Specialized in manufacturing compensators, expansion joints, baffle doors

Nine times out of ten people in the dry plumbing business have dealt with expansion joints. But if you ask what the process principle of non-metallic compensator is, many people will be stunned for a moment, and then say vaguely: Isn't it just a piece of cloth sewn in a circle? Alas, if this is heard by the old master of dry craftsmanship, he can laugh angrily.

Non-metal compensators and metal expansion joints, who does whose work? The metal expansion joint absorbs the displacement by the deformation of the bellows, has high pressure resistance and long life, but it is sensitive to fatigue and has nothing to do with corrosive smoke. Non-metallic compensator is different, it relies on multi-layer flexible fabric to bear displacement, corrosion resistance, high temperature resistance, large compensation amount, especially suitable for large-diameter low-pressure pipeline. The non-metallic compensator can just catch the large displacement and corrosive medium that the metal expansion joint can't carry. The two are not a substitution relationship, but a division of labor relationship-metal work, non-metal really can't do it; Nonmetal work, metal really can't catch it.

Skin is not a piece of cloth: What role does each layer play in a multi-layered composite structure

The skin is the core of a non-metallic compensator, but it is never a single-layer structure. You go and take a look at the non-metallic expansion joint (fabric fiber expansion joint) of a regular manufacturer. The skin is at least four or five layers from the inside to the outside, and each does its own thing.

The innermost layer is a temperature-resistant layer, which directly contacts the medium, either silicone rubber glass fiber cloth or fluororubber composite cloth, which is responsible for one thing: blocking high-temperature smoke. In the middle is the insulation layer, usually with ceramic fiber blanket or glass fiber wool, to trap the heat and prevent it from jumping out. Further outside is the structural reinforcement layer, high-strength glass fiber cloth or polyester canvas, which bears the tensile force and ensures that the whole skin is not torn apart. The outermost layer is a weather-resistant layer, fluororubber or silicone rubber coating, which is anti-UV, anti-aging and rainwater erosion.

How to fit between layers? It's not just brushing glue and sticking it. There is a parameter called peel strength in the composite process. What range will be controlled by regular enterprises? There are test records before leaving the factory. You take a piece of skin that has been used for a year and tear it apart. If the layers are cleanly separated, then the composite pressure is not enough at the beginning, and the temperature is not in place.

Skeletons and frames: What makes soft connections hold up to the pressure? There's something special about it here

The skin itself is soft, so what does it rely on to keep its shape? By the skeleton. The frames of non-metallic compensators are generally divided into two types: one is a metal frame, welded with angle steel and channel steel, wrapped around the skin, and bearing the internal pressure thrust and moment; The other is an internal guide tube, which is inserted inside the pipeline to guide the flow direction of the medium and protect the skin from being directly washed by the airflow.

There must be a gap between the guide tube and the skin. Some people don't understand and think that the tighter you lean against it, the better the seal. On the contrary, the skin deforms when it works, and if the guide tube presses against it, it will wear out in a few movements. The size of the gap is particular, and it is calculated according to the displacement and medium temperature during design, not determined by slapping the forehead.

How to connect frame and skin? Most are compressed with strips and bolts. The spacing between bolts will not be too large, and it is easy to leak if it is too large; It's also not too small, too small to install troublesome. If you look at the four corners of the rectangular non-metallic expansion joint, that position is most prone to problems-there are many skin folds at the corners, and the pressing force is uneven. If you steal a little work, it will start to leak after a year and a half. Therefore, a better manufacturer will thicken the strip at the corner, or use an arc transition, which is a real skill in technology.

From blanking to molding: What are the key steps of the process of non-metallic compensator?

The blanking of non-metallic compensators is not difficult or simple. The round one is easy to handle, just expand it and count it as a fan; The rectangular one is troublesome. The four corners have to be cut, and the incision angle and overlap length are all particular. The material is not cut correctly, and the skin is full of pleats when it is loaded, and it won't take long to crack from fatigue.

