Specialized in manufacturing compensators, expansion joints, baffle doors

Two days ago, a buddy who designed flue gas pipelines called and asked, "How to calculate the resistance of the expansion joint? I calculate the pressure drop according to the straight pipe section. If the pump is selected small, the test run will trip." Alas, this sounds familiar. Too many people have stumbled in the selection of expansion joints-the system pressure drop is quite accurate, but they have slapped their heads in the expansion joints. Today, let's talk about this matter clearly. After you understand it, you can pat your chest and say that you can count.

First, don't rush to set the formula-what problem does the resistance calculation solve?

I calculate this resistance, what is it for?
To put it bluntly, there are two purposes: first, ensure that the head of the pump or fan is sufficient, so that the system will not be pressurized or the flow rate is insufficient; Second, avoid the pipeline stress exceeding the standard due to excessive local resistance, which will tear the expansion joint.

To put it bluntly, the expansion joint is installed in a pipe, and the fluid passing through it will inevitably cause pressure loss. If this loss is ignored, the chosen pump will either trip like the guy above, or the big horse-pulled cart will waste the electricity bill. Calculate it accurately, and you can match an economical and reliable system.

Second, the physical nature of resistance: the expansion joint is not a straight pipe, and the fluid turns a corner here

What is the structure inside the expansion joint? Take the most commonUniversal corrugated expansion jointFor example, the ripples raise one by one, and when the fluid flows through it, it is like driving a car on a washboard road-every wave peak has to change direction, accelerate, and slow down, and energy is lost in the process. This loss belongs to local resistance, which is completely different from the friction resistance along the pipeline.

If you think about it, a straight pipe of the same caliber and a section of corrugated expansion joint, at the same flow rate, the pressure drop of the latter may be 5 to 10 times that of the former. Why? Because the fluid repeatedly "bends" in the ripples, a large number of vortices are created. This kind of local resistance cannot be applied hard by Darcy's formula, but has to be calculated by a special resistance coefficient method.

Then how to quantify? The empirical formula is as follows: Δ P = ξ · (ρ · v²/2), where ξ is the drag coefficient, ρ is the medium density, and v is the flow rate. The difficulty is all about this.

Third, different types of expansion joints, the resistance difference is greater than you think (metal corrugation vs non-metal vs sleeve)

Different types of expansion joints have very different internal geometries, and the drag coefficients can be different by an order of magnitude.

• Metal corrugated expansion joint(For example, those used in the power station industryHigh temperature axial expansion joint、Large diameter thick wall expansion joint): The ripple bulge is obvious, and the fluid turbulence is violent. The resistance can be two or three times different from the one with a guide tube and the one without one. More on that later.
• Non-metallic expansion joints (fabric fiber expansion joints): The interior is generally a smooth rectangular or circular channel, without obvious corrugated bumps, and the resistance is much less than that of metal corrugated expansion joints. However, it depends on the fabric to bear pressure, and the flow rate cannot be too high, usually controlled within 15m/s.
• Sleeve type pipe expansion joint: The displacement is compensated by the sliding of the inner and outer sleeves, and the inner channel is basically straight through, with the smallest resistance, which can be calculated almost according to the straight pipe section. But seals are prone to wear and tear, which is another matter.

So don't take all models with one formula. You're choosingCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointWhen the structure is more complicated, the drag coefficient has to be corrected according to the specific wavenumber and waveform.

4. Calculate it hand-in-hand: empirical formula + key parameter values

Okay, straight to the case. Assume a steam pipeline with nominal diameter DN300, design flow rate of 25m/s and medium density of 0.6kg/m³. Select a steam pipeline with guide tubeUniversal corrugated expansion joint, Single Wave.

Check the drag coefficient ξ. According to industry experience, ξ of single-wave metal corrugated expansion joint with guide tube is generally between 0.5 and 1.2. Take 1.0 for conservative points. Without the deflector, ξ can soar above 3.0.

Calculate the pressure head. ρ ·v²/2 =0.6×25²/2=187.5 Pa.

