Specialized in manufacturing compensators, expansion joints, baffle doors

What are deflectors used for? What happens if it falls off- -starting with the expansion joint structure

Let's talk about the basics first. The compensator (that is, the expansion joint) usually has a guide tube inside it. This thing looks inconspicuous, but it really doesn't work without it. The guide tube is installed on the inside of the bellows, and it mainly does two things: first, it guides the medium to flow smoothly and reduces turbulence and resistance; The second is to protect the bellows from being directly washed by high-speed media. Think about it, if the thin wall of the bellows is directly attacked by steam or acid with particles, it won't take long to perforate and leak. As soon as the guide tube falls off, the bellows will be worn out and the vibration will exceed the standard, and the whole expansion joint will fail, and the pipeline system will suffer. Two days ago, a customer called and said that a high-temperature axial expansion joint in their factory had been used for less than half a year, and the guide tube fell off, and they were anxious to change the whole machine. I said don't worry yet, take it apart and have a look. The high probability is one of the following five reasons.

Reason 1: The flow rate of the medium is too high or the flow direction suddenly changes, and the guide tube is "blown" down

This is the most common culprit. If the flow rate of the medium in the pipeline is too fast, or the flow direction changes suddenly due to the sudden opening and closing of the valve, the guide tube will be subjected to a huge impact force. Especially in some power station steam pipelines, the flow rate can soar to more than 40m/s. If the guide tube is not reinforced well, the weld will be directly torn. Don't forget, the deflector itself is a cantilevered construction – one end is welded to the end tube and the other is freely suspended. High-speed fluid blows at it like a water gun, vibrating repeatedly, and fatigue cracking is a matter of time. What about that? When selecting the model, you have to calculate the flow rate and pressure drop, not just look at the diameter. For example, the design flow rate of the guide tube used on the double hinge expansion joint of the air-cooled island vacuum pipeline has to be stuck, and if it exceeds it, the tube has to be thickened or changed into a multi-section support structure. Many design institutes have stepped on this pit.

Reason 2: The installation direction is reversed and the arrow is white marked? The guide tube is flushed directly

There are arrows on the expansion joints to mark the flow direction of the medium, but some people don't look at it. The installation drawing saves trouble, or the on-site pipeline is complicated, and there are a lot of cases where the installation is reversed. When the guide tube is designed, only one-way flow is considered-the medium enters from the bell mouth of the guide tube and flows out from the straight tube section. Install it in turn, and the medium directly hits the blind end of the guide tube, which bears all the dynamic pressure like a plug, so it is strange that it doesn't fall off. A more hidden problem is that some manufacturers have opened guide holes or pressure relief grooves on the guide tube, and reverse installation will turn these holes into "attack points" and accelerate local erosion. Therefore, before installation, be sure to check the flow direction mark on the general corrugated expansion joint or external pressure single axial expansion joint, and then test the pressure after installation. You don't know, we have encountered it several times after sales. When we removed it, we saw that the scour marks on the inside of the guide tube were obvious, and the direction of the arrow was reversed.

Reason 3: Welding quality is not up to standard, or fatigue fracture, especially in high temperature working conditions

There is a welding connection between the guide tube and the end tube, and the quality of the weld directly determines the life. There are three common problems: insufficient height of the welding foot, unfused, and the presence of pore slag inclusion. These defects may be fine at normal temperature and low pressure. Once high temperature and high pressure steam is introduced, as soon as the thermal stress of the weld comes up, the crack will expand in a few days. Especially for high-temperature axial expansion joints, when the working temperature exceeds 400℃, the difference of creep strength between the base metal and the welding material will be magnified, and the weld at the root of the guide tube becomes the weakest link. In addition, don't forget that the pipeline system starts and stops frequently, and the alternating stress generated by the thermal expansion and contraction cycle will also make the weld fatigue. According to some data, more than 60% of the failure positions in the heat-affected zone of the weld are in the cases where the guide tube of the expansion joint of thermal pipeline falls off. Want to save trouble? Then honestly ask the manufacturer to do non-destructive testing, such as radiation or ultrasound, and don't save that little money.

