Specialized in manufacturing compensators, expansion joints, baffle doors

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.

On the side of the smoke duct of power plant or the ash discharge pipe of cement plant, you often see a flat "cloth bag" joint with bolts pressing the flange. That is the non-metallic compensator, also called the non-metallic expansion joint (fabric fiber expansion joint). Many purchasers ask with pipeline parameters: What is a non-metallic compensator? What's the difference from the metal expansion joint? Let's say that it is a flexible section that can absorb thermal displacement, vibration and installation errors of the pipe, but the main force is not a bellows, but a layer of composite fabric skin.

Start with the name: What is the difference between metal expansion joint and corrugated expansion joint?

Metal expansion joints and corrugated expansion joints, essentially the same thing, absorb displacement by the deformation of stainless steel bellows. They have strong pressure bearing capacity and high temperature resistance, making them particularly useful on circular steam pipes. However, on large-section rectangular pipes such as smoke ducts in power plants and ash discharge pipes in cement industries, metal ones are not effective-high cost, limited displacement, and fear of acid-alkali corrosion. Masters prefer to use non-metallic expansion joints for three reasons: they are cheap, corrosion-resistant and can absorb three-dimensional displacement. Especially the fabric fiber skin has great elasticity, and the lateral, vertical and angular displacements can all be eaten. The metal bellows can only give an axial direction, and it will be blinded when it encounters complicated displacements.

Pick open to see the structure: Non-metallic compensators are not as simple as a piece of cloth

When disassembled, the non-metallic compensator consists of a skin (non-metallic elastic material), thermal insulation layer, stainless steel wire mesh, frame flange and flow guide tube. Each layer has a division of labor: the skin is the sealing core, and fluororubber, silicone rubber or polytetrafluoroethylene are commonly used; The insulation layer is made of ceramic fiber blanket to block the high-temperature smoke, otherwise the high temperature will directly burn the skin; The stainless steel wire mesh is the skeleton, which enhances the compression and tear resistance of the skin; Frame flanges are responsible for pipe connections; The guide tube is placed inside to allow the airflow to pass smoothly and avoid the medium directly washing the skin. Rectangular non-metallic expansion joint and circular non-metallic expansion joint have the same structural law, that is, the flange shape is different, and the rectangular one is more suitable for large-section air duct.

Take the working conditions for themselves: Where can big things happen without non-metallic compensators

Behind the desulfurization flue gas baffle door, the flue gas is wet and contains sulfur, and the metal bellows corroded and wore out within a few months after being installed. At this time, the non-metallic skin is on top, and it is resistant to acid and alkali and chloride ions. There are also rectangular air ducts that can't be reached by high-temperature axial expansion joints, such as the flue at the tail of the boiler, which has a large cross section, high temperature and more than one displacement direction, and there is no way for metal at all. What about the rubber compensator? Afraid of high temperature, it will age and crack when it exceeds 150℃. The non-metallic compensator is insulated with ceramic fiber, and the skin surface can carry it to 600℃. Do you know how to choose?

Don't underestimate the selection: JB/T 12235-2015 is full of hard indicators

The national standard JB/T 12235-2015 for non-metallic expansion joints frames the temperature resistance grade, displacement calculation and fatigue life. The standard is divided into several grades according to the working temperature. When selecting the type, the thermal expansion of the pipeline should be calculated first, and then the corresponding fatigue life requirements in the standard should be checked. What to choose for skin material? Depending on the temperature and corrosiveness of the medium, the flue gas has a high sulfur content and a fluororubber composite layer, which ordinary silicone rubber can't bear. What about insulation thickness? According to the inner wall temperature backward, let the outer surface temperature of the skin be controlled in a safe range. There is an on-site case: when selecting the ash discharge pipe in a cement plant, it was greedy for cheap, and the insulation layer was thinned. After half a year's operation, the surface temperature of the skin soared to more than 80 degrees, which directly burned and cracked. Another case is the compensator at the desulfurization inlet. The skin is made of ordinary silicone rubber, which ages and leaks after three months. It stops after replacing the fluororubber PTFE composite skin. The price of the wrong choice is not just as simple as air leak.

Both installation and repair are particular: only when you can install and repair can you use the full life

Install the non-metallic compensator, first look at the direction of the guide tube, which must follow the flow direction of the medium. Install the guide tube backwards for a ride, and the skin will be torn apart after a few strokes. Do you want to do pre-stretching? The non-metallic compensator has low stiffness, so it generally does not need to be pre-stretched like metal, and it can be kept in a free state when installed. But the tie rod nut must be adjusted properly-the tie rod is locked when transported, and it must be loosened after installation, so that the expansion energy saving can expand and contract freely, and guess what? Many sites forget to loosen the nut, the compensator becomes a rigid connection, and the displacement of the pipeline is all stuck in the equipment, which cracks within a few days.

After two or three years of operation, the skin ages and cracks. Look at the damage area first. Small breaks can be repaired locally and treated with special glue and patch cloth; If a large area is torn or the flange is detached, replace the whole skin directly. Compared with the metal expansion joint, the non-metal replacement cycle is short, about 3-5 years, and the metal bellows can last 10 years. However, non-metal is cheap, lightweight, quick to disassemble and assemble, low labor cost, and lower comprehensive maintenance cost in the corrosive smoke environment. Alas, understanding what a non-metallic compensator is is much more important than choosing a model in a daze.

Find out first: What equipment does GB/T 12777 manage, and why is pipeline design inseparable from it

Old rules, first answer this most direct question: What is the standard of gb/t 12777? The full name is General Technical Conditions for Expansion Joints of Metal Corrugated Pipe, which specifies the design, manufacture, inspection and acceptance requirements of expansion joints of metal corrugated pipe. To put it bluntly, all the corrugated metal compensating elements used in pipes that can absorb thermal expansion and contraction displacement are within its jurisdiction.

But please note that it tubes a "metal bellows expansion joint", which is not non-metallic or rubber. If you use it to cover fabric fiber expansion joints, you will be wrong. The design basis of general-purpose corrugated expansion joints, corrugated expansion joints for power station industry and high-temperature axial expansion joints often mentioned in our industry can't get around this standard.

Then why can't pipeline design do without it? Think about it, the elongation of a steam line from cold to hot may be tens or even hundreds of millimeters. If you don't give it a device to compensate for displacement, the pipeline will transmit force to the equipment interface. This is how the accident of flange cracking and pipe shelf displacement comes from. GB/T 12777 sets rules for this "compensation device", how to calculate the displacement, how to test the fatigue life and how to control the stiffness. With this rule, the design institute can draw drawings and the manufacturer can produce them.

Some of the hardest technical requirements in the standard: How to specify displacement, fatigue life and stiffness

In this standard, the core is nothing more than three things: displacement, fatigue life and stiffness. Let's talk about it one by one.

Displacement, which refers to how much axial, lateral, and angular deformation is absorbed by expansion energy savings. The standard gives the requirement of displacement capacity for bellows with different wave patterns, but its precondition is that the bellows cannot be unstable or excessive plastic deformation. This is not determined by patting the head, it needs to be verified by type test.

Fatigue life is the highlight. When the bellows expansion joint works under repeated displacement, the most afraid is fatigue fracture. GB/T 12777 stipulates that the fatigue life should not be less than a certain number of cycles under design displacement. Do you know what that means? When you select the model, if you only give the displacement, regardless of the fatigue life, the manufacturer can make a "disposable" product for you. It can really move when installed, but it leaks after hundreds of cycles. Therefore, when purchasing, you have to write the number of cycles clearly, such as 1500 times or 10,000 times, which directly determines the number of layers and thickness of the bellows.

The stiffness is directly linked to the force on the pipe. The greater the stiffness, the greater the reaction force when the expansion joint deforms, and the higher the load on the pipe support and equipment interface. There is a calculation formula for stiffness in the standard, and it is also required to measure it at the time of factory inspection. This is why you can't only look at the displacement when selecting the model. Excessive stiffness will transmit the force back to the equipment, and the deformation will not absorb much, but create new stress. These three parameters are tied together, and it is meaningless to take any of them separately. And guess what? Many old masters who have used expansion joints for decades will still stumble on them when selecting models. The problem is often that the fatigue life is neglected.

What is the difference between GB/T 12777 and GB/T 14525 and JB/T 12235? Don't choose the wrong one

To plug in here, many people confuse GB/T 12777 with GB/T 14525. In fact, the latter is "General Technical Conditions for Corrugated Metal Hoses", which pipes metal hoses-that is, flexible pipe sections with mesh sleeves for absorbing displacement or reducing vibration and noise. The expansion joints of metal hoses and bellows look similar, but the use scenarios are different: hoses are mainly used in pump ports, steam turbine inlet and outlet, etc., where vibration isolation or small displacement compensation is needed, while expansion joints are specially used to compensate the axial displacement, lateral displacement and even angular displacement of pipelines.

There is also JB/T 12235, the machinery industry standard for non-metallic expansion joints, which applies to compensators made of fabric fibers, rubber or PTFE. If your project medium is sulfur-containing flue gas, or low temperature but extremely corrosive working conditions, you may want to refer to JB/T 12235 instead of GB/T 12777.

The metallic type is classified as GB/T 12777 pipe, and the non-metallic and rubber have different standards. If you choose a metal bellows expansion joint on the flue gas desulfurization pipeline, it may be corroded for less than two years; On the other hand, it is even more unreliable to install non-metallic expansion joints on the high-temperature and high-pressure steam pipe network.

How to compare this standard in actual selection: landing cases from metal hoses to various expansion joints

Two days ago, a customer who was doing power plant renovation asked with a drawing, saying that a section of his air-cooled island vacuum pipeline needed to compensate for the lateral displacement, and asked me to match it with a model. I opened the drawing and saw that the working condition was vacuum, the temperature was about 200°C, and the displacement requirement was ±50mm in the transverse direction. Under this working condition, it is suitable for you to use a small tie rod transverse expansion joint, because its bellows is constrained by the tie rod, which can only absorb the transverse displacement without generating the main fixed bracket thrust. This selection logic corresponds to the standard, that is, the clause on lateral displacement and angular displacement in GB/T 12777-the bellows cannot bear torque, and the structural design of tie rod and hinge must follow the relevant requirements in the standard.

Another scene is the flue gas pipeline at the head and tail of the kiln in the cement industry. High-temperature axial expansion joint or straight pipe pressure balance expansion joint are often selected under such working conditions as large temperature fluctuation, large dust and limited installation space. The advantage of the pressure balance type is that the internal pressure thrust of the bellows is cancelled by the balance structure, and the blind plate force will not be transmitted to the fixed bracket. This design principle and calculation method are also regulated by GB/T 12777. If you don't look at the standard and choose by experience, and use a large tie rod to forcibly absorb the axial displacement, the stress analysis report of the pipeline may not pass the examination at all.

Back to the metal hose. It is not that with GB/T 12777, any pipe can be used, the flexible connection of the pump inlet and outlet, and the vibration isolation of the instrument pipeline. Where the metal hose should be used, the metal hose should be used honestly. They are complementary relationships, not alternative relationships. The PTFE-lined hose and vacuum special hose in the station take the GB/T 14525 route. The material, mesh structure, pressure level and expansion joint are not the same algorithm.

Pits that are easy to step on during procurement and acceptance: How to read standard versions, material reports and type tests

The last link is also the easiest place to step on pits. Do you think it will be fine if you get the goods and accept them according to the standards? That's naive.

Look at the standard version first. A new version of GB/T 12777 was released in 2024, replacing the previous 2008 version. The old version of the standard has a historical problem-the requirements for fatigue life and safety factor are relatively lax. The new version has made adjustments to the design fatigue life and safety factor requirements. If you only write "in accordance with GB/T 12777" in the contract, without noting the next year number, the supplier will do it according to the 2008 version for you, and you can't find fault. However, the special inspection institute may not recognize it when the project is accepted. Therefore, the full name and year number of the standard must be stated in the contract.

Look at the material report again. Stainless steel plate, the raw material of corrugated pipe, must have a warranty, and the data of material, furnace batch number and mechanical properties must be complete. Here is a detail: many suppliers can't produce the intergranular corrosion test report of stainless steel, but this is precisely one of the main reasons for the failure of bellows under high temperature conditions. You don't want trachoma leakage six months after the tube is installed, do you? It is not excessive to ask the supplier to provide the chemical composition analysis report of raw materials during acceptance.

Finally, there is the type test. GB/T 12777 stipulates that the type test, including hydraulic pressure test, air tightness test, stiffness test and fatigue life test, should be carried out when the new product is finalized. However, note that many factories only do factory inspection and do not do type test, which are not the same thing. When you purchase large diameter thick wall expansion joints or double hinge expansion joints for vacuum pipes for nuclear power plants, it is best to explicitly request a copy of the type test report in the technical agreement. Let me reveal to you the bottom. There are only a few manufacturers in the industry who have really done complete type tests. If you ask to read this report, the supplier's foundation will be figured out at once.

In the final analysis, understanding GB/T 12777 is not to memorize the provisions, but to not be fooled when selecting the model, not missing items when signing the contract, and not being blinded during acceptance. The standard is a tool, and when you use it well, your plumbing system will run steadily for twenty years.

The term Flange Thermal Displacement is too common in pipelines in power plants, cement plants, and chemical plants. To put it bluntly, as soon as the pipe is heated, the flange connection jumps. You think the flange isn't tightened? Not. You thought it was an aging gasket? Not all of them. The real driving force behind the scenes was that the thermal expansion displaced the pipe, and the flange faces were crooked.

The flange surfaces are staggered, and the bolts are tight and loose; The spacer is squeezed out in a small section that looks like a tongue sticking out; Not long after the pipeline started, the flange seeped, tightened the bolts for several days, and then leaked again after a while. What's more interesting is that some flanges are good in the cold state, but they leak when they heat up, and they leak badly when the temperature difference is big. For this kind of problem, you only change the gasket and add torque, which is a cure for the symptoms but not the root cause.

What happens if you don't compensate? Leaky flanges and cracked pipes are no small matter

The consequence of carrying hard is to run and leak if it is light, and to say it is an accident if it is heavy. The flange thermal displacement is not absorbed, and the stress in the pipeline system will always accumulate. The flange bolts are subjected to additional bending stress, the gaskets are repeatedly squeezed, and the sealing surface fails sooner or later. If the medium is high-temperature steam or corrosive gas, leakage is a potential safety hazard.

The plumbing itself isn't much better. The stress is concentrated in the elbow, tee and fixed bracket, and it is possible that the weld cracks and the pipe wall cracks after a long time. Moreover, the thermal displacement is repetitive, and the pipeline is repeatedly subjected to fatigue load every time it starts and stops and the load fluctuates. A friend from a cement factory said when chatting that their kiln head pipeline cracked three times a year, welded and cracked again, and finally solved it with a compensator. As a result, the life of the equipment is reduced and the maintenance cost rises.

To compensate for flange thermal displacement, should I use corrugated expansion joint or metal hose?

This is a type selection problem, depending on the displacement type and working condition. Corrugated expansion energy saving absorbs axial displacement, transverse displacement, and even angular displacement, depending on the structural form. The universal corrugated expansion joint is suitable for absorbing axial displacement; Compound hinge transverse expansion joint specializes in transverse displacement; If the pipe diameter is large and the pressure is low, nonmetallic expansion joints (fabric fiber expansion joints) can also be used, and the temperature resistance and compensation amount are not bad.

What about metal hoses? Its advantages are good flexibility, can absorb multi-dimensional displacement, and also take into account vibration reduction. If flange thermal displacement is accompanied by significant mechanical vibration, or the line has installation deviations to absorb, metal hoses are more appropriate. However, the pressure resistance and axial stiffness of metal hoses are different from those of corrugated expansion joints. In the case of large diameter and high pressure, they must be carefully calculated.

So, who exactly to choose? Depends on which displacement dominates your pipes. Mainly axial, with corrugated expansion joints; Multidimensional direction, with vibration, with metal hose. If this measurement is wrong, the compensator will be installed for nothing.

Three parameters that are most easily overlooked in model selection: displacement, pressure and temperature

In many projects, the drawings mark how many millimeters the thermal displacement of the flange is, but many people start to check the model when they get the parameter table, and three key parameters are taken over.

• The amount of displacement.Many customers only give a total thermal elongation of the pipe, without distinguishing between axial or transverse. The design of the expandable joint is based on the displacement direction. If you report the axial displacement as the lateral displacement, the selected product will not have that ability at all. It is necessary to give the axial displacement, the transverse displacement, and preferably the angular displacement data respectively.
• Stress.The pressure here is not as simple as the design pressure of the pipeline. It is necessary to clarify whether it is the test pressure, the operating pressure or the maximum transient pressure. The bellows of the corrugated expansion joint is a thin-walled piece, and the higher the pressure, the lower the fatigue life. When we make corrugated expansion joints for power station industry, the pressure parameters must be accurate to two decimal places after MPa, and the stress calculation of corrugated pipes is completely different.
• Temperature.Temperature directly affects the allowable stress of the corrugated pipe material. For the same expansion joint, the allowable displacement can be more than 50% different under the working conditions of 200℃ and 400℃. In addition, in high-temperature working conditions, we have to consider whether internal insulation layer or guide tube is needed, otherwise the bellows will directly face the high-temperature medium, and the life will be greatly reduced. The specific function of the expansion joint guide tube is to protect the inner wall of the bellows and reduce the direct action of high-speed fluid erosion and temperature.

These three parameters are not filled in by patting the head. Without which, manufacturers can't accurately calculate the fatigue life.

Installation and Predisplacement: The Details That Make the Compensator Really Work

Select the right type, install and pull the crotch, still leak.

Pre-displacement is the most easily overlooked link in the installation of corrugated expansion joint. The so-called pre-displacement means that during cold installation, the expansion joint is pre-stretched or compressed for a certain distance in the opposite direction of hot displacement, so that it just returns to the neutral position in hot state. The purpose of this is to make full use of the displacement capacity of the expansion joint and avoid the displacement exceeding the limit in the hot state.

How to adjust the tie rod nut on the expansion joint? In the transportation state, the expansion joint is generally locked with a tie rod to prevent damage to the bellows during transportation. Once installed in place, loosen the tie rod nut to allow the expansion joint to move freely. Some construction teams don't understand that the nut is directly welded or not loosened, and the compensator is equal to a rigid short joint, which can't absorb any displacement. The role of the expansion joint tie rod is to transport support and protection, not to keep you permanently locked.

In addition, the fixing brackets and guide brackets of the pipeline system must be set according to the design requirements. The expansion joint can only compensate its own section. If the fixed bracket is not firmly placed, the displacement distribution of the whole pipe system will be messed up, and the compensator may be pulled out by the displacement exceeding the design value.

A Case of Practical Treatment of Pipes in a Cement Plant

Two years ago, there was a cement plant project. The inlet and outlet pipeline of kiln head waste heat boiler, DN800 pipe, operated at 350℃, and the flange connection leaked repeatedly. From the initial quarterly tightening of bolts to the repair welding once a month, the maintenance personnel really couldn't bear it.

The axial displacement is about 18mm and the transverse displacement is about 6mm. The pressure is not high, 0.15MPa. After we bring the data back to the manufacturer for calculation and analysis, the suggestion is: install a metal corrugated expansion joint in cement industry near the heat source side of the pipeline to absorb axial displacement; At the same time, a section of metal hose is connected in series on the pipe section on the other side to absorb lateral offset and equipment vibration.

Pre-displacement was made during installation, and cold state pre-stretched 8mm. After it was put into operation, I stared at it for more than three months, and the flange surface was clean and never leaked again. Later, the customer reported that the labor and material cost spent on leak treatment in the previous quarter was almost the price of half a compensator.

The problem of flange thermal displacement is solved at one time by choosing the right compensation scheme, so there is no need to fight against leakage every year. Carry it hard, that's saving up bills for the next downtime. Don't be vague when it is time to put on the compensator, but the premise is that the scheme is accurate and installed correctly.

Two days ago, I met a customer. The corrugation of the expansion joint of a high-temperature steam pipeline in their factory was obviously a little crooked, but it didn't leak, so I wanted to last until the maintenance period. As a result, it didn't last until it broke in the middle of the night, and the whole line stopped. This is not an exception.

Three variations, distinguish first and then judge

Do you want to find out the deformation and scrapping standard of metal expansion joint? The first step is not to measure the dimensions, but to distinguish the deformation properties. Elastic deformation, can rebound after displacement recovery, this does not constitute scrapping; Plastic deformation is permanent deformation, and the wave distance can't go back if it is elongated. However, slight plasticity may not fail immediately, so we must continue to observe it. The real danger is instability deformation-the bellows buckles laterally or distorts in the plane under the action of pressure, and the corrugations seem to have been screwed, which is basically directly judged. GB/T 12777 and EJMA specifications give design guidelines, telling you how to calculate when designing, but on-site judgment depends on specific measurement and observation.

Hard indicators for on-site disposal judgment

On-site judgment on several hard indicators, reach any one directly change:

• The wave pitch variation of bellows exceeds the design value by more than 15%, or the wave height has obvious local depressions/bumps;
• Cracks, penetrating corrosion, pitting and perforation appear on the bellows surface;
• Signs of cracking in the connection or end welds.

These do not meet the condition of "making do with it". Many masters can make a rough judgment just by looking at the shape of the ripples-the troughs are squeezed together, the peaks collapse, and the ripples are obviously asymmetric-but it is more reliable to measure the wave distance with a caliper and check the gap between the troughs with a gauge.

Invisible deformation is more dangerous than thought

Pressure-induced plane instability and column instability. The manifestation of plane instability is that a certain wave bulge of the bellows is convex, as if it is blown up alone; Column instability is the whole bellows bending to one side. These two instabilities don't leak immediately, but the bellows have lost their ability to absorb displacement, and continued service will only accelerate fatigue fracture. In this case, it is recommended to replace it directly regardless of whether it leaks or not. If the lateral displacement exceeds the limit, which causes the end flange to deflect, it is necessary to combine the tie rod and hinge structure to judge whether it is damaged jointly-for example, the single hinge of the transverse expansion joint of the compound hinge is worn out, and the whole structure is stressed incorrectly.

Different working conditions have different scrap scales

The scrap scale corresponding to different working conditions varies greatly. In the high-temperature creep environment, even if the deformation of bellows is less than 15%, the cumulative creep deformation continues to increase, so the remaining life should be considered. Uniform thinning to the lower wall thickness limit in corrosive media is more dangerous than simple deformation. There are also stainless steel bellows that are prone to stress corrosion cracking in chloride ion environment-this kind of crack is often penetrating, and the appearance deformation may not be obvious, but it has been wasted. So don't just focus on the size, the material and media environment should be looked at together.

Pits that are easy to step on in practice

Someone screwed the tie rod nut to "correct" the deformation. As a result, the displacement that should be freely absorbed became a binding force, and the force distribution of the bellows changed completely, accelerating the failure. Others forcibly fix the bellows by welding steel plates on the outside-this operation will change the original flexible compensation path of the bellows. It looks like it is fixed, but it is actually creating a new stress concentration point. In addition, the guide tube wears out or falls off. Although the bellows itself is not deformed, the medium directly washes the trough, which will soon reduce the leakage, which is also an indirect signal of scrapping. In daily inspection, priority is given to the outlet of the guide tube, the low point of the bellows and the trough position near the bracket, which have the most problems.

Finally, give a bottom line

If your measured deformation data does not meet the standard, or you are unsure whether it is unstable, then it will be disposed of as scrapped. The cost of replacing a metal expansion joint is much lower than the cost of downtime for maintenance caused by pipe leakage. Especially for high-temperature axial expansion joints and straight pipe pressure balance expansion joints used in key pipelines, once the bellows fails, the consequence is not as simple as leakage. Don't wait until you miss it before you regret it.