Specialized in manufacturing compensators, expansion joints, baffle doors

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.

Excessive limit is not just "screwing too tight" as simple

Two days ago, a buddy from a thermal power plant complained to me that their power station industry leaked after installing corrugated expansion joints for only three months. When removed, the limit nut is pressed against the end plate, and the bellows is directly pulled into an "eight" shape. I am too familiar with this scene-many people's first reaction is that the quality of the expansion joint is not good, but in fact, the root is most likely at the limit.

The limiting devices (tie rods, nuts, ear plates) were originally designed to protect the bellows from excessive stretching or compression during transportation or installation. However, if you regard the limit as a dead block, or the amount of pre-compression/pre-stretching is not accurate during installation, the displacement direction changes as soon as the pipeline runs, and the limit changes from "bodyguard" to "shackle". And the result? The actual displacement of the bellows exceeds the design value, which ranges from corrugation instability and weld cracking to scrapping of the whole expansion joint or even pulling off the pipe. Don't ask me how I know-I have been in this business for more than ten years, and I have seen more than I have cultivated.

The root is mostly in the installation stage, so don't rush to throw the pot to the product

"The cold tightening amount is adjusted according to the design value", but how many on-site workers screw it directly to the end to save trouble? Or a general-purpose corrugated expansion joint was selected during the selection, and as a result, the actual thermal displacement of the pipeline was 30% larger than the design value, so the limit was naturally "stuck". There is also a common pit: the pipe support is loose or the foundation sinks, which leads to the deviation of the displacement direction, and the limiting device becomes the main stress point instead. And guess what? 90% of cases can be solved by adjusting the tie rod nut without replacing new parts at all.

So don't doubt the product when you come up. Ask yourself first: Did you find out the function of the expansion joint tie rod during installation? The tie rod is only for transportation and protection, and must be adjusted to the working position after installation. Does the screw of the expansion joint need to be removed? The answer is: the transport screw must be disassembled, and the limit nut should be cleared.

How to judge whether the limit is really "too big"? Don't rely on the feel

Take a caliper to measure the actual distance from the peak to the trough of the bellows, and compare the allowable compression/tensile limit given by the manufacturer. For example, in the product information of our site, the general-purpose corrugated expansion joint usually has the maximum compression amount and stretching amount marked. Under normal conditions, there should be a 3-5mm gap between the tie rod nut and the ear plate. Once it is dead, it means that the limit is too large. There is another local method: after shutting down, remove one nut and push and pull the expansion joint by hand. If you can move easily, it means that the original limit is fine. If you don't move at all... then you can basically confirm the diagnosis.

Remember, feel can deceive, but data won't. The function of the expansion joint guide tube also needs to be clarified-it is responsible for guiding the flow direction of the medium and protecting the bellows. If the limit is too large, the bellows will be deformed, and the guide tube will be easily damaged.

The solution is divided into three steps. Don't come up and move the knife

First step, loosen the nut. Turn the limit nut back, leaving the free clearance required by the design. For details, please refer to our article "How to Adjust the Tie Rod Nut of the Expansion Joint". Pay attention to re-check the pipe alignment after loosening to prevent deflection. In the second step, if the bellows has been permanently deformed (uneven wave pitch) after loosening the nut, then you have to consider replacing it. In the third step, for the excessive limit caused by insufficient pre-stretching, you can re-do cold tightening: use a jack to push the pipe to the designed displacement position, and then lock the nut.

If the conditions on the scene don't allow for cold tightness, don't force it. We also have double hinge transverse expansion joint or straight tube pressure balance expansion joint, which can absorb multi-directional displacement through its own structure and avoid single-point limit overload. The metal corrugated expansion joint in cement industry has been wear-resistant for the dust environment, and the high-temperature axial expansion joint comes with a guide tube and an insulation layer-choosing the right model can save half of the trouble.

Prevention is always less worried than remedy

In the selection stage, don't only look at the nominal diameter. Calculate the thermal expansion, installation temperature and ambient temperature of the pipeline clearly, and then select the corresponding model according to the expansion joint model and size comparison table. For example, steam pipeline, see what category steam pipeline belongs to, and then choose high-temperature axial type or straight pipe pressure balance type. Friends of cement factory remember to read the page of metal corrugated expansion joint in cement industry, wear-resistant design is not a gimmick. Also, it is recommended to read the article on the role of the expansion joint guide tube, which can help you avoid at least 30% of the selection pits.

By the way, the adjustment of the expansion joint tie rod during installation is a technical job. Our website has detailed steps for the correct installation method of large tie rod expansion joint, and the last step is to make sure to adjust the limit nut back to the working position. Don't be like that thermal power plant buddy who thinks that if you tighten it, everything will be fine.

When do you have to change? Don't hesitate

The bellows has visible cracks, bulges, serious uneven wave pitch, or the sound of media leakage-don't hesitate to change it directly. Note that the new one should match the displacement of the original pipeline, and don't buy a larger size to limit it back. Our double straight pipe bypass pressure balance expansion joint or curved pipe pressure balance expansion joint here can absorb greater displacement in a limited space, which is especially suitable for old pipeline transformation. If you are really too lazy to calculate the data, you can send us the on-site photos and pipeline parameters, and directly recommend the model for you-we do this every day, which is much more reliable than you groping for yourself.

Calculate the thermal displacement first: temperature change is the starting point, installation temperature is a pit

Metal expansion joint calculation method? The first step is always thermal displacement. The formula is simple enough to recite: Δ L = α × L × Δ T. The linear expansion coefficient α is 0.012 mm/m·℃ for carbon steel, 0.016 for stainless steel and 0.018 for copper. Just note it. The pipe length L is dead, but how to take Δ T? A lot of people are stuck here.

