Specialized in manufacturing compensators, expansion joints, baffle doors

The expansion joint is a "small part" in the pipeline system, but once it strikes, the whole line has to stop. How to judge how old it is to the extent that it should be replaced? Don't wait for it to leak before panicking. Everyone in this industry should know the following methods of judgment.

1. Why do expansion joints age? Understand the aging mechanism first

Expansion joint aging is not metaphysics,Physical and chemical factorsEach and a half. Metals-such as general-purpose corrugated expansion joints and high-temperature axial expansion joints-mainly carry fatigue and corrosion. Repeated action of alternating stress, coupled with chloride ions and sulfide in the medium, cracks will occur in the bellows over time. Non-metallic ones are more troublesome, such as non-metallic expansion joints (fabric fiber expansion joints) and rubber compensators. Ultraviolet rays, ozone and temperature will make rubber hard and fabric fibers delaminate. To put it bluntly, high temperature, high pressure and corrosive media are three "killers". Only by accelerating aging and figuring out these can the judgment have a basis.

The desulfurization flue gas baffle door used in power plant desulfurization system, the non-metallic expansion joint next to it, the medium temperature is high and the acid condensate is also present, and the life time is often less than half of the design value. This is the result of the acceleration of dielectric corrosion superposition temperature.

2. Aging signals visible to the naked eye

No matter how professional the testing is, the first step is always to look. Rust pits on the surface of the metal expansion joint and fine cracks at the trough of the bellows are typical signals. Non-metallic expansion joints are more pronounced: cracked, sticky, bulging surfaces, or layered blistering of fabric layers. In addition, pay attention to whether there is any abnormal displacement in the installation position, such as the deformation of the guide tube and the loosening of the tie rod nut (how to adjust the specific question and answer on how to adjust the tie rod nut of the expansion joint), which may mean that the internal structure can no longer hold up.

Two days ago, a customer reported that the non-metallic expansion joint next to the flue gas baffle door had a large area of delamination in three months, and when it was disassembled, it was seen that the fibers were carbonized. Did you say it was miserable? This kind of thing can be detected by the naked eye, and there is no need to wait for the detection data.

For directly buried (fully buried) expansion joints, the surface cannot be seen, but it can be checked whether there is abnormal uplift or settlement on the ground, and indirectly judge whether the interior fails.

3. Performance data will not lie: measure stiffness and displacement

If your eyes can't see correctly, talk to the data. Once the stiffness of the expansion joint (refer to the article on the stiffness and calculation formula of the bellows) changes by more than 20%, it will basically fail. How to measure? Measure axial stiffness with a simple loading device or directly compare factory reports. The displacement compensation ability is also a hard indicator-if the expansion joint cannot be restored to its original position in actual operation, it means that the bellows has undergone plastic deformation.

The corrugated expansion joints and compound hinge transverse expansion joints used in the power station industry should be regularly tested for displacement rebound. Don't be too troublesome, you can screen out 80% of the hidden dangers with one test. And guess what? Some factories are too troublesome, and after three years of accident, the bellows got stuck, the stress of the whole pipeline exceeded the standard, and the brackets were crooked.

There is also the system stress exceeding the standard caused by stiffness attenuation, which can be known by measuring the stress of the pipe support with a strain gauge. Don't wait for the bracket to deform before checking, then the cost will double.

4. Key points of aging judgment of different types of expansion joints

Different materials and structures have different "dead points" of aging. Don't look for non-metals with the one you use for metals.

