Specialized in manufacturing compensators, expansion joints, baffle doors

Compensator deformation? Don't be in a hurry to say "broken."

Has this ever happened to you? Not long after the pipeline was running normally, the compensator was bulging, the bellows were twisted into twists, and the rubber joints were bulging like balloons. Most people's first reaction is "the quality of this compensator is not good". But I want to say that 90% of the deformation has nothing to do with the product itself, but the stress is not released. General-purpose corrugated expansion joints, rubber compensators, non-metallic expansion joints... There are all kinds of strange deformation forms, and there are only a few reasons at the root. Today, let's take a look at them one by one.

1. Working condition design deviation-the selection parameters are far from the site

High-temperature pipeline selects common general-purpose corrugated expansion joint. Once the temperature rose, the ripples crept directly-not a day or two, but slowly collapsed. There are also insufficient pressure levels, and when the medium impacts, it bulges or even bursts. And guess what? A standard expansion joint is used in the steam pipeline of a chemical plant, and the design temperature is 350℃. In actual operation, it often rushes to 420℃. After three months, the wall thickness of bellows is thinned by 40%. When selecting the model, you must keep an eye on the medium temperature, pressure, displacement, especially the applicable temperature range of high-temperature axial expansion joints and non-metallic expansion joints. Don't be too troublesome, list the media parameters clearly and check them against the product manual-this step can't be saved.

Let's talk about stress. The pressure rating should be high rather than low when selecting the model. For example, steam pipe water hammer, instantaneous pressure can soar to double the design value. Do you think choosing a 1.6MPa one is enough? It may actually rush to 3.2MPa. If the directly buried (fully buried) expansion joint is not insulated, the repeated freezing and thawing of moisture in the soil can also make the ripples crack-this is another "selection bias": the environmental conditions are ignored.

2. Improper installation and constraint setting-the number one killer of human factors

The fixed supports are not welded firmly, the guide brackets are too spaced apart, and the tie rod nuts are screwed randomly-these operations are the number one human factors of deformation. Two days ago, I met a customer. The flue gas pipeline of the power plant was equipped with a compound hinge transverse expansion joint. As a result, the guide bracket was installed inclined, the transverse displacement of the compensator was locked, and the bellows was twisted into a twist. At the scene, the installer didn't look at the drawings at all, and the bracket was crooked by five centimeters.

Always check the adjustment method of the expansion joint tie rod nut before installation. Many products leave the factory with locked rods to protect transportation and installation. After installation, it should be loosened or removed according to the instructions, otherwise the compensator will not work properly. Does the screw of the expansion joint need to be removed? The answer is: it depends on the working conditions. If it is an axial type, the transport screw is usually removed; If it is a hinge type, the tie rod nut should be adjusted to the design clearance. Don't think tightening is done.

3. Media corrosion and overtemperature and overpressure-invisible knife

Acid, alkali, salt spray or sulfide in smoke can directly chew through the bellows. Although PTFE-lined hoses and PTFE compensators are corrosion-resistant, they fail when the temperature exceeds 180℃. Tsk, many people think that "PTFE is all-powerful". As a result, the flue gas temperature rushed to 250℃, and fluoroplastics directly softened and deformed.

The water hammer in the steam pipe is more concealed. The instantaneous pressure of the water hammer can soar to twice the design value, and the bellows has no time to release, which leads to direct plastic deformation. This is not the worst-if the directly buried (fully buried) expansion joint is not insulated, the moisture in the soil will freeze and thaw repeatedly, the bellows will become brittle at low temperature, and micro-cracks will appear in a freeze-thaw cycle. You say it was wrong or not?

4. Mechanical vibration and fatigue damage-resonance is a chronic poison

The vibration frequency of the pump, fan and air-cooled island equipment matches the natural frequency of the compensator? Once it resonates, the bellows bends tens of thousands of times a day, and it will crack within three months. The double hinge expansion joint of air-cooled island vacuum pipeline is especially important for fatigue life. Many designers only consider thermal displacement and ignore vibration frequency. And the result? After the high-frequency displacement accumulates, the appearance does not change when you look at it, but in fact, the micro-cracks are already covered.

How to solve it? Manufacturers are required to provide stiffness calculation formula and fatigue life curve during model selection. When installing, add vibration damping bearings in the pipe system, or replace with rubber compensators to absorb vibration. Rubber PTFE compensator also has a certain vibration reduction effect, but attention should be paid to the medium temperature.

