Specialized in manufacturing compensators, expansion joints, baffle doors

1. Design is not about patting the head: how to determine the parameters of bellows?

Many engineers come up and ask, "Give me an expansion joint for DN200", as if buying screws-IMHO, this is the wrong way. The expansion joint is designed to push down from the four dimensions of medium, temperature, pressure and displacement of the pipeline system. For example, the temperature of the steam pipe is always three or four hundred degrees, and the pressure is also high. For this kind of working condition, we have to use the steam pipe in our stationHigh temperature axial expansion joint； If you change to a flue gas system, such as the pipeline behind the desulfurization tower, which contains sulfur, chlorine and particles, you have to consider wear resistance and corrosion-the materials and structures of expansion joints used in the cement industry and the power station industry are completely different.

Do you want to add the deflector? Look at the flow rate and particle content. In our question and answer, we specifically talked about the function of the guide tube: high-speed fluid carries particles, and without the guide tube, the bellows wall will wear out in a few months. In turn, the clean medium runs at low speeds, and the guide tube increases local resistance instead. There is also a common rollover scene in engineering: the installation direction of the guide tube is reversed, and the arrow is pointed opposite to the flow direction of the medium, resulting in erosion and perforation. Alas, this job can't be based on feeling.

How to calculate stiffness? There is a question and answer in our product library that specifically talks about the calculation formula of bellows stiffness, which involves wave number, wave height, wall thickness and material elastic modulus. Designers have to substitute these parameters into the verification, not just set a model number. Take a real figure: the same DN200, 16 waves, wave height 40mm and wave height 50mm, the stiffness difference is more than 30%. The stiffness of the expansion joint you selected is too great to absorb the displacement, and the pipeline stress explodes; It's too small and can't hold internal pressure, so it's easy to become unstable. So, don't slap your head.

2. If you choose the right type, you will have fewer accidents than half

There are various expansion joints on the market, and there are more than 30 kinds listed in our station:Universal corrugated expansion joint、External pressure single axial expansion joint、Compound hinge transverse expansion joint、Straight pipe pressure balanced expansion joint、Curved tube pressure balance expansion joint... each corresponds to a different displacement compensation mode and pipe arrangement. And guess what? Choose the wrong type, from leakage to broken pipeline.

Two days ago, I met a customer. The steam pipeline used a general-purpose type. As a result, the axial thrust pushed the fixed bracket crooked-the general-purpose type can only absorb axial displacement, but your pipeline still has lateral displacement! Later replaced withCompound straight pipe bypass pressure balanced expansion jointTo solve the problem. There is also a large tie rod expansion joint, which is specialized in absorbing lateral displacement. If you use it as an axial direction, the tie rod is a display. Under high temperature conditions, should the bellows be insulated? It is necessary, otherwise the bellows will be directly exposed to radiant heat, and the austenitic stainless steel will be sensitized, and intergranular corrosion will come to your door sooner or later.

Another side door-directly buried pipe, you can't dig it up and inspect it, can you? Then you have to useDirect buried (fully buried) type expansion jointWith its own waterproof jacket and guide structure. There is also an air-cooled island vacuum pipe, the displacement is three-dimensional, ordinary expansion joints can't handle it at all, have to go upDouble hinge expansion joint for air-cooled island vacuum pipeline。 You see, it is not an exaggeration to say that if you choose the right type, there will be half the accidents.

3. Material and Manufacturing Process: Don't Save Money on Bellows

Austenitic stainless steel is usually selected as the material of corrugated pipe, such as 304 and 316L, but the environment containing chloride ions (such as desulfurization flue gas) has to be upgraded-with super austenite or nickel-based alloy. We have them in our stationPTFE-lined hoseTo deal with strong corrosive media, such as hydrochloric acid and hydrofluoric acid pipelines, a package of PTFE layer, everything will be fine. However, note that the pressure and temperature resistance of PTFE lining are limited. If the temperature exceeds 200℃ or the pressure fluctuates greatly, the PTFE layer may peel off.

In the manufacturing process, hydroforming and mechanical forming have their own advantages and disadvantages. Hydroformed corrugated pipe has uniform wall thickness and high fatigue life, but also high cost; Mechanical forming is cheap, but it is easy to thin at the trough. The thickness control of corrugated pipe is the key-the fatigue life of the same batch of materials, with a wall thickness difference of 0.1mm, may be twice as bad. The heat treatment process cannot be saved, and the solution treatment can eliminate the processing stress, otherwise the bellows will crack within a few days after being installed.