Reason 4: Improper material selection of guide tube, loosening caused by corrosion or thermal expansion

This pit often comes out of projects with complex media composition. For example, the desulfurization flue gas pipeline contains a large amount of SO₂ and moisture. If the guide tube is made of ordinary 304 stainless steel, pitting and stress corrosion cracking will come soon. For another example, the material of the guide tube is different from that of the end tube, and the thermal expansion coefficient is different. Under long-term high-temperature operation, there will be a gap at the connection, and the guide tube will shake inside and finally fall off. In equipment such as desulfurization flue gas baffle door or rubber PTFE compensator, the material of the guide tube is best matched with the corrosiveness of the medium-316L should be used, and the nickel-based alloy should not be hesitated. Another point: the roughness of the inner wall of the guide tube also affects the erosion resistance. In order to save money, some small factories use unpolished hot-rolled plates, and rough particles on the face wall adhere, which aggravates local corrosion. Be more careful when making PTFE compensator. Although PTFE guide tube is corrosion-resistant, it has low strength and large thermal expansion. The fixed structure must be specially designed, otherwise it will be squeezed and deformed as soon as the temperature rises, and loosened as soon as the temperature drops.

On-site Remediation and Prevention-How to Avoid Secondary Shedding from Type Selection to Overhaul

What if the deflector has dropped? Don't rush to scrap water-based, you can try on-site repair. But only if the bellows itself is not damaged. If only the guide tube falls off, you can remove the end flange, remove the residue, and reweld or rivet the new guide tube in place. It should be noted that the repaired guide tube should be made of the same material and thickness as the original design, and the welding process should also match. However, if the bellows has been washed and thinned, it is more practical to replace it with a new one, such as choosing a double hinge transverse expansion joint or a curved tube pressure balance expansion joint with its own reinforcement structure.

The flow rate is controlled within 15m/s (depending on the medium), the flow direction is clearly marked and must be checked on site, the welding is carried out according to NB/T 47014 standard, and the guide tube material is at least one grade higher than the base metal in corrosion resistance. During maintenance, open the inspection hole on the expansion joint at least once a year, and use an endoscope to see the wear of the inner wall of the guide tube and the state of the weld. Don't wait until there is an abnormal noise or a sudden change in pressure drop to check, then it is often too late. Every point you attach importance to the deflector, the life of the expansion joint will be three points longer. Is that the truth?

1. First understand: why is the insulation layer of the compensator easy to break, and what are the consequences of breaking it

The insulation layer of the compensator is damaged for two reasons, to put it bluntly: first, the environment is too bad, and second, the construction itself is not in place. Think about it, the pipe expands and contracts by heat, and the compensator itself expands and contracts there. If the insulation layer is too tightly wrapped, too hard, or the wrong material is chosen, and it shakes with the bellows, can it not crack?

What's even more ruthless is the high-temperature flue gas and steam pipeline-as soon as the temperature rises, if the insulation material is not temperature-resistant enough, it will directly burn and carbonize, losing the heat insulation effect. There are also those open-air pipes. When the rain soaks, the insulation cotton absorbs water and becomes heavier, which is crushed. The metal bellows are in a humid environment for a long time, and the corrosion is accelerated, and the life span is directly cut off by half.

Consequences? Three words: money for nothing. Once the insulation layer is broken, the waste of energy is a trivial matter. The key is that the compensator bellows or fabric fiber layer is directly exposed to high temperature and corrosive media, and the compensators bought by hundreds or thousands may be scrapped in a few months. What's more troublesome is that once the insulation layer falls off and blocks the pipeline, or the baffle door gets stuck, the whole system has to be shut down for emergency repair.

So,Compensator insulation repair？ It is not just a layer of cotton to finish the job, every step has to be taken seriously.

