Specialized in manufacturing compensators, expansion joints, baffle doors

Flue displacement is not "move if you want": first distinguish the three kinds of displacement clearly

Everyone who is engaged in engineering knows that the flue looks like a thick steel pipe, but it is actually very temperamental. Once the high-temperature flue gas passes, the pipeline expands and contracts by heat, and it is common for the flue of tens of meters to elongate by more than ten centimeters. Saying "can the flue be displaced?" Sounds like a simple nod-and-shake question, but actually it is clear what kind of displacement it is-thermal displacement, settlement displacement, or installation error.

Thermal displacement is best understood, that is, when the temperature changes, the tube expands and contracts by itself. The troublesome point of settlement displacement is often that the flue passes through the wall, floor, or is connected to large equipment such as dust collector and desulfurization tower. As soon as the equipment foundation sinks, the flue is forcibly deviated. The installation error is the most frustrating. After the on-site pipeline is welded, it is found that it is out of alignment, and it is forcibly pulled and aligned. As a result, the internal stress is all suppressed on the flange.

The properties of these three displacements are completely different. Thermal displacement has a rule to follow, and settlement displacement depends on geological reports and structural settlement observation data. Installation error belongs to man-made problem. You haven't even figured out what kind of displacement it is, and the rest of the compensation scheme is all useless.

How to calculate the displacement amount? Don't slap your head, look at these three numbers first

Calculating the displacement is not about slapping your head to estimate a "almost". Look at the direction of the pipe first. The longer the straight pipe section, the greater the thermal elongation. The calculation formula is Δ L = α × L × Δ T. The linear expansion coefficient α of carbon steel is 0.012mm/ (m·℃), and the flue gas temperature of a 30-meter flue rises from 20℃ to 200℃, and the elongation is calculated to be about 65mm.

Look at the temperature again-there is a pit here. Many people directly substitute the flue gas temperature, ignoring one detail: there is an insulation layer on the outer wall of the flue, and the actual wall temperature is much lower than the flue gas temperature. Whether the wall temperature or the flue gas temperature should be taken during design is directly related to the size of the expansion joint selection. Finally, look at the position of the fixed bracket. The fixed bracket divides the pipeline into independent compensation sections, and the elongation of each pipe section is the amount that the expansion joint needs to absorb.

It is useless for you to only give a total displacement, you have to tell the manufacturer which section is right for which section. The expansion joint is installed between point A and point B. It only absorbs the thermal displacement of the section A-B, and it can't control the other sections.

What to Absorb Displacement: Don't Think of Expansion Joints as Magic Balm

When it comes to choosing a compensator, I am most afraid of a "general" model conquering the world. The flue is mostly rectangular with low pressure, high temperature and large displacement, so it is the first choiceMetal rectangular expansion jointOrRectangular non-metallic expansion joint。

The advantage of non-metallic expansion joint (fabric fiber expansion joint) lies in the large amount of compensation displacement, the transverse displacement of tens of millimeters can be achieved in a single wave, and it is resistant to high temperature and corrosion. The wet and acid working condition of desulfurization flue is particularly suitable. Metal rectangular expansion joints are more suitable for high temperature and need to bear certain pressure occasions, with strong pressure bearing capacity, but relatively small displacement compensation ability.

Some people say it worksUniversal corrugated expansion jointOkay, okay? No way. The circular bellows is fine for use in circular pipelines. It is hard sleeved on the rectangular flue, and the stress is concentrated at the four corners, so the bellows can easily tear. The flue is the flue, don't use the idea of pipe compensator to trap it.

How does the flue gas baffle door cooperate with the expansion joint? The key is "don't hold back"

The function of the flue gas baffle door is to block or regulate the flue gas flow, and it cannot absorb displacement by itself. There are many problems on the spot, that is, both sides of the baffle door are directly welded, and the expansion joint is installed at the other end of the baffle door and the equipment. As a result, the baffle door becomes a fixed anchoring point, and all thermal displacements are pressed on the door frame. Over time, the door panel is deformed and stuck, and it is not closed tightly.

One side of the flue gas baffle door is provided with a fixing bracket, the other side is provided with a guide bracket, and the expansion joint is arranged on the side away from the baffle door. In this way, the force on the baffle door is minimized, and the expansion joint can work normally. Think about it, after all, the flapper door is a device that needs to be moved. If you sandwich it between two large displacements, isn't that causing trouble for yourself?