Although non-metallic expansion joints are cheap, they have limited temperature and pressure resistance. We have them in our stationNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointIt is particularly good for flue gas duct compensation, but don't take it as a substitute for metal-if the temperature exceeds 400℃ or the pressure exceeds 0.1MPa, non-metals basically can't bear it. I'm really unsure. Reference standards: JB/T 12235-2015 for non-metallic ones, and national standards for metallic ones. But then again, the standard is only the bottom line. When the working conditions are severe, materials must be selected beyond the standard.

4. Core principle of selection: pipe layout and compensation ability should be matched

Expansion joint is not a single part, it is a whole system with pipe bracket, guide bracket and fixed bracket. When designing, calculate the amount of displacement absorbed by each expansion joint-axial, transverse and angular displacement, not just the total displacement. For example: a section of L-shaped pipe has a thermal elongation of 20mm between the fixed brackets on both sides, but there is a lateral offset of 5mm at the corner of the pipe. You only plug an axial expansion joint, who will eat the lateral displacement? It can only be carried by the pipe itself, and as a result, the flange bolts are bent.

What about that? Draw a sketch of the pipe system, mark all thermal displacements, and then select the minimum and most reasonable number of expansion joints. For the multi-directional displacement of air-cooled island vacuum pipeline, it is right to use double hinge expansion joint. Small tie rod, large tie rod, duplex, bypass…the more complex the structure, the higher the price, but also the greater the safety redundancy. Don't stress out to save two bucks-we have a customer, oneRotary compensatorThe problem that can be solved is that there are three general types of non-fortress. As a result, the fixed bracket is pushed down twice, and the maintenance cost is three times more expensive than the equipment.

Expansion joints of complex structure such asCurved tube pressure balance typeAndStraight pipe pressure balance typeThey come with their own balanced bellows, which can offset the internal pressure thrust, and the fixed bracket can be made much lighter. However, accordingly, the installation space should be reserved enough, and the spacing of the guide brackets is strictly limited-almost, the pressure balancing effect is greatly reduced. How to choose? There is no universal formula, but there is one principle: the compensation ability should be 20% ~30% greater than the actual displacement, leave a margin, and don't stick with the boundary.

5. Installation and Operation and Maintenance: Real Cases of Detail Rollover

Expansion joint installation, first thing to look at the direction of the arrow. We specifically said in our question and answer that the direction of the arrow of the expansion joint must point to the direction of the medium flow. Install backwards? The guide tube holds the fluid, directly forming vortex erosion, and it will inevitably perforate within three months. There is also the problem of the tie rod nut-the screw used for transportation and fixation, should it be removed after installation? Look at the type. Some expansion joints (such as the general type) have tie rods that are transport fixings and must be removed after installation, otherwise they limit displacement. But those working tie rods, like the large tie rod expansion joints, must be retained, they are responsible for withstanding internal pressure thrust. We specifically talked about whether the expansion joint screw needs to be removed in our Q&A, so don't get confused.

The deflector was installed in the wrong direction. As I said before, I'll emphasize it again here: there is more than one case, all of which are low-level errors. There are also rubber compensators and PTFE compensators, which have requirements for installation torque-use a torque wrench and twist according to the torque given by the manufacturer. Screw too tightly will damage the flange surface, and screw too loosely and leak. From our stationrubber compensatorAndRubber PTFE compensatorAll have recommended installation torque values, just follow them.

Finally, talk about operation and maintenance. Check the bellows regularly every year for cracks, corrosion and thinning. Shine the trough with a flashlight, and replace it immediately if there is a crack. Don't wait for the pipe to burst before you remember maintenance-there is a petrochemical plant, and the expansion joint has not been checked for five years. One day, in the middle of the night, there was a "bang", and the steam leakage burned two people. Tsk, do you think you can save this money? The service life can't just look at the nominal name of the manufacturer, but also the working conditions: the chlorine-containing environment may be scrapped in two or three years, and clean steam can be used for more than ten years. Therefore, regular inspection is more important than any type selection.

External pressure metal corrugated expansion joint, to put it bluntly, is a compensator that makes the bellows bear the medium pressure outside. You may ask: Why should good bellows be left outside? This has to start with its "arch-enemy"-internal pressure.

What is the difference between external pressure and internal pressure structure? Looking at the original intention of design from the angle of force

Internal pressure expansion joint, the bellows is directly soaked in the pipeline medium, and the pressure is pushed from the inside to the outside. At this time, the bellows wall is subjected to circumferential tensile stress, just like blowing air into a balloon, and the peaks and troughs are in tension state. The external pressure type is just the opposite-the medium pressure acts on the outer surface of the bellows, and the inside of the bellows is actually normal pressure or low pressure. And guess what? In this way, the bellows wall is mainly subjected to compressive stress, not tensile stress.

