Specialized in manufacturing compensators, expansion joints, baffle doors

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.

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.