Specialized in manufacturing compensators, expansion joints, baffle doors

What are the classifications of expansion joints? First distinguish between metals and non-metals

Many customers ask "What are the classifications of expansion joints?" In fact, the roughest way to divide them is to look at the materials-metal and non-metal. Metal expansion joints are seen most often, likeUniversal corrugated expansion joint、Corrugated expansion joint for power station industry、Large diameter thick wall expansion jointThe bellows inside are all stainless steel or alloy materials, which are hard-core in pressure and temperature resistance. The same cannot be said for non-metallic expansion joints, such asNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointThe main body is fiber fabric plus silicone coating, which is mainly used in the occasion of low pressure, large cross section and violent temperature fluctuation of smoke duct.

These two are not substitute relationships. Metal can carry pressure and temperature, but the reaction force is large when compensating displacement; Non-metal has good flexibility and corrosion resistance, but weak pressure bearing capacity. You have to take metal to do non-metal work, and the cost goes up; Taking non-metals to top the high temperature and high pressure is looking for trouble. If the material layer is not selected correctly, no matter how advanced the structure is, it will be useless.

According to the structure: the respective doorways of bellows, sleeve and rotary compensator

The structure determines how the expansion joint absorbs the displacement.Bellows typeIs the absolute mainstream, compensated by ripple deformation. There are many patterns in this, there are single, duplex, with hinges, and with pull rods. LikeCompound hinge transverse expansion joint、External pressure single axial expansion jointIt's all based on this logic.

Sleeve type pipe expansion jointIt relies on the relative sliding of the inner and outer sleeves to compensate the axial displacement, has simple structure and low cost, but the sealing performance is not as good as that of bellows, and is suitable for those pipelines with unclean medium and low pressure.Rotary compensatorIt is another way, which absorbs the axial displacement by converting the axial displacement into rotating motion through the cooperation of a pair of rotating cylinders in the bending section of the pipe. It is especially used in long-distance conveying of steam pipe network, with large compensation amount and small pressure drop.

When choosing a structure, it depends on the direction and space of the pipe. If there are many straight pipe sections, choose axial type; if there are elbows, consider hinge type or rotary compensator. Where the space is compact, the sleeve type is easy to install-it doesn't mean that the more advanced the more suitable, it is the last word that it can be installed and compensated in place.

According to the compensation function: axial, transverse, angular and pressure balance type

This level of classification is the real test of selection skills.

Axial typeThe compensator mainly absorbs the axial expansion and contraction of the pipe, such asUniversal corrugated expansion jointIt is a typical axial product. Several are arranged on straight pipe sections, so thermal expansion and contraction are not afraid.Transverse typeThe good thing is that it absorbs displacement perpendicular to the axis of the pipe,Compound hinge transverse expansion jointThat's what we do.Angular typeWell, with a combination of hinges, the pipe swings at a certain angle, likeDouble hinge expansion joint for air-cooled island vacuum pipelineIt is particularly useful in air-cooled islands with large displacements and multi-directional changes.

If the pressure in the pipeline is high and the blind plate is strong, the ordinary axial compensator will transmit the pressure thrust to the fixed support, which must be made as thick as the city wall. That's when you have to use itStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint。 Its principle is that a balancing bellows is built in, which eats the blind plate force itself, and the main fixing bracket basically does not bear extra thrust. There are more complicatedCompound straight pipe bypass pressure balanced expansion jointWhich is suitable for the working condition that requires a large compensation amount and does not want the bracket to be stressed. Do you think this classification is important or not? Wrong choice, pipeline vibration, bracket deformation, later is all trouble.

According to the working conditions of the industry: special goods such as power stations, cement and desulfurization

As soon as the industry working conditions come up, the expansion joint has to be "customized". Smoke air duct of power station boiler, high temperature, large cross section, generally usedMetal rectangular expansion jointOrNon-metallic expansion jointWhich not only absorbs thermal displacement but also reduces vibration. Of the cement industryMetal Corrugated Expansion Joints in Cement Industry, that has to be wear-resistant, the dust particles at the head and tail of the kiln are fast, and the material and wall thickness of the bellows have to be increased.

