Specialized in manufacturing compensators, expansion joints, baffle doors

Find out first: the expansion joint is fixed, is the pipeline fixed or the expansion joint itself?

Many customers came up and asked, "How to fix the metal expansion joint?", and then planned to hold the expansion joint body with a bracket. This is a mistake of principle. The expansion joint itself is a flexible element used to absorb the thermal displacement of the pipe. If you hold it tightly down, how can it deform? Doesn't that mean pretending for nothing?

What really should be fixed is the pipe. The pipes on both sides of the expansion joint need to be firmly locked by the fixed bracket, so that all the displacements caused by thermal expansion and contraction are concentrated in the "buffer zone" of the expansion joint. On the contrary, the expansion joint body cannot bear any external restraining force, it only needs to expand and contract freely along the displacement of the pipe. In other words, the fixed bracket holds the pipe, the guide bracket holds the direction, and the expansion joint only deforms.

How to arrange the fixed bracket and the guide bracket? Distance, force and position are all special

The layout principle of the fixed bracket, in one sentence: divide the pipe into several independent compensation pipe sections. There can only be at most one expansion joint in each pipe section, and the fixing bracket must be arranged at both ends of the expansion joint, and the distance should be close enough. Why? Because if the fixed bracket is too far from the expansion joint, the friction force of the middle pipeline itself and the blind plate force generated by the medium pressure will be applied to the expansion joint, which will cause the compensation amount to fail at least and the bellows to crack at worst.

Taking the common general-purpose corrugated expansion joint as an example, the distance between the first guide bracket and the end of the expansion joint should be controlled within 4 times the nominal diameter, and the distance between the second guide bracket and the first guide bracket should not exceed 14 times the nominal diameter. Intermediate guide brackets should be set at intervals at the back. The specific distance depends on the stiffness and allowable deflection of the pipe. This is not a head-slapping number, it comes from the pipe system stress analysis. CAESAR II is commonly used in engineering for checking, but you have to know at least these basic limits.

In terms of force, the fixed bracket should be able to withstand the blind plate force generated by the medium pressure, the friction force of the pipeline, the elastic reaction force of the expansion joint and the weight of the pipeline. Blind plate force is a big head. The higher the pressure and the larger the caliber, the greater the force. Therefore, large-diameter thick-walled expansion joints, fixed brackets of the level of corrugated expansion joints used in power station industry, and concrete foundations must be calculated by tons, so they must not be saved.

The tie rod/nut on the expansion joint should be removed or not during installation? How to tune it?

It's a high-frequency problem. The expansion joint leaves the factory with tie rods and nuts, mainly to prevent the bellows from being bruised and strained during transportation and hoisting. Once installed in place, the tie rod nut must be loosened to allow the expansion energy saver to deform freely. If you don't loosen it, the expansion joint will be locked, and the displacement of the pipe will be stuck in the system, and the flange, weld and equipment mouth will all suffer.

When will the tie rod be used? Two scenarios. First, if the expansion joint needs to bear the blind plate force generated by internal pressure, and no pressure balanced expansion joint is designed, the tie rod can be used as a restraining element, but note that the tie rod at this time is not for you to tighten, but is adjusted to the allowable displacement range. Second, the working conditions that require pre-stretching or pre-compression before installation are realized by the tie rod nut. The adjustment method is: first loosen the locking nut, rotate the adjustment nut to compress or stretch the bellows to the specified amount, and then tighten the locking nut. The specific pre-displacement depends on the difference between the installation temperature and the calculated temperature of the pipeline. Don't screw it blindly.

How to adjust the expansion joint tie rod nut, we specifically talked about it in our previous article, and here we emphasize the core-after the adjustment, the locking nut must be tight back, otherwise the vibration during operation will cause loosening and the tie rod will lose its function.