Δ P = ξ ×187.5=1.0×187.5 ≈ 188 Pa. This is just the loss of a single wave. If you choose a multi-wave expansion joint (such asCompound straight pipe bypass pressure balanced expansion joint), also multiply by the wavenumber, but pay attention to the interwave interference, not a simple linear superposition-usually corrected by the square root of the wavenumber.

The contrast system allows pressure drop. If the outlet pressure of the fan is only 500Pa, an expansion joint will dry out 188Pa, plus the loss of the pipe valve, it is definitely not enough. At this time, either change to a low-resistance type with a guide tube, or increase the diameter to reduce the flow rate.

5. Guide tube, flow rate, medium-those "invisible" factors that affect resistance

Just now, we mentioned the guide tube. What is this thing for?Specific Function of Expansion Joint Guide TubeIt is to let the fluid take a relatively straight path inside the corrugation, so as to avoid direct impact on the root of the corrugation to generate large vortex. With it, the drag coefficient can be reduced to one-third or even lower than without it. So don't be reluctant to give up that cost, especially for high-speed airflow pipelines, the guide tube is a power-saving artifact.

Medium viscosity. Gas and water are very different. The gas has low density and low viscosity, and the resistance mainly comes from eddy loss; Liquids such as water or oil have high density and high viscosity. In addition to local resistance, the friction loss of corrugated wall should not be underestimated. When calculating liquid pipeline, it is recommended to refer toMetal hose pressure standardEmpirical data in, or do CFD simulation directly.

In addition, the higher the flow rate, the better. The flow rate doubles and the resistance becomes quadrupled (because v²). Some people choose small diameter expansion joints to save money, but as a result, the pressure drop is too large, and the pump consumes much more energy than the equipment money saved. Tsk, the gain outweighs the loss.

6. Completion of calculation is not equal to completion-common cases of resistance rollover in engineering

Let's tell me a few real rollover scenes.

Case A: The desulfurization flue of a power plant was usedNon-metallic expansion jointIt would have been fine, but the design flow rate was mentioned above 20m/s, and as a result, the fabric layer was torn by the airflow after half a year of operation. Turning back and calculating, the local pressure drop exceeds the standard, resulting in negative pressure fluctuation and fabric fatigue damage. Later replaced with a deflectorMetal rectangular expansion jointJust steady.

Case B: Steam pipeline of a chemical plant, selectedExternal pressure single axial expansion jointThe pressure drop of the deflector is not counted, and as a result, the system safety valve jumps frequently. Finally, it was found that the corrugated root inside the expansion joint accumulated scale, the actual circulation area shrank by 30%, and the resistance tripled. So regular inspections and cleaning are also important.

And guess what? These rollovers all have one thing in common: the expansion joint is regarded as "part of the pipe" in the design stage, and the resistance is not counted separately at all. In fact, the expansion joint is a local resistance member in the pipeline, which must be listed separately in the pipeline hydraulic calculation table.

How is the resistance of the expansion joint calculated?Remember this formula: Δ P = ξ · (ρ V²/2), then honestly check ξ, set the flow rate, and check the system. Don't slap your head, don't be lazy. Understand these steps, and selection is no longer metaphysics.

How does the wave pitch of the bellows change? Full interpretation from mold to application

A few days ago, a customer who made power station pipelines called and asked, "How did you adjust the wave pitch of your bellows? I see 20mm marked on the drawing. Can you change it to 15mm for me?" This problem is actually quite typical-many people think that the wave pitch is a fixed size, and the manufacturer can set as much as he wants. Today, let's break this matter apart and explain it clearly, from mold to manufacturing to type selection, explain it clearly at once.

Wave length is not randomly determined: first understand what this parameter is for

Wave distance, to put it bluntly, is the axial distance between two adjacent peaks (or troughs). It directly determines one of the core capabilities of the bellows-how many waves can be stuffed into a unit length. The more wavenumbers, the greater the amount of compensation; The larger the wave pitch, the higher the stiffness of the individual waves.