There is another detail that is easy to overlook —Round Flap Door (Double Seal)This type of equipment should be installed with sufficient maintenance space. Don't install the expansion joint against the baffle door in order to save land. When the bolts can't be twisted, it's too late to regret it.

Common displacement errors in construction: the fixed bracket is not done correctly, and everything is installed in vain

I have seen many construction sites, and the expansion joint is installed, and the selection is correct, but it leaks soon after running. When I removed it, the bellows was twisted out of shape. The reason is simple – the fixing bracket is not done right.

Fixing the bracket is not just finding a steel beam and welding it. What it does is hold the pipe firmly in place and allow the thermal displacement to go in the direction you intend. If the strength of the fixed bracket is not enough, as soon as the pipe is heated, the bracket deforms first, and the expansion joint is forced to absorb the displacement exceeding the design value, and over-compensation, the bellows will soon be fatigued and cracked.

The guide bracket is also particular. The guide bracket ensures that the pipe can only move along the axial direction and does not allow it to run blindly laterally. If the guide brackets are too spaced apart, the pipe will sag under the action of gravity, and the expansion joint bears a bending moment, and this injury is permanent.

And guess what? Some projects even welded the fixed bracket to the expansion joint and directly welded the compensator to death. Then why install expansion joints? It's better to seal it with a blind plate flange.

Selection and Installation Checklist: Leave Maintenance Space for the Project

After so many years of dealing with flue, summarize a few checking points, and you will basically not make a big mistake by following the inspection.

• First, check whether the design displacement direction matches the type of expansion joint: axial type for axial displacement, hinge type or transverse type for transverse displacement, and compound hinge type for angular displacement. If you choose the wrong type, change it directly.
• Check whether the position of the fixed bracket is consistent with the design drawings, whether the solder joint is firm, and whether it is damaged by subsequent construction.
• Verify that the direction of the sleeve in the expansion joint is consistent with the flow direction of the medium-this direction is wrong, the medium directly flushes the bellows, and it won't take long to wear out.
• See if there is any reserved maintenance space around the expansion joint, and whether the outer protective cover of the bellows is pressed down by the insulation layer.
• In the wet smoke environment of non-metallic expansion joints, it is necessary to check whether the skin is aging and cracked, and whether there is condensate water on the surface that penetrates into the insulation cotton.

In addition, when the expansion joint is installed, the tie rod or transportation fixture preset by the manufacturer must be removed after the pipeline hydraulic test and heat insulation are completed. If it is dismantled early, the installation displacement will be consumed in advance, and there will be no margin for trial operation. It is dismantled late, and the expansion joint will not work, which is no different from a rigid pipe.

Can the flue be displaced? It can be done, but first you have to distinguish the type clearly, calculate the displacement clearly, select the correct compensator, and make the bracket solid. The four links are interlocked, and if one is missing, there will be problems. Don't wait until a leak is repaired, when the rework cost will be enough for you to reinstall several sets of expansion joints.

When the customer called, he asked in the first sentence: What material is cheap and has good sealing for the expansion joint? To be honest, every time I hear this question, I have to ask first: What medium goes in your pipeline here, what is the temperature and what is the pressure? It wasn't an argument, it was a question that really couldn't be answered in one sentence.

As soon as I came up, I rushed to "the cheapest" to choose the material, and eight out of ten turned over the car behind. Let's break up several common materials and crush them into pieces. After reading it, you will know what to choose.

First, recognize these materials

The expansion joints on the market are roughly divided into these categories according to their materials.

• rubber compensator: Suitable for low pressure, normal temperature, large diameter pipeline, mainly depends on elastic deformation of rubber to absorb displacement, the price is generally the lowest.
• Non-metallic expansion joints (fabric fiber expansion joints): It is made of multi-layer fabric fiber composite, and its high temperature resistance is better than that of rubber. It is commonly used in low pressure and high temperature occasions such as flue gas and air duct.
• Universal corrugated expansion joint (metal): Typical stainless steel bellows structure, with strong pressure and temperature resistance, is the main force in complicated working conditions.
• PTFE compensatorAndPTFE-lined hose: PTFE material is resistant to strong corrosion, but the price is also expensive.

In addition to the above, there are various special products such as metal hoses, metal rectangular expansion joints, high-temperature axial expansion joints, etc. It is also to compensate for the displacement of the pipeline. The material is different, and the price can be several times or even ten times different. But the key is not who is cheap, but whether it is suitable or not.