Taboo: Take the expansion joint as a pipe support and hanger. These wrong installation methods should be avoided

The most typical mistake I have seen is that someone welds the pipe bracket directly to the expansion joint bellows, or uses a pipe clamp to pick up the expansion joint body to bear the load. The wall thickness of corrugated pipe is usually only a few tenths to a few millimeters. If you use it as a hanger, who will perforate if it doesn't perforate?

Another type of error is that the guide bracket is installed in the wrong direction. The role of the guide bracket is to limit the radial displacement of the pipe and allow axial displacement. As a result, the snap ring of the guide bracket and the pipe were welded to death on the spot, or the gap was left too small. As soon as the pipe expanded by heat, the guide bracket became a fixed bracket, and the expansion joint was crushed to death. The correct method is: the inner diameter of the snap ring of the guide bracket should be slightly larger than the outer diameter of the pipe, leaving an axial sliding space, but the gap should not be too large, otherwise it will not play a guiding role.

In addition, when installing the expansion joint, don't forcibly use the expansion joint to compensate for the installation error in order to adjust the pipeline counterpart deviation. Corrugated pipe has high requirements for coaxiality, and if it is forcibly installed in misalignment, the local stress will increase exponentially, and it will not take long for fatigue cracking.

Differences in Fixation of Different Expansion Joint Types: What to Pay Attention to for Axial Type, Transverse Type and Pressure Balance Type

Different types of expansion joints, the arrangement logic of fixed brackets and guide brackets is different.

Axial expansion joints, such as general corrugated expansion joints and external pressure single axial expansion joints, mainly absorb axial displacement. The fixed brackets at both ends must firmly lock the pipeline, and the guide brackets must be arranged in the middle according to the aforementioned spacing to ensure that the expansion joint expands and contracts along the axis direction.

Transverse expansion joints, such as double-hinged transverse expansion joints and double-hinged expansion joints for air-cooled island vacuum pipelines, absorb lateral displacement through hinge or universal joint structures, but do not absorb axial displacement themselves. Such expansion joints usually need to be used in groups of two or three. The layout of the fixed bracket should especially consider the bending moment generated by lateral displacement, and the direction of the hinge pin should be consistent with the displacement direction, otherwise the hinge will be pinned and the expansion joint will be wasted.

Pressure balance expansion joints are even more interesting, such as straight pipe pressure balance expansion joints, curved pipe pressure balance expansion joints and compound straight pipe bypass pressure balance expansion joints. They come with their own balanced bellows, which can offset the blind plate force generated by internal pressure, so the force on the fixed bracket can be greatly reduced. However, the guide bracket still cannot be saved, because the balanced expansion joint is more sensitive to lateral displacement and angular displacement, and once deflection occurs, the balancing cavity is easily unstable.

All fixing brackets and guide brackets must be in place prior to installation of the expansion joint. During the pipeline hydraulic test, the fixed brackets at both ends of the expansion joint should bear the blind plate force under the test pressure, which is greater than normal operation, so the welds and anchor bolts of the brackets must be re-checked before the test.

If there is a problem with the metal expansion joint, don't rush to replace it with a new one. It is simple to change another, but the cost is high, the downtime is long, and it can actually be repaired in many cases. The point is-you have to figure out what kind of injury it is. Is the bellows fatigued? Or is it cracked by the media? Or was it strained by brutal operation during installation? These three situations are dealt with in completely different ways.

Two days ago, I met a customer who said that a general-purpose corrugated expansion joint on their steam pipeline was leaking. When I removed it, the corrugation was concave but not cracked. This situation does not need to be changed at all, it can be rectified with pressure correction. But if it is another kind-intergranular corrosion cracks appear on stainless steel bellows, then it is useless for you to use pressure correction. After the external shaping is completed, the internal cracks are still there, and they will still leak once pressurized. In this case, you have to repair welding, or directly replace the bellows section. So the first step is not hands-on, but judgment.

Three things to test before recovery. If one thing is missing, it will be in vain

After judging the type of damage, don't rush to get started. Do three things first, otherwise the cultivation will be in vain.

