Why Can't Plumbing Systems Live Without Corrugated Metal Expansion Joints? Understand this displacement compensation thing, and you'll know how critical it is
The pipeline, which looks dead, is actually "moving" all the time. As soon as the temperature changes, the metal pipe will elongate or shorten-thermal expansion and contraction, as taught in junior high school physics. However, in actual working conditions, the temperature difference is always hundreds of degrees Celsius, and the thermal expansion of a pipeline tens of meters long may reach more than ten or even tens of millimeters. If you don't leave a "gasping" place for the pipeline, the stress will directly destroy the flange, crack the weld, and even push down the equipment bracket.
Expansion joint corrugated metal does this job. It is like a joint made of a spring, and uses bellows to absorb axial, transverse and even angular displacement of the pipe. Think about it, 600℃ steam in the main steam pipeline of the power station, high-speed airflow with dust in the pipeline of the cement plant, and various acid and alkali media in the chemical plant-in these scenarios, a reliable expansion joint corrugated metal is the safety valve of the system. This site hasUniversal corrugated expansion joint、High temperature axial expansion joint、Corrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement IndustryAnd other products, all of which are specially designed for different working conditions.
Why do many expansion joints leak after two or three years of use? There is only one reason-the three hard indicators were not pegged during the selection. Which three? Look down.
Three hard indicators of selection: pressure level, temperature range and displacement-how to correspond to your own working conditions?
Let's start with stress. The pressure resistance of corrugated metal of expansion joint directly depends on the wall thickness, number of layers and material of corrugated pipe. If the pressure level is selected low, the bellows will be burst; If the choice is high, the cost will be wasted, and the increase of stiffness will affect the compensation ability. Generally, low pressure (below 0.25MPa) uses a single layer wave, and medium and high pressure (0.6-2.5MPa) requires multiple layers or even reinforcing rings. For example, the pressure of the main steam pipe of a power station is usually above 10MPa. At this time, multiple layers of stainless steel bellows must be used, such as this station'sCorrugated expansion joint for power station industry, wall thickness and number of layers are calculated according to ASME standards.
Temperature is more critical. High temperatures will reduce the yield strength of the metal and creep will also occur. Ordinary 304 stainless steel starts to oxidize and peel above 450℃, and 316L can last to about 600℃. If the medium temperature exceeds 800°C, you have to use a superalloy, such as Incoloy 800. The temperature of the preheater pipeline in the cement industry is usually above 900 ℃. At this timeMetal Corrugated Expansion Joints in Cement IndustryThe interior must be lined and insulated, otherwise the bellows will burn through directly.
The amount of displacement is the ultimate "just need". The wave number of the corrugated metal of the expansion joint determines how much displacement it can absorb. But don't be greedy for too many-too many wavenumbers, low axial stiffness, easy instability; Too few waves, not enough displacement. Regular manufacturers will give you a "displacement-life" curve, telling you the fatigue life corresponding to each wave. It is an industry practice to set aside a 20% margin when selecting models.
The three indicators are also coupled to each other. High pressure often requires thick walls, but thick walls will lead to increased stiffness and affect displacement absorption capacity. How to balance? It is not complicated-give the working condition parameters (pressure P, temperature T, displacement Δ L) to the manufacturer and let them calculate with professional software. The rules in our business are:Universal corrugated expansion jointSuitable for regular working conditions, special working conditions must be customized.
How to match different industries? High-temperature steam in power stations, dust washing in cement, and corrosive media in chemical industry have their own particularities
In the power station industry, especially in thermal power generation, high-temperature and high-pressure steam pipelines are the protagonists. The steam temperature is 550-600℃, the pressure is 15-20MPa, and the temperature changes drastically when the pipeline is started and shut down, which is easy to produce low frequency and large displacement. At this time to useCorrugated expansion joint for power station industryOrHigh temperature axial expansion jointThe structure is usually lined with an inner cylinder to guide the flow and prevent high-speed steam from washing the bellows. Another point: Power station pipelines mostly adopt large tie rod structures, which are used to absorb lateral displacement, such as this station'sLarge tie rod expansion jointRelated Products.