The desulfurization system is more special. The flue gas contains sulfide, and the temperature changes greatly. Ordinary metal bellows can't bear corrosion, so it is commonly usedPTFE compensatorOr a metal hose lined with PTFE, such asPTFE-lined hose。 Also matched with the expansion jointDesulfurization flue gas baffle door、Flue gas baffle doorThese valve equipment have to be considered together when selecting, otherwise the connection size will not match, and the scene will be blinded.

Quick locking categories? Look at these key parameters

Then how do you quickly judge which category to use? Don't rush, just three steps.

The first step is to look at the medium and temperature. High-temperature flue gas and corrosive gas, directly leaning in the direction of non-metal or tetrafluorine lining; High temperature and high pressure steam, honestly choose metal corrugated expansion joint.

The second step is to calculate the compensation amount. Is the axial displacement large? Is there any lateral displacement? If there are multiple directions, preference is given to hinged type or rotary compensators. Like a direct pipeline, then you have to goDirect buried (fully buried) type expansion jointThis goods is directly buried in the soil, and there is a protective sleeve on the outside, so you can't make do with ordinary expansion joints.

The third step is to look at the pipeline constraints. If the position of the bracket has been set, it is necessary to choose the pressure balance type according to the thrust that the bracket can bear. Two days ago, I met a customer who was doing a steam pipe network project. When I came up, I asked, "What are the classifications of expansion joints?" I asked him how to arrange the bracket, and he looked confused. You have to understand that classification is not just a difference in name, each structure corresponds to different force models and installation conditions.

The classification table looks simple, but there are many things involved in the actual selection. The four dimensions of material, structure, function and working condition are cross-combined, and dozens or hundreds of schemes can be discharged. But as long as you grasp the core-the temperature and pressure of the medium, the magnitude of the displacement direction, and the force constraints of the pipeline-you won't deviate. Understanding these, the expansion joint classification is no longer a table, but a selection map in your hand.

Find out first, what kind of equipment is the metal expansion joint?

Metal expansion joints, also called corrugated compensators, are flexible connectors mounted on pipes. It mainly relies on the deformation of the bellows to absorb the displacement and vibration caused by thermal expansion and contraction of the pipeline, so as to avoid the pipeline being cracked or broken. To put it bluntly, it just puts an elastic joint on the pipe.

Are metal expansion joints and corrugated compensators the same thing? Yes, and no. Say "yes" because the core of the vast majority of metal expansion joints is the bellows; Say "no" because there are members of the metal expansion joint family of non-corrugated structures such as sleeve types and rotary compensators. So you can think of corrugated compensators as the mainstream branch of metal expansion joints, but don't draw an equal sign.

Classified by displacement: axial, transverse, angular, you should know these faces

What types of metal expansion joints are included? The most commonly used division method is by absorbing displacement.

Axial typeExpansion and contraction of the absorbent pipe along the axis. Representative products are:

• Universal corrugated expansion joint: The most basic and widely used model, suitable for pipelines with ordinary temperature and pressure.
• High-temperature axial expansion joint: with heat insulation layer and high-temperature resistant material, it can serve hot air ducts above 500℃.
• External pressure single axial type expansion joint: Bellows bear external pressure, good stability, suitable for high pressure steam pipeline.
• Straight Pipe Pressure Balanced Expansion Joint: By balancing the bellows to offset the blind plate force, it is suitable for equipment with fixed ends and unable to bear pressure thrust.

Transverse typeAbsorbs displacement perpendicular to the axis of the pipe. The double hinge transverse expansion joint is typical-two hinge groups are used to restrain the bellows so that it only bends to the side and cannot twist around itself.

Angular typeAbsorbs the angular displacement at the bend of the pipe. Curved pipe pressure balance type expansion joint is a representative of this kind, which is mostly used in the corner of L-shaped or Z-shaped pipe.

Special needs according to working conditions: These "non-mainstream" models specialize in solving difficult problems

Some pipes are unusual, so they have to use special expansion joints.

Direct buried (fully buried) type expansion joint— — The heating pipeline directly buried underground cannot be equipped with an inspection well. The shell of this expansion joint should be made into a structure that can withstand soil pressure, and the interface must be waterproof. It is the only option when directly buried heating pipes are laid.

Sleeve type pipe expansion joint— — Rely on the relative sliding of the inner and outer sleeves to compensate the displacement, which is suitable for low-pressure large-diameter pipelines. The price is much cheaper than that of corrugated pipelines, but the sealing performance is a soft rib and needs regular maintenance.