5. Manufacturing defects and transportation damage-no one can save it when the source is rotten

When the bellows is formed, the wall thickness is uneven, the corrugation roundness is not enough, or the stiffness test is not done before leaving the factory, and the original shape is directly revealed when installed. Also, it was flattened and bumped during transportation, and it was welded without inspection on the spot. It would be strange if there was no accident. A large number of large-diameter thick-walled expansion joints were purchased in a certain project. After the arrival, the workers didn't unpack them because they were troublesome, and directly hoisted and welded them. As a result, all three bellows had pits and all cracked after two months of operation.

When purchasing, ask for the stiffness calculation formula and material report. When receiving the goods, take a caliper to measure the wave distance and total length. Don't be too troubled. A copy of the metal hose size comparison table, expansion joint model and size must be prepared on site. If you fail the acceptance, you will return the goods directly. Don't take chances.

Prevention Guide: Three Steps

• Type selection check: The medium temperature, pressure, displacement amount and environmental conditions (freeze-thaw, corrosion) are matched one by one. High temperature axial expansion joint for high temperature, PTFE-lined or PTFE-lined compensator for corrosion, rubber compensator or damped for high frequency vibration.
• Installation Specifications: The fixed support and guide bracket shall be constructed according to the drawing. The tie rod nut is operated according to the adjustment method of the tie rod nut of the expansion joint, and the transport screw is disassembled when it is necessary.
• Periodic inspection: Is there any crack, bulge or corrosion on the surface of the bellows? Direct burial type to check whether the insulation layer is intact? The vibration frequency of air-cooled island double hinge expansion joint is measured every six months.

Causes of compensator deformation? Complicated is also complicated, and simple is also simple-it is nothing more than the five lines of type selection, installation, medium, vibration and quality. If you stomp on each of them, the probability of deformation will be reduced by 90%. Remember: compensators are not consumables, they are life-savers. Don't wait until something goes wrong to regret it.

Wave height and wave distance, what the hell are they talking about?

Don't be fooled by the words "wave height" and "wave distance". To put it bluntly, wave height is the vertical height of a ripple from peak to valley-you can think of it as the amplitude of the undulation of a wave; Wave distance is the horizontal distance between two adjacent peaks (or troughs). Placed on the bellows of the compensator (that is, the expansion joint), these two parameters directly determine how "soft and hard" it is and "how much displacement it can swallow".

The larger the wave height, the larger the spring coil diameter, the easier the bellows to deform, and the ability to compensate displacement is strong, but the pressure bearing capacity will decrease; The larger the wave pitch, the larger the pitch equivalent to the spring, the more rigid the bellows and can carry higher pressure, but the number of corrugations required under the same displacement is less. Therefore, selecting wave height and wave distance is essentially to find a balance between "flexible compensation" and "stable pressure resistance".

When designing pipeline, selection of compensator wave height and pitch? Who has the final say? Don't worry, read a negative textbook first.

What happens if the wave height and wave distance are selected wrong? A real case of pipeline failure

Two years ago, there was a problem with the steam pipeline of a chemical plant. The design temperature is 400℃, the pressure is 1.6MPa, and the displacement is 80mm. They chose a general-purpose corrugated expansion joint, which has a large wave height and a small wave pitch-to put it bluntly, it is "too soft". Results Less than three months after operation, the bellows became unstable, the wave crest collapsed, and the air leaked directly. When the scene was removed, the corrugation spacing had been squeezed together and completely lost its elasticity.

What's the problem? If the wave height is large, although it can easily cope with the axial displacement of 80mm, the local stress of the bellows wall is too high due to the small wave pitch. Under the superposition of high temperature and pressure, the creep of the material is accelerated and the fatigue failure is advanced. This is the typical pit of "compensator wave height and wave distance selection? Only focusing on displacement, ignoring pressure". Selection is never about patting your head, you have to break up the working condition parameters one by one.

Three core factors influencing the selection of wave height and pitch: pressure, displacement and fatigue life

There are only three things that really determine the height and distance of the wave:Design pressure, compensated displacement, expected fatigue life。 Let's disassemble them one by one.