2. The "physical examination" before repair is indispensable-metal vs non-metal compensator, what is the difference between the key points of the examination

Everyone knows the principle of checking before repairing, but most people are planted in the "wrong direction of checking".

bump intoUniversal corrugated expansion jointOrHigh temperature axial expansion joint

This kind of metal compensator, after you peel off the insulation layer, focus on whether there are cracks, corrosion pits and scratches on the surface of the bellows. Especially the welding heat-affected zone and trough position, which is the stress concentration point, is most prone to problems. Shine it with a flashlight to see if there are any bright dots-that could be a sign of intergranular corrosion. Measure the wall thickness again. If the thinning exceeds 20% of the designed wall thickness, don't repair it and change it.

WhileNon-metallic expansion joints (fabric fiber expansion joints)Orrubber compensator, the main points of the inspection are completely different. This kind of compensator relies on fabric fiber layer or rubber layer to seal and displace. After you peel off the heat insulation, you should follow the direction of the fiber to see if there is delamination, fluffing and penetrating damage. The rubber compensator should also be touched to see if it is sticky and hardened-pinch it with your hand, and if it doesn't bounce back, it means that it is seriously aging, and it won't last long after repairing it.

Also, don't leak the deflector. Whether metallic or non-metallic, the role of the guide tube is to prevent the media from directly washing the bellows or fabric layer. If the guide tube deforms and falls off, no matter how well the insulation layer is repaired, the inside will still be blown out.

3. Practical points: How to avoid damaging the bellows or fabric fiber layer when dismantling the old insulation layer

In this link, on-site workers are most likely to overturn. The reason is simple: the old insulation layer is exposed to wind and sun, and it often sticks to the surface of the compensator-especially those insulation structures fixed with asphalt or resin glue. When disassembled, a layer of skin is accidentally taken off the surface of the bellows.

When dismantling the old insulation layer, only wooden or copper scrapers and plastic shovels can be used for tools. It is prohibited to directly use shovels and crowbars. RightMetal rectangular expansion jointOrRectangular non-metallic expansion jointFor this kind of special-shaped parts with many corners, it is necessary to use a soft brush with low-pressure compressed air to blow off the floating ash first, and then peel it off manually to avoid hard objects getting stuck in the trough.

Don't touch the tie rod nut when removing the insulation. Especially the tie rod nut of the expansion joint, which is used for preload adjustment. As soon as you move, the displacement of the compensator changes. Take a picture before disassembling and note down the location, and you can only restore it when you put it back.

ForRubber PTFE compensatorIts lining PTFE layer is extremely thin, and it is afraid of high temperature and scratches. When dismantling the insulation, shovel along the axial direction of the pipe, and don't pull it horizontally. If it is torn, then the compensator insulation layer repair is half in vain.

4. Choose the right insulation material and structure: High-temperature axial expansion joint and rubber compensator have different requirements for insulation layer

The thicker the insulation material is not the better, nor is it the more expensive. The key is to look at the compensator type.

Such asHigh temperature axial expansion jointThe working temperature is often above 500℃, and generally rock wool and glass wool are directly burned into ash. Ceramic fiber blanket or aerogel felt must be used, and there should be a layered structure: the inner layer is made of high-aluminum ceramic fiber (temperature resistance 1260℃), and the outer layer is made of ordinary ceramic fiber or aluminum silicate wool. How to calculate the thickness? The principle is to make the external surface temperature not exceed 50℃, and at the same time leave the expansion space of the bellows-don't plug it, and leave at least an annular gap of 10~15mm.

Whilerubber compensatorOrRubber PTFE compensatorFeeling the other way around. Rubber is afraid of high temperature, but the purpose of heat preservation is not to keep the rubber warm, but to prevent external heat sources (such as the high-temperature pipe next to it) from baking the rubber. In this case, the insulation layer should be wrapped outside the rubber compensator, but it must be made into a detachable structure and fixed with stainless steel cable ties or quick buckles, which is convenient to open and check the state of the rubber body during maintenance.

As forDirect buried (fully buried) type expansion jointOrSleeve type pipe expansion jointThe insulation layer directly contacts the soil and groundwater, and it should be waterproof in addition to insulation-the outer protective layer has to use three-layer PE anti-corrosion structure, otherwise once the water vapor penetrates, the insulation will become cold and accelerate corrosion.