For high temperature and high pressure steam pipelines, internal pressure bellows are easily scrapped in advance because of fatigue cracks (tensile stress is an accelerating factor). The external pressure type moves the bellows between the outer sleeve and the end pipe, and the medium goes from the inside. The bellows "blows cold air" outside (actually, it is also insulated), and the stress state becomes pressurized. The compression resistance of stainless steel corrugated pipe is much stronger than the tensile resistance, so the external pressure type can withstand higher pressure and temperature under the same working conditions. This is not something to think about, but a pile of decades of engineering data.

The core components of external pressure single axial expansion joint: bellows, end tube, guide tube and outer sleeve

Let's take one apartExternal pressure single axial expansion joint(This thing is in the product information of this site), and see what it looks like inside.

• Bellows: Core elastic element, typically multilayered thin wall pressed with austenitic stainless steel (304 or 316L). Because it is an external pressure structure, the number of layers can be made less than that of the internal pressure type, so that the stiffness is reduced and the compensation amount is greater.
• End tube: The two sections of pipe connecting the pipe, divided into the left end pipe and the right end pipe. One end is fixed, and the other end moves with the thermal displacement of the pipe.
• guide tube: It is installed inside the medium flow channel to protect the inner wall of the bellows from being washed by high-speed fluid. Here is a key: the direction of the arrow of the guide tube must point to the flow direction of the medium, and if it is installed backwards, it is equivalent to not installed. Regarding this, the question and answer of this site "The specific function of the expansion joint guide tube" has a detailed explanation.
• Outer sleeve: External pressure type unique "armor". A circle of thick tubes is sleeved outside the bellows, and the medium pressure acts on the outer surface of the bellows through the outer sleeve. The outer sleeve also plays a supportive and protective role to prevent the bellows from being damaged by external forces.

To put it bluntly, the external pressure single axial type means that the medium takes the "inner channel" and the bellows takes the "outer channel", which does not directly contact each other, but the pressure is transmitted to the bellows through the outer sleeve. The structure is not complicated, but if you want to design it stable and air-free, there are many doorways.

What scenarios is this thing mainly used in? Power station, cement, steam pipeline... Don't choose the wrong model

You thought it was a universal part? Wrong. External pressure metal corrugated expansion joint has its "home field".

The most classic ones are the main steam pipeline and reheating steam pipeline of the power station. The pressure of these pipes is above 10 MPa and the temperature is up to 600 °C. Under this working condition, the life of the internal pressure type may only be two or three years, and it can easily run for ten years if it is replaced with the external pressure type. The reason just mentioned-compressive stress conditions are more resistant to fatigue.

What about the cement industry? The high-temperature fan outlet and the flue gas pipeline at the head and tail of the kiln have large temperature fluctuations and dust. The external pressure type expansion joint can be made into wear-resistant lining because the guide tubeMetal Corrugated Expansion Joints in Cement IndustryDesign experience, can withstand abrasion. However, note: if the medium contains sulfur oxides (such as wet flue gas after desulfurization), you have to consider corrosion-resistant materials or PTFE lining. At this time, external pressure type is not necessarily cost-effective.

Air cooled island vacuum pipe. There is a "double hinge expansion joint of air-cooled island vacuum pipeline" in this product, which is a special structure. If it is only a simple axial displacement, the external pressure single axial type is also a candidate-because the external pressure bellows bears external compressive stress in vacuum state, and its stability is better than that of the internal pressure type.

Find out whether the medium pressure in your pipe is pushing from the inside to the outside or from the outside to the inside. The pressure of the steam pipe comes from the inside, and the external pressure type is used; The pressure of the water pipe is also internal, so it is not wrong to use the external pressure type, but the cost is high. It is more economical to use the general corrugated expansion joint in low pressure and normal temperature applications.

Three hard indexes in model selection: pressure grade, compensation amount and stability check

Choose external pressure expansion joint, not just look at the caliber to place an order. Three hard indicators, one less is waiting for rework:

First, the pressure rating.The nominal pressure of the external pressure type depends not on how much internal pressure the bellows can withstand, but on how much pressure the sealing structure between the outer sleeve and the end tube can carry. Commonly used grades PN6, PN10, PN16, PN25. If the design pressure of the pipeline is 2.5MPa, choose PN25, but don't forget to check the critical pressure of instability of the bellows under external pressure. This calculation is mentioned in the "Stiffness and Calculation Formula of Bellows" in our station. Simply put, the wall thickness, wave height and wave pitch of the bellows determine whether it can maintain its shape under external pressure. If the pressure rating is chosen low, the bellows will crush like a flattened can.