Second, where is the price gap?

Under the same caliber and pressure level, the price ranking is roughly like this:

Rubber compensator cheapestNon-metallic expansion joints are second, general-purpose corrugated expansion joints are more expensive, and PTFE compensators and PTFE-lined hoses are the most expensive. The root reason for the expense lies in the material cost and manufacturing process. PTFE materials are difficult to form, and metal bellows need precision welding and heat treatment, so the price will naturally go up.

However, cheap does not equal a good deal. Rubber compensator does save money on normal temperature water, but if it is used to pass 130℃ steam, the life may be shortened from five years to five months, and the cost will be higher after repeated replacement. Don't just focus on the purchase unit price, the comprehensive life and replacement frequency are reliable.

Third, what does sealing depend on?

The sealing performance is good or bad, the material is the foundation, but the structural design is the key.

The non-metallic expansion joint is compounded by multi-layer fabric fibers and sealed by flange compression. This structure performs well on low-pressure flue gas pipes, but if you force it on high-pressure pipes, the fiber layer will blow through, which is embarrassing.

The medium is guided away to reduce the direct erosion of the fluid to the inside of the bellows and reduce the risk of the bellows being worn out. Think about it, once the bellows breaks and leaks, the whole pipe suffers.

One more word, likeDouble-sealed single-axis circular baffle doorThe sealing effect of this kind of product with double seal structure is much more stable than that of ordinary single seal, which is of great significance to the strict air duct or flue gas system.

Fourth, how to choose?

If the large-diameter pipeline with low pressure and normal temperature is not afraid of corrosion,rubber compensatorOrNon-metallic expansion jointThe price/performance ratio is really high, the price is cheap, and the sealing performance is sufficient.

If the temperature and pressure are high and the medium is corrosive, don't save this money. Choose honestlyUniversal corrugated expansion jointOr simply onPTFE compensator。 Although the initial investment is large, the seal is reliable and durable. For example, the steam pipelines in the power station industry and the kiln head and kiln tail air ducts in the cement industry have very bad working conditions, and the requirements for sealing and durability are extremely high. Generally, metal corrugated expansion joints or high-temperature axial expansion joints with corresponding working conditions are directly used.

Some working conditions are in the middle, such as the flue gas pipeline of desulfurization system, which is not high in temperature but corrosive, so it has to be comprehensively considered. The corrosion resistance of PTFE is indeed good, but if the diameter is large, the non-metallic expansion joint is matched with a suitable anticorrosive material lining, which is more economical and practical. Therefore, there is no universal answer to the selection, and it must be determined according to the working conditions one by one.

Don't use a rubber compensator to top high-temperature steam。 If there is such a plan to save money, it will leak in a few days. When the time comes, the loss of shutdown for maintenance will be enough to buy hundreds of compensators.

5. Finally, pour cold water on a basin

Whether the seal is good or not depends half on the installation. This point goes to the root.

The flange bolts must be tightened evenly and alternately diagonally, and the other side cannot be tightened after one side is screwed to death. The pipeline cannot be biased, and the pre-deformation of the expansion joint must be done according to the requirements given by the manufacturer, so do it right.

Two days ago, I met a customer who said that the expansion joint I bought was leaking, so let us pay for it. As a result, after the sales visit, the bellows was burned by welding slag during installation. Did you say that this matter was caused by material? Not. What's even more outrageous is that some people hoist them with wire ropes directly on the bellows, and the good products are just scrapped.

No matter how expensive the material is, if the installation is not standardized, it will still leak. Save what don't save installation, don't make do with crooked pipelines. Rework when something goes wrong will not only spend more money, but also delay production. Many customers come to the factory to discuss explanations after leaking them. In all probability, the bellows was screwed or bruised during installation, which is not a matter of material at all.

What material expansion joint is cheap and has good sealing performance? The answer is never on the price list, but in your own working condition parameters and installation quality. Know the materials, understand the working conditions, and then take the installation seriously, and the money is worth it.

Find out first: the expansion joint is fixed, is the pipeline fixed or the expansion joint itself?

Many customers came up and asked, "How to fix the metal expansion joint?", and then planned to hold the expansion joint body with a bracket. This is a mistake of principle. The expansion joint itself is a flexible element used to absorb the thermal displacement of the pipe. If you hold it tightly down, how can it deform? Doesn't that mean pretending for nothing?

