Specialized in manufacturing compensators, expansion joints, baffle doors

1. There is no fixed answer to the life of metal bellows expansion joint: design, working conditions and materials have the final say

Two days ago, a customer called and asked, "How many years will your bellows expand and save energy?" I asked him, "What working conditions do you use it in? What is the temperature? Is the medium corrosive?" He was stunned. To be honest, there is no standard answer to this question. Metal bellows expansion joint life? It is not a true or false question at all, but a comprehensive question determined by design, working conditions and materials.

The same general-purpose corrugated expansion joint, placed on a clean steam pipe, may run steadily for ten years; It is not unusual to throw it into the dusty environment of the cement industry and scrap it in three years. So don't believe those sales talk that pat your chest and say "guarantee for 10 years"-those are theoretical values measured under certain conditions.

Five core factors affecting life: temperature, pressure, medium, displacement frequency and installation quality

If the expansion joint of metal bellows fails, 90% of them can't escape these five pits.

• Temperature: If the design temperature exceeds 10℃, the fatigue life may be directly discounted by 50%. For example, corrugated expansion joints are used in power station industry. If high-temperature axial expansion joints run above 600℃ for a long time, the material must be heat-resistant alloy, and ordinary stainless steel can't last a maintenance cycle.
• pressure: The higher the internal pressure, the greater the circumferential stress on the bellows wall. Frequent pressure fluctuations can easily lead to low-cycle fatigue.
• Medium: Chloride ions and sulfides in the chemical industry are invisible killers. PTFE-lined hoses or PTFE compensators are specially designed to deal with corrosive media, but if you touch hydrochloric acid with ordinary stainless steel, the life is calculated by day.
• Displacement frequency: A pipeline that starts and stops several times a day has a much shorter life than a pipeline that runs continuously. Frequent expansion and contraction is equal to repeatedly bending the wire, and it will break sooner or later.
• Installation quality: Most overlooked. During installation, the tie rod bolts are not removed, or the pre-offset is not done correctly, and the expansion joint directly enters the limit state before it runs. I saw a scene two days ago, and the screw of the expansion joint of the big tie rod was not disassembled, but it was directly welded to death-that would compensate for a fart.

3. Life difference in different application scenarios: from high temperature of power station to cement dust, to chemical corrosion

Come on, let's talk about a few practical scenarios.

Power station industry: Boiler outlet, turbine bypass, high temperature, high pressure, and frequent start-and-stop. In this scenario, the power station industry uses corrugated expansion joints and air-cooled island vacuum pipeline double-hinged expansion joints, and the design life is usually about 10 years. However, in fact, due to overtemperature and overpressure, it is the norm to change a batch in 5-8 years.

Cement industry: There is much dust, strong grinding property, and high temperature. If the metal corrugated expansion joint in cement industry is not added with wear-resistant guide tube, the corrugated pipe will be impacted by particle flow and perforated quickly. The specific function of the guide tube is to protect the bellows body, but the guide tube itself is also a wear part.

Chemical industry: Corrosive media are enemy number one. PTFE-lined metal hoses and rubber PTFE compensators are standard here. But even with a PTFE compensator, the liner needs to be checked regularly to see if it has fallen off-once the medium penetrates the metal matrix, the rate of corrosion will make you doubt your life.

Flue gas desulfurization system: The non-metallic expansion joint (fabric fiber expansion joint) behind the desulfurization flue gas baffle door has a long life, but the acid dew point corrosion of the metal corrugated expansion joint is particularly serious in the low temperature and high humidity environment of wet desulfurization. We advise customers to give preference to rubber compensators or non-metallic compensators in this case.

4. Common failure modes: fatigue fracture, corrosion perforation, instability deformation, which one have you encountered?

For so many years, what I have seen the most isFatigue fracture。 Under the cyclic displacement of the bellows, micro-cracks gradually appear in the stress concentration areas of the peaks and valleys, and finally there is a click. And guess what? Many owners have to wait until they leak before changing, so why did they go early?

Secondly,Corrosion perforation, especially point corrosion. Stainless steel is most afraid of chloride ions. Even if the concentration is very low, it will penetrate quickly at the stress concentration point. Therefore, on chemical pipelines, we recommend using vacuum special hoses or directly buried (fully buried) expansion joints plus external protection.