Rotary compensator— — The displacement is absorbed by the rotation of a pair of rotating arms, and the compensation amount is large. It is especially suitable for long-distance direct burial laying of steam pipelines, which saves brackets compared with traditional expansion.

Large diameter thick wall expansion joint— — The diameter exceeds 1 meter and the wall thickness is increased, which is specially used in large-flow and low-pressure pipelines such as flue gas desulfurization and induced draft fan outlet of power station. Ordinary bellows are easily unstable under this working condition, so it is necessary to use reinforced design.

Does a metal hose count as a metal expansion joint? Close relatives, but don't mix them

This is the easiest pit to step on when selecting a model. Metal hoses and metal expansion joints look similar and have similar names, but their functional emphases are different: expansion joints mainly absorb thermal displacement, and metal hoses are mainly used to connect equipment with relative motion, such as vibration damping joints at pump inlet and outlet, and compensate for installation deviations.

You use the metal hose as an expansion joint, it has few corrugations, short fatigue life, and will crack soon; Conversely, use the expansion joint as a hose, which is not soft enough, and the vibration of the equipment will still be transmitted. When purchasing, read the working condition description clearly: if there is thermal displacement, choose expansion joint; if there is vibration or frequent disassembly, choose metal hose.

Inferred from industry application: Which type should I choose for power station, cement and desulfurization?

Another dimension-the industry pushes back. The working conditions of different industries vary greatly, and products have long been "specialized".

Corrugated expansion joint for power plant industryThe main steam and the reheating steam pipeline have high pressure and high temperature, and the external pressure single axial type or the high temperature axial type are preferably selected; Air-cooled island vacuum pipeline is a large-diameter low-pressure vacuum condition, and the double-hinge expansion joint of air-cooled island vacuum pipeline is needed, which has low stiffness and good seal.

Metal Corrugated Expansion Joints in Cement Industry: The flue gas at the head and tail of the kiln has a large dust content and a drastic temperature fluctuation. When selecting the general-purpose type or the high-temperature axial type, the guide tube should be thickened to prevent the dust from washing and wearing the bellows.

Desulfurization system: The flue gas of wet desulfurization contains acidic corrosive medium, which cannot be carried by ordinary stainless steel, so it is necessary to use PTFE-lined hose or PTFE compensator as an anticorrosive layer; At the same time, the desulfurization flue gas baffle door should be matched with the double seal design. At this time, the expansion joint must be matched with the baffle door.

So, what are metal expansion joints included? The answers are different by displacement, special needs and industry. However, there is one principle that remains unchanged: first find out the pipeline medium, temperature, pressure and displacement, and then compare it with the specific models in the product information of this site. Don't ask "which is cheaper". The wrong choice of expansion joint can lead to air leakage or pipeline overturning. Which one is lighter and which one is heavier, don't I need to say?

What are you most afraid of when choosing metal expansion joints? I am afraid that the customer will only say "Quote me the price of DN600" on the phone. After quoting, ask "why is it so expensive". Because the selection parameters are incomplete, manufacturers can only quote according to the most conservative and largest specifications. Can this price not be high?

What about that? If you want to buy a suitable expansion joint, don't rush to report the caliber. Find out the following 5 things.

Don't be in a hurry to report the caliber, first calculate how much displacement the pipeline should absorb

The core function of the expansion joint is to absorb displacement. The amount of displacement was not calculated clearly, and the rest was all for nothing.

The thermal expansion of a pipeline is generally calculated by multiplying the linear expansion coefficient of the pipeline by the temperature difference and then multiplying the length of the pipeline. The linear expansion coefficient of carbon steel is approximately 0.012 mm/m·°C, and the stainless steel is slightly higher. For example: a 30-meter-long carbon steel pipeline, when the medium temperature rises from 20°C to 200°C, the axial thermal expansion is 30×180×0.012=64.8mm.

This is not over, the ambient temperature during installation, equipment vibration, and displacement caused by pipeline foundation settlement have to be counted. If you only know the diameter of the pipe and don't know how to calculate it, ask the manufacturer for a "Quick Check Table of Pipeline Thermal Displacement" and check it. But only if you have to prepare the pipeline direction diagram and clearly mark the positions of the fixed bracket and sliding bracket.