Pressure: Determining the upper limit of wave height

The higher the pressure, the smaller the wave height. Why? Large wave height means that the curvature radius of the bellows wall is large, and the film stress is large when it is subjected to internal pressure, which is easy to bulge or burst. Therefore, for high-pressure working conditions (such as the main steam pipeline of the power station, the pressure is above 10MPa), bellows with small wave height and large wave pitch are generally selected, such asHigh temperature axial expansion jointOrExternal pressure single axial expansion joint。 Conversely, it is fine to use large wave heights for low-pressure pipes (such as flue gas pipes), likeRectangular non-metallic expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)It does not rely on the metal corrugation to bear pressure at all, and the constraint of wave height and wave distance is much smaller.

Displacement: a function of wave pitch and number of ripples

When the compensation displacement is large, the common practice is to increase the number of ripples instead of blindly increasing the wave height. Because excessive wave height is sensitive to pressure, the wave distance can be appropriately amplified. Such asUniversal corrugated expansion jointIn the selection table, for every 10mm increase in axial displacement, 1~2 waves are usually added. The wave pitch should be selected to ensure that there is sufficient gap between the corrugations, so as to avoid the collision of wave peaks during compression-the collision will directly lead to stress concentration and fatigue life drop.

Fatigue Life: The "Referee" of Model Selection

A lot of designers ignore this. ASTM or national standards have clear requirements for the fatigue life of compensators (such as 1000 cycles). The collocation of wave height and wave pitch directly affects the maximum stress amplitude of bellows, and then determines the life. As mentioned in the product information on our siteStiffness and Calculation Formula of BellowsIt is emphasized in (QA1) that the stiffness is inversely proportional to the cubic power of the wave height and directly proportional to the first power of the wave distance. When the wave height increases slightly, the stiffness decreases sharply, but the stress does not necessarily decrease-full stress check is needed. Therefore, applications with high fatigue life requirements (such as those for air-cooled island vacuum pipesDouble hinge expansion joint for air-cooled island vacuum pipeline), must take into account both wave height and wave distance, and iteratively calculate with special software. And guess what? The root cause of the failure of many factories is that only an "experience value" was taken during the fatigue life check.

How to choose under different working conditions? From general purpose to high temperature to non-metallic compensators

After talking about theory, talk about practice. According to the existing product series of this site, the selection of several typical scenes is sorted out:

• General Low Pressure Pipeline (Water, Oil, Low Pressure Steam): SelectUniversal corrugated expansion jointThe wave height is 0.1 to 0.15 times of the nominal diameter, and the wave distance is 0.8 to 1 times of the wave height. For example, DN100 pipeline has a wave height of 10~15mm and a wave pitch of 8~15mm. When the pressure is lower than 1.0MPa, it can still be usedrubber compensatorOrRubber PTFE compensatorThe wave height and pitch are determined by rubber molding, so there is no need to worry about it.
• High temperature and high pressure steam pipeline (main steam and reheated steam of power station): Must go onHigh temperature axial expansion jointOrCorrugated expansion joint for power station industry。 The wave height should not exceed 20mm (even above DN600), and the wave distance should be appropriately increased (15~25mm) to reduce the film stress. At the same time, the internalexpansion joint guide tubeAvoid high-speed steam scour ripples.
• Cement/flue gas industry (large displacement, low pressure, dusty): PreferredNon-metallic expansion jointOrRectangular non-metallic expansion joint。 In fact, the concept of wave height and wave pitch is not applicable to fabric fibers, but the "band width" and "interlayer spacing" in structural design are similar to logic: the band is too wide (equivalent to too large wave height) and it is easy to dust, and the compensation is too narrow (the wave pitch is too small). This type of product is pressed byNational standard for non-metallic expansion joints(JB/T 12235-2015) can be selected.
• Corrosive media (acid, base)：PTFE-lined hoseOrPTFE compensatorEnter the stage. Wave height and wave pitch are limited by the molding ability of PTFE. Usually, the wave height is small (8~12mm), and the wave pitch is uniform, so it cannot be over-stretched, otherwise the PTFE layer will crack.

The wave height and wave distance of different working conditions are very different, so you can't take a template everywhere in the selection. Two days ago, a customer asked: What is the selection of compensator wave height and wave distance? Can you give me a watch directly? I say yes, but only if you list the pressure, temperature, displacement, number of cycles-otherwise the table given is waste paper.