Unstable deformationWhat? That's caused by too much pressure or skewed installation. The bellows is like being twisted and twisted, with uneven wave pitch and bulging directly in severe cases. Structures with restraints, such as double hinge transverse expansion joints and straight tube pressure balance expansion joints, are designed to prevent instability. However, some masters regard the tie rod as a reinforcement when installing it, which should be dismantled or not, and the result becomes an accomplice.

5. Practical operation method of prolonging life: one-stop selection, installation, inspection and maintenance

To say a thousand words, if you want to prolong the life of metal bellows expansion joint, you have to start from the root.

Don't make do with the selection: The working condition parameters must be given accurately. For high temperature, choose high-temperature axial expansion joint, for corrosion, choose PTFE-lined or rubber compensator, and for large diameter and high pressure, choose large-diameter thick-walled expansion joint. Don't buy a general-purpose model cheaply to handle harsh conditions-that's equivalent to running a rally with a Xiali.

Installation should be standardized: How to adjust the expansion joint tie rod nut? It's all clearly written in the instructions, but field workers often get it wrong. Remember: the transport tie rod should be removed after installation, and the adjustment tie rod should adjust the pre-bias according to the compensation amount. Does the screw of the expansion joint need to be removed? Depending on the situation, but by no means means welded to death.

Patrol inspection cannot be omitted: Look at the bellows surface every month for cracks, corrosion spots and deformation. Focus on checking whether the guide tube is loose and whether the tie rod is bent. The national standard JB/T 12235-2015 for non-metallic expansion joints also stipulates the periodic inspection cycle, so just follow it.

Maintaining records is done: What day it was changed, what model, and the change of working condition parameters-take a note, and there will be a basis for the next change. Products such as rotary compensators and sleeve pipe expansion joints are relatively simple to maintain, but don't ignore them.

The life of a metal bellows expansion joint is like asking how many years a car can run- -how you drive it, where you drive it, and how you maintain it. Choose the right product, install the right position, and check it out regularly. There will be no problem for ten or eight years; On the contrary, it is not unusual to lie down in a nest in three or two months. What scenario are you using for the equipment at hand? Comparing the above few, you probably have an idea in your mind.

Where are metal expansion joints fixed? — — Detailed explanation of installation position and fixation method

Two days ago, I received a call from a power station customer, saying that the corrugated expansion joint they installed in the power station industry leaked after only half a year of operation. I asked him: How do you arrange the fixed brackets? The other party was stunned. Fixed? Isn't it just welded to the pipe?Tsk, this is the typical use of expansion joints as rigid parts.Today, let's break this matter apart and explain it clearly-where and how to fix the metal expansion joint.

1. The expansion joint is not something you can fix if you want to: first understand why it needs to be "positioned"

Absorbs thermal displacement, absorbs vibration, absorbs installation errors.But it does not carry the self-weight of the pipeline, nor does it carry the pressure thrust.Therefore, the so-called "fixing" is not to lock it to the pipe with bolts, but to draw a range of motion for it with brackets, tie rods or restraints-let the bellows only move where it should move, and stay honestly where it shouldn't move. To put it bluntly, fixing is to draw a working area for the expansion joint, not to weld it to death. What's the difference between welding it to death and using a steel pipe? The bellows are either pulled and cracked or compressed to permanent deformation.

Many manufacturers' installation manuals state that "guide brackets must be set at both ends of the expansion joint", but the on-site construction team saves trouble and directly lets the expansion joint support itself on the pipeline. And the result?Corrugated pipes are fatigued and cracked in a few months, and the repair cost is dozens of times more expensive than the bracket money.This account has to be clearly settled.

2. Fixed bracket and guide bracket: The two brothers have different division of labor

Fixation bracket (main fixation point)It is the anchoring point of the pipeline, which should carry the thrust generated by the pressure of the pipeline, the thermal expansion thrust, and the self-weight of the pipeline. Whileguide bracketJust one thing: limit the lateral displacement of the pipe and allow it to slide freely along the axis. Expansion energy saving is mounted between two fixed brackets, or one end can be fixed and the other end free. Take the general-purpose corrugated expansion joint and high-temperature axial expansion joint in our station for example. When installing, guide brackets must be set at both ends, otherwise the pipe will twist like a snake, and the corrugated pipe will soon yaw and fail.

