Specialized in manufacturing compensators, expansion joints, baffle doors

What Are the Ways of Fabric Connection? Complete solution of connection process of fabric fiber expansion joint in industrial pipeline compensation

The "fabric connection" we talk about, in the field of industrial pipelines, most likely refers to the joint treatment between non-metallic expansion joints (fabric fiber expansion joints) and pipelines. This type of equipment is usually used in flue gas, hot air and dust ducts, and relies on flexible bands of multi-layer fiber fabric and rubber composite. The connection method determines the sealing and life of the whole compensation system. If you choose the wrong one, the later maintenance can make people collapse.

Flanged connection: the most versatile, but the details cannot be sloppy

Flange connection is the most common way in the field-welding flanges at both ends of the expansion joint, flanges on the pipeline, gaskets in the middle, and the bolts are tightened. It is convenient to disassemble and assemble, and does not need to move the pipeline during maintenance, and directly loosen the bolts and change the ring belt. But the pit is also here: the flange surface and bolt materials are not selected correctly, and it will rust to death in half a year. For example, the PTFE-lined metal hose matching the desulfurization flue gas baffle door has strong acidic medium in the flue gas. The flange has to be made of 316L stainless steel, and the bolts have to be coated with Dacro. Otherwise, it will be too late to break the bolts when disassembling them. The sealing gasket is also cheap. Graphite composite gasket is used for high-temperature flue gas and PTFE gasket is used for low-pressure and normal temperature, otherwise the leakage rate will double directly.

Welding connection: once and for all, but don't expect to change it later

Welding connection is to weld the metal frame of the expansion joint directly to the pipe, without bolts and gaskets. Advantages? Extremely low leakage rate, suitable for high temperature, high pressure and long-term non-maintenance occasions. For example, the power station industry uses corrugated expansion joints, high-parameter steam pipelines, with a temperature above 500℃ and a pressure of 10MPa. Do you dare to use flanges? The spacer blew off long ago. There are also high-temperature axial expansion joints, and the working conditions are continuously stable, so they don't have to be cared for for ten or eight years after welding. The cost is that you have to cut the pipe during maintenance, welding, flaw detection and heat treatment, and the labor cost is more than 3 times higher than that of flange. Therefore, welding is only suitable for those scenarios where "install it and don't plan to disassemble it".

Clamp/hoop connection: a money-saving tool with low voltage and large diameter

The clamp connection looks simple-apply sealant to the end faces of the two pipes, put a rubber compensator or rubber PTFE compensator on it, and use a hoop on the outside. Quick installation, one person can do it in half an hour. It is especially suitable for smoke and air ducts with low pressure (below 0.1MPa) and large diameter (above DN1000). Cement plant denitrification flue loves to use this trick, saving money and construction period. However, the disadvantages are also obvious: the sealing ring is a consumable part, and the rubber ages quickly. If the ambient temperature exceeds 80℃, it can't bear it, so it has to be replaced every two or three years. If the medium contains particulate dust, it will wear faster. To put it bluntly, the clamp is a combination of "cheap, fast and short-lived".

Adhesive connection: Patching is the technical job

The loop belt of non-metallic expansion joint (fabric fiber expansion joint) itself is multi-layer composite, and the loop belt and the metal frame have been vulcanized or bonded into one body when the manufacturer leaves the factory. The scene is just docking the frame with the pipe flange. But what if the ring belt is partially damaged during maintenance? Can't change the whole thing, can you? The sheet has to be patched on site with a special adhesive. This job looks simple, but it is actually very demanding on curing time and temperature. I met a customer two days ago, saying that it opened the glue after two months of mending. I asked: What is your curing temperature? He said that the workshop was 5℃ in winter, and it was used directly after gluing. That certainly won't work! Cure at room temperature for at least 24 hours, and ensure that the ambient temperature is not lower than 15℃. If it is a high-temperature zone, it is necessary to use heat-resistant epoxy glue, which can withstand the temperature above 150℃ after curing. Tsk, not many people pay attention to this detail.