5. Reassembly and sealing: How to wrap special-shaped parts such as baffle doors and rectangular expansion joints to prevent air leakage

Reassembling insulation layer is a technical work, especially for special-shaped parts. You're wrapped so tight that the compensator can't move; If you pack it too loosely, air and heat leakage outside equals a white pack.

Let's startFlue gas baffle doorOrDesulfurization flue gas baffle door。 This kind of equipment itself has flange connection and operation mechanism, and the insulation layer cannot cover the transmission connecting rod and flange bolt. The correct method is: only cover the valve body, and leave a bare area of 250mm at the connection between the operating mechanism and the flange, which is convenient for daily inspection and tightening. When wrapping, lay with aluminum silicate cotton felt in layers and staggered seams, the thickness of each layer does not exceed 50mm, and then fix it with stainless steel strip, and add 0.5mm thick aluminum skin as a rain cover on the outside-the joints of aluminum skin are tightly blocked with silicone sealant.

Metal rectangular expansion jointAndRectangular non-metallic expansion jointThe difficulty lies in the four corners. The right angle position can easily become an air leak point. I generally suggest using prefabricated special-shaped insulation modules, making the corners into R-angles, pressing and molding with molds, and directly stuck on them on the spot. If there is no prefabrication condition, fill the corner gap with high-temperature-resistant sealant (calcium silicate-based, temperature resistance above 1000℃), and then compress it with fiber blanket.

After reinstalling, do a thermal imaging test. After running for half an hour, scan it with an infrared thermal imaging camera, and find that the temperature is abnormally high, which means that the insulation is not tightly sealed, so make it up quickly.

6. Life management after repair: Don't wait for the next time it is broken before repairing, keep an eye on these three positions in daily inspection

After the insulation is repaired, it is not once and for all. If you keep an eye on three parts, you can eliminate the accident in the bud.

The joint between the insulation layer and the metal pipe or flange. Here, the temperature difference is the largest, the thermal stress is the most concentrated, and it is easiest to crack. Inspect once a month and press the seam with your hand to see if it feels loose. If the temperature gradient suddenly changes by infrared detection, local repair should be made immediately.

Drain hole (if any) at the bottom of the compensator. Especially for outdoor installationExternal pressure single axial expansion jointOrCompound hinge transverse expansion jointClogging of the bottom drainage hole may cause condensed water to accumulate and damage the insulation layer. During the inspection, poke the drainage hole with a thin wire to confirm that it is unblocked.

The exterior of the insulation layer at the crest of the bellows. This is the place that is most severely washed by displacement. Over time, the insulation cotton will be rubbed into balls like marshmallows. You gently press the wave crest with your palm. If it feels empty underneath (the insulation layer is separated from the bellows), it means that the fixed structure has failed and has to be tightened again.

Compensator Insulation Repair?It is not a one-time thing, but a system from inspection, material selection to construction and maintenance. Follow these six steps for each repair, which can prolong the life of the compensator by at least 3 times.

Let's talk about the phenomenon first-what does compensator failure look like?

Who in the dry plumbing business hasn't touched a compensator strike a few times? The most common scenes: cracks and bulges appear on the surface of the bellows, or a hole is simply torn from the weld, and the medium is sprayed all over the workshop. More seriously, the guide tube fell off, the tie rod nut collapsed, and even the whole expansion joint was deformed by the lateral thrust-I tell you, that scene was no different from the scene of a car accident.

Two days ago, a buddy from the cement factory called and said that they had installed a machine on the kiln head pipeMetal Corrugated Expansion Joints in Cement IndustryIn less than three months, fatigue cracks appeared in the bellows. After removing it, the groove has been worn out at the peak position, which is obviously caused by high-frequency and low-amplitude vibration. And even more outrageous: a power station project, on a steam pipelineHigh temperature axial expansion jointBecause the transport screw was not disassembled during installation, it was run directly with the restraint bolt, and as a result, the bellows was forcibly pulled and cracked.

Nine times out of ten, these failure cases are not the problems of the products themselves-the mines buried in design selection and the pits stepped on in on-site installation are the real culprits.

