Specialized in manufacturing compensators, expansion joints, baffle doors

Expansion joints without pre-deformation when they leave the factory will be scrap iron at the site

The factory state of the metal expansion joint directly determines whether it can work after being installed. Many people receive the goods and see that the bellows are shiny and the paint is intact, and they feel that everything is okay-wrong. If the expansion joint is not pre-deformed when it leaves the factory, it is basically a decoration when transported to the site, and may even become a potential safety hazard of the pipeline system.

Why do you say that? Because the displacement compensation ability of bellows is achieved by the elastic deformation of corrugations. If the bellows is at a completely free length when leaving the factory, once the pipe heats up after installation, the bellows will be forcibly compressed (or stretched) from the free state to absorb the heat displacement. The problem is that the fatigue life of bellows is calculated according to the number of cycles, and it is installed directly in the free state, which is equivalent to overdrawing a part of the displacement quota that should be left for cold state to hot state in advance. For example, if a general-purpose corrugated expansion joint with a design compensation of 100mm is pre-stretched by 50mm before leaving the factory, it will be in a semi-compressed state in the cold state, and then compressed by 50mm in the hot state. If it is not pulled when it leaves the factory, it will be installed directly at the site. When it is hot, it will be compressed by 100mm, the stress of the bellows will be directly doubled, and the life will be reduced by a cliff.

So, what is the factory status of metal expansion joints? It is not "what you receive", but "what the manufacturer should be according to your working conditions". This point is confused, and there are pits behind it.

Several common conditions at the time of leaving the factory, which one do you have

Expansion joints with different working conditions have completely different factory conditions. There are generally four types.

The first, the free state.This is most commonly found in small diameter, low pressure, and little temperature change. For example, some non-metallic expansion joints (fabric fiber expansion joints) used in ventilation ducts have low stiffness, and their absorption displacement mainly depends on structural rotation rather than corrugation stretching, so they don't need pre-deformation when they leave the factory. However, note that the expansion joints of metal bellows, especially the high-temperature axial expansion joints used in power stations and cement industries, will not leave the factory in a free state.

The second is pre-stretched or pre-compressed.This is the most particular category. Straight pipe pressure balance expansion joint, external pressure single axial expansion joint, curved pipe pressure balance expansion joint, etc. Before leaving the factory, the manufacturer will pre-stretch or compress the bellows for a certain distance according to the cold tightness (also called cold drawing) of the pipeline. Pre-stretching is used to compensate for thermal expansion (pipes become longer when heated), and pre-compression is used to compensate for thermal shrinkage (pipes become shorter when cooled, such as cryogenic lines). How much to pull and how much to press are written on the drawings and nameplates. Just work according to the drawings during installation.

The third type, with a limiting device.Many tie rod and hinge expansion joints will be equipped with temporary limit screws or limit blocks when they leave the factory to prevent the bellows from being accidentally stretched, compressed or twisted during transportation and hoisting. For example, compound hinge transverse expansion joint and compound straight pipe bypass pressure balance expansion joint have large transverse displacement. Once the bellows is twisted during transportation, the corrugations will produce plastic deformation, and the whole process will be wasted. The limit device is used to lock the displacement, and then remove it after installation and alignment at the site.

The fourth type, with protective support.Large-diameter thick-walled expansion joints, directly buried (fully buried) expansion joints, such as large self-weight, thick bellows wall and large wave height, will be wrapped with a protective sleeve or support frame on the outer ring of the bellows when leaving the factory to prevent bumps, extrusions and scratches on the corrugated surface during transportation. Because any pit or scratch in the bellows is a potential stress concentration point.

The factory position of the tie rod and nut, whether to dismantle or not

This question has been asked countless times in the background. Every time I rhetorically ask: Have you seen the nameplate?

There are two kinds of tie rods on the expansion joint, one isStructural tie rod(Permanent tie rod), such as the tie rod on the single hinge expansion joint and the large tie rod expansion joint. This set of rods participates in the force and is used to bear the blind plate force generated by the internal pressure of the pipeline. The nut on this tie rod is in a locked state, which is set after leaving the factory. It must not be loosened or disassembled during installation. You remove it, and the pipe is pressed, and the bellows bulge out like a balloon.