Three elements of model selection: temperature, pressure and space

Which one do you choose? There is no "best", only "most suitable". Staring at three numbers first —Medium temperature: Basically say goodbye to rubber and ordinary bonding when it exceeds 300℃. Flange bolts should pay attention to high-temperature creep, and welding is more reliable.Pressure rating: Flanges or welding are preferred above 0.1MPa, and clamps below 0.1MPa can also be made up.Installation space: The narrow clamp is easier to use than the flange, such as the double hinge expansion joint of air-cooled island vacuum pipeline, which has a compact space and can't be sealed with a clamp, so it can only be welded.

Cement industry metal corrugated expansion joint, pipeline temperature fluctuation large, dust. Some people tried to save trouble with welding. As a result, two years later, the ring belt wore out and had to be replaced. The pipe had to be cut, and the kiln was shut down for three days, resulting in a loss of millions in output value. If the flange connection was used at the beginning, with the double-sealed single-axis circular baffle door, the sealing grade could reach more than 99.9%, and it would take half a day to change the ring belt. On the other hand, the double hinge expansion joint of air-cooled island vacuum pipeline, welding is the only choice in high vacuum scenario, and the flange leakage rate can't pass the test at all. Once the vacuum degree drops, the power generation efficiency plummets.

Don't be fooled, turn your own standards

To put it bluntly, many people can't even figure out the temperature threshold. If you are not sure, go directly to this site"National Standard for Non-Metallic Expansion Joints"Or compare the article "The Specific Function of Expansion Joint Guide", which is much more realistic than listening to bragging. The way of fabric connection seems simple, but behind it is the comprehensive consideration of material science, thermodynamics and sealing technology. If you choose the right one, the pipeline will run safely for ten years; If you choose wrong, half a year will make you doubt your life.

DN1000 not 1000mm in diameter? Understand the definition of nominal diameter first

When many people saw DN1000 for the first time, their first reaction was "the diameter of the tube is 1 meter". To be honest, I thought so when I first entered the industry, but I was fooled once by the drawings-the flange I actually bought couldn't fit. The "DN" in DN1000 is an abbreviation for Nominal Diameter, which is essentially aNominal size, not measured values. Take steel pipe as an example. The outer diameter corresponding to DN1000 is commonly 1016mm or 1020mm (see the standard, such as GB/T 1047 or ASME B36.10), while the inner diameter depends on the wall thickness. For example, a pipe with an outer diameter of 1020mm with a wall thickness of 10mm has an inner diameter of 1000mm; If the wall thickness is 12mm, the inner diameter will be 996mm. So here's the question: When you buy the expansion joint or hose of DN1000, do you have the flange according to the outside diameter, or do you have the flow according to the inside diameter? The answer is – look at the FLAND standard. If you are equipped with the national standard PN series flange, the sealing surface size of DN1000 flange and the center circle diameter of bolt hole are fixed, which has nothing to do with the actual wall thickness of the pipe. But if you come across the American Standard Class series, you have to count separately. Bottom line: Don't regard DN as the measured value. It is a code name. When selecting the model, be sure to check the detailed size of the connection end.

How to use DN1000 in large diameter pipe compensation?

DN1000 is particularly common in large-diameter pipelines in thermal power, steel and chemical industries. Which products will be marked with this size? Just a few examples:Large diameter thick wall expansion joint、Metal hose、Desulfurization flue gas baffle door。 Two days ago, the purchase of a desulfurization project asked me: Is the flue gas baffle door of DN1000 1 meter in diameter? I said you should understand it this way, but the actual door panel size depends on the inner diameter of the air duct and the thickness of the lining. The door frame size of the baffle door is usually larger than the DN1000 by a circle, because it needs to leave a sealed space. Another exampleMetal hoseThe metal hose of DN1000 is usually used for circulating water pipelines or flue gas pipelines in power plants. The number of bellows layers and the number of net strands are directly linked to the pressure resistance grade. WhileDesulfurization flue gas baffle doorThe DN1000 (list number 19) refers more to the standard diameter of the flange connection port. You have to confirm whether it is a single-axis double-baffle or a double-seal single-axis circular baffle door. Their structure and sealing methods are completely different.