Let's talk about the straight pipe pressure balance expansion joint-it comes with its own tie rod or hinge, and the fixed point design must strictly follow the drawings, so it can't be taken for granted.

3. The "fixation" posture of different expansion joints is very different

Different types of expansion joints, the fixing methods are very different, don't take the same set of schemes everywhere.

• Axial expansion joint(For example, external pressure single axial type and general type): The fixed bracket must be welded to the main fixed points at both ends of the bellows, and guide brackets must be added at every other section in the middle. The spacing is calculated according to the stiffness and thermal displacement of the pipe.
• hinged expansion joint(For example, double hinge of double hinge and double hinge of air-cooled island vacuum pipeline): It itself restricts the angular displacement through the hinge structure, and the fixed bracket only needs to resist the thrust, and the distance between the guide brackets can be appropriately relaxed.
• Pressure balanced expansion joint(Straight pipe pressure balance type, curved pipe pressure balance type): There is a balanced bellows or tie rod inside, and the thrust of the fixed bracket is very small, and even a small bracket can be used, but the guide bracket must be in place, so that the pipe cannot deviate laterally.
• Sleeve expansion joint: The fixed point is set at the outer tube end of the sleeve, and the inner tube slides freely. At this time, special attention should be paid to whether the sealing packing is pressed and biased.
• Treatment of tie rods/screws: Many customers ask "Do you want to dismantle the screw of the expansion joint". Let me say the conclusion directly: the transport screw is for protection, and it should be adjusted according to the pre-tension or pre-compression requirements after installation. For the specific torque value, refer to the product manual, and it is not just twisted for a few turns.

4. The most common pit stepped on the site: Take the expansion joint as a pipeline support

They used the corrugated expansion joint in the power station industry as a pipe support, and the corrugated pipe directly pressed against the weight of the pipe, and it leaked in half a year. The right thing to do is simply —The expansion joint itself does not bear any vertical loadAll pipe weights must be carried by brackets or hangers. For large-diameter thick-walled expansion joints, they must be fixed with temporary supports during installation, and then remove the temporary parts after the pipeline is welded and the bracket is adjusted in place. Otherwise, the bellows will be compressed and deformed before it even works.

When the expansion joint is installed near the elbow, the fixed bracket cannot be directly welded to the elbow, but should be set on the straight pipe section behind the elbow, otherwise the stress at the elbow will be concentrated and the bellows will soon be torn.

5. Where exactly is it fixed? A table to state the recommended location

No more forms, just talk about dry goods:

• Straight pipe section: The fixed brackets are arranged at both ends of the expansion joint, and the distance is generally no more than 6 meters (specifically adjusted according to the pipe diameter and medium temperature).
• Near elbow: The fixed bracket is arranged on the straight pipe section behind the elbow to avoid stress concentration at the elbow.
• Close to equipment entrance and exit: For example, pumps, valves, cooling towers, the equipment itself can be used as a fixed point, but the thrust allowed by the nozzle of the equipment must be checked. Some equipment nozzles are fragile and deform with a slight top.
• buried pipelineFor example, for directly buried fully buried expansion joints, reinforced concrete fixed piers are usually poured at both ends to prevent the compensator from being torn by soil displacement.
• High temperature region: Referring to the application of metal corrugated expansion joint in kiln head pipeline in cement industry, the fixed bracket should avoid the high-temperature section, otherwise the steel of the bracket will soften and fail first.

6. To summarize a hard truth: the fixation of the expansion joint is not "locking", but "guiding"

Without proper guidance and fixing, there is a high probability that the bellows will be scrapped within a few months.When designing and selecting, it is best to send the pipeline layout drawing to the manufacturer, so that they can help check the force of the fixed bracket. There are various installation guides and technical parameters of expansion joints in our station, such as the adjustment method of expansion joint tie rod, the function of guide tube, etc. How much maintenance cost can you save by turning it over before installation? Do the math for yourself.

So, the next time someone asks, "Where is the metal expansion joint fixed," you can tell him:Fixed to brackets, fixed to drawings, fixed to your heart – not locked with brute force, but guided by knowledge.

First, bellows, the heart of the expansion joint, how to withstand thermal expansion and contraction?

