Specialized in manufacturing compensators, expansion joints, baffle doors

Why do non-metallic compensators need regular maintenance? Structural characteristics and working environment determine its vulnerability

Non-metallic compensator, also called non-metallic expansion joint or fabric fiber expansion joint, mainly absorbs heat displacement, vibration and noise reduction in pipeline system. It consists of a flexible fabric layer, insulation layer, sealing layer and metal frame. This structure is inherently "tender" than metal bellows-the fabric layer is afraid of high temperature aging, acid and alkali corrosion, and mechanical scratching. What about the work environment? Desulfurization flue gas, hot air pipeline, cement kiln tail... are all working conditions with high temperature, dust and corrosive media. If you think about it, it would be strange if you were burned by smoke and fire every day.

Two days ago, I met the equipment supervisor of a cement factory.Rectangular non-metallic expansion jointIt took two years to leave it alone. As a result, the smoke was directly sprayed out, and the production line stopped for three days. Replacing a compensator itself is not expensive, but the loss of downtime is hundreds of thousands a day. So don't wait for a leak to remember, regular maintenance is about spending a small amount to save a lot of money.

Daily inspection: see, listen and touch, find problems in three steps

You don't need any sophisticated instruments for daily inspections, just use your eyes, ears and hands well.

look

Stand at a safe distance and glance at the whole first. See if the fabric layers are bulging, cracking, delaminating? Is there any rust or deformation in the metal frame? Pay special attention to welds and connecting bolts – usually the first thing that goes wrong is a loose bolt, resulting in a loose seal. Also, see if the deflector (if any) has any signs of wear and tear from the media. You can flip through our previous articles for the specific function of the guide tube, which will not be expanded here.

Listen

A normal non-metallic compensator runs with little to no sound. If you hear a "hiss" of air leakage, or a "click" of metal friction, it is 80% because the seal is broken or the frame is misaligned. Note when listening: Is it in a position with large displacement? For example, near the elbow and next to the fixed bracket.

Touch

After the machine is stopped (or with insulated gloves), touch the surface temperature of the fabric layer with your hand. If the local temperature is significantly higher than the surrounding temperature, it means that the internal insulation may collapse or break, and the heat is transferred directly to the external surface. Don't be careless at this time, the high temperature will accelerate the aging of the fabric, and it won't take long to leak.

Practical maintenance: cleaning, tightening and anti-corrosion, one cannot be missed

What to do after finding a minor problem? Three steps: clean, fasten, and prevent corrosion.

Cleaning: Use a soft brush or compressed air to clean up dust and scale on the surface of the compensator. It is strictly forbidden to scrape with a hard metal blade-once the fabric layer is scratched, it is the future leak point. If the medium is viscous (such as desulfurization slurry), rinse it with clean water and dry it before subsequent treatment.

fastening: Check all connecting bolts and tie rod nuts. Note: How to adjust the expansion joint tie rod nut? It's not that the tighter you screw it, the better. The function of the tie rod is to limit the excessive displacement and at the same time ensure the design compensation amount. If you tighten too tightly, it limits the normal displacement, and the pipe stress will transfer to the weak link. Standard practice: Pre-tighten to the design value and then lock the loose nut. You can refer to the manufacturer's installation instructions, or the Q&A of "The Function of Expansion Joint Tie Rod" we wrote before.

antisepticMetal frames, especially carbon steel, rust quickly in wet or corrosive environments. Find the corrosion spot, polish and remove rust first, and then apply temperature-resistant anti-corrosion paint twice. Pay attention to avoid the fabric layer, the lacquer will swell and harden when it touches the fabric, which is not worth the loss. For stainless steel frames (such asDesulfurization flue gas baffle doorMatching compensator), check pitting and stress corrosion cracks, if they appear, it will be a big trouble.