Media, pressure, temperature: three-piece set that determines the material and number of layers of corrugated pipe

Looking at these three parameters together, the material, wall thickness and number of layers of the bellows are basically determined.

At this end of the medium, 304 stainless steel is enough for ordinary steam, hot water and compressed air; For the medium containing chloride ions, 316L is used; In this strong corrosion condition of desulfurization flue gas, corrosion-resistant alloys or non-metallic expansion joints are often selected. The composition of the medium must be written in full, don't just mention a "smoke". The content of SO₂, NO and dust in the smoke is different, and the corrosiveness is several times worse.

If the pressure is high, the wall thickness has to be added; When the temperature is high, the permissible stress of the material decreases, and the compensation amount is also affected. For example, a steam pipeline with a pressure of 1.6MPa and a temperature of 350°C usually uses 2 to 3 layers of bellows; At the same temperature but the pressure dropped to 0.1MPa, a single layer is enough. The more layers, the greater the stiffness and the higher the price, so there is no need to blindly pursue multiple layers.

On the other hand, don't leak the deflector. The flow rate of the gas medium is high, and the bellows are easily eroded without the guide tube. If the medium contains particulate matter, the anti-wear structure at the front end of the guide tube should also be considered.

Axial, lateral or angular displacement? Choose the universal or hinged type

If the displacement direction is not clear, the chosen structure must be wrong.

The pipeline mainly expands and contracts along the direction of the pipeline, that is, axial displacement. Choose general corrugated expansion joint, also called axial expansion joint. If the pipeline takes an L-shaped or Z-shaped bend, and there is displacement in both horizontal and vertical directions, a single axial type can't be handled at this time, so we have to consider the double hinge transverse expansion joint, or the use of two hinge expansion joints together.

Angular displacement is common at equipment connections, such as steam turbine exhaust port and fan inlet and outlet. Hinge type expansion joint or double tie rod type is a good choice under this working condition.

How to judge? To put it bluntly, just one sentence: telescope and contract along the direction of the tube to choose the general-purpose type; If the pipe is bent and the joint is deflected, you have to use one with hinges or duplex structure. Don't expect to eat all the displacements with a general-purpose type. That's called hard carrying, not compensation.

Tie rod, hinge or pressure balance? Constrained structural relationship support stress

The function of expansion joint is not only "telescopic", but also "controllable force".

The bellows will produce a huge thrust under the action of pressure. If this thrust is all thrown to the fixed bracket, how thick must the bracket be? Therefore, there should be a restraint structure inside the expansion joint to share-this is the meaning of the existence of tie rod, hinge and pressure balance.

The universal expansion joint does not come with its own constraints, and the internal pressure thrust must be carried by the main fixed bracket. Where the pipeline is complicated and the bracket is difficult to make, use straight pipe pressure balance expansion joint or curved pipe pressure balance expansion joint, which can offset the internal pressure thrust, and the force acting on the bracket is much smaller. It is often used on pipelines in cement plants and power stations.

Some people think that the expansion joint with a tie rod can replace the fixed bracket. It wasn't. The tie rod constrains the displacement of the expansion joint itself, not the thrust of the pipeline. The bracket has to do what it needs to do, but the force is different.

High temperature flue gas, large diameter, vacuum pipeline: how to feed in special working conditions

Once the working condition is special, the standard products often can't stand it, so they have to be "added".

High-temperature flue gas, when the temperature reaches above 600°C, metal bellows are basically useless-material creep and oxidative corrosion will quickly shorten the life. At this time, the candidate non-metallic expansion joint (fabric fiber expansion joint) is more suitable, with high temperature resistance, good flexibility and certain sound absorption and vibration isolation effect. Non-metallic expansion joints are often used in desulfurization flues, which are used together with flue gas baffle doors.

Large-diameter pipes, such as flues or air ducts above DN3000, metal bellows are expensive to manufacture, heavy in weight, and laborious in construction and hoisting. Rectangular non-metallic expansion joints are more common schemes under such operating conditions. Of course, if the customer specifies metal, it has to use large-diameter thick-walled expansion joints, and the reinforcing rings and side plates have to be thickened.