What really should be fixed is the pipe. The pipes on both sides of the expansion joint need to be firmly locked by the fixed bracket, so that all the displacements caused by thermal expansion and contraction are concentrated in the "buffer zone" of the expansion joint. On the contrary, the expansion joint body cannot bear any external restraining force, it only needs to expand and contract freely along the displacement of the pipe. In other words, the fixed bracket holds the pipe, the guide bracket holds the direction, and the expansion joint only deforms.

How to arrange the fixed bracket and the guide bracket? Distance, force and position are all special

The layout principle of the fixed bracket, in one sentence: divide the pipe into several independent compensation pipe sections. There can only be at most one expansion joint in each pipe section, and the fixing bracket must be arranged at both ends of the expansion joint, and the distance should be close enough. Why? Because if the fixed bracket is too far from the expansion joint, the friction force of the middle pipeline itself and the blind plate force generated by the medium pressure will be applied to the expansion joint, which will cause the compensation amount to fail at least and the bellows to crack at worst.

Taking the common general-purpose corrugated expansion joint as an example, the distance between the first guide bracket and the end of the expansion joint should be controlled within 4 times the nominal diameter, and the distance between the second guide bracket and the first guide bracket should not exceed 14 times the nominal diameter. Intermediate guide brackets should be set at intervals at the back. The specific distance depends on the stiffness and allowable deflection of the pipe. This is not a head-slapping number, it comes from the pipe system stress analysis. CAESAR II is commonly used in engineering for checking, but you have to know at least these basic limits.

In terms of force, the fixed bracket should be able to withstand the blind plate force generated by the medium pressure, the friction force of the pipeline, the elastic reaction force of the expansion joint and the weight of the pipeline. Blind plate force is a big head. The higher the pressure and the larger the caliber, the greater the force. Therefore, large-diameter thick-walled expansion joints, fixed brackets of the level of corrugated expansion joints used in power station industry, and concrete foundations must be calculated by tons, so they must not be saved.

The tie rod/nut on the expansion joint should be removed or not during installation? How to tune it?

It's a high-frequency problem. The expansion joint leaves the factory with tie rods and nuts, mainly to prevent the bellows from being bruised and strained during transportation and hoisting. Once installed in place, the tie rod nut must be loosened to allow the expansion energy saver to deform freely. If you don't loosen it, the expansion joint will be locked, and the displacement of the pipe will be stuck in the system, and the flange, weld and equipment mouth will all suffer.

When will the tie rod be used? Two scenarios. First, if the expansion joint needs to bear the blind plate force generated by internal pressure, and no pressure balanced expansion joint is designed, the tie rod can be used as a restraining element, but note that the tie rod at this time is not for you to tighten, but is adjusted to the allowable displacement range. Second, the working conditions that require pre-stretching or pre-compression before installation are realized by the tie rod nut. The adjustment method is: first loosen the locking nut, rotate the adjustment nut to compress or stretch the bellows to the specified amount, and then tighten the locking nut. The specific pre-displacement depends on the difference between the installation temperature and the calculated temperature of the pipeline. Don't screw it blindly.

How to adjust the expansion joint tie rod nut, we specifically talked about it in our previous article, and here we emphasize the core-after the adjustment, the locking nut must be tight back, otherwise the vibration during operation will cause loosening and the tie rod will lose its function.

Taboo: Take the expansion joint as a pipe support and hanger. These wrong installation methods should be avoided

The most typical mistake I have seen is that someone welds the pipe bracket directly to the expansion joint bellows, or uses a pipe clamp to pick up the expansion joint body to bear the load. The wall thickness of corrugated pipe is usually only a few tenths to a few millimeters. If you use it as a hanger, who will perforate if it doesn't perforate?

Another type of error is that the guide bracket is installed in the wrong direction. The role of the guide bracket is to limit the radial displacement of the pipe and allow axial displacement. As a result, the snap ring of the guide bracket and the pipe were welded to death on the spot, or the gap was left too small. As soon as the pipe expanded by heat, the guide bracket became a fixed bracket, and the expansion joint was crushed to death. The correct method is: the inner diameter of the snap ring of the guide bracket should be slightly larger than the outer diameter of the pipe, leaving an axial sliding space, but the gap should not be too large, otherwise it will not play a guiding role.

In addition, when installing the expansion joint, don't forcibly use the expansion joint to compensate for the installation error in order to adjust the pipeline counterpart deviation. Corrugated pipe has high requirements for coaxiality, and if it is forcibly installed in misalignment, the local stress will increase exponentially, and it will not take long for fatigue cracking.