How to deal with common faults? Leakage, deformation, aging, right seat

Leakage, deformation, aging. It is handled differently.

leak: Confirm the leak location first. If it is on the fabric layer, the small hole can be temporarily treated with a special patch of repair glue (be careful to be temperature and pressure resistant). Don't mend the big tear, just change the components. If the leak is at the connection of the metal frame to the pipe, check that the seal gasket is damaged or that the flange surface is flat. Don't screw the bolts when you come up-if they are uneven, they won't seal them even if they break them.

deformation: Bulging of the fabric layer of the non-metallic compensator, usually due to collapse of the internal insulation, or the operating temperature exceeding the design value. At this time, you have to stop the machine and open it to check the insulation. Don't think about patching, the internal structure is broken and the external fabric won't last long. In addition, the deformation of the frame (such as the corner of the rectangular compensator tilting) is generally caused by excessive pipe thrust or installation deviation, so it is necessary to recalculate the pipe stress and adjust the bracket.

Aging: The fabric layer has a large area of cracking, hardening and loss of elasticity. Don't hesitate to change it directly. Aging compensators have a sharp decrease in strength and may burst at any time. To judge the degree of aging, you can pinch the edge of the fabric with your hand-if it breaks when pinched, it is "osteoporosis".

From the beginning of installation, it determines the ease of later maintenance

This view may be a little counter-intuitive, but the fact is: if you don't install it correctly, the maintenance behind you will exhaust you. There was a chemical factory before,Non-metallic compensatorIt was installed very close downstream of the pipe elbow, and as a result, the compensator was subjected to additional bending moment during operation, and six were replaced in three years. Normal installation considerations include:

• Ensure that the pipes on both sides of the compensator are coaxial and the deviation is within the allowable range (refer to the model and size table of the expansion joint).
• The installation direction should be correct: Pay attention to whether the medium flow direction is consistent with the direction of the arrow of the expansion joint, and the insulation layer will be blown askew if it is reversed.
• Temporary supports to be removed: Tie rods or supports may be installed for the convenience of transportation when leaving the factory, which must be removed after installation, otherwise the compensator will not work.
• Pre-stretching (or pre-compression): If it is cold installation, the compensator should be pre-adjusted according to the ambient temperature. See the design drawings for specific values. If it is not adjusted, the compensator will bear additional tensile stress when the temperature is low in winter.

Think about it, these details are done in place, and the post-maintenance workload is directly halved. Didn't do it? Then just wait to wipe your ass every few months.

When is it time to get a new one? Don't cause trouble just to save money

Many bosses think that "continue to use it if it can still be used", but the non-metallic compensator not only decreases in performance at the end of its life, but also may suddenly burst. Then when do you have to change?

First, achieve the designed service life.According to the national standard for non-metallic expansion joints (JB/T 12235-2015), the service life under normal working conditions is generally 3-5 years. If it is overrun, it is recommended to do regular water or air pressure tests, but don't expect it to last long.

Second, irreparable damage occurs.For example, large-area ablation of fabric layer, serious deformation of metal frame, cracking of weld seam, etc. Don't think about welding and repairing-once the structural integrity of the non-metallic compensator is destroyed, the reliability after repair is greatly reduced.

Third, there is no cure for leakage.Patched more than three times and still leaking? Don't worry about it, get a new one. The indirect cost of leak repair per shutdown is much higher than the cost of replacement. Don't count this account? Ask the cement factory in front.

By the way, when replacing a new one, it is recommended to check the pipe bracket and adjacent equipment (such as flue gas baffle door, electric plug-in insulation door, etc.) simultaneously. Often, the old compensator is broken because the pipe displacement exceeds the standard, and the root problem is not solved, so the new one dies early.

Don't wait for the smoke to remember. Look, listen, touch regularly, tighten what should be tight, clear what should be cleared, and change what should be changed. The non-metallic compensator is actually quite solid-as long as you have given it "due care".