In the cement industry, what is difficult to deal with is dust and high temperature. The raw powder in the kiln tail preheater pipe, with high wind speed and hard particles, hits the bellows directly, and it takes less than half a year to wear out. So the cement industryMetal Corrugated Expansion Joints in Cement IndustryWear-resistant liners must be added-usually ceramic patches or wear-resistant steel plates, and the thickness of the guide tube must be thickened. In addition, the inlet and outlet pipelines of cement vertical mill and powder separator vibrate greatly, so the expansion joint with damping device should be selected.
In the chemical industry, corrosive media is the number one enemy. If it is a strong acid such as hydrochloric acid and sulfuric acid, ordinary stainless steel can't bear it at all. This is the time to use itPTFE-lined hoseOrPTFE compensatorUse PTFE to separate the metal bellows from the medium. If it is a high temperature acidic gas (such as a flue gas desulfurization system), useDesulfurization flue gas baffle doorCooperate with non-metallic expansion joints to isolate. This site has a rich chemical product line, make sure to provide media composition and concentration before selecting, don't take it for granted.
Installing That Thing: Where Does the Deflector Arrow Head? How much do you adjust the tie rod? Is the screw disassembled or not?
The expansion joint is not just soldered directly when bought. 90% of the early failures are in the installation.
First, the direction of the arrow of the guide tube. The guide tube inside the corrugated metal of the expansion joint has an arrow on it, which represents the direction of the medium flow. The arrows must be directed downstream, that is, in the direction of the medium flow. Why? Because the role of the guide tube is to let the medium flow through the middle, avoiding direct flushing of the bellows. If it is installed backwards, the medium will drill into the gap between the bellows and the guide tube, causing vortex erosion, which will leak in a few months. This site also specifically talked about in the question and answerSpecific Function of Expansion Joint Guide TubeYou can look at it carefully.
Second, how to adjust the tie rod? Expansion joints with tie rods (such as double hinge type, curved tube pressure balance type), and the nuts on the tie rods are used to adjust the amount of pre-tension. Generally speaking, when installing, the expansion joint should be compressed to half the design length and then locked with a tie rod. The aim is to pre-stretch in the cold state, and when the pipe is thermally expanded, the expansion joint just returns to the neutral position. The specific adjustment value depends on the installation instructions given by the manufacturer. Never screw it yourself-screwing it too tightly will crush the bellows.
Third, should the transportation screw be disassembled? This question is asked by the most people. The answer is: it has to be dismantled. The transportation screw is used to prevent the expansion joint from shaking and deforming during transportation, and must be completely dismantled after being installed in place. If not dismantled, the screw will limit the deformation of the bellows when the pipe expands thermally, causing the stress to concentrate at the weld, causing cracking. Don't make such low-level mistakes.
Expansion joint leaks after three years of use? Most of the problems lie in these places (with life extension methods)
Two days ago, I met a customer, saying that the cement pipeline expansion joint of their factory leaked in less than three years, and it still leaked after changing it twice. When I looked at the pictures of the scene, the deflector was full of pits, which must have been installed backwards. Again, the medium temperature is 900℃, but they use ordinary 304 stainless steel. Yeah, the choice was wrong.
- Insufficient selection parameters: There is no margin for pressure, temperature and displacement, and the bellows works in the extreme state for a long time, and the fatigue life drops sharply.
- Wrong material selection: A material that is not resistant to corrosion is used under corrosive media, or a material that is not resistant to oxidation is used under high temperature.
- Installation failure: The guide tube is installed backwards, the tie rod is wrongly adjusted, and the screw is not disassembled-as mentioned before.
- External additional stress: The sinking of the pipe support and the excessive cold tightening of the pipe system exerted unplanned force on the expansion joint.
How to extend life? First, when selecting a model, ask the manufacturer to clearly tell him the number of cycles you expect (such as 1,000 times or 10,000 times). Second, strictly follow the drawings when installing, especially the direction of the guide tube and the pre-stretching amount of the tie rod. Third, check it regularly. When shutting down for maintenance every year, check whether there are cracks and corrosion pits on the surface of the bellows, and whether the guide tube is worn. If you find that the deflector is worn more than 1/3 of the thickness, change it quickly.
Expansion joint corrugated metal is not a metaphysics, it is the "joint" in pipeline engineering. If you choose the right one, you will have no problem using it for ten or eight years; If you choose the wrong one, change it every day. Pressure, temperature, displacement-none of these three numbers can be missed.