The first piece, wall thickness measurement.Use an ultrasonic thickness gauge to dot the peaks and valleys at multiple points. Don't only measure one or two points. The wall thickness of one circle of bellows may not be uniform, and media erosion and corrosion thinning are often concentrated on one side. Multi-point measurement can be used to find out the real situation. Wall thickness reduction by more than 10% of the design wall thickness? Then don't fix it, just change it.

The second piece, the hydraulic test.The test pressure is 1.5 times the design pressure, and the pressure is maintained for 10 minutes. At this time, my eyes were kept close to see if there was any leakage or residual deformation. Many expansion joints "look okay, but leak when used", that is, the hydraulic test failed.

Third, displacement review.Compare the factory pre-stretch amount, measure the actual length of the current expansion joint, calculate how much displacement it has absorbed, and whether there is any margin. If it exceeds the design compensation amount, it is useless for you to finish repairing the bellows. The thermal displacement of the pipe has not been digested at all, and it will be broken again soon after installation.

Deflector tube. Many people only look at the bellows and forget to check whether the guide tube is worn out by the medium erosion. Once the guide tube is worn out and the high-speed fluid directly washes the inner wall of the bellows, that is the real murderer. You look fine with the bellows on the outside, but the inside has been hollowed out.

Different structures, restoration methods vary greatly

Axial type, hinge type and pressure balance type have different structures, different failure modes and essential differences in recovery means.

The loss of elasticity of axial expansion joint is mostly due to the attenuation of bellows stiffness. How to recover? By pressing the bellows into a predetermined position by re-pre-deforming, that is, cold tightening, a portion of the elastic displacement capacity can be restored. To put it bluntly, it is to "re-teach" the bellows and make it remember its range of elasticity.

What about the double hinge transverse expansion joint? If the hinge pin is worn and the clearance becomes larger, the light reset is useless. Once the clearance between the pin and the bushing exceeds tolerance, the motion trajectory of the expansion joint is deviated. You pull it back in place, and it will deviate back once it runs. In this case, the pin shaft and bushing must be replaced, and the hinge clearance must be repaired to the design tolerance range before it is truly recovered.

Straight pipe pressure balance type expansion joints are the most troublesome. When the middle cylinder is deformed, you can't pull the bellows directly-that will completely mess up the force relationship between the balancing bellows and the working bellows. The correct way is to disassemble it, check the compression and rebound of the balanced bellows and the working bellows respectively, judge which side has a problem, and then deal with it in a targeted manner.

Field repair of weld leakage and bellows crack

Small cracks, length not exceeding one third of the circumferential wavelength, can be repaired with argon arc welding. Before welding, polish the crack out of the groove to expose fresh metal, and do coloring flaw detection after welding. Here's a detail:Avoid arcing on bellows crest。 The crest is where the stress is most concentrated, and starting an arc there is equivalent to planting a new crack source by hand. The grade of the welding rod must match the base metal. If the wrong welding rod is used in the stainless steel corrugated pipe, new cracks will appear immediately in the heat affected zone.

However, if the cracks appear in large pieces and groups, it means that there is something wrong with the material or medium. The most common culprit is chloride ion stress corrosion. At this time, even if you make up all the cracks, it is useless to restore them to the original material-the corrosion conditions haven't changed, and they cracked again not long after. In this case, either lining with a corrosion resistant layer or directly replacing it with a material resistant to chloride ion corrosion. Hard supplement, it can't be made up.

What about corrosion only occurring in local areas? You can cut the damaged bellows section, replace it with a new one, and butt it with a ring weld. However, it should be noted that the newly replaced bellows section must be completely consistent with the wave pitch, wave height, material grade and layer number of the original bellows, otherwise the stiffness will not match and the force will be uneven, and the newly replaced section will become the weakest point.

