Specialized in manufacturing compensators, expansion joints, baffle doors

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".

1. Why are non-metallic expansion joints old and bad? — — Aging, scratching and delamination of fabric layer, first find out the type of damage before talking about repair

Non-metallic expansion joints (also called fabric fiber expansion joints) are widely used in flue gas, air duct and desulfurization systems. They do not rely on the deformation of metal corrugations to absorb displacement like metal corrugated expansion joints, but rely on multi-layer flexible fabrics and thermal insulation cotton to compensate. Bad methods are also different-there are only three common ones: aging and brittleness of the fabric layer, external scratches or wear, and delamination and bulging between layers. There is also a hidden one, which is that the internal insulation cotton has been blown out and hollowed out. On the surface, it looks unbroken, but in fact it has collapsed.

So before looking for a restoration unit, you should open it yourself and have a look. Even if it's just a hole, cut a small piece to see if the surface silicone coating is cracked or the fiberglass cloth underneath has been powdered. The type of damage determines the repair method: surface scratches can be repaired locally, but if the delamination range is large and the cotton layer collapses, it is not a question of whether to repair it or not, but the whole circle has to be replaced.

2. Repair or replace? — — Calculate material costs, downtime losses and residual life, and don't spend money with a slap on your head

Many users hear that it costs 20,000 yuan to replace it and 8,000 yuan to repair it, and they immediately make a decision to repair it. But have you calculated downtime costs? The shutdown loss of a kiln may be several thousand dollars an hour. It takes two or three days to repair a non-metallic expansion joint, dismantle the old, clean the dust, polish, lay up and heat close. If there are spare parts on site, it can be done in one day. Compare the two sides, which is cost-effective is not just the material cost.

Besides, the residual life has a good foundation. If this expansion joint has been used for five years, and the fabric layer is powdered in a large area, you spend 8,000 to finish it and last another year? Then it's better to just replace it with a new one. On the other hand, if only a small piece is rubbed on the outside and the inner layer is intact, then you spend some money to repair it, and it will last for another three to five years. This account should be settled clearly before deciding.

Third, what to look for in a restoration unit? — — It is not possible to do it with a sewing machine, it depends on the standard of non-metallic expansion joints, interlayer structure and field process

The non-metallic expansion joint looks like a cloth bag with flanges, but experts know that the interlayer structure is the lifeblood. Temperature-resistant layer, heat insulation layer, airtight layer and reinforcement layer, the material, thickness and overlap direction of each layer are particular. The national standard JB/T 12235-2015 clearly states that after repair, at least the displacement, pressure resistance and temperature resistance should be guaranteed not lower than the original design. But some restoration units have never even heard of this standard. Isn't it a blind delay to take an industrial sewing machine to fill the outer layer of cloth for you?

Therefore, when choosing a repair unit, the first thing is to see if he understands the standard. Another, ask him if he can provide the interlayer structure diagram. If he can't say it, he will block it as soon as possible. Also, you have to ask about the process-is the site cold glued or heat bonded? Is there any vulcanization equipment? What material is used for coating? These key points are cut corners and you can't tell at all.

4. Is on-site repair reliable? — — The complete process from demolition to heat-sealing and vulcanization, as well as common corner-cutting techniques

On-site repair is not impossible, but it depends on how to do it. The serious process should be: first set up a shelf (if it is a high-altitude pipeline), completely remove the old failure layer, and expose the metal flange surface; Then remove rust, clean up, and apply adhesive; Then lay layer by layer according to the original structure-first lay insulating cotton, then lay airtight layer, then lay reinforcing cloth, and finally lay surface weather-resistant layer; Each layer should be compacted and bubble removed, and the corners should be fixed with pressing strips and bolts; Finally do heat sealing or vulcanization treatment. It takes a day and a half for a skilled team to finish the whole process.

But how do some people cut corners? The most outrageous thing I have ever seen is to wrap the outside of the old non-metal expansion joint directly with a layer of silicone cloth, and not tear down the old one or clean it up. Just a rub of putty is done. There is also a work team, and the insulation cotton is not laid, so two layers of cloth are sewn on it. On the third day of starting the machine, the air leaks and whines. You ask why they don't come by the standards? Answer: Party A urged tightly. Squeezing? Then don't quote the price for the repair.

5. How to accept after repair? — — Dimensional tolerance, airtightness test and warranty period agreement, all of which must be missed

After the repair, you don't just sign and leave. You have to check and accept it. Prerequisite the size-the length of the expansion joint, the flange spacing and the parallelism of the flange surface. If these tolerances are exceeded, it will be stiff when installed, and the life will be directly reduced in half. Measure the airtightness again, use compressed air or smoke cakes, and focus on whether there is any air leakage at the corner strips.

The warranty period must be stated in the contract. The warranty period of non-metallic expansion joint repair is generally one year in the industry, and better units dare to write two years. If the other party says "warranty? We don't engage in warranty", then you don't even look at it, just change to the next one. In addition, in the maintenance report taken back, the name, thickness, overlap amount and batch number of each layer of materials must be clearly listed, so that future problems can be traced back.

Non-metallic expansion joint repair unit? Easy to find, more to make do with. You first clear out the above five things, and then ask the other person with questions. Only those who can answer and come up with a plan can talk about the next step. If you dare to quote without watching the scene, most likely you want to sign it first. Equipment maintenance, the most afraid thing is the three words "said yes", written clearly in black and white, is responsible for your own equipment.

What are the classifications of expansion joints? First distinguish between metals and non-metals

Many customers ask "What are the classifications of expansion joints?" In fact, the roughest way to divide them is to look at the materials-metal and non-metal. Metal expansion joints are seen most often, likeUniversal corrugated expansion joint、Corrugated expansion joint for power station industry、Large diameter thick wall expansion jointThe bellows inside are all stainless steel or alloy materials, which are hard-core in pressure and temperature resistance. The same cannot be said for non-metallic expansion joints, such asNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointThe main body is fiber fabric plus silicone coating, which is mainly used in the occasion of low pressure, large cross section and violent temperature fluctuation of smoke duct.

These two are not substitute relationships. Metal can carry pressure and temperature, but the reaction force is large when compensating displacement; Non-metal has good flexibility and corrosion resistance, but weak pressure bearing capacity. You have to take metal to do non-metal work, and the cost goes up; Taking non-metals to top the high temperature and high pressure is looking for trouble. If the material layer is not selected correctly, no matter how advanced the structure is, it will be useless.

According to the structure: the respective doorways of bellows, sleeve and rotary compensator

The structure determines how the expansion joint absorbs the displacement.Bellows typeIs the absolute mainstream, compensated by ripple deformation. There are many patterns in this, there are single, duplex, with hinges, and with pull rods. LikeCompound hinge transverse expansion joint、External pressure single axial expansion jointIt's all based on this logic.

Sleeve type pipe expansion jointIt relies on the relative sliding of the inner and outer sleeves to compensate the axial displacement, has simple structure and low cost, but the sealing performance is not as good as that of bellows, and is suitable for those pipelines with unclean medium and low pressure.Rotary compensatorIt is another way, which absorbs the axial displacement by converting the axial displacement into rotating motion through the cooperation of a pair of rotating cylinders in the bending section of the pipe. It is especially used in long-distance conveying of steam pipe network, with large compensation amount and small pressure drop.

When choosing a structure, it depends on the direction and space of the pipe. If there are many straight pipe sections, choose axial type; if there are elbows, consider hinge type or rotary compensator. Where the space is compact, the sleeve type is easy to install-it doesn't mean that the more advanced the more suitable, it is the last word that it can be installed and compensated in place.

According to the compensation function: axial, transverse, angular and pressure balance type

This level of classification is the real test of selection skills.

Axial typeThe compensator mainly absorbs the axial expansion and contraction of the pipe, such asUniversal corrugated expansion jointIt is a typical axial product. Several are arranged on straight pipe sections, so thermal expansion and contraction are not afraid.Transverse typeThe good thing is that it absorbs displacement perpendicular to the axis of the pipe,Compound hinge transverse expansion jointThat's what we do.Angular typeWell, with a combination of hinges, the pipe swings at a certain angle, likeDouble hinge expansion joint for air-cooled island vacuum pipelineIt is particularly useful in air-cooled islands with large displacements and multi-directional changes.

If the pressure in the pipeline is high and the blind plate is strong, the ordinary axial compensator will transmit the pressure thrust to the fixed support, which must be made as thick as the city wall. That's when you have to use itStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint。 Its principle is that a balancing bellows is built in, which eats the blind plate force itself, and the main fixing bracket basically does not bear extra thrust. There are more complicatedCompound straight pipe bypass pressure balanced expansion jointWhich is suitable for the working condition that requires a large compensation amount and does not want the bracket to be stressed. Do you think this classification is important or not? Wrong choice, pipeline vibration, bracket deformation, later is all trouble.

According to the working conditions of the industry: special goods such as power stations, cement and desulfurization

As soon as the industry working conditions come up, the expansion joint has to be "customized". Smoke air duct of power station boiler, high temperature, large cross section, generally usedMetal rectangular expansion jointOrNon-metallic expansion jointWhich not only absorbs thermal displacement but also reduces vibration. Of the cement industryMetal Corrugated Expansion Joints in Cement Industry, that has to be wear-resistant, the dust particles at the head and tail of the kiln are fast, and the material and wall thickness of the bellows have to be increased.

The desulfurization system is more special. The flue gas contains sulfide, and the temperature changes greatly. Ordinary metal bellows can't bear corrosion, so it is commonly usedPTFE compensatorOr a metal hose lined with PTFE, such asPTFE-lined hose。 Also matched with the expansion jointDesulfurization flue gas baffle door、Flue gas baffle doorThese valve equipment have to be considered together when selecting, otherwise the connection size will not match, and the scene will be blinded.

Quick locking categories? Look at these key parameters

Then how do you quickly judge which category to use? Don't rush, just three steps.

The first step is to look at the medium and temperature. High-temperature flue gas and corrosive gas, directly leaning in the direction of non-metal or tetrafluorine lining; High temperature and high pressure steam, honestly choose metal corrugated expansion joint.

The second step is to calculate the compensation amount. Is the axial displacement large? Is there any lateral displacement? If there are multiple directions, preference is given to hinged type or rotary compensators. Like a direct pipeline, then you have to goDirect buried (fully buried) type expansion jointThis goods is directly buried in the soil, and there is a protective sleeve on the outside, so you can't make do with ordinary expansion joints.

The third step is to look at the pipeline constraints. If the position of the bracket has been set, it is necessary to choose the pressure balance type according to the thrust that the bracket can bear. Two days ago, I met a customer who was doing a steam pipe network project. When I came up, I asked, "What are the classifications of expansion joints?" I asked him how to arrange the bracket, and he looked confused. You have to understand that classification is not just a difference in name, each structure corresponds to different force models and installation conditions.

The classification table looks simple, but there are many things involved in the actual selection. The four dimensions of material, structure, function and working condition are cross-combined, and dozens or hundreds of schemes can be discharged. But as long as you grasp the core-the temperature and pressure of the medium, the magnitude of the displacement direction, and the force constraints of the pipeline-you won't deviate. Understanding these, the expansion joint classification is no longer a table, but a selection map in your hand.

Find out first, what kind of equipment is the metal expansion joint?

Metal expansion joints, also called corrugated compensators, are flexible connectors mounted on pipes. It mainly relies on the deformation of the bellows to absorb the displacement and vibration caused by thermal expansion and contraction of the pipeline, so as to avoid the pipeline being cracked or broken. To put it bluntly, it just puts an elastic joint on the pipe.

Are metal expansion joints and corrugated compensators the same thing? Yes, and no. Say "yes" because the core of the vast majority of metal expansion joints is the bellows; Say "no" because there are members of the metal expansion joint family of non-corrugated structures such as sleeve types and rotary compensators. So you can think of corrugated compensators as the mainstream branch of metal expansion joints, but don't draw an equal sign.

Classified by displacement: axial, transverse, angular, you should know these faces

What types of metal expansion joints are included? The most commonly used division method is by absorbing displacement.

Axial typeExpansion and contraction of the absorbent pipe along the axis. Representative products are:

• Universal corrugated expansion joint: The most basic and widely used model, suitable for pipelines with ordinary temperature and pressure.
• High-temperature axial expansion joint: with heat insulation layer and high-temperature resistant material, it can serve hot air ducts above 500℃.
• External pressure single axial type expansion joint: Bellows bear external pressure, good stability, suitable for high pressure steam pipeline.
• Straight Pipe Pressure Balanced Expansion Joint: By balancing the bellows to offset the blind plate force, it is suitable for equipment with fixed ends and unable to bear pressure thrust.

Transverse typeAbsorbs displacement perpendicular to the axis of the pipe. The double hinge transverse expansion joint is typical-two hinge groups are used to restrain the bellows so that it only bends to the side and cannot twist around itself.

Angular typeAbsorbs the angular displacement at the bend of the pipe. Curved pipe pressure balance type expansion joint is a representative of this kind, which is mostly used in the corner of L-shaped or Z-shaped pipe.

Special needs according to working conditions: These "non-mainstream" models specialize in solving difficult problems

Some pipes are unusual, so they have to use special expansion joints.

Direct buried (fully buried) type expansion joint— — The heating pipeline directly buried underground cannot be equipped with an inspection well. The shell of this expansion joint should be made into a structure that can withstand soil pressure, and the interface must be waterproof. It is the only option when directly buried heating pipes are laid.

Sleeve type pipe expansion joint— — Rely on the relative sliding of the inner and outer sleeves to compensate the displacement, which is suitable for low-pressure large-diameter pipelines. The price is much cheaper than that of corrugated pipelines, but the sealing performance is a soft rib and needs regular maintenance.

Rotary compensator— — The displacement is absorbed by the rotation of a pair of rotating arms, and the compensation amount is large. It is especially suitable for long-distance direct burial laying of steam pipelines, which saves brackets compared with traditional expansion.

Large diameter thick wall expansion joint— — The diameter exceeds 1 meter and the wall thickness is increased, which is specially used in large-flow and low-pressure pipelines such as flue gas desulfurization and induced draft fan outlet of power station. Ordinary bellows are easily unstable under this working condition, so it is necessary to use reinforced design.

Does a metal hose count as a metal expansion joint? Close relatives, but don't mix them

This is the easiest pit to step on when selecting a model. Metal hoses and metal expansion joints look similar and have similar names, but their functional emphases are different: expansion joints mainly absorb thermal displacement, and metal hoses are mainly used to connect equipment with relative motion, such as vibration damping joints at pump inlet and outlet, and compensate for installation deviations.

You use the metal hose as an expansion joint, it has few corrugations, short fatigue life, and will crack soon; Conversely, use the expansion joint as a hose, which is not soft enough, and the vibration of the equipment will still be transmitted. When purchasing, read the working condition description clearly: if there is thermal displacement, choose expansion joint; if there is vibration or frequent disassembly, choose metal hose.

Inferred from industry application: Which type should I choose for power station, cement and desulfurization?

Another dimension-the industry pushes back. The working conditions of different industries vary greatly, and products have long been "specialized".

Corrugated expansion joint for power plant industryThe main steam and the reheating steam pipeline have high pressure and high temperature, and the external pressure single axial type or the high temperature axial type are preferably selected; Air-cooled island vacuum pipeline is a large-diameter low-pressure vacuum condition, and the double-hinge expansion joint of air-cooled island vacuum pipeline is needed, which has low stiffness and good seal.

Metal Corrugated Expansion Joints in Cement Industry: The flue gas at the head and tail of the kiln has a large dust content and a drastic temperature fluctuation. When selecting the general-purpose type or the high-temperature axial type, the guide tube should be thickened to prevent the dust from washing and wearing the bellows.

Desulfurization system: The flue gas of wet desulfurization contains acidic corrosive medium, which cannot be carried by ordinary stainless steel, so it is necessary to use PTFE-lined hose or PTFE compensator as an anticorrosive layer; At the same time, the desulfurization flue gas baffle door should be matched with the double seal design. At this time, the expansion joint must be matched with the baffle door.

So, what are metal expansion joints included? The answers are different by displacement, special needs and industry. However, there is one principle that remains unchanged: first find out the pipeline medium, temperature, pressure and displacement, and then compare it with the specific models in the product information of this site. Don't ask "which is cheaper". The wrong choice of expansion joint can lead to air leakage or pipeline overturning. Which one is lighter and which one is heavier, don't I need to say?

What are you most afraid of when choosing metal expansion joints? I am afraid that the customer will only say "Quote me the price of DN600" on the phone. After quoting, ask "why is it so expensive". Because the selection parameters are incomplete, manufacturers can only quote according to the most conservative and largest specifications. Can this price not be high?

What about that? If you want to buy a suitable expansion joint, don't rush to report the caliber. Find out the following 5 things.

Don't be in a hurry to report the caliber, first calculate how much displacement the pipeline should absorb

The core function of the expansion joint is to absorb displacement. The amount of displacement was not calculated clearly, and the rest was all for nothing.

The thermal expansion of a pipeline is generally calculated by multiplying the linear expansion coefficient of the pipeline by the temperature difference and then multiplying the length of the pipeline. The linear expansion coefficient of carbon steel is approximately 0.012 mm/m·°C, and the stainless steel is slightly higher. For example: a 30-meter-long carbon steel pipeline, when the medium temperature rises from 20°C to 200°C, the axial thermal expansion is 30×180×0.012=64.8mm.

This is not over, the ambient temperature during installation, equipment vibration, and displacement caused by pipeline foundation settlement have to be counted. If you only know the diameter of the pipe and don't know how to calculate it, ask the manufacturer for a "Quick Check Table of Pipeline Thermal Displacement" and check it. But only if you have to prepare the pipeline direction diagram and clearly mark the positions of the fixed bracket and sliding bracket.

Media, pressure, temperature: three-piece set that determines the material and number of layers of corrugated pipe

Looking at these three parameters together, the material, wall thickness and number of layers of the bellows are basically determined.

At this end of the medium, 304 stainless steel is enough for ordinary steam, hot water and compressed air; For the medium containing chloride ions, 316L is used; In this strong corrosion condition of desulfurization flue gas, corrosion-resistant alloys or non-metallic expansion joints are often selected. The composition of the medium must be written in full, don't just mention a "smoke". The content of SO₂, NO and dust in the smoke is different, and the corrosiveness is several times worse.

If the pressure is high, the wall thickness has to be added; When the temperature is high, the permissible stress of the material decreases, and the compensation amount is also affected. For example, a steam pipeline with a pressure of 1.6MPa and a temperature of 350°C usually uses 2 to 3 layers of bellows; At the same temperature but the pressure dropped to 0.1MPa, a single layer is enough. The more layers, the greater the stiffness and the higher the price, so there is no need to blindly pursue multiple layers.

On the other hand, don't leak the deflector. The flow rate of the gas medium is high, and the bellows are easily eroded without the guide tube. If the medium contains particulate matter, the anti-wear structure at the front end of the guide tube should also be considered.

Axial, lateral or angular displacement? Choose the universal or hinged type

If the displacement direction is not clear, the chosen structure must be wrong.

The pipeline mainly expands and contracts along the direction of the pipeline, that is, axial displacement. Choose general corrugated expansion joint, also called axial expansion joint. If the pipeline takes an L-shaped or Z-shaped bend, and there is displacement in both horizontal and vertical directions, a single axial type can't be handled at this time, so we have to consider the double hinge transverse expansion joint, or the use of two hinge expansion joints together.

Angular displacement is common at equipment connections, such as steam turbine exhaust port and fan inlet and outlet. Hinge type expansion joint or double tie rod type is a good choice under this working condition.

How to judge? To put it bluntly, just one sentence: telescope and contract along the direction of the tube to choose the general-purpose type; If the pipe is bent and the joint is deflected, you have to use one with hinges or duplex structure. Don't expect to eat all the displacements with a general-purpose type. That's called hard carrying, not compensation.

Tie rod, hinge or pressure balance? Constrained structural relationship support stress

The function of expansion joint is not only "telescopic", but also "controllable force".

The bellows will produce a huge thrust under the action of pressure. If this thrust is all thrown to the fixed bracket, how thick must the bracket be? Therefore, there should be a restraint structure inside the expansion joint to share-this is the meaning of the existence of tie rod, hinge and pressure balance.

The universal expansion joint does not come with its own constraints, and the internal pressure thrust must be carried by the main fixed bracket. Where the pipeline is complicated and the bracket is difficult to make, use straight pipe pressure balance expansion joint or curved pipe pressure balance expansion joint, which can offset the internal pressure thrust, and the force acting on the bracket is much smaller. It is often used on pipelines in cement plants and power stations.

Some people think that the expansion joint with a tie rod can replace the fixed bracket. It wasn't. The tie rod constrains the displacement of the expansion joint itself, not the thrust of the pipeline. The bracket has to do what it needs to do, but the force is different.

High temperature flue gas, large diameter, vacuum pipeline: how to feed in special working conditions

Once the working condition is special, the standard products often can't stand it, so they have to be "added".

High-temperature flue gas, when the temperature reaches above 600°C, metal bellows are basically useless-material creep and oxidative corrosion will quickly shorten the life. At this time, the candidate non-metallic expansion joint (fabric fiber expansion joint) is more suitable, with high temperature resistance, good flexibility and certain sound absorption and vibration isolation effect. Non-metallic expansion joints are often used in desulfurization flues, which are used together with flue gas baffle doors.

Large-diameter pipes, such as flues or air ducts above DN3000, metal bellows are expensive to manufacture, heavy in weight, and laborious in construction and hoisting. Rectangular non-metallic expansion joints are more common schemes under such operating conditions. Of course, if the customer specifies metal, it has to use large-diameter thick-walled expansion joints, and the reinforcing rings and side plates have to be thickened.

The vacuum pipe is different again. Under vacuum conditions, the corrugated pipe should withstand external pressure, and its ability to resist instability is the key. Double hinge expansion joints are often used in air-cooled island vacuum pipelines, which have one more constraint in structure and good stability. The special vacuum hose is equipped with metal mesh sleeves outside the bellows, which is also the truth.

It is much more efficient to give the manufacturer a selection parameter table than to call back and forth

When communicating with the manufacturer, give the following information at once:

• Pipe diameter DN, connection mode (flange or welding)
• Type and value of displacement: What are the axial, transverse and angular displacements in mm/degree
• Name and composition of the medium, especially with corrosive components
• Design pressure MPa, Design temperature °C, Operating temperature °C
• Installation position indication (straight pipe section, near elbow, equipment interface)
• Arrangement of fixed bracket and sliding bracket
• Is there any thermal insulation layer and any outer sheath requirements

All this information is given, and the manufacturer can make the plan and quotation in half an hour. The time saved is much more efficient than calling back and forth to urge questions.

There is no "universal model" for metal expansion joints, only "fit or not". Only when the selection parameters are clear and the working condition boundary is clear can the manufacturer give you a reliable scheme. If you are not sure, send the parameter table directly to the manufacturer-let the engineer judge whether the scheme is valid or not.