Second, the amount of compensation.External pressure single type axial type mainly absorbs axial displacement. The amount of compensation depends on the wavenumber of the bellows. The compensation capacity of each wave is generally 10~25mm (related to the diameter and wave height). For example, the thermal elongation of the pipe is 80mm, which requires at least 4 waves. However, with more wavenumbers, the overall length increases, and the risk of instability also increases. Therefore, don't blindly add waves, and use duplex structures or tie rods to limit lateral displacement when necessary.

Third, stability check.This is a pit unique to the external pressure type. The outer surface of the bellows is surrounded by the outer sleeve, and if the gap between the inner diameter of the outer sleeve and the outer diameter of the bellows is too large, the bellows will develop "circumferential instability"-the crest collapses inward. Solution: Control the clearance to not exceed 5 times the wall thickness of the bellows, or add a guide ring. Many manufacturers are lazy in this link, and as a result, they are leaked after six months of operation.

Frequently stepped pits in installation and operation and maintenance: tie rod adjustment, guide tube direction, pre-deformation treatment

No matter how good things are, it will be useless if you install them wrong. Let's talk about a few real rollover cases:

Tie rod adjustment.External pressure single type axial type usually with tie rod (this site FAQ "Expansion joint tie rod function" has an explanation). The role of the tie rod is to fix the bellows length during transportation and installation, preventing it from expanding and contracting freely. However, many construction teams directly weld or screw the tie rod to death, forgetting to loosen the nut before the pipeline is put into operation. As a result, as soon as the pipeline heated up, the expansion joint did not move at all, and the bellows was forcibly stretched to plastic deformation, which leaked the next day. Correct practice: After installation and before pressure test, adjust the tie rod nut to the reserved compensation amount position (such as pre-tension amount), and then lock the back cap. For the specific adjustment method, see "How to adjust the expansion joint tie rod nut".

Direction of the guide tube.The arrow points to the direction of the media flow, a cliche that somebody commits every year. When the guide tube is installed backwards, the medium directly washes the root of the bellows, and the wear speed is doubled. Especially the dusty airflow, which can be worn out in two months.

Pre-deformation treatment.When steam pipeline is installed in cold state, the expansion joint needs to be pre-stretched (compressed) to offset the displacement difference from cold state to hot state. The external pressure expansion joint itself is longer than the internal pressure type, and it is easy to press the bellows into a "bamboo shape" during pre-stretching. Experience: Calculate the thermal elongation first, then pre-stretch (or pre-compress) according to the compensation amount of 60% ~70%, and then lock the tie rod. Release slowly after commissioning.

Alas, two days ago, I met a customer. The steam pipeline used an external pressure expansion joint. As a result, the guide tube was installed in the reverse direction, and the tie rod was not loose. It exploded in a month. Lost hundreds of thousands. You say it was wrong or not? Therefore, structural selection is technology, installation, operation and maintenance is conscience, and both are indispensable.

How is it different from a regular expansion joint? — — Find out what the word "Wanxiang" means first

To be honest, people who came into contact with this thing for the first time would most likely be misguided by the word "Wanxiang". You think it can turn 360 degrees like a joint? Don't be naive. The core of a real universal metal expansion joint is two sets of hinge structures, one set swinging in the plane and the other set swinging in the vertical plane. When combined, it can absorb the displacement of the pipe in any direction-but note that it is "angular displacement", not "rotation".

Common expansion joints, such as those common in our stationUniversal corrugated expansion jointOrAxial expansion joint, they can only absorb displacement in one direction. You don't expect the axial type to carry the lateral force, and the tube will look at you sooner or later if you hard-fit it. The advantage of the universal type is that once the two hinges cross, it can be folded when the pipe expands and contracts thermally, no matter which side it moves up, down, left and right. To put it bluntly, it is a "flexible joint" to prepare the direction of complex pipelines.

Then it andCompound hinge transverse expansion jointWhat's the difference? The latter is also two hinges, but is relatively simple in construction and is typically used to absorb lateral displacement. The universal type has one more degree of freedom and can absorb angular displacement from two different planes at the same time. Which to choose depends on the actual deviation direction of the pipeline-don't expect one model to dominate the world.