But this stuff isn't a patter on the head. A high-temperature axial expansion joint is used in steam pipeline, and a metal corrugated expansion joint in cement industry is used in kiln tail pipeline. The logic of wave pitch is completely different. The former has to bear high temperature and high pressure, and the wave distance is too small to easily concentrate stress; The latter is mainly to absorb thermal displacement, and a larger wave pitch will be more resistant to fatigue.

So you see, the wave pitch is essentially a "performance adjustment knob". Wherever the knob is twisted, the stiffness, compensation amount and fatigue life all change accordingly. Then how is this knob twisted in the manufacturing process?

Manufacturing: How to Control Wave Pitch in Hydroforming and Mechanical Forming

Hydroforming and mechanical forming (also called roll forming). The wave length control logic is completely different.

Hydroforming: The pipe blank is filled with high-pressure oil, and the pipe wall is pushed up by oil pressure to form waves. The wave pitch is determined by the axial spacing of the mold-the groove engraved on the mold, the groove pitch is the wave pitch. Because of the uniform oil pressure and full wave crest, hydroforming is suitable for small wave distance and multi-wave number products, such as universal corrugated expansion joints and metal hoses. If you want to change the wave pitch under this process, you have to change the mold or adjust the spacer ring on the mold. So once shaped, the wave distance is basically locked.

Mechanical forming: Roll the pipe wall by a pair or several pairs of rollers, and gradually press out the corrugations. The wave pitch is controlled by the ratio of the feed speed and the rotation speed of the roller. The advantage is that it can be adjusted flexibly-the same set of molds, adjusting the speed ratio can enlarge or reduce the wave distance. The disadvantage is that the material requirements are high, and the stainless steel thin-walled pipe is easy to wrinkle. Many large-diameter thick-walled expansion joints and external pressure single-type axial expansion joints are mechanically formed, because such products often need customized wave pitches to match specific compensation requirements.

The bellows will have a rebound during the molding process. There will be a difference of 0.3-0.8mm between the actual wave distance and the mold size, and experienced masters will count the rebound amount in advance in the mold design. After you receive the product, measure it, and the wave distance can generally be controlled within ± 0.5mm.

When the wave pitch changes, the performance changes completely: the relationship between stiffness, compensation amount and fatigue life

Let's start with stiffness. The larger the wave pitch, the larger the bending radius of the single-wave metal material, and the stronger the ability to resist deformation – the stiffness is naturally high. The reason is very simple, just like a spring, the larger the lap gap, the harder it is to press. For straight pipe pressure balance expansion joint, which relies on stiffness to balance the thrust, the wave pitch is often designed to be too large. On the other hand, the transverse expansion joint of the compound hinge needs a large amount of compensation, and the wave pitch has to be small, so that the unit length can accommodate more waves.

What about the amount of compensation? Total compensation amount = single wave compensation amount × wave number. When the wave distance is smaller, the wave number increases at the same length, and the total compensation amount increases. However, the amount of single wave compensation is not fixed-the single wave with large wave distance can be pulled more open, so the two should be calculated comprehensively. Take our commonly used bellows as an example: if the wave pitch is changed from 20mm to 16mm, the total compensation can be increased by about 30%, but the fatigue life may be reduced by 15%-20%. How to choose? Depends on the working conditions.

Fatigue life is the most sensitive piece. When bellows expand and contract frequently (such as double hinge expansion joint for air-cooled island vacuum pipe), if the wave pitch is too small, the stress concentration at the wave root will increase, and cracks often start there. I handled a case in which the customer temporarily changed the wave pitch by two millimeters, but it leaked after eight months of use-it was replaced with the original design wave pitch, and it was fine for three years. Therefore, when making non-standard customization, don't just focus on the compensation amount, the fatigue life is the ceiling.

Practical Selection: How to Select Wave Distance under Different Working Conditions

Give you a few specific scenarios, and you will understand by comparing them:

• High temperature and high pressure steam pipeline(Corrugated expansion joint for power station industry): The wave pitch is recommended to be 18-25mm. This range can take into account both stiffness and fatigue life, and it is more stable with multi-layer corrugated structure.
• Corrosive media transport(Lined with PTFE hose and PTFE compensator): The wave pitch should be too large, 22-30mm. Because the lining layer is brittle, the wave distance is too small to break the PTFE layer easily.
• Large displacement and low frequency working condition(Direct buried expansion joint, rotary compensator): The wave pitch can be as small as 12-15mm. It mainly depends on the compensation amount of the wave number pile. Anyway, it only acts dozens of times a year, so fatigue is not a problem.
• Smoke duct system(Non-metallic expansion joint, rectangular non-metallic expansion joint): In fact, metal bellows are not used in this occasion, but if you use metal rectangular expansion joint in the flue, the wave pitch will be larger-more than 30mm, because soot particles are easy to accumulate in the trough, and if the wave pitch is too small, it will fail.

In addition, to be honest, many customers only look at the diameter and pressure when selecting models, and don't mention the wave distance at all. The wave distance column on the sales bill is often blank. But if you think about it, the price difference between the wave pitch of 18mm and 22mm of the same general-purpose corrugated expansion joint can be 20%, and the performance is completely different. Therefore, the next time you inquire, it is best to tell the manufacturer your "displacement + working frequency" directly, and the manufacturer can help you calculate the appropriate wave distance.

How to deal with wave pitch abnormality during installation and maintenance

Finished installing on site and found that something was wrong with the wave distance? Don't panic, first judge whether it is caused by manufacturing deviation or installation stress. Take a ruler and measure the wave distance at three different positions:

• If the deviation is within ±0.5mm, normal, rebound influence.
• If the deviation exceeds 1mm and is concentrated on one side, there is a high probability that there is a problem with the pipeline alignment during installation, and the bellows is pulled off. At this time, you have to loosen the expansion joint tie rod nut and readjust the axial position of the pipe. For specific adjustments, you can turn our previous article "How to Adjust the Expansion Joint Tie Rod Nut".
• If the whole bellows is axially compressed and the wave pitch generally becomes smaller, it means that the cold drawing amount of the pipeline is not done correctly, or the medium temperature exceeds the design value. After parking and cooling down, observe it. If the wave distance can't be restored, you have to change parts-forcibly running with illness will accelerate fatigue cracking.

It is found that the wave pitch is significantly larger during maintenance, usually because plastic deformation (over-stretching) has occurred. This is basically hopeless, just replace it with a new one. Remember to add a limit tie rod (such as the tie rod structure in the expansion joint of the large tie rod) to the bellows when selecting the model next time to prevent accidental over-displacement.

The more accurate the wave distance is, the better. Some high-end customers take vernier calipers for acceptance, and they have to be stuck to a whole millimeter, which is actually unnecessary. As long as within the design tolerance, priority is given to ensuring that the compensation amount and stiffness meet the standard-after all, bellows is a "flexible element", and being too real may turn out good things.

Regarding the wave distance, if you still want to talk about specific working conditions, call our technical department directly. We have more than 20 kinds of drawings of expansion joints, from high-temperature axial type to directly buried type, from metal hose to PTFE compensator, and how to determine the wave pitch has ready-made case reference. Don't worry, don't fool you into choosing the most expensive, only help you choose the most correct one.

First, the core value of metal expansion joint: it is not just "compensating displacement"

When many people mention metal expansion joints, their first reaction is "absorbing thermal expansion". Yes, this is the basic skill. But its benefits go beyond that – vibration and noise reduction, preventing pipeline thrust from damaging equipment, adapting to complex spatial layouts…To put it bluntly, it is the "flexible joint" of the entire pipeline system. Without it, as soon as the high-temperature steam pipe is hot, the flange weld will crack; When the equipment vibrated, the whole pipeline shook. Two days ago, I met a customer. The pump unit in the factory building always had problems. I changed the machine seal three times, and finally found that the pipeline stress was not eliminated. Add a universal corrugated expansion joint, and the problem is solved directly. So don't underestimate this thing. It is an "invisible life-saving talisman" in power stations, cement, chemical industry and desulfurization industries.

