What exactly is the use of industrial metal expansion joint? Don't think it's just a "spring."
When many people come into contact with this thing for the first time, they always think that it is a large spring-the pipe can expand and contract when it is hot and reset when it is cold. Tsk, that's half right. Industrial metal expansion joints are indeed used to absorb thermal displacement, but their more important task is to protect pipes and equipment from being torn by huge thermal stresses. Think about it, the temperature of a steam pipe hundreds of meters long soars from normal temperature to 500℃, and the expansion amount is tens of centimeters. Hard carry? The flange bolt collapsed first, and the pipe frame followed. At this time, the expansion joint is the role of "unloading force".
But it is not simply elongated and shortened. Depending on the direction of displacement, you can choose axial, lateral, or angular. Like what we always sayUniversal corrugated expansion jointWhich is suitable for axial expansion and contraction of straight pipe segments; If the pipeline is complicated and requires lateral displacement, it is necessary toCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 The benefit of the pressure balanced type is that the blind plate force generated by the internal pressure is not transmitted to the fixed bracket, which is particularly critical for large diameter, high pressure lines.
So, stop using it as a spring. The spring only pipes in one direction, and the expansion joint pipes in axial, transverse and angular directions. It is also necessary to distinguish the coupling influence of pressure and temperature on the rigidity of bellows. I met a customer two days ago. Before buying, I only asked "what is the maximum compensation amount". When I arrived at the scene, I found that the medium contained chloride ions, and the corrugated pipe material was 304, which cracked in three months. What is the difference between this and choosing the wrong spring?
The three most overlooked issues in model selection: temperature, displacement, and fatigue life
Let's talk about temperature first. Many people only look at the design temperature, not the temperature fluctuation range and heating rate. For example, the main steam pipeline of a power plant rises rapidly when it is started, and the temperature difference between the inner and outer walls of the bellows is large, which will generate thermal stress. At this time, if you chooseHigh temperature axial expansion jointWithout adequate wall thickness transition or heat treatment process, local stress concentration directly leads to early cracking. The temperature also affects the allowable stress of the material-the strength of 304 is discounted in half at 600℃. If you calculate the wall thickness according to the allowable stress at normal temperature, it will be strange if you don't overturn.
Let's talk about displacement. The displacement given by the design institute is often the theoretical value, ignoring the actual installation deviation and the elastic deformation of the pipeline support and hanger. We have seen a project. The axial displacement on the drawing is 30mm, but the actual displacement is 45mm when measured on site, because the fixed bracket next door has slipped. When selecting the model, it is recommended to reserve the margin according to 1.25 times of the theoretical value, and clarify whether it is multi-directional combined displacement or single direction. for exampleStraight pipe pressure balanced expansion jointSpecially absorbs axial displacement and cannot withstand large lateral misalignment.
Finally, fatigue life. The standard generally requires that the fatigue life of bellows be ≥1000 times, but you have to calculate the number of temperature cycles and the number of pressure cycles. Some chemical plants start and stop more than 200 times a year, and with the daily tiny pulsation, the actual number of cycles is much higher than the design value. When selecting the model, ask the manufacturer for the fatigue life curve (S-N curve). Don't just listen to the words "warranty for ten years". A ten-year warranty does not mean that the bellows can carry a ten-year cycle, but the manufacturer is willing to change it for free-but the loss of a shutdown may be a hundred times more expensive than the expansion joint itself.
Record of rollover at installation site: the tie rod is not removed, the guide frame is not installed, and the direction of the guide tube is reversed
Okay, I'll admit, every visit to the scene is like opening a blind box. Most common rollover scenarios:Tie rod not removed。 In order to protect the bellows from deformation during transportation and installation, the manufacturer will install transportation rods at both ends of the expansion joint. After installing the pipeline at the site, the tie rod must be loosened or removed. However, many construction teams don't know that if they test the pressure of the pipeline directly, the tie rod is still screwed tightly-the expansion joint is locked, and there is no way to compensate for the displacement at all. As a result, as soon as the temperature rose after the pressure test, the pipe arched up and the bellows exploded.
The second isGuide frame not installed。 Especially for axial expansion joints, guide brackets must be provided on the pipeline to ensure the displacement along the axis direction. The spacing between the guide brackets is strictly calculated, and it is not done by just welding an angle steel. If the guide frame is missing, the pipe will begin to bend laterally as soon as it is hot, and the bellows will twist, and the plane will be instable and directly scrapped.
And the third isThe direction of the guide tube is reversed。 The role of the guide tube is to guide the medium to flow smoothly through the bellows and prevent high-speed airflow or particles from washing the corrugations. Arrows are usually marked on the guide tube pointing to the direction of the medium flow. When the arrow is installed backwards, it is equivalent to regarding the guide tube as a throttle plate, and the medium rushes directly into the corrugated trough, and the wear speed doubles. We encountered a cement factory and it was installed backwardsMetal Corrugated Expansion Joints in Cement IndustryThe guide tube wore through the trough in three months, and dust flew all over the sky.