Performance verification after recovery, and pits for reloading

Finished fixing, don't rush to go online. First do the air tightness test at room temperature, and then use the working medium to do the withstand pressure test. The airtightness test is to check leakage, and the pressure test is to check strength. Each of the two checkpoints is indispensable.

When reloading, there are a few pits to avoid.

First, the tie rod nut must be loosened. The tie rod of the expansion joint is used for fixing during transportation and installation, and must be in a free state during operation. Many expansion joints are "restored and broken" because the user locks the tie rod too dead, turning the axial compensation into a rigid constraint-the force of thermal expansion and contraction is all held in the bellows, so it's weird if it's not bad. The correct way is to let the expansion joint be in a free state after recovery, and only use the tie rod when adjusting the cold tightness.

Second, check whether the fixing bracket and guide bracket are in correct position. If the fixed bracket is not fixed firmly and the guide bracket slips off, the expansion joint will bear additional force outside the design, and this force is continuous and repeated, which has a great influence on the life of the bellows. How to Recover Metal Expansion Joints? Repairing the bellows is only the first step, and if the systemic problem is not solved, it will still break again in the same place.

In the final analysis, the expansion energy saving can't be restored, depending on whether you can accurately judge the type of damage, repair it strictly according to the process, and eliminate the root cause of the damage. These three steps are in place, and the restored expansion joint will be used for another three to five years. If you want to save trouble, just weld and tighten the tie rod no matter what injury, then you might as well buy a new one.

Non-Metallic Expansion Joint Disadvantages? Don't wait until you leak to understand these pits. After more than ten years of pipeline compensation, I have seen too many projects stumble on non-metallic expansion joints-it is not that it is bad, but that many people only focus on its advantages of cheap and large displacement resistance, and throw its shortcomings behind them. Today, if you don't brag or black, explain these pits clearly one by one.

Pressure capacity is a flaw

The core structure of non-metallic expansion joints (fabric fiber expansion joints) is a fiber fabric composite layer plus a metal frame, which is subjected to pressure by flexible bands? Thinking too much. Its pressure-bearing skeleton is essentially a fiber cloth and sealing layer, not a rigid metal shell. When the pressure is slightly higher, the circle belt will bulge like blowing a balloon, and if it is higher, it will tear directly.

In practical applications, non-metallic expansion joints can usually only be used in pipeline systems below 0.1MPa, and many smoke and air duct working conditions even only have a micro positive pressure of several thousand Pa. Compared with the metal corrugated expansion joint, the stainless steel corrugated pipe can easily carry 1.6MPa, and the special corrugated expansion energy saving for high-pressure steam pipeline can achieve more than 10MPa. And guess what? Someone dared to install a non-metallic expansion joint on the compressed air line, but the loop belt burst after half an hour of turning on. Have you ever seen a pressure cooker seal with canvas? No, no.

Temperature resistance is not infinite

The limit of fabric fibers lies there. The long-term temperature resistance of conventional silica gel-coated glass fiber products is ≤250℃, fluororubber ≤200℃ and polytetrafluoroethylene ≤150℃. Even if it is compounded with high silicon oxygen or ceramic fiber, and the limit is about 1000℃, it is also a special customization, and the cost is high. If it runs at a temperature close to the upper limit for a long time, the fiber will gradually embrittle and powder, and finally leak.

Two days ago, I met a buddy from a cement factory. The outlet temperature of the high-temperature fan frequently rushed above 900℃, and there was no margin left in the design. The non-metallic ring belt was ablated like a ragged rag in less than a year, and the scene was terrible. Therefore, leave at least 20% of the temperature margin when selecting the model, and don't take the upper limit as a normal working condition.

Easy to corrode and wear? Sulfur, chlorine, and dust in the medium are all enemies

In scenes such as flue gas baffle door and desulfurization flue gas baffle door, the non-metallic belt is not faced with clean air, but a mixture containing SO₂, chloride ions and high humidity acidic gas. Once the acid penetrates into the fiber layer, it directly corrodes the internal metal frame and connecting bolts. Dusty airflow washes at high speed, and the surface of the ring belt is like sandpaper polishing, and it is worn through from the weak point first.