Field case: DN1000 corrugated expansion joint for power station industry for steam pipeline in a power plant

A while ago, I went to the site of a 660MW unit project, and several steam pipelines were installedCorrugated expansion joint for power station industry(5th in the list), the specification is DN1000. When the installation team finished hoisting according to the drawings and was ready to tighten the tie rod nut, the technician suddenly found that the wave number and pressure level on the expansion joint nameplate did not match the design. And guess what? The drawing requires 4 waves with a withstand pressure of 2.5MPa, while the actual arrival is 6 waves and 1.6MPa. Although the flange hole spacing is all right, more wave number means that the axial stiffness becomes smaller, and it is easy to be compressed and unstable under the large thrust of steam pipeline. In the end, I could only return it for exchange, and the construction period was delayed for three days. So don't think that everything will be fine if DN1000 is matched. The wave number, pressure level and temperature compensation must be checked item by item. Especially for high-temperature steam pipelines, it depends on the material of the guide tube (lining tube) -if it is ordinary carbon steel, it can't withstand more than 500℃, so stainless steel has to be used.

Are metal hoses and rubber compensators interchangeable for DN1000?

Basically not, hard change is equivalent to digging a hole for yourself.Metal hose(The first in the list) relies on bellows and stainless steel mesh sleeve to bear pressure, and the maximum temperature resistance can reach above 600℃ (depending on the material, such as 316L or Incoloy 800), and the pressure resistance can also reach more than ten MPa. Whilerubber compensator(List 9) The main body is rubber + ply, and the temperature resistance is generally over 100℃, and the pressure does not exceed 1.6MPa. You said it was used on the steam pipe of the power plant, and the rubber one melted directly as soon as it was put on. On the other hand, for some low-pressure and normal-temperature circulating water pipes, metal hoses are ok, but the cost is several times higher, so it is unnecessary. There is also a pit: rubber compensators usually have hard flanges at both ends and a rubber sphere in the middle, which cannot bear the weight of the pipe when installed; Metal hoses can be properly supported. Be sure to ask about the medium temperature, working pressure and displacement type before selecting the model. Don't just look at the word DN1000 to place an order.

Three Must-Ask Questions When Purchasing DN1000 Products

First question: What is the connecting flange standard?Is it the national standard GB/T 9119 or the Ministry of Chemical Industry HG/T 20592? Or is it the American Standard ANSI B16.5? Each standard flange outer diameter, number of bolt holes, hole center circle diameter are different. The same supplier may match the national standard by default, but what you get is the American standard flange, and the on-site nozzle has to be reprocessed if it doesn't match. Second question: How to calculate the compensation amount? The expansion joint of DN1000, with axial compensation amounts of 50mm and 100mm, is completely two structures, and the wave number, layer number and wave height are different. Don't just listen to verbal promises. Third question: Does the guide tube have to be fitted? Many people ignore this.Specific Function of Expansion Joint Guide Tube(From Article 7 of FAQ on this site) is to protect the bellows from being washed by high-speed media, while reducing the flow resistance. For flue gas or steam pipelines containing particles, the life of bellows without guide tube is cut in half directly. However, having a guide tube will increase the pressure drop, which can be omitted if the medium is clean and the flow rate is low.

Let's talk about what is expansion joint plane instability

To put it bluntly, when the bellows is subjected to internal pressure, the corrugations are not axially compressed or stretched honestly, but like a flattened can, the plane direction of the corrugations is twisted, wrinkled or even bent laterally. This thing is called plane instability in engineering, and it is also called "bellows instability" in the industry. Have you ever seen the kind of ripples that were originally neat in circles suddenly turn into crooked twists? That's the typical symptom. Understanding this can avoid big problems in the pipeline system.

For example, if the pressure of the corrugated expansion joint used in the power station industry is not accurately calculated in the design, it will change from a ring to an ellipse with corrugation, or even partially deflate. That's not a cosmetic problem, it's a precursor to structural failure.

Where did the plane instability come from?

Excessive internal pressure and insufficient design stiffness. For example, when selecting the type, the working pressure is not accurately calculated, or the corrugated plate is too thin, or the ratio of wave height and wave pitch is unreasonable. When you think about it, bellows are inherently dependent on the geometry of corrugations to absorb displacement. Once the pressure exceeds its critical value, it is like a human spine suddenly bends under excessive compression-it can't hold up. In addition, if there are additional lateral forces or torsional stresses during installation, instability can also be induced. Two days ago, I met a customer, saying that the general-purpose corrugated expansion joint used in power stations was crooked after half a year of use. When I checked the drawings, the wave number was large but the wall thickness didn't keep up, and the typical pressure exceeded the limit.