Bellows is the most critical deformation element in metal expansion joints, none of them. Usually, stainless steel sheet is used, after hydraulic or mechanical forming, and corrugations are pressed one by one. The parameters of wave peak, wave pitch and number of layers directly determine the compensation amount-how much it can pull in the axial direction, how much it can bias in the lateral direction, and how much pressure it can hold.

The general-purpose corrugated expansion joint absorbs displacement by single-layer or multi-layer corrugated pipe, which has simple structure and is suitable for conventional pipelines. But if you run into a high-pressure steam pipe, you have to use an external pressure single axial expansion joint-its bellows are placed in an external medium and have a higher pressure capacity. To put it bluntly, the corrugated shape and wall thickness of the bellows are its "muscles", and the fatigue life must be calculated when designing. Metal expansion joints cannot be used as permanent parts, and usually have a life cycle of more than 100,000 times. This number should be remembered when selecting the model.

Will the bellows be damaged by the medium? Which brings us to the next part.

Second, end connection and guide tube-how to weld the expansion joint to the pipeline without letting the medium damage the inside?

The pipe and flange are the interfaces connecting the pipeline, and the material must match the pipeline-carbon steel to carbon steel, alloy steel to alloy steel, don't match randomly. The guide tube (also called lining tube) is installed on the inner wall of the bellows, and it has two functions: first, it reduces the direct erosion of the bellows by high-speed flowing medium (steam and flue gas); The second is to reduce the eddy current of the medium and reduce the flow resistance.

Corrugated expansion joints used in power station industry and metal corrugated expansion joints in cement industry are usually equipped with guide tubes because of the great impact of dust or high-temperature gas. The thickness and length of the guide tube are determined according to the diameter of the pipe and the flow rate of the medium, and not all pipes can be added casually. For example, the expansion joint matched with the desulfurization flue gas baffle door is highly corrosive in the medium containing sulfide, and the material of the guide tube should be stainless steel or lined with PTFE. Two days ago, I met a customer and asked about the specific function of the expansion joint guide. In fact, I was afraid that the bellows would be worn out.

3. Tie rod, hinge and limiting device-the expansion joint is not a rubber band, and the displacement direction must be controlled

The expansion joint cannot be moved, and must absorb the displacement according to the design direction. The core difference between tie rod expansion joints and hinge expansion joints (such as double hinge transverse expansion joints and double hinge expansion joints) lies in the restraint direction. The tie rod is used to absorb the axial displacement while limiting the lateral displacement to prevent the bellows from becoming unstable; The hinge allows angular or lateral displacement in a particular plane.

The high temperature axial type expansion joint has a guide ring inside to ensure that the bellows only expands and contracts along the axis. After installation, the tie rod nut needs to be adjusted to the predetermined length-two days ago, a customer asked how to adjust the tie rod nut of the expansion joint. In fact, it is to keep the bellows in the designed cold and tight position to avoid exceeding the allowable range during operation. Think about it, if the tie rod is not adjusted properly, and the bellows is directly pushed to death as soon as it expands thermally, wouldn't it be wasted?

4. Different types of structural differences-from single axial direction to pressure balance, what are the doors?

Metal expansion joint compensators are not a "one mold" thing. For example, the straight pipe pressure balance expansion joint eliminates the blind plate force through two sets of bellows and balance rings, which is suitable for large-diameter pipelines; While the curved tube pressure balance type expansion joint is used at the bend. The double straight pipe bypass pressure balance expansion joint is suitable for the space constrained.

The structure of sleeve pipe expansion joint (also called packing box expansion joint) is completely different-it relies on the sealing packing and the sliding of inner and outer sleeves to compensate the displacement, but the pressure and temperature resistance are limited, so it is only used for low-pressure and low-temperature pipelines. There are also rotary compensators, directly buried expansion joints, large-diameter thick-walled expansion joints... Each structure has applicable scenarios. Which you choose depends on the direction of the pipe, pressure rating, displacement type, and site space. Don't think that the more complicated the structure, the better. There is no need to use the pressure balance type for the general-purpose corrugated expansion energy-saving thing.