Differences in Fixation of Different Expansion Joint Types: What to Pay Attention to for Axial Type, Transverse Type and Pressure Balance Type

Different types of expansion joints, the arrangement logic of fixed brackets and guide brackets is different.

Axial expansion joints, such as general corrugated expansion joints and external pressure single axial expansion joints, mainly absorb axial displacement. The fixed brackets at both ends must firmly lock the pipeline, and the guide brackets must be arranged in the middle according to the aforementioned spacing to ensure that the expansion joint expands and contracts along the axis direction.

Transverse expansion joints, such as double-hinged transverse expansion joints and double-hinged expansion joints for air-cooled island vacuum pipelines, absorb lateral displacement through hinge or universal joint structures, but do not absorb axial displacement themselves. Such expansion joints usually need to be used in groups of two or three. The layout of the fixed bracket should especially consider the bending moment generated by lateral displacement, and the direction of the hinge pin should be consistent with the displacement direction, otherwise the hinge will be pinned and the expansion joint will be wasted.

Pressure balance expansion joints are even more interesting, such as straight pipe pressure balance expansion joints, curved pipe pressure balance expansion joints and compound straight pipe bypass pressure balance expansion joints. They come with their own balanced bellows, which can offset the blind plate force generated by internal pressure, so the force on the fixed bracket can be greatly reduced. However, the guide bracket still cannot be saved, because the balanced expansion joint is more sensitive to lateral displacement and angular displacement, and once deflection occurs, the balancing cavity is easily unstable.

All fixing brackets and guide brackets must be in place prior to installation of the expansion joint. During the pipeline hydraulic test, the fixed brackets at both ends of the expansion joint should bear the blind plate force under the test pressure, which is greater than normal operation, so the welds and anchor bolts of the brackets must be re-checked before the test.

If there is a problem with the metal expansion joint, don't rush to replace it with a new one. It is simple to change another, but the cost is high, the downtime is long, and it can actually be repaired in many cases. The point is-you have to figure out what kind of injury it is. Is the bellows fatigued? Or is it cracked by the media? Or was it strained by brutal operation during installation? These three situations are dealt with in completely different ways.

Two days ago, I met a customer who said that a general-purpose corrugated expansion joint on their steam pipeline was leaking. When I removed it, the corrugation was concave but not cracked. This situation does not need to be changed at all, it can be rectified with pressure correction. But if it is another kind-intergranular corrosion cracks appear on stainless steel bellows, then it is useless for you to use pressure correction. After the external shaping is completed, the internal cracks are still there, and they will still leak once pressurized. In this case, you have to repair welding, or directly replace the bellows section. So the first step is not hands-on, but judgment.

Three things to test before recovery. If one thing is missing, it will be in vain

After judging the type of damage, don't rush to get started. Do three things first, otherwise the cultivation will be in vain.

The first piece, wall thickness measurement.Use an ultrasonic thickness gauge to dot the peaks and valleys at multiple points. Don't only measure one or two points. The wall thickness of one circle of bellows may not be uniform, and media erosion and corrosion thinning are often concentrated on one side. Multi-point measurement can be used to find out the real situation. Wall thickness reduction by more than 10% of the design wall thickness? Then don't fix it, just change it.

The second piece, the hydraulic test.The test pressure is 1.5 times the design pressure, and the pressure is maintained for 10 minutes. At this time, my eyes were kept close to see if there was any leakage or residual deformation. Many expansion joints "look okay, but leak when used", that is, the hydraulic test failed.

Third, displacement review.Compare the factory pre-stretch amount, measure the actual length of the current expansion joint, calculate how much displacement it has absorbed, and whether there is any margin. If it exceeds the design compensation amount, it is useless for you to finish repairing the bellows. The thermal displacement of the pipe has not been digested at all, and it will be broken again soon after installation.

Deflector tube. Many people only look at the bellows and forget to check whether the guide tube is worn out by the medium erosion. Once the guide tube is worn out and the high-speed fluid directly washes the inner wall of the bellows, that is the real murderer. You look fine with the bellows on the outside, but the inside has been hollowed out.

Different structures, restoration methods vary greatly

Axial type, hinge type and pressure balance type have different structures, different failure modes and essential differences in recovery means.

