Specialized in manufacturing compensators, expansion joints, baffle doors

First, let me answer the most straightforward question: Non-metallic compensators are not universal

It's not like standard flanges or bolts, which can be fitted when you take them to the right size. The design logic of non-metallic compensator (that is, we often call non-metallic expansion joint and fabric fiber expansion joint) is tailor-made around the working conditions-temperature, pressure, medium, displacement and installation space, each of which directly determines the structural selection and material matching.

If you take a non-metallic compensator used in the flue of a power plant and install it on the cold air duct of the cement industry, something will probably happen. It's like buying shoes, size 42 feet will make do with a size 43, but let you run a marathon in rain boots, try it?Are non-metallic compensators universal? The answer is clear: not universal.

Then why do some people always think it's "universal"?

Because from the appearance, the non-metallic compensator is just a circle of skin and frame, which seems to have little technical content. Actually, the skin is a layered structure: fluororubber, silicone rubber, PTFE, ceramic fiber, fiberglass cloth, and each layer has its own task-temperature resistance, corrosion resistance, pressure bearing, sealing.

Although they are all non-metallic expansion joints, the number of skin layers can differ by several layers between those used behind the smoke baffle door and those used next to the high-temperature axial expansion joint. The same is true for rubber compensators and rubber PTFE compensators. The PTFE layer is specially used to deal with strong corrosive media. If you use an ordinary rubber compensator to pass concentrated sulfuric acid, it is not a type selection, but a mine planting for the pipeline.

Let's talk about the displacement compensation ability

Axial expansion, lateral deflection and even angular displacement can be done, which is incomparable to metal corrugated expansion joints. But how much it can absorb depends on the design. Rectangular non-metallic expansion joints are usually used in rectangular smoke ducts, and another set of algorithms is matched with circular baffle doors.

You can't regard all non-metallic compensators as universal parts just because of the nominal "large displacement". The pre-deformation amount, the setting of the guide tube and the stiffness of the frame given in the design all directly affect the actual compensation effect. Two days ago, I met a customer who used the double-hinged expansion joint originally designed for air-cooled island vacuum pipeline on ordinary hot air pipeline. As a result, the vibration exceeded the standard-that thing was metal, not an occasion where non-metallic compensators could be replaced casually. On the contrary, non-metallic ones could not be used everywhere.

The installation location and environment cannot be ignored

Non-metallic compensators are common in desulfurization systems, dust removal systems, power station flues and cement production lines, because of large temperature fluctuations, dust and corrosive gases in these working conditions. But if you use it on pipelines with vacuum requirements, you have to weigh it-the non-metallic skin has limited pressure bearing capacity, and it is easy to deflate under negative pressure conditions. At this time, you may have to look at the vacuum special hose or the external pressure single axial expansion joint.

In other words, before selecting a non-metallic compensator, first list the working condition parameters clearly: what the medium is, how high the temperature is, whether the pressure is positive or negative, whether there is acid-alkali corrosion, which is the displacement direction of the pipeline, and whether there is any space limit in the installation position. Only when these parameters are all together can the manufacturer give you the appropriate skin material and structure. If one parameter is missing, the scheme may deviate.

In the final analysis, the non-metallic compensator is a "non-standard custom" flexible connector

It does look similar in many industries, but material formulations, structural details, and connection methods vary widely. The real professional approach is to find the manufacturer to come up with a plan with the working condition parameters, instead of taking a photo to draw a ladle with a gourd. The national standard JB/T 12235-2015 only stipulates the technical requirements and test methods, and does not say that you can install it everywhere if you buy a standard product.

Are non-metallic compensators universal? — Not universal. But you treat it like a custom piece, and it can dry better than a metal compensator under the right conditions. When it comes to model selection, it is never about looking at pictures, but about data.

Next time someone tells you, "Just buy a nonmetallic compensator and install it", ask him: What is the working temperature? Does the medium contain sulfur? What is the displacement amount? If he can't answer it, you can treat it as a joke.

What is the thrust when the blind plate is blocked? Calculation method of thrust of blind plate of expansion joint

When pipeline pressure test, purging or system isolation, once the blind plate is sealed, the medium pressure directly acts on the cross section of the blind plate. This force is transmitted along the pipe to the expansion joint, the bracket, and the fixing point. You go to the site and look at the pipe sections that have problems-the expansion joint tie rod is bent, the bracket is displaced, and the fixing point weld is cracked-nine times out of ten, the root cause is the blind plate thrust is not clearly calculated.