The vacuum pipe is different again. Under vacuum conditions, the corrugated pipe should withstand external pressure, and its ability to resist instability is the key. Double hinge expansion joints are often used in air-cooled island vacuum pipelines, which have one more constraint in structure and good stability. The special vacuum hose is equipped with metal mesh sleeves outside the bellows, which is also the truth.

It is much more efficient to give the manufacturer a selection parameter table than to call back and forth

When communicating with the manufacturer, give the following information at once:

• Pipe diameter DN, connection mode (flange or welding)
• Type and value of displacement: What are the axial, transverse and angular displacements in mm/degree
• Name and composition of the medium, especially with corrosive components
• Design pressure MPa, Design temperature °C, Operating temperature °C
• Installation position indication (straight pipe section, near elbow, equipment interface)
• Arrangement of fixed bracket and sliding bracket
• Is there any thermal insulation layer and any outer sheath requirements

All this information is given, and the manufacturer can make the plan and quotation in half an hour. The time saved is much more efficient than calling back and forth to urge questions.

There is no "universal model" for metal expansion joints, only "fit or not". Only when the selection parameters are clear and the working condition boundary is clear can the manufacturer give you a reliable scheme. If you are not sure, send the parameter table directly to the manufacturer-let the engineer judge whether the scheme is valid or not.

How to weld metal expansion joint? This question looks simple, but in fact there are many pits. I met a customer two days ago. Within two months of installing the expansion joint I took back, the root of the weld leaked. When disassembled, the typical group pair edge misalignment exceeds the standard + the heat input is too large, which burns through the thin-walled bellows.

Everyone in this business knows that metal expansion joint welding is completely different from ordinary pipe welding. The bellows has thin walls, low stiffness and is sensitive to heat. If you don't pay attention to it, you will crack or burn through the weld. Today, we will explain these five key points thoroughly according to the actual working experience.

1. Look at the material before welding: the wrong selection of welding material will directly waste parts

Don't weld the pipe when you get it. Find out if the material is 304, 316L, Inconel or carbon steel first. The welding processes of these types of materials vary a hundred and eighty thousand miles.

Most of the general corrugated expansion joints and bellows in our station are made of 304 or 316L stainless steel. This kind of austenitic stainless steel is easy to weld, but attention should be paid to controlling the interlayer temperature, which is best if it does not exceed 150℃. Some high-temperature axial expansion joints are made of Inconel alloy, such as Inconel 625 or 800H. This material must be made of nickel-based welding wire, such as ERNiCrMo-3, and the heat input must be strictly controlled, otherwise the corrosion resistance of the weld at high temperature will be directly scrapped. Carbon steel is relatively solid, but preheating and post-weld insulation cannot be saved.

Before selecting welding materials, check the materials of bellows and connectors according to the product information. What materials are used for the product, the welding materials will follow the match, don't guess by experience.

2. Groove and assembly pair: Thin-walled parts are most afraid of edge misalignment

When the expansion joint port is paired with the connector, the bevel angle, blunt edge and clearance must be according to the process card. The wall of the bellows is thin, usually only 0.5mm ~3mm. When the groove is opened wide, it will directly melt through, and when the groove is opened small, the root will not melt through.

When actually working, many people are planted in the wrong side. The specification requires that the amount of misalignment should be controlled within 10% of the wall thickness. What do you mean? For example, if the wall thickness is 2mm, the amount of misalignment cannot exceed 0.2mm. Beyond this number, the root of the weld seam is prone to unfusion, and the surface looks like it is welded. In fact, there is delamination inside, and it will leak when it is pressed.

When assembling, use special fixtures, do not hold your hands and fix them by spot welding. The bellows itself cannot bear the concentrated load, and if it is hard broken when assembling, the corrugation deformation will be wasted.

3. How to choose the welding method: argon arc welding is standard

Thin-walled bellows are all argon arc welded, which is the iron law. The heat input of argon arc welding is controllable and the arc is stable, which can ensure the back shaping. Thick-walled parts such as large-diameter thick-walled expansion joints can be filled with argon arc bottoming + manual arc welding, which is efficient, but the bottoming layer must be argon arc welded.

Note that when filling and cover welding, the arc should not directly face the bellows, and try to be as close to the connecting side, so as to reduce the damage of high temperature to the bellows. The welding current should be adjusted according to the material and thickness. 304 stainless steel, about 1mm bellows, current control at 50~70A. Inconel material is lower, and you have to use the back to protect the gas.

There is no cookie cutter routine for welding methods, but there is one principle: the smaller the heat input, the better, the faster the better, and the thinner the wire, the better.

4. Welding sequence and deformation control: longitudinal seam first and then annular seam

What are bellows most afraid of? Welding splashes and arc scratches. Splash on the ripples is a corrosive starting point. Therefore, before welding, the bellows surface must be blocked with protective cloth or white iron sheet.

In the welding sequence, the longitudinal seam is welded first, and then the circumferential seam is welded. The longitudinal seam is the weld of the bellows itself. If there are drawings, it must be argon arc welded, and it should be smoothed after welding. In circumferential welding, symmetrical welding or segmented desolding is done to reduce local heat buildup. Sectional desoldering, each section is 200~300mm, and the next section is welded after one section is cooled down.

During on-site welding, protective gas should be passed through the inside of the expansion joint. Some people say that it doesn't make sense to save money, but as a result, the back shaping is a mess, and the oxide scale falls off one by one. Remember, the back side is not well formed, and the weld life is directly folded in half. The back of the stainless steel bellows is protected with argon gas, and the purity is not less than 99.99%.

5. Post-welding inspection and pressure test: Don't skip that step

A weld appearance inspection is only the first step. Biting edges, surface pores, crater cracks, these can be seen by the naked eye is definitely unqualified. But what is more critical is internal defects, which must be nondestructive tested according to standards.

For corrugated expansion joints used in power station industry and expansion joints used in pressure pipelines, the welds generally require 100% RT or UT. RT is filming, UT is ultrasound. Conditionally, use phased array, which is more efficient. We provide a complete report on the welded expansion joints in our station, as long as the customer requests.

The same goes for pressure testing. The pressure test of metal expansion joint should be carried with a restraining device to prevent excessive extension of bellows. When the pressure is tested, it is low first and then high, and the pressure is increased gradually. If the pressure gauge has been calibrated, the measuring range should be 1.5~2 times the test pressure. During the hydraulic test, the internal air should be discharged, otherwise the pressure will be unstable.

How to weld metal expansion joint? In fact, the core is one sentence: treat the bellows as a precision part, and every parameter is true. There is no shortcut to welding, but according to the above five steps, the scrap rate can be reduced by more than half. Don't bother, the cost of rework is much higher than the meticulous effort when welding.

Two days ago, I met an engineer who was doing heating network design. He asked me with the drawings, "What is the unit of angular displacement of the expansion joint? Why do some of the drawings write ° and some of them write mm?" I saw that on his expansion joint selection sample, the angular displacement column was marked with "±5°", while the other company gave it with "±12mm". If you compare these two directly, something will go wrong. Today, break this parameter apart and crumble it into pieces to explain clearly.

What exactly is angular displacement? First distinguish the difference between it and lateral displacement and axial displacement

What the expansion joint does in the pipeline, to put it bluntly, is to absorb displacement. There are three kinds of displacement: axial displacement is the expansion and contraction along the axis of the pipe, transverse displacement is the misalignment perpendicular to the axis, and angular displacement-is the relative rotation between the flanges at both ends of the expansion joint. You can think of it as the pipe "folded at the waist", and the size of this folded corner is the angular displacement.

Understanding this is critical. Many people confuse angular displacement with lateral displacement, which is actually two different things. The lateral displacement is translation and the angular displacement is rotation. For example, a piece of equipment is lifted up by 20mm because of thermal expansion. If the pipe connecting it turns a corner, the expansion joint may actually bear an angular displacement of several degrees, instead of a simple 20mm lateral displacement. You just report a "lateral displacement of 20mm" to the manufacturer, and people really can't choose a model for you.

Is the unit of angular displacement only degrees (°)? Why do some drawings say millimeters?

The standard unit of angular displacement is degrees (°), which is nothing to contend with. But why do some drawings say millimeters? Because in many manufacturers' samples, angular displacement is expressed by "equivalent lateral displacement". What do you mean? It is to convert this angular displacement into the arc displacement at one end of the bellows.