"Mining" in Type Selection and Design

Compensator failure analysis The first step is always to see if the design parameters are correct first. Many purchasing diagrams save trouble, just buy a general model according to the nominal diameter of the pipeline and install it. For example, the steam pipeline, the medium temperature is 400℃, the pressure is 1.6MPa, and the result is a conventional oneUniversal corrugated expansion jointThe wall thickness is not enough, the wave number is not correct, and the compensation amount is not enough, and the deformation exceeds the limit directly when the machine is started.

Calculation of the amount of displacement. Pipe thermal expansion is calculated according to temperature, pipe length and material, but many people only calculate axial displacement, ignoring lateral and angular displacement. EspeciallyCorrugated expansion joint for power station industryThe direction of the pipe system is complicated and there are many turns. If only a single axial type is equipped, the lateral displacement that should be absorbed has no place to go, and the bellows will be stressed by different forces, and the fatigue life will plummet.

There is also a "stealth bomb" by design-media corrosion. The flue gas in the desulfurization system contains sulfide, and the temperature is not high but the humidity is high. If ordinary stainless steel bellows is selected, chloride ion stress corrosion cracking is almost inevitable. This kind of working condition can be usedPTFE compensatorOrLined with PTFE hose to isolate metal from corrosive media. But the price is a little more expensive, and Party A often wants to save this money, and as a result, it spends ten times the money to change the pipe later.

"Man-made knife" in installation and operation and maintenance

The design is fine, but it can still mess up when you get to the scene. The most typical one I've seen: the expansion joint is installed, but the transport screw is not removed. The customer said that "I was afraid of air leakage and kept insurance". As a result, once it was run, the bellows was stretched to the limit and directly plastically deformed and scrapped. Remember, allStraight pipe pressure balanced expansion jointAndExternal pressure single axial expansion jointThe tie rods and nuts on the drawings are marked as "removed after installation", and none of them can be left.

Another fatal operation-forcing counterparts. The pipeline installation error is big. Some construction teams use hoists to hard-pull the pipes to align them, and use the expansion joint as a jack. The bellows is forcibly twisted or stretched, residual stress has been created inside, and it cracks not long after running. In this case, you should addManual plug-in insulation doorOrDouble-sealed single-axis circular baffle doorTo adjust the flow path, don't save that money.

What about the operation and maintenance link? Inspection is a mere formality. Many people take a flashlight and shine it around, and if they see that there is no air leak, it is done. In fact, the micro-cracks, indentations and corrosion pits on the bellows are simply not visible with the naked eye. Wall thickness testing and non-destructive testing must be done regularly. Especially those operating at high temperature and pressureSleeve type pipe expansion jointOrRotary compensatorIf the sealing surface is not replaced in time after it is worn, the leakage accident will occur in minutes.

Different "death methods" are different under different working conditions

There are many strange failure modes, but they can be summarized in several kinds: fatigue failure, corrosion failure, overpressure blasting and instability buckling. Different working conditions, the main reasons are completely different.

Power Plant Steam Pipeline-High-frequency thermal cycling coupled with vibration, is most prone to low-cycle fatigue. We did a diagnosis for a thermal power plant, and they used the pipelinesCompound hinge transverse expansion jointToo many wave numbers lead to insufficient overall stiffness and direct instability under the impact of pipeline water hammer. Later replaced withDouble hinge expansion joint for air-cooled island vacuum pipelineThe problem was solved only after the limit device was installed.

Flue Gas Desulfurization Systems – Corrosion is the number one killer. For the import and export of a desulfurization towerNon-metallic expansion joint (fabric fiber expansion joint)It could have been acid and alkali resistant, but when installing, the orientation of the fabric layer was reversed, and the medium penetrated from the weak side, and it rotted out in three months. On this occasion, apart from choosing the rightDesulfurization flue gas baffle doorAndRubber PTFE compensatorYou must also pay attention to the installation direction sign.

Direct buried pipeline-the groundwater level is high, the soil is corrosive, and ordinary metal bellows can't bear it at all. Must chooseDirect buried (fully buried) type expansion joint, plus anti-corrosion coating and sacrificial anode protection. Last year, a thermal pipