Specialized in manufacturing compensators, expansion joints, baffle doors

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.

Find out first: How did pre-compressed 50% come from?

When many on-site masters heard about the installation of expansion joints, their first reaction was to "press half first and then talk about it". Where did this habit come from? Some people say that it is to save trouble, some people say that they are afraid of insufficient heat expansion, and others think that the more they press, the more durable the bellows will be. Do you look through the design manual and find the "pre-compression 50%" rule? Can't find it. The expansion joint is not a spring, the displacement capacity of the bellows is a fixed stroke, and the pre-compression of 50% just hard presses the bellows to the middle position, which sounds like a "centering safety" and actually blocks both sides.

Two days ago, I met a customer and told me that the high-temperature axial expansion joint in their factory began to leak after less than half a year of installation. When I removed it, there was a crack at the bottom of the bellows trough. Ask how to pre-press it during installation? The answer is "pressed by 50% as usual". You calculate the actual thermal elongation of the pipe again, and it only takes 20% to press at full load. With this 50% pressed in, the bellows is in the wrong initial position from day one, and it would be weird if nothing happened.

Disadvantage 1: The axial compensation ability is sacrificed, and the bellows will be broken if the thermal displacement of the pipeline is slightly larger

What does it mean to pre-compress 50%? Assume that the rated axial displacement of the bellows is ±40mm and the total stroke is 80mm. If you press 40mm in advance, the remaining compressible stroke is only 0, and the stretching stroke is only 40mm. What if the actual thermal expansion of the pipe is 50mm? The bellows was either pressed to the limit or pulled overhead, and the crest of the wave cracked directly. Products such as general-purpose corrugated expansion joints and high-temperature axial expansion joints have a good displacement range when they leave the factory. The amount of pressure should be calculated according to the actual thermal displacement, which is not one size fits all.

Amount of pre-compression = (actual working displacement/2) - (displacement that has occurred at installation). It's better for you to press 50% when you come up, which is equivalent to throwing half of the compensation ability calculated by the design institute. For example, if you buy shoes in size 43, but don't wear a size 42, you still say "it will loosen after you hold it up"-who do you blame for worn feet?

Disadvantage 2: The corrugated pipe is in a high stress state for a long time, and the fatigue life falls by a cliff

Bellows absorb displacement by elastic deformation of wave peaks and valleys. Pre-compressed by 50%, the trough is pressed deeper, the crest is stretched more open, and the stress level does not rise linearly, but doubles up. For stainless steel bellows, the direct problem caused by high stress is stress corrosion cracking-even if there is a little chloride ion in the medium, the crack can penetrate the wall thickness in a few months.

Some people take "pre-compression can extend life" as an example, so don't be led off. What are the conditions for pre-compression to extend life? It is the amount of pre-compression that just offsets the cold tight displacement during installation, allowing the bellows to return to the middle position during operation. For example, the installation temperature of the pipeline is 20℃, the operating temperature is 200℃, and the calculated elongation is 60mm. When the pipeline is installed, it is pre-pulled or preloaded by 30mm, and it just completes half a cycle during operation. This is called "cold tightness", not "blind pressure". If you press 50%, if it does not match the calculated value, the bellows will always work in an over-limit state, and the fatigue life will be directly reduced by an order of magnitude.

Disadvantage 3: Instability or buckling is easy to occur after installation deviation and medium pressure are superimposed

In addition to absorbing displacement, bellows have to withstand the thrust generated by internal pressure. After 50% pre-compression, the initial deflection of the bellows is already present and a large chunk of the stability margin is eaten. At this time, once there is pressure fluctuation, water hammer, or even just normal pressure fluctuation in the pipeline, the bellows may become laterally unstable-the pipe itself does not move, and the bellows itself "bulges". Instability is not slowly leaking, but instantaneous twisting and deformation, and the whole pipeline system has to stop.

Particular attention should be paid to directly buried expansion joints and externally pressurized single axial expansion joints, which have strict limitations on pre-deformation. There is soil constraint around the direct buried type, and if it is pressed in the wrong direction, the lateral force of soil will directly crush the bellows; External pressure single axial type external pressure bellows itself works in the reverse state. If you precompress it by 50%, it will add insult to injury. The pre-deformation data in the installation manual are calculated by the manufacturer one by one, not copied.

So how much exactly should we pre-compress?

Amount of pre-compression = (actual working displacement/2) - (displacement that has occurred at installation). It is recommended that you copy this formula next to the device. For example: the maximum thermal elongation of the pipeline is calculated to be 80mm, and it has been elongated by 10mm at ambient temperature during installation, so the pre-compression amount is 80/2-10=30mm. This is called on-demand preloading.

If the pipeline has no thermal displacement at all, such as some lined pipelines, the pre-compression amount is 0, and compression instead creates stress. If it is cold after installation and heats up after operation, the pre-compression amount should be greater than 0. On the contrary, if it is installed in a hot state and cools down after operation, it has to be "pre-stretched", not compressed.

Finally, let's be honest

Expansion joint pre-compression is a technical job, not a matter of patting your head to set proportions. If you have installed a pre-compressed 50% expansion joint, quickly go through the running record to see if there are any abnormalities in temperature and pressure, and then squat down to see if there are any cracks, bulges or scratches on the bellows surface. If you find anything wrong, contact the manufacturer immediately for re-accounting. If you haven't installed it yet, you are unsure about the selection or installation. Don't be superstitious about the "experience value" of the old master. Find an expert who makes expansion joints to calculate it with a calculation book, which will save much money than changing the pipe afterwards.

First, let me answer the most straightforward question: Non-metallic compensators are not universal

It's not like standard flanges or bolts, which can be fitted when you take them to the right size. The design logic of non-metallic compensator (that is, we often call non-metallic expansion joint and fabric fiber expansion joint) is tailor-made around the working conditions-temperature, pressure, medium, displacement and installation space, each of which directly determines the structural selection and material matching.

If you take a non-metallic compensator used in the flue of a power plant and install it on the cold air duct of the cement industry, something will probably happen. It's like buying shoes, size 42 feet will make do with a size 43, but let you run a marathon in rain boots, try it?Are non-metallic compensators universal? The answer is clear: not universal.

Then why do some people always think it's "universal"?

Because from the appearance, the non-metallic compensator is just a circle of skin and frame, which seems to have little technical content. Actually, the skin is a layered structure: fluororubber, silicone rubber, PTFE, ceramic fiber, fiberglass cloth, and each layer has its own task-temperature resistance, corrosion resistance, pressure bearing, sealing.

Although they are all non-metallic expansion joints, the number of skin layers can differ by several layers between those used behind the smoke baffle door and those used next to the high-temperature axial expansion joint. The same is true for rubber compensators and rubber PTFE compensators. The PTFE layer is specially used to deal with strong corrosive media. If you use an ordinary rubber compensator to pass concentrated sulfuric acid, it is not a type selection, but a mine planting for the pipeline.

Let's talk about the displacement compensation ability

Axial expansion, lateral deflection and even angular displacement can be done, which is incomparable to metal corrugated expansion joints. But how much it can absorb depends on the design. Rectangular non-metallic expansion joints are usually used in rectangular smoke ducts, and another set of algorithms is matched with circular baffle doors.

You can't regard all non-metallic compensators as universal parts just because of the nominal "large displacement". The pre-deformation amount, the setting of the guide tube and the stiffness of the frame given in the design all directly affect the actual compensation effect. Two days ago, I met a customer who used the double-hinged expansion joint originally designed for air-cooled island vacuum pipeline on ordinary hot air pipeline. As a result, the vibration exceeded the standard-that thing was metal, not an occasion where non-metallic compensators could be replaced casually. On the contrary, non-metallic ones could not be used everywhere.

The installation location and environment cannot be ignored

Non-metallic compensators are common in desulfurization systems, dust removal systems, power station flues and cement production lines, because of large temperature fluctuations, dust and corrosive gases in these working conditions. But if you use it on pipelines with vacuum requirements, you have to weigh it-the non-metallic skin has limited pressure bearing capacity, and it is easy to deflate under negative pressure conditions. At this time, you may have to look at the vacuum special hose or the external pressure single axial expansion joint.

In other words, before selecting a non-metallic compensator, first list the working condition parameters clearly: what the medium is, how high the temperature is, whether the pressure is positive or negative, whether there is acid-alkali corrosion, which is the displacement direction of the pipeline, and whether there is any space limit in the installation position. Only when these parameters are all together can the manufacturer give you the appropriate skin material and structure. If one parameter is missing, the scheme may deviate.

In the final analysis, the non-metallic compensator is a "non-standard custom" flexible connector

It does look similar in many industries, but material formulations, structural details, and connection methods vary widely. The real professional approach is to find the manufacturer to come up with a plan with the working condition parameters, instead of taking a photo to draw a ladle with a gourd. The national standard JB/T 12235-2015 only stipulates the technical requirements and test methods, and does not say that you can install it everywhere if you buy a standard product.

Are non-metallic compensators universal? — Not universal. But you treat it like a custom piece, and it can dry better than a metal compensator under the right conditions. When it comes to model selection, it is never about looking at pictures, but about data.

Next time someone tells you, "Just buy a nonmetallic compensator and install it", ask him: What is the working temperature? Does the medium contain sulfur? What is the displacement amount? If he can't answer it, you can treat it as a joke.