Surface peeling → fiber fracture → perforation leakage. In a desulfurization project, acid-resistant coating belt was not used in order to save money. Half a year later, the belt was rotten like honeycomb briquettes, the baffle door was not closed tightly, and the whole system had serious air leakage. If there is sulfur, chlorine and dust in the medium, you have to choose the corresponding corrosion-resistant and wear-resistant composite layer. Don't think that all non-metallic expansion joints are the same.

Installation and replacement are not that simple

Many people think that non-metallic expansion joints are flexible, so they can be installed casually. Wrong! Precisely because of its softness, it has more demanding requirements for pre-stretching and limiting than metal expansion joints. Rectangular non-metallic expansion joints and round non-metallic expansion joints must be adjusted according to the designed cold drawing value when installing-the cold drawing amount is not enough, there is no compensation margin in the hot state, and the ring belt is pulled and cracked; The amount of cold drawing is too large, and it is squeezed into wrinkles when it is cold, and it doesn't take long to wear out.

The limit screw and the transport fixture should not be disassembled blindly. Some on-site workers try to save trouble. As soon as they come up, they screw off all the limit bolts. As soon as the pipeline expands slightly, the ring belt directly tears open a big hole. After the installation is completed, the adjustment of the limit device should be adjusted, and the removal of the limit device should be removed according to the manufacturer's instructions. Flexibility is not casual, but more demanding.

Lifespan Anxiety: Exactly How Many Years Will It Last?

There is no uniform answer to this question, but it varies greatly. Clean air, temperature no more than 150℃, no corrosive medium, it is normal to use for 5 to 8 years; However, if the medium contains sulfur and dust, and the temperature fluctuates frequently, it is not uncommon to leak in 1 to 3 years. The key lies in selection and maintenance.

Select the material and structure of the ring belt according to the pressure, temperature and media composition during the selection, strictly pre-stretch and limit the position during installation, and regularly check the surface of the ring belt for wear, blister and hardening during operation. Don't wait for a leak to pat your thigh. At that time, you can only stop the furnace for emergency repair, and the loss is dozens of times more expensive than the expansion joint itself.

Where is there a perfect compensator? In high-pressure working conditions, metal corrugated expansion joints are honestly selected, and it is the turn of non-metal expansion joints to play in large displacement and low-pressure scenarios. Understanding the shortcomings is not to deny it, but to use it for a long time.

Every time a customer asks "Which metal expansion joint is better?" With the parameters of the pipeline, I have to ask first: Does your pipeline run steam or smoke? What's the temperature? How stressful? Have you calculated the compensation amount? I don't mean to make things difficult for people, but I can't recommend them without asking them clearly. The expansion joint is never the more expensive the better, but the more matched the better. Today, I will explain the five questions that must be understood before choosing a model.

First distinguish the type: what kind of expansion joint are you buying

Metal expansion joints are a large category, and there are many types below. Someone took the data of the flue gas pipeline to buy a general-purpose corrugated expansion joint, and it leaked within two months after installing it. When they turned back, they scolded the product. In fact, the selection was wrong.

According to the structure and working principle, there are several common categories:Corrugated expansion joint(Including a general-purpose corrugated expansion joint, a high-temperature axial expansion joint, a straight tube pressure balance expansion joint, a curved tube pressure balance expansion joint, etc.),Sleeve type pipe expansion joint、Rotary compensator。 The bellows absorbs the displacement by corrugation deformation, the sleeve slides by the inner and outer tubes, and the rotary compensator absorbs the displacement of the pipeline by rotation, each of which has its own application scenarios.