In the molding stage, the most critical process is the press-fitting of skin and flange. Before pressing, a sealing strip should be placed on the flange surface, and then the skin should be placed on it, and the tension around it should be adjusted to be uniform, and then it should be gradually pressed with pressing strips and bolts. This "step-by-step" is important-you have to twist the bolts diagonally alternately and press them into place three to four times. If you press them too fast or too hard, the internal fibers of the skin will be unevenly stressed, and the local stretch will be overstretched, which will greatly reduce the life span.

What about welding? To say welding is also a key step. Should frame welds be fully welded or intermittently welded? Most manufacturers do intermittent welding, because the frame will deform during use, and full welding is easy to crack. However, the splicing corners of the frame must be fully welded, where the force is concentrated, and intermittent welding can't bear it. These details, the drawings won't tell you at all.

By the way, there is also the pressure test before leaving the factory. Pressure testing is not as simple as hitting it with an air pump to see if it leaks. According to JB/T 12235-2015 standard, non-metallic expansion joints should be tested for airtightness, and there are clear requirements for pressure holding time and test pressure. Some factories try to save trouble. Even if there is any bulge on the surface of the skin with the naked eye, it is done. What can this find out? The needle eyes and tiny holes in the inner fibrous layer are simply invisible to the naked eye.

Principle: Why is non-metallic compensator the first choice in power plant desulfurization and cement flue?

Say response to use. The temperature of the flue gas in the desulfurization system of the power plant is not extremely high, but it contains sulfur, chlorine and a large number of slurry particles, which is highly corrosive and abrasive. Metal Expansion Joints In this environment, bellows are quickly corroded and perforated. The skin of the non-metallic compensator is fluororubber composite glass fiber cloth, which is naturally acid-and alkali-resistant. In addition, the guide tube separates the airflow, and the use environment is bad. It can still last for three to five years.

What about the cement flue? Mainly due to large dust and large temperature fluctuations. The kiln tail flue gas can reach 400℃, and the temperature changes drastically when the machine is started and shut down. Thermal fatigue cracks of metal expansion joints are common under this working condition. The non-metallic compensator itself is flexible, and the thermal stress can't be transmitted at all. Moreover, the insulation layer of the skin can effectively insulate the temperature, and the frame temperature will not be too high.

There are also those installed next to the desulfurization flue gas baffle door at the flue outlet, which have a narrow space and complicated pipelines. The non-metallic compensator can be made into a rectangular and non-standard shape, which takes up little installation space and is convenient for maintenance. Would you like to try a different metal expansion joint? For such a large bellows assembly, there is simply not enough maintenance space.

To put it bluntly, the process principle of non-metallic compensator is not so mysterious, that is, the word "flexibility" is made to the extreme: the temperature, corrosion and displacement are all digested by multilayer composite materials, and then the pressure, erosion and structural stability problems are covered by metal frames and guide tubes. However, the more simple things sound, the more details hide the door. Those invisible press-fit tensions, compound temperatures, and bolt tightening sequences are the watershed that determines whether it will last for three or ten years.

When you purchased the non-metallic compensator, did you ask the manufacturer about the thermal aging data of each layer of material of the skin? If you haven't asked, remember to ask next time. With this question, half of the suppliers who only know how to fight prices can be screened out.

Who in the dry plumbing business hasn't seen a metal expansion joint? Universal corrugated expansion joint and high-temperature axial expansion joint, these metal parts are indeed the main force in steam and hot water pipelines. But if you put it on the smoke pipe, it will crack for you in less than three months. Why? Because of some working conditions, metal bellows are inherently unable to bear it. At this time, you have to rely on a non-metallic compensator-but don't rush to think of it as a "cheap stand-in", which solves the problem that metal parts can't handle at all.