Its core work: multi-directional displacement one shoulder, pipeline stress does not run blindly

What are plumbing systems most afraid of? Stress from thermal expansion. Especially those pipes that turn around. Once the temperature heats up, no one knows where the pipes are squeezed. What is the role of universal metal expansion joint? Simple and crude-swallow all the displacements in multiple directions, let the thrust concentrate on the fixed bracket, and don't let the stress jump at the welds and flanges.

Take the main steam pipe in the power station for example. The temperature can soar to more than 500 degrees, and the pipe can expand by several millimeters per meter. The total displacement of a pipeline of tens of meters is scary to death. If only the axial type is used, the lateral displacement cannot be managed; If two hinges are combined, it takes up too much space. One solution for universal type, two hinges are arranged crosswise, and then cooperatetie rod(Remember the "role of expansion joint tie rod" in the FAQ?) To withstand pressure thrust, bellows only absorb displacement and perform their respective duties.

Deflector tube. This thing is inSpecific Function of Expansion Joint Guide TubeAs mentioned in, the main thing is to prevent high-speed airflow from washing the bellows. The design of the guide tube inside the universal type is more particular, because there are many displacement directions, and the guide tube must swing with the hinge, leaving a large gap to leak air and a small one to get stuck. Many small factories in this piece can't do well, which directly leads to a discount in life.

Which occasions must use the universal type? The real demand of power stations, cement and desulfurization

Before talking about the application scenario, let's first look at a set of data: in the smoke duct system of domestic thermal power plants, the amount of universal expansion joint accounts for more than 30% of the total number of pipeline compensators. Why? Because the flue tends to be winding, and the space is often limited, it is impossible to install large-size double hinges. You go and seeCorrugated expansion joint for power station industryMany of that kind of products are universal.

The same goes for kiln tail pipes in the cement industry. High temperature, dusty, multi-directional displacement, ordinary axial type can't carry two months before leaking. Use the universal type, matchingMetal Corrugated Expansion Joints in Cement IndustryHigh temperature resistant design, there are many problems in five years. Two days ago, a customer who was doing the denitrification transformation of cement kiln tail told me that they used to use a non-metallic compensator, which was replaced every six months. After replacing it with a universal metal bellows, the number of downtime was directly cut in half.

Look at the desulfurization system again. Near the flue gas baffle door at the outlet of the desulfurization tower, the pipeline has to bear both thermal expansion, fan vibration and corrosion prevention. useDesulfurization flue gas baffle doorWith universal expansion joints, it is a common configuration. Note that the desulfurization environment medium contains acidity, and the material must be 316L or higher. At this timePTFE-lined hoseOrPTFE compensatorIt can also go up, but the advantages of universal metal lie in higher temperature resistance and stronger pressure-you can't always use PTFE in 200℃ flue gas, can you?

Don't step on pits when selecting: universal type is not universal type, installation details determine life

The name "Wanxiang" is too easy to let people down their guard. The most outrageous case I have ever seen is that the universal type is used as a long and short telescoper, stuffed directly between two fixed brackets, and as a result, the bellows is pressed into a twist. Remember that the universal type can only absorb angular displacement and a small amount of lateral displacement,Unable to absorb axial displacement。 If the pipe has a large axial expansion and contraction, you need to matchAxial expansion jointOrStraight pipe pressure balanced expansion joint。

Transport screw. In order to fix the bellows when leaving the factory, several screws will be welded-this point is clearly stated in the FAQ "Do the screws of expansion joints need to be disassembled". Many on-site workers try to save trouble and don't dismantle it, or half of it is dismantled. As soon as the system heats up the screw, the bellows gets stuck and it fails directly. Solution: After installation, all must be removed and marked.

Adjustment of tie rod nuts. Gimbal typically hasLarge tie rodWhich is used to withstand the thrust generated by the internal pressure while limiting the excessive elongation of the bellows. The tie rod nut cannot be screwed to death and must be adjusted according to the design pre-compression amount. It is written in the answer of "How to Adjust the Tie Rod Nut of the Expansion Joint" that the nut is usually screwed to the marked position when it is cold, leaving a thermal expansion margin. Screwed to death is equivalent to locking the bellows, and the consequence is that all the stress of the pipeline runs to the equipment.

Talk about something real: fault cases and common misunderstandings, spend less money after reading it

Let's start with a real malfunction. Two universal expansion joints were used in the outlet pipeline of the grate cooler of a cement plant, which burst one after another within half a year. On disassembly, longitudinal cracks appear in the bellows sidewalls — typical lateral displacements exceeding design values. The designer patted his head to measure the thermal expansion of the pipeline, but ignored the forced displacement caused by fan vibration. How to fix it? Replace withCompound straight pipe bypass pressure balanced expansion jointThe vibration displacement is directed to structures with stronger absorption capacity. Hey, I spent an extra two hundred thousand to buy a lesson.

It is believed that the universal type can be installed in any direction. It is true that it is more flexible than a regular expansion joint, but it has its own "neutral face" — the center of the hinge's swing. If the pipe centerline deviates from this neutral plane during installation, the displacement will increase exponentially and the bellows life will plummet. Therefore, pipeline stress analysis must be done before installation, and don't rely on experience alone.

Also, don't combine the universal type withRotary compensatorMix up. The rotary compensator absorbs the displacement by rotation, which is suitable for the thermal expansion of long straight pipes; Universal type mainly solves the scenario of space constraint and multi-directional displacement. You use it on asphalt pipes? Then you have to chooseSleeve type pipe expansion joint。 If you choose the wrong one, you will spend money for nothing.

If your pipe is only displaced in two directions and there is enough space, use two setsHinge transverse expansion jointOften cheaper than a gimbal and easier to maintain. Don't be superstitious about the word "universal", it is only a tool, not a omnipotent. Understanding the displacement boundary conditions of your own pipeline and then selecting the type is better than anything else.

How much does the metal expansion joint DN250 expand and contract exactly? Stop being fooled by parameter tables

Expansion and contraction amount of metal expansion joint DN250, no fixed value。 You are holding a model and wondering how long it will stretch? That's the same as asking "how fast can this car go"-depends on whether it's a 1.5L self-priming or a 4.0T twin-turbo. The DN250 is only a nominal diameter, and the amount of expansion and contraction depends on wave number, wave height, wall thickness, material, pressure class, and temperature. Take the usual ones on our stationUniversal corrugated expansion jointFor example, the axial compensation amount is usually between a few tens of millimeters and two hundred millimeters, but when changed toHigh temperature axial expansion jointOrExternal pressure single axial expansion jointAnd the data is completely different.

"Why can your DN250 only compensate 80mm, but that one can reach 150mm?" I said you should first see whether the working pressure of his target is 0.1MPa or 1.6MPa, and whether the temperature is normal temperature or 500℃. The same pipe, the working condition is three times different, can the expansion and contraction amount be the same? This is not a fraud of the parameter table, but the selection logic is not right at all.

To understand the scaling amount, first understand these key parameters

The first one iswavenumber-The number of twists and turns on the bellows. The more waves, the greater the expansion and contraction, but the pressure bearing capacity will decrease. The second isWave height and wall thicknessThe higher the wave and thinner the wall, the better the flexibility, but the fatigue life will be shortened. And the third isMaterials-Stainless steel 304, 316L or high temperature resistant alloy? When the temperature exceeds 400℃, the allowable stress of ordinary 304 should be folded in half, and the temperature correction coefficient must be included in the calculation of expansion and contraction.

The same DN250 is used on 0.25MPa flue gas pipeline and 1.0MPa steam pipeline, and the design safety factor is completely different. Think about it, if the low-pressure expansion joint is forced into the high-pressure pipeline, and the bellows will crack as soon as it deforms, it will be useless no matter how much expansion and contraction is. We have a customer in the cement industryMetal Corrugated Expansion Joints in Cement Industry, the medium temperature is 600℃, ask if DN250 can achieve 120mm compensation amount-of course, but you have to use double-layer bellows and guide tube, and you have to pressPressure balance typeDesign. Simply heap wave number? If it can't bear the high temperature and internal pressure, it will be scrapped in three months.

How to calculate the expansion and contraction amount of DN250 expansion joints with different structural types?

This is the easiest place to overlook.Single axial type(For example, the general corrugated expansion joint) can only absorb axial displacement, and the expansion and contraction amount mainly depends on the wave number stacking. But if you need to absorb lateral displacement or angular displacement at the same time, you have to useCompound hinge transverse typeOrDouble straight pipe bypass pressure balance type。 For example, for the same DN250, the axial compensation amount of the single axial type may be 150mm, but if it is replaced with the double hinge type, its axial compensation may only be 30mm, but it can absorb the angular displacement of ±5°-these are completely two kinds of compensation logics.

AndStraight pipe pressure balanced expansion jointIt relies on two sets of bellows to offset the internal pressure thrust, and no matter how large the expansion and contraction is designed, the pipe will not fly. When you choose a model, you have to ask clearly what direction of displacement the pipeline should compensate, otherwise you can't install it when you buy it back, and the rework cost is much more expensive than the expansion joint itself.

