Specialized in manufacturing compensators, expansion joints, baffle doors

Are expansion joints metal or not? Don't answer in a hurry

Is the expansion joint metal? Don't rush to answer "yes", and don't insist on "no". Everyone who has worked on pipelines knows that the expansion joint is not a single material at all. It is a family with various members, including all-metal, pure non-metal, and rubber and PTFE. You only ask "Is it metal?" It's like asking "Is the car burning oil?" The hybrid car shook its head when it heard it.

Expansion joints are both metallic and non-metallic. Metal expansion joints are the mainstream, but non-metallic expansion joints are better in some working conditions. If you only look at the material to draw a conclusion, nine times out of ten, you will step on a pit.

From bellows to non-metallic expansion joints, what is the difference in materials

The core of metal expansion joint is corrugated pipe, which is generally stamped with stainless steel and alloy steel, and absorbs the thermal displacement of pipe by elastic deformation of metal. The general corrugated expansion joint, high-temperature axial expansion joint and corrugated expansion joint for power station industry of this station are all representatives of this road. The bellows can be made into single layer or multi-layer, and the guide tube and tie rod can be added. The more complicated the structure, the stronger the compensation ability.

What about non-metallic expansion joints? Take rectangular non-metallic expansion joints and non-metallic expansion joints (fabric fiber expansion joints) for example. The main body is several layers of fiber fabric, with thermal insulation cotton sandwiched in the middle, and both ends are fixed with metal flange frames. Rubber compensator and PTFE compensator are more direct, even the bellows are not used, all depending on the flexibility of rubber or PTFE material.

As soon as the materials are changed, the performance logic completely changes. Metals rely on ductility to carry displacement, while non-metals rely on fabric deformation and internal structure sliding. Metals can carry hundreds or thousands of degrees of high temperature, but non-metals can't carry that high temperature, but they have good thermal insulation performance, are not afraid of acid-alkali corrosion, and even have a much longer life than metals in some media.

Metal expansion joint and non-metallic expansion joint, what problems do they solve

High temperature, high pressure and large displacement compensation. Steam pipeline, power station boiler air supply system, cement kiln head and tail, the temperature in these places is always five or six hundred degrees Celsius, and the pressure is not low. Only metal bellows can withstand it. For example, the metal corrugated expansion joint in the cement industry is specially designed for this working condition. There are also directly buried (fully buried) expansion joints, which have to bear external pressure and groundwater when buried in the soil, and non-metals can't do this job at all.

What about non-metallic expansion joints? It does not take the high-temperature and high-pressure route, and its home field is large-size, low-pressure, vibration isolation and noise silencing occasions. For example, the desulfurization flue has a cross-section of several meters wide. Do you try it with a metal bellows? Costs are frighteningly high and processing is difficult. At this time, the rectangular non-metallic expansion joint came on the field, which was easily done. Another example is the fan inlet and outlet. The rubber compensator can absorb vibration and reduce noise. PTFE compensator is the nemesis of strong corrosive media. The metal is directly wasted when it meets hydrochloric acid and concentrated sulfuric acid, but PTFE is unscathed.

See? No one is superior to anyone, only whether it is suitable or not. You take the metal expansion joint to catch the corrosive acid, and it will be perforated in three months; Take the non-metallic one to carry the high-temperature steam and burn it through directly. Wrong choice, no one can hold up.

Don't just look at the material when selecting the model. These working conditions are more critical

There is a big circle in front. In fact, the core is one sentence: don't worry about "whether the expansion joint is metal or not", first find out your own working conditions. What is the medium? How hot is the temperature? How stressful? Is the displacement axial, lateral or angular? How wide is the pipe size? Is there enough installation space for tie rods? These are the hard indicators that determine the selection.

Take an example. The same thermal pipeline, with a temperature of 200℃ and a pressure of 1.6MPa, has no problem choosing a metal expansion joint; However, if the medium is sulfur-containing flue gas, the temperature is not high, but it is easy to crystallize and block, then non-metallic expansion joint is more suitable. For example, when the installation space is limited, the rotary compensator and the lateral expansion of the double hinge save energy and a lot of space, but they are all metal. Do you want to use non-metal instead? There are no doors.