High temperature and corrosion resistance: the natural enemy of metal bellows, a special session of fabric fibers

The temperature of the flue gas pipeline of thermal power plant, the tail of cement kiln and the original flue of desulfurization system always rises to hundreds of degrees, and the medium is full of sulfide and chloride ions. In this environment, pitting and stress corrosion cracking of metal bellows are common phenomena. If you change to 316L or 254SMO, the cost goes up, but the lifespan is still not good.

The same cannot be said for non-metallic expansion joints (also called fabric fiber expansion joints). Its flexible fabric layer is laminated layer by layer with high-temperature resistant materials such as ceramic fiber and glass fiber, which can steadily hold high-temperature smoke and reduce the surface temperature. A thermal power customer told me that the original metal expansion joint was replaced with a non-metal one once every six months, and there was no air leakage after three years of use.

Non-metallic expansion joint ≠ rubber compensator ≠ PTFE compensator. The temperature resistance of rubber compensator generally does not exceed 100℃, the polytetrafluoroethylene compensator has first-class corrosion resistance but weak pressure, while the energy saving of fabric fiber expansion can be carried above 1000℃, but it can hardly bear the pressure. Before selecting, distinguish whether your pipeline is high-temperature flue gas or corrosive liquid. Don't mix it.

Large displacement compensation and low rebound force: rectangular pipes can only rely on it

For pipes with large thermal expansion, metal compensators are not impossible, but you have to calculate how many waves and how long the guide tube is needed. What if I don't have enough space? Especially for rectangular smoke ducts, have you ever seen a metal bellows with a square section? Even if you do it, that stress distribution is enough for you to have a headache.

The non-metallic compensator has low stiffness, and a fabric loop can absorb tens of millimeters of multidimensional displacement, which can be digested in axial, transverse and angular directions. In the flue gas system of the flue duct of the power station and the tail of the cement kiln, the thermal expansion of the pipeline is so large that the metal compensator can't be put down at all, and the rectangular non-metallic expansion joint is easily handled by the multi-layer structure-the outer fabric holds the medium, the middle insulation layer insulates the heat, and the inner deflector protects the fabric from being washed by the airflow. Think about it, a rectangular air duct is several thousand millimeters wide and has an expansion of several hundred millimeters. How can metal parts compare with it?

One more word, the reaction force of the non-metallic compensator on the pipeline is extremely small, which means that the load of the bracket and the fixed point is small, and the civil foundation doesn't have to be so bulky, which can save a lot of money.

Vibration isolation, noise reduction and tightness: Invisible helpers for operational stability

Pipeline vibration and noise are pain points in the operation and maintenance of power plants and cement plants. When the fan started and the baffle door opened and closed, the metal compensator transmitted the vibration hard, and after a long time, the weld cracked and the flange loosened. The damping characteristics of non-metallic compensators are inherently superior. They can absorb vibration energy and suppress both noise and shock.

And the non-metallic compensator and the baffle door are natural partners. You look at the flue gas baffle door, round baffle door (double seal), electric plug-in insulation door of the desulfurization system, and the flexible connections on both sides of these devices are almost exclusively non-metallic expansion joints. Why? Because the baffle door needs a certain degree of freedom when it moves, the non-metallic compensator is soft and not stuck, and it can also ensure the sealing performance of the system, and the door frame will not be squeezed and deformed due to thermal expansion and contraction. If a metal compensator is used, the displacement when the door panel opens and closes will easily break the bellows.

Pits that are easy to step on when selecting models: Don't just look at the maximum compensation amount on the sample

The non-metallic compensator looks simple in structure, and there are many examples of rollover during model selection.