The external pressure single type axial expansion joint was selected, but the actual operating pressure exceeded the design pressure by 1.5 times, and the instability probability increased greatly. The design pressure is 1.0MPa, but when it actually runs to 1.6MPa, 80% of them will have problems-this is not metaphysics, but common sense of mechanics.

The dangers of plane instability are no joke

Failure and air leakage are mild, and pipe burst is severe. Especially in some high-temperature and high-pressure pipelines, such as corrugated expansion joints and high-temperature axial expansion joints used in power station industries, once they become unstable, the corrugation breaks and leads to media leakage, which may directly cause shutdown accidents and even personal safety risks. Do you think the loss is big? Moreover, instability often does not happen suddenly, and there will be deformation accumulation in the early stage, but once it passes the critical point, the collapse speed is extremely fast. So don't wait until you hear an abnormal noise or see a noticeable distortion before dealing with it, it's already too late.

A cement factory used a batch of metal corrugated expansion joints in cement industry. In the third month after installation, it was found that the corrugations were partially depressed, and the operator didn't take it seriously. As a result, it burst directly on the sixth day, and high-temperature gas was ejected. Fortunately, no one came close at that time. This matter was later resumed, that is, the wave height ratio exceeded the standard and the plane stability was not enough.

How to judge whether the expansion joint has plane instability?

The simplest and crudiest way-eyes. If it is found that the corrugations of the bellows have irregular wavy wrinkles and local depressions, or the whole bellows rotates like a twist, it is basically unstable. More professionally, you can measure the change of outer diameter and wave pitch of bellows, or monitor it with a strain gauge. In addition, it is also critical to compare the design parameters: if the actual operating pressure exceeds 1.5 times the design pressure, the probability of instability increases greatly. For example, the external pressure single axial expansion joint, with a design pressure of 1.0MPa, actually runs to 1.6MPa, and 80% of them will have problems.

Scratches or abnormal wear on the inner wall of the guide tube. Because the corrugation deforms after instability, it will rub the guide tube. Therefore, regularly checking the state of the guide tube is also one of the basis for judgment. The product information of this site mentions the specific functions of the expansion joint guide tube, one of which is to protect the corrugation, but it can also reflect the health of the corrugation.

To prevent plane instability, the core is four words: choose the right one and use it well

When selecting the type, don't just look at the pressure grade and diameter, but also check the stability coefficient of the bellows. Like straight pipe pressure balance expansion joint or double hinge transverse expansion joint, because the structure itself has constraints, the risk of instability will be lower. However, if it is a general-purpose corrugated expansion joint or a large-diameter thick-wall expansion joint, special attention should be paid to the matching of wave height and wall thickness. Install a guide tube when necessary-the guide tube can not only protect the inner wall, but also reduce the direct impact of the medium on the corrugation, indirectly reducing the risk of instability. Concentricity should also be ensured during installation, and the pipe should not be pulled and dragged.

In some scenarios, such as the double-hinged expansion joint of air-cooled island vacuum pipeline for air-cooled island vacuum pipeline or the high-temperature axial expansion joint for high-temperature pipeline, the stability check must be done during design, and it can't only rely on experience. In addition, although the materials of metal rectangular expansion joints and non-metal expansion joints are different, the principle of plane instability is the same-the instability of corrugated structure is a geometric problem and does not change due to the material.

The product drawings of many small factories are cheap with thin materials, or they are not designed according to standards. It doesn't take long for users to buy them back and use them before they go wrong. When making products in our industry, such as metal rectangular expansion joints and non-metal expansion joints, stability must be regarded as a hard index. If you find that the expansion joint on the pipe looks wrong one day, don't hesitate to stop it and check it quickly. After all, a pipeline costs millions and an expansion joint costs thousands of pieces. Is it a loss to scrap the whole line because of instability?

For expansion joints that have been installed, strain gauges can be attached to the bellows surface to record the deformation regularly. Once it is found that the strain value of a certain side corrugation continues to increase, it indicates that instability is occurring. This is much more reliable than observing it with the naked eye. Especially for high-risk scenarios such as power stations and chemical industries, early warning saves too much money than remediation afterwards.