"Does the non-metallic compensator do heat insulation? I see that the metal bellows next door are covered with a thick layer, so I'm afraid of something going wrong." This problem is actually quite typical-I have seen too many people want to insulate non-metallic compensators as soon as they come up, and as a result, they not only spend money in vain, but also destroy the compensators. Today, let's make it clear when to add it and when not to add it.

1. Find out what material the non-metallic compensator itself is made of first

Non-metallic compensator (also called non-metallic expansion joint, fabric fiber expansion joint), the main body is laminated by flexible composite materials such as silicone cloth, fluorine adhesive tape and glass fiber. To put it bluntly, it is itself a set of multi-layer insulation structures. The temperature resistance of ordinary silicone cloth can reach about 200℃, and that of fluorine tape can reach above 250℃. In many operating conditions, the temperature resistance margin of the skin itself is sufficient.

Then why do some people always think they need to add insulation? Because everyone's mind is all about metal corrugated expansion joints-such asCorrugated expansion joint for power station industryOrMetal Corrugated Expansion Joints in Cement IndustryThe metal shell conducts heat quickly and must be insulated to prevent scalding and save energy. However, the skin of the non-metallic compensator itself is a bad conductor of heat. If it is piled up blindly for heat preservation, it may flatten the ripples and hinder the displacement, and the compensator will be scrapped in advance.

2. Under what circumstances must non-metallic compensators be insulated?

You can't talk to death. Under some working conditions, it is really impossible not to do heat insulation. There are three scenarios:

• High temperature operating condition: The medium temperature exceeds the allowable long-term use temperature of the skin. For example, the temperature of flue gas pipe is 400℃, ordinary silicone cloth can only carry 200℃, and fluorine tape can only carry 250℃-at this time, an insulation layer must be added on the outside of the compensator to reduce the surface temperature of the skin.
• Low temperature operating condition: likeDouble hinge expansion joint for air-cooled island vacuum pipelineThe medium is cold, and the outer wall is prone to condensation or even ice. What is done at this time is to keep the cold, not keep the heat.
• Human body protection: likeDesulfurization flue gas baffle doorThe temperature of the nearby non-metallic compensator may be less than 100℃, but it is safer to add a detachable insulation sleeve to prevent scalding. The environmental inspection also recognizes this.

What is the medium temperature? How much is the skin material resistant to temperature? What is the ambient temperature? The difference is calculated before we do it.

3. If the insulation is not done correctly, the life of the compensator will be directly reduced by half

Tsk, I met a client two days ago.Rectangular non-metallic expansion jointIt was covered with a layer of hard rock wool and then reinforced with cement. And guess what? The compensator barely stretched freely and ripped in a few months. The core function of non-metallic compensator is to absorb displacement. What is the difference between wearing a hard shell on it and welding it to death?

The insulation material must be soft and compressible, such as aluminum silicate fiber blanket. And it can't be pressed too firmly, let alone the insulation material can be stuffed into the corrugated gap-that is equivalent to jamming all the movable joints of the compensator. Remember an iron rule: leave at least 10mm gap between the insulation layer and the compensator, or use a metal shell for movable connection.

4. Correct practice: layered design + detachable structure

The standard practice is divided into three steps, and one step less may overturn:

1. Moisture-proof insulation layer: Wrap a layer of aluminum foil or Teflon cloth outside the skin of the non-metallic compensator to prevent moisture penetration.
2. Flexible insulation layer: Lay aluminum silicate fiber blanket, the thickness is calculated according to the temperature difference, generally 50-100mm.
3. Protective shell: Use stainless steel or galvanized iron sheet as the shell, but it must be made into the style of two-half buckle and fixed with buckle-convenient for disassembly and assembly during maintenance, otherwise the whole insulation layer will be smashed next time you change the skin.

Some high temperature operating conditions, such as we giveHigh temperature axial expansion jointThe plan is to directly customize the non-metallic compensator with thermal insulation interlayer-the thermal insulation cotton is pre-installed between the skin and the protective net when it leaves the factory, and only the butt flange is needed on the site, thus saving the trouble of on-site construction. Is it much less worry-free?

5. In the final analysis, heat preservation is not the end, temperature control is

Judge whether the non-metallic compensator should be insulated or not. Don't just look at the name. First calculate three sets of numbers: medium temperature, ambient temperature, and allowable heat loss. For example, the temperature of the flue gas pipe is 400℃, and the non-metallic skin can only carry 250℃, so insulation must be added to reduce the skin surface below 200℃. However, if the flue gas temperature is only 150℃, the skin itself can cope with it, and adding thermal insulation is a pure waste of money.

In addition, the overall insulation design of the pipeline system must be considered. Non-metallic compensators are often the weak points in the whole insulation link-many projects age and leak in advance because the insulation is not done well, and finally the whole system has to be reworked. So, when you are unsure, directly ask the manufacturer for the temperature-pressure curve, which is much more reliable than guessing.

If you are still unsure, welcome to inquire. Our sideNon-metallic expansion joint (fabric fiber expansion joint)、Double hinge expansion joint for air-cooled island vacuum pipelineThey are all with detailed technical parameters, and you can know whether you should do heat insulation or not.

1. Why can high-temperature resistant non-metallic compensators withstand high temperatures? Materials are key

To be honest, when many people come into contact with this thing for the first time, their first reaction is: Can something made of fabric and rubber hold hundreds of degrees of high temperature? Don't worry, let's look at the materials first. High-temperature resistant non-metallic compensators usually adopt multi-layer composite structure, the core of which is the flexible fabric layer in the middle, such as silicone rubber coated glass fiber cloth, polytetrafluoroethylene film and ceramic fiber felt. Silicone rubber itself can withstand temperature to about 250°C, but with the addition of glass fiber and ceramic fiber, the working temperature is directly raised to 600°C or even above 1000°C. And guess what?"Non-metallic expansion joint (fabric fiber expansion joint)" mentioned in the product information of this siteThe more layers, the stronger the heat insulation and temperature resistance.

There is also a layer that is easily overlooked-the internal deflector. Many non-metallic compensators will be equipped with metal guide tubes, which not only guide the airflow to reduce erosion, but also block the high-temperature smoke from licking directly to the fabric surface. This thing is kind of like armor, minimizing the damage from heat radiation. So you see, the confidence of high temperature resistance is not blown out, but layered materials.

2. Temperature upper limit and pressure level: Don't just listen to propaganda, look at national standards and actual measurements

Some manufacturers in the market say that their products can carry 1200°C, but when you ask about the pressure level, they hesitate. According to the National Standard for Non-metallic Expansion Joints JB/T 12235-2015, the upper limit of working temperature of non-metallic compensator is related to the medium, pressure and displacement, not just by looking at the numbers. How do the measured data come from? Take a practical case: The outlet pipeline of the rotary kiln in a cement plant isRectangular non-metallic expansion jointWith a medium temperature of 850°C and a pressure of ±3kPa, the fabric loop belt was changed once after more than two years of continuous operation.

In terms of pressure, non-metallic compensators are naturally inferior to metal because of their flexible structure, so they are generally used in low-pressure flue duct systems (within 0.1MPa). But it has a unique skill: it can absorb multi-dimensional displacements (axial, radial and angular), and it has minimal thrust on the pipe. So don't use it as a pressure vessel, it's more like an elastic soft joint.

What about the performance of high temperature resistant non-metallic compensator? Bottom line: look at the working condition match. For scenarios below 600°C, low pressure, and large displacements, it is much more reliable than a metal compensator.

Third, just with the front of the metal compensator: who is more cost-effective and who is more durable?

Two days ago, I met a customer who insisted on using it on a 300°C flue.High temperature axial expansion jointI asked him why he didn't use non-metallic ones. He said he thought the metal was durable. As a result, the metal bellows should not only choose heat-resistant alloy steel (the cost doubles), but also consider the corrosion problem, and there are many welding points, which makes maintenance troublesome.