The other istransport tie rod(temporary tie rod), which only serves the transportation and hoisting stages. It is usually made of thinner screws or angle steel, with one end welded to the end pipe and the other end locked with a nut to the flange or tube sheet. The role of this tie rod is to maintain the factory length of the bellows while locking in the amount of pre-tension/pre-compression. After installation and alignment, the tie rod must be removed or loosened to the extent that it is completely unstressed. If it is not disassembled, the expansion joint becomes a rigid short joint, which can't absorb any displacement. All the thermal stress of the pipeline is transferred to the equipment and bracket, and the root of the bellows will crack after a few heat cycles.

To judge what kind of tie rod it is, look at two points: first, thickness, structural tie rods are generally thick, and high-strength bolts are used; The second is to see if there is a mark for adjusting the spacing of nuts. The transport tie rod is usually marked with the words "remove after installation" with paint. Really unsure, turn out the factory information, it is clearly written on it.

What records are left behind by factory inspection to be considered qualified

The factory status of the metal expansion joint is not only the physical posture, but also the inspection record. When a qualified expansion joint leaves the factory, these things must be attached with the box: hydraulic test record, air tightness test record (when gas medium is required), stiffness test data and displacement data.

Hydraulic pressure test, generally according to the design pressure of 1.25 times or 1.5 times the water pressure, hold the pressure for 30 minutes without leakage, this is a hard index. Air tightness test is used for pipelines transporting flammable, explosive or toxic media, and the requirements are stricter. Usually, low-pressure air tightness of 0.05~0.1MPa or helium mass spectrometry leak detection is directly performed. The most critical stiffness data are axial stiffness, transverse stiffness and bending stiffness. These three values are directly calculated into pipeline stress analysis. Whether the data is correct or not, and whether the values deviate greatly from the design must be verified on the spot. The displacement data refers to the maximum compensation amount allowed by this expansion joint in all directions, axial, transverse and angular directions. Comparing it with the drawings, you can know whether the manufacturer has cut corners.

After you get this information, don't rush to put it away, check it with the product nameplate. The nominal diameter, design pressure, design temperature, compensation amount and factory number on the nameplate should be aligned with the certificate. If you don't, call back to the manufacturer.

Shipping and storage, which state parameters can be quietly destroyed

There is a saying that "it is good when you leave the factory, but it is bad when you go to the construction site". During the period from the factory to the installation of metal expansion joints, there are three state parameters that are most prone to problems: pre-stretching amount, limiting device and surface protection.

What does the amount of pre-stretch maintain? By the limit rod. However, the practice of many construction sites is that the equipment is thrown in the yard directly in the open air, exposed to the sun and soaked in rain. If the limit tie rod is exposed to humid environment for a long time, the thread will rust, and it can't be removed even if it wants to be removed during installation. Rust is not the worst thing. The worst thing is that someone tries to save trouble by cutting the tie rod directly with gas cutting-that's how the expansion joint is treated as scrap iron.

For surface protection, there is generally stainless steel or spray protective layer on the outside of the bellows, but if the sling is bound at the position of transportation and hoisting without a rubber pad, the wire rope will be directly corrugated, and a strangulation mark will be a crack source. In a dusty environment such as cement plants and power plants, dust accumulates in the trough of corrugated pipes. If it absorbs moisture and chloride ions are enriched, stainless steel corrugated pipes can't withstand pitting corrosion.

Pad sleepers below, above 30cm from the ground, cover with rainproof cloth on top, oil the limit rod regularly to prevent rust, and don't pile any heavy objects on the bellows. There are not many projects that can be done with such a small thing.

On-site acceptance three-minute self-inspection checklist

No nonsense, just give you a set of moves.

• Touching the nameplate:Is there a nameplate? Are the model, pressure, temperature, compensation amount and factory number on the nameplate clear and unaltered? Expansion joint without nameplate, refuse to sign for it.
• Second, look at the ripples:Are there any pits, scratches, welding slag splashes and arc damage on the corrugated surface? Is the corrugation spacing even? If the wave distance is wide or narrow, it means that plastic deformation has occurred during transportation, and the product will be returned.
• Three pairs of limits:Pre-tensioned/pre-compressed expansion joint, is the limit rod intact and the locking nut loose? Is the paint mark on the tie rod clear? If the mark is gone, confirm with the manufacturer what the pre-deformation amount is.
• Quad core records:Are there any factory hydraulic test reports, stiffness data and displacement data? Are the data consistent with the nameplate? Is the stamp on the document complete?
• 5. Check the attachments:Is the deflector installed? Is the direction of the guide tube consistent with the direction of the medium flow (along the direction of the medium flow)? Is the arrow on the nameplate in the same direction as the deflector?