What is the difference between non-metallic expansion joints and rubber joints?

"That expansion joint on my desulfurization flue is cracked. Can you use a rubber joint to top it up first?" When I heard this, I quickly stopped it-brother, this is not to save trouble, but to dig a hole for myself. Non-metallic expansion jointsFabric fiber expansion joint) andrubber compensator(Commonly known as rubber joint), it does look a bit like it, and it is all soft connection. But the lining is far from it. One relies on multi-layered fabrics and metal frames to carry high-temperature smoke, and the other relies on rubber elastomers to absorb vibration and displacement. Today, I'll break the difference between the two and make it clear.

Let's see what they're all made of

Non-metallic expansion jointThe main body is a composite layer of silicone cloth and fluorine adhesive cloth, with stainless steel wire mesh sandwiched in the middle, and metal frame fixed on the outside. This thing can withstand 1000°C and higher-like oursThe high-temperature axial expansion joint is specialized in this. Whilerubber compensatorWhat? Pure rubber or cloth-sandwiched rubber, formed by vulcanization process. The highest temperature resistance is 100-200℃, and it will be soft, cracked and aged anymore. One is like armor, the other is like a rubber band — armor can block fire, rubber bands can only block small vibrations.

Performance data is put up, don't just feel it

Non-metallic expansion joints have a large amount of compensation, can absorb multi-directional displacement, and are especially suitable for rectangular pipes-at this time, they must be usedRectangular non-metallic expansion joint。 Rubber joints are better at vibration and noise reduction, but the pressure resistance is limited, generally PN0.6-1.6MPa. When encountering corrosive media, such as sulfur flue gas and acid-alkali solution, rubber joints are easy to age and crack. What about that? Can be usedRubber PTFE compensatorCome to the rescue, PTFE lining can carry corrosion, but the temperature resistance still can't keep up. In the final analysis, the selection depends on three numbers: temperature, medium and compensation direction. Whichever is wrong, something goes wrong in three months.

Exactly what scenario should it be used in?

Cement plants, flue ducts of power plants, boiler outlets-these places must use non-metallic expansion joints because of high temperature resistance, corrosion resistance and flexible structural compensation. OurDesulfurization flue gas baffle door、Electric plug-in insulation door、Manual plug-in insulation doorThese equipment interfaces are usually used with non-metallic expansion joints instead of rubber joints. In turn, the inlet and outlet of water pump, air conditioning pipeline, water treatment system-these places have low temperature, stable pressure, and mainly vibration isolation. Rubber compensator is suitable, which is cheap and easy to change. It is not impossible for you to install non-metallic expansion joints on the water pump, but the cost is high and the material is overused, so why bother?

Don't confuse the names, let alone the standards

Some peers call non-metallic expansion joints "fabric compensators" and rubber joints "rubber expansion joints", but they must not be mixed in function. Especially high-temperature dusty media-such as the flue gas at the outlet of power plant boilers, the temperature can reach above 800℃, and there is a lot of dust washing. Rubber joint up? It will crack in three months, and the ash is pasted to death. The non-metallic expansion joint is designed according to the national standard JB/T 12235-2015, multi-layer fabric + metal frame, much longer life. And guess what? In a previous project, the customer changed the rubber joint himself, and it leaked in less than half a year. The loss of shutdown was more expensive than buying ten non-metallic expansion joints. To summarize in one sentence: look at the temperature, the medium and the compensation direction. If you choose the wrong one, you will dig a hole for yourself.

One more word, our product line in our station is very complete- -Metal hose、PTFE-lined hose、Corrugated expansion joints(general purpose type, direct burial type, pressure balance type, hinge type...), andRound flapper door、Double-sealed single-axis circular baffle doorWait. If you are unsure of the specific working conditions, just throw the parameters over, and we will help you calculate according to JB/T standard. Don't guess for yourself, let alone use rubber joints to carry the flames — it really can't carry it.

1. The core basis of grading: pressure, temperature and compensation amount

After doing pipeline compensation for so many years, the most asked thing is: "How to divide the grade of metal expansion joints?" In fact, in the final analysis, there are three hard indicators: pressure, temperature and compensation amount. These three parameters basically lock in which level of product you should use.

