Specialized in manufacturing compensators, expansion joints, baffle doors

Let's talk about the temperature first: it's not just filling in a number

Two days ago, a customer came to me with a purchase order that said "temperature 200℃" and asked me if I could do it. I asked him back: Is this 200℃ the medium temperature or the design temperature? Steam pipes or thermal oil? Is it a continuous 200℃ run or an occasional flush? He was stunned for a moment and said, "This…the manufacturer asked me to fill it in."

And the result? Less than two months after installing the equipment, the bellows leaked. When disassembled, the material has creep cracks. The problem is not the manufacturer, but the "200℃"-he filled in the medium temperature, but the design temperature required for model selection is the highest temperature of the medium plus the safety margin, which is dozens of degrees different. How to write compensator selection temperature? If you really think you can just fill in the numbers, then there are pits behind.

First thing: The three temperatures must be clearly distinguished

Medium temperature, design temperature, and ambient temperature are three things, not the same thing.

Medium temperatureIt is the actual degree of fluid in the pipe, which is easy to understand.Design temperatureIt is the highest temperature that the compensator can bear. When selecting the model, the highest temperature of the medium plus the safety margin is taken-for example, the highest operating temperature of the steam pipeline is 250℃, and the design temperature is at least 280℃ or even 300℃.Ambient temperatureIt is the external environment. Outdoor pipelines are exposed to minus 20℃ in winter and 60℃ in summer, which affects the selection of insulation layer and material toughness.

Many people just write the medium temperature and are done. The design temperature is not written, the ambient temperature is not mentioned, and the manufacturer can only guess. If you guess right, you are lucky. If you guess wrong, whose equipment is scrapped?

The second thing: Metal and non-metal, the way to write temperature is two sets of logic

Metal corrugated expansion joints (such as general corrugated expansion joints and high-temperature axial expansion joints) absorb displacement by elastic deformation of corrugated shell, and temperature directly determines the allowable stress of material. 304/316L can hold up at room temperature, and above 400℃, the allowable stress is visible to the naked eye. At this time, either change the alloy material with higher temperature resistance, or increase the number of layers or wall thickness to compensate for the strength loss. Therefore, when selecting the metal compensator, the temperature should be clearly written as "maximum operating temperature", and whether there is an instantaneous overtemperature condition should be indicated-for example, when the steam pipe is heated, it may instantly rush above the design value, which directly affects the fatigue life.

The temperature resistance of non-metallic expansion joints (fabric fiber expansion joints, rectangular non-metallic expansion joints) depends on the skin material. Silicone coated glass fiber cloth, polytetrafluoroethylene film, ceramic fiber felt, each material has different temperature resistance upper limit: fluororubber can only reach around 200℃, and ceramic fiber can reach above 1000℃. Selecting and filling temperature of non-metallic expansion joint, you can't just write an average value. You have to write clearly the sustained temperature, peak temperature and duration-the non-metallic skin burns through this thing, which is often caused by instantaneous overtemperature, not slow aging. It looks reasonable for you to write "350℃". In actual working conditions, the bypass rushes to 600℃ for half an hour, and the skin burns through directly. Whose do you think this belongs?

Third thing: Temperature and pressure are bound, don't disassemble and write

The same general-purpose corrugated expansion joint can withstand 2.5MPa at room temperature, and may not even be able to withstand 1.0MPa at 400℃. When the temperature is high, the yield strength of the material decreases and the creep rate accelerates. Therefore, the temperature and pressure in the selection parameter table must be paired: "200℃/1.6MPa" and "400℃/0.6MPa"-this is called working condition pair, not two separate numbers. If only one temperature is written without the corresponding pressure, the manufacturer can't calculate the fatigue life and stability at all.

And the temperature cycle frequency. Alternating hot and cold several times a day is completely different from starting and stopping only once a year, and the fatigue life calculation is completely different. Each time the steam pipeline starts and stops is a temperature cycle, and each cycle of the bellows consumes a fatigue life. When writing the temperature, write the working system clearly: continuous operation or intermittent operation, and start and stop several times a day. It's much more important than struggling with those few degrees of error.