Second, how to "save life" of metal expansion joint under high temperature and high pressure working conditions

The thermal expansion and contraction of pipelines are physical laws, and hard carrying will only lead to accidents. Metal expansion joints rely on bellows structure and absorb displacement by elastic deformation. For example, high-temperature axial expansion joint, which is specially used to deal with axial expansion and contraction; The transverse expansion joint of compound hinge solves the transverse offset. For example, the corrugated expansion joint used in the power station industry has a steam temperature of five to six hundred degrees and a pressure of more than ten MPa. Ordinary rubber compensators can't bear it at all, so stainless steel or heat-resistant alloy metal bellows must be used. Here's a key point: the deflector. Many customers don't know what it is for-it can guide high-speed airflow away, prevent the bellows from being directly washed, and its life can be several times worse. To put it bluntly, if you choose the right model and material, the metal expansion joint is the "fuse" of the system.

3. Selection of different scenarios: from power station to cement, don't choose the wrong one

And guess what? Many people hold an expansion joint parameter and want to be universal in all occasions. That's a dream. In the flue gas pipeline of power station industry, the temperature is high and the pressure fluctuates greatly. Double-sealed single-axis circular baffle door is commonly used with large-diameter thick-walled expansion joint. In the cement industry, there is much dust and severe wear. The metal corrugated expansion joint in the cement industry has to be lined with wear-resistant lining. There are also desulfurization flue gas baffle doors, which are highly corrosive, and lined with PTFE hose or PTFE compensator is the correct solution. As for the vacuum pipeline of air-cooled island, the double hinge expansion joint of air-cooled island vacuum pipeline has to be used to resist negative pressure. So don't think about it once and for all, you have to choose according to the medium, temperature, pressure and displacement direction. If you are not sure, go directly to the manufacturer for the model selection table, or refer to the metal expansion joint weight table to estimate the model.

4. Installation does not mean completion: those pits of installation and post-maintenance

The screw must be removed! Many novices are afraid of trouble, so they leave the tie rod nut unremoved. As a result, the expansion joint can't expand and contract freely, so it is installed for nothing. The role of the expansion joint tie rod is to protect the bellows during transportation, and once in place, it must be loosened or removed. There is also the direction of the arrow-that is the direction of the medium flow, and the guide tube will block if it is installed backwards. Also pay attention to the coaxiality when installing, deviation will lead to stress concentration. What about post-maintenance? Regularly check whether there are cracks and corrosion points on the surface of the bellows, especially the external pressure single axial expansion joint with external sleeve, which is easy to accumulate water and rust. To put it bluntly, three points for selection and seven points for installation, and the remaining ninety points for daily inspection. Don't wait until it leaks before replacing it, it will be expensive.

V. Life and cost: How long can an investment last?

The design life of metal expansion joints is usually calculated by the number of cycles, such as 10,000 or 20,000. How long can it actually run? Depends on working conditions. It is also stainless steel bellows, which may be fine for more than ten years when used on clean steam pipes; Used in desulfurization flue gas containing chloride ions, it will be perforated in half a year. So don't just look at the price, but whether the material matches. For example, vacuum special hoses are used in high vacuum systems, which have extremely strict requirements for leakage rate, and are expensive but must be used. Rubber compensators are cheap but have limited temperature and pressure resistance. The initial investment of metal expansion joint is high, but the frequency of maintenance and replacement is low, and the whole life cycle cost is cost-effective. You taste, you taste. Finally, let me add: the expansion joint and the compensator are the same thing, so don't be fooled by different names. Anyone who knows the business knows that if you choose the right thing, the system will be safe and stable; Wrong choice, headache every day.

First, find out what kind of metal expansion joint you are facing

The name metal expansion joint is actually a general term. What you have in your hand may beUniversal corrugated expansion joint、High temperature axial expansion joint, could also beDirect burial type、External pressure single type axial type, even large-diameter thick-walled expansion joints. Different structures correspond to different working conditions: high-temperature axial type for high-temperature steam pipeline, direct burial type for buried pipeline, and occasions where transverse displacement needs to be absorbedCompound hinge transverse expansion joint。 Get the wrong model, and it will be a ticking time bomb.

Before selecting, look at the pipeline medium temperature, pressure and compensation direction-is it axial, transverse or angular displacement? For example, the power station industry usesCorrugated expansion joint for power station industryHigh temperature resistance and fatigue resistance are usually required, while the cement industryMetal Corrugated Expansion Joints in Cement IndustryTo deal with dust and abrasion. Each product has a clear application scenario, so don't mix it.

2. Four things before installation: arrow, guide tube, pull rod and bracket

Many people's first reaction when they get the expansion joint is to weld it to the pipe. Stop it. The first step is to look at the direction of the arrow on the expansion joint cylinder. That arrow represents the flow direction of the medium. If the direction is installed in the opposite direction, the guide tube will become a throttle plate, which will increase the pressure loss at least and cause the bellows to flush and fail at least —The direction of the arrow of the expansion joint refers toMedia flow, this common sense rollover case I have seen no less than ten times.

The second step is to check whether the guide tube is in good condition. The role of the deflector is to protect the bellows from being directly washed by high-speed media, especially fluids with particles (such as the pipe behind the smoke baffle door). Once the guide tube falls off, the bellows life plummets. Step 3, confirm the position of the tie rod nut. Adjustment nuts on tie rods are usually in a pre-tensioned or pre-compressed state when they leave the factory to compensate for installation errors. Once installed in place, these nuts should be loosened or adjusted according to the design requirements-how exactly? refer toHow to adjust the tie rod nut of expansion jointThe core principle of that question and answer is: let the expansion joint be in the design pre-displacement in the cold state, and only in the hot state can it work normally.

Step 4, check the pipe support. The expansion joint itself does not bear the weight of the pipe, so it is necessary to set up a fixing bracket and a guide bracket. The lack of guide brackets can cause the expansion joint to twist, which is the most overlooked mistake in installation. And guess what? Some construction sites did not install guide brackets, and three months later, the bellows was directly twisted into a twist.

3. Hard rules of upper pipe welding: torque and misalignment are not allowed

Before welding the expansion joint to the pipe, make sure that both ends of the pipe have been adjusted to the same axis. Forcibly using expansion joints to compensate for pipeline installation deviations (such as non-parallel flanges and uneven gaps) is equivalent to keeping the bellows in a torsional state after welding. The biggest fear of metal bellows is torsional loads-it is good at tensile compression, but its ability to resist torsion is almost zero.

The correct thing to do is to position the pipe with a jack or a reversed chain so that the flanges or welded joints at both ends of the expansion joint are naturally aligned. After welding, remember to review the straightness of the expansion joint. If the deviation exceeds 1% of the nominal diameter of the pipe, it should be reworked. Also, if you useDouble-sealed single-axis circular baffle doorOrElectric plug-in insulation doorIn this type of sealed structure, welding heat may damage the internal seal and require cooling measures. Alas, I've seen cases where seals melt directly after welding on site, and the rework cost is enough to buy three new expansion joints.

4. Debugging stage: Is the tie rod disassembled or not?

It was the most asked question on the scene.Do you need to remove the screw of the expansion joint？ First of all, be clear: the tie rod (or screw) plays a protective role during transportation and installation, preventing the bellows from excessively elongating or compressing. Once installed in place, whether to remove it depends on the expansion joint type.

For general-purpose corrugated expansion joints, if the design allows for axial displacement, then the locking nut on the tie rod needs to be loosened to allow the bellows to expand and contract freely. But if the tie rod is used aslimit tie rodDesign (such as double hinge transverse expansion joint), it cannot be removed at will, but the nut position should be adjusted according to calculation. How to judge? Look at the product nameplate or drawing: If the value of "pre-deformation" is marked, it is usually necessary to adjust the nut to the specified scale in the cold state. For specific operation, first loosen the nut on one side, and then symmetrically adjust the other side to avoid force on one side. After adjustment, check the bellows for skew. To put it bluntly, whether the pull rod is removed or not depends on whether it plays a "bodyguard" or a "traffic police"-the bodyguard can be withdrawn, but the traffic police have to keep the command.

5. Fatal misunderstanding in operation: the insulation layer cannot wrap the bellows

In order to keep heat at many sites, the expansion joints are wrapped tightly with rock wool. This is a big no-no. The inside of the bellows is a cavity structure, which needs heat dissipation. If the insulation layer is directly applied to the bellows surface, local superheating zones will be formed and material fatigue will be accelerated. The correct way is: the insulation layer should only be made on the pipes at both ends of the expansion joint, and the expansion joint itself should be exposed, or a detachable insulation sleeve should be used.

In addition, for high-temperature axial expansion joints and corrugated expansion joints used in power station industry, we should also pay attention to whether there is condensed water accumulation inside the bellows-when starting at low temperature, if the medium is steam, the condensed water may form a water hammer and impact the bellows instantly. The solution is to set the trap in the low position. Is that the truth? Just like when a person wears a cotton-padded jacket, he can't wrap his head around him, so he must leave the heat dissipation port.

6. The maintenance cycle is very simple: look at the displacement and fatigue times

Metal expansion joints are limited life components.Service life of expansion jointDepends on the actual number of cycles rather than simply the time. For example, an expansion joint with a design life of 1,000 full displacements can be used for about three years if it starts and stops once a day; But if the pipe vibrates frequently (like connecting the pump outlet), it can fail in a few months.

During maintenance, focus on checking whether there are cracks at the crest of the bellows and corrosion pits on the surface. ForLarge diameter thick wall expansion jointAnddirectly buried expansion joint, due to being buried in the soil or difficult to disassemble, it is recommended to reserve inspection holes or use models with leak detection during installation. If you useRotary compensatorOrSleeve type pipe expansion jointThe maintenance method is different-this type of product relies on sliding seals and needs regular grease injection. In short, different structures have their own rules. Don't apply the same maintenance method to all expansion joints.

Two days ago, I met a customer who installed a general-purpose expansion joint on the steam main pipe, without a guide bracket, and the insulation layer was still wrapped tightly. Three months later, the bellows cracked, and I came to ask "How to use the metal expansion joint?"-I can only say that the wrong installation is equal to white installation, and one step of type selection, installation and debugging can't be saved.

What Are the Ways of Fabric Connection? Complete solution of connection process of fabric fiber expansion joint in industrial pipeline compensation

The "fabric connection" we talk about, in the field of industrial pipelines, most likely refers to the joint treatment between non-metallic expansion joints (fabric fiber expansion joints) and pipelines. This type of equipment is usually used in flue gas, hot air and dust ducts, and relies on flexible bands of multi-layer fiber fabric and rubber composite. The connection method determines the sealing and life of the whole compensation system. If you choose the wrong one, the later maintenance can make people collapse.

Flanged connection: the most versatile, but the details cannot be sloppy

Flange connection is the most common way in the field-welding flanges at both ends of the expansion joint, flanges on the pipeline, gaskets in the middle, and the bolts are tightened. It is convenient to disassemble and assemble, and does not need to move the pipeline during maintenance, and directly loosen the bolts and change the ring belt. But the pit is also here: the flange surface and bolt materials are not selected correctly, and it will rust to death in half a year. For example, the PTFE-lined metal hose matching the desulfurization flue gas baffle door has strong acidic medium in the flue gas. The flange has to be made of 316L stainless steel, and the bolts have to be coated with Dacro. Otherwise, it will be too late to break the bolts when disassembling them. The sealing gasket is also cheap. Graphite composite gasket is used for high-temperature flue gas and PTFE gasket is used for low-pressure and normal temperature, otherwise the leakage rate will double directly.

Welding connection: once and for all, but don't expect to change it later

Welding connection is to weld the metal frame of the expansion joint directly to the pipe, without bolts and gaskets. Advantages? Extremely low leakage rate, suitable for high temperature, high pressure and long-term non-maintenance occasions. For example, the power station industry uses corrugated expansion joints, high-parameter steam pipelines, with a temperature above 500℃ and a pressure of 10MPa. Do you dare to use flanges? The spacer blew off long ago. There are also high-temperature axial expansion joints, and the working conditions are continuously stable, so they don't have to be cared for for ten or eight years after welding. The cost is that you have to cut the pipe during maintenance, welding, flaw detection and heat treatment, and the labor cost is more than 3 times higher than that of flange. Therefore, welding is only suitable for those scenarios where "install it and don't plan to disassemble it".

Clamp/hoop connection: a money-saving tool with low voltage and large diameter

The clamp connection looks simple-apply sealant to the end faces of the two pipes, put a rubber compensator or rubber PTFE compensator on it, and use a hoop on the outside. Quick installation, one person can do it in half an hour. It is especially suitable for smoke and air ducts with low pressure (below 0.1MPa) and large diameter (above DN1000). Cement plant denitrification flue loves to use this trick, saving money and construction period. However, the disadvantages are also obvious: the sealing ring is a consumable part, and the rubber ages quickly. If the ambient temperature exceeds 80℃, it can't bear it, so it has to be replaced every two or three years. If the medium contains particulate dust, it will wear faster. To put it bluntly, the clamp is a combination of "cheap, fast and short-lived".

Adhesive connection: Patching is the technical job

The loop belt of non-metallic expansion joint (fabric fiber expansion joint) itself is multi-layer composite, and the loop belt and the metal frame have been vulcanized or bonded into one body when the manufacturer leaves the factory. The scene is just docking the frame with the pipe flange. But what if the ring belt is partially damaged during maintenance? Can't change the whole thing, can you? The sheet has to be patched on site with a special adhesive. This job looks simple, but it is actually very demanding on curing time and temperature. I met a customer two days ago, saying that it opened the glue after two months of mending. I asked: What is your curing temperature? He said that the workshop was 5℃ in winter, and it was used directly after gluing. That certainly won't work! Cure at room temperature for at least 24 hours, and ensure that the ambient temperature is not lower than 15℃. If it is a high-temperature zone, it is necessary to use heat-resistant epoxy glue, which can withstand the temperature above 150℃ after curing. Tsk, not many people pay attention to this detail.

Three elements of model selection: temperature, pressure and space

Which one do you choose? There is no "best", only "most suitable". Staring at three numbers first —Medium temperature: Basically say goodbye to rubber and ordinary bonding when it exceeds 300℃. Flange bolts should pay attention to high-temperature creep, and welding is more reliable.Pressure rating: Flanges or welding are preferred above 0.1MPa, and clamps below 0.1MPa can also be made up.Installation space: The narrow clamp is easier to use than the flange, such as the double hinge expansion joint of air-cooled island vacuum pipeline, which has a compact space and can't be sealed with a clamp, so it can only be welded.

Cement industry metal corrugated expansion joint, pipeline temperature fluctuation large, dust. Some people tried to save trouble with welding. As a result, two years later, the ring belt wore out and had to be replaced. The pipe had to be cut, and the kiln was shut down for three days, resulting in a loss of millions in output value. If the flange connection was used at the beginning, with the double-sealed single-axis circular baffle door, the sealing grade could reach more than 99.9%, and it would take half a day to change the ring belt. On the other hand, the double hinge expansion joint of air-cooled island vacuum pipeline, welding is the only choice in high vacuum scenario, and the flange leakage rate can't pass the test at all. Once the vacuum degree drops, the power generation efficiency plummets.

Don't be fooled, turn your own standards

To put it bluntly, many people can't even figure out the temperature threshold. If you are not sure, go directly to this site"National Standard for Non-Metallic Expansion Joints"Or compare the article "The Specific Function of Expansion Joint Guide", which is much more realistic than listening to bragging. The way of fabric connection seems simple, but behind it is the comprehensive consideration of material science, thermodynamics and sealing technology. If you choose the right one, the pipeline will run safely for ten years; If you choose wrong, half a year will make you doubt your life.