Let's start with stress. The wall thickness, number of layers and wave height of bellows used in low pressure (0.1~0.6MPa) and high pressure (≥2.5MPa) are completely different from the same concept. For example, the general corrugated expansion joint has a general design pressure within 1.0MPa; However, the corrugated expansion joint used in the power station industry is 2.5MPa or even higher at every turn, so the corrugated pipe must be multi-layered, reinforced ring, and even armored. The pressure level directly determines the safety margin and fatigue life.

The temperature is tougher. 304 stainless steel can hold up to 100℃ at room temperature, but above 600℃, you have to use superalloys (such as Inconel 625), and you have to consider creep strength. Two years ago, a cement factory customer installed a high-temperature axial expansion joint on the kiln tail flue, with a nominal temperature of 800℃. As a result, ordinary 304 material was selected, and it cracked in three months-the wrong temperature grade was selected, which was equal to dry for nothing.

The amount of compensation is interesting. Axial compensation, lateral compensation and angular compensation, the compensation capabilities corresponding to different structural forms are far different. The straight pipe pressure balance expansion joint mainly absorbs axial displacement, while the compound hinge transverse expansion joint specializes in transverse displacement. The bigger the compensation amount is not the better. A large compensation amount means that the bellows is longer and has more wavenumbers, so the stiffness will come down and it is easy to become unstable. Therefore, when selecting a grade, the compensation amount must match the stiffness.

2. Looking at the grade from the standard: national standard, industry standard and manufacturer definition

When it comes to standards, many purchases say "just follow the national standard", but the national standard is actually very fine. The national standard for metal expansion joints is mainly GB/T 12777 (General Technical Specifications for Metal Bellows Expansion Joints), but don't expect this standard to cover all scenarios. For example, for metal corrugated expansion joints in cement industry, you have to refer to JC/T 967 (metal corrugated expansion joints for cement industry pipelines); Corrugated expansion joints used in the power station industry will involve DL/T 5185 (Technical Regulations for the Design of Bellows Compensators for Pipelines in Thermal Power Plants). The non-metallic expansion joint has JB/T 12235-2015, which is different from metal and non-metal, and the grading logic is different.

Manufacturer-defined grades are even more interesting. Domestic first-line brands, such as the products listed on our station, all have internal code names. For example, the general-purpose corrugated expansion joint is divided into PN0.6, PN1.0 and PN1.6 according to the pressure when it leaves the factory; However, it is also PN1.6. Different manufacturers may have differences in the number of bellows layers, the wall thickness of the end tube and the material of the guide tube. Therefore, it is not enough to only look at the "pressure level" of the manufacturer's trademark, but also the corresponding structural details.

For a desulfurization flue gas baffle door project, "Expansion joint grade: PN1.0, temperature resistance 200℃" is written on the drawings of the design institute. Results During on-site installation, it was found that there was a large amount of condensed acid in the flue gas, and 304 couldn't bear it at all. Finally, it was replaced with PTFE-lined hose, and the grade was redefined from the perspective of materials-this incident reminds us that the standard grade is only the foundation, and the "soft parameters" of medium corrosiveness and medium phase state are the real hidden grade lines.

3. Typical products corresponding to different grades: from general-purpose to power station/cement industry specific

Don't copy books about grading, it depends on the actual application scenarios. Let's pull out the product line and compare it:

• General scenario of low pressure and low temperature:Universal type corrugated expansion joint, metal hose, rubber compensator. The design pressure of these products is generally ≤1.0MPa and the temperature is ≤300℃, which is suitable for HVAC, water supply and drainage, and general industrial pipelines. Typical features are single-layer bellows, no guide tube or simple guide tube, cheap and frequent change.
• Medium-voltage and medium-temperature utilities:High temperature axial type expansion joint, external pressure single type axial type expansion joint, sleeve type pipe expansion joint. These products can have a pressure of 2.5MPa and a temperature of about 450℃, which are commonly used in steam pipelines and thermal pipe networks. Note that there will be a guide tube here, whose function is to prevent the high-temperature medium from directly washing the bellows-the specific function of the expansion joint guide tube, as specifically mentioned in our Q&A, is the key component to protect the bellows from high-speed fluid wear.
• High pressure and high temperature working conditions:Corrugated expansion joints for power station industry, metal corrugated expansion joints for cement industry and directly buried (fully buried) expansion joints. The pressure of the main steam pipeline of the power station is 10MPa, and the temperature exceeds 600℃. It must be designed with multi-layer bellows + external pressure balance. The flue gas temperature at the outlet of rotary kiln in cement industry is high and dusty, so it is necessary to use wear-resistant guide tubes and high-temperature resistant alloys.
• Special Structure:Double hinge transverse type expansion joint, curved tube pressure balance type expansion joint, air-cooled island vacuum pipe double hinge expansion joint. These are in the "custom-on-demand" rating, there are no standard models, and they must be designed one by one according to the pipe stress analysis.