The essence of blind plate thrust is that the pressure acts on the area to produce an axial force. The formula is spread out in one sentence:F = P × A。 F is the blind plate thrust in N; P is the medium pressure in MPa; A is the effective pressure area of the blind plate in mm². However, in practice, how to take a and which value to use P are all particular.

The formula itself is not complicated, but the complexity lies in the value of the parameter

F = P × A, Anyone can write this formula. The difficulty is: Is the diameter of the blind plate calculated according to the inner diameter of the pipe or the middle diameter of the sealing surface?

In most cases, the blind plate is installed between two flanges, and the medium pressure acts on the gasket sealing position, which is the middle diameter of the flange sealing surface, not the inner diameter of the pipe. Take DN400 pipe as an example. The inner diameter is 400mm, and the medium diameter of the sealing surface may reach 410mm. The area difference between the two is about 5%. On high-pressure pipelines, this gap is converted into thrust of several tons.

If the expansion joint is close to the blind plate, the bellowsEffective areaIt should also be included in the calculation. The effective area of the bellows is the equivalent area of the axial thrust generated by internal pressure on the bellows. This value is directly given in the product sample, and it is taken according to the manufacturer's data, so there is no need to push it yourself. Universal corrugated expansion joint, straight pipe pressure balance expansion joint, compound hinge transverse expansion joint, all types of samples have this parameter. Can't find it? Call the manufacturer to ask for it, don't estimate it yourself.

Different types of expansion joints have completely different force paths

The same blind plate thrust, mounted on the general corrugated expansion joint and mounted on the straight pipe pressure balance expansion joint, the calculation result may be the same, but the force distribution is very different.

The universal corrugated expansion joint itself has no self-restraining ability, and the bellows will produce a great axial thrust under the action of pressure, which is all borne by the main fixed bracket. In other words, if the bracket can't hold it, the expansion joint will be installed in vain. Two days ago, I met a customer. The steam pipeline of DN500 in the workshop was pressure tested, and the blind plate was sealed. The general corrugated expansion joint tie rod next to it was directly bent into a bow, because the bracket was not designed according to the thrust.

Straight pressure balanced expansion joint is different. It relies on its own working bellows and balanced bellows to offset the internal pressure thrust. The blind plate thrust is mainly carried by the equipment itself, and the requirements for fixed brackets are much smaller. The double hinge transverse expansion joint bears the internal pressure thrust by the tie rod, but it can only absorb the transverse displacement, and the blind plate thrust will still be transmitted to the bracket. The structure of external pressure single axial type expansion joint determines that it has strong external pressure capacity, but it can't reduce the thrust of blind plate.

This expansion joint you used, can it withstand blind plate thrust, or can it only absorb displacement? This question is asked wrong, and there is all trouble behind it.

Several Key Corrections in Practice

The test pressure tends to be higher than the operating pressure. For pipelines with a design pressure of 1.0MPa, the hydraulic test may be 1.25 times or even 1.5 times. At this time, the blind plate thrust must be calculated according to the pressure test pressure, not according to the operating pressure. It is quite safe to calculate according to the operating pressure. When you press it, the bracket will be wasted directly.

The temperature correction cannot be ignored either. When high-temperature pipeline is running, the allowable stress of materials will decrease. If the allowable stress at normal temperature is used in the calculation of blind plate thrust, it must be reduced accordingly under high-temperature working conditions. How to take the safety factor? It depends on the type of bracket-the fixed bracket is generally 1.5, and the guide bracket and sliding bracket are 1.1 to 1.2. Don't blindly enlarge it, and don't stiffen a coefficient. How much should be taken according to the actual working conditions.

Example: A DN400 steam pipeline

Assume a DN400 steam pipeline with a design pressure of 1.6MPa and a hydraulic test pressure of 2.0MPa. The inner diameter of the tube is about 400mm, the middle diameter of the sealing surface of the blind plate is 410mm, and the area is about 132000mm². Calculated at 2.0MPa:

F =2.0×132000=264000 N, Approximately 26.9 tonne force.

That's not a small number, right? If a general-purpose corrugated expansion joint is installed on the pipeline, the main fixed bracket must solidly hold the thrust of 26.9 tons; If replaced with straight pipe pressure balance type expansion joint, the force of the support can be reduced greatly. This comparison tells you directly: the calculation of blind plate thrust is not just a mathematical problem, but directly determines the bracket design and expansion joint selection.

Three of the easiest things to overlook on the scene

First, is the temporary bracket near the blind plate strong enough during pressure test? Many temporary brackets are welded with several steel pipes, which look quite stable, but when calculated as thrust, it is completely insufficient. Don't save this.

Second, is the limit device that comes with the expansion joint adjusted in place before the pressure test? The tie rod of the double hinge transverse expansion joint is loose when it leaves the factory, and the nut must be locked tightly before pressure test. If you don't lock it tightly, the pull rod will push the bend directly during the pressure test, and it will be too late to cry.

Third, the blind plate should be positioned a little farther away from the expansion joint, so that the thrust can first pass through the natural corner of the pipe to digest a part. If it can't be avoided, it is necessary to add a temporary fixing bracket between the expansion joint and the blind plate.

Calculation method of blind plate thrust of expansion joint? After all, F = P × A, the formula itself is worthless. What is valuable is that you know when to use which P, which A, and who exactly carries the calculated force. Understand these, and then talk about the design of the bracket and the selection of the expansion joint, so that it is not easy to go wrong.

Nine times out of ten people in the dry plumbing business have dealt with expansion joints. But if you ask what the process principle of non-metallic compensator is, many people will be stunned for a moment, and then say vaguely: Isn't it just a piece of cloth sewn in a circle? Alas, if this is heard by the old master of dry craftsmanship, he can laugh angrily.

Non-metal compensators and metal expansion joints, who does whose work? The metal expansion joint absorbs the displacement by the deformation of the bellows, has high pressure resistance and long life, but it is sensitive to fatigue and has nothing to do with corrosive smoke. Non-metallic compensator is different, it relies on multi-layer flexible fabric to bear displacement, corrosion resistance, high temperature resistance, large compensation amount, especially suitable for large-diameter low-pressure pipeline. The non-metallic compensator can just catch the large displacement and corrosive medium that the metal expansion joint can't carry. The two are not a substitution relationship, but a division of labor relationship-metal work, non-metal really can't do it; Nonmetal work, metal really can't catch it.

Skin is not a piece of cloth: What role does each layer play in a multi-layered composite structure

The skin is the core of a non-metallic compensator, but it is never a single-layer structure. You go and take a look at the non-metallic expansion joint (fabric fiber expansion joint) of a regular manufacturer. The skin is at least four or five layers from the inside to the outside, and each does its own thing.

The innermost layer is a temperature-resistant layer, which directly contacts the medium, either silicone rubber glass fiber cloth or fluororubber composite cloth, which is responsible for one thing: blocking high-temperature smoke. In the middle is the insulation layer, usually with ceramic fiber blanket or glass fiber wool, to trap the heat and prevent it from jumping out. Further outside is the structural reinforcement layer, high-strength glass fiber cloth or polyester canvas, which bears the tensile force and ensures that the whole skin is not torn apart. The outermost layer is a weather-resistant layer, fluororubber or silicone rubber coating, which is anti-UV, anti-aging and rainwater erosion.

How to fit between layers? It's not just brushing glue and sticking it. There is a parameter called peel strength in the composite process. What range will be controlled by regular enterprises? There are test records before leaving the factory. You take a piece of skin that has been used for a year and tear it apart. If the layers are cleanly separated, then the composite pressure is not enough at the beginning, and the temperature is not in place.

Skeletons and frames: What makes soft connections hold up to the pressure? There's something special about it here

The skin itself is soft, so what does it rely on to keep its shape? By the skeleton. The frames of non-metallic compensators are generally divided into two types: one is a metal frame, welded with angle steel and channel steel, wrapped around the skin, and bearing the internal pressure thrust and moment; The other is an internal guide tube, which is inserted inside the pipeline to guide the flow direction of the medium and protect the skin from being directly washed by the airflow.

There must be a gap between the guide tube and the skin. Some people don't understand and think that the tighter you lean against it, the better the seal. On the contrary, the skin deforms when it works, and if the guide tube presses against it, it will wear out in a few movements. The size of the gap is particular, and it is calculated according to the displacement and medium temperature during design, not determined by slapping the forehead.

How to connect frame and skin? Most are compressed with strips and bolts. The spacing between bolts will not be too large, and it is easy to leak if it is too large; It's also not too small, too small to install troublesome. If you look at the four corners of the rectangular non-metallic expansion joint, that position is most prone to problems-there are many skin folds at the corners, and the pressing force is uneven. If you steal a little work, it will start to leak after a year and a half. Therefore, a better manufacturer will thicken the strip at the corner, or use an arc transition, which is a real skill in technology.

From blanking to molding: What are the key steps of the process of non-metallic compensator?

The blanking of non-metallic compensators is not difficult or simple. The round one is easy to handle, just expand it and count it as a fan; The rectangular one is troublesome. The four corners have to be cut, and the incision angle and overlap length are all particular. The material is not cut correctly, and the skin is full of pleats when it is loaded, and it won't take long to crack from fatigue.

In the molding stage, the most critical process is the press-fitting of skin and flange. Before pressing, a sealing strip should be placed on the flange surface, and then the skin should be placed on it, and the tension around it should be adjusted to be uniform, and then it should be gradually pressed with pressing strips and bolts. This "step-by-step" is important-you have to twist the bolts diagonally alternately and press them into place three to four times. If you press them too fast or too hard, the internal fibers of the skin will be unevenly stressed, and the local stretch will be overstretched, which will greatly reduce the life span.

What about welding? To say welding is also a key step. Should frame welds be fully welded or intermittently welded? Most manufacturers do intermittent welding, because the frame will deform during use, and full welding is easy to crack. However, the splicing corners of the frame must be fully welded, where the force is concentrated, and intermittent welding can't bear it. These details, the drawings won't tell you at all.

By the way, there is also the pressure test before leaving the factory. Pressure testing is not as simple as hitting it with an air pump to see if it leaks. According to JB/T 12235-2015 standard, non-metallic expansion joints should be tested for airtightness, and there are clear requirements for pressure holding time and test pressure. Some factories try to save trouble. Even if there is any bulge on the surface of the skin with the naked eye, it is done. What can this find out? The needle eyes and tiny holes in the inner fibrous layer are simply invisible to the naked eye.

Principle: Why is non-metallic compensator the first choice in power plant desulfurization and cement flue?

Say response to use. The temperature of the flue gas in the desulfurization system of the power plant is not extremely high, but it contains sulfur, chlorine and a large number of slurry particles, which is highly corrosive and abrasive. Metal Expansion Joints In this environment, bellows are quickly corroded and perforated. The skin of the non-metallic compensator is fluororubber composite glass fiber cloth, which is naturally acid-and alkali-resistant. In addition, the guide tube separates the airflow, and the use environment is bad. It can still last for three to five years.

What about the cement flue? Mainly due to large dust and large temperature fluctuations. The kiln tail flue gas can reach 400℃, and the temperature changes drastically when the machine is started and shut down. Thermal fatigue cracks of metal expansion joints are common under this working condition. The non-metallic compensator itself is flexible, and the thermal stress can't be transmitted at all. Moreover, the insulation layer of the skin can effectively insulate the temperature, and the frame temperature will not be too high.

There are also those installed next to the desulfurization flue gas baffle door at the flue outlet, which have a narrow space and complicated pipelines. The non-metallic compensator can be made into a rectangular and non-standard shape, which takes up little installation space and is convenient for maintenance. Would you like to try a different metal expansion joint? For such a large bellows assembly, there is simply not enough maintenance space.

To put it bluntly, the process principle of non-metallic compensator is not so mysterious, that is, the word "flexibility" is made to the extreme: the temperature, corrosion and displacement are all digested by multilayer composite materials, and then the pressure, erosion and structural stability problems are covered by metal frames and guide tubes. However, the more simple things sound, the more details hide the door. Those invisible press-fit tensions, compound temperatures, and bolt tightening sequences are the watershed that determines whether it will last for three or ten years.

When you purchased the non-metallic compensator, did you ask the manufacturer about the thermal aging data of each layer of material of the skin? If you haven't asked, remember to ask next time. With this question, half of the suppliers who only know how to fight prices can be screened out.

Who in the dry plumbing business hasn't seen a metal expansion joint? Universal corrugated expansion joint and high-temperature axial expansion joint, these metal parts are indeed the main force in steam and hot water pipelines. But if you put it on the smoke pipe, it will crack for you in less than three months. Why? Because of some working conditions, metal bellows are inherently unable to bear it. At this time, you have to rely on a non-metallic compensator-but don't rush to think of it as a "cheap stand-in", which solves the problem that metal parts can't handle at all.

High temperature and corrosion resistance: the natural enemy of metal bellows, a special session of fabric fibers

The temperature of the flue gas pipeline of thermal power plant, the tail of cement kiln and the original flue of desulfurization system always rises to hundreds of degrees, and the medium is full of sulfide and chloride ions. In this environment, pitting and stress corrosion cracking of metal bellows are common phenomena. If you change to 316L or 254SMO, the cost goes up, but the lifespan is still not good.

The same cannot be said for non-metallic expansion joints (also called fabric fiber expansion joints). Its flexible fabric layer is laminated layer by layer with high-temperature resistant materials such as ceramic fiber and glass fiber, which can steadily hold high-temperature smoke and reduce the surface temperature. A thermal power customer told me that the original metal expansion joint was replaced with a non-metal one once every six months, and there was no air leakage after three years of use.

Non-metallic expansion joint ≠ rubber compensator ≠ PTFE compensator. The temperature resistance of rubber compensator generally does not exceed 100℃, the polytetrafluoroethylene compensator has first-class corrosion resistance but weak pressure, while the energy saving of fabric fiber expansion can be carried above 1000℃, but it can hardly bear the pressure. Before selecting, distinguish whether your pipeline is high-temperature flue gas or corrosive liquid. Don't mix it.

Large displacement compensation and low rebound force: rectangular pipes can only rely on it

For pipes with large thermal expansion, metal compensators are not impossible, but you have to calculate how many waves and how long the guide tube is needed. What if I don't have enough space? Especially for rectangular smoke ducts, have you ever seen a metal bellows with a square section? Even if you do it, that stress distribution is enough for you to have a headache.

The non-metallic compensator has low stiffness, and a fabric loop can absorb tens of millimeters of multidimensional displacement, which can be digested in axial, transverse and angular directions. In the flue gas system of the flue duct of the power station and the tail of the cement kiln, the thermal expansion of the pipeline is so large that the metal compensator can't be put down at all, and the rectangular non-metallic expansion joint is easily handled by the multi-layer structure-the outer fabric holds the medium, the middle insulation layer insulates the heat, and the inner deflector protects the fabric from being washed by the airflow. Think about it, a rectangular air duct is several thousand millimeters wide and has an expansion of several hundred millimeters. How can metal parts compare with it?

One more word, the reaction force of the non-metallic compensator on the pipeline is extremely small, which means that the load of the bracket and the fixed point is small, and the civil foundation doesn't have to be so bulky, which can save a lot of money.

Vibration isolation, noise reduction and tightness: Invisible helpers for operational stability

Pipeline vibration and noise are pain points in the operation and maintenance of power plants and cement plants. When the fan started and the baffle door opened and closed, the metal compensator transmitted the vibration hard, and after a long time, the weld cracked and the flange loosened. The damping characteristics of non-metallic compensators are inherently superior. They can absorb vibration energy and suppress both noise and shock.

And the non-metallic compensator and the baffle door are natural partners. You look at the flue gas baffle door, round baffle door (double seal), electric plug-in insulation door of the desulfurization system, and the flexible connections on both sides of these devices are almost exclusively non-metallic expansion joints. Why? Because the baffle door needs a certain degree of freedom when it moves, the non-metallic compensator is soft and not stuck, and it can also ensure the sealing performance of the system, and the door frame will not be squeezed and deformed due to thermal expansion and contraction. If a metal compensator is used, the displacement when the door panel opens and closes will easily break the bellows.

Pits that are easy to step on when selecting models: Don't just look at the maximum compensation amount on the sample

The non-metallic compensator looks simple in structure, and there are many examples of rollover during model selection.

• Pressure rating: The fabric layer is not a steel plate, and the force it can withstand under positive pressure is limited. If the pressure exceeds 0.1MPa, you have to choose carefully. Use non-metals hard in high-pressure conditions, which is a joke about safety.
• Upper temperature limit: If the sample is written at 500℃, do you think everything will be fine? The medium temperature and the surface temperature of the fabric are two different things. If the thickness of the insulation layer is not correct, the outer surface will still be hot, and the inner fabric will still age.
• Media composition: There are acid and alkali components in the flue gas, the wrong choice of fabric material, and the corrosion is faster than metal. If it contains tar and dust particles, it is necessary to add a deflector plate and a purge port.
• Installation pre-deformation: Non-metallic expansion joints generally have preloading or preloading when they leave the factory. If the on-site installation can't match the temperature, they will be directly hard-installed, and the compensation amount will be completely wasted.

In addition, refer to the national standard JB/T 12235-2015 for non-metallic expansion joints. Don't just focus on the maximum compensation amount on the sample, but also the fatigue life and flange load have to be counted. Some manufacturers give samples that only write "maximum displacement", but that is the limit value, not the allowable value under fatigue life. You choose according to the limit value, and the fabric will crack within a few months of installing it. When the time comes, you will stop working and change parts, and the loss will be great.

To be honest

Non-metal compensators are not universal pieces-high-pressure steam pipes require high elastic reaction force. If you hard on non-metal, it will definitely pull your crotch. On the other hand, non-metals are given priority to pipelines and equipment connections with high temperature, large displacement, low pressure and corrosive media, and the right choice can save a lot of operation and maintenance costs. This is a multiple choice question, not a true or false question. Figure out the working conditions and calculate the parameters. Non-metallic compensators can save you not only money for spare parts, but also the awful time of being woken up by the phone in the middle of the night to deal with leaks.

Cold draw amount of metal expansion joint? Don't laugh, this is not a question that everyone can answer. When the master of dry pipeline installation sees "cold drawn 50mm" marked on the drawing, most of them will scratch their heads: What does this have to do with the compensation amount? Which way? What happens if you pull too much?

Let's get this straight first.

What is cold drawing amount: Understand the difference between it and compensation amount and pre-displacement

The compensation amount is the total maximum thermal displacement of the pipe from cold to hot, which is the design value and is the part of the deformation that the expansion joint is going to "eat". And what about the amount of cold pull? It is an artificial pre-tension (or pre-pressing) of the expansion joint in the opposite direction of thermal displacement during installation. To put it bluntly, it is to "advance" part of the thermal deformation in advance.

Pre-displacement is the broader term and includes cold drawing and cold pressing. Cold drawing is only the most common type of pre-displacement-for thermal expansion displacement of pipes. Don't be confused by these names: the amount of compensation is "to happen" and the amount of cold pull is "you did it ahead of time". One is the formula in the textbook, and the other is the wrench at the installation site.

Why Cold Draw: What is the true displacement of thermal expansion and contraction?

Some people think that the expansion joint is installed and it's done? Why do you pull it cold?

Settle the account for you. A section of 30-meter-long steam pipeline, the working temperature is 200℃, the ambient temperature during installation is 20℃, and the linear expansion coefficient of carbon steel is calculated according to 12×10⁻⁶/℃:

Δ L =12×10⁻⁶ ×30000mm × (200-20) =64.8mm

More than 63mm. If you install the bellows directly from its natural length, it will have to carry a compression of 65mm when it is hot. The wave pitch is pressed to the limit, and the fatigue life is directly cut in half. What about the cold pull half? 32mm pre-compression +32mm hot compression, the cycle amplitude is small, and the fatigue life is doubled up.

How to determine the amount of cold drawing: calculate step by step according to the temperature, length of pipe section and position of fixed bracket

Δ L = α × L × (T_Work-T_Environment)

α is the linear expansion coefficient (about 12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel), L is the length of the pipe section between the two fixed brackets, and T is the temperature difference. After calculating the total thermal displacement Δ L, 50% to 70% of Δ L is taken as the cold drawing amount, which is a commonly used "cold drawing ratio".

• Steam pipeline, temperature difference> 150℃, cold pull ratio 50% ~65%
• Hot water pipeline, temperature difference 80~150℃, cold pull ratio 60% ~70%
• Low temperature or temperature difference

Of course, this is only an estimation method. The project also depends on the position of the fixed bracket, the direction of the pipeline and the number of elbows. If there are several bends in the same pipe section, the thermal displacement is no longer pure axial, and the cold drawing amount has to be corrected according to the results of stress analysis software. You are marked with the amount of cold drawing on the drawing, and you are done with it, but you know how it came from, and you have a bottom in your heart when checking the calculation.

How to operate on site: pre-compression, tie rod, jack, measurement record

The operation process is not difficult to say, but the details are the worst.

In the first step, remove the expansion joint from the package and check that the set nut on the transport protection tie rod is loose. In the second step, the cold drawing direction is determined according to the drawing. When the pipe heats up, it extends, so the bellows is pre-compressed during installation; Pipes are pre-stretched if they are cold shrunk, such as cryogenic pipes.

The third step is practical execution. If the diameter is small and the cold drawing amount is less than 30mm, just screw the tie rod nut directly. If the diameter is large or the cold drawing amount exceeds 50mm, don't screw it hard. Use a jack and an auxiliary frame to push it. Two people on the side will tighten the flange bolts diagonally at the same time, so that the bellows will deform evenly, otherwise the wave pitch will leak later.

Step 4, record. Don't be too troublesome-ambient temperature, pipe wall temperature, length of corrugated pipe before cold drawing, length after cold drawing, actual cold drawing amount and number of adjustment turns of tie rod are all written into the construction record. Which link is wrong, these data are clues to find the cause.

Consequences of mistaken cold drawing amount: bellows instability, bracket exceeding limit, nozzle leakage

Two years ago, there was a project of thermal pipe network, the general-purpose corrugated expansion joint of DN600. During the installation, the workers didn't draw it cold, so they welded it directly. Three months into operation, the bellows cracked at the trough, and the steam blew out far away. When I removed it, I saw that the wave pitch had been pressed together, and the bellows was completely unstable. When the manufacturer sent an engineer to the scene, the first thing he asked was: Has the cold drawing been done?

Even if the amount of cold pull is too large, it won't work. The pre-compression is excessive, and the bellows is close to the yield limit in the cold state. Before it is put into production, the fatigue life has been burned by a large part. The axial force bearing by the fixed bracket will also exceed the design value, and there have been cases where the guide bracket has fallen off.

Bellows instability, bracket force exceeding limit, nozzle leakage-any one of these three things is a shutdown accident.

Cold Drawing Quantity and Matching of Different Types of Expansion Joints

Different structures of expansion joints have different concerns in cold drawing operation.

Universal corrugated expansion jointMost commonly, the structure is simple, just adjust the tie rod nut in place directly when cold drawing. Be careful not to install the deflector in the opposite direction, and the medium should flow in the direction of the arrow.

External pressure single axial expansion jointThe bellows is on the outside of the guide tube and has a protective sleeve on the outside. When cold drawing, you can't see the state of the bellows from the outside, only the scale mark on the outer cylinder. The advantage of this structure is that the bellows is not directly washed by the medium, but you must look at the marking line when pulling it in cold, and don't forget to lock the nut after pulling it in place.

Straight pipe pressure balanced expansion jointThe most particular. It has a balancing bellows and a working bellows inside, which relies on the self-balancing of internal pressure and thrust. If the cold drawing amount is only calculated according to the thermal displacement, and the force on the balance end is not considered, the actual displacement of the bellows after installation will not match the design value. It is suggested that when this model is cold drawn, the bellows on both sides should be pre-displaced at the same time, and only 20% ~30% of the cold drawing amount should be adjusted each time, and the operation should be alternated so that the pressure balancing system should always be in a symmetrical state.

AndCompound hinge transverse expansion jointIt absorbs lateral displacement by the change of hinge angle. There is no such thing as axial cold pulling, but the rotation direction of hinge should be found according to the drawings when installing, and the reason is similar.

Okay, that's it. The formula of this set of cold-pulled things is not complicated, but the difficulty lies in that someone has to be serious in every link. Next time you go to the construction site and see that the bellows is crooked before installation, don't be surprised, that may be the right cold drawing site.