Actual selection: How to determine the expansion and contraction according to the thermal displacement of the pipeline?

Don't check Baidu, calculate the thermal elongation first. The formula is simple: Δ L = α × L × Δ T. Carbon steel pipeline α ≈0.012mm/m·℃. Assuming that DN250 pipeline is 30m long and the temperature difference is 200℃, the thermal elongation is 0.012×30×200=72mm. Then the axial compensation amount of the expansion joint you choose should be at least greater than 72mm, leaving 20% safety margin, that is, about 90mm.

But it is not enough to look at the axial direction-if the line has elbows that cause lateral displacement, then considerCompound hinge transverse expansion jointOrUniversal hinge type。 We have products on our site that deal with this combined displacement. In addition, don't forget the internal pressure thrust: DN250 has a blind plate force of about 49kN at 1.0MPa. If you don't use the pressure balance type, the fixed bracket has to hold this force, otherwise the pipe will fly out. Last year, there was a case in a power plant. They chose the ordinary axial type to install it in the position of a large-diameter thick-walled expansion joint, but the bracket collapsed during pressure test-a painful lesson.

Don't make mistakes about the pits that are easy to step on during installation, the guide tube and the tie rod

Let's startguide tubeIt is on the inner wall of the bellows, and its function is to guide the high-speed fluid away to avoid the bellows from being washed and worn. weFAQThe role of the guide tube is talked about in detail. Don't underestimate the thin plate-without it, the expansion joint of DN250 is used in the dusty flue gas pipe, and it will be worn out in half a year. Againtie rod nut adjustmentMany construction teams do not disassemble the transportation fixings after installation, or screw the tie rod nut to death, resulting in the expansion joint losing its compensation function. On expansion jointArrow DirectionIt is the flow direction of the medium, and the guide tube will not play a role if it is installed backwards. We still have clients who putDouble-sealed single-axis circular baffle doorIt's used as an expansion joint. Laughing to death. That thing is used for truncation, not compensation. Therefore, be sure to check the product nameplate before installation to confirm whether it is a general-purpose type or a high-temperature axial type, and do not mix it.

In addition,Do you need to remove the screw of the expansion joint？ The transportation fixing parts must be disassembled, but the adjusting nut on the pull rod should not be disordered-how to adjust it specifically,FAQThere are also instructions in. In short, installation is not by brute force, but by looking at nameplates and drawings.

Summary: Don't just look at parameters in type selection, working conditions are the fundamental

The diameter of DN250 can be expanded from 30mm to 300mm. The key is that you should give the working pressure, temperature, medium, pipe direction and fixed bracket position. Instead of guessing blindly on the parameter table, it is better to directly find the manufacturer to produce a calculation book. All expansion joint products on our station-includingUniversal corrugated expansion joint、High temperature axial expansion joint、Straight pipe pressure balanced expansion joint、Large diameter thick wall expansion joint、Compound hinge transverse expansion jointAnd so on-all support customization according to GB/T 12777 standard. You provide the working conditions, and we will calculate them for you. The data speaks for itself, and it is not false.

How much expansion and contraction of the metal expansion joint DN250 does not depend on the model, but what exactly your pipe wants it to carry. Wrong choice, no matter how beautiful the parameters are, they are still scrap iron.

What are single and triple waves? Find out the structure before choosing

The core of the metal expansion joint is that bellows, and the wave number is the number of corrugations. Single-wave expansion joints have only one ripple, and three waves are three ripples-it sounds like crap, but many people run to ask for the price without even figuring it out.

Structurally, the single-wave expansion joint is more like a "spring sheet", and the wall thickness, wave height and wave distance are all fixed. The three-wave expansion joint is formed by three single waves welded in series or integrally, and there is a transition section between each wave. To put it bluntly, a single wave is "one section" and a three wave is "three sections strung together".

But don't think that three waves are as simple as three single waves stacked. The wave number has changed, and the stiffness, compensation amount and pressure resistance of the whole expansion joint are all another set of logic. And common products on the market, such as ourUniversal corrugated expansion jointSingle-wave and three-wave are also different in end structure and guide tube design.

Performance Differences: How Different Are Compensation Amount, Stiffness and Pressure Resistance

The axial displacement absorbed by single wave energy is usually only a few millimeters to more than ten millimeters, and the three wave energy is up to tens of millimeters. But what about the cost? The stiffness drops with the wave number-single wave stiffness is high, three wave stiffness is low. Some people think that low stiffness is so soft and wrong.