Under the same caliber, the price is generally only 1/3 to 1/2 of that of metal. More importantly, its installation accuracy requirements are low, and the on-site cutting adjustment is not distressed. Moreover, it is convenient to repair-if the fabric ring belt is broken, it can be replaced directly, unlike metal bellows that have to dismantle a bunch of flanges. What about longevity? Under normal working conditions (the temperature is lower than the design value, and the medium has no corrosion), the fabric layer can be used for 5-8 years, which is much longer than the fatigue life of metal bellows at high temperature.

But it's not without its flaws. Non-metallic compensators are afraid of sharp hard objects scratching, afraid of direct burning by open flame. So it fits flue gas, hot air, dust ducts, not steam ducts (unless specifically designed). When selecting a model,Product Center of this siteCompare the parameters well, and don't mess with the Mandarin Duck Spectrum.

4. Selection and rollover site: size, displacement and installation direction must not be missing

Let me tell you the truth. In order to save trouble, a steel mill bought a non-metallic compensator and installed it directly. As a result, the fabric burst after one week's operation. Why? The amount of displacement is not counted correctly! The thermal expansion length of the pipe is calculated to be 50mm, and the compensator they bought can only absorb 30mm, so it's weird that it doesn't burst.

Diameter (DN size), compensated displacement (axial/transverse/angular), operating temperature。 If one is missing, it can overturn. Such asRectangular non-metallic expansion jointIt is used on rectangular flue. When installing, it should be noted that the direction of the guide tube must be consistent with the flow direction of the medium, otherwise the guide tube will be washed askew and the fabric will be directly exposed to the high-temperature airflow. There is also the installation direction: non-metallic compensators usually can't withstand pressure thrust, so the tie rod bolt should be adjusted to the appropriate tension (refer to the FAQ "How to adjust the tie rod nut of the expansion joint"), and it can't be screwed to death or too loose.

In addition, where to choose the location is also very particular. Try to avoid installing it behind the elbow or in the turbulent zone downstream of the valve, otherwise the airflow impact will shorten the fabric life. If you can't avoid it, then install an internal anti-scour board (some manufacturers design it as standard, and some have to be customized). You bought the compensator with or without a deflector? This is inSpecific Function of Expansion Joint Guide TubeThere is a detailed explanation in the Q&A, so it is recommended to take a closer look before placing an order.

5. How many years can the life span last? Practical recommendations for maintenance and replacement

The lifetime of non-metallic compensators is greatly affected by temperature fluctuations. If the medium temperature is stable, the aging of the fabric layer is slow, and it is normal to use it for 6 years. However, if it starts and stops frequently and the temperature rises and falls sharply, it may have to be changed in 3 years. Simple maintenance: check the fabric surface every six months for any cracks, aging and brittleness, and any leakage traces. By the way, use a flashlight to see if the internal guide tube is deformed or fallen off.

Once you find bulging or partial ablation of the fabric, don't hesitate to change it quickly. Note when changing: Before dismantling the old ring belt, first confirm that the pipeline has cooled down and relieved pressure, safety first. When installing the new ring belt, the strip bolts should be symmetrically tightened to prevent uneven force. If you can't handle it yourself, go directly to the manufacturer for after-sales service. Many non-metallic compensator suppliers provide on-site replacement services.

After all, what is the performance of high-temperature resistant non-metallic compensators? Depends if you know how to use it. Temperature resistance of materials is a hard index, while installation and maintenance is a soft effort. Get these points thoroughly, and it is definitely the "utility pie" in your plumbing system.

Let's talk about the conclusion first: Is the non-metallic compensator resistant to high temperatures?

