Specialized in manufacturing compensators, expansion joints, baffle doors

Calculate the thermal displacement first: temperature change is the starting point, installation temperature is a pit

Metal expansion joint calculation method? The first step is always thermal displacement. The formula is simple enough to recite: Δ L = α × L × Δ T. The linear expansion coefficient α is 0.012 mm/m·℃ for carbon steel, 0.016 for stainless steel and 0.018 for copper. Just note it. The pipe length L is dead, but how to take Δ T? A lot of people are stuck here.

Does the installation temperature use the ambient temperature or the actual pipe temperature? I'll tell you, directly based on the local extreme temperature difference. For example, outdoor pipelines in the north, -30℃ in winter and +40℃ in summer, Δ T is 70℃. Don't worry about what month it is when it is installed, leave an allowance. The expansion amount of the pipe is considered small, the expansion joint is hard, and the bellows is directly torn. A while ago, a customer used a general-purpose corrugated expansion joint on a steam pipeline. The installation temperature was 20℃, the actual steam was 160℃, and Δ T was only 140℃. As a result, the actual operating temperature of the pipeline fluctuated to 180℃, and the expansion amount exceeded 20%. The bellows cracked after half a year.

The wrong calculation of pressure and thrust causes the expansion joint to be scrapped directly

The thrust generated by the pressure is F = P × A, and this A is the effective area of the bellows. However, the effective area calculation methods of different structures are completely different. For example, the effective area of the general corrugated expansion joint is equal to the circular area corresponding to the average diameter of the corrugated pipe; However, the effective area of the external pressure single axial expansion joint should be calculated according to the inner diameter of the external pressure cylinder, not the bellows itself. There's a big pit here: the blind plate force created by pressure, and the pipe can twist like noodles if it is not withstood by the tie rod or the main fixing bracket.

When calculating pressure thrust, don't forget that the medium may still have impact force. Water hammer effect when steam pipe starts, instantaneous pressure may be twice the working pressure. You calculate the thrust according to the steady-state pressure, but the fixed bracket is not designed enough, and the expansion joint tie rod breaks directly. Last year, in a chemical plant, the straight pipe pressure balance expansion joint was not counted as a water hammer, the fixed bracket was pushed and displaced by 30mm, and the bellows was unstable and scrapped.

Compensation amount, stiffness and fatigue life-how to break the iron triangle?

These three parameters are constrained by each other. The greater the amount of compensation, the softer the bellows (lower stiffness), but the fatigue life will be shortened. The national standard requires that the fatigue life is not less than 1000 times, which is the basic line. In practice, we generally design corrugated expansion joints and high-temperature axial expansion joints for power station industry according to 10,000 times. How to calculate? Reference to EJMA standards, combining wave pitch, wave height and wall thickness.

A bellows with the same caliber DN300 and a wave pitch of 40mm is 30% more stiff than a wave pitch of 50mm, but the fatigue life may double. The wall thickness increases by 0.3mm, the stiffness increases obviously, and the compensation ability decreases. It takes repeated iteration to find the balance point. Two days ago, someone took a double hinge transverse expansion joint and asked for a compensation amount of 100mm. I saw that the wall thickness was only 1.5mm, the wave number was 8, and the fatigue life was only 500 times. It is recommended that the wave distance be widened, the wall thickness be 1.8mm, the wave number be increased to 12, and the life time be mentioned only 8,000 times.

Don't use one algorithm for all models

The calculation ideas of straight pipe pressure balance expansion joint and double hinge transverse expansion joint are completely different. The pressure balanced type counts the thrust cancellation generated by internal pressure-it has two bellows itself, one absorbing displacement and the other canceling the pressure thrust. However, the hinge type should consider the coupling of angular displacement and lateral displacement, not simple superposition. For example, the transverse expansion joint of compound hinge, the angle change between two hinge groups will produce axial and lateral displacement at the same time, and it is necessary to draw a displacement vector diagram to calculate it clearly.