• Metal expansion joint(Including direct burial type, straight pipe pressure balance type, curved pipe pressure balance type, etc.): Focus on the thinning of bellows trough thickness,Ultrasonic thickness gauge is the most accurate。 Large diameter thick wall expansion joints also have to check the weld parts, especially the tie rod joints.
• Rubber compensator and rubber PTFE compensator: The hardness tester brushes and brushes, and it is basically wasted if the hardness increases by more than 20 degrees; Press again to see if there is any elastic restoring force. After the rubber is aged, it becomes hard and can't compensate for the displacement, so the hard brace will crack.
• Non-metallic expansion joints (fabric fiber expansion joints, rectangular non-metallic expansion joints): Pull a piece of fiber to see the strength, or use water pressure to check the sealing performance. The non-metallic expansion joint matched by the flue baffle door is even worse. It runs in high-temperature flue for one year, and the aging rate is three times faster than that indoors.
• Special scenario: Desulfurization flue gas baffle door, air-cooled island vacuum pipeline double hinge expansion joint, these should consider medium corrosion and vacuum fatigue, and it is easy to miss judgment by appearance alone. The fine cracks caused by vacuum fatigue can't be seen clearly with the naked eye, so they have to be tested by penetration.

In addition, the rotary compensator, sleeve-type pipe expansion joint and other poor maintainability, once aging, the repair cost is higher than replacement. So judge more decisively.

5. Break when you break: When should you change instead of repair?

If you find aging, don't always think about making do. Several hard indicators are well held:

• The crack depth of the metal expansion joint exceeds the wall thickness by 10%;
• The leakage test of the non-metallic expansion joint is unqualified, or the damaged area of the fabric layer is more than 10%;
• The stiffness attenuation causes the system stress to exceed the standard (it will be known by measuring the force of the pipeline support with a strain gauge);
• The surface crack depth of the rubber component exceeds 2mm.

At this time, don't worry about "how long it can be used", and go directly to the manufacturer to replace it. For example, rotary compensator, sleeve-type pipe expansion joint, maintenance requires dismantling the pipeline, and the labor cost is more expensive than new parts. Remember, the life of expansion joints is usually only 60%-80% of the design life (refer to the Q&A on the service life of expansion joints), so making spare parts plans in advance is the way to save money. Don't wait until the middle of the night to make a call in a hurry. At that time, doubling the spot price will delay the construction period.

The core of the aging judgment method of expansion joint is "eye diligence, hand diligence and data diligence". Regular inspections and records are better than anything else.

Let's tell the truth first. Two days ago, a purchaser from a chemical plant complained to me, saying that their hot water circulation pipeline used the expansion joint for less than half a year, and suddenly it cracked, and the scene was sprayed in a mess. I asked him if he heard a loud noise, if the pipe was shaking violently, and he said yes, and when the valve was closed, it was like someone had smashed a hammer in the pipe. Okay, solved the case- -water hammer.

The water hammer is called "invisible killer" in the industry. It doesn't take its time like corrosion, wear and tear, it is an instant crit. Think about it, the high-speed flowing liquid is suddenly stopped by a valve or pump, and the kinetic energy is instantly converted into a pressure wave, which bounces back and forth in the pipeline. The peak pressure can reach several times or even ten times the normal working pressure. If the pipe can't hold it, just blow it up.

How does water hammer happen? Understand this "invisible killer" first

A long water pipe, the water is running at a speed of 3 meters per second, and suddenly you shut the valve sharply-the water can't stop. Its forward pressure is blocked by the valve, and the pressure wave begins to propagate upward at the speed of sound from the valve. When it hits the elbow, tee and blind plate, it will be reflected back and form an oscillation. How violent is the peak of this pressure wave? The measured data show that the peak water hammer value of DN300 steel pipe can soar to more than 15kg/cm² under the normal working pressure of 3kg/cm². It's weird that the pipe doesn't crack.

What about that? There are two ways to solve the problem from the root: one is to control the operation, such as slowly closing the valve and installing a slow-closing check valve; The second is to rely on equipment to carry it hard-the expansion joint is the protagonist here.

Expansion Energy Saving Holds Water Hammer? The principle is actually very simple

The core component of the expansion joint is the bellows. Whether it is metal or non-metal, it essentially uses elastic deformation to absorb the displacement and vibration generated by the pipe. When the pressure wave rushes over, the bellows will compress or stretch like a spring, converting kinetic energy into deformation energy, thus reducing the peak pressure. The principle sounds simple, but whether you can hold it depends on the selection and design.