Resistant, and can carry better than metal compensators under some working conditions. But there is a premise-you have to choose the right material. When many people hear the word "non-metal", the first reaction in their minds is "plastic things are soft when exposed to heat". Tsk, that impression has to be changed. Non-metallic compensators (also called fabric fiber expansion joints and non-metallic expansion joints) are not simple plastic at all. Its main body is composite materials: multi-layers of fabric fiber cloth, rubber, fluoroplastics and insulation cotton are superimposed together, and each layer performs its own duties. Therefore, whether it is resistant to high temperatures or not depends entirely on which "formula" you pick.

The secret of high temperature resistance is hidden in materials: how many degrees can it carry from silica gel to tetrafluorine?

Let's take apart several kinds of fabrics commonly used in nonmetallic compensators, and the upper temperature limit is clear at a glance:

• Silicone cloth/fluororubber cloth: The continuous working temperature is 200℃ ~250℃, and it can reach 300℃ in a short time. Suitable for hot air duct, low pressure steam.
• Polytetrafluoroethylene (PTFE/Tefluoro): Temperature resistance range-200℃ ~ +250℃, but above this temperature, PTFE will decompose to produce toxic gases. This site'sPTFE compensatorAndPTFE-lined hoseIs a typical representative, suitable for corrosive media and low temperature occasions.
• Ceramic fiber cloth (aluminum silicate fiber): This thing is the real main force of high temperature resistance, and the long-term use temperature can reach more than 1000℃. With stainless steel wire reinforcement, it is commonly used in flue gas pipeline and boiler exhaust system. stationaryNon-metallic expansion joint (fabric fiber expansion joint)Use this kind of material in large quantities.
• Glass fiber cloth + silicone rubber coating: The common temperature resistance range is 250℃ ~400℃, taking into account both flexibility and sealing.

So you see, there are corresponding products for the temperature resistance span of non-metallic compensators from 150℃ to 1200℃, and the key lies in the thickness of the middle insulation layer and the number of fabric layers. Listed in the stationHigh temperature axial expansion jointIt belongs to the metal category, but if you need super-large displacement and high temperature, non-metals are more flexible.

The scenario decides the choice: Which kind to use for flue gas pipes? Which kind of high-temperature steam should I use?

Two days ago, I met a customer who was doing desulfurization project. When I came up, I asked, "Is the non-metallic compensator resistant to high temperatures? The flue gas temperature is 350℃". I said no problem, but it has to be equipped with ceramic fiber lining + stainless steel wire mesh reinforcement layer, and the outer layer is protected against acid corrosion with fluororubber cloth. This operating condition is selectedRectangular non-metallic expansion jointThe most suitable, can absorb three-dimensional displacement, flange installation also saves space.
Conversely, if it is high temperature steam (such as the main steam pipe of a power station, the temperature is 540℃), guess what?Non-metallic expansion joints N/A。 High-temperature and high-pressure steam has strong permeability, and the non-metallic layer is easy to bulge and delaminate. At this time, honestlyCorrugated expansion joint for power station industryOrHigh temperature axial type expansion joint. Non-metallic compensator is most used for low-pressure saturated steam (≤150℃) on steam pipeline. If it exceeds this number, metal compensator is the correct solution.

Compared with metal compensators, what is the advantage of non-metal high temperature resistance?

The upper limit of temperature resistance of metal compensators (such as stainless steel bellows) is very high. Ordinary 304 stainless steel can carry 600℃, and Inconel alloy can even reach 900℃. But nonmetallic compensators have a unique skill- -Thermal shock resistance。 Metals are prone to fatigue cracks under quenching and sudden heating, while non-metals are more solid because of their soft material and multilayer structure that can buffer thermal stress. For example, the temperature of the flue gas pipeline of the waste incinerator instantly drops from 800℃ to 200℃, and the metal expansion joint may crack in two or three years.Non-metallic expansion joint (fabric fiber expansion joint)It's normal to use it for five or six years. In addition, the non-metal compensator also has its own vibration and noise reduction function, and the metal compensator has to be added with an additional damper. But what about the cost? Non-metals have weak pressure bearing capacity, so they are generally only used at low pressure (≤0.1MPa), and they still have to be used when high pressure is neededUniversal corrugated expansion jointOrStraight pipe pressure balanced expansion joint。