Can I use the general-purpose one for power station projects? Of course not, let alone that industry standards don't allow it, and the materials themselves can't bear it. The level is wrong, it either leaks or explodes, and there is no intermediate option.

4. Pits that are easy to step on when selecting grades: the influence of material, stiffness and guide tube

Material selection only depends on pressure and not temperature. The common mistake is the stainless steel bellows of PN1.6, which works at 500℃, and thinks it's okay-in fact, 304 starts to accelerate oxidation above 425℃, so it is recommended to use 321 or 347. Therefore, when selecting a grade, the material grade must be confirmed separately.

Ignore stiffness. The stiffness and calculation formula of bellows are explained in detail on our site, but many selection personnel only look at the compensation amount. When the compensation amount is large, the stiffness will be small, and the force feedback of the pipeline on the equipment will become larger, which may pull out the equipment joint. This thing of stiffness determines that the expansion energy saving can't be "soft" enough to absorb displacement without transmitting excessive thrust. If conditions permit, let the manufacturer provide the stiffness value, and then do the pipeline stress calculation.

The guide tube is reversed or not installed. The function of the guide tube is not only to guide the flow, but also to reduce the fluid excitation and prevent the deposition of particles. However, in some projects, in order to save costs, the guide tube was removed. As a result, the inner wall of the bellows was blown away by high-speed steam, and it leaked in one year. Conversely, the deflector cannot be thickened at will-too thick will increase the local stress and lead to a decrease in the fatigue life of the bellows. It's a balancing point.

Two days ago, I met a customer who made gas turbines, and chose a special vacuum hose for use in the exhaust pipe. The grade was enough, but there was no guide tube. As a result, the particulate matter in the flue gas wore the bellows out. Alas, such lessons are all too common.

5. Practical suggestions: How to determine the grade of expansion joint according to working conditions

After so much verbose, here are some steps that can be used directly:

Step 1: List the working condition parameters.Media, pressure (design/operating pressure), temperature (design/operating temperature), type of displacement (axial/transverse/angular) and amount of displacement, nominal diameter of pipe.
Step 2: Determine the pressure rating.According to the table of GB/T 12777, find the number of bellows layers and wave height corresponding to DN and design pressure. Note: The allowable stress will be reduced at high temperature, so the pressure level should be corrected according to the high temperature working condition.
Step 3: Select the structural form.According to the displacement type: axial displacement, select straight tube pressure balance expansion joint or general type; Lateral displacement selection compound hinge transverse type; Pressure balance type of angular displacement curve selection tube; For large diameter and low pressure applications, sleeve-type pipe expansion joint or multi-wave type can be considered.
Step 4: Check the material..Temperature Step 5: Confirm whether you want a guide tube.If the medium flow rate is> 10m/s and contains particles, or the medium temperature is> 400℃, a guide tube must be added. The material of the guide tube is generally the same or higher than that of the bellows.
Step 6: Find the manufacturer to confirm the stiffness.Especially in scenarios where the pipeline is sensitive to the thrust of the equipment, such as connecting steam turbines and compressors, the greater the stiffness, the greater the thrust, which may push the base of the equipment.

In fact, how to divide the grade of metal expansion joint? In the final analysis, it is these three dimensions: load (pressure + temperature + displacement), environment (corrosion + wear + scour), and function (safety + life). You sort out these three points, and you can't go wrong with your selection. If you are still worried, just talk to our engineer with the parameters. Don't slap your forehead to set the grade-after all, the cost of installing this thing and then dismantling it is not a little.