5. How Structural Design Affects Actual Type Selection-Inferring Structural Parameters Back According to Working Conditions

When selecting a metal expansion joint compensator, you must first find out the pipeline medium (temperature, pressure, corrosiveness), displacement (axial, transverse, angular) and connection mode (welding or flange). For example, the expansion joint matched with the desulfurization flue gas baffle door requires corrosion resistance and sulfide resistance, usually with stainless steel or PTFE-lined hose. On the air-cooled island vacuum pipeline, more dual-hinge expansion joints are used to absorb multi-directional displacement.

The more complex the structure is, the better-large-diameter thick-walled expansion joints are used in high-pressure steam lines, but the general-purpose type for small-diameter water pipes is enough. Remember: structural rationality directly affects the life. Before selection, it is best to check the fatigue times according to the stiffness calculation formula of the bellows, so as to avoid the problem of "leaking after two or three years of installation".

Finally, the structure of the metal expansion joint compensator is disassembled here, and the core is one sentence: the bellows determines the compensation ability, the guide tube protects the interior, the tie rod hinge controls the direction, and different structures are selected for different working conditions. After understanding these, go to the product manual and it will be clear at a glance.

1. The underlying logic of material classification: Which gear does the metal expansion joint count?

In your company's ERP system, which category does the metal expansion joint hang under? If the answer is "hardware machinery" or "plumbing fittings", congratulations, at least it didn't stray. But in reality, I have seen it stuffed into "rubber products", and some into the "valve" category-is it outrageous? What's even more outrageous is that some people bought it for power stations according to the model of "universal corrugated expansion joint". As a result, the pressure didn't match, and the scene exploded directly.

The material classification of metal expansion joints depends essentially on your management dimension. From the general direction, there are materials (metal vs non-metal), functions (compensator vs pipeline accessories) and industries (general-purpose vs power/chemical-specific). Each dimension is reasonable, but once mixed, the purchase order becomes a guessing game.

One setCorrugated expansion joint for power station industryThe material is 316L stainless steel, with a design pressure of 2.5MPa and a temperature of 600℃. If you classify it as "general hardware", the warehouse manager may put a piece with the bolt at hand, and can't find it after looking through it for a long time during the next maintenance-because no one remembers what the goods are called. However, if it is classified as "power station boiler accessories" according to industry classification, the retrieval efficiency is much higher.

Second, from the perspective of functional use: is it a pipeline accessory or a compensator? The two do not conflict

Are expansion joints and compensators the same thing or not? Before, a customer asked me, "I want to buy a compensator. If you recommend an expansion joint for me, will you make a mistake?" I directly threw the explanation in the Q&A library — —Expansion joints and compensators actually refer to different designations of the same type of equipment in industrial piping systems。 You said it was a pipe attachment? Yes, because it is installed on a pipe and is part of the piping system. You said it was a compensator? Right, too, because its core function is to absorb thermal displacement and vibration.

This characteristic determines its "double identity" in material classification. In the material codes of many enterprises, it appears under two categories: "pipeline accessories" and "mechanical compensation components" at the same time. Such asUniversal corrugated expansion jointIt can be classified as "bellows expansion joint" standard in GB/T 12777, and it can also be written as "pipe flexible joint" in the purchase list.

So, don't dwell on the name. You just have to remember: the function of a metal expansion joint is to compensate for displacement, and the installation position is on the pipe-these two points determine that its attribution is not single. If you have to choose one of the two, it is recommended to follow the industry practice: the electric power and chemical industries are used to call them "compensators" and are classified as "pipeline accessories"; However, the machinery and HVAC industries prefer "expansion joints" and are classified as "hardware machinery".

III. Classification according to industry standards: What do you say about JB/T and GB/T? You can't go wrong with referring to these

Talking about classification can't get around national standards. There are several standards related to metal expansion joints:GB/T 12777Is the general technical condition of metal bellows expansion joint;JB/T 12235Then for non-metallic expansion joints. Although the latter is non-metallic, many projects will manage metals and non-metals under one material category.

Then how to reference it? It's simple: see what standards your device performs. Such asHigh temperature axial expansion jointUsually designed according to GB/T 12777, it corresponds to "GB/T standard parts" or "general mechanical parts" in the material classification table. If the purchase order says "JB/T 12235-2015", you have to look for non-metallic or fabric fiber products, such asNon-metallic expansion joint (fabric fiber expansion joint)。

But standards can only help 60%. The remaining 40% depends on your management system. Some enterprises will putDirect buried (fully buried) type expansion jointTo "buried pipeline attachments", and putRotary compensatorListed separately under "Special Compensators". At this time, it is easy to miss the subdivision categories only by relying on standard coding.