The loss of elasticity of axial expansion joint is mostly due to the attenuation of bellows stiffness. How to recover? By pressing the bellows into a predetermined position by re-pre-deforming, that is, cold tightening, a portion of the elastic displacement capacity can be restored. To put it bluntly, it is to "re-teach" the bellows and make it remember its range of elasticity.

What about the double hinge transverse expansion joint? If the hinge pin is worn and the clearance becomes larger, the light reset is useless. Once the clearance between the pin and the bushing exceeds tolerance, the motion trajectory of the expansion joint is deviated. You pull it back in place, and it will deviate back once it runs. In this case, the pin shaft and bushing must be replaced, and the hinge clearance must be repaired to the design tolerance range before it is truly recovered.

Straight pipe pressure balance type expansion joints are the most troublesome. When the middle cylinder is deformed, you can't pull the bellows directly-that will completely mess up the force relationship between the balancing bellows and the working bellows. The correct way is to disassemble it, check the compression and rebound of the balanced bellows and the working bellows respectively, judge which side has a problem, and then deal with it in a targeted manner.

Field repair of weld leakage and bellows crack

Small cracks, length not exceeding one third of the circumferential wavelength, can be repaired with argon arc welding. Before welding, polish the crack out of the groove to expose fresh metal, and do coloring flaw detection after welding. Here's a detail:Avoid arcing on bellows crest。 The crest is where the stress is most concentrated, and starting an arc there is equivalent to planting a new crack source by hand. The grade of the welding rod must match the base metal. If the wrong welding rod is used in the stainless steel corrugated pipe, new cracks will appear immediately in the heat affected zone.

However, if the cracks appear in large pieces and groups, it means that there is something wrong with the material or medium. The most common culprit is chloride ion stress corrosion. At this time, even if you make up all the cracks, it is useless to restore them to the original material-the corrosion conditions haven't changed, and they cracked again not long after. In this case, either lining with a corrosion resistant layer or directly replacing it with a material resistant to chloride ion corrosion. Hard supplement, it can't be made up.

What about corrosion only occurring in local areas? You can cut the damaged bellows section, replace it with a new one, and butt it with a ring weld. However, it should be noted that the newly replaced bellows section must be completely consistent with the wave pitch, wave height, material grade and layer number of the original bellows, otherwise the stiffness will not match and the force will be uneven, and the newly replaced section will become the weakest point.

Performance verification after recovery, and pits for reloading

Finished fixing, don't rush to go online. First do the air tightness test at room temperature, and then use the working medium to do the withstand pressure test. The airtightness test is to check leakage, and the pressure test is to check strength. Each of the two checkpoints is indispensable.

When reloading, there are a few pits to avoid.

First, the tie rod nut must be loosened. The tie rod of the expansion joint is used for fixing during transportation and installation, and must be in a free state during operation. Many expansion joints are "restored and broken" because the user locks the tie rod too dead, turning the axial compensation into a rigid constraint-the force of thermal expansion and contraction is all held in the bellows, so it's weird if it's not bad. The correct way is to let the expansion joint be in a free state after recovery, and only use the tie rod when adjusting the cold tightness.

Second, check whether the fixing bracket and guide bracket are in correct position. If the fixed bracket is not fixed firmly and the guide bracket slips off, the expansion joint will bear additional force outside the design, and this force is continuous and repeated, which has a great influence on the life of the bellows. How to Recover Metal Expansion Joints? Repairing the bellows is only the first step, and if the systemic problem is not solved, it will still break again in the same place.

In the final analysis, the expansion energy saving can't be restored, depending on whether you can accurately judge the type of damage, repair it strictly according to the process, and eliminate the root cause of the damage. These three steps are in place, and the restored expansion joint will be used for another three to five years. If you want to save trouble, just weld and tighten the tie rod no matter what injury, then you might as well buy a new one.

Non-Metallic Expansion Joint Disadvantages? Don't wait until you leak to understand these pits. After more than ten years of pipeline compensation, I have seen too many projects stumble on non-metallic expansion joints-it is not that it is bad, but that many people only focus on its advantages of cheap and large displacement resistance, and throw its shortcomings behind them. Today, if you don't brag or black, explain these pits clearly one by one.

Pressure capacity is a flaw

The core structure of non-metallic expansion joints (fabric fiber expansion joints) is a fiber fabric composite layer plus a metal frame, which is subjected to pressure by flexible bands? Thinking too much. Its pressure-bearing skeleton is essentially a fiber cloth and sealing layer, not a rigid metal shell. When the pressure is slightly higher, the circle belt will bulge like blowing a balloon, and if it is higher, it will tear directly.