Measured Data Speak: Relationship between Wave Height and Wave Pitch and Compensator Stiffness

We use a set of typical data to illustrate the problem (based on the factory laboratory measurement, non-theoretical formula calculation). Take 304 stainless steel bellows with wall thickness of 1.2mm and corrugated inner diameter DN150 as an example:

• Scheme A: Wave height 15mm, wave pitch 12mm → axial stiffness about 400N/mm, single wave compensation amount 5mm, fatigue life 3000 times (at 1.0MPa).
• Scheme B: wave height 20mm, wave pitch 12mm → axial stiffness drops to 220N/mm, single wave compensation amount 7mm, but fatigue life drops to 1200 times.
• Scheme C: The wave height is 15mm, the wave pitch is 18mm → the axial stiffness rises to 600N/mm, the single wave compensation is only 3.5mm, and the fatigue life is reduced to 800 times due to stress concentration.

See? The wave height increases, the stiffness decreases exponentially, and the compensation amount goes up, but the fatigue life is severely sacrificed. The wave pitch increases, the stiffness increases, but the compensation amount is too small, more ripples are needed, and the cost goes up. The core of the selection is to find the practically available intersection in this triangle (stiffness-compensation-life).

This is why professional manufacturers (such as the technical accumulation behind the 23 types of products in our station) are doing itExpansion joint selectionWhen, finite element will be used to assist calibration, rather than relying on hand calculation alone.

Summary: Selection is not patting the head, just follow these four steps

After so much verbose, it is summarized into a four-step operation guide. Next time you design the pipeline, you will go directly against it:

1. Explicit boundary conditions: Pressure (maximum operating pressure + hydraulic test value), temperature (maximum + minimum), displacement (axial/transverse/angular, note thermal expansion and contraction and installation deviation), allowable fatigue life (usually 1000 times or more).
2. Initial wave height and wave distance: According to the pressure level, refer to the industry experience (such as high-pressure wavelet height, low-pressure large wave height), choose a gear first. Recommended wave height for high pressure =0.03~0.06 times nominal diameter, 0.08~0.15 times for medium and low pressure.
3. Calculate the number of corrugations required: Divide the total displacement by the allowable compensation amount of single wave (note that the safety factor is 0.6~0.7) to get the minimum number of corrugations, and then multiply by the wave pitch to get the effective length of the bellows. While considering space constraints.
4. Fatigue checking and fine-tuning: The maximum stress is calculated using software or a formula (e.g. EJMA standard) and the fatigue curve of the material is compared. If the fatigue life is not enough, the wave height should be reduced or the number of corrugations should be added, and the single wave compensation amount should be reduced. Go back and adjust the wave distance again, and iterate again and again.

Compensator wave height and pitch selection? There is no standard answer to this question, only the answer "best for your working conditions". Don't be superstitious about the so-called "universal type"-find a professional manufacturer. After all, if something happened to the pipe, it was not as simple as replacing the bellows.

Why is the end structure of the compensator more critical than the corrugated body?

Two days ago, a customer who was doing a flue gas desulfurization project came over, and the pipeline wasDesulfurization flue gas baffle doorAndNon-metallic expansion jointAs a result, the end flange gasket was selected wrong, and it leaked in less than two months. You say it was wrong or not? The end of the compensator is the "joint" of stress transmission in the pipeline system, which not only has to bear the medium pressure and temperature displacement, but also has to bear the on-site installation error-if the workers are slightly misaligned, the end has to be carried hard. Once there is a problem with the end structure, no matter how good the bellows is, it will be for nothing. So don't just stare at the number of ripples, the end design is the "one-vote veto" link.

There are only a few common end structures, but the selection doorway is very deep

Most commonlyFlanged connection, applicable to mostUniversal corrugated expansion jointAndMetal hose。 Here's the pit: flange standards (HG/T, GB, ANSI) must match the pipe. Some people try to save trouble. When they buy it, they find that the flange bolt holes don't match. Expanding or repairing welding on the spot is equivalent to digging a hole for themselves. Once the sealing surface is deformed, it will be a matter of time before it leaks.

welded endCommonly used in high temperature and high pressure applications, such asCorrugated expansion joint for power station industryAndHigh temperature axial expansion joint, direct butt welding with pipeline, high strength but not detachable, can only be cut during maintenance. This structure requires extremely high welders' skills, and the weld flaw detection fails? Then you might as well use a flange.