IV. Material coding from the perspective of procurement: Why do some people classify it as "hardware machinery" and others as "pipeline fittings"?

And guess what? Same oneCompound hinge transverse expansion jointCompany A's code is FJ-001 (hardware and machinery), but Company B's code is GD-023 (pipeline fittings). There is one reason: the management caliber is different.

When most enterprises use ERP classification, they will refer to the equipment attribution department. If the expansion joints are procured by the equipment department, they are customary to classify them by mechanical components, and the coding prefix may be "M" or "05"; If procured by engineering, it tends to be classified by pipeline system and the coding prefix becomes "P" or "08". This "dual-track system" is very common, but the pit is also here-when calling across departments, there is often an oolong of "I obviously bought it, but I can't find it in the system".

How to avoid it? I suggest doing two things: first, add functional keywords to the material description, such as "metal expansion joint | compensated displacement | for pipeline"; Second, establish a mapping table to correspond the codes of hardware machinery and pipeline fittings. Don't bother, aLarge diameter thick wall expansion jointEasy tens of thousands of dollars, the return cost of buying the wrong is astonishing.

5. Product examples of this site: from general-purpose to power station-specific, to help you understand the classification boundary

It's boring to talk about theory. You can understand where the boundary of classification is by looking directly at the products on our site.

• Universal corrugated expansion joint: The most basic classification, suitable for normal temperature and low pressure pipelines, belongs to the category of "general pipeline fittings". The code is usually classified as "bellows series" under the category of hardware machinery.
• Corrugated expansion joint for power station industry: Specially used for boiler, steam turbine system, the material and design pressure are special. Such products are often divided into separate categories, called "special compensators for power stations" or "accessories for power equipment", which are different from ordinary expansion joint codes.
• Metal Corrugated Expansion Joints in Cement Industry: Wear resistant, high temperature resistant, with guide tube structure. It can be counted as "cement machinery accessories" or "pipeline accessories", depending on your company's main business. If it is purchased by a cement plant, it is more inclined to be classified as "process pipeline parts" in spare parts.
• Sleeve type pipe expansion joint: The structure is simple, used in low-pressure applications, and bellows type is completely different. Many ERPs put it under the "sleeve class" under "pipe fittings".

Can you see the way? For the same product, change the industry, and the classification logic will change. So before buying, find out the classification system of your company first, and then take a seat.

6. What are the consequences of wrong classification? A real case tells you

They are in urgent need of a batch for overhaulDouble hinge expansion joint for air-cooled island vacuum pipelineAccording to the old habit, the buyer searched in the category of "pipeline fittings", and the result was that the general axial bellows was bought back. After installation, it was found that the axial compensation was not enough, and the vacuum degree could not be hit-because the double-hinge expansion joint absorbed the lateral displacement, the axial tube could not be used at all.

Finally, it was shut down for 3 days, which resulted in heavy direct losses. The reason is that the procurement classification is wrong: he classifies the expansion joint of "vacuum pipeline special" under "general pipeline accessories", and the system won't remind you of the performance difference between these two classifications at all.

What kind of material does metal expansion joint belong to?The answer depends on your management granularity-you can think of it as a pipeline attachment or a compensator, but never think of it as "whatever it looks like".

Classification is for easy finding, but also for easy use. Before buying, it is better to ask the supplier, "Is this product consistent with the parameters of the XXX model you sold before?"

Find out the difference between single wave and three wave first-structure determines performance

Two days ago, a customer who made steam pipelines called and asked, "Which one is better, single wave or three wave?" I asked him back, "What's the pressure of your pipe? What is the displacement?" He was stunned for a moment and said, "I see that the compensation amount of three waves on the parameter table is greater. Is it right to choose three waves?" Alas, this kind of thinking is horrible. Which is better, single wave or triple wave of metal expansion joint? It's not a question of "who's stronger" at all, but what kind of structure you need for your working conditions.