In large-diameter low-pressure scenes such as flue gas pipes and air ducts, non-metallic expansion joints or rectangular expansion joints are usually used; Steam pipelines and chemical pipelines use metal corrugated expansion joints; Direct buried (fully buried) type expansion joint for buried pipeline; Dual hinge expansion joint for vacuum special hose or air cooled island vacuum pipe for vacuum pipe. If the working conditions are wrong, no matter how expensive it is, it will be useless. Just like you can't wear leather shoes to run a marathon.

Three hard indicators: compensation amount, pressure and temperature. None of them will work

These three parameters are the selected foundation. The amount of compensation is not enough, and the bellows is pulled to the limit and directly torn; When the pressure exceeds, the bellows bulges or even bursts; The upper temperature limit leaves no margin, the creep of the material accelerates, and the life of the material decreases by a cliff.

For example, the hot air duct of a power station boiler has a temperature of 350 °C, a pressure of 5 kPa and a lateral displacement of 40 mm. If you take a general-purpose expansion joint with normal temperature and pressure to install it, the wall thickness of the bellows can't withstand high-temperature oxidation at all, and intergranular corrosion cracks will appear in less than a few months. This working condition must be at least a high-temperature axial expansion joint, or a special corrugated expansion joint for power station industry, and the material must be temperature-resistant alloy.

Media corrosiveness is often a "hidden killer". For corrosive media such as desulfurization flue gas and chemical tail gas, the material of corrugated pipe must be upgraded to 316L or even higher, and lined with PTFE if necessary. Otherwise, in less than two years, the bellows will be corroded and perforated. A cement factory customer chose 304 material for cheap at first. As a result, the flue gas with excessive SO₂ content directly corroded the bellows, and the loss of shutdown maintenance was far greater than the saved material cost.

Look at the structure selection: how to go the pipeline, you can follow the selection

There are many structural types of expansion joints, but in fact, they correspond to the displacement form of the pipeline. The selection logic is very simple: in which direction the pipeline moves, you choose the expansion joint that absorbs the displacement in which direction.

• Mainly axial displacementThe pipeline is straight, short and has large thermal expansion-choose axial type, such as external pressure single axial expansion joint, which has good external pressure structure stability and is suitable for high pressure working conditions.
• Lateral displacement is mainly, Z-shaped pipe section or L-shaped pipe section-choose transverse type, such as compound hinge transverse expansion joint, and two hinge groups cooperate to absorb transverse displacement.
• Multiple directional displacement need to be absorbed simultaneouslyBut the thrust cannot be too large-choose the universal hinge type or the double hinge type. Single hinges can only absorb angular displacement in one plane, while universal hinges can absorb spatial angular displacement.
• Afraid that the pipeline pressure thrust is too great and the equipment will be damaged-choose the pressure balanced type, such as the straight pipe pressure balanced expansion joint or the curved pipe pressure balanced expansion joint. The blind plate force generated by the internal pressure of the bellows is balanced by the internal structure, and no additional thrust is applied to the equipment.
• Space is constrained to fit large corrugated expansion joints— — Consider the rotary compensator or sleeve-type pipe expansion joint, which is small in size and large in compensation.

Don't take a universal corrugated expansion joint and try to solve all the problems. The general-purpose type is cheap, but it has certain compensation ability in three directions, and the compensation amount in each direction is not large. When the working conditions are complicated, it is hard to use it, just like taking a Swiss Army knife to cut down trees. It is capable, but it is extremely inefficient and easy to collapse.

The material and manufacturing process determine how long it lasts

In the same working condition, some people use expansion joints for five years, while others leak after half a year. What's the difference? Material and craftsmanship.

Let's talk about the material first. Although the main materials of 304 and 316L are austenitic stainless steel, their molybdenum content is different, and their chloride ion corrosion resistance is a grade poor. Desulfurization, seawater, chemical industry, 316L is the bottom line. If the chloride ion in the medium is high, it has to be upgraded. When the temperature exceeds 800℃, ordinary stainless steel won't work, so high-temperature resistant alloys have to be used, and the manufacturing process is completely different.