These five steps can be completed in a little more than two minutes. If there is a problem, take photos on the spot, write records, notify the manufacturer, and don't drag it out until the day of installation before unpacking.

What is the factory status of metal expansion joints? It is the pre-deformation position calculated by the manufacturer according to your working conditions, and it is the reference point to ensure the safe operation of the bellows during the life cycle. The first thing you do after receiving the goods is not to rush to hoist them, but to confirm that these "states" are still there.

Engineers, who hasn't dealt with pipelines? The pipe is a thing that stretches when it is hot, shrinks when it is cold, and vibrates when the pump is turned on. If you forcefully solder it to death at both ends, the interface will leak at least, and the equipment will be damaged at worst. What about that? Install an expansion joint.

What is an expansion joint?To put it bluntly, it is a section of "elastic joint" installed in the middle of the pipe, which is specially used to absorb the thermal expansion and contraction, vibration and installation deviation of the pipe. You look at those steam lines, flue gas pipes, chemical plants, and you can see a guy with a rippled belly at some distance, and that's probably it.

Are expansion joints and compensators the same thing? Let's make this name clear first

"Is the expansion joint the same as the compensator?" The answer is: the same. These are the two names of the same thing. They are both Expansion Joint in English. There are also compensators and Expansion joints in the industry, so no one has to correct anyone. You just have to know that when you see the words "corrugated compensator", "pipe compensator" and "expansion joint", you know what you know.

Expansion energy saving absorbs displacement, but it does not produce displacement. It's not the same thing as a spring damper, and it's not the same thing as a hose. Metal hoses are mainly used to absorb vibration and end displacement, while expansion joints are components on pipelines that absorb multi-dimensional displacement in a system. These two functions overlap, but the application scenarios are very different.

What does an expansion joint rely on to absorb displacement? What is the use of bellows, guide tube and tie rod

Take apart a typical corrugated expansion joint and you'll find just three things at the core.

**Bellows** is the heart. It is a corrugated shell made of thin-walled stainless steel plate by hydraulic or mechanical forming, which absorbs the expansion and contraction of the pipeline by elastic deformation of the corrugation. The more ripples, the greater the displacement that can be absorbed, but the pressure bearing capacity will decrease accordingly. So when you look at those expansion joints used in high-pressure working conditions, the corrugations are relatively shallow and the number of layers is large, just to take into account both strength and flexibility.

The **guide tube** is an inner liner. It is mounted inside the bellows and is in direct contact with the medium. What is the role? The first is to reduce the erosion wear of the bellows when the medium flows, and the second is to reduce the fluid resistance. Think about it, there are potholes inside the bellows, and the medium flow will definitely run around in the past. With the guide tube to straighten the channel, the flow will be much smoother. There is another detail-the installation direction of the guide tube is particular. Generally, it should be installed along the direction of medium flow. If it is installed backwards, it will not only fail to divert the flow, but also may blow the bellows out.

**Puller** is the bodyguard. It is attached to the end plates at both ends of the expansion joint, restricting the bellows from expanding and contracting only in the axial direction, preventing it from twisting and twisting. Like the transverse expansion joint of compound hinge and the pressure balance expansion joint of straight pipe, the tie rod system is more complicated, and the purpose is to make the bellows deform according to the designed direction instead of turbulently.

Metallic, non-metallic, sleeve type... What are the common types of expansion joints and what working conditions are they suitable for

There are many ways to classify expansion joints. According to the material, the three most common categories are metal, non-metal and sleeve type.

Metal corrugated expansion jointIs the absolute workhorse. It is resistant to high temperature and high pressure, and is suitable for steam pipeline, power station smoke air duct and chemical pipeline. There are general corrugated expansion joints, external pressure single axial expansion joints, double hinge transverse expansion joints and straight pipe pressure balance expansion joints. The name sounds around, and the core idea is one: choose different structural forms according to the layout of the fixed bracket of the pipeline and the displacement direction that needs to be absorbed. For example, the metal corrugated expansion joint used in the cement industry has to intentionally strengthen the wear-resistant design, because the cement dust wear is too severe.

Non-metallic expansion jointAlso called fabric fiber expansion joint or rectangular non-metallic expansion joint, the flexible non-metallic material is used as a loop belt. It is not afraid of large displacements, and is especially suitable for rectangular pipes, such as the inlet and outlet of desulfurization tower, the flue near the flue gas baffle door. Non-metallic expansion joints also have an advantage that metal can't compare-good thermal insulation performance and stronger corrosion resistance, which is suitable for flue gas environment containing acid and alkali. Of course, it has limited pressure resistance and cannot be used on high-pressure pipelines.

Sleeve type pipe expansion jointTook another path. It relies on the relative sliding of the inner and outer sleeves to absorb displacement, has simple structure and low cost, and is mainly used for buried pipelines or heating pipelines with low pressure and large diameter. The disadvantage is also obvious-the seal is prone to failure and the packing needs to be maintained regularly.

There is also a category calledRotary compensatorIt absorbs the thermal displacement of the pipeline through the rotation of the rotating cylinder, and can top several ordinary expansion joints on a long-distance steam pipeline, but the installation accuracy requires high requirements, and not everyone can play it.

Don't just look at the name when choosing a model. Also called "expansion joint", the design logic of corrugated expansion joint and double hinge expansion joint of air-cooled island vacuum pipeline used in power station industry is completely different from the same channel. The former has to withstand the smoke erosion of hundreds of degrees Celsius, while the latter has to be kept sealed in a vacuum environment. The material, wavenumber and wall thickness are all calculated separately.

When choosing expansion joint, only look at the caliber: pressure, temperature and displacement are the key parameters

Two days ago, I met a customer who was doing HVAC engineering. When I came up, I asked, "How much is the expansion joint of DN500?" I said, don't ask for a price in a hurry. What pressure, temperature and displacement does your pipeline want to absorb? These numbers can't be determined, and the quotation is meaningless.

When choosing the expansion joint, the diameter is only the basic parameter, and the real core is the following three:

• Stress.The pressure capacity of bellows is determined by wall thickness, number of layers and wave height. The expansion joint of PN16 is used on the pipeline of PN25, and the bellows bulges for you in minutes.
• Temperature.When the temperature is high, the allowable stress of stainless steel plummets. 800℃ flue and 200℃ steam pipe, the materials of corrugated pipes are completely different. Heat-resistant alloy steel should be used in high temperature conditions, and even heat insulation layer should be added.
• The amount of displacement.Axial displacement, transverse displacement and angular displacement should be calculated separately. Axial expansion joint can absorb axial expansion and contraction, and you have to let it eat lateral deviation, and it will be fatigued and cracked within two months.

Fatigue life。 Expansion joints are flexible pieces that consume life every time they expand or contract. The design life is usually calculated according to 1000 to 5000 cycles. If you start and stop the boiler frequently, the fatigue life of the bellows has to be longer, so don't save this money.

Poor installation of expansion joints is equal to buying for nothing: the easiest pit to step on in installation and maintenance

Expansion joint, three points depends on type selection, seven points depends on installation. If you don't pay attention to it, no matter how expensive the expansion joint is, it will be useless.

The first pit isThe tie rod nut is not adjusted properly。 The nut on the tie rod when the expansion joint leaves the factory is locked, that is in shipping condition. When installing, loosen the positioning nut so that the bellows can expand and contract freely. Some people try to save trouble, and they weld the pipe directly without even loosening the nut. The expansion joint becomes a decoration, and the pipe should crack. On the other hand, if it is a pressure-balanced expansion joint, the adjustment of the tie rod nut is particularly specific, so it can't be completely loosened at once, so the bellows on both sides must be stressed evenly.

The second pit isInstall the guide tube in reverse direction。 As mentioned earlier, the guide tube is directional. Install it backwards, and the medium flushes the bellows directly, which can wear out in a few months.

The third pit isForget the pre-deformation。 When the ambient temperature at the time of installation is far from the operating temperature of the pipe, the expansion joint should be pre-stretched or pre-compressed. For example, if the steam pipeline installed in winter runs at more than 100 degrees, the expansion joint has to stretch half of the compensation amount in advance, otherwise the displacement will be exceeded after operation.

The fourth pit isSave the bracket。 The expansion joint is not a universal glue, it can only absorb the thermal displacement of the pipeline itself, and the fixed bracket and guide bracket of the pipeline should be set. If the bracket is not set correctly, the expansion joint will be forced to bear the force it should not bear, and its life will be greatly reduced.