Large diameter thick wall expansion joints are more troublesome. The increase of wall thickness leads to the increase of stiffness and the decrease of compensation ability, but the pressure thrust is large. You have to assume a wall thickness first, calculate the compensation amount, check the fatigue life, and adjust the wall thickness or wave number if it fails, and iterate repeatedly. Some designers try to save trouble and directly use empirical formulas. As a result, the stress of the pipeline exceeds the limit and the vibration is abnormally large.

Guide tube and tie rod nut are not decorations

The function of the guide tube is to reduce the erosion of the medium, especially for high temperature and high speed steam. But it affects the effective area and the pressure loss – the inner diameter variation must be taken into account in the calculation. Many people directly calculate according to the inner diameter of the bellows, ignoring the space occupied by the guide tube. As a result, the actual pressure loss is more than 30% greater than the calculated value. The adjustment of the tie rod nut directly determines the direction of pre-deformation, and if it is installed backward, the expansion joint will fail in advance. How to adjust the expansion joint tie rod? The principle is: when cold tight, the tie rod nut compresses the bellows, so that it has a pre-stretch in the installed state, so that when the working temperature rises, the bellows can be both compressed and stretched, and the service life is longer.

Actual Combat Checklist: Don't wait for an accident to regret it

After getting the medium temperature, pressure, pipe diameter and direction diagram given by the customer, follow this order: first draw the displacement vector diagram to clarify the axial, transverse and angular displacements; Calculate thermal displacement and pressure thrust; Then the structure type is selected according to the working conditions-high temperature axial type for high temperature, large diameter thick wall expansion joint for large diameter, and transverse type of compound hinge for absorbing multi-directional displacement; Finally, check the fatigue life, which is not less than the design life.

Two days ago, a customer used the double straight pipe bypass pressure balance expansion joint on the steam pipeline. As a result, the lateral displacement was not counted. After the thermal expansion of the pipeline, the lateral offset of 15mm was produced, and the expansion joint was hard, so the bellows leaked after three months. You say it was wrong or not? The lateral displacement is only 15mm, and it can be solved by adding a double hinge transverse expansion joint. Why save that money?

Metal expansion joint calculation method? To put it bluntly, there are three parameters: thermal displacement, pressure thrust and fatigue life. If one is wrong, it will wait for rework. Selection is not a chance, it is calculated.

1. Why insulate the metal expansion joint? Not just to save energy

Many people think that the insulation layer for the pipe is just to save some steam money. However, in the metal expansion joint, the significance of heat preservation is far more than energy saving. If you think about it, the core function of the expansion joint is to absorb the thermal displacement of the pipe-the bellows has to expand and contract freely to work. If the temperature is too high, the creep strength of the bellows material (such as 304 or 316L stainless steel) will drop dramatically. For example, when 304 stainless steel is at 550℃, the allowable stress is only about 1/3 of the normal temperature. At this time, the heat insulation is not in place, and the bellows may be fatigued and cracked in advance before the design life.

A more insidious problem is condensation corrosion. If the insulation layer of steam pipe is damaged, the local temperature drops below the dew point, and the water vapor condenses into acidic water, which directly corrodes the bellows wall. I have seen a corrugated expansion joint used in a power station industry in a power plant. Because the outer iron sheet of thermal insulation was not sealed well, the rain seeped in and stuffed, and it rusted out in two years. Therefore, insulation is not only to save coal and gas, but also to protect the expansion joint from heat damage and chemical corrosion. To put it bluntly, if the insulation is not done well, it is only a matter of time before the expansion joint is scrapped.

Second, how to choose insulation materials? Don't let the heat "burn through" your protective layer

When choosing insulation materials, first look at the temperature resistance grade. Don't listen to the manufacturer's bragging that "composite silicate felt can withstand 1000℃", you have to leave an allowance in actual working conditions. For steam pipes (usually ≤350℃), rock wool or aluminum silicate fiber wool is enough, with a density of more than 120kg/m³. However, if it is flue gas and high-temperature gas, such as the kiln tail flue gas connected by the metal corrugated expansion joint in the cement industry, the temperature can reach 600~900℃. At this time, ceramic fiber blanket or aerogel felt must be used. Aerogels have low thermal conductivity and are three times more insulating than rock wool at the same thickness, but they are expensive-you have to calculate the full life cycle cost.