AndThreaded connection, mostly used for small caliberrubber compensatorOrSpecial hose for vacuum。 But don't use threads for large-diameter pipes-the torque control is not good, and the ends are cracked directly. I have seen the rubber compensator of DN300, and the threaded ends are cracked like spiderwebs. Therefore, the thread is only suitable for small diameter and low pressure occasions.

The cooperation between the end and the guide tube is a detail that many people ignore

The inside of the end of the expansion joint is usually addedguide tube(The role of the guide tube is specifically talked about in the question and answer of this site), and its core task is to guide the flow direction of the medium and protect the bellows from erosion. But how to fix the end of the guide tube? Is it welded to the end tube or is it a sliding fit?

RightDirect buried (fully buried) type expansion jointAndExternal pressure single axial expansion jointFor example, the guide tube must be integrated with the end structure. Otherwise, when the guide tube is displaced during thermal expansion, the bellows will be scratched, and the scratch is the stress concentration point, and the fatigue life will be directly folded in half. And guess what? Some customers remove the guide tube in order to save costs, and as a result, the medium directly impacts the root of the bellows, which is perforated in one year. Is it a good deal to save hundreds of dollars and lose tens of thousands?

Under different working conditions, the design difference of end structure can be ridiculously large

Such asPTFE-lined hoseThe end must be flanged and coated, otherwise there will be gaps between the PTFE lining and the metal end tube, and the corrosive medium will bulge when it seeps in. What should I do if I bulge? You can only change the whole root. Another exampleRectangular non-metallic expansion jointThe end portion is a frame structure, and the fabric fibers are fixed by beads and bolts. Here, the bolt hole spacing design should be accurate. If you screw it too tightly, you will crush the fiber and loosen the air leakage. I have seen a steel mill project where the bolt spacing was 50mm larger, and as a result the fibers were clattered by the wind and worn out in three months.

AndRotary compensatorThe end is actually a rotary seal structure, using high-temperature-resistant graphite packing, which is completely different from our conventional welded end. The compression amount and replacement space of the packing must be considered in the design, otherwise the torque will be large and the seal will fail.

Several typical scenarios of end failure, how many have you encountered?

The first type：Large diameter thick wall expansion jointCracked end weld. It is mostly because the thrust of the pipe system is not calculated clearly, and the weld bears extra bending moment. When designing, only the axial thrust is calculated, and the lateral wind load and earthquake load are forgotten. It is strange that the weld does not crack.

The second type：Compound hinge transverse expansion jointThe end bolts are loose and the hinge plate is displaced causing the end tube to jam. This kind of problem often lies in the vibration condition, the bolts are loosened when they vibrate without anti-loosening washers.

The third most injustice：Manual plug-in insulation doorAndElectric plug-in insulation doorThe end interface, because there is not enough operating space during installation, the board can't be pulled out during later maintenance. In the design stage, the installation space and maintenance channel must be taken into account. Don't wait until the wrench can't be stuffed in.

Here is a practical suggestion: The end structure design must be combined with product model and standard

Don't think about taking a generic scheme for all projects. For example, if you chooseCurved tube pressure balance expansion jointThe end is to bear the weight of the pressure thrust balancing mechanism; chooseDouble hinge expansion joint for air-cooled island vacuum pipelineHow much Pa does the end seal structure have to vacuum? These parameters have to be determined at the drawing stage. From the product list of this siteRubber PTFE compensatorToSleeve type pipe expansion jointEach kind of end design has particular attention. Rework when something goes wrong? That cost is more than doubled-construction delays and losses from production stoppages are enough to buy several sets of equipment.

Therefore, the next time you design a compensator, first think clearly how to connect the ends. If the connection method is not selected correctly, the rest will be all in vain. It's as simple as that.

Find out first: What does compensator plane instability look like?

To put it bluntly, plane instability means that when the bellows is subjected to internal pressure or axial displacement, the bellows no longer expands and contracts along the axis honestly, but bends, twists and even rolls over in the plane perpendicular to the axis like a twist. Imagine: an originally straight metal bellows suddenly becomes S-shaped or wavy, with asymmetric wrinkles between the corrugations-this is the typical look of plane instability.

This instability is not the slow grinding of metal fatigue cracks, it often occurs instantaneously. Once it appears, the compensator is basically scrapped, and in severe cases, it directly leads to pipeline rupture, media leakage, and even the whole pipe system is paralyzed. The most exaggerated case we have ever seen in actual engineering is: the cement industryMetal Corrugated Expansion Joints in Cement IndustryLess than two months after it was put into operation, the bellows was completely twisted into a ball like a kneaded can.

