Specialized in manufacturing compensators, expansion joints, baffle doors

I met a customer two days ago. I just installed a batch last yearflue expansion jointIt leaked before it was used for a full maintenance cycle. When I removed it, I saw that the guide tube was washed up by the smoke like a knife-it was installed backwards. No one looked at the direction of the arrow. You say it was wrong or not? There is no problem with the equipment itself. As soon as there is a mistake in the installation link, it will be completed later. Today, break these five steps into pieces and explain them clearly. Follow them, and at least step on half the pit.

First, recognize the model number and arrow direction before installation

Don't rush up when you get the expansion joint. One glance at the nameplate, one glance at the arrow. That arrow is not an ornament, it represents where the medium flows. Once the orientation of the guide tube is reversed, the high-temperature dusty flue gas directly washes the root of the bellows, and it will wear out in less than a few months. Common ones areMetal rectangular expansion jointAndNon-metallic expansion joints (fabric fiber expansion joints)— — The former carries high temperatures, such as the flue of power station boilers; The latter is corrosion resistant, and the wet flue gas working conditions in the desulfurization system all depend on it. If you select the model correctly, you must confirm that the arrow is consistent with the actual pipe flow direction before installation.

In addition, the nuts on the tie rods are in a pre-tensioned state when they leave the factory. In order to protect the bellows from being stretched or compressed during transportation, the manufacturer will adjust a fixed position. Don't loosen it in advance, and adjust it according to the drawings when the hoisting is in place.

Second, bracket arrangement is the real technical activity

Many people think that the expansion joint is a flexible joint, and it is finished by welding it. And the result? Once the pipe heats up, the bellows are either jacked to death or pulled apart. The problem is with the bracket arrangement. Both sides of the flue expansion joint must be setFixed bracketAnd in the middleguide bracketEnsure that the displacement only goes in the design direction.

Used by a cement plantMetal Corrugated Expansion Joints in Cement IndustryNo guide brackets were installed at both ends, and the thermal expansion twisted the bellows into a twist, which was scrapped in two months. The strength of the fixed bracket should be able to withstand the pressure thrust of the pipeline. The expansion joint itself does not bear this force, it is only responsible for absorbing displacement. Don't let the expansion joint carry it on its own, it can't carry it.

3. Hoisting alignment and welding sequence

Never use wire rope to directly strangle the corrugated part when hoisting. When the wire rope is strangled, the corrugation is deformed, the local stress is concentrated, and the life is directly discounted. The correct way is to hold the end tube with a sling and lift it smoothly. The gap between counterparts should be uniform, generally controlled at 2~4mm, too big or too small.

Weld one end first and naturally cool to room temperature before welding the other end. Both sides are welded simultaneously or continuously, and the thermal stress will deform the bellows. If it is a flange connection, tighten it symmetrically with a torque wrench, and the sealing gasket should be temperature-resistant and corrosion-resistant-non-metallic expansion joints are usually equipped with fluorine rubber pads, so don't use ordinary asbestos pads for cheap. Don't rush to remove the tie rod after welding, wait until the pipeline system is completely fixed.

4. Cold tightening amount and tie rod adjustment

The operating temperature of the flue is always several hundred degrees Celsius, and the thermal expansion of the pipe is very large. Therefore, when leaving the factory, the expansion joint often hasPre-stretchingOrPre-compressionThis value is called cold tightness. The tie rod nuts must be adjusted according to the values specified in the design drawings during installation. Tighten the locking nut after adjustment to prevent loosening.

After the system heats up to the design temperature, the transport rod needs to be removed. How to dismantle it specifically? This site has a special question and answer "Does the screw of the expansion joint need to be disassembled", which is very clear. Anyway, remember: without removing the tie rod, the bellows can't expand and contract freely, and the pressure is all held on the corrugation, which will bulge soon.

V. Post-installation acceptance and common pits

An airtightness test must be done before smoking. Focus on checking the weld and flange surface, and use soapy water or leak detector. During trial operation, observe whether the displacement is smooth, and stick your ear to the pipe to listen to any abnormal noise. If it is found that one side of the expansion joint is bulging and the other side is overstretched, 80% of it is that the guide bracket is stuck or the tie rod is not adjusted in place.

Expansion joints near desulfurization flue gas baffle doors are particularly prone to corrosion. Wet flue gas contains sulfuric acid and sulfurous acid, and the corrosion rate is several times faster than dry flue gas. SoDesulfurization flue gas baffle doorThe matching expansion joint must be checked to see if the anti-corrosion coating is intact. Even if a small piece of the coating falls off, it can be pierced in half a year.

Install this work, the details make or break. The drawings do not match the actual object? Then stop and ask, don't push it. Find a reliable manufacturer (such as us), and do a thorough technical briefing, which will be much less worried later.

1. Core question: Will the expansion joint of desulfurization flue expand and contract? The answer is yes, but the key depends on how you understand "telescoping".

Two days ago, a customer came to me and asked, "Does the expansion joint of desulfurization flue expand and contract?" I asked him back, "What do you understand about expansion and contraction?" He was stunned for a moment and said that it was like a spring to pull and contract. In fact, the working conditions of desulfurization flue are not so simple.

The expansion joint will, of course, "expand and contract", but this "extension" does not necessarily mean axial elongation, and "contraction" does not necessarily mean axial compression. In the desulfurization system, the diameter of the flue is often two or three meters, and the temperature rises from normal temperature to 150℃ or even higher. The thermal expansion of the flue wall is calculated in millimeters, but what is more troublesome is the relative displacement between pipes-angular, lateral and even torsional. So,Does the desulfurization flue expansion joint telescope?The answer is: it must be able to adapt to multi-dimensional displacement, not just a single direction of expansion and contraction.

Look at our product listMetal rectangular expansion joint、High temperature axial expansion joint, andNon-metallic expansion joint (fabric fiber expansion joint)The structure is designed to absorb axial and radial displacements. So don't expect the expansion joint to go straight like a rubber band, it's more like a joint, flexible but not blind.

Second, why do you have to scale? -Those invisible thermal expansion, contraction and vibration displacement in the desulfurization flue

When the desulfurization system is in operation, fluctuation of flue gas temperature is the norm. Although the net flue gas temperature after gypsum slurry washing has dropped, the original flue gas section may be as high as 160℃. The flue is made of steel structure, and the linear expansion coefficient is about 12×10⁻⁶/℃. You calculate: a 30-meter-long flue, when the temperature rises by 100℃, will elongate by 36mm. If this force is held hard, it will take minutes for the weld to crack and the bracket to deform.

In addition to thermal expansion and contraction, there is more covert-fan vibration. When the induced draft fan and the booster fan are started, the vibration frequency is not high but the amplitude is large, and the flue shakes with it. Coupled with the flue self-weight, wind load, seismic load... these displacements are superimposed, and the elasticity of the pipeline itself can't bear it at all.

What about that? Expansion joints are meant to "eat" these displacements. For example, our commonUniversal corrugated expansion jointThe bellows can absorb axial displacement through compression and stretching of the corrugations;Compound hinge transverse expansion joint andCurved tube pressure balance expansion jointThen specifically deal with lateral and angular displacements. To put it bluntly, a flue without expansion joints is like a car chassis without shock absorption.

Third, metal or non-metal? Material Selection and Structural Difference of Expansion Joint in Desulfurization Flue

This question is chosen wrong, and the post-maintenance costs can make you cry. The desulfurization flue is a typical "three highs and one low" environment: high temperature (above 100℃ in some areas), high humidity (close to saturation), high corrosion (SO₂, HCl gas and condensed acid), and low pressure (slightly positive pressure or negative pressure).

Metal expansion joint(e.g.Corrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement Industry) has the advantages of high strength, strong pressure bearing capacity, and can adapt to large displacements. But the downside is also terrible: corrosion. After desulfurization, the net flue gas humidity is high, the pH value of condensed water is low, and pitting corrosion may occur in 316L stainless steel. We generally suggest addingPTFE-lined hoseOr spray an anti-corrosion coating protective layer.

WhileNon-metallic expansion joint(includingrubber compensator、Rubber PTFE compensator、PTFE compensatoras well asNon-metallic expansion joint (fabric fiber expansion joint)) is becoming more and more popular in desulfurization flues. Why? Because non-metallic materials are inherently corrosion resistant and flexible, they can absorb multi-dimensional displacements. Especially like fabric fiber expansion joint, the main body is made of silicone cloth, glass fiber wool, polytetrafluoroethylene film, etc., and the temperature resistance can reach 250℃, and the inner line is a guide tube (refer to our question and answerSpecific Function of Expansion Joint Guide Tube) It also prevents airflow from directly washing the skin. However, the short board of non-metal is low pressure. Fortunately, the desulfurization flue is a low-pressure system, which matches perfectly.

If the temperature exceeds 250°C and the pressure is greater than 0.1 MPa, preference is given to metal expansion joints with anti-corrosion liners; If the temperature is within 200℃ and the pressure is low, non-metals are the king of cost performance. From our stationRectangular non-metallic expansion jointThere are many application cases in desulfurization system of thermal power plant, and the effect is stable.

Fourth, everything will be fine if the expansion joint is installed? Common Failure Modes and Maintenance Points

Many customers think that they can be installed for ten years, but accidents happen in two years. The most failures I encounter are three:

• Corrosion perforation: If the metal bellows is not treated with anti-corrosion treatment, condensed acid will deposit at the bottom of the corrugation and leak quickly. So chooseUniversal corrugated expansion jointAsk about the material and processing process.
• Fatigue cracking: Frequent vibration leads to stress concentration in bellows. Such asLarge tie rod expansion jointIf the tie rod nut is not aligned during installation (see Q&AHow to adjust the tie rod nut of expansion joint), will limit the displacement and instead accelerate fatigue.
• Non-metallic skin tear: The reason is often that the design of the guide tube is unreasonable or the airflow rate is too high. We have a case where a power plant usedNon-metallic expansion jointThe wind speed exceeded 15m/s. As a result, the end of the guide tube was not fixed, and the skin was worn out in three months.

Regular inspection, pay attention to whether there are corrosion marks or abnormal deformation on the surface of the expansion joint; Check that the tie rod is loose (ref.Do you need to remove the screw of the expansion joint, the transport screw after installation and commissioning must be removed); For non-metallic ones, see the skin for bulging or aging cracks. In addition,Service life of expansion jointThe general design is 10-15 years, but the actual depends on the working conditions. Suggested cooperationFlue gas baffle doorAndDesulfurization flue gas baffle doorRealize maintenance and isolation, and facilitate shutdown and replacement.

At the end of the day,Does the expansion joint of the desulfurization flue expand and contract？ It must stretch and shrink, but more importantly, choose the right type, install the right position and take care of it regularly. Understand these points, and select models without stepping on pits.

How to remove square flue expansion joint? Look at the type before you do it

Two days ago, a customer called and said that a square flue expansion joint in the factory was leaking, and he was anxious to change it. As a result, it was stuck halfway through the dismantling-the bolts were rusty, the tie rod couldn't be twisted, and the fabric layer was torn. He asked me: How should you dismantle this thing when you sell it?

Good question. How to disassemble the square flue expansion joint is really not just a matter of twisting it with a wrench. Before dismantling, regardless of the type and working conditions, hard dismantling will only pull and deform the flange surface, and even make the pipe bracket crooked. Today, I will explain the two common methods of disassembling square expansion joints, breaking them apart and crumbling them.

First, there are two main types of square flue expansion joints: metal rectangular and rectangular non-metal

Metal rectangular expansion jointAndRectangular non-metallic expansion joint。

Metal rectangular expansion joint, structurally is corrugated pipe plus flange, with tie rod and nut. Used for high-temperature and high-pressure flue gas pipelines, such as power station boiler outlets and desulfurization flues. The material of corrugated pipe is usually stainless steel, and the temperature resistance can reach above 600℃.

Rectangular non-metallic expansion joint, the main body is a fabric fiber layer (non-metallic expansion joint, also called fabric fiber expansion joint), with a guide tube inside and clamped by a metal frame outside. It has lower temperature resistance, but a large amount of compensation, which is suitable for low-pressure, large-size flues, such as dust collection pipes in steel mills.

These two things, the disassembly method is completely different things. You take the idea of dealing with metal parts to dismantle non-metal, and the fabric layer is ripped off as soon as you pull.

2. Three issues that must be clearly understood before disassembly: medium, pressure and bolt state

Before you start, ask yourself three questions. If you can't answer them, don't touch the wrench.

First, what is the medium?Is it hot smoke, cold air, or corrosive gas? If it is wet flue gas after desulfurization, which contains sulfuric acid condensate, the metal bolt may have been corroded so that only one layer of skin is left, and it will break when twisted.

Second, is there still pressure in the tube?Some maintenance drawings save trouble, and the flange is removed before the system is completely relieved. If the square flue expansion joint is disassembled under pressure, the flange surface suddenly separates, and the high-pressure gas will collapse the bolt like a bullet. It's no joke, every year there are cases of injuries as a result.

Third, what is the current state of the bolt?First, take a flashlight to see if there is rust, deformation, and whether the tie rod nut has been welded to death. There's a simple way to judge: tap the head of the bolt with a wrench and listen to the sound-crisp instructions have room for loosening, stuffy instructions have rusted to death.

3. Disassembly of metal rectangular expansion joint: loosen the tie rod first, and then disassemble the flange

There is a core principle in the disassembly sequence of metal rectangular expansion joints:Release the pre-tightening force first, then remove the connecting bolts。

Step 1: Loosen the tie rod nut.The metal rectangular expansion joints are equipped with tie rods, which are used to limit the excessive stretching of the bellows. When disassembling, first loosen all the nuts at both ends of the pull rod, so that the bellows is in a free state. Be careful not to dismantle all, leave one or two buckles to prevent falling off.

Step 2: Diagonal method to remove flange bolts.Many people are used to dismantling them one by one from one side. As a result, the flange is unevenly stressed, and the remaining bolts are stuck. The correct way to do this is: first remove the two bolts of the diagonal, then remove the other pair of diagonals, step by step like removing a tire. Every time you remove one, check the flange surface for misalignment.

Step 3: Top off the flange surface.After the bolts are all removed, the flanges may still stick together because the gaskets will carbonize and bond after high-temperature operation. Gently wedge with a flat shovel from the flange gap and shake left and right to separate the two flange surfaces. Don't smash the flange ear plate with a sledgehammer, which will deform the bolt hole.

The worker cut the bolt directly with a cutting gun. As a result, the cutting flame burned through the bellows and the whole expansion joint was scrapped. The wall thickness of the corrugated pipe of the metal rectangular expansion joint is usually only 0.5~1.5mm, and it is worn as soon as it is ironed.

4. Disassembly of rectangular non-metallic expansion joint: Pay attention to the fabric layer and guide tube

Rectangular non-metallic expansion joints are much more delicate. It is a metal press frame on the outside, a fabric fiber layer on the inside, and a flow guide tube in the middle.

Step 1: Remove the bolts of the press frame.First remove the metal press frame bolts that hold down the fabric layer. Note: This kind of bolts are usually large in number and closely spaced, so it is easy to leak when disassembled by one person. It is recommended to use a marker to make a mark between the press frame and the flange, so that it can be aligned when put back.

Step 2: Carefully separate the fabric layers.After using the fabric fiber layer for a period of time, it will be smoked hard by smoke and lose its elasticity. Do not pull hard when disassembling. Use scissors to cut off the attached spacer along the edge of the press frame. If there are tar-like deposits, spray some loosening agent first.

Step 3: Take out the guide tube.Rectangular non-metallic expansion joints usually have a guide tube inside, which is used to guide air flow and protect the fabric layer. The guide tube is inserted in the flue. When pumping out, it should follow the axial direction and not skew, otherwise it will scratch the anticorrosion layer on the inner wall of the flue.

If the fabric layer of non-metallic expansion joint has been broken, the insulation cotton inside will leak out. Before dismantling, confirm whether there is any insulation cotton leakage. If so, you need to wear a dust mask and goggles first-the glass fiber sticks to the skin and itches unbearably.

5. What about the bolts that can't be removed? Several practical treatment methods

When removing square flue expansion joints, screw rust is the most common pit. I'll list a few measured methods that work:

• Osmotic loosening agents.Spray WD-40 or special bolt loosener on the root of the thread and wait 15 minutes before screwing. If it is a high-temperature bolt, it works better with bolt coolant (instantaneous cooling creates cold shrinkage gap).
• The tapping method.Use a copper or wood hammer to tap the side of the bolt head a few times to shatter the rust layer. Don't use a hammer, it will round the bolt head.
• Heating method.Heat the nut part with oxyacetylene flame (be careful to avoid bellows), use the principle of thermal expansion and contraction to expand the nut, and then twist it quickly. This method can be used for the tie rod nut of the metal rectangular expansion joint.
• Grooving method.If it can't be twisted, use an angle grinder to cut a straight slot in the head of the bolt, and then screw it with a large screwdriver. This is a destructive means. After removing the bolts, it is wasted, but it keeps the flange.

Don't try to clamp the bolt rod with a pipe plier and turn it hard, which will break the screw rod and make it harder to remove it if it breaks in the bolt hole.

6. Don't rush to suit after dismantling, check these parts first

It was finally removed, so don't try to put on new pieces quickly. Spend 10 minutes checking the following places to save you the hassle of reworking later.

Is the flange surface flat?Lean against it with a ruler to see if there is any deformation. The flange of the square flue expansion joint may warp after long-term use because it is subjected to thermal stress. If the deformation exceeds 1mm/m, the new expansion joint will not be sealed tightly.

Are the bolt holes elongated?The bolt holes will become oval at high temperatures. When this is found, it is necessary to ream the hole with a reamer or re-drill the hole after repair welding.

Is the deflector intact?If it is a rectangular non-metallic expansion joint, check the removed deflector for wear. The head of the guide tube is the easiest to wear. Once perforated, the airflow will directly scour the fabric layer, resulting in the newly replaced expansion joint breaking in half a year.

Is there a settlement in the bracket?Settlement of the pipe supports at both ends of the expansion joint can cause the new expansion joint to be forcibly twisted when installed. Measure the actual distance between the two flanges, and compare it with the length of the new expansion joint. If the error exceeds 5mm, the bracket should be adjusted first.

In the final analysis, the disassembly of the square flue expansion joint depends on strength and experience. You have mastered these steps, and the next time you meet them, you will have the bottom in your heart. If you encounter particularly tricky working conditions-for example, all the bolts are rusted in the blind hole and the flange is seriously deformed-you should find technical support from the manufacturer, and don't carry it hard.

Is the flue expansion joint installed without distinction between positive and negative? I told him that it was a score. Not all expansion joints have a direction, but once there is a direction requirement, they are installed backwards, ranging from wear and air leakage to the whole section of corrugation tearing. Let's break it up and break it down today.

Why do some flue expansion joints have to be oriented?

The key is in the deflector. This thing fits inside the corrugation, like a bell mouth, and functions to guide the flow of the medium and reduce scour and vortex. Think about it, there is much dust in the flue gas pipeline and the flow rate is fast. If the guide tube is installed in the reverse direction, the airflow will go straight to the root of the corrugation, and the wear speed will more than double. Like the one we stand onUniversal corrugated expansion joint、High temperature axial expansion jointThe direction of the guide tube is a hard requirement-the opening must point to the direction of the medium. Why? Because the medium enters from the large port and exits from the small port, the guide tube can smoothly guide the airflow into the corrugation to avoid direct impact.

A high-temperature axial expansion joint was installed backwards. After less than three months of operation, the corrugated root grinded out grooves visible to the naked eye, and the air leakage rate soared to 15%. You say it was wrong or not?

What about the expansion joint without the deflector?

For example, pure rubberrubber compensator, andNon-metallic expansion joints (fabric fiber expansion joints), there is no metal diversion structure inside, and it doesn't matter whether it is positive or negative in theory. However, note that some non-metallic expansion joints will thicken the wear-resistant layer on the upward surface in order to resist erosion. At this time, the manufacturer will mark an arrow on the product, and the arrow points to the flow direction of the medium. If you ignore the arrows, the wear-resistant layer will be installed on the back, which is basically equivalent to wasting money.

In addition,Rubber PTFE compensatorSuch composite products sometimes make a fuss about the lining. In short, don't just look at the appearance, you have to look at the manufacturer's label.

How to judge if there is a direction? Three methods

Look for arrow marks on the outer wall or flange of the product. We often say that "the direction of the arrow of the expansion joint refers to", that is, the direction of the medium flow and the direction of the small mouth of the guide tube. In our stationMetal rectangular expansion joint、Corrugated expansion joint for power station industryWhen you leave the factory, an arrow will be marked in a prominent position, so you can tell it at a glance.

What if there were no arrows? Take apart the flange to see the internal structure. The guide tube is like a bell mouth, and the small mouth is facing the direction of the medium. Otherwise, look forLarge diameter thick wall expansion jointDrawings-This type of heavy equipment generally has installation instructions, and the manufacturer will mark the flow direction.

Then I bought itSleeve type pipe expansion jointIs there any direction? Sleeve type is also divided, but most of them rely on sealing packing, and the direction has little influence. But the corrugated ones are basically particular.

What if I install it backwards? Don't make do

For flue gas pipelines, especially desulfurization systems or high-temperature flues, the inverted guide tube will expose the ripples directly to the dusty gas stream. Doubling the wear rate is light, and in severe cases, it can trigger corrugation tearing, resulting in the outage of the entire pipeline. We handled aDesulfurization flue gas baffle doorThe matching expansion joint was installed backwards by the user, and after half a year, the air leakage exceeded the standard, so it had to be stopped for a new one.

It is recommended to reinstall in the correct direction after shutdown. If the site can't be dismantled (for example, it has been welded or the space is limited), you can consider installing an anti-wear plate on the outside, but it will treat the symptoms rather than the root cause. The guard plate can block for a while, but can't stop long-term scouring. Eventually it will have to be changed.

At the end of the day, take ten minutes to verify directions before installation

It is much more cost-effective than replacing an expansion joint of tens of thousands of dollars in the later period. Whether it isRectangular non-metallic expansion jointStillDirect buried (fully buried) type expansion jointThe rules are the same: if there is an arrow, look at the arrow, if there is no arrow, look at the guide tube, and if there is no mark, ask the manufacturer for drawings. Don't pretend by your feeling, the industry is just like that.

So back to the question of "Is there a positive and negative for the flue expansion joint"-the answer is: those with guide tubes and those marked by arrows must be distinguished between positive and negative; Pure rubber or fabric with no guide tube and no marks, usually not distinguished, but also pay attention to the direction of the wear layer. The heavier the equipment, the worse the working conditions, the less careless. Remember: When you get the right direction, the pipeline can run for the long term.

1. What does the expansion joint at the end of the flue do?

Many people think that the expansion joint at the end of the flue is a "telescopic tube", which is used to offset thermal expansion and contraction. Actually, what it does is more complicated than you think. Especially in power stations, cement and chemical industries, the end of the flue is connected to a dust collector, desulfurization tower or induced draft fan, and the pipeline is full of high-temperature flue gas, as well as dust and corrosive gases. The expansion joint should not only absorb axial displacement, but also carry radial offset and angular displacement. Think about it, the flue is hundreds of meters long. As soon as the temperature rises, the pipe will not only become longer, but also be distorted due to uneven settlement of the support and wind load. If the expansion joint is not selected correctly, it will either get stuck or leak, and in serious cases, the equipment will be directly pulled out.

Many on-site faults are not the poor quality of the expansion joint at all, but the parameters are not calculated correctly. Therefore, don't turn over the sample to select the model as soon as you come up, and honestly find out the three basic parameters: temperature, pressure and displacement.

2. Three bottoms that must be found out before calculation: temperature, pressure and displacement

These three parameters are like oil, salt, sauce and vinegar for cooking. If one dish is missing, it will be wasted.

• Design temperature: It is not the operating temperature, but the limit temperature. For example, the smoke is normal at 250℃, but it may rush to 350℃ when it starts and stops. You have to calculate it according to 350℃. In addition, consider the temperature gradient-the temperature difference between the inner and outer walls of the flue will lead to uneven expansion, especially the fiber layer of non-metallic expansion joints, which will burn up when the temperature is too high.
• Design pressure: Most flue systems are slightly positive or negative pressure, but don't be careless. The entrance area of the desulfurization tower may have an instantaneous positive pressure impact due to the fan surge. One project once selected an ordinary non-metallic expansion joint, and as a result, the pressure fluctuation directly tore the skin. It is an industry rule that the pressure parameter should be 1.5 times the maximum working pressure as the safety margin.
• displacement amount: Here are the most pits. The displacement includes thermal expansion displacement, installation error displacement and foundation settlement displacement. Thermal expansion can be calculated by formula (detailed in the next section), but the installation error and settlement are estimated by experience, and it is generally recommended to reserve a margin of ±10mm. For example, the horizontal section of the flue is 20 meters long, and the thermal expansion is calculated to be 45mm, so the displacement capacity should be at least 60mm when you select the model.

3. How to calculate thermal expansion? The formula is simple, but the pits lie in the details

Δ L = α × L × Δ T. α is the linear expansion coefficient, about 12×10⁻⁶/°C for carbon steel and about 17×10⁻⁶/°C for stainless steel; L is the length of the pipe in mm; Δ T is the temperature difference in °C. For example: a section of carbon steel flue with a length of 15 meters, rising from the installation temperature of 20°C to the operating temperature of 300°C, Δ L =12×10⁻⁶ ×15000×280=50.4mm. ok, the numbers are out, and then what? Here comes the pit.

The pipe is not a whole one, there are elbows, tees, and baffle doors in the middle. The elbow itself will absorb part of the displacement, but in actual calculation, it is usually conservatively calculated according to the length of the straight pipe section, and safety comes first. The second pit: multiple pipes in parallel flue, such as double flue, each pipe has different expansion amount, so the expansion joint can't be selected independently, so the mutual constraints between pipe bundles have to be considered. Third Pit: If the end of the flue is connected toFlue gas baffle door(For example, the desulfurization flue gas baffle door or electric plug-in insulation door of this station), the frame stiffness of the baffle door is limited, and the thrust generated by the expansion joint will deform the door body. At this time, it is necessary to select the expansion joint with a tie rod to offset the pressure thrust.

So don't worry and be happy after calculating the values, ask yourself: Do I have any of these "invisible killers" in my plumbing system?

Fourth, the key to selection: non-metallic expansion joint or metal rectangular expansion joint, which should you use?

Non-metallic expansion joint (fabric fiber expansion joint)AndMetal rectangular expansion joint。 How to choose? In one sentence: large displacement, low pressure, high temperature flue gas, with non-metal; For small displacement, medium and high pressure, wear resistance and erosion resistance are needed, and metal rectangles are used.

The advantages of non-metallic expansion joints are large compensation (a single layer of skin can absorb 100mm axial displacement), high temperature resistance (silica gel cloth + ceramic fiber layer can reach 1000℃), and no thrust-because the fiber layer has basically no stiffness. However, its weakness is poor compression resistance. If the positive pressure exceeds 0.05MPa, it should be cautious, and there should be no sharp particles to scour, otherwise the skin will wear out quickly. Therefore, the end of the dusty flue, such as the electrostatic precipitator outlet, often needs to be addedguide tubeProtection (the guide tube can prevent the flue gas from directly eroding the internal parts of the expansion joint, which is specifically mentioned in the FAQ of this site).

The metal rectangular expansion joint has high stiffness, high pressure and long life, but the manufacturing cost is high, and the compensation per wave is limited (generally, a single wave can only absorb 10~20mm). If your flue size is very large, such as a rectangle of 2 meters by 3 meters, the metal rectangular expansion joint is usually made of a multi-wave structure, and it should be equipped withtie rodTo withstand internal pressure thrust. The function of the tie rod is to limit the excessive stretching of the expansion joint and prevent the instability of the bellows (refer to the question and answer of this site "The function of the expansion joint tie rod").

In addition, don't forget that some projects will also use the two in combination: metal rectangular expansion joints for the axial direction of the flue, and non-metallic ones for angular displacement.

5. Don't be in a hurry to place an order after calculation: do you want to count the guide tube, tie rod and fatigue life?

The parameters have been calculated and the model has been selected, but there are still a few things to confirm before placing an order, otherwise it may break after three days of installation.

• guide tube: High-frequency wear conditions must be matched. The thickness and material of the guide tube should also be selected according to the flue gas temperature. Generally, the temperature resistance of carbon steel guide tube is ≤500℃, which exceeds that of stainless steel.
• Tie rod and limit structure: When the metal rectangular expansion joint is installed, how to adjust the tie rod nut is a technical job (this site has special questions and answers). Remember: After installation, the tie rod nut should be loosened to half of the designed displacement, otherwise the expansion joint will not expand or contract normally.
• Fatigue life: Flues that start and stop frequently, such as peak shaving units in thermal power plants, expansion joints may experience multiple temperature cycles every day. The fatigue life of metal bellows is inversely proportional to the displacement, and generally requires at least 1000 cycles. Although the non-metallic expansion joint has no fatigue problem, the aging life of the skin is usually 3~5 years, so it needs to be replaced regularly.
• Medium corrosivity: The wet flue gas after desulfurization contains sulfuric acid condensate, which has strong corrosion to metals. That's when you have to goPTFE-lined hoseOrPTFE compensatorOr choose a corrosion-resistant non-metallic skin.

Oh yeah, there's another one that's easy to overlook-pressure thrust. The internal pressure of the pipeline acting on the effective area of the expansion joint will generate a large axial thrust. If the tie rod or fixed bracket is not installed, this force will destroy the adjacent equipment. Especially for large-size flues, the pressure thrust can easily reach tens of tons, so don't take it seriously.

VI. A practical case: the complete calculation process from parameters to selection

Two days ago, I met a customer, the kiln tail flue of a cement plant, with a rectangular cross-section of 1.8m ×1.2m, a length of 25 meters, and a material of Q235. Working conditions: Flue gas temperature 350℃, maximum 400℃ at start-up, pressure-0.03MPa (negative pressure), horizontal installation, two ends connected respectivelyRound Flap Door (Double Seal)AndFlue gas baffle door。 Displacement requirements: axial thermal expansion + installation allowance total 80mm, transverse displacement estimated ±5mm.

Step 1: Calculate thermal expansionΔ L =12 x 10⁻⁶ x 25000 x (400-20) =114 mm. With a reserved margin of 10mm, the total displacement is 124mm. However, note that there are baffle doors at both ends of the flue, which are rigid, and the actual absorption of the expansion joint is 120mm.

Step 2: Judge positive and negative pressuresUnder negative pressure conditions, the internal pressure thrust is very small, and the pull rod is not needed. However, negative pressure may cause the skin to be concave, and a support ring needs to be added inside the non-metallic expansion joint.

Step 3: Type selectionConsidering the temperature of 400℃, large displacement, negative pressure and rectangular section, the first choiceRectangular non-metallic expansion jointThe skin structure adopts ceramic fiber + silica gel cloth + stainless steel wire mesh, which has a temperature resistance of 450℃, and a guide tube (carbon steel, δ =3mm) is installed to protect the inner wall. Since the lateral displacement is only ±5mm, the non-metallic expansion energy saving is easily absorbed.

Step 4: CheckCheck the product information of non-metallic expansion joint in this site, select the model with a length of 500mm, and the axial compensation amount can reach 150mm, which meets the requirements. The length of the guide tube is designed according to 1.2 times the inner diameter of the pipe to avoid flushing. Finally, confirm the size of the flange of the baffle door interface, and the order can be placed if it matches.

After walking down the whole process, you will find:How to calculate the expansion joint at the end of the flueThis matter was not just a formula. There are living working condition details behind each parameter of temperature, pressure and displacement, and if one is missed, the car may overturn. But as long as you follow this logic step by step, even doing it the first time can reduce your selection mistakes by 90%.