Common faults under practical working conditions: cracking, instability and corrosion of bellows-causes and countermeasures
crackingMost are fatigue or stress corrosion. Fatigue cracking often occurs in troughs because that is the stress concentration zone. Countermeasures: Increase the wave number to reduce the single wave displacement, or switch to multi-layer bellows (multi-layer structure has better fatigue resistance). Stress corrosion is more common in chlorine-containing media (such as desulfurization flue gas), and corrosion-resistant materials must be changed, such asPTFE-lined hoseOr duplex stainless steel. Oh, yeah,Desulfurization flue gas baffle doorMatching expansion joints often have similar problems, and fluoroplastic lining is a common means.
InstabilityIt is divided into plane instability and columnar instability. Plane instability is manifested by corrugation distortion, which is usually caused by excessive internal pressure or unreasonable design of guide tube, which leads to media impact. Columnar instability is the whole expansion joint bending down like noodles due to the lack of guide brackets or excessive axial force of the pipe. Countermeasure: Increase the rigidity of the guide frame spacing, or chooseExternal pressure single axial expansion joint— — The external pressure structure itself has strong ability to resist columnar instability.
corrosionThere are many kinds, pitting corrosion, intergranular corrosion, crevice corrosion. Pitting corrosion is common in the accumulation of chloride ions on the surface of stainless steel; Intergranular corrosion originates from welding heat affected zone sensitization, and the solution is to choose low carbon or stabilized stainless steel (316L or 321). Gap corrosion often occurs at the connection between bellows and end pipe, so it is necessary to ensure the welding quality and make pickling passivation.
What do you see for the length of life? Don't be fooled by the "ten-year warranty"
The four words "ten years of warranty" sound very practical, right? But you have to see what you insure. Most manufacturers only guarantee that the body does not leak, but not the fatigue life. Moreover, the warranty terms often require "use according to the manufacturer's installation guidance". As long as there is a little irregularity on your site (for example, the tie rod is not disassembled and the medium flow rate exceeds the standard), the manufacturer directly refuses to guarantee. More importantly, lifespan is not a concept of time, but a concept of number of cycles.
Materials, structures, working conditions. The material determines the corrosion and temperature resistance grades; Structure (wave height, wave pitch, number of layers, wall thickness) determines stiffness and fatigue performance; Working conditions (temperature, pressure, displacement frequency, medium corrosivity) determine the actual decay rate. Simple judgment: If the same pipeline is usedrubber compensatorAndMetal expansion joint, rubber ages much faster, but fatigue cracks in metals tend to be more concealed. It is recommended to check the bellows surface once a year: use penetration detection to detect micro-cracks, and use thickness gauge to detect wall thickness thinning. Don't wait until you leak to know something is wrong.
Industry Case: Differences in the selection of three lines of power station, cement and chemical industry
Power station industryThe biggest features are high temperature, high pressure and large diameter. Main steam pipeline at boiler outlet, temperature above 540℃, pressure above 10MPa. We recommendCorrugated expansion joint for power station industryTypically high temperature axial or pressure balanced, the material is P91 or Incoloy 800. This kind of expansion joint is most afraid of material creep and thermal fatigue, so creep analysis must be done during design, and the solution treatment after bellows forming must be strictly controlled.
Cement industryThe pain point is that dust and high temperature alternate. The temperature of the exhaust gas pipe at the end of the kiln fluctuates greatly, and it contains a lot of abrasive dust. At this timeMetal Corrugated Expansion Joints in Cement IndustryWear-resistant guide tubes must be installed, and the material is heat-resistant steel (such as 310S or 321). In addition,Rectangular non-metallic expansion jointIt is also often used in cement plants, but the upper limit of temperature resistance of non-metals is low, so it is not suitable for high-temperature sections.
Chemical industryThe most complex, the medium is all kinds of strange. There are strong acids, strong bases, organic solvents, and cryogenic pipelines. Chemical pipelinePTFE-lined hoseAndPTFE compensatorVery common, used to prevent corrosion. In addition, when the pressure level is high and it is necessary to compensate for multi-directional displacement,Compound straight pipe bypass pressure balanced expansion joint。 However, chemical projects should pay attention to electrostatic grounding: metal bellows are grounded through flange bolts. If the bolts are loose or coated with anti-rust paint, the resistance exceeds the standard and may cause explosion.
To say a thousand words, industrial metal expansion joints are never auxiliary machines that are bought and installed. Selection, installation and maintenance are indispensable. What pits have you stepped on?