Single wave, as the name implies, is a ripple; Three waves are three ripples connected in series. The structural differences directly determine their mechanical properties. There are few single wave corrugations, the wall thickness can be made thicker (of course, it is also related to the material and molding process), and the overall stiffness is large. Three waves are equivalent to three springs strung together, and each ripple shares a part of the deformation. The overall flexibility is large, and the compensation amount will naturally go up. However, large flexibility does not mean that everything will be fine-the force between the three ripples is uneven, and the ripples near the ends tend to bear greater stress, which is a flaw of fatigue life.

Therefore, don't just look at the "compensation amount of 100mm" written on the sample and feel that the three waves have won. You have to break it apart and crumble it to see: How many cycles of life is this compensation measured? How much is the design pressure? How hot is the temperature? The combination of these parameters is called the real working condition.

Single wave: Large stiffness and small displacement, suitable for high pressure and small displacement scenarios

The typical application scenario of single-wave expansion joint, let me make an analogy: like the main steam pipeline of a power station boiler, the pressure is always more than ten MPa or even higher, and the temperature is five or six hundred degrees, but the thermal displacement of the pipeline is actually not large-maybe only a few millimeters to ten millimeters. With a single wave at this time, the advantage comes out. The single wave has high stiffness and can resist high pressure without instability; At the same time, because the wave number is small, the displacement of each wave is relatively concentrated, so as long as the design stress is within the allowable range, the fatigue life is easy to guarantee.

We have one on our siteHigh temperature axial expansion jointIt is used a lot in the petrochemical industry, which is a typical single-wave or multi-wave customization. However, in high-pressure scenarios, engineers generally prefer single-wave-with-thick-wall design, which can reduce the overall stress level. In addition,Universal corrugated expansion jointThere are also single-wave models in, which are suitable for the working conditions of "high pressure, small displacement and thick pipeline". It's easy to say, but you have to calculate when you actually choose the modelStiffness and Calculation Formula of BellowsIn that formula, the wave number n is placed directly on the denominator-the fewer the wave number, the greater the stiffness. You taste, you taste.

The structure is simple and there are few leakage points. Because of the many welding parts in three waves (there are usually circumferential welds between waves or formed integrally, but the crest and trough stresses are concentrated), the quality control points in the manufacturing process are also denser. For scenarios such as nuclear power and chemical industry that pursue the ultimate reliability, single wave is a safe choice.

Three waves: good flexibility and large compensation, but fatigue life and stability have a price

Three more waves. Its core selling point is the large amount of compensation. For example, some long-distance thermal pipelines, or scenarios with large installation errors, require the expansion joint to absorb the axial displacement of tens or even hundreds of millimeters. That is when three or more waves (such as four or five waves) come in handy. I've seen aCorrugated expansion joint for power station industryIn the case, the pipe diameter is DN1200, the temperature difference is 200℃, the calculated thermal elongation is 80mm, and the four-wave structure is finally selected. If a single wave is forcibly used, the wall thickness will go up to the sky, and the displacement of a single wave is too large, and the stress will long exceed the limit.

But the cost is also obvious. First of all, the axial stiffness of the three waves is low, and it is prone to columnar instability under high pressure-that is, the ripples bulge like balloons and lose their load-bearing capacity. So the three waves usually need to cooperatetie rodOrguide tubeTo enhance stability. We specifically explained it in the FAQ on our siteFunction of expansion joint tie rod: The tie rod is not used to compensate for displacement, but to restrain the pressure thrust and prevent the expansion joint from excessively elongating or twisting. Nine times out of ten, the three-wave products should be equipped with tie rods, or even designed toStraight pipe pressure balanced expansion jointOrCompound hinge transverse expansion jointTo balance the blind force.

Secondly, fatigue life. With the same displacement of 10mm, a single wave may experience 100,000 cycles without any problem. Because the displacement of each wave division of the three waves is only 3.3mm, it stands to reason that the life is longer-but in fact, because of uneven stress distribution (edge wave stress concentration), the test data often show that the fatigue safety factor of the three waves is lower than that of the single wave. Therefore, in occasions with frequent pulsating pressure (such as compressor inlet and outlet pipelines), choosing three waves should be extra cautious, and it is best to do a fatigue life check.

How to make a decision on the actual selection? Three dimensions of hard indicators to help you set

Having said all that, how do you choose? I summarize three hard indicators. If you take this to set your working conditions, it will be almost inseparable.