In addition, it should be noted that the insulation material cannot contain chloride ions that are harmful to stainless steel. The chlorine content of some inferior rock wool exceeds the standard, and chloride will precipitate when exposed to water, which will lead to stress corrosion cracking of bellows. I recommend that the chloride ion content test report be required at the time of purchase (standard general requirement ≤25ppm). Let's talk about the actual details: the thicker the thickness of the insulation layer, the better. After the thickness exceeds the critical point, the outer layer dissipates heat and increases the cost. Generally, 100~150mm for steam pipeline and 200~250mm for high-temperature flue gas. When you are not sure, you can find the manufacturer to calculate the heat loss. Don't slap your head.

Third, the easiest pit to step on during construction: the wave section, guide tube and tie rod can't be wrapped

This is the core problem of metal expansion joint insulation. Many people directly wrap the thermal insulation cotton on the bellows, and then pack it with iron sheet-it's done, and the expansion joint is directly wasted. Why? Because the axial expansion and contraction and lateral displacement of the bellows require space. If the insulation layer is stuck between the peaks and valleys, once the pipeline expands thermally, the bellows will be hardly supported by the insulation material, which will reduce the compensation amount at the light and squeeze the corrugated shell into deformation at the worst.

Correct practice: There must be an air gap between the insulation layer and the bellows.How to do it specifically? First, make an independent layer of detachable thermal insulation box (also called thermal insulation cover) outside the expansion joint, make the box body with stainless steel sheet, fill the inside with thermal insulation cotton, and keep a gap of 50~80mm between the box body and the outer wall of the bellows. This insulates without restricting movement. Don't forget the deflector-it is installed inside the expansion joint and is responsible for deflecting the medium and protecting the bellows. The guide tube itself does not need heat insulation, but during construction, do not stuff the heat insulation material into the annular gap between the guide tube and the bellows, otherwise the guide tube will deform, the medium vortex will intensify, and even cause vibration.

And tie rods and nuts. The tie rod on the general corrugated expansion joint and the large tie rod expansion joint is used to bear the internal pressure thrust and cannot be wrapped in the insulation layer. Because the tie rod needs to be checked regularly for looseness (to see if the nut is displaced), if the package is dead, the inspector can't see it at all. The correct way is to expose the pull rod and adjustment nut, and the insulation box only covers the main section of the bellows. Similarly, the reinforcing ribs on the rectangular expansion joint must also leave a heat dissipation gap.

4. What should I do if the insulation layer fails? Look at these three signals for daily inspection

The insulation layer will not break suddenly, and it will wear out slowly. Keep an eye on three points in daily inspection:

• Abnormal increase in surface temperature:Sweep the outer wall of the insulation box with an infrared temperature measuring gun. If the temperature somewhere is more than 20℃ higher than the adjacent temperature, it means that the insulation cotton has fallen off or water has entered. Especially the corrugated section of the high-temperature axial expansion joint is most prone to hot spots.
• Deformation or bulging of outer sheet:If the stainless steel plate used in the insulation box bulges, water may accumulate in it. The expansion of water vapor under heat will push the iron sheet up and aggravate corrosion at the same time. Seeing this situation, quickly take it apart to check whether the insulation cotton is wet.
• Insulation box fixing loose:The expansion joint vibrates when it runs, and the hoops and bolts will slowly loosen. During the inspection, push the insulation box with your hand. If the shaking is obvious, tighten it again.

Expansion joints on steam pipes, recommended once a month; High-temperature flue gas (such as near the desulfurization flue gas baffle door) is best seen every Monday because of the high risk of corrosion. If the insulation layer is found to be damaged, don't drag it, and replace it immediately when the machine is shut down for maintenance. Otherwise, the little labor cost you save won't be enough to pay for an expansion joint.