Fourth thing: Case is straightforward than theory

Some time ago, a cement factory purchased a non-metallic expansion joint for the flue gas pipeline. The working condition table wrote "temperature 350℃", which looked fine. I asked: Is there an instantaneous high temperature in the kiln tail flue gas? He said that yes, when the kiln is shut down for maintenance, the bypass is opened, and the flue gas temperature can rush to 600℃ for about half an hour.

That's the key message. The material cost difference between the skin selected according to 350℃ continuous and 600℃ instantaneous working conditions is more than double that of the material selected according to 600℃ continuous working conditions. If the communication is not in place, the manufacturer will continue to design according to 600℃, and you will spend 600℃ money; The manufacturer designs according to 350℃, and what you get is 350℃ goods, which will be burned through as soon as the bypass is opened.

Practical suggestion: Write this in the temperature column of the selection single

Don't bother, just fill in these:

• Medium temperature range: xx℃ ~ xx℃
• Design temperature: xx℃
• Instantaneous maximum temperature: xx℃/duration xx minutes
• Ambient temperature: xx℃
• With or without insulation: Yes/No

If you are not sure, ask the manufacturer directly, explain the working conditions thoroughly, and let the other party help you judge. Compensator is a waste of money if you choose it big, and a safety hazard if you choose it small. Only when the temperature parameters are written accurately can the equipment be installed and used for a long time. How to write compensator selection temperature? The answer is four words: write the whole working condition.

Find out one thing first: Does big thermal expansion test the bellows or the frame?

When many people talk about expansion joints, their eyes are fixed on the bellows-how good the material is, how much the wavenumber is, and how much the temperature resistance is. But have you ever thought that the bellows are cracked and crushed. In many cases, it is not the bellows itself, but the frame can't hold it first. If the frame is not hard enough, the bellows will be completely messed up under the force, and no matter how good the stainless steel bellows is, it will be in vain.

To put it bluntly, in the face of great heat expansion, the frame is the one who carries the matter. The bellows is responsible for deformation and absorption of displacement, and the frame is responsible for restraining the messy forces such as blind plate force, internal pressure thrust and thermal stress. The frame broke down, and the bellows were the scapegoat.

Three mainstream choices of profile frames: carbon steel, stainless steel and non-metal composite frames, each with its own destiny

Carbon steel, stainless steel, non-metal composite. Carbon steel is cheap and high in strength, but its temperature resistance is limited. If it exceeds 350℃, alloy steel or lining with heat insulation layer must be applied. Stainless steel is good in temperature resistance and corrosion resistance, but it is expensive, and the stiffness will drop at high temperature, depending on the specific grade. What about non-metallic composite frames? Light weight, strong corrosion resistance, but weak pressure bearing capacity, generally used in low pressure and large cross-section smoke duct occasions.

Choosing a frame is not the more expensive the better, but to see who can hold the bottom line at what temperature and load. When you take the carbon steel frame to carry the smoke at 800℃, it will be oxidized and peeled in two days, the stiffness will plummet, and the bellows will suffer.

Take the power station and cement industry to talk about things: How to choose the frame of high-temperature axial expansion joint and metal rectangular expansion joint

The flue at the tail of power station boilers and cement kilns can expand by tens or even hundreds of millimeters. This kind of place is commonly usedHigh temperature axial expansion joint, the bellows are Inconel or high nickel, but what about the frame? Many design institutes only give a Q235B channel steel, as long as it is strong enough. And the result? After half a year of operation, the frame was deformed, the bellows was forcibly twisted and pulled, and the weld cracked.

Metallic rectangular expansion joints in the cement industry are more typical. The stiffness of the rectangular section itself is poor. If the frame is not reinforced by box girders or stiffened plates, the out-of-plane deformation will directly press on the bellows as soon as the internal pressure comes up. Two days ago, I met the owner of a cement factory, saying that their flue expansion joint had changed the bellows three times a year, and then it was replaced with an integral stainless steel frame with reinforcing ribs, and there was no accident.

What happens if the frame is not stiff enough? Measured data tell you the common reasons why bellows are cracked

The axial expansion joint of DN500 has a design pressure of 0.25MPa and a temperature of 600℃. The frame is made of No. 14 channel steel and the bellows stiffness is 120N/mm. During operation, the thermal displacement is 60mm, and the lateral deformation of the frame reaches 8mm. As a result, the actual bending stress of the bellows is 40% higher than the design value-the fatigue life is directly cut by half. There are a lot of cases in the air duct and desulfurization pipeline of power stations.

The bellows were pulled and cracked, and eight out of ten were caused by frame deformation. You think that the quality of the bellows is not good, but in fact, the stiffness of your frame is not enough. During the design, only the strength was checked, but not the stiffness. When the frame is bent in the hot state, the bellows is not pure axial compression, but the composite force of tension + bending + torsion. Who can bear it?

Don't just look at temperature resistance, but also look at the restraint mode-tie rods, hinges, pressure balance, and the frame structure changes accordingly

What material to choose for the frame is one thing, the structural form is another. Tie rod expansion joint, the frame to bear the reaction force from the tie rod, then you have to calculate the buckling of the tie rod and the strength of the connecting ear plate. Hinged type, hinge seat must be installed on the frame, the force points are more concentrated, and the local stiffness requirements are higher.

The pressure balance type is more complex, such asStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion jointThe forces of the middle bellows and the balanced bellows should be self-balanced through the frame. If the frame stiffness of this structure is insufficient, the bellows are involved with each other, and it is easy to become unstable. Do you think the frame is nothing more than a few steel plates welded together? When it was actually designed, it was full of doorways.

Type selection suggestions: What working conditions use carbon steel frame and what working conditions must use stainless steel or non-metal frame

The flue gas temperature is lower than 350℃, the pressure is not high, the medium is not corrosive, and the carbon steel frame is enough, but remember to add coating or temperature-resistant paint. If the temperature exceeds 400℃, or there is sulfide corrosion (such as the flue behind the desulfurization flue gas baffle door), don't hesitate to use a stainless steel frame, at least 304L, and 316L or higher for the high-temperature section.

For large-diameter rectangular flue, such as cement kiln tail and power station induced draft fan outlet,Rectangular non-metallic expansion jointIt was more appropriate at this time. The non-metallic expansion joint itself absorbs the displacement by fabric fibers. The frame is made of non-metallic composite materials or carbon steel frames lined with stainless steel, which is light in weight and corrosion resistant. The key is that the cost is much lower than that of all stainless steel.

Let's talk moreMetal rectangular expansion jointThis kind of dust collector used in the import and export of power plants, the frame must be made of reinforcing ribs, and it is best to weld hot-rolled steel into an integral frame, instead of welded thin-walled steel plates. Don't ask me how I know – I've seen too many cases of rectangular corrugated plates tearing due to insufficient frame rigidity.

So, back to the question at the beginning: What profile frame can eat big heat expansion? The answer is: a frame with sufficient rigidity, right material and smart structure. Don't just focus on the bellows. If you choose the right frame, the expansion joint can be really worry-free. Next time a manufacturer blows to you that "our bellows use imported materials", you should ask first: What materials do you use for your frame? How do you check the stiffness? If he can't answer it, you know it.

The expansion joint leaks, which is upsetting to watch and more troublesome to deal with. But don't rush to dismantle it, first find out where the water came from. The location of the leakage point is different, and the reason is completely different from the modification of the method.

First distinguish the leakage point: bellows, weld, flange or connection

Get a leaky expansion joint and turn around it first. Water stains, rust, crystalline salt, will help you guide the way. There are only four common leakage points: bellows body, circumferential weld, flange sealing surface, and the connection between pipe and pipe.

Bellows leaks, usually with pinholes, cracks, or corrosion pits in the walls. Weld leakage mostly occurs on longitudinal seams or annular seams. If you look closely, you can find pores and unfused. Flange leaks, usually at the gasket position, with wet marks or dripping. As for the leakage of the pipe, it is often where the pipe butts with the expansion joint, and there is a problem with the weld or thread. And guess what? Many users report for repair and say that "the expansion joint is leaking", which is actually a flange leak, which has nothing to do with the half dime of the bellows.

Corrugated pipe leakage: corrosion, fatigue and stress corrosion cracking are the main causes

The bellows is the heart of the metal expansion joint. If it leaks, the problem is usually more serious. Three conditions are the most common.

The first is corrosion. There are chloride ions, sulfide in the medium, or improper chemicals are used during cleaning, and the inner wall of the bellows will pitting and intergranular corrosion. 304 stainless steel is very fragile in chlorine-containing environment, so you have to change 316L or higher grade material at this time.

The second is fatigue. The pipeline expands and contracts repeatedly, and every time the bellows expands and contracts, the corrugated root bears alternating stress. The number of cycles is more, and the crack will initiate from the trough and slowly penetrate the wall thickness. In particular, it should be noted that the constraints underground are more complicated than those above ground, and the fatigue life may be much lower than the design value.

The third is stress corrosion cracking. If the tie rod nut is adjusted too tightly and forcibly matched during installation, the bellows will take on duty with extra stress. If you encounter corrosive media again, the crack will break through in a few days. Here is a data: the 304 bellows of the circulating water pipeline of a power plant leaked after less than eight months. After metallographic analysis, it shows transgranular cracks and typical stress corrosion. Later, it was replaced with 254SMO material resistant to chloride ions, and the problem was solved.

Leaky welds and flanges: improper installation, loose bolts and gasket failure

Weld leakage is most likely a problem of welding quality. In the ring seam between the bellows and the connecting pipe, if the current is too large during welding, the welding wire is damp, or the slag is not thoroughly cleaned, there will be pores and slag inclusions. This kind of leakage point, all defects must be dug out before repair welding, otherwise it will be repaired in vain.

Flange leakage, don't blame the gasket yet. Check the bolts first. Have you ever encountered this situation: tightening the bolts a few turns, it didn't leak at the time, and then it leaked again two days later? That's because the flange is unevenly stressed, one side is crushed to death, and the other side is still open. The correct way to do this is to tighten diagonally, adding to the specified torque in two or three times. Gasket failure should be distinguished whether it is aging, creep or corroded by the medium. The rubber gasket quickly hardens in front of high-temperature oil products, so it can be held by replacing it with a metal winding pad or a PTFE pad.

The expansion joint was in use, and the weld between the connector and the pipe cracked. This is often due to the lack of pipe frame support in the pipeline system, and the expansion joint is regarded as a force bearing member. The expansion joint compensates for displacement, it is not responsible for load-bearing. The addition of sliding brackets is different.

Non-metallic expansion joint water leakage: fabric layer aging, skeleton corrosion and lax sealing

Don't think that only the metal expansion joints will leak. The non-metallic expansion joint (fabric fiber expansion joint) leaks, and the precursor is more concealed. It generally has several layers of structure: fluororubber or silicone coated cloth as airtight layer, ceramic fiber as heat insulation layer, and the outermost is stainless steel wire mesh and skin. Water seeped out, indicating that the airtight layer was broken.

Airtight layer aging is a common occurrence. The flue gas temperature fluctuates greatly, and the coated cloth alternates hot and cold repeatedly, and it begins to become brittle and cracked in about two years. If you notice powdery shedding on the surface of non-metallic expansion joints, that's a sign of aging. At this time, when it rains again, water seeps in along the fibrous layer.

Don't ignore skeletal corrosion either. The frames and platens of non-metallic expansion joints, if stainless steel, will still rust in sulfur-containing flue gas. If the platen bolts are rusty and the skin can't be pressed tightly, water will seep from the edges. The treatment method is to clean the bolts regularly, apply anti-jamming agent, and change to high-strength alloy bolts if necessary.

The problem of lax sealing mostly lies in the installation link. The sealant between the skin and the flange is not evenly applied, or the wave shape of the pressure plate is wrong, which will leave gaps. When installing, it must be pressed tightly according to the manufacturer's torque sequence. Don't try to save trouble and only screw a few points.

Treatment plan and prevention: explain from type selection, material to installation and maintenance

Water leaks are not done once and for all by plugging them. You have to hold it down from the source.

In the selection stage, first see the working conditions clearly. What is the medium? What's the temperature? How stressful? Is it corrosive? If it is a flue gas pipeline, it has to use non-metallic expansion joints (fabric fiber expansion joints) to absorb three-dimensional displacement, which is resistant to high temperature and has large compensation. If it is a steam pipe, metal corrugated expansion joints are more suitable, such as general purpose corrugated expansion joints or high temperature axial expansion joints, which can withstand higher pressures and temperatures. Wrong choice, it is only a matter of time before the water leaks.

In terms of material, for media with high chloride ion content, don't use 304, use 316L or 254SMO. When the temperature exceeds 400℃, stainless steel bellows should consider high-temperature creep. At this time, heat-resistant alloy can be selected. If the medium is particularly dirty and there are many particles, add a guide tube to avoid direct flushing of the bellows.

In terms of installation and maintenance, remember three things: First, the expansion joint tie rod nut, adjust it according to the requirements after installation, some need to be removed, some need to be loosened, don't screw it. Second, the flange connection should be symmetrically tightened and the moment should be uniform. The third is to make regular inspections, touch your hands and see, and find rust spots or wet marks, so deal with them as soon as possible. Small leaks do not make up, big leaks suffer.

In case of urgent work and no suitable spare parts on hand, can you temporarily block it? You can use pipe hoops and rubber plates to hoop the leakage point of the bellows first, but that is only an emergency and can't last for a month. What is really reliable is to replace the leaky expansion joint. Even if it is just a weld leak, it is recommended to replace it as a whole. Welding quality is such a matter, the site conditions are difficult to guarantee, and the risk of repair is much greater than that of replacing it with a new one.

Expansion joint leaks are no trivial matter and involve pipeline safety and production continuity. Distinguish the leakage points, see the working conditions clearly, choose the right products, and standardize the installation. After this set of combination boxing is played down, the probability of water leakage can be reduced by more than half. If you are annoyed by this matter, go through the above steps first, and there is a high probability that you can find out the root cause of the disease.

Are metal compensators pipe fittings? Not. But if you go around the construction site, eight out of ten purchasers will buy it as a pipe fitting. Why? It looks like it-with flanges at both ends and a stainless steel bellows in the middle. When welded to the pipe, it looks like an elbow and a tee. But if you really choose the type according to the idea of pipe fittings, trouble will come.

Pipe fittings do "connect" and compensators do "compensate"

Let's talk about what pipe fittings are first. Elbow, tee, four-way, flange, blind plate, these things do the work of connecting and changing direction. They are rigid parts, and the shape they are when installed is whatever, and the thermal expansion and contraction of pipes have nothing to do with them. What about metal compensators? Its serious name is corrugated expansion joint, which belongs to the flexible element of pipeline. Its core function is to absorb thermal displacement, reduce vibration and noise. You can see from the product classification of this site that general-purpose corrugated expansion joints, external pressure single axial expansion joints and straight pipe pressure balance expansion joints are all under the category of compensator, and they don't go along with pipe fittings at all.

To put it bluntly, the pipe fitting is rigid connection, the compensator is flexible compensation, and the force in the pipeline is completely different. You weld a compensator at the elbow and want it to absorb the displacement that the elbow can't absorb? The direction is reversed.

Why do people keep confusing them?

On the one hand, the metal compensator does have connectors and flanges at both ends, and the installation method is almost the same as the pipe fitting-welded, bolted, and done. On the other hand, some manufacturers themselves scream, mixing "metal compensator" with "metal hose". This station is clearly distinguished: metal hose and PTFE-lined metal hose are specially designed to absorb vibration and lateral displacement; Whether the compensator is a general-purpose type or a high-temperature axial type, it mainly carries axial displacement. The two things look alike, but the functional emphasis is completely different. Confused, the selection straight away.

Two days ago, a customer came to me with the drawing of a metal hose and asked me if it could be used as an expansion joint. I said that if there is axial thermal displacement at your installation position, the hose can't hold it at all, and the corrugated pipe will become unstable as soon as it is compressed. Only then did he realize that he had taken the two things seriously.

Ask Yourself Three Questions Before Choosing a Model

Ordinary pipe fittings have no displacement compensation ability, which is the most essential difference between it and compensator. Metal compensators are divided into particularly fine displacement types: axial type, transverse type, hinge type and pressure balance type, each with its own applicable scenarios. When selecting, ask yourself three questions first: How much thermal displacement does the pipe go? What is the pressure rating? What is the medium?

Take an example. The steam pipeline has high temperature and large expansion amount, so it is necessary to use high-temperature axial expansion joint or external pressure single axial expansion joint; If the space available in the middle of the pipe is limited, it is necessary to consider the double hinge transverse expansion joint or the curved pipe pressure balance expansion joint. If you only report the nominal diameter and pressure and let the manufacturer deliver the goods, it is equivalent to asking the doctor to prescribe the medicine based on his height, which is unreliable. The medium has to be made clear. If the corrosive medium should be lined with PTFE, it has to be lined with PTFE. Take ordinary stainless steel to carry it hard, and it will leak in less than two years.

The installation link can better see that it is not a pipe fitting

Pipe fittings are simple to install, welded or threaded, and the position is right. Compensators are different. Before installation, check the amount of pre-stretching or pre-compression. When hoisting, the bellows can't be stressed, let alone take it to bear the self-weight of the pipeline. For example, sleeve-type pipe expansion joints and rotary compensators, the installation requirements are stricter, and they are slightly sloppy, ranging from leakage to direct scrapping of bellows instability. You said it was a pipe fitting. How could there be so much attention to pipe fittings?

So, exactly how to buy it?

Back to the topic – are metal compensators pipe fittings? The answer is clear: no. The difference between it and pipe fittings is not only that its functions are different, but that the whole design logic is different. Pipe fittings are standard parts and can be chosen at will; The compensator is a non-standard customized part, and the compensation amount must be calculated according to the working conditions.

If you buy the compensator as a pipe fitting in the pipeline design stage, only look at the nominal diameter and pressure, and there is a high probability that something will go wrong. The correct way is to hand over the parameters of pipeline layout, medium temperature and installation position to the manufacturer, and let them help you calculate the compensation amount and choose the appropriate model. Each of the double hinge transverse expansion joints and straight pipe pressure balance expansion joints in this station is designed for specific working conditions. Just take a pipe fitting instead, and don't even think about it.

What are fabric expansion joints? I'll let you know the truth first

In the dry pipeline industry for more than ten years, every once in a while, someone asks with drawings: What is this thing? Why does it look like a canvas pocket? Guess what, that's what we're talking aboutfabric expansion joint, also called non-metallic expansion joint, fabric fiber expansion joint. To put it bluntly, it is a device that absorbs the thermal expansion, contraction and vibration displacement of the pipe by a ring of flexible fabric. You think of it as a "cartilage head" added in the middle of the pipe, and the pipe can twist wherever it wants, but the stress can't be transmitted, and the equipment is safe.

You don't look at the word "fabric" in its name, you think it's a rag. This set of things is used in high-temperature air ducts of power plants and cement plants, and it is more useful than metal bellows at critical moments. Don't believe it? Look down.

Take it apart and see: What is wrapped in this "cartilage head"?

Take a standard rectangular non-metallic expansion joint, for example. It is usually a four-layer structure, and each layer does a different job.

• Fabric band (skin)— The outermost layer was also the core. It is generally made of fluororubber, silicone rubber coated glass fiber cloth or polytetrafluoroethylene material, which is responsible for sealing and absorbing displacement. If this layer breaks, the whole expansion joint will be wasted.
• Thermal insulation-Close to the inside of the loop band and fill it with ceramic fiber felt or glass wool. The flue gas pipe is often four to five hundred degrees. Without its insulation, the rubber coating on the outside will melt in a few minutes.
• guide tube— — This thing is made of metal. It is inserted into the pipe to divert the airflow and prevent the high-temperature smoke from directly washing the fabric band. Don't underestimate it. Without the guide tube, the life of the loop can be shortened by half.
• Frame (flange)— — That is, the upper, lower, left and right four angle steel or channel steel welded frames, and the pipe flange is screwed to death with bolts. Frame strength determines how much pressure the expansion energy saving is subjected to.

Each floor is coordinated with the work. You asked if the insulation can be saved? I advise you not to save it, save the circle belt burning through, and change it once enough for you to buy ten layers of insulation cotton.

Compared with the metal corrugated expansion joint, what is the difference?

Since metal expansion joints are so strong, why do you have to use fabric expansion joints in flue gas ducts and air ducts? There's a deep doorway here.

The metal corrugated expansion joint absorbs displacement by the deformation of the bellows, but it has a fault: it is sensitive to fatigue life. The temperature fluctuates greatly in the flue gas pipeline, and there is acid-alkali corrosion. Once the metal corrugated pipe is cracked by fatigue, air leakage and ash leakage is a trivial matter, and shutdown for maintenance is a big trouble. Moreover, the displacement of metal pipes is limited, and metal bellows with large cross-sections such as rectangular air ducts are heavy and expensive to make.

The same cannot be said for fabric expansion joints. It compensates the displacement by the deformation of the flexible ring belt, which can move in three dimensions and absorb in axial and transverse directions. Moreover, it has great damping, can reduce vibration and noise, and its price is much cheaper than that of metal bellows of the same caliber. The disadvantage is that the pressure bearing capacity is poor, and it is generally used in air ducts and flues from several kPa to more than ten kPa. High pressure occasions? Then you should honestly use metal.

Which industries are using it? Tell me a few real scenarios

In the past few years, I ran the scene, and I saw these three places the most:

Boiler air duct of power plant。 The rectangular air duct that comes out of the boiler blower has high temperature and big vibration. What does the thermal displacement rely on to absorb? Commonly used non-metallic expansion joints. Especially the corrugated expansion joint used in the power station industry has been seen in both low-temperature and high-temperature sections, but the air duct inlet section is almost all fabric type.

Cement plant kiln head and kiln tail.The temperature of the flue gas from the rotary kiln can reach above 350℃, and the dust content is large. There are many metal corrugated expansion joints in the cement industry, but the section between the kiln tail and the dust collector is basically rectangular non-metallic expansion joints. The reason is simple: too much dust accumulates on the metal ripples, which will jam the ripples. The fabric band is not afraid of this, and the slight dust accumulation will fall off as soon as it is shaken.

Desulfurization system.The flue gas at the inlet and outlet of the desulfurization tower has high humidity, contains sulfide, and is extremely corrosive. At this time, candidate metal bellows, you have to keep an eye on the material, and the cost will rise. With fabric expansion joints, the outer layer of fluororubber and PTFE coating itself is corrosion-resistant, and the cost performance will come up.

There are also those flue gas baffle doors and desulfurization flue gas baffle doors next to, and fabric expansion joints are also liked at pipe connections. Why? When the baffle door expands and contracts, the pipes on both sides pull each other, and no soft connection has ripped the bolts early.

Focus on these five parameters in model selection, and don't be fooled by sales

Some sales come up and blow "our family has five layers of skin sandwich, the thickest in the world". You would be naive to listen to him. When choosing fabric expansion joints, just keep an eye on these five numbers:

• Diameter dimensions and profile.Rectangular or circular? What is the flange port size? This is not accurate, and the rest is all useless.
• Design displacement amount.Axial stretch, lateral offset, angular displacement, how many millimeters are each? How much displacement you give, how wide cloth you have to use for the loop belt, which directly determines the price.
• Medium temperature.The materials used at 150℃ and 500℃ are completely different things. Don't just look at the upper limit of "temperature resistance 800℃", but ask what is the long-term continuous working temperature.
• Pressure rating.The air duct is generally slightly positive pressure, but what in case there is negative pressure? If the negative pressure is large, the ring belt will deflate, and it will fail.
• Media composition.Does it contain sulfur? Does it contain dust? Are there any corrosive gases? This has the greatest impact on coating material selection.

These five parameters are not clear. After listening to the sales speech, you have a high probability of buying expensive or wrong.

Why do you have to change it after a few years of use? How to check daily?

To be honest, fabric expansion joints are consumables. Metal bellows can be used for more than ten years. In the environment of high temperature, dust and acid-alkali corrosion, the life span of fabric belts is generally three to five years. They run with high load every day, and they have been changed in two years. Don't expect it to be done once and for all. You have to leave room for replacement when designing it.

For daily inspection, we focus on three places: first, whether there are bulges, cracks and exposed fibers on the surface of the ring belt, second, whether the flange bolts are loose, and third, whether there is any abnormal noise and air leakage. You take a flashlight to shine on the inside of the ring belt. If you see that the color of the insulation layer is black and brittle, it means that it is running warm, and you have to arrange a replacement quickly.

There is also particular attention when changing. The old loop is removed, and the frame should be cleaned without burrs. When the new ring belt is installed, the bolts should be twisted diagonally and the force should be even, otherwise the force will deviate to one side, and the ring belt should be torn again.

In the final analysis, fabric expansion joints are not advanced technology, but they rely on those few layers of cloth to chew hard bones that metal expansion joints can't handle. The next time someone asks you, "What's a fabric expansion joint?", you tell him: That's the guy in the plumbing system who knows the best "make the sea brighter".