Low stiffness means that under the same pressure, the bellows is easier to deform, and it is easy to fatigue if the deformation is large. What about pressure resistance? Because the wall thickness of single wave is relatively thicker (under the same caliber), the withstand voltage is usually higher than that of three wave. For example, with the same design of DN200 and 0.6MPa, the blasting pressure of a single wave can reach more than 3MPa, while the blasting pressure of three waves may only be 2.5MPa. It's not an absolute value, but the trend is there.

When the three-wave expansion joint is subjected to lateral displacement, each wave will "share" the deformation, but at the same time, it is also prone to inter-wave interference. We doExternal pressure single axial expansion jointIt has been encountered that if the wave distance design of the three waves is unreasonable, the outer wave will be deflated. That's a real thing.

Application scenario disassembly: Which working condition must use single wave? Which scenario is more stable with three waves?

Is the medium in your pipes under high pressure? Does the temperature fluctuate a lot? If the answer is "high pressure, small displacement", then a single wave is the lifeblood. For example, the compact space on the steam pipeline has a displacement of 5mm and a pressure of 1.6MPa. If you use three waves, it will be easy to tremble. We've done it for the power station industryCorrugated expansion joint for power station industry, the main steam pipeline is all single wave plus external pressure structure, why? Because the safety margin is sufficient.

On the other hand, if it is low pressure and large displacement-such as flue gas pipes and air-cooled islands, three waves are the right way. For example,Double hinge expansion joint for air-cooled island vacuum pipeline, the tube is tens of meters long, the thermal expansion and contraction are tens of millimeters, and the single wave can't hold it at all. The three waves cooperate with the hinge structure, which is stable. There are also hot air ducts in the cement industry, which have high temperature but low pressure, and three-wave belt guide tubes, which can double their life.

The pressure is not high but frequent vibration compensation is required. For example, the pump outlet can absorb multi-directional displacement with three waves, but be careful not to choose the wrong model — —Compound hinge transverse expansion jointThis is what it does specifically, and it has nothing to do with wave number.

The most common mistake made in model selection: blindly pursuing wave number or looking only at price

Two days ago, I met a customer and said, "I want three waves because the compensation is large"-as a result, his pipeline pressure was 2.5MPa and the temperature was 400℃, and the three waves cracked after three months. Later, the columnar instability caused by internal pressure was checked. You say it was wrong or not?

Just look at the price. Single wave is cheap and three wave is expensive, so choose single wave. As a result, the displacement was not enough, and the tube deformed the expansion joint by hard pulling, and the guide tube flew. Tsk, we see too many of these things.

Selecting the wave number is not a pat on the head. You have to calculate the compensation requirement first, then inverse the stiffness, and then check the pressure and fatigue life. We have an internal formula: single wave for small displacement and high pressure, three waves for large displacement and low pressure, and double waves or multi-layer waves for the middle zone. Oh, yeah,Large diameter thick wall expansion jointSometimes single wave is also used, because the wall thickness can be very thick and the pressure resistance is easy.

Professional advice: Introduce the wavenumber from the pipeline parameters, and place an order after calculating it clearly

How on earth is that calculated? The first step is to get the pipeline design parameters: medium, pressure, temperature, pipe diameter, pipe length and fixed bracket position. The second step is to calculate the amount of thermal displacement. Step 3, determine if you need axial, lateral, or angular compensation.

A section of DN300 steam pipeline with a length of 20 meters, a temperature difference of 200℃ and an axial displacement of about 30mm. Pressure 0.8 MPa. If a general-purpose corrugated expansion joint is used, the rated compensation amount of single wave is up to 10mm, and three single waves need to be connected in series, but the stiffness increases after series connection, and multiple sets of tie rods are required. It's better to go straight to three waves and get it done in one. However, it should be noted that the three waves at 0.8MPa should be checked whether plane instability will occur. Usually, we will add a guide sleeve or selectStraight pipe pressure balanced expansion joint。

Don't believe that "three waves are definitely better than single waves" or "single waves are safer". Each wavenumber has its reason for existence. We doHigh temperature axial expansion jointWhen, the wave number will be adjusted for different temperature segments: three waves below 600℃, and a single wave is used to thicken the wall above 600℃, because the material creeps at high temperature, and multiple waves are easy to be uneven.

If you are still unsure, send the pipeline parameters to our technical department, and they will calculate it with finite element, which is ten times better than slapping your own head. After all, which is better, single-wave or three-wave metal expansion joints, is never a multiple-choice question, but an arithmetic question.