5. Comparison of insulation schemes under different working conditions: steam, flue gas and high-temperature gas are different

The insulation strategies of different media vary greatly. Let's go through them one by one:

Steam pipeline:The medium temperature is generally 250~400 ℃, and the pressure is medium and low. Insulation scheme recommended: rock wool + stainless steel insulation box, thickness 100~150mm. Note that the steam pipeline has hydrophobic requirements, and a drainage hole (a small hole of Φ 6mm is enough) should be left at the bottom of the insulation box to avoid the accumulation of condensed water. If the external pressure single axial expansion joint is used, the external pressure cylinder also needs to be insulated, but the guide sleeve should be ensured to slide freely.

Flue gas pipeline:Flue gas baffle doors and expansion joints commonly found in power stations and cement industries. The flue gas temperature fluctuates greatly (200~700℃), and contains sulfur and dust. The insulation material must be acid-resistant ceramic fiber with a thickness of more than 200mm. It is recommended to use a double-layer structure during construction: the inner layer is high-temperature resistant ceramic fiber blanket, and the outer layer is waterproof aerogel felt. Non-metallic expansion joints (fabric fiber expansion joints) that are in direct contact with smoke usually do not need external insulation, but rely on their own multilayer insulation structure. However, the metal rectangular expansion joint must be wrapped, and a rain cover should be added outside the insulation box-because the smoke will condense due to the decrease in temperature and corrode the bellows.

High temperature gas (e.g. hot air, inert gas):Temperatures may exceed 900°C. In this case, it is recommended to use aerogel composite insulation structure, or simply make internal insulation-spray high-temperature insulation coating on the inner wall of the expansion joint. However, the internal insulation will affect the installation of the guide tube, so it needs to be specially designed. For example, the double hinge expansion joint of air-cooled island vacuum pipeline, the medium is a mixture of high-temperature steam and air, and anti-freezing should be considered outside, and the thickness of insulation layer may exceed 300mm.

Finally, there is no universal "universal insulation scheme". When selecting, be sure to send the medium, temperature, pressure and environmental conditions of your pipeline to the expansion joint manufacturer, and ask them to help calculate the insulation thickness and structure-don't copy other people's schemes by your own feeling. After all, how to insulate metal expansion joints is ultimately an engineering problem, not a mathematical problem.

Last summer, the desulfurization flue of a coastal power plant was shut down for maintenance. The reason was dumbfounded-the flange gasket was aging, and the bolt preloading force was uneven. The flue gas came out of the gap, and the whole line was forced to land the load. Lao Wang, who is in charge of the maintenance, told me, "The corrugated pipe material cost a lot of money, and the result was planted in the connection method." Indeed, non-metallic expansion joints (fabric fiber expansion joints, rubber compensators, rectangular non-metallic expansion joints) are good at carrying heat expansion and cold contraction in flue ducts and desulfurization systems, but if the connection method is not selected correctly, none of them can run away from air leakage, running and shutdown.

1. Flanged connection: the most common pit, often the deepest

Flange connection is the most commonly used way of non-metallic expansion joints, none of them. Regardless of the siteNon-metallic expansion joint (fabric fiber expansion joint)StillRectangular non-metallic expansion jointThe mating flanges will be equipped when they leave the factory. But when installed on site, three details are prone to rollover.

flange surface flatness— — I have seen the on-site pipe flange tilt up by 3mm after welding, and the workers firmly plugged the asbestos gasket flat. As a result, it leaked one week after it was put into operation. Correct practice: the flatness error of the matching flange is ≤1mm/m, otherwise it should be leveled with sealant or machining.

bolt preload force-Many people think that "the tighter it is screwed, the less it leaks", which may be right for metal pipes, but it is a disaster for non-metal expansion joints. The loop tape (fabric fiber layer) is pressed too tightly by the bolt, the edge stress is concentrated, and it tears in a few months. This site'sRectangular non-metallic expansion jointClearly mark the torque value in the installation manual. For example, the preload force of M16 bolts should be controlled at 80-100N·m. Don't rely on the feel.

