? The most direct answer is: it can work stably under high temperature, high pressure and high frequency alternating working conditions. But these three indicators are not isolated, they contain each other.
Let's start with temperature resistance. General 304 stainless steel bellows, the long-term working temperature is about 400℃. When you encounter the high temperature of seven to eight hundred degrees such as power plant boilers and cement kiln tails, you have to use itHigh temperature axial expansion jointThe material is changed to Incone 625 or GH3030, and then lined with thermal insulation. And guess what? In our stationCorrugated expansion joint for power station industryAndThe metal corrugated expansion joint in cement industry is specially designed for this kind of scene, and the thickness of insulation layer and the number of corrugated layers have been optimized.
In this part of pressure resistance, the wall thickness, wave height and number of layers of bellows directly determine the upper limit of pressure bearing. Such asLarge diameter thick wall expansion jointThe wall thickness of a single layer can reach more than 2mm, and the design pressure starts at 1.6MPa. But when the stress is high, the fatigue life will be discounted-there is a balance.
Fatigue resistance is the real "invisible killer". The pipeline starts and stops, steam hammers and vibrates every day, and the bellows expands and contracts back and forth. According to the standard, the fatigue life of general bellows is about 1000~3000 times. But this is only a theoretical value. In actual use, corrosion, stress concentration and installation deviation will greatly shorten the life. Therefore, the real test of quality is the comprehensive performance of these three hard indicators.
How to choose between different working conditions? Universal type, high temperature type and pressure balance type all have their own unique skills
When it comes to model selection, don't expect one product to conquer the world. Let's take a look at the three most typical scenarios:
Scenario 1: Normal temperature, low pressure, ordinary pipeline.For example, heating water systems, usingUniversal corrugated expansion jointThat's enough. Simple structure, affordable price, axial compensation amount 20~50mm, easy to do. Of course, if the pipe has a lateral displacement, it can be consideredDuplex hinge transverse type expansion joint.
Scenario 2: High temperature, corrosion, large displacement.For example, desulfurization flue and high-temperature fan outlet. Here the medium temperature is high and there is an acidic condensate. Conventional bellows can't hold it, so you have to use itHigh temperature axial expansion jointAdd corrosion resistant coating. In addition,Desulfurization flue gas baffle doorUsed in conjunction, it can cut off smoke and facilitate maintenance. Also pay attention to large-diameter pipes, rectangular cross-sectionsMetal rectangular expansion jointMore suitable, we have special products in our station.
Scenario 3: High pressure, strictly limited displacement.For example, steam main pipe, pump outlet. The thrust generated by the internal pressure of the pipeline is huge. If the axial expansion joint is directly installed, the pipe frame must be made like a bunker. At this timePressure balanced expansion jointIt comes in handy-canceling the internal pressure thrust from each other through two bellows, without the need for extra fixing brackets. Breaking it down, there areA straight tube pressure balance type expansion joint,Curved tube pressure balance expansion joint、Double straight pipe bypass pressure balance type expansion joint. Their core difference lies in the arrangement of bellows, which should be determined according to the direction of pipeline and space size.
In addition, for buried pipelinesDirect buried (fully buried) type expansion joint, directly buried in the soil, the outer layer of antiseptic is particularly well done. Vacuum pipes are usedSpecial hose for vacuumAnd high sealing requirements.
Exactly how long do metal expansion joints last? Influencing factors and measured data
As soon as this question is thrown up, many customers will ask. To be honest, there is no standard answer, but you can give a rough range of judgment.
Look at the design criteria first. GB/T 12777 stipulates that the fatigue life of bellows is calculated according to the number of cycles. Under normal working conditions, the design life is 1000 times, but in actual tests, many high-quality products can run more than 3000 times. We tested a batch beforeUniversal corrugated expansion jointWith an axial displacement of 30mm at a normal temperature of 0.6MPa, the crack appeared only after 3500 cycles.
- Temperature:Fatigue life drops by about half for every 100°C rise. At 400℃, the life may only be 20% of normal temperature.
- Pressure fluctuations:Frequent pulse pressure is more harmful than stable pressure. For example, the water hammer of the steam pipe may make the local stress of the bellows exceed the limit at one time.
- Corrosive media:Chloride ions and sulfides will accelerate stress corrosion cracking. In the desulfurization and denitrification system, if the material selection is wrong, it may leak in half a year.
Therefore, the actual service life can range from a few months to more than a dozen years. The key is to choose the right model in the early stage and install it correctly in the later stage. Such asrubber compensatorThe life span is relatively short, generally 3~5 years; However, if the metal expansion joint is stable in working condition and properly maintained, it will be used for 8~10 years without any problem.
Details that are easy to overlook in installation and maintenance-you can't go wrong with tie rods, deflectors, and arrows
A while ago, there was a customer who finished installingLarge tie rod expansion jointLater, it was found that the compensation was not enough, and the pipeline was still crooked. After investigating for a long time, it turned out that the tie rod nut was locked too dead, and the bellows couldn't move at all. Tsk, this kind of low-level mistake is actually quite common.
The function of the tie rod, as mentioned in the frequently asked questions and answers-"The function of the tie rod of the expansion joint" makes it very clear: it only bears the internal pressure thrust, and does not limit the displacement of the bellows. When installing, the tie rod nut should be reserved for adjustment clearance, and cannot be screwed to the end. How to tune it? The article "How to Adjust the Tie Rod Nut of Expansion Joint" gives the specific steps: first pre-tighten to the end, and then loosen the thread length corresponding to the displacement.
Look at the deflector again. Many people think it is just a shield, but it is not. As explained in "The Specific Function of Expansion Joint Guide Tube", the guide tube has two key functions: first, drainage, allowing the medium to pass smoothly and reducing vortex erosion; The second is to protect the inner wall of the bellows from being worn by particulate matter. Note when installing: the direction of the arrow must be consistent with the flow direction of the medium-the article "The direction of the arrow of the expansion joint refers to" specifically said that the arrow is the flow direction mark. If the guide tube is installed backwards, it becomes a throttle device, and the bellows will be impacted.
- Before on-site welding, be sure to remove the expansion joint or protect it, and the welding slag splash will burn through the bellows.
- When hoisting, you can't directly strangle the bellows with ropes, but use soft belts or lift from the end pipe.
- After installation, remember to dismantle the limit screw for transportation-"Does the screw of the expansion joint need to be dismantled" answers this question. If it is not dismantled, the expansion joint will be completely locked, which will not compensate.
To be honest, the structure of metal expansion joints is not complicated, but the details determine success or failure. Choose the right model, install it in the right direction, and adjust the tie rod, and it can serve safely for many years. If these details are not paid attention to, the failure rate skyrockets.
Finally, if you are choosing a model for a pipe, you might as well take a look at our siteMetal hose、PTFE-lined hose、Rotary compensatorAnd other product series, there is always an adapted scheme.