For example, the product of this siteUniversal corrugated expansion jointThe design is mainly to absorb the axial displacement of thermal expansion and contraction, and the wall thickness and wave pitch of its bellows are determined according to the thermal displacement conditions. The pressure wave of a water hammer is a high-frequency shock, which is completely different from the slowly changing load of thermal displacement. You take the universal model to carry the water hammer, which is like taking an umbrella to block a bullet-not that it is completely useless, but it is easy to break through.

Type selection door: Not any expansion joint can prevent hammer

Then what should I use? The key lies in three parameters: pressure grade, stiffness and fatigue life.

• Pressure rating: Do not select according to the working pressure, at least 1.5 times the peak pressure of the water hammer. For example, the working pressure is 4kg/cm², and the peak value of water hammer is calculated as 12kg/cm², so the nominal pressure of the expansion joint has to start with PN16.
• stiffness: The smaller the stiffness, the stronger the ability to absorb shocks. But the stiffness is too low and it is easy to become unstable, so we have to find a balance. Generally recommendedExternal pressure single axial expansion jointIts bellows is subjected to external pressure, has good stability, and the stiffness can be made relatively low, which is suitable for absorbing shocks.
• Fatigue life: Water hammer is a cyclic load, with few times but huge amplitude. The ordinary thermal displacement design may only require a fatigue life of 1000 times, but it is recommended to do more than 10000 times for water hammer resistance. This site'sStraight pipe pressure balanced expansion jointAndCompound hinge transverse expansion jointThere are redundancies in fatigue design, which can cope with such shocks.

In addition, the design of the guide tube cannot be ignored. The guide tube can guide the medium smoothly through the bellows, avoid vortex impact, and at the same time, it can also prevent high-pressure waves from directly impacting the root of the bellows. In the product information of this site,Corrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAll of them have been strengthened on the diversion tube, so you can see the specific structure diagram.

If it is a corrosive medium or high temperature working condition, you have to consider the material. Such as usingPTFE-lined hoseOrPTFE compensatorThey are corrosion resistant, but their pressure-bearing capacity is relatively weak, so they need to be installed with a limit tie rod. How to adjust the limit pull rod? You can refer to "How to Adjust the Tie Rod Nut of Expansion Joint" we wrote before. Simply put, loosen the nut after installation, let the bellows expand and contract freely, and tighten it to the set pre-stretching amount.

Installation and configuration: wrong location, wasted effect

If you choose the right product, it will be useless if you can't install it. Under water hammer conditions, there are several iron laws for the installation of expansion joints:

• The fixing bracket must be strong enough: The expansion joint itself does not bear the blind plate force, and the blind plate force should be carried by the fixed bracket. Under the impact of water hammer, if the stiffness of the fixed bracket is not enough, the whole pipe will be pushed away, and the expansion joint will directly exceed the displacement limit and tear. The main fixing point should be set near the valve, elbow and reducing diameter.
• The spacing between guide brackets should be calculated well: In order to prevent the pipe from swinging laterally under the action of pressure waves, the spacing of the guide brackets shall not exceed 80% of the allowable span of the pipe. Especially for horizontal pipe sections, if the spacing is too large, the pipe will swing up like a fish's tail and twist the expansion joint.
• Pre-stretch/pre-compression should be accurate: If there is a large difference between the installation ambient temperature and the working temperature, it is necessary to pre-stretch or pre-compress according to the calculated value. For specific operation, please refer to the steps in "Correct Installation Method of Large Tie Rod Expansion Joint". The core is to loosen the nut → pull or press to the predetermined size → lock the nut.
• Don't reverse the direction of the expansion joint: Many products have arrows on them, which is where the medium flows. If the direction of the arrow is wrong, the position of the guide tube will be reversed, and the high-pressure wave will directly hit the inside of the corrugation, which is easy to tear.

Two days ago, another customer asked me that they had installed the pipelinerubber compensatorIs it more resistant to water hammer than metal? The rubber compensator does have a good vibration damping effect, but its pressure resistance is not as good as the metal bellows, and it ages quickly. If your water hammer has high pressure and high frequency, it is recommended to use itMetal expansion jointRubber is suitable for low pressure and small amplitude applications, such as water pump inlet and outlet.