Gasket selection— — The flue gas temperature is high and contains sulfur, so ordinary rubber gaskets can't bear it.Rubber PTFE compensatorCommonly used integral flange + PTFE gasket, corrosion resistance and good fit. But note: the gasket should not be too thick, otherwise the bolt will not be compressed firmly; It should not be too thin, otherwise the compensation joint will be easily displaced when vibrating. According to JB/T 12235-2015 standard, the gasket compression ratio is controlled at 25%-30% to be the most stable.

Used in a cement plantNon-metallic expansion joint (fabric fiber expansion joint)Connecting the cyclone outlet, the workers tried to save trouble and screwed all the flange bolts to the maximum torque. As a result, the ring belt tore a hole along the bolt hole, and the smoke was sprayed with dust for half an hour. Later, I changed the preload wrench and never happened again.

2. Connection between clamp and buckle edge: save money? Temperature resistance passes first

Many people like to connect low-pressure large-diameter pipelines with clamps or buckles, which are cheap and convenient to disassemble. But here's the paradox: You think clamps save money and hassle? Wrong, the temperature resistance rating does not match the fastener material, and it will come loose in a few months.

Clamps are mostly used in circular pipes, such as the flue section behind the flue gas baffle door. However, the bolts and spring sheets of the clamps will creep at high temperatures-when the temperature exceeds 200℃, the elastic modulus of ordinary carbon steel buckles decreases by 30%. A stainless steel 304 clamp was used in a desulfurization project. Half a year later, the bolts were loosened due to thermal cycle, and acid fog emerged at the interface. Later, it was replaced with a 310S clamp to stabilize.

This site'sRectangular non-metallic expansion jointThe structure of buckling edge + strip is mostly used: the edge of the expansion joint band is flanged, embedded in the flange groove, and the outside is fixed with an entire strip. This design is particularly friendly to the four corners of a rectangular pipe, and there will be no case where the clamp can't press the R corner. However, if the material of the strip is not selected well, it will also roll over-ordinary galvanized strip will rust and wear in the wet desulfurization environment for three months. It is recommended to use 316L or FRP composite strips.

Have you ever used a failed clamp case on your project? Chat in the comments section.

3. Hot vulcanization bonding and integral molding: Don't be superstitious about the word "sticky"

Rubber non-metallic expansion joints, such as this station'srubber compensatorAndRubber PTFE compensatorThe connection method is commonly bonding or integral vulcanization molding. However, many purchases are biased by sales talk: "We use imported glue, which sticks firmly!"-Not enough bonding strength? That's because you didn't follow the standards.

High temperature flue gas section (> 120℃) must use hot vulcanization process。 The essence of vulcanization is that rubber molecules are crosslinked with reinforcing materials to form chemical bonds, and the peel strength can reach more than 50N/mm. In the normal temperature section (≤80℃), cold adhesion and special primer can be used, and the peel strength can also reach ≥25N/mm as required by JB/T 12235-2015.

I met a customer who used a cold-stick rubber compensator in a hot air duct (180℃) of a power plant, and the bonding surface degummed after two weeks. During the maintenance, it was discovered that the cold adhesive glue softened at high temperature and couldn't hold it at all. So,Not all non-metallic expansion joints can be bonded— — When selecting the model, be sure to ask clearly: What is the medium temperature? Is it frequent start and stop?

Integral molding is to directly mold rubber or PTFE into a compensator with flanges, such as this station'sPTFE compensator, the flange and the corrugated body are formed at one time, there is no bonding interface, and the reliability is higher. But the cost is also high, and it is not suitable for oversized sizes (such as rectangular sections above 5 meters).

4. Special structure: pipe welding and composite seal

In some operating conditions, the standard connection method is simply not sufficient. For example, double seal structure-learn from this siteRound Flap Door (Double Seal)The idea that the non-metallic expansion joint can also be made into two layers of seal inside and outside. The inner layer is covered with fabric fiber skin to absorb displacement, and the outer layer is protected by metal deflector + temperature-resistant sealing strip. This structure is commonly found in high temperature fan inlets and outlets that require zero leakage.