The wall thickness of bellows is ignored by many people. The thinner the wall thickness, the better the flexibility, the greater the compensation amount, but the smaller the pressure resistance and corrosion resistance margin. The thicker the wall thickness, the stronger the pressure bearing capacity, but the greater the stiffness, the weaker the compensation capacity. There is a balance point here, which has to be calculated according to actual working conditions. Tell you to pat your head to fix the wall thickness of the manufacturer, as soon as possible black.

Fatigue life is another hard indicator. Corrugated pipe absorbs displacement by corrugated deformation. Every time it is deformed, the root of corrugated pipe is subjected to alternating stress. How many cycles can be withstood in the design life, the manufacturer must do a fatigue test and issue a test report. Under normal circumstances, the fatigue life required by the standard is more than 1000 times, and good ones can achieve thousands of times. If you don't ask this data clearly, you will find the manufacturer when the bellows is cracked, and they will send you off when they say "the working conditions have changed".

Don't just focus on quotes when picking manufacturers. These questions must be asked

Do you really dare to use the one with the low quote? The expansion joint is a "air bag" in the pipeline system, and the pressure, temperature and displacement are all pressed on it. Once it fails, the whole system has to stop. Compared with the loss of shutdown, what is hundreds of dollars more expensive?

Just ask three questions.

First, are there any cases of similar working conditions? The working conditions of power plants, steel plants and chemical plants have their own tempers. Manufacturers who have done similar projects have their experience points directly. Ask him to send you a few cases and ask about the use effect by the way.

Second, can you produce a calculation book? The regular manufacturer will calculate the selection of expansion joints according to the pipe system parameters provided by you, and produce a calculation book to explain the verification results of compensation amount, pressure and temperature. If you can't come up with a calculation book, you basically rely on experience to make a decision at your own risk.

Third, how fast is the after-sales response? Problems with expansion joints are often emergency shutdown. Can the manufacturer respond within 24 hours, arrive within 48 hours, or at least give an emergency treatment plan. Bargains without after-sales guarantee are "cheap to buy and expensive to use" in the end.

So which metal expansion joint is better? The answer is clear: if it is suitable for working conditions, the right material is used, the manufacturing process is excellent, and the manufacturer is reliable, that's good. There is no shortcut to model selection. Only by confirming the parameters one by one and asking the questions one by one can we avoid detours and spend less money.

Why Pipelines "Can't Hold" Thermal Expansion and Cold Contraction

Everyone knows that the pipe elongates when heated, but why can a carbon steel pipe, whose temperature rises from normal temperature to 200℃, eject several tons or even more than ten tons of force when it is several tens of meters long?
Because the two ends of the pipe are tightly restricted by the fixed bracket and the equipment interface, the thermal elongation can't be released, and the force is fully pressed on the pipe wall, ranging from flange leakage to torn the equipment interface. The work of the compensator is not simply "absorbing displacement"-displacement is just the appearance. What it really wants to solve is the force balance of the pipe system, so that the deformation caused by thermal expansion and contraction has a place to go, instead of carrying it hard.

Therefore, when selecting a compensator, look at the system constraints first, and then the displacement amount. You just count the elongation and not the reaction force, just like looking at the scene of a car accident without looking at the speed, it doesn't make sense.

Bellows are the soul

The core of the corrugated expansion joint is the bellows. Take the general-purpose corrugated expansion joint, for example, it relies on the elastic deformation of the corrugation to "eat" the axial displacement-the corrugation is compressed, the displacement is absorbed, and then the pipe is pulled back to its original position by the elastic restoring force. This principle sounds simple, but the shape, number of layers, and material of the ripples directly determine the compensation ability.

External pressure single-type axial expansion joint is clever in putting the bellows outside, and internal pressure acts on the end, not the corrugated inner wall. In this way, columnar instability can be avoided, and it is suitable for large displacement and high pressure scenarios. What is the amount of compensation? It is not determined by patting the head. The ripple stiffness, single wave displacement and fatigue life restrain each other. For example, if the design life requires 1000 full displacement cycles, the single wave displacement must be controlled within a certain value; If you have to overdisplacement, your life will drop exponentially. Many scene ripples cracked after two or three years of use. Nine times out of ten, it was only greedy for large compensation when selecting the model, not fatigue.

Ripples alone are not enough

While the bellows absorbs displacement, it will generate a huge blind plate force, which will be transmitted to the fixed bracket and adjacent equipment. The pressure balance type digests the blind plate force by itself through the internal balance corrugated structure or tie rod system, and the pipeline and equipment don't have to carry extra force.

What about the curved tube pressure balance type? It is specially used in elbow sections to absorb lateral displacement while also balancing pressure and thrust. Compound hinge transverse expansion joints rely on hinges to constrain axial displacement, and only allow transverse deformation to occur-it works where there is not enough space and axial displacement is large.

However, if you have simple working conditions and low pressure, it is easier to use a rotary compensator. It uses the rotation angle to change the direction of the pipeline, has a large compensation amount and low flow resistance, and is used a lot in steam pipelines. People don't rely on the corrugated hard top, but "borrow their strength".

Nonmetallic and metallic, compensation logic is completely different

Non-metallic expansion joints (fabric fiber expansion joints) and rubber compensators are made of flexible bodies, which absorb multi-directional displacement by the flexibility of fabric or rubber itself, and can also absorb shock and noise. For example, in the low-temperature and strong corrosive environment of desulfurization flue, the metal bellows can't bear acid, but the non-metallic material is solid, so it is convenient to replace. The vibration damping effect of rubber compensator at the inlet and outlet of pump is simply incomparable to that of metal parts.

But you don't expect non-metals to withstand high pressure and heat. Don't stick metal on those occasions, and vice versa. If the material is chosen wrong, the compensator is a pile of scrap iron.

With the compensator all will be fine? Wrong

The direction of the guide tube is installed backwards, and the medium directly washes the inner wall of the corrugation, which will wear out in less than half a year. The tie rod nut is used for positioning when it leaves the factory. After installation, it must be adjusted according to the instructions, not to be welded to death. Installing pre-stretching is even more specific-for the expansion joint that requires pre-stretching, you don't do it, it looks fine in the cold state, but it overdisplaces when it runs in the hot state.

These details are not done in place, and the compensator not only fails to compensate, but becomes a weak point in the pipeline. We have seen too many cases, and customers said, "The quality of your products is not good". As a result, when we looked at the scene, the draw rod was not disassembled, the guide tube was installed backwards, and the bracket was in the wrong position. It was purely the installation and backing the pot.

Don't just look at the compensation amount when selecting a model

Medium, temperature, pressure and fatigue life have to be calculated together. For corrugated expansion joints used in power station industry, the typical working condition is high-temperature steam, so the thermal displacement and the number of fatigue cycles must be calculated. The expansion joint matching the desulfurization flue gas baffle door contains sulfur and dust in the medium, so it is necessary to consider corrosion resistance and wear resistance. At this time, it is more reliable to use non-metal expansion joint or metal rectangular expansion joint with guide tube.

Two days ago, I met a customer and said, "Our compensation amount is enough, why is it still bad?" When asked, there were particles in the medium, and the bellows were directly worn out. You can't just look at the amount of compensation, media abrasion is more deadly than displacement. Therefore, when selecting the model, give all the working condition parameters, and don't just lose a "compensation amount of 50mm".

The compensator looks like a small part, but it does the work of "moving a thousand pounds". You treat it as an ordinary joint, and it will give you a look; If you treat it as a precision bullet, it will keep your pipeline safe for twenty years. The truth is this truth, the key is not to think of its "compensation" simply.