• Pressure rating: The fabric layer is not a steel plate, and the force it can withstand under positive pressure is limited. If the pressure exceeds 0.1MPa, you have to choose carefully. Use non-metals hard in high-pressure conditions, which is a joke about safety.
• Upper temperature limit: If the sample is written at 500℃, do you think everything will be fine? The medium temperature and the surface temperature of the fabric are two different things. If the thickness of the insulation layer is not correct, the outer surface will still be hot, and the inner fabric will still age.
• Media composition: There are acid and alkali components in the flue gas, the wrong choice of fabric material, and the corrosion is faster than metal. If it contains tar and dust particles, it is necessary to add a deflector plate and a purge port.
• Installation pre-deformation: Non-metallic expansion joints generally have preloading or preloading when they leave the factory. If the on-site installation can't match the temperature, they will be directly hard-installed, and the compensation amount will be completely wasted.

In addition, refer to the national standard JB/T 12235-2015 for non-metallic expansion joints. Don't just focus on the maximum compensation amount on the sample, but also the fatigue life and flange load have to be counted. Some manufacturers give samples that only write "maximum displacement", but that is the limit value, not the allowable value under fatigue life. You choose according to the limit value, and the fabric will crack within a few months of installing it. When the time comes, you will stop working and change parts, and the loss will be great.

To be honest

Non-metal compensators are not universal pieces-high-pressure steam pipes require high elastic reaction force. If you hard on non-metal, it will definitely pull your crotch. On the other hand, non-metals are given priority to pipelines and equipment connections with high temperature, large displacement, low pressure and corrosive media, and the right choice can save a lot of operation and maintenance costs. This is a multiple choice question, not a true or false question. Figure out the working conditions and calculate the parameters. Non-metallic compensators can save you not only money for spare parts, but also the awful time of being woken up by the phone in the middle of the night to deal with leaks.

Cold draw amount of metal expansion joint? Don't laugh, this is not a question that everyone can answer. When the master of dry pipeline installation sees "cold drawn 50mm" marked on the drawing, most of them will scratch their heads: What does this have to do with the compensation amount? Which way? What happens if you pull too much?

Let's get this straight first.

What is cold drawing amount: Understand the difference between it and compensation amount and pre-displacement

The compensation amount is the total maximum thermal displacement of the pipe from cold to hot, which is the design value and is the part of the deformation that the expansion joint is going to "eat". And what about the amount of cold pull? It is an artificial pre-tension (or pre-pressing) of the expansion joint in the opposite direction of thermal displacement during installation. To put it bluntly, it is to "advance" part of the thermal deformation in advance.

Pre-displacement is the broader term and includes cold drawing and cold pressing. Cold drawing is only the most common type of pre-displacement-for thermal expansion displacement of pipes. Don't be confused by these names: the amount of compensation is "to happen" and the amount of cold pull is "you did it ahead of time". One is the formula in the textbook, and the other is the wrench at the installation site.

Why Cold Draw: What is the true displacement of thermal expansion and contraction?

Some people think that the expansion joint is installed and it's done? Why do you pull it cold?

Settle the account for you. A section of 30-meter-long steam pipeline, the working temperature is 200℃, the ambient temperature during installation is 20℃, and the linear expansion coefficient of carbon steel is calculated according to 12×10⁻⁶/℃:

Δ L =12×10⁻⁶ ×30000mm × (200-20) =64.8mm

More than 63mm. If you install the bellows directly from its natural length, it will have to carry a compression of 65mm when it is hot. The wave pitch is pressed to the limit, and the fatigue life is directly cut in half. What about the cold pull half? 32mm pre-compression +32mm hot compression, the cycle amplitude is small, and the fatigue life is doubled up.

How to determine the amount of cold drawing: calculate step by step according to the temperature, length of pipe section and position of fixed bracket

Δ L = α × L × (T_Work-T_Environment)

α is the linear expansion coefficient (about 12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel), L is the length of the pipe section between the two fixed brackets, and T is the temperature difference. After calculating the total thermal displacement Δ L, 50% to 70% of Δ L is taken as the cold drawing amount, which is a commonly used "cold drawing ratio".