Select and avoid pits: Don't just look at temperature, but also at pits such as pressure, medium and life

Many people think that "it's enough to withstand temperature", but it leaks after three months of installation. Why? The pressure is over the limit! The pressure rating of non-metallic compensators usually does not exceed 0.1MPa, and if there is a positive or negative pressure fluctuation inside the pipe, the flange sealing surface can easily be blown open. Besides, the flue gas containing acidic gas must be lined with PTFE or fluororubber, otherwise the fiber cloth will be corroded through. From our stationRubber PTFE compensatorIt is to solve this corrosion + medium temperature working condition.
There is also the problem of life-the fabric layer of non-metallic compensator is a consumable, and the normal life is 3~5 years, while the metal compensator can do it for more than 10 years. Therefore, if it is a key pipeline that runs non-stop for a long time (such as the main steam of a power plant), don't use non-metals cheaply; If it is an auxiliary pipe that is easy to maintain and replace (such as dust removal system and flue bypass), non-metal will be very fragrant. And don't forgetNational Standard for Non-Metallic Expansion Joints JB/T 12235-2015When selecting, compare the temperature-pressure curve in the standard, and don't use it beyond the parameters.
In one sentence: Non-metallic compensators do withstand high temperatures, but it depends on how high, what medium and how much pressure. Don't be fooled by the name, it's the exact opposite of "intolerant to high temperatures"-it's pressure, not temperature, that really limits positioning.

How to connect a metal expansion joint? Find out which model to use first

I received a phone call from a customer two days ago, saying that the steam pipe leaked only three months after installing the expansion joint. I asked him what model he used, and the answer was "just an ordinary metal expansion joint". Tsk, that's the problem —Method of joining metal expansion jointIt's not a matter of screwing a wrench. If you choose the wrong model, the rest will be for nothing.

The universal corrugated expansion joint is suitable for pipelines with axial displacement as the main; Large tie rod expansion saves energy and carries lateral displacement, but tie rod is the core; Pressure balance type (such as straight pipe pressure balance expansion joint and curved pipe pressure balance expansion joint) is specially designed to deal with high pressure and large diameter, so that the blind plate force does not spread outside. There are also high-temperature axial expansion joints, external pressure single axial expansion joints, double hinge transverse expansion joints... Each structure determines its joining method difference. If you use the high-temperature type as a general-purpose type, the bellows will burn through directly. This is not a joke, but a real thing.

Key checks before installation: Should tie rods and screws be removed or left?

Many masters get the expansion joint, and when they see the tie rod and adjusting screw, they mutter: Is this a transportation protection or a permanent part?

For universal corrugated expansion joints and high temperature axial expansion jointsThe limit tie rod and adjustment screw that came with the factory must be removed or loosened to the designed position before installation. Why? Because their role is to prevent the bellows from being overcompressed or stretched when transported and hoisted. After the pipeline docking is completed, if the tie rod is still screwed to death, the expansion joint will lose its compensation function, which is no different from a section of steel pipe. Two days ago, in a power plant project, the workers didn't dismantle the tie rod, and the bellows bulged directly during the pressure test-the pressure was carried by the tie rod, but the bellows was damaged.

The large tie rod expansion joint and the tie rod on the pressure balance expansion joint are structural parts and cannot be disassembled! Because the big tie rod has to bear the blind plate force generated by internal pressure, you remove the tie rod, and the bellows deforms instantly. Therefore, be sure to look at the nameplate and drawings before installation, and distinguish between "transportation limit" and "working tie rod".