Does the installation temperature use the ambient temperature or the actual pipe temperature? I'll tell you, directly based on the local extreme temperature difference. For example, outdoor pipelines in the north, -30℃ in winter and +40℃ in summer, Δ T is 70℃. Don't worry about what month it is when it is installed, leave an allowance. The expansion amount of the pipe is considered small, the expansion joint is hard, and the bellows is directly torn. A while ago, a customer used a general-purpose corrugated expansion joint on a steam pipeline. The installation temperature was 20℃, the actual steam was 160℃, and Δ T was only 140℃. As a result, the actual operating temperature of the pipeline fluctuated to 180℃, and the expansion amount exceeded 20%. The bellows cracked after half a year.

The wrong calculation of pressure and thrust causes the expansion joint to be scrapped directly

The thrust generated by the pressure is F = P × A, and this A is the effective area of the bellows. However, the effective area calculation methods of different structures are completely different. For example, the effective area of the general corrugated expansion joint is equal to the circular area corresponding to the average diameter of the corrugated pipe; However, the effective area of the external pressure single axial expansion joint should be calculated according to the inner diameter of the external pressure cylinder, not the bellows itself. There's a big pit here: the blind plate force created by pressure, and the pipe can twist like noodles if it is not withstood by the tie rod or the main fixing bracket.

When calculating pressure thrust, don't forget that the medium may still have impact force. Water hammer effect when steam pipe starts, instantaneous pressure may be twice the working pressure. You calculate the thrust according to the steady-state pressure, but the fixed bracket is not designed enough, and the expansion joint tie rod breaks directly. Last year, in a chemical plant, the straight pipe pressure balance expansion joint was not counted as a water hammer, the fixed bracket was pushed and displaced by 30mm, and the bellows was unstable and scrapped.

Compensation amount, stiffness and fatigue life-how to break the iron triangle?

These three parameters are constrained by each other. The greater the amount of compensation, the softer the bellows (lower stiffness), but the fatigue life will be shortened. The national standard requires that the fatigue life is not less than 1000 times, which is the basic line. In practice, we generally design corrugated expansion joints and high-temperature axial expansion joints for power station industry according to 10,000 times. How to calculate? Reference to EJMA standards, combining wave pitch, wave height and wall thickness.

A bellows with the same caliber DN300 and a wave pitch of 40mm is 30% more stiff than a wave pitch of 50mm, but the fatigue life may double. The wall thickness increases by 0.3mm, the stiffness increases obviously, and the compensation ability decreases. It takes repeated iteration to find the balance point. Two days ago, someone took a double hinge transverse expansion joint and asked for a compensation amount of 100mm. I saw that the wall thickness was only 1.5mm, the wave number was 8, and the fatigue life was only 500 times. It is recommended that the wave distance be widened, the wall thickness be 1.8mm, the wave number be increased to 12, and the life time be mentioned only 8,000 times.

Don't use one algorithm for all models

The calculation ideas of straight pipe pressure balance expansion joint and double hinge transverse expansion joint are completely different. The pressure balanced type counts the thrust cancellation generated by internal pressure-it has two bellows itself, one absorbing displacement and the other canceling the pressure thrust. However, the hinge type should consider the coupling of angular displacement and lateral displacement, not simple superposition. For example, the transverse expansion joint of compound hinge, the angle change between two hinge groups will produce axial and lateral displacement at the same time, and it is necessary to draw a displacement vector diagram to calculate it clearly.

Large diameter thick wall expansion joints are more troublesome. The increase of wall thickness leads to the increase of stiffness and the decrease of compensation ability, but the pressure thrust is large. You have to assume a wall thickness first, calculate the compensation amount, check the fatigue life, and adjust the wall thickness or wave number if it fails, and iterate repeatedly. Some designers try to save trouble and directly use empirical formulas. As a result, the stress of the pipeline exceeds the limit and the vibration is abnormally large.

Guide tube and tie rod nut are not decorations

The function of the guide tube is to reduce the erosion of the medium, especially for high temperature and high speed steam. But it affects the effective area and the pressure loss – the inner diameter variation must be taken into account in the calculation. Many people directly calculate according to the inner diameter of the bellows, ignoring the space occupied by the guide tube. As a result, the actual pressure loss is more than 30% greater than the calculated value. The adjustment of the tie rod nut directly determines the direction of pre-deformation, and if it is installed backward, the expansion joint will fail in advance. How to adjust the expansion joint tie rod? The principle is: when cold tight, the tie rod nut compresses the bellows, so that it has a pre-stretch in the installed state, so that when the working temperature rises, the bellows can be both compressed and stretched, and the service life is longer.

Actual Combat Checklist: Don't wait for an accident to regret it

After getting the medium temperature, pressure, pipe diameter and direction diagram given by the customer, follow this order: first draw the displacement vector diagram to clarify the axial, transverse and angular displacements; Calculate thermal displacement and pressure thrust; Then the structure type is selected according to the working conditions-high temperature axial type for high temperature, large diameter thick wall expansion joint for large diameter, and transverse type of compound hinge for absorbing multi-directional displacement; Finally, check the fatigue life, which is not less than the design life.

Two days ago, a customer used the double straight pipe bypass pressure balance expansion joint on the steam pipeline. As a result, the lateral displacement was not counted. After the thermal expansion of the pipeline, the lateral offset of 15mm was produced, and the expansion joint was hard, so the bellows leaked after three months. You say it was wrong or not? The lateral displacement is only 15mm, and it can be solved by adding a double hinge transverse expansion joint. Why save that money?

Metal expansion joint calculation method? To put it bluntly, there are three parameters: thermal displacement, pressure thrust and fatigue life. If one is wrong, it will wait for rework. Selection is not a chance, it is calculated.