• Steam pipeline, temperature difference> 150℃, cold pull ratio 50% ~65%
• Hot water pipeline, temperature difference 80~150℃, cold pull ratio 60% ~70%
• Low temperature or temperature difference

Of course, this is only an estimation method. The project also depends on the position of the fixed bracket, the direction of the pipeline and the number of elbows. If there are several bends in the same pipe section, the thermal displacement is no longer pure axial, and the cold drawing amount has to be corrected according to the results of stress analysis software. You are marked with the amount of cold drawing on the drawing, and you are done with it, but you know how it came from, and you have a bottom in your heart when checking the calculation.

How to operate on site: pre-compression, tie rod, jack, measurement record

The operation process is not difficult to say, but the details are the worst.

In the first step, remove the expansion joint from the package and check that the set nut on the transport protection tie rod is loose. In the second step, the cold drawing direction is determined according to the drawing. When the pipe heats up, it extends, so the bellows is pre-compressed during installation; Pipes are pre-stretched if they are cold shrunk, such as cryogenic pipes.

The third step is practical execution. If the diameter is small and the cold drawing amount is less than 30mm, just screw the tie rod nut directly. If the diameter is large or the cold drawing amount exceeds 50mm, don't screw it hard. Use a jack and an auxiliary frame to push it. Two people on the side will tighten the flange bolts diagonally at the same time, so that the bellows will deform evenly, otherwise the wave pitch will leak later.

Step 4, record. Don't be too troublesome-ambient temperature, pipe wall temperature, length of corrugated pipe before cold drawing, length after cold drawing, actual cold drawing amount and number of adjustment turns of tie rod are all written into the construction record. Which link is wrong, these data are clues to find the cause.

Consequences of mistaken cold drawing amount: bellows instability, bracket exceeding limit, nozzle leakage

Two years ago, there was a project of thermal pipe network, the general-purpose corrugated expansion joint of DN600. During the installation, the workers didn't draw it cold, so they welded it directly. Three months into operation, the bellows cracked at the trough, and the steam blew out far away. When I removed it, I saw that the wave pitch had been pressed together, and the bellows was completely unstable. When the manufacturer sent an engineer to the scene, the first thing he asked was: Has the cold drawing been done?

Even if the amount of cold pull is too large, it won't work. The pre-compression is excessive, and the bellows is close to the yield limit in the cold state. Before it is put into production, the fatigue life has been burned by a large part. The axial force bearing by the fixed bracket will also exceed the design value, and there have been cases where the guide bracket has fallen off.

Bellows instability, bracket force exceeding limit, nozzle leakage-any one of these three things is a shutdown accident.

Cold Drawing Quantity and Matching of Different Types of Expansion Joints

Different structures of expansion joints have different concerns in cold drawing operation.

Universal corrugated expansion jointMost commonly, the structure is simple, just adjust the tie rod nut in place directly when cold drawing. Be careful not to install the deflector in the opposite direction, and the medium should flow in the direction of the arrow.

External pressure single axial expansion jointThe bellows is on the outside of the guide tube and has a protective sleeve on the outside. When cold drawing, you can't see the state of the bellows from the outside, only the scale mark on the outer cylinder. The advantage of this structure is that the bellows is not directly washed by the medium, but you must look at the marking line when pulling it in cold, and don't forget to lock the nut after pulling it in place.

Straight pipe pressure balanced expansion jointThe most particular. It has a balancing bellows and a working bellows inside, which relies on the self-balancing of internal pressure and thrust. If the cold drawing amount is only calculated according to the thermal displacement, and the force on the balance end is not considered, the actual displacement of the bellows after installation will not match the design value. It is suggested that when this model is cold drawn, the bellows on both sides should be pre-displaced at the same time, and only 20% ~30% of the cold drawing amount should be adjusted each time, and the operation should be alternated so that the pressure balancing system should always be in a symmetrical state.

AndCompound hinge transverse expansion jointIt absorbs lateral displacement by the change of hinge angle. There is no such thing as axial cold pulling, but the rotation direction of hinge should be found according to the drawings when installing, and the reason is similar.

Okay, that's it. The formula of this set of cold-pulled things is not complicated, but the difficulty lies in that someone has to be serious in every link. Next time you go to the construction site and see that the bellows is crooked before installation, don't be surprised, that may be the right cold drawing site.

Let's talk about the conclusion first: temperature compensation, let's talk about the score situation

Give the answer directly-whether temperature compensation is required for non-metal tensile non-destructive testing depends on what your testing purpose is. If you just want to compare the consistency of the quality of the same batch of products relatively, there is little difference between compensation or not; However, if you want to take the data measured at room temperature to check the design load and do safety assessment, the data will be wasted without temperature compensation.

This is not a big deal. After doing this for a long time, you will find that the dispute on the spot is often not whether there is a problem with the material itself, but that the testing conditions do not match the actual working conditions, and no one recognizes the data.

Tensile properties of non-metallic materials, how sensitive are they to temperature? — Starting with fabric fibers and rubber

Let's take the common things as an example. The tensile strength, elastic modulus and elongation at break of glass fiber cloth, fluororubber and silicone rubber coatings used in fabric fiber expansion joints are completely different at 20℃ and 150℃.

Take rubber as an example. As soon as the temperature rises, the movement of molecular segments intensifies, the hardness decreases, and the tensile strength drops considerably. Some formulas have a strength of only about 60% of normal temperature at 100℃. For materials such as PTFE (polytetrafluoroethylene), the fluctuation of mechanical properties caused by temperature changes cannot be ignored. It is hard and stiff at room temperature. As soon as the temperature approaches its upper limit of use, softening and creep will all come.

So you see, the mechanical properties of non-metallic materials are a curve that changes with temperature, not the single point at normal temperature.

What do standards say? Is there a clear answer in JB/T 12235-2015

JB/T 12235-2015 "Non-metallic Expansion Joint" standard, it has requirements for the tensile property test of laminated composite materials and non-metallic elastic materials. The standard specifies the test methods, such as sample preparation, loading rate and test environmental conditions. However, the standard does not give a ready-made correction formula for temperature compensation.

Why? Because there are many grades and mixed formulas of non-metallic materials, each manufacturer's fabric fiber and rubber system are different, so it can't be converted with a uniform coefficient. The standard is to give this initiative to the tester: when you measure, you have to record as much ambient temperature as it is-the implication is that you have to take responsibility for this temperature difference yourself.

To put it bluntly, the standard gives the methodological framework, but it doesn't answer the engineering judgment for you.

Actual scene: What should I do if the pipeline operating temperature vs the detection ambient temperature is 20℃?

Two days ago, I also met a customer. The rectangular non-metallic expansion joint used in their project had a designed operating temperature of 180℃. The on-site equipment was shut down for maintenance, removed and sent to the laboratory for tensile non-destructive testing. As soon as the laboratory air conditioner was turned on, the ambient temperature was 22℃. When the test report came out, the data was pretty, and the breaking strength was completely up to standard. But the problem is-180℃ and 22℃, the difference is 158℃.

The tensile strength measured at room temperature of 22℃ and the actual bearing capacity at high temperature of 180℃ are not the same at all. If you use this data to prove that this expansion joint still has enough safety margin at 180℃, then this report is not convincing. The difference of 20℃ is already obvious, not to mention this temperature difference of hundreds of degrees Celsius at every turn.

Therefore, the closer the detection conditions are to the real working conditions, the more the data can explain the problem. Can't do it? That also has to quantify the effect of temperature clearly, not when it doesn't exist.

In what cases can you not compensate? In what circumstances must compensation be made?