Why does a good expansion joint suddenly "convulse" and become like this?

What factors are most likely to push the compensator into "plane instability"?

The reason is only three words-press, bend and twist. But breaking it down, there are a few things to blame:

• Internal pressure too high: Pressure is the direct pusher of plane instability. When the bellows is subjected to internal pressure, it will produce circumferential stress, which will bulge the corrugation outward. When the pressure exceeds a certain critical value, the corrugation will lose its stability. To put it bluntly, it is like blowing the balloon over-blown, and the balloon wall begins to bulge locally.
• Excessive axial displacement: The compensator is designed to absorb thermal displacement, but if the actual displacement exceeds the design value, the bellows is overcompressed or stretched, and the spacing between the corrugations changes too much, which can also induce instability. You see those expansion joints that are crushed and asymmetrical like an accordion on the spot, and nine times out of ten, the axial displacement is exceeded.
• Installation deviation: This one is too common. The center line of the pipeline is not aligned, and the expansion joint is forcibly screwed on; Or the fixing bracket is not done properly, causing the expansion joint to bear additional bending moment. Two days ago, I met a customer who said that they had installed a power station projectCorrugated expansion joint for power station industryAs a result, it collapsed directly during the pressure test-after checking, the eccentricity of the pipeline was two centimeters different during installation.
• Insufficient support: The bellows itself is a flexible element. Without suitable guide brackets and fixing brackets, it will twist like a snake without a skeleton. Especially those with long ripples, multiple wavenumbers, such asUniversal corrugated expansion jointIf the limiting device is lacking in the middle, the probability of plane instability soars.

The calculation formula of plane instability is not so mysterious. The key is to look at these parameters

Many designers are big when they mention the calculation formula, but the core calculation of plane instability is actually not complicated. At present, the formula in the American Association of Expansion Joint Manufacturers (EJMA) standard is commonly used in engineering:

Critical pressure P_cr = (0.34× π × E × t²) / (L_b × D_m²)
Where: E — — elastic modulus of corrugated pipe material, t — — wall thickness of single layer of corrugated pipe, L_b — — total length of corrugated pipe, D_m — — average diameter of corrugated pipe.

Do you see that? It is four parameters that really determine the instability boundary:Material stiffness (E), wall thickness (t), length (L_b) and average diameter (D_m)。 The longer the length and the larger the diameter, the lower the critical pressure and the easier the instability. On the contrary, the thicker the wall thickness and the harder the material, the stronger the ability to resist instability.

But many people just use the formula and ignore another implicit condition- -Wave distance and wave height ratio。 In practical engineering, the design of bellows should not only meet the critical pressure, but also consider the cumulative effect brought by wave number. Like aStraight pipe pressure balanced expansion jointIf there are too many wave numbers, even if the single wave calculation is qualified, the whole may be unstable. Therefore, there is an empirical value in the industry: the safety factor of single wave instability is usually above 1.5, and the overall safety factor of instability is above 2.0.

What if the calculation is unqualified? The easiest way is to reduce the wave number or increase the wall thickness. But don't forget that the increase in wall thickness will make the stiffness greater and the compensation ability decrease. This is a game-you have toCompensation amount, pressure class, structure dimensionsFind a balance between.

Wrong selection, inadequate installation, no matter how accurate the calculation is, it is useless-a guide to avoiding pits in actual combat

The formulas were so wild that they were all in vain when they arrived at the scene- -I've seen this a lot. Here are a few pits that you'd better avoid:

• Ignore the influence of temperature on elastic modulus during model selection: Many designers calculate it at room temperature and think everything will be fine. However, with four or five hundred degrees of steam in the power station pipeline, the E value of stainless steel will drop by more than 30%. The critical pressure was also cut in half. Therefore, under high temperature working conditions, selectHigh temperature axial expansion jointThe elastic modulus at high temperature must be used for recalculation.
• Misuse of tie rod as support: Some scene drawings save trouble, takeexpansion joint tie rodUsed as a guide bracket. The tie rod can only limit axial displacement and cannot resist lateral bending. The initial stage of plane instability is lateral bending, and the tie rod can't stop it at all. The right thing to do is to install separate guide brackets, one every two to three wave pitches.
• Excessive cold tightening operation: Pre-tensioning or pre-compression (cold tightening) is to reduce the stress in the working state, but the amount of cold tightening is too large, which is equal to initially giving the bellows an additional displacement. If the design margin of the compensator itself is insufficient, the cold tightening directly triggers instability. There's a case whereDouble hinge expansion joint of air-cooled island vacuum pipeThe cold tightness exceeded the design value by 20%, and it became unstable after one week of operation.
• Ignore wall thickness reduction caused by media corrosion: The environment of desulfurization flue is highly corrosive, if the materials are selected incorrectly, such as usingNon-metallic expansion joint (fabric fiber expansion joint)However, without considering acid-alkali corrosion, the wall thickness gradually becomes thinner, and the original critical pressure fails. Regular thickness measurement is a life-saving means.

Talk about the cases of rollover due to plane instability in power stations and cement industries

Let's start with the power station. A 300MW unit was installed on the main steam pipelineCompound hinge transverse expansion jointThe design pressure is 4.0 MPa and the temperature is 540 °C. After half a year's operation, the inspection found that the bellows had obvious local bulging, which was orange peel-shaped. After stopping for inspection, it was found that the plane instability of the bellows had occurred, and the corrugation spacing changed from a uniform 10mm to 8mm on one side and 12mm on the other. The cause of the accident is very typical: the elastic modulus used in design is the normal temperature value, and the high temperature attenuation is not considered; In addition, the length of the bellows is too long, and the critical pressure is only 10% higher than the working pressure, so the safety factor is not enough. Finally, the whole expansion joint was scrapped, and the front and rear pipelines were replaced, and production was stopped for three days, resulting in a loss of seven figures.

Let's say cement. Used in the cement industryMetal Corrugated Expansion Joints in Cement IndustryIt is often arranged at the outlet of the preheater, and the air duct has large diameter, high temperature and large dust content. I have a project to chooseSingle-axis double-flapper doorWith an expansion joint, but the guide tube of the expansion joint (i.e.Specific Function of Expansion Joint Guide TubeThe liner mentioned in) is worn out so badly that the bellows are directly exposed to the high-temperature dusty airflow. Dust accumulates in the trough, destroying the uniform force of the bellows. Coupled with the large spacing between pipe supports and hangers, the expansion joint is subjected to additional bending moment, and finally the bellows has serious twisting and instability. From the photos at the scene, the bellows looks like a kneaded paper ball, which is terrible to see. The solution is to use insteadRectangular non-metallic expansion jointIt is equipped with wear-resistant guide tube and two guide brackets are added at the same time.

Alas, these lessons were all earned for real money. Plane instability calculation is not armchair, it is directly related to the safety of pipeline system and project cost. When you select or check the expansion joint next time, don't just turn through the sample. Go through the pressure, temperature, displacement, material and support conditions, and then make the safety factor-at least 80% of the pits can be avoided.

1. Why do we have to do a pre-displacement? Can you not set it up?

Two days ago, I met a customer who had a directly buried (fully buried) expansion joint on the steam pipeline. After installing it according to the drawings, the flange collapsed directly after half an hour of startup. The problem is that the pre-displacement is set to zero-which is equivalent to having the compensator directly carry the heat expansion on the cold pipe. To put it bluntly, pre-displacement means that the compensator "eats" part of the displacement in advance when installed, so that it is in the middle position at the working temperature, so as to avoid stretching one side to the limit and compressing the other side to death. The thermal pipeline design code clearly states that after calculating the thermal elongation, the pre-displacement is generally about 50%. However, the real working conditions are complicated, and 50% is only a starting point. If you don't set the pre-displacement, it is equivalent to leaving the bellows at the extreme position in the cold state. As soon as the temperature rises, it will either break or crush. So don't ask "can you not set it up", ask is waiting for rework.

2. How to calculate the pre-displacement? The formula is not complicated, but don't slap your head with the data

Δ X = α × L × Δ T × K. α is the linear expansion coefficient, carbon steel is about 0.012mm/m·℃, L is the length of the pipe section, Δ T is the difference between the working temperature and the installation temperature, and K is the pre-displacement coefficient (usually 0.5~0.7). But don't think you'll be done with a formula. The sensitivity of the rotary compensator and the large tie rod expansion joint to the pre-displacement is completely different. The rotation compensator itself absorbs the displacement by rotation, and if the pre-displacement is too small, the rotation angle will exceed the limit; However, the pressure balance expansion joint of straight pipe has stricter requirements for axial displacement, and if the pre-displacement is larger, the bellows may be pressed out of plastic deformation. Therefore, after calculating the theoretical value, we have to check it again according to the article "Stiffness and Calculation Formula of Bellows" in the product information of this site, combined with the allowable displacement of the specific model. Otherwise, the number you calculate may just be the fatigue limit point of the bellows, and it will be wasted once it is used.