One more word on maintenance. If the protective sleeve on the outside of the metal bellows is broken and rainwater seeps in, the bellows will easily rust and pitting. The ring belt of non-metallic expansion joint is even more delicate. If the cotton head is broken, it should be repaired in time, otherwise it will tear a big hole when the wind blows. It is much more cost-effective to take a second look during regular inspections than to stop production for emergency repair afterwards.

With all that said, back to the question at the beginning:What is an expansion joint?It is the elastic joint in the pipeline system that silently carries all the thermal expansion and contraction. Understand its principle, type and selection logic, and when you look at the pipeline drawings, you will have a bottom in your heart. Next time someone comes to ask for a price with a caliber, you can also ask him the other way around, "What is your operating pressure?"

Two days ago, at the power plant site, the old master pointed to a huge rectangular pipe and told me: Install this non-metallic expansion joint, and be careful not to scratch the fabric layer. I was stunned for a moment-the non-metallic expansion joint he was talking about clearly said "non-metallic compensator" on the drawing. The same thing, two names, and that's just the beginning. Later, I discovered that this thing has a bunch of aliases in the industry: fabric fiber expansion joints, flue compensators, non-metallic flexible joints... Let's just figure this out today.

The "compensator" and the "expansion joint" are originally the same thing

Why does the same thing have so many names? In fact, the root is that the words "compensator" and "expansion joint" are the same thing. Just like people have big names and nicknames, pipeline flexible compensation equipment also has two official names. Not to mention, by structure, there are also rectangular non-metallic expansion joints, a variant specifically used on rectangular flues. When customers call to inquire, eight out of ten say "I want non-metallic expansion joints", and the remaining two say "non-metallic compensator", but when it comes to the drawing, it may say "fabric fiber expansion joints" again-because its main body is indeed superimposed and pressed from high silicon fiber cloth and fluororubber coated cloth.

It's not the same thing as a metal hose

One of the most confused purchasers I've ever seen came to me with the drawings, said they needed a "soft connection" and asked me if it was a metal hose. It was immediately apparent that he had confused a non-metallic compensator with a metal hose. Metal hoses are corrugated pipe structures, which are resistant to high pressure, while non-metal compensators rely on fabrics and flexible materials to absorb displacement, which are resistant to high temperatures and corrosion, but have weak pressure bearing capacity. They are like raincoats and jackets. They can both block water, but they are actually used in two ways. In the flue ducts of power stations, cement plants and desulfurization systems, non-metallic compensators are the protagonists.

Where did the "flue compensator" come from? Because the flue gas baffle door and desulfurization flue gas baffle door are equipped with non-metallic compensators nine times out of ten. The flue gas coming out of the boiler is hot and contains sulfur, and the metal bellows can't bear the corrosion at all, while the non-metallic materials are like fish in water. The old masters were too lazy to say the full name, so they shouted "flue compensator" directly. The new technicians were confused when they heard it: Is this the same thing as non-metallic compensator? Yes, that's it. There is also one called "non-metallic flexible compensator", which emphasizes its flexible characteristics, but it is actually the same thing.

The name "fabric fiber expansion joint" is shouted loudest in the workshop

Let's talk about the term "fabric fiber expansion joint". In the production workshop of non-metallic expansion joints, the most conspicuous thing is the layers of fabrics-glass fiber cloth, polytetrafluoroethylene film and aluminum silicate cotton felt. The old master looked at these materials and casually shouted "fabric fiber expansion joint". This name is also recognized in the standard. JB/T 12235-2015 is the national standard for non-metallic expansion joints, which clearly defines it as a flexible compensation device. So when you hear "fabric fiber expansion joint", don't doubt it, it's the brother of the non-metallic compensator.

With so many names, these are the ones that you really need to report clearly when placing an order

After all, there are so many names, but when you want to buy it, you have to clarify the key parameters: diameter size, pressure, temperature, medium, displacement, and whether it is circular or rectangular. Because the structural details of rectangular non-metallic expansion joints are different from those of circular joints, the installation methods are also different. Don't just say "I want a non-metallic compensator", that's not saying it.

The next time you hear someone shout "flue compensator" or "fabric fiber expansion joint", you will know what kind of thing he is referring to-non-metallic, flexible, high-temperature resistant, specialized in thermal expansion and contraction of smoke and air ducts. If you are still unsure, just throw the working condition parameters over, and let's smooth them out together.