The arc length is equal to the radius times the radian. If the radius of rotation of the bellows is 500mm and the angular displacement is 2°, the corresponding arc length is 500×2× π/180≈17.5mm. But there is a big pit here: different expansion joint structures have different radii of gyration at all. The same 2° angular displacement may be several times different in millimeters between a single hinge expansion joint and a large double hinge transverse expansion joint.

So when you see mm written on the drawing, don't rush to convert it. You have to find out where this mm comes from, whether it is the equivalent lateral displacement calculated by the manufacturer according to its own product structure, or it refers to the swing arc length of a certain point of the bellows. If you take this mm directly as an angular displacement to select the model, then you can't install it on the spot.

Generation of angular displacement from bellows structure: Why are single hinges different from double hinges

Angular displacement is not produced for nothing, it has to be guided by hinge structure. Single hinge expansion joint, that is, a hinge is installed at both ends of the bellows, which can only rotate in one plane, and its ability to absorb angular displacement is very limited. The compound hinge transverse expansion joint has two sets of hinges and a section of pipe in the middle. The bellows can undergo more complicated deformation and absorb large angular displacement and transverse displacement.

For example, the double hinge expansion joint used on the vacuum pipeline of the air-cooled island has a more specific structure. The two hinges cooperate with the bellows to deal with the thermal expansion of large-area horizontal pipelines. You see, for the same angular displacement, the magnitude that a single hinge and a double hinge can absorb is completely different. Therefore, when selecting a model, don't just look at an "angular displacement of 2°", but you have to confirm what structure it is. If you give the manufacturer an angular displacement of 4 °, the double hinge can be easily handled, but if you take the single hinge to carry it, the bellows will become unstable sooner or later.

How to read angular displacement values? Common unit misunderstandings and conversion traps in engineering type selection

Get a copy of the parameter table of the expansion joint. The angular displacement column says "±6°", and you can read it directly as "plus or minus 6 degrees". However, the pipeline layout drawings issued by some design institutes will mark the angular displacement as "6mm/m", which is easy to be confused. 6mm/m is actually the slope, which when converted into an angle is arctan (0.006) ≈ 0.34°. Don't think of it as a simple 6mm displacement.

Think of the unit of angular displacement "°" on the sample as radians. If anyone thinks of 5° as 5 rad, the selected expansion energy saving is twisted into a twist. Remember, the default unit of angular displacement in engineering is degrees, not radians, or millimeters.

Then how to check whether the tonnage given by the manufacturer is correct? Let me tell you a way: you first calculate the total angular displacement required for the pipeline, then look at the allowable angular displacement of each bellows on the manufacturer's sample, and then multiply it by the number of sets of hinges. If it is a single hinge, a bellows carries an angular displacement; If it is a duplex hinge, each set of hinges shares a portion. Don't expect one bellows to eat all the angles.

Type selection practical combat: the cooperation relationship between angular displacement and tie rod and hinge structure, and how to raise capital to manufacturers

The tie rod and hinge are the "seat belt" of the expansion joint. The tie rod is responsible for bearing the internal pressure thrust and preventing the bellows from being pushed open by the pressure; The hinge is responsible for constraining the direction of displacement, so that the angular displacement goes honestly along the designed plane. Without tie rods and hinges, a bellows alone can't absorb angular displacement alone.

Therefore, when selecting and raising capital, you need to provide the manufacturer with these things: pipe diameter, design pressure, design temperature, medium, and the size and direction of angular displacement. The angular displacement must be clearly written as "degrees" and explain which plane it is rotating. It is best to attach a simplified diagram. Don't just say "need to absorb angular displacement". The manufacturer is not a god and can't guess the direction of your pipeline.

The universal corrugated expansion joint itself does not absorb angular displacement, so it can only be carried by the flexibility of the bellows itself, and its life will drop suddenly if it is carried too much. If you want to absorb angular displacement, choose a compound hinge transverse expansion joint or a special hinge expansion joint. If you want to save that little money, it will be more troublesome to replace the corrugated pipe if it cracks in the future.

What is the unit of angular displacement of the expansion joint? The standard answer is one word: degrees. However, understanding the structural logic behind it, the conversion trap and the cooperation relationship with the hinge tie rod will prevent the selection from overturning. Next time I see mm written on the sample, ask more "how to convert it", which is much more reliable than stuffy selection.