Comparison of real cases: How big is the gap between putting it right and putting it wrong

Last year, there was a steam condensate recovery system in a paper mill, with a design pressure of 6kg/cm² and an operating temperature of 95℃. They originally used a certain brand of general-purpose corrugated expansion joint, which took about 8 months. After an emergency pump stop, a DN250 expansion joint on the main pipeline exploded, and the corrugated pipe directly tore open a 30cm hole. The scene was filled with water vapor, almost injuring people.

The peak water hammer is at least 20kg/cm², while the nominal pressure of the expansion joint they use is only PN10, and the fatigue life design is only 500 times. Moreover, the spacing between the guide brackets is too large during installation, and the expansion joint is twisted by transverse sliding of the pipeline during impact, and the stress at the root of the bellows is concentrated, which leads to direct brittle fracture.

Later we changed the one we recommendedExternal pressure single axial expansion jointWith a nominal pressure of PN25 and a nominal fatigue life of 12,000 times, a fixed bracket is added on the outlet side of the valve, and the spacing between the guide brackets is reduced from 6 meters to 3.5 meters. I haven't had any accidents for more than two years now. Is that the truth? Choosing the right model and installing the right position doesn't cost much, but it saves a lot of worry.

Another case is the steam turbine bypass pipeline of a power plant, high temperature and high pressure steam, and the risk of water hammer is also high. They usedStraight pipe pressure balanced expansion jointThis structure has its own pressure balance cavity, which can effectively offset the blind plate force. At the same time, the bellows adopts multi-layer thin-wall design, which has good flexibility and obvious effect of absorbing shock waves. After using it for three years, when I opened it and inspected it, I couldn't even find cracks on the surface of the bellows, and there was only slight oxidation on the top. Therefore, don't save that little money, spend more time on the selection, and the maintenance cost can save a lot.

Finally, water hammer protection can not be solved by expansion joints alone, but also by integrating pipe system design, operation process and bracket configuration. However, as the last line of defense, the expansion joint can save your life if you choose it right, but if you choose it wrong, it will be a hidden danger. If there are any specific project conditions, feel free to ask. Let's calculate the parameters together and don't let the pipeline crack again.

Can the expansion joint be buried in the ground? Don't jump to conclusions yet

Two days ago, I met a customer, and he was in a hurry on the phone: "Our heating pipeline was buried with ordinary expansion joints, and it leaked in one heating season. Can you help me?" Alas, I have seen this too much. In order to save that procurement cost, many Party A directly buried the general-purpose corrugated expansion joint in the soil, feeling that "it's invisible when buried anyway". And the result? As soon as the groundwater bubbles and the soil is pressured, the bellows become unstable, corroded and fractured by fatigue, and the whole set of pipes have to be dug out and reworked.

Can expansion joints be used buried in the ground?The answer is: ordinary ones can't work, but the "directly buried (fully buried) expansion joint" of this station is specialized in this job.

Why can't ordinary expansion joints bear the underground environment?

Standard metal corrugated expansion joints (such as general corrugated expansion joints and high temperature axial expansion joints) are designed without considering external loads at all. It works by default in overhead pipes or pipe trenches with no soil pressure around, no vehicle crushing, and no groundwater infiltration. You bury it, which is equivalent to asking a man in a suit to carry the sandbag-the bellows are directly deformed by earth pressure, and coupled with poor drainage, the rate of corrosion doubles.

In order to save investment, a thermal power company in the north directly buried the general-purpose corrugated expansion joint, and the backfill soil was mixed with gravel. In the first winter of operation, the bellows was pitted by gravel, coupled with water vapor corrosion, and the weld cracked. Finally, the whole pipe section was scrapped, and the excavator dug for three days before replacing it with new parts. It is more than three times more expensive than buying the direct burial type at the beginning.