3. Different compensators, the "character" of pre-displacement is far different

Metal corrugated expansion joints and rubber compensators are not the same thing. The elastic modulus of the rubber compensator is low, so it is easy to bulge if the pre-displacement is set large, and it can't protect if the pre-displacement is set small. What about PTFE-lined hoses? The PTFE layer itself is afraid of stretching, and the safety margin on the stretched side must be prioritized for pre-displacement. Let's talk about the corrugated expansion joint-high-temperature and high-pressure steam pipeline used in power station industry. The pre-displacement amount often needs to be more than 60% of the calculated thermal displacement, because the temperature difference impact during startup and shutdown will bring additional dynamic displacement. On the contrary, the metal corrugated expansion joint in cement industry has much dust and slow temperature fluctuation. It is enough to set the pre-displacement to 40%, but it is easy to get stuck by dust accumulation if it is set too much. You see, it is also called a compensator, and the pre-displacement "character" in different scenarios can be twice as different. So don't conquer the world with a single coefficient.

Fourth, at the installation site, how to "adjust" the pre-displacement?

Armchair is over, talk about work. The pre-displacement is typically performed by a tie rod or screw. For example, for the expansion joint of a large tie rod, the screw will be locked in the pre-stretched position when leaving the factory. When installing, compress or stretch the expansion joint to the pre-displacement required by the drawing, and then lock the tie rod nut. There is a pit here: many people read the question and answer "How to adjust the tie rod nut of the expansion joint" on this site, thinking that just twist it a few times. In fact, you have to measure the bellows length variation with a dial gauge or vernier caliper, accurate to the millimeter. For example, the universal corrugated expansion joint requires a pre-compression of 10mm. When you screw the screw with a wrench, you must monitor the flange spacing at both ends at the same time. Don't rely on the feel. In addition, if there is a compensator near such equipment as electric plug-in plate isolation door, the mechanical impact when the isolation door is opened and closed should be considered in the pre-displacement, leaving an additional margin. Otherwise, when the door was closed, the pre-displacement would be directly knocked out.

5. The most terrible rollover cases, guess what?

One power plant uses a curved tube pressure balance expansion joint, and the pre-displacement direction is set reverse-what should be compressed is set to stretch. As a result, the bellows was squeezed into twists and scrapped directly after operation. Second: a chemical plant selected an external pressure single axial expansion joint, and the pre-displacement completely copied the theoretical value. It was not considered that the ambient temperature during installation was 35℃ (the designed installation temperature was 20℃), which was equal to the actual pre-displacement being about 15% less. Two months later, the compensator end weld cracked. Third (this one is the most outrageous): some people also calculate non-metallic expansion joints (fabric fiber expansion joints) according to the pre-displacement coefficient of metal. The creep characteristics of fabric fibers are completely different from those of metals, and excessive pre-displacement leads to early fatigue tearing of the fabric layer. Therefore, don't be lazy. The product manual of each compensator has a recommended pre-displacement range and correction factor, but some people don't look at what is in their pocket. Is that the truth?

6. Summarize an iron law: Pre-displacement setting is a "live", but it has to be done according to the rules

There is no universal formula, but there is a basic logic: first calculate the thermal elongation-confirm the displacement capacity boundary of this type of compensator according to the product data of this site-consider the installation temperature correction-accurately adjust with tools in the field-finally make cold state marks and record. If you are really unsure, go directly to the manufacturer to set the parameters. For example, pressure balance expansion joint, compound hinge transverse expansion joint and other complicated structures, the pre-displacement setting is often linked to pipeline stress analysis, not a matter of screwing a few screws. Remember, if the compensator is selected correctly and the pre-displacement is set wrong, the whole system will still be played out. Therefore, the next time someone asks you "How to set the pre-displacement of the compensator?", you will throw these six words to him: calculate, correct, repair, adjust, remember and ask. One step less, and it may collapse.