Detailed explanation of installation steps: key points of centering, welding and fixing brackets

Step 1, Align. The pipe flange surfaces must be parallel, and the deviation must be controlled within 1-2mm. If you take the level to hit it, if the bellows is twisted, the life will be directly discounted by 50%. Step 2, welding. If it is a welded expansion joint, fix it by spot welding first, observe the deformation amount, and then weld it in layers, and the thickness of each layer does not exceed the diameter of the welding electrode. The welding current should not be too large to prevent the heat-affected zone from burning the thin wall of the bellows. It is recommended to wrap a damp cloth around the outer wall of the bellows for cooling. The third step is to fix the bracket. Remember one principle: the first bracket at both ends of the expansion joint must be a fixed bracket or a guide bracket to prevent the axial thrust of the pipe from pushing the expansion joint askew. EspeciallyDouble hinge expansion joint of air-cooled island vacuum pipeFor this large-size product, the spacing of brackets must be made according to the design drawing, and one less bracket may lead to hinge failure.

Many on-site diagrams save trouble, and the expansion joint is directly welded to the unfixed pipe. As a result, as soon as the temperature rises, the pipe drifts with the expansion joint, and the compensation amount is all wasted. And guess what? Finally reworked and removed, the bellows has been permanently deformed.

Differences in joining methods of different types of expansion joints

• Universal corrugated expansion joint: Axial displacement, remove the limit pull rod during installation to ensure the free expansion and contraction of the bellows. Pay attention to the direction of the deflector-the arrow must point to the direction of the media flow, otherwise the deflector will be washed off.
• Large tie rod expansion joint: For lateral displacements, such as L-pipe corners. The pull rod cannot be removed during installation, and the nut should be adjusted so that the pull rod is in the force balance position. The FAQ asks "How to adjust the expansion joint tie rod nut"? Generally, it is pre-tightened to 80% of the design length first, and then fine-tuned after the pipeline hydraulic pressure test.
• Straight pipe pressure balanced expansion joint: Working bellows and balancing bellows at both ends with tie rods in the middle. During installation, the three chambers must bear pressure at the same time, so the tie rod should never move. Moreover, this kind of expansion joint usually comes with its own fixed bracket, and only a guide bracket needs to be set on the spot.
• Curved tube pressure balance expansion joint: More complex construction for elbow sections that need to absorb combined axial and lateral displacements. Pay attention to the coaxiality of flanges at both ends when installing, deviation more than 2mm will cause bellows to twist.
• Compound hinge transverse expansion joint: Consists of two hinges and a set of bellows that absorb only the lateral displacement in the plane. The hinge pins must be lubricated during installation and are not allowed to withstand torsional torque.

And there's alsoExternal pressure single axial expansion jointIts bellows is on the outside, subjected to external pressure, and the installation direction has strict requirements. The exhaust hole on the housing must face upward, otherwise the condensate will not drain out-this is where many novices roll over.

Post-installation commissioning and acceptance: displacement inspection and leak test

Measure the length of the expansion joint in the cold state with a dial gauge or vernier caliper and record the initial value. Then introduce the medium (water or gas), slowly raise the temperature to the working temperature, and measure the length again to see if the actual displacement is within the design range. If the actual displacement is larger than the design value, it means that the pipe fixing bracket has failed or the spacing of the guide bracket is too large. If there is no change, congratulations, you may have forgotten to remove the limit rod.

Leak tests must also be done. For corrugated expansion joints used in power station industry or metal corrugated expansion joints in cement industry, it is usually required to hold pressure at 1.5 times the design pressure for 30 minutes without leakage. Small diameter can use compressed air with soapy water, large diameter can use ultrasonic leak detector. Pay special attention to weld seams and bellows crests-that is where cracking is most likely.

Method of joining metal expansion jointIt is not an isolated operation, it has to be matched with pipe design, support arrangement, and media characteristics. The stress analysis of the same model, used in steam pipes and used in flue gas pipes, is completely different. So, don't use "universal type" as a universal plaster. If you are not sure, flip through the product information on the station, or ask the manufacturer's technical engineer directly-it is much more troublesome than thinking about it yourself.