• Pressure dimension: Design pressure> 2.5MPa, preference is given to single wave, especially pipes with larger nominal diameter. The higher the pressure, the more critical the stability. You check itMetal expansion joint weight tableIt will be found that the wall thickness of multi-wave products under the same caliber is often a little thinner than that of single-wave products-that is to reduce the stiffness in exchange for compensation, but it becomes a short board under high pressure.
• Displacement dimension: The required axial compensation amount exceeds the single wave limit compensation amount (usually the maximum single wave can be 20~30mm, depending on the diameter and wall thickness), then multiple waves must be used. However, note that the compensation amount is not greater, the better. If it exceeds 50mm, it is recommended to consider duplex structure or hinge-type expansion joint, such asCompound straight pipe bypass pressure balanced expansion jointInstead of a bunch of wave numbers.
• Temperature/Media Dimension: Under high temperature (> 400℃) environment, the material creep is intensified, the stress relaxation at the peak of multi-wave is faster, and the life decay is obvious. At this time, the stress level of single wave is lower, which is relatively more advantageous. Corrosive media (such as flue gas desulfurization systems) should be consideredPTFE-lined hoseOrPTFE compensatorMore wavenumbers will increase the risk of medium retention, and single waves are easier to clean.

Waste heat power generation pipeline of a cement plant, pressure 1.0MPa, temperature 350℃, displacement 30mm. According to the above three dimensions: the pressure is not high, the displacement is medium, and the temperature is high. At this time, single wave can also be done, but the wall thickness needs to be thickened, which leads to an increase in cost; Three waves can also be done, but it needs to be equipped with a strong tie rod. Eventually we recommended to our clientsMetal Corrugated Expansion Joints in Cement IndustryThe three-wave zone guide tube scheme in China has been used by customers for three years and good feedback. There is no absolute answer, it is all calculated.

Don't ignore the matching: the guide tube, pull rod and installation direction can't be wrong

Choosing the right wave number is only the first step, and the matching accessories are the details that determine success or failure. Guide tube, we have a special article on our stationSpecific Function of Expansion Joint Guide Tube— — It is mainly used in pipelines with high medium flow rate (such as steam and flue gas), so as to prevent ripples from being directly washed by high-speed fluids, and at the same time, it can reduce flow resistance and vibration. Single-wave products are less equipped with guide tubes, because they have great stiffness and are not afraid of erosion; However, three-wave products, especially large-diameter ones, have deflectors that are almost standard. Otherwise, the high-speed airflow will wear through the thin ripples, and it will be impossible to change them.

The adjustment of the tie rod is also critical.How to adjust the tie rod nut of expansion joint？ Remember one sentence: tie rods are used to adjust pre-tension or pre-compression after installation is complete, not to lock up expansion joints. Many on-site workers screw the tie rod to death directly, which is equivalent to turning the expansion joint into a rigid connection, so why do you need it? The correct way to do this is: according to the ambient temperature at the time of installation, calculate the amount of pre-tension required for the pipe, adjust the nut position, and then let the expansion joint expand and contract freely. Because of the low axial stiffness of the three-wave structure, the installation direction is easy to reverse.The direction of the arrow of the expansion joint refers toMedia flow direction, the arrow must be pointed to the fixed bracket or end point, otherwise the deflector will drive and cause vibration.

If the pipeline has large lateral displacement, the three-wave structure is prone to the combined failure mode of "column instability + plane instability". This is the time to considerCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointInstead of simply changing the wave number. Type selection is a systematic project, and wave number is just one of the variables.

Summary: There is no absolute good or bad, only suitable or unsuitable working conditions

Which is better, single wave or triple wave of metal expansion joint? Did you notice that the question itself is not valid. Single wave is an epee without edge, specializing in high pressure and small displacement; Three waves are seven-foot soft swords, which focus on big compensation flexibility. You took the soft sword to cut the iron plate, and the blade collapsed; You use the epee to play tricks, and you break your wrist. So next time someone asks you "which is better" with the parameter table, you will throw him three words: depending on the working conditions.

If you are really unsure, just roughly sieve it according to the three dimensions (pressure, displacement and temperature) I mentioned above, and then find a reliable manufacturer to report the plan. Our product line in our site is very complete, fromMetal hoseToNon-metallic expansion jointToAll kinds of baffle doorsEach has an applicable boundary. Again: there are no perfect expansion joints, only matching designs.