Stress is also a key watershed. Non-metallic expansion joints usually withstand low pressure, generally from several kilopascals to tens of kilopascals, and metal expansion energy saving bears several megapascals or even higher. Choosing the wrong material can lead to premature aging or pipeline cracking and leakage. Two days ago, I met a customer, who chose a non-metallic one for cheap. As soon as the operating pressure came up, the fabric bulged and scrapped directly. Therefore, the material is only the appearance, and the working condition is the essence.

Common Myth: Are expansion joints, compensators, metal hoses the same thing?

Many customers call the expansion joint compensator, which is no problem. The two are different names for the same type of equipment. But metal hoses are different. Some people always mix them together. Expansion joint is a section of flexible connection in the middle of rigid pipeline, welded or flanged at both ends, mainly absorbing displacement; A metal hose itself is a bendable tube for use in vibrations, deflections, or situations where frequent movement is required. The price, structure, and use are all different. There are metal hoses, PTFE-lined metal hoses and vacuum special hoses in this station. They are two product lines with expansion joints, so they can't be mixed.

Another myth is to use bellows as expansion joints. In fact, bellows is only a flexible component in the expansion joint, and there must be bellows in the metal expansion joint, but the bellows itself is not equal to the expansion joint. Similarly, a non-metal expansion joint also has a metal frame in it, and you can't say it is a metal expansion joint just because it comes with a metal frame. Therefore, the standard answer to the question "Is the expansion joint metal?" Is two words: look at the model.

Next time someone asks you if the expansion joint is metal, you directly ask him: Which kind are you asking?

Two days ago, the equipment supervisor of a thermal power plant called me, saying that the non-metallic expansion joint at the outlet of their induced draft fan was leaking, and the noise at the scene was so loud that the air volume was obviously not enough. He asked me if I wanted to replace it with a new one directly. I asked him not to rush to change it, and checked it in order, and finally solved the problem for less than 500 yuan. I have seen too many similar cases. Today, I will write the investigation ideas clearly. If you encounter them, just follow them.

Air leakage is no trivial matter: energy consumption and corrosion will come to your door

Let's talk about the harm first, otherwise you don't take it seriously. The most direct impact of non-metallic expansion joint leakage is the increase of fan power consumption. If a part of the sealed flow rate of the flue gas system is missed, the fan has to increase the power to make up. A simple comparative data: for every 1% increase in air leakage rate, fan power consumption increases by about 2% to 3%. It is normal for the air duct matched by a 300,000-kilowatt unit to consume hundreds of kWh of electricity an hour. The extra electricity bill in one year is enough to replace several expansion joints.

Even worse is corrosion. When the flue gas leaks out, along with the condensed water, sulfur dioxide and chloride ions are all attached to the surrounding pipelines and equipment surfaces. We have seen a case where the pipe bracket was next to the air leak point. In less than half a year, the corrosion layer of the bracket peeled off like fish scales. If you really have to wait until the equipment is thoroughly corroded before repairing it, the cost is not to be stopped by another expansion energy saving.

Find the leakage point first: Look at it with a flashlight, and you will know it with the smoke method

There is no need to guess the location of the air leak, it is just those few places.

First, the skin surface. The skin of non-metallic expansion joint is a fabric fiber layer, which is washed and aged in high-temperature smoke for a long time, and is most prone to tiny cracks and even holes. Use a flashlight to stick to the skin slowly, and see that the light transmitting point is the leakage point. When the crack is fine and can't be seen, take a piece of paper and rub it on the surface of the skin. The place that is sucked is the air leakage point.

The second place, the flange seal. Where the flange surface is connected with the steel pipe, the sealing gasket is aged and deformed or not compacted during installation, so the gap will come out. This is not easy to find by looking outside. Take a smoke pipe and walk along the flange to see where the smoke drifts.

The third place, the beading. The bead is used to press the skin tightly on the flange frame. When the bolts on the bead are loose or the bead itself is deformed, the skin will come out of the groove, and the air leakage is not small. None of these locations are difficult to check. It takes ten minutes.

Emergency treatment: Patches and bolts have their own specifics

Find the leakage point, and decide whether to fill or replace it depending on the situation.

Small cracks or pinholes on the skin can be patched with high-temperature repair tape or silicone cloth, which can last for a while. Note that the surface is cleaned before repairing, and the dust and oil stains are not wiped off. The tape is also a white sticker, and it will fall off after a few days. This method is suitable for temporary repair, and it is no problem to last until the planned shutdown for maintenance, but it is not a long-term solution.

If there is an air leak caused by a loose bolt, don't screw it when you come up. Let's first see if there are any signs of displacement on either side of the flange-thermal expansion will pull the bolts loose, especially in the thermal power plant case I mentioned earlier. The normal practice is to loosen the bolt, adjust the natural position of the expansion joint, and then tighten it symmetrically according to the standard torque. Direct hard screwing may deform the pressing strip, or tear the skin folds, which is really the worse the more you repair it.

Radical cure plan: change when you should, don't make up for it blindly

What circumstances don't make up? The skin is aging and hardening in a large area, the surface cracks are densely packed, or the flange and bead have been deformed. At this time, it is not something that can be solved by repair. It's safer to replace it directly with a new one.

Rectangular non-metallic expansion jointIt is suitable for rectangular air ducts, such as the inlet and outlet of induced draft fan and the inlet and outlet of desulfurization tower;Non-metallic expansion joint (fabric fiber expansion joint)It covers circular and special-shaped pipes, and the fabric fiber skin itself is the core sealing layer and displacement compensation layer.

Don't just look at the interface size when selecting a model. The three parameters of medium temperature, working pressure and displacement amount determine the number of layers and material structure of the skin. For example, if the flue gas temperature is 200 °C or less, a conventional combination of silica gel cloth + glass fiber is sufficient; If the smoke temperature exceeds 350℃, fluororubber or ceramic fiber layer should be considered. The pressure is simple. Generally, the pressure of flue gas duct is not high, but it should be noted that the skin is easily deflated under negative pressure conditions, so a supporting skeleton has to be added.

Don't forget matching inspection: lax baffle door will aggravate air leakage

Expansion joint air leakage is sometimes its own problem, but half the probability is related to supporting equipment. Two days ago, I met a customer. The expansion joint was replaced with a new one or leaked. After checking it, the problem was in the upstream and downstream baffle doors.

The desulfurization flue gas baffle door or ordinary flue gas baffle door is not closed tightly, which causes the system pressure fluctuation during operation, and the skin of the expansion joint is repeatedly agitated, and fatigue cracks appear. Double-seal structures such as circular baffle doors (double seals) and double-seal single-axis circular baffle doors, although the sealing grade is high, the wear of the sealing surface must be checked regularly. It is best to open and close the manual plug-in type insulation door or the electric plug-in type insulation door once when the machine is shut down for maintenance to see if there is any sticking. The whole air duct is a system, and the expansion joint is just one of the links. It protects the duct, and you have to protect it.

Routine Maintenance: Spend Your Effort at Usual Times

At the end of the day, non-metallic expansion joints shouldn't wait until there is an air leak to attract attention. During daily inspection, take a flashlight to see if there are any scratches or cuts on the surface of the skin, especially pay attention to whether there are any places scratched by the insulation iron sheet. The torque of the flange bolt is checked every six months. Don't be too troubled. The standard requires that the torque value be as much as it is. Don't rely on the feel. The fixing bolt of the guide tube is also easy to loosen. After falling off, the high-temperature smoke directly washes the inner layer of the skin, and the wear speed is more than ten times the normal.

According to JB/T 12235-2015 standard, in the inspection items of non-metallic expansion joints, the skin condition, flange connection and the integrity of the guide tube are listed. If you do maintenance in the front, it is very common for an expansion joint with passing quality to last three or five years, and the air leakage problem has nothing to do with you. If we wait for the air leak to deal with it, then the money for repairing it is almost enough to buy a new one.

So, what should I do if I encounter air leakage in non-metallic expansion joints? First, reassure yourself and go through it in the above order: find the leakage point, assess the severity, deal with it urgently, change it when you need to, check the supporting equipment, and then adjust the daily maintenance strategy. In most cases, there is no need to go to war at all.

Flue displacement is not "move if you want": first distinguish the three kinds of displacement clearly

Everyone who is engaged in engineering knows that the flue looks like a thick steel pipe, but it is actually very temperamental. Once the high-temperature flue gas passes, the pipeline expands and contracts by heat, and it is common for the flue of tens of meters to elongate by more than ten centimeters. Saying "can the flue be displaced?" Sounds like a simple nod-and-shake question, but actually it is clear what kind of displacement it is-thermal displacement, settlement displacement, or installation error.

Thermal displacement is best understood, that is, when the temperature changes, the tube expands and contracts by itself. The troublesome point of settlement displacement is often that the flue passes through the wall, floor, or is connected to large equipment such as dust collector and desulfurization tower. As soon as the equipment foundation sinks, the flue is forcibly deviated. The installation error is the most frustrating. After the on-site pipeline is welded, it is found that it is out of alignment, and it is forcibly pulled and aligned. As a result, the internal stress is all suppressed on the flange.

The properties of these three displacements are completely different. Thermal displacement has a rule to follow, and settlement displacement depends on geological reports and structural settlement observation data. Installation error belongs to man-made problem. You haven't even figured out what kind of displacement it is, and the rest of the compensation scheme is all useless.

How to calculate the displacement amount? Don't slap your head, look at these three numbers first

Calculating the displacement is not about slapping your head to estimate a "almost". Look at the direction of the pipe first. The longer the straight pipe section, the greater the thermal elongation. The calculation formula is Δ L = α × L × Δ T. The linear expansion coefficient α of carbon steel is 0.012mm/ (m·℃), and the flue gas temperature of a 30-meter flue rises from 20℃ to 200℃, and the elongation is calculated to be about 65mm.

Look at the temperature again-there is a pit here. Many people directly substitute the flue gas temperature, ignoring one detail: there is an insulation layer on the outer wall of the flue, and the actual wall temperature is much lower than the flue gas temperature. Whether the wall temperature or the flue gas temperature should be taken during design is directly related to the size of the expansion joint selection. Finally, look at the position of the fixed bracket. The fixed bracket divides the pipeline into independent compensation sections, and the elongation of each pipe section is the amount that the expansion joint needs to absorb.

It is useless for you to only give a total displacement, you have to tell the manufacturer which section is right for which section. The expansion joint is installed between point A and point B. It only absorbs the thermal displacement of the section A-B, and it can't control the other sections.

What to Absorb Displacement: Don't Think of Expansion Joints as Magic Balm

When it comes to choosing a compensator, I am most afraid of a "general" model conquering the world. The flue is mostly rectangular with low pressure, high temperature and large displacement, so it is the first choiceMetal rectangular expansion jointOrRectangular non-metallic expansion joint。

The advantage of non-metallic expansion joint (fabric fiber expansion joint) lies in the large amount of compensation displacement, the transverse displacement of tens of millimeters can be achieved in a single wave, and it is resistant to high temperature and corrosion. The wet and acid working condition of desulfurization flue is particularly suitable. Metal rectangular expansion joints are more suitable for high temperature and need to bear certain pressure occasions, with strong pressure bearing capacity, but relatively small displacement compensation ability.

Some people say it worksUniversal corrugated expansion jointOkay, okay? No way. The circular bellows is fine for use in circular pipelines. It is hard sleeved on the rectangular flue, and the stress is concentrated at the four corners, so the bellows can easily tear. The flue is the flue, don't use the idea of pipe compensator to trap it.

How does the flue gas baffle door cooperate with the expansion joint? The key is "don't hold back"

The function of the flue gas baffle door is to block or regulate the flue gas flow, and it cannot absorb displacement by itself. There are many problems on the spot, that is, both sides of the baffle door are directly welded, and the expansion joint is installed at the other end of the baffle door and the equipment. As a result, the baffle door becomes a fixed anchoring point, and all thermal displacements are pressed on the door frame. Over time, the door panel is deformed and stuck, and it is not closed tightly.

One side of the flue gas baffle door is provided with a fixing bracket, the other side is provided with a guide bracket, and the expansion joint is arranged on the side away from the baffle door. In this way, the force on the baffle door is minimized, and the expansion joint can work normally. Think about it, after all, the flapper door is a device that needs to be moved. If you sandwich it between two large displacements, isn't that causing trouble for yourself?

There is another detail that is easy to overlook —Round Flap Door (Double Seal)This type of equipment should be installed with sufficient maintenance space. Don't install the expansion joint against the baffle door in order to save land. When the bolts can't be twisted, it's too late to regret it.

Common displacement errors in construction: the fixed bracket is not done correctly, and everything is installed in vain

I have seen many construction sites, and the expansion joint is installed, and the selection is correct, but it leaks soon after running. When I removed it, the bellows was twisted out of shape. The reason is simple – the fixing bracket is not done right.

Fixing the bracket is not just finding a steel beam and welding it. What it does is hold the pipe firmly in place and allow the thermal displacement to go in the direction you intend. If the strength of the fixed bracket is not enough, as soon as the pipe is heated, the bracket deforms first, and the expansion joint is forced to absorb the displacement exceeding the design value, and over-compensation, the bellows will soon be fatigued and cracked.

The guide bracket is also particular. The guide bracket ensures that the pipe can only move along the axial direction and does not allow it to run blindly laterally. If the guide brackets are too spaced apart, the pipe will sag under the action of gravity, and the expansion joint bears a bending moment, and this injury is permanent.

And guess what? Some projects even welded the fixed bracket to the expansion joint and directly welded the compensator to death. Then why install expansion joints? It's better to seal it with a blind plate flange.

Selection and Installation Checklist: Leave Maintenance Space for the Project

After so many years of dealing with flue, summarize a few checking points, and you will basically not make a big mistake by following the inspection.

• First, check whether the design displacement direction matches the type of expansion joint: axial type for axial displacement, hinge type or transverse type for transverse displacement, and compound hinge type for angular displacement. If you choose the wrong type, change it directly.
• Check whether the position of the fixed bracket is consistent with the design drawings, whether the solder joint is firm, and whether it is damaged by subsequent construction.
• Verify that the direction of the sleeve in the expansion joint is consistent with the flow direction of the medium-this direction is wrong, the medium directly flushes the bellows, and it won't take long to wear out.
• See if there is any reserved maintenance space around the expansion joint, and whether the outer protective cover of the bellows is pressed down by the insulation layer.
• In the wet smoke environment of non-metallic expansion joints, it is necessary to check whether the skin is aging and cracked, and whether there is condensate water on the surface that penetrates into the insulation cotton.

In addition, when the expansion joint is installed, the tie rod or transportation fixture preset by the manufacturer must be removed after the pipeline hydraulic test and heat insulation are completed. If it is dismantled early, the installation displacement will be consumed in advance, and there will be no margin for trial operation. It is dismantled late, and the expansion joint will not work, which is no different from a rigid pipe.

Can the flue be displaced? It can be done, but first you have to distinguish the type clearly, calculate the displacement clearly, select the correct compensator, and make the bracket solid. The four links are interlocked, and if one is missing, there will be problems. Don't wait until a leak is repaired, when the rework cost will be enough for you to reinstall several sets of expansion joints.

When the customer called, he asked in the first sentence: What material is cheap and has good sealing for the expansion joint? To be honest, every time I hear this question, I have to ask first: What medium goes in your pipeline here, what is the temperature and what is the pressure? It wasn't an argument, it was a question that really couldn't be answered in one sentence.

As soon as I came up, I rushed to "the cheapest" to choose the material, and eight out of ten turned over the car behind. Let's break up several common materials and crush them into pieces. After reading it, you will know what to choose.

First, recognize these materials

The expansion joints on the market are roughly divided into these categories according to their materials.

• rubber compensator: Suitable for low pressure, normal temperature, large diameter pipeline, mainly depends on elastic deformation of rubber to absorb displacement, the price is generally the lowest.
• Non-metallic expansion joints (fabric fiber expansion joints): It is made of multi-layer fabric fiber composite, and its high temperature resistance is better than that of rubber. It is commonly used in low pressure and high temperature occasions such as flue gas and air duct.
• Universal corrugated expansion joint (metal): Typical stainless steel bellows structure, with strong pressure and temperature resistance, is the main force in complicated working conditions.
• PTFE compensatorAndPTFE-lined hose: PTFE material is resistant to strong corrosion, but the price is also expensive.

In addition to the above, there are various special products such as metal hoses, metal rectangular expansion joints, high-temperature axial expansion joints, etc. It is also to compensate for the displacement of the pipeline. The material is different, and the price can be several times or even ten times different. But the key is not who is cheap, but whether it is suitable or not.

Second, where is the price gap?

Under the same caliber and pressure level, the price ranking is roughly like this:

Rubber compensator cheapestNon-metallic expansion joints are second, general-purpose corrugated expansion joints are more expensive, and PTFE compensators and PTFE-lined hoses are the most expensive. The root reason for the expense lies in the material cost and manufacturing process. PTFE materials are difficult to form, and metal bellows need precision welding and heat treatment, so the price will naturally go up.

However, cheap does not equal a good deal. Rubber compensator does save money on normal temperature water, but if it is used to pass 130℃ steam, the life may be shortened from five years to five months, and the cost will be higher after repeated replacement. Don't just focus on the purchase unit price, the comprehensive life and replacement frequency are reliable.

Third, what does sealing depend on?

The sealing performance is good or bad, the material is the foundation, but the structural design is the key.

The non-metallic expansion joint is compounded by multi-layer fabric fibers and sealed by flange compression. This structure performs well on low-pressure flue gas pipes, but if you force it on high-pressure pipes, the fiber layer will blow through, which is embarrassing.

The medium is guided away to reduce the direct erosion of the fluid to the inside of the bellows and reduce the risk of the bellows being worn out. Think about it, once the bellows breaks and leaks, the whole pipe suffers.

One more word, likeDouble-sealed single-axis circular baffle doorThe sealing effect of this kind of product with double seal structure is much more stable than that of ordinary single seal, which is of great significance to the strict air duct or flue gas system.

Fourth, how to choose?

If the large-diameter pipeline with low pressure and normal temperature is not afraid of corrosion,rubber compensatorOrNon-metallic expansion jointThe price/performance ratio is really high, the price is cheap, and the sealing performance is sufficient.

If the temperature and pressure are high and the medium is corrosive, don't save this money. Choose honestlyUniversal corrugated expansion jointOr simply onPTFE compensator。 Although the initial investment is large, the seal is reliable and durable. For example, the steam pipelines in the power station industry and the kiln head and kiln tail air ducts in the cement industry have very bad working conditions, and the requirements for sealing and durability are extremely high. Generally, metal corrugated expansion joints or high-temperature axial expansion joints with corresponding working conditions are directly used.

Some working conditions are in the middle, such as the flue gas pipeline of desulfurization system, which is not high in temperature but corrosive, so it has to be comprehensively considered. The corrosion resistance of PTFE is indeed good, but if the diameter is large, the non-metallic expansion joint is matched with a suitable anticorrosive material lining, which is more economical and practical. Therefore, there is no universal answer to the selection, and it must be determined according to the working conditions one by one.

Don't use a rubber compensator to top high-temperature steam。 If there is such a plan to save money, it will leak in a few days. When the time comes, the loss of shutdown for maintenance will be enough to buy hundreds of compensators.

5. Finally, pour cold water on a basin

Whether the seal is good or not depends half on the installation. This point goes to the root.

The flange bolts must be tightened evenly and alternately diagonally, and the other side cannot be tightened after one side is screwed to death. The pipeline cannot be biased, and the pre-deformation of the expansion joint must be done according to the requirements given by the manufacturer, so do it right.

Two days ago, I met a customer who said that the expansion joint I bought was leaking, so let us pay for it. As a result, after the sales visit, the bellows was burned by welding slag during installation. Did you say that this matter was caused by material? Not. What's even more outrageous is that some people hoist them with wire ropes directly on the bellows, and the good products are just scrapped.

No matter how expensive the material is, if the installation is not standardized, it will still leak. Save what don't save installation, don't make do with crooked pipelines. Rework when something goes wrong will not only spend more money, but also delay production. Many customers come to the factory to discuss explanations after leaking them. In all probability, the bellows was screwed or bruised during installation, which is not a matter of material at all.

What material expansion joint is cheap and has good sealing performance? The answer is never on the price list, but in your own working condition parameters and installation quality. Know the materials, understand the working conditions, and then take the installation seriously, and the money is worth it.

Find out first: the expansion joint is fixed, is the pipeline fixed or the expansion joint itself?

Many customers came up and asked, "How to fix the metal expansion joint?", and then planned to hold the expansion joint body with a bracket. This is a mistake of principle. The expansion joint itself is a flexible element used to absorb the thermal displacement of the pipe. If you hold it tightly down, how can it deform? Doesn't that mean pretending for nothing?

What really should be fixed is the pipe. The pipes on both sides of the expansion joint need to be firmly locked by the fixed bracket, so that all the displacements caused by thermal expansion and contraction are concentrated in the "buffer zone" of the expansion joint. On the contrary, the expansion joint body cannot bear any external restraining force, it only needs to expand and contract freely along the displacement of the pipe. In other words, the fixed bracket holds the pipe, the guide bracket holds the direction, and the expansion joint only deforms.

How to arrange the fixed bracket and the guide bracket? Distance, force and position are all special

The layout principle of the fixed bracket, in one sentence: divide the pipe into several independent compensation pipe sections. There can only be at most one expansion joint in each pipe section, and the fixing bracket must be arranged at both ends of the expansion joint, and the distance should be close enough. Why? Because if the fixed bracket is too far from the expansion joint, the friction force of the middle pipeline itself and the blind plate force generated by the medium pressure will be applied to the expansion joint, which will cause the compensation amount to fail at least and the bellows to crack at worst.

Taking the common general-purpose corrugated expansion joint as an example, the distance between the first guide bracket and the end of the expansion joint should be controlled within 4 times the nominal diameter, and the distance between the second guide bracket and the first guide bracket should not exceed 14 times the nominal diameter. Intermediate guide brackets should be set at intervals at the back. The specific distance depends on the stiffness and allowable deflection of the pipe. This is not a head-slapping number, it comes from the pipe system stress analysis. CAESAR II is commonly used in engineering for checking, but you have to know at least these basic limits.

In terms of force, the fixed bracket should be able to withstand the blind plate force generated by the medium pressure, the friction force of the pipeline, the elastic reaction force of the expansion joint and the weight of the pipeline. Blind plate force is a big head. The higher the pressure and the larger the caliber, the greater the force. Therefore, large-diameter thick-walled expansion joints, fixed brackets of the level of corrugated expansion joints used in power station industry, and concrete foundations must be calculated by tons, so they must not be saved.

The tie rod/nut on the expansion joint should be removed or not during installation? How to tune it?

It's a high-frequency problem. The expansion joint leaves the factory with tie rods and nuts, mainly to prevent the bellows from being bruised and strained during transportation and hoisting. Once installed in place, the tie rod nut must be loosened to allow the expansion energy saver to deform freely. If you don't loosen it, the expansion joint will be locked, and the displacement of the pipe will be stuck in the system, and the flange, weld and equipment mouth will all suffer.

When will the tie rod be used? Two scenarios. First, if the expansion joint needs to bear the blind plate force generated by internal pressure, and no pressure balanced expansion joint is designed, the tie rod can be used as a restraining element, but note that the tie rod at this time is not for you to tighten, but is adjusted to the allowable displacement range. Second, the working conditions that require pre-stretching or pre-compression before installation are realized by the tie rod nut. The adjustment method is: first loosen the locking nut, rotate the adjustment nut to compress or stretch the bellows to the specified amount, and then tighten the locking nut. The specific pre-displacement depends on the difference between the installation temperature and the calculated temperature of the pipeline. Don't screw it blindly.

How to adjust the expansion joint tie rod nut, we specifically talked about it in our previous article, and here we emphasize the core-after the adjustment, the locking nut must be tight back, otherwise the vibration during operation will cause loosening and the tie rod will lose its function.

Taboo: Take the expansion joint as a pipe support and hanger. These wrong installation methods should be avoided

The most typical mistake I have seen is that someone welds the pipe bracket directly to the expansion joint bellows, or uses a pipe clamp to pick up the expansion joint body to bear the load. The wall thickness of corrugated pipe is usually only a few tenths to a few millimeters. If you use it as a hanger, who will perforate if it doesn't perforate?

Another type of error is that the guide bracket is installed in the wrong direction. The role of the guide bracket is to limit the radial displacement of the pipe and allow axial displacement. As a result, the snap ring of the guide bracket and the pipe were welded to death on the spot, or the gap was left too small. As soon as the pipe expanded by heat, the guide bracket became a fixed bracket, and the expansion joint was crushed to death. The correct method is: the inner diameter of the snap ring of the guide bracket should be slightly larger than the outer diameter of the pipe, leaving an axial sliding space, but the gap should not be too large, otherwise it will not play a guiding role.

In addition, when installing the expansion joint, don't forcibly use the expansion joint to compensate for the installation error in order to adjust the pipeline counterpart deviation. Corrugated pipe has high requirements for coaxiality, and if it is forcibly installed in misalignment, the local stress will increase exponentially, and it will not take long for fatigue cracking.

Differences in Fixation of Different Expansion Joint Types: What to Pay Attention to for Axial Type, Transverse Type and Pressure Balance Type

Different types of expansion joints, the arrangement logic of fixed brackets and guide brackets is different.

Axial expansion joints, such as general corrugated expansion joints and external pressure single axial expansion joints, mainly absorb axial displacement. The fixed brackets at both ends must firmly lock the pipeline, and the guide brackets must be arranged in the middle according to the aforementioned spacing to ensure that the expansion joint expands and contracts along the axis direction.

Transverse expansion joints, such as double-hinged transverse expansion joints and double-hinged expansion joints for air-cooled island vacuum pipelines, absorb lateral displacement through hinge or universal joint structures, but do not absorb axial displacement themselves. Such expansion joints usually need to be used in groups of two or three. The layout of the fixed bracket should especially consider the bending moment generated by lateral displacement, and the direction of the hinge pin should be consistent with the displacement direction, otherwise the hinge will be pinned and the expansion joint will be wasted.

Pressure balance expansion joints are even more interesting, such as straight pipe pressure balance expansion joints, curved pipe pressure balance expansion joints and compound straight pipe bypass pressure balance expansion joints. They come with their own balanced bellows, which can offset the blind plate force generated by internal pressure, so the force on the fixed bracket can be greatly reduced. However, the guide bracket still cannot be saved, because the balanced expansion joint is more sensitive to lateral displacement and angular displacement, and once deflection occurs, the balancing cavity is easily unstable.

All fixing brackets and guide brackets must be in place prior to installation of the expansion joint. During the pipeline hydraulic test, the fixed brackets at both ends of the expansion joint should bear the blind plate force under the test pressure, which is greater than normal operation, so the welds and anchor bolts of the brackets must be re-checked before the test.