What is special about the directly buried expansion joint? To put it bluntly, wearing "body armor"

The "directly buried (fully buried) expansion joint" of this station has three layers of protection in structure:
First, the outer sheath is an integral welded carbon steel or stainless steel casing that directly bears soil pressure and ground dynamic loads (such as car rolling).
Second, the interior is filled with insulation material to prevent heat loss, especially suitable for steam pipes and thermal pipes.
Third, the end seal structure can prevent groundwater from seeping into the bellows and avoid electrochemical corrosion.

Note that it is not the same thing as a "sleeve-type pipe expansion joint" or a "rotary compensator". Sleeve type is sealed by packing, which is easy to leak after long-term operation; The rotary compensator absorbs displacement by rotation, but the installation space requires a large amount. Direct buried expansion joints are specially designed for fully buried environments, so don't mix them.

How many of the three easiest pits to step on when selecting models?

Pit 1: Only look at nominal pressure, regardless of external load.Some people think that since the directly buried expansion joint is called "directly buried", just choose a pressure level. SO WRONG! With a buried depth of 2 meters and a buried depth of 0.5 meters, the soil pressure difference is several times. If a car or excavator passes through the ground, the dynamic load is even more terrible. The pressure bearing capacity must be re-checked according to the actual buried depth and ground use.

Pit 2: Ignoring the axial thrust, the fixed bracket is weak.Both ends of buried pipes must be provided with sufficiently strong fixing brackets. In many projects, in order to save costs, the bracket is welded with thin steel plates. As a result, when the pipeline expands thermally, the axial thrust directly pushes the bracket askew, and the bellows is squeezed to be twisted and deformed. Remember: When the directly buried expansion joint compensates for displacement, the axial thrust must be borne by the fixed bracket, and the strength of the bracket cannot be sloppy.

Pit 3: The anti-corrosion grade is lazy, but the insulation layer accelerates corrosion.If there is an insulation layer in the directly buried part, once the moisture penetrates, the insulation material will become a "water absorber", and the risk of electrochemical corrosion is higher than that of exposed metal. Epoxy coating or cathodic protection is recommended, especially in areas with large burial depth and high groundwater table.

If you don't keep an eye on the installation details, no matter how good the expansion joint is, it will be useless

Backfill soil must be tamped in layers, each layer not more than 30 cm thick, and there must be no large pieces of rubble or sharp objects. At least 50cm of operating space should be left on both sides of the expansion joint to facilitate later maintenance-don't bury the bolts in concrete, and you can't screw them even if you want to.

The soil can be covered only after the pressure test is completed. The pressure test pressure is generally 1.5 times of the design pressure, and the pressure can be kept for 30 minutes without leakage before the soil can be filled. The thickness of the covering soil must be according to the design drawings, and it cannot be thickened at will. We have done a project, and the customer buried the directly buried expansion joint 2 meters deep (the design was only 1.2 meters) to save trouble. As a result, the compensation amount was not enough, and the pipeline collapsed directly from the interface during thermal expansion.

Not all buried pipes need to be buried directly, but thermal steam pipes must be honestly selected

For example, the circulating water pipes of power plants and the conveying pipes of cement plants have low medium temperature and low pressure, so it can be done with "rubber compensator" or "polytetrafluoroethylene compensator" with anticorrosion treatment. However, thermal pipelines and steam pipelines-especially those with a medium temperature exceeding 200℃ and a pipeline length of several hundred meters-honestly choose the "directly buried (fully buried) expansion joint" of this station, and don't joke about its service life.

Not sure about the selection? You can ask our technical department for the parameter table of buried working conditions, fill in the soil type, buried depth, ground load, medium temperature and pressure clearly, and we will give you a calculation book. After all, the final answer to the question "Can expansion joints be buried and used?" Depends on whether you choose the right product or not.

Section 1: The "Safety Illusion" of Overchoosing-Why do many people think bigger is better?

When choosing expansion joints in the project, many people hold the mentality of "rather big than small". I think with a larger caliber and more wavenumber, the compensation ability will definitely be stronger and the system will be safer. Doesn't that sound reasonable? But the actual failure case slapped the face. Two days ago, I talked to a master who did power plant maintenance. He said that several of the corrugated expansion joints replaced on their pipelines were fatigue cracked because of the large selection. The principle is actually very simple: the larger the size of the expansion joint, the greater the stress generated when it is subjected to internal pressure, especially the position of wave peaks and troughs. Moreover, after the selection is large, the actual displacement of the system may be much smaller than the design value, and the corrugation is in a state of "not having enough food" for a long time, so the creep and stress concentration of the material are more obvious. Tsk, is it the opposite of intuition?

Section 2: What trouble will you cause if you elect big? — From instability to air leakage, don't try to hide from any

Let's start with instability. The expansion section is large, especially for large-diameter thick-walled expansion joints or external pressure single axial expansion joints. If the nominal diameter exceeds the actual pipeline by a lot, and the end constraints are not adjusted accordingly, plane instability or columnar instability will easily occur. Once the bellows becomes unstable, it is only a matter of time before the seal fails. Let's talk about fatigue life. The life of the expansion joint depends on the number of cycles and stress levels. If a large expansion joint is selected, the actual stress value of the bellows may exceed the design standard by more than 30%-the original designed life of 100,000 times will crack after 20,000 times. There is also the problem of installation space: the expansion joint is selected to occupy more axial space, and the layout of pipe bracket and guide bracket has to be changed accordingly, or it may interfere with adjacent equipment. Not to mention the cost. Stainless steel materials cost twice as much, and flanges, tie rods and guide tubes are all upgraded, which is pure waste.

Section 3: Under different working conditions, the "lethality" of excessive selection has its own emphasis

The corrugated expansion joint used in the power station industry is often on high-temperature and high-pressure steam pipelines. If it is selected large, the wall thickness of the corrugated pipe will be relatively thin and the pressure resistance will decrease. The metal corrugated expansion joint used in the cement industry contains more dust in the medium. If the flow rate is larger, the dust is easier to accumulate in the trough and the corrosion is aggravated. If a non-metallic expansion joint (fabric fiber expansion joint) is used, the consequence of choosing a large one is more direct-the tension bearing by the fabric layer is uneven, and it will bulge and tear in the short term. For the rubber compensator, choosing a large one will also make the rubber body crack by fatigue. Let's take a look at the rotary compensator and sleeve-type pipe expansion joint. Such products are sealed by packing. If the casing gap is larger, the sealing packing can't be pressed tightly, and air leakage is common. After all, every type of expansion joint has its best working size range, and blindly choosing large is digging a hole for yourself.

Section 4: What exactly is called "choosing the right"? Don't slap your head

Many people get the pipeline layout drawing, and as soon as they see the thermal displacement of 200mm, they directly choose an expansion joint with a rated compensation of 250mm. This approach is only one-third correct. The correct selection process should be: first clarify the medium, temperature, pressure and nominal diameter of the pipeline, then calculate the thermal elongation and cold tightness, and also consider the installation error and additional displacement of the end point. For general-purpose corrugated expansion joints, a safety margin of 10% ~15% should be left when selecting the model, but it should never exceed 20%. If it is a high-temperature axial expansion joint, the allowable stress of the material at the highest operating temperature should also be checked. There is also one that is easy to overlook-the deflector. For example, the diameter of the selected expansion joint is larger than that of the pipeline, and the size of the guide tube has to change accordingly. Otherwise, the medium will directly wash the inner wall of the bellows and wear it out in a few months. Therefore, the selection is not done by looking at the numerical value, but the guide tube, the tie rod and the limit structure have to be taken into account.

Section 5: What if I have already installed it? Can you remedy it if you elect it big?

This problem is case by case. If the expansion joint is only a small amount too large (say one gauge larger than the actual pipe) and space allows, you can limit the amount of compression of the bellows by adjusting the tie rod nut, or add pre-tension/pre-compression to match the actual displacement. For structures with guide barrels, the guide barrels can also be replaced to improve flow path matching. However, if you choose more than two specifications, or there are signs of instability, then don't hesitate to change it directly. The cost of forcibly making do is far greater than that of replacing a new one-the loss of shutdown and production and the risk of safety accidents can't be afforded. In addition, a reminder: the screw and tie rod of the expansion joint are not decorations, and they must be adjusted in place according to the actual displacement before installation. For details, you can check the question and answer "How to adjust the tie rod nut of the expansion joint" we wrote before.

Section 6: The most afraid thing about model selection is "taking it for granted"

The expansion joint is not the bigger the better, nor the smaller the better, it is "just right" the best. Different working conditions, different media and different pipeline layout correspond to different models and sizes of expansion joints. For example, high-temperature axial type is used for high-temperature pipelines, straight pipe pressure balance type is used for large temperature differences, compound hinge transverse type is used for transverse displacement, and directly buried (fully buried) expansion joint is used for buried in soil. Each product has its own "temper". Before the selection, it is best to communicate with the technicians of the manufacturer and give all the working condition parameters. Remember: Does inflated excerpts have an impact when they are big? The answer is clear — not only impactful, but potentially fatal.

First, what is the metal expansion joint used for? — Don't be fooled by the name, the core is two words: compensation

Two days ago, I met a customer who came up and asked, "Is the metal expansion joint the same as the expansion joint on the radiator?" Alas, it means about the same, but it is far from that simple. To put it bluntly, as soon as the temperature in the pipeline system changes, the pipe will expand and contract by heat. The result of hard pulling is that the interface collapses and the equipment is crooked. Metal expansion joints do just that — they absorb displacement and release stress. You can call it compensator. These two words are common in the industry (refer to Article 14 of the FAQ in our site). There are two words at the core: compensation. Compensate for axial expansion and contraction, lateral offset, and even angular torsion. Without it, steam pipes and heat pipe networks will show you a look in minutes.

What we produceUniversal corrugated expansion jointIn chemical pipelines, they are soaked in high-temperature steam every day, and expand and contract hundreds of times all year round, relying on the elastic deformation of the thin walls of the bellows. AndCorrugated expansion joint for power station industryThe temperature of the main steam pipe of the power plant can soar to five or six hundred degrees. Without it, the pipe would have collapsed early.

Second, there are so many structural classifications, which ones are commonly used by us? — Bellows, sleeves, non-metals, each with its own temper

When you stand in our warehouse, you can see that the densely packed expansion joints are actually divided into three categories according to their structure: bellows type, sleeve type and non-metallic type.

Bellows expansion jointIt is the main force, relying on the deflection of ripples to absorb displacement. There are many sub-classes: such asHigh temperature axial expansion jointSpecialized in carrying high temperature;Straight pipe pressure balanced expansion jointIt can balance the blind plate force and be used on large pipes under pressure;Compound hinge transverse expansion jointIt can absorb lateral displacement and fit where the space turns. AndExternal pressure single axial expansion jointExternal pressure structure can withstand higher pressure, which is common in petrochemical industry.

Sleeve type pipe expansion jointIt is another way, which relies on the sliding between sleeves to compensate for the displacement. The structure is simple but the sealing requirements are high. It is generally used in low pressure and large displacement occasions, such as heating pipe network.

Besides, non-metallic ones, we have them on our stationNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion joint。 This thing is made of fabric and rubber, which can absorb multi-dimensional displacement and is corrosion-resistant, especially in smoke ducts and desulfurization systems. Oh, yeah,rubber compensatorAndRubber PTFE compensatorAlso in this category, the vibration damping effect is first-class.

Each structure has its own temper. If you choose the wrong one, the oil and water leakage in two years will be light, and the whole pipeline system may collapse.

Third, if you don't look at these three points in type selection, you will most likely step on pits-medium, temperature and displacement, one cannot be missed

After so many years of explaining metal expansion joints, I have seen too many people who pat their heads and select models. Let me tell you, first figure out these three parameters:

• Medium: What fluid goes? Strong corrosive, must be usedPTFE-lined hoseOrPTFE compensator； For dusty flue gas, wear resistance and guide tube structure should be considered.
• Temperature: Temperature directly determines the material. Use 304 stainless steel at room temperature, and you have to use heat-resistant alloy for five or six hundred degrees. Our oneHigh temperature axial expansion jointThe upper limit of the design temperature can reach more than 1000℃.
• displacement amount: How does the pipeline go? How much is axial telescoping? How big is the lateral offset? Every direction should be calculated clearly. Such asDouble hinge expansion joint for air-cooled island vacuum pipelineSpecially designed for vacuum large diameter pipeline, can absorb large angular displacement.

Don't think that these three values are enough to know. In actual engineering, they will affect each other. For example, high temperature + corrosive media, optical material selection is a knowledge. You choose an ordinary stainless steel bellows, and it won't take long to corrode and perforate. So don't be lazy and give your working condition parameters to the manufacturer honestly.

4. In installation and daily maintenance, the most easily overlooked details-the orientation of the guide tube, the adjustment of the pull rod, and whether the screw can be removed or not

Installing expansion joints looks simple, but the details determine the longevity. The first easy mistake to make is to install the deflector in the reverse direction. We have a FAQ on our siteSpecific Function of Expansion Joint Guide TubeThe guide tube is used to guide the flow direction of the medium and protect the bellows from being washed and worn. The direction of the arrow must be consistent with the flow direction of the medium (refer to Article 10 of Q&A). Install it backwards, and the guide tube is a plug, which will aggravate the wear.

How to adjust the tie rod nut? Many customers receive the expansion joint and find that there are tie rods and nuts fixed at both ends, thinking that it is used for reinforcement, and it will not be disassembled. Wrong! These tie rods (also called transport tie rods) are used to prevent the bellows from deforming during transportation. After being installed in place, the nut should be loosened, adjusted to the working position, or removed directly (refer to Q&A 12 and 13). Otherwise, the expansion joint can't expand and contract freely at all. What is the difference between it and a steel pipe?

AndCorrect installation method of expansion joint of large tie rodThe large tie rod structure is to bear the internal pressure thrust, and the tie rod must be adjusted to the designed length during installation, and cannot be screwed randomly. Routine maintenance is to regularly check the bellows for cracks and corrosion spots, especially thoseDesulfurization flue gas baffle doorNext to the expansion joint, the acidic environment is most prone to problems.

5. Common faults and misunderstandings: Why did it leak after two years of use? — — Fatigue, corrosion, installation stress, take the right seat

"The quality of your expansion joint is not good. It leaked in two years." Every time I heard such words, I always asked about the working condition first. Nine times out of ten, it is a problem of type selection or installation. There are three common reasons for leakage:

• Fatigue failure: If the pipeline vibrates frequently, or the design displacement is smaller than the actual, the bellows bends repeatedly, and the metal fatigue causes cracks. This situation can be consideredMetal hoseOrSpecial hose for vacuumTo dampen the vibration.
• Corrosion perforation: There are chloride ions and sulfide in the medium, and stainless steel still can't bear it. Either reline PTFE or choose high nickel alloy. WeMetal Corrugated Expansion Joints in Cement IndustryOften encounter sulfur-containing flue gas, the surface has to be made with anti-corrosion coating.
• Installation stress: The pipeline is forced to be installed at the opposite side, or the tie rod is not loosened, and the force of the expansion joint exceeds the design. When installing, make sure that the expansion joint is in a free state, and don't pull hard.

Think that the expansion joint is "all-purpose" and can absorb all displacement. Virtually every model has a cap on the ability to compensate. Such asDirect buried (fully buried) type expansion jointSpecially used for directly buried pipelines, the axial compensation is large but the lateral compensation is limited. If the pipe turns too much, you have to cooperateRotary compensatorOrCurved tube pressure balance expansion joint。

This thing of expansion joint, three points depends on quality, seven points depends on selection and installation. Don't just buy a general-purpose model to save money and trouble. When you're not sure, flip through the "Explanation of metal expansion joint"Series articles, or directly find technology to talk about working conditions. Save those few minutes of communication, and the maintenance fee later will be enough for you to buy ten expansion joints.