In practical applications, non-metallic expansion joints can usually only be used in pipeline systems below 0.1MPa, and many smoke and air duct working conditions even only have a micro positive pressure of several thousand Pa. Compared with the metal corrugated expansion joint, the stainless steel corrugated pipe can easily carry 1.6MPa, and the special corrugated expansion energy saving for high-pressure steam pipeline can achieve more than 10MPa. And guess what? Someone dared to install a non-metallic expansion joint on the compressed air line, but the loop belt burst after half an hour of turning on. Have you ever seen a pressure cooker seal with canvas? No, no.

Temperature resistance is not infinite

The limit of fabric fibers lies there. The long-term temperature resistance of conventional silica gel-coated glass fiber products is ≤250℃, fluororubber ≤200℃ and polytetrafluoroethylene ≤150℃. Even if it is compounded with high silicon oxygen or ceramic fiber, and the limit is about 1000℃, it is also a special customization, and the cost is high. If it runs at a temperature close to the upper limit for a long time, the fiber will gradually embrittle and powder, and finally leak.

Two days ago, I met a buddy from a cement factory. The outlet temperature of the high-temperature fan frequently rushed above 900℃, and there was no margin left in the design. The non-metallic ring belt was ablated like a ragged rag in less than a year, and the scene was terrible. Therefore, leave at least 20% of the temperature margin when selecting the model, and don't take the upper limit as a normal working condition.

Easy to corrode and wear? Sulfur, chlorine, and dust in the medium are all enemies

In scenes such as flue gas baffle door and desulfurization flue gas baffle door, the non-metallic belt is not faced with clean air, but a mixture containing SO₂, chloride ions and high humidity acidic gas. Once the acid penetrates into the fiber layer, it directly corrodes the internal metal frame and connecting bolts. Dusty airflow washes at high speed, and the surface of the ring belt is like sandpaper polishing, and it is worn through from the weak point first.

Surface peeling → fiber fracture → perforation leakage. In a desulfurization project, acid-resistant coating belt was not used in order to save money. Half a year later, the belt was rotten like honeycomb briquettes, the baffle door was not closed tightly, and the whole system had serious air leakage. If there is sulfur, chlorine and dust in the medium, you have to choose the corresponding corrosion-resistant and wear-resistant composite layer. Don't think that all non-metallic expansion joints are the same.

Installation and replacement are not that simple

Many people think that non-metallic expansion joints are flexible, so they can be installed casually. Wrong! Precisely because of its softness, it has more demanding requirements for pre-stretching and limiting than metal expansion joints. Rectangular non-metallic expansion joints and round non-metallic expansion joints must be adjusted according to the designed cold drawing value when installing-the cold drawing amount is not enough, there is no compensation margin in the hot state, and the ring belt is pulled and cracked; The amount of cold drawing is too large, and it is squeezed into wrinkles when it is cold, and it doesn't take long to wear out.

The limit screw and the transport fixture should not be disassembled blindly. Some on-site workers try to save trouble. As soon as they come up, they screw off all the limit bolts. As soon as the pipeline expands slightly, the ring belt directly tears open a big hole. After the installation is completed, the adjustment of the limit device should be adjusted, and the removal of the limit device should be removed according to the manufacturer's instructions. Flexibility is not casual, but more demanding.

Lifespan Anxiety: Exactly How Many Years Will It Last?

There is no uniform answer to this question, but it varies greatly. Clean air, temperature no more than 150℃, no corrosive medium, it is normal to use for 5 to 8 years; However, if the medium contains sulfur and dust, and the temperature fluctuates frequently, it is not uncommon to leak in 1 to 3 years. The key lies in selection and maintenance.

Select the material and structure of the ring belt according to the pressure, temperature and media composition during the selection, strictly pre-stretch and limit the position during installation, and regularly check the surface of the ring belt for wear, blister and hardening during operation. Don't wait for a leak to pat your thigh. At that time, you can only stop the furnace for emergency repair, and the loss is dozens of times more expensive than the expansion joint itself.

Where is there a perfect compensator? In high-pressure working conditions, metal corrugated expansion joints are honestly selected, and it is the turn of non-metal expansion joints to play in large displacement and low-pressure scenarios. Understanding the shortcomings is not to deny it, but to use it for a long time.