Such asHigh temperature axial expansion jointNon-metal alternative-welded to the pipe with stainless steel through pipe, covered with multiple layers of skin (silicone cloth + ceramic fiber + stainless steel wire mesh), the design logic of layered overlap is simple: each layer is responsible for a function-the innermost layer is insulated, the middle layer is pressurized, and the outer layer is corrosion-resistant. The weld position must be staggered to avoid thermal stress concentration. After one project did so, it was extended from quarterly maintenance to biennial maintenance.

In addition,Double hinge expansion joint for air-cooled island vacuum pipelineAlthough it is mostly used in metal pipes, its hinge compensation concept can also be transplanted to non-metals: the universal joint structure allows the displacement in three directions, and the composite gasket is added at the connecting flange, which has a very stable effect.

To summarize – there is no best way to connect, only the most suitable. If the medium temperature is lower than 150 ℃ and the pressure is slightly positive, the flange connection is sufficient; Large-diameter low-pressure buckling strip; Vulcanization or integral molding for high temperature; If you want zero leakage, go to double seal. This site'sNon-metallic expansion jointCovering fabric fiber, rubber, PTFE and other materials, the connection methods can be customized from flange, clamp to vulcanization bonding. The key is to select according to parameters-the temperature, pressure and displacement are clearly reported, and we will help you match the scheme without running.

How exactly do metal expansion joints expand? Physical processes from bellows deformation to displacement absorption

When many people come into contact with the metal expansion joint for the first time, they will stare at the circle of corrugated pipes in a daze: this thing looks like the drain pipe of a washing machine. How can it swallow the thermal expansion and contraction of tens of millimeters of pipes? In fact, the core principle is very simple-relying on the elastic deformation of the bellows itself to "eat" the displacement. Think of it as a retractable spring tube, except that the spring is made of stainless steel or alloy, and the shape of the corrugations has been specially calculated to bend repeatedly without fatigue fracture while withstanding internal pressure and temperature.

Each corrugation (also called crest and trough) of the bellows will bend and stretch when stressed, and multiple corrugations are accumulated together, and the compensation amount comes out. Take what we always sayUniversal corrugated expansion jointFor example, the single wave compensation amount is typically between 2 and 8 mm, depending on the wave height and wall thickness. A set of more than ten waves of expansion joints can easily handle tens of millimeters of axial expansion and contraction. To put it bluntly, this is to accumulate small deformations into macroscopic displacements-the same principle as stacking Arhats. One person doesn't have much height on tiptoe, but more than a dozen people can reach the roof after stacking them. Isn't this the principle?

The direction of displacement in the pipe is not only axial. The steam pipe heats up and the pipe elongates along the length, which is axial. However, sometimes the pipeline is deformed or turned due to the bracket, and there will be lateral misalignment or angular deflection. At this time, it is not easy to rely on the axial expansion joint alone. Hard installation is equivalent to letting the bellows twist to work, and it will crack in a few cycles.

What to do? Engineers added tie rods, hinges or universal rings outside the bellows to forcibly limit the deformation direction of the bellows. Such asCompound hinge transverse expansion jointTwo sets of bellows are used to cooperate with the hinge structure to absorb the lateral displacement;Curved tube pressure balance expansion jointThen the internal pressure of the bellows on both sides is self-balanced, and the angular displacement is absorbed, while the pressure thrust is not transmitted to the fixed bracket. You see, the same bellows, with different restraints, can make more than a dozen functions-the products listed on our website, likeStraight pipe pressure balance type、Double straight pipe bypass pressure balance type、External pressure single axial expansion jointThey are all derived from this idea.

The elastic limit of metal is just that little. Will it not crack when bent repeatedly? Good question. Here we have to talk about the fatigue life design of the expansion joint. The bellows will be pre-strained before leaving the factory, and the wall thickness at the trough is usually a little thicker than the peak, in order to transfer the stress concentration zone to the peak position. In addition, the guide tube is an accessory-the guide tube is sleeved on the inner wall of the bellows, which can not only guide the flow of the medium without washing the ripples, but also reduce the vibration caused by turbulence (for specific functions, please refer to the Q&A in our station).

The service life of the expansion joint is generally calculated according to 1,000 to 10,000 full cycles in design. Under actual working conditions (such as how many times a year the power plant pipeline starts and stops), it is no problem to use it for twenty or thirty years. Two days ago, a customer asked the gas bellows to be changed every few years. In fact, it was civil gas hoses, which are different from industrial metal expansion joints. Civil gas hoses have age requirements, and the type test used in industry is carried out according to the standard JB/T 12235. As long as it does not exceed the design displacement and pressure, it can be checked regularly, and there is no need to pat your head to change them.

When installing the expansion joint, what exactly is that arrow direction and tie rod nut for? The direction of the arrow refers to the direction of the flow of the medium, and its main function is to prevent the direct impact of the medium on the ripples caused by the reverse installation of the guide tube. The tie rod nut is locked in the transportation and installation stage, and the purpose is to pre-compress or pre-stretch the expansion joint to the middle position to avoid the on-site pipeline being misaligned; After all the pipes are connected and the fixing bracket is welded, loosen the nut to the specified position (usually the product description will give the distance between the two nuts), otherwise the expansion joint is locked and there is no room for expansion at all. You say it was wrong or not?

To put it bluntly, the principle of metal expansion joint expansion is not mysterious. The difficulty is to choose the right model and install it correctly according to whether it is axial, transverse or angular displacement in the pipeline system. Like those used in the power station industryCorrugated expansion joint for power station industryCement industryMetal Corrugated Expansion Joints in Cement Industry, the working temperature is high, the dust is heavy, and it will be equipped with heat insulation layer and wear-resistant lining-but in the final analysis, the corrugations of the bellows are still bending honestly, bearing all the thermal stress for the pipe. Therefore, the next time you see the bulging bellows on the pipe, you will know that it is not a beautiful decoration, but a hard-core part that actually carries displacement with your body.

Why do you have to have regular inspections? — Don't wait until you miss it to regret it

Two days ago, I met a brother from a power plant. Their desulfurization tower was exportedNon-metallic expansion joints (fabric fiber expansion joints)Suddenly it was torn, and the hot smoke was wrapped in slurry and directly leaked out. After the production line was stopped for 3 days, the light loss was a small million. You say it was wrong or not? In fact, this kind of accident can be completely avoided-as long as it is checked regularly, it will only take a few hours to replace one if the signs of aging of the fabric layer and corrosion of the metal frame are found in advance.

Non-metallic compensators (also called non-metallic expansion joints and fabric fiber expansion joints) are widely used in smoke and air ducts, desulfurization systems and cement production lines, because they are resistant to high temperatures, corrosion resistance and can absorb large displacements. However, its soft belt is tossed by high temperature, dust, acid and alkali gases all the year round, and its aging rate is much faster than that of metal parts. When you see air leakage, smoke leakage and abnormal noise before dealing with it, the internal structure is often hurt. SoHow to check a non-metallic compensator?This problem is actually one of the core lessons of pipeline maintenance.

Appearance inspection: Seeing, touching and listening, three steps to find out the big fault

Look-scan details with your eyes

Standing on the side of the compensator, first look at the surface of the fabric for bulging, scratches, fluffing, and whitening. Bulging usually means debonding between layers, and scratches deep may have damaged the inner PTFE or rubber film. Pay special attention to the corners of rectangular non-metallic expansion joints-that position has the most stress concentration and is most likely to crack first. Look at whether the metal flange is corroded and the weld is cracked.

Touch-to feel with your hands

Put on the gloves and touch the circle along the surface of the circle band. If it feels hard and sticky locally, it means that the material has degraded. Rubber compensator or rubber PTFE compensator is especially afraid of this-it will be brittle and cracked when it is hard, and it will be corroded by the medium when it is sticky. The normal feel should be uniform elasticity and smooth surface.

Listen-Find abnormalities with your ear

When the equipment is running, get closer and listen to whether there is a hissing air leak or a puff bulge. Experienced masters know where they leak as soon as they hear it-the hissing sound is mostly due to the fabric needle eye being enlarged, and the puff sound may be the displacement of the internal cushion.

After these three steps are completed, 70% to 80% of the surface problems can be found out. However, some internal injuries have to rely on these methods.

Dimension and displacement compensation ability check-measure to know whether there is failure

Nonmetallic compensators are not only soft connections, they absorb the thermal expansion and mechanical displacement of the pipe. Think about it, the pipe can stretch tens of millimeters when it is hot, and it all depends on this soft guy's deformation to digest it. If it loses its elasticity due to aging, or does not leave enough margin during installation, the consequence will be pipe stress hard top, flange crack, or even equipment.

How to check? Simple: Measure the actual length and width of the compensator under shutdown, and compare with the factory design dimensions. If the amount of compression has exceeded 80% of the design maximum, or the amount of stretch is almost to the end, then you have to consider replacing it. Common standards, such as JB/T 12235-2015 "Non-metallic Expansion Joints", specify the displacement limit and fatigue life.

In addition, don't forget to look at the guide tube (if any) -the specific function of the expansion joint guide tube is to protect the fabric from being washed by high-speed airflow. If it corrodes and collapses, the compensator life is directly discounted in half.

Sealability and airtightness test-a dozen pressures, all leaks appear

The front ones are all visual and feel, and the real tough battle is the airtightness test. Especially in the desulfurization flue gas baffle door, air-cooled island system and other occasions with high sealing requirements, the high air leakage rate not only increases the energy consumption, but also may lead to scaling and corrosion of peripheral equipment.

Close the valves on both sides of the non-metallic compensator, seal one end with a blind plate, and give 0.05~0.1MPa compressed air to the other end (don't overpressure, the fabric can't bear to rush), and then coat each seam, flange surface and bolt hole with soapy water. Where bubbles are the leakage spots.

Before the test, confirm that the compensator has no obvious bulge or tear, otherwise it will collapse directly when the wind takes a dozen, which will be dangerous. The pressure holding time should be at least 5 minutes, and the pressure should not drop before it is qualified. If there are not many leaks, you can try to plug them with special repair glue; But if the fabric leaks in a large area, or several leaks are strung together, don't toss it, just replace it with a new one.

Judgment and treatment of common faults-which can be repaired by themselves and which must be replaced

Come on, go straight to the dry goods:

• Small scratches and slight fluffing on the fabric surface: Apply a layer with silicone rubber or fluororubber repair agent, and it can top for a while when it dries. However, the premise is that the covering layer is not damaged, and the medium temperature does not exceed the temperature resistance range of the repair agent.
• Air leakage due to loose flange bolts: This can be tightened by itself, and the torque value is tightened 2~3 times in diagonal order. Don't screw to death at once, it will easily deform the flange.
• Local bulging and delamination of circular zone: It indicates that the fabric and inner layer have been detached. Don't fix it, just change it. Because the bulge will get bigger and bigger and break sooner or later.
• Metal frame corrosion perforation: Look at the corroded area. If it is only a small piece, you can apply anticorrosive paint after repair welding and polishing; If a large area is thinned, the whole frame has to be replaced.
• Fabric distortion due to displacement exceeding limit: First check whether the pipe support is displaced or the original selection is too small. If it is a bracket problem, adjust the bracket; If it is a type selection problem, the expansion joint with a larger compensation amount must be replaced.

At the end of the day,How to check a non-metallic compensator?Not going through the motions, but turning "examination" into "diagnosis". Each inspection records the appearance status, size data, and leakage, and compares the historical trend. For example, the compression amount in the last quarter was only 40%, and this time it reached 60%, which means that the aging has accelerated-the spare parts replacement plan can be arranged six months in advance.

Don't wait for the smoke to panic. Follow this list, and your non-metallic compensator (whether it is a rectangular non-metallic expansion joint, rubber compensator, or PTFE compensator) will last three or five more years, and it will be perfectly fine.