Causes of Metal Expansion Joint Deformation? Don't wait for the pipeline to tear to find an answer

Two days ago, I met an old customer, who called and said that there was an abnormal noise in the expansion joint bellows in the power plant, like something scraping in the pipe. The inspector dared not get too close for fear of something going wrong. When the machine was stopped and disassembled, the bellows had already bulged a bag, and there were still a few fine cracks at the folds. If you delay it for another two days, the whole pipeline will have to be changed.

The deformation of the metal expansion joint is really not a small problem. Many people think that it is just a soft connection, which can be bent and stretched. When they see that something is wrong, it is often too late. Let's get it straight. Why does it deform? What should I do if I see the deformation?

Find out first: What exactly does the expansion joint deformation look like, is it bulging, twisting or flattening?

Deformation is not a single kind, there are three common types:bulge(Local bulge, like a tumor),Twisted(The axial twisting force is crooked),flattening(radially recessed). Bulging mostly occurs at the peak of the bellows, which happens when the medium pressure exceeds or the wall thickness of the bellows is corroded and thinned. The distortion is mostly due to the lateral displacement exceeding the design value, or the pipeline system does not make a reasonable guide bracket. Flattening, basically, the external pressure is regarded as the internal pressure in vacuum conditions, or it is smashed by external force during installation.

There is also a hidden deformation called "instability"-the bellows looks like there is no obvious bulge, but the wave distance is uneven. Some waves are close to each other, and some waves can be pulled away. This is the most insidious, because it can't be seen on the outside, but the fatigue life has been greatly reduced.

One of the culprits: the hidden danger left during installation-the cold tightening was not done correctly and the tie rod was not removed

Let's play back the most common metamorphosis cases. A steam pipeline in a chemical plant is equipped with a general-purpose corrugated expansion joint. Less than two months after operation, the corrugated pipe is twisted. The manufacturer sent someone to take a look, and the problem was there: the cold tightening amount was simply not enough during installation, and the pre-stretching was not pulled enough. As soon as the pipeline heated up, the expansion joint directly received the compression limit.

Another high-frequency operation error is that the tie rod is not disassembled. Some expansion joints with tie rods, such as double hinge transverse expansion joints, the tie rods are used for transportation protection when they leave the factory, and must be loosened or adjusted according to design after being installed in place. Some construction teams find it troublesome, or don't understand it at all, so they directly fix the tie rod with nuts in place. As a result, the displacement of the pipeline was not absorbed, and it was all on the bellows, so it was strange that it did not deform.

To go a little further, many scenes are now usedExternal pressure single axial expansion jointTo absorb large axial displacement, the stability of this structure itself is good, but if you don't pay attention to the distance between the flow sign and the guide bracket when installing, it will still give you a look.

The second culprit: the invisible killer of media and working conditions-overtemperature, corrosion and water hammer can't escape

Changes in operating conditions are the ones that are most often overlooked. You think the design temperature is 300℃, but it soars to 350℃ in actual operation. The allowable stress of bellows material falls straight down, so it's weird that it doesn't bulge. There is also corrosion. The sulfide in the flue gas pipe is dilute sulfuric acid when it meets condensed water. The corrosion rate of stainless steel bellows in this environment is more than ten times higher than that in the air. To withstand this environment, you have to rely on non-metallic expansion joints or PTFE-lined metal hoses to support it.

Water hammer This problem is especially common on steam pipes. Condensed water accumulated in the pipes, and when the steam came, the water was pushed away and slammed into the elbow or expansion joint. This impact load is not constant, but the instantaneous pressure can reach several times the normal operating pressure. Once or twice is fine, but more times, the trough position of the bellows will produce micro-cracks.

By the way, if the medium is powder or slurry with particles, the role of the guide tube is very critical. The guide tube can prevent the particles from directly washing the bellows, but many projects save the guide tube in order to save costs. As a result, the bellows is worn out, and then the whole becomes unstable and deformed. This account is really not cost-effective after calculation.

The third culprit: the design selection is not well matched-the rigidity and fatigue life of corrugated pipe are not determined by patting the head

The selection is the most controversial. Some people think that the expansion joint can be connected, and the more bellows layers, the better. That's really not the case. The stiffness and fatigue life of bellows are a pair of contradictions. If the stiffness is small, it will easily deform and become unstable, while if the stiffness is large, it will not absorb the displacement. The design of each expansion joint has a strict calculation basis, including compensation amount, pressure level, temperature, media characteristics and number of cycles. You take a general-purpose corrugated expansion joint with a design life of 1,000 times and use it in a system that starts and stops once an hour, and it will be scrapped for you in three months.

Let's talk about the choice of pressure balanced expansion joint. If the pipeline pressure is high and the axial displacement is large, the straight pipe pressure balance expansion joint is a safe choice, which can balance the internal pressure thrust and reduce the thrust on the fixed bracket. However, some people try to save money and use ordinary axial expansion joints to carry the pressure and thrust, and finally the bellows and bracket suffer together.

What to do after transformation? Can it be fixed? Or just change? How to prevent it next time?

Bulge, crack, bellows perforation, no repair, new one。 If the twist is not serious, if it is just the wrong adjustment of the tie rod, you can re-adjust the tie rod nut to see if the bellows can be restored. But to be honest, once the permanent plastic deformation occurs, the fatigue life of the bellows has been greatly reduced, and even if the shape is coming back, the heart is not at ease.

Then how to prevent repeat offense when changing? Three words: depending on the working conditions. Check the cold tightness before installation, find out whether the tie rod is for transportation or function, and check the guide bracket spacing. During operation, attention should be paid to the abnormal working conditions such as over-temperature, over-pressure and water hammer. Don't just look at the appearance during regular inspections. Use a flashlight to check the bellows trough for salt, rust or cracks-these things look inconspicuous from the outside.

In the procurement process, make the working condition parameters clear, and let the manufacturer help you choose the model. For example, the corrugated expansion joint in the power station industry and the corrugated expansion joint in the cement industry have completely different working conditions. The former should carry the creep temperature and pressure cycle of the main steam, while the latter should prevent the erosion and erosion of high-temperature dust. Choosing the right type is much more important than remediation after the fact.

The metal expansion joint is not a display on the pipe, it is the breathing joint of the pipe. Its deformation is that the pipeline system is sending a distress signal with its body. Don't ignore it, let alone wait for the pipeline to rip to find an answer.

Many customers always feel that the compensator is installed once and for all. Especially the non-metallic compensator (also called fabric fiber expansion joint), because it doesn't look "tough" like the metal corrugated expansion joint, it feels delicate and easy to break. In fact, on the contrary, the maintenance difficulty of non-metallic compensators is not technical, but that you don't use the right method.

"How to maintain non-metallic compensator?" This question, our after-sales engineers have to be asked hundreds of times a year. Today, I disassembled it and crumpled it to make it clear. Each one is the experience hammered out in the actual project.

Find out the structure of non-metallic compensator first, then talk about maintenance

If you don't even know what layers it consists of, maintenance is busy. The non-metallic compensator is not a whole cloth sleeve, it is a flexible joint made of multiple layers of different materials, and mainly depends on the skin to absorb thermal displacement and vibration. Common structures include:

• Skin (non-metallic band)— — The outermost sealing layer, usually glass fiber cloth coated with fluororubber, silicone rubber or polytetrafluoroethylene, depends on it for temperature resistance and corrosion resistance.
• Thermal insulation-The interior is filled with ceramic fiber blanket or glass wool, which is used to block high temperature and protect the skin.
• guide tube— — Install inside the pipeline to guide the flow of media and avoid high-temperature airflow directly washing the skin. This is the same as the guide tube of the metal expansion joint.
• Flange plates and bolts— — Metal parts that press the skin against the pipe flange, by which standard non-metallic expansion joints are installed.

So what is the nature of maintenance? Is to protect the skin, keep the seal, and prevent the bolts from loosening. You remember the structure in your mind, and you will naturally understand it later.

Where to look for daily inspections? Focus on checking these parts

Inspection is really not difficult, just see with your eyes. It is suggested that in the order of "from the outside to the inside", more than a dozen compensators on a pipeline can be checked once in half an hour.

First Lookskin surface。 Any bulging, delamination, cracking, scratching? The most common injury is abrasion-dust particles, driven by airflow, sand the skin like sandpaper. If the outer fiberglass cloth is found to have broken yarns or exposed yarns, it means that the skin has begun to deteriorate.

Second Lookflange bolt。 This one is the easiest to overlook, but goes wrong the most. Under the temperature change of non-metallic compensator, the thermal expansion and contraction of bolts produce stress relaxation, which has to be tightened once in one or two months. You try the wrench one by one. If you can twist it half a turn, even if the torque is insufficient.

Third Lookguide tube。 Extend a flashlight from the entrance of the compensator and shine it in to see if the guide tube is curled and fallen off. Just two days ago, a cement factory customer reported that the compensator was hot. When he disassembled it, he saw that half of the guide tube was burned through, and the flue gas was sprayed directly on the skin from the gap. It was found that the insulation layer of the whole pipeline would have to be scrapped a few days later.