Let's start with the situation that you can't do it. The first is the factory comparative inspection of the same batch of products-everyone measures it together at room temperature, and the measured values are horizontally comparable. Anyway, the benchmarks are the same. The second is routine arrival acceptance. You mainly want to confirm that the product is not damaged in production and transportation, and whether there is any obvious deterioration. The benchmark judgment at room temperature is enough.

However, in the following cases, it is recommended to make temperature correction honestly:

• The design safety margin under high temperature working conditions is inferred by using the detection data, such as non-metallic expansion joints (fabric fiber expansion joints) or rubber compensators on flue ducts and desulfurization systems, and the operating temperature is generally above 100℃;
• In failure analysis, it is necessary to correspond the normal temperature detection data with the temperature working condition at the time of failure to find out the real reason;
• For life assessment or fatigue performance analysis, temperature directly affects the creep and relaxation behavior of materials;
• It involves horizontal comparison and selection and replacement between different suppliers and different material systems-the temperature response characteristics are different, and it is easy to choose wrong if you are not on the same benchmark.

There are also products such as PTFE-lined metal hoses and PTFE compensators. The mechanical attenuation of PTFE materials at high temperatures should not be underestimated. If the temperature change correction has been done or not, the results can be an order of magnitude worse.

Three implementable suggestions for inspectors and design institutes

First, the ambient temperature and the sample temperature must be marked on the test report. Don't underestimate this line of words. If there is a dispute, this is the basis. There is no compulsory unified temperature compensation formula in JB/T 12235-2015, so make the condition record work solid.

Second, if conditions permit, put the sample at the target operating temperature for pretreatment before testing, and then test it after constant temperature for a period of time. Even if it can't be completely isothermal loaded, it is a big step towards real working conditions.

Third, when the design institute selects non-metallic expansion joints, if the material testing data obtained is measured at room temperature, don't directly put it into the high-temperature working condition. The performance data of the material system at the corresponding temperature level should be requested from the manufacturer, or the calibration calculation after temperature correction should be requested. This is particularly critical for fabric fiber expansion joints and rubber compensators.

Back to the title question-does non-metallic tensile non-destructive require temperature compensation? The answer is not a simple "yes" or "no", but you have to clarify what the data is used for first. When used in the right place, room temperature data is also valuable; If you use the wrong scenario, no matter how beautiful the numbers are, you can't support the safety margin.

Find out what non-metallic compensator is first: compared with metal compensator, where is it stronger and weaker?

The core structure of non-metallic compensator, also called non-metallic expansion joint (fabric fiber expansion joint), is flexible fabric band, thermal insulation filler layer and metal frame. It is different from the metal corrugated compensator: metal absorbs displacement by the deformation of the corrugated thin wall, while non-metal absorbs displacement by the flexibility of the fabric band.

First, the amount of compensated displacement is large, especially the lateral and angular displacement, which is often several times that of metal; Second, the thrust is small, because the stiffness of the fabric loop is extremely low, and the load requirement on the pipe support is low; Third, it has good corrosion resistance and can resist corrosive media such as sulfide and chloride ions in flue gas. There are also shortcomings-poor pressure bearing capacity, which can generally only be used in low pressure or even slightly negative pressure conditions, and fabric belts are consumables, and their life is not as good as that of metal bellows.

Therefore, the use of non-metallic compensator is not to replace metal, but to make up the shortcomings of metal. The following working conditions are its home field.

Application 1: Large-diameter low-pressure pipes such as air ducts and flues of power plants can be easily compensated by rectangular cross sections

The air ducts and flues of power plants are often two meters, three meters or even larger in diameter, and many of them are rectangular in cross section. Metal corrugated compensator for this occasion? Material and molding costs are ridiculously high, and metal bellows are sensitive to cross-sectional shape, making rectangular bellows extremely difficult to manufacture.

Nonmetallic compensators don't have this concern. Its ring belt is flat, and it is made into a rectangular cross section by the frame. This is how rectangular non-metallic expansion joints come from. Large diameter and low pressure just avoid the short board of non-metal pressure difference, and give full play to its advantages of large compensation and small thrust.

And guess what? A rectangular flue of DN4000, the heat displacement may reach fifty or sixty millimeters, and the non-metallic compensator can easily absorb it. What about the metal ones? Honestly calculate the fatigue life.

Use 2: How can fabric fibers and rubber materials hold high temperature and corrosive smoke environment?

Flue gases and corrosive gases are another typical type of operating conditions. The temperature of the flue gas at the inlet of the desulfurization tower may reach 150℃, as well as corrosive components such as SO₂, SO₃ and HCl, and dust washing in some places.

The band material of the non-metallic compensator is a composite structure such as glass fiber, polytetrafluoroethylene and silicone rubber, which can withstand the temperature above 200℃, and its corrosion resistance is much better than that of metal. Metal bellows are prone to stress corrosion cracking in chloride ion environment, but non-metallic bands become more and more solid.

In addition, the insulation layer of the non-metallic compensator can be designed as a multi-layer structure, which can not only compensate for displacement, but also reduce heat loss. This kind of structural design flexibility, metal compensator can't compare.

Application 3: Vibration and noise reduction of equipment interface, and "fault tolerance" function of installation deviation

The inlet and outlet of rotating equipment such as pumps, fans and compressors can't avoid vibration. If the pipes are connected rigidly, vibrations are transmitted directly to the piping system, and both noise and stress are large.

The fabric band of the non-metallic compensator itself has damping effect, which can absorb part of high-frequency vibration. A non-metal compensator is installed at the outlet of the fan, and the vibration transmission rate can be reduced by an order of magnitude. Rubber compensator and rubber PTFE compensator are the same way, which is suitable for vibration reduction of fluid media pipelines.

There is also a particularly practical use – fault tolerance. During on-site installation, the pipeline alignment deviation is several millimeters or even more than ten millimeters, which is normal. Hard connections can't be installed, so soft connections are much more convenient. The non-metallic compensator can absorb the installation deviation, which saves the user the time of field adjustment, which is considerable.

Application 4: Special applications in desulfurization, dust removal and other systems, used with baffle doors and other equipment

Flue gas baffle door, electric plug-in type insulation door, manual plug-in type insulation door of desulfurization system, these devices will have thermal displacement in the pipeline when switching and starting and stopping. The baffle door body is rigid and cannot absorb displacement, so it must be digested by the peripheral compensator.

Here non-metallic compensators are pretty much the only option. The temperature of desulfurization flue gas is not high but it is extremely corrosive, and the cross section of pipeline is often rectangular, so the space is limited. The non-metal compensator can be made into a flat shape, connected on both sides of the baffle door, and used in cooperation with the desulfurization flue gas baffle door, which just meets the technical requirements.

The same goes for dusting systems. The fabric ring belt compensator at the inlet and outlet of the dust collector can not only compensate the displacement, but also block the vibration transmission and protect the body of the dust collector.

Pits to be avoided in model selection: pressure, temperature, displacement, none of which can be taken for granted

Finally, talk about selection. What are non-metallic compensators most afraid of? Stress. Some people think that the fabric loop is soft anyway, so the pressure should not matter, right? Wrong. Under positive pressure, the ring belt will bulge, and the skeleton will be subjected to additional pressure load, which will tear if the design is not good.

Temperature is also a red line. Silicone rubber bands and fluorine rubber bands have different temperature resistance grades, and they will age and crack if they exceed the upper limit. The displacement is more accurate, and the deformation of the ring belt must be controlled within the safe range, otherwise creep fatigue will shorten the life.

The usefulness of non-metallic compensators?In the final analysis, it is born for specific working conditions such as large diameter, low pressure, high corrosion, large displacement and vibration reduction. When selecting the model, give the working condition parameters to the manufacturer and let the engineer help you check it-don't take it for granted.