Remember, the skin is a consumable and can be replaced if it is broken. However, if the deflector and the insulation layer are broken, it is a systemic problem, and the maintenance cost is completely different.

The timing of cleaning and replacing non-metallic compensator

The topic of cleaning is quite controversial in the industry. Here is my view:If you can not wash it, don't wash it. Don't clean the dust.

Why? Once the surface coating of non-metallic skin is corroded by chemical cleaning agent, the sealing performance is degraded directly. Are the solvents you use to remove oil or rust? How much acidity and alkalinity? Don't be smart until the manufacturer gives advice.

If the dust accumulation is too thick and affects the heat dissipation, just purge it with dry compressed air, control the pressure within 0.2MPa, and keep a distance of more than 30cm from the skin. If there is really oily stain, brush lightly with a soft bristle brush dipped in neutral soap, then quickly rinse with clean water and air dry, and the process is controlled within 10 minutes.

So when should I change it?Penetrating cracks or air leakage in skinSwitch it directly without discussion. If only the surface coating is powdered, it can last another month or two, but there must be a replacement plan. There is a rough empirical figure for reference: the skin life of non-metallic compensator is about 3~5 years under normal temperature flue gas conditions; Under the high temperature above 300℃ or the working condition containing sulfur and alkali, the service life is shortened to 1~2 years. This is not said by patting the head, but summarized with reference to JB/T 12235-2015 standard and a large number of engineering cases.

Maintenance Myth: These Actions Can Instead Damage the Compensator

The following operations are the "negative teaching materials" seen repeatedly at the after-sales site.

Myth#1: The tighter the bolts are tightened, the better.The skin is sealed by the uniform pressure of the pressure plate. If you tighten the bolts on one side, the pressure plate is warped, and the other side leaks. The correct method is to pre-tighten diagonally several times, and finally tighten it uniformly with a torque wrench. The torque value refers to the requirements of the installation drawings.

Myth 2: If you find small cracks, use glue to mend them.Not only does this fail to restore the mechanical properties of the structure, but the solvent in the glue will further damage the skin coating. Many customers take structural glue on them by themselves, and as a result, the cracks get bigger and bigger.

Myth 3: Ignore maintenance during system downtime.When the machine is shut down, the residual medium in the pipeline (especially sintering flue gas and desulfurization slurry) will crystallize or corrode the skin inside the compensator. If the machine is shut down for more than one week, it is recommended to open the drain outlet to drain the accumulated liquid, and purge it with low-pressure steam if necessary.

To add, a non-metallic compensator cannot be used as a pipe support point. In some sites, in order to save supports, the weight of the pipeline is pressed on the compensator, and the skin is overloaded every day, and its life is more than halved.

Maintenance cycle and record management under different working conditions

There is no uniform standard for maintenance cycle, it depends on your operating environment. We usually divide our customers into three categories:

• Normal operating conditions(The temperature is lower than 200℃, the medium is non-corrosive and dust-free): Inspect once a quarter, and tighten the bolts once every six months.
• bad working conditions(High temperature and high dust, or containing acid, alkali and sulfur components): Inspect once a month, and tighten the bolts every quarter. Under this working condition, the surface temperature of the skin is measured with an infrared thermometer, which is much more reliable than hand touch.
• Extreme operating conditions(For example, near the desulfurization flue gas baffle door and the smoke exhaust system of the power station): See it once a week, focusing on observing whether there are any traces of acid dew point corrosion. By the way, the skin material of the non-metallic compensator in the desulfurization system must be acid-resistant fluororubber coating, otherwise it will be useless no matter how diligent the maintenance is.

Records management, don't bother. Each compensator establishes an independent ledger, which contains three lines: installation date, problems found in previous inspections, and details of replacement parts. With two or three years of data, you can find out the loss law of the compensator on your pipeline and prepare the wearable parts in advance.

To put it bluntly, the answer to the question "how to maintain non-metallic compensators" is twelve words:Understand structure, check frequently, do not move, keep records。

Although this article is about maintenance, if you encounter problems at the selection or installation level, don't shoulder it yourself. Whether the compensator can live long or not is decided from the day of selection and installation. If you are unsure of the specific working conditions, talk to the manufacturer's technical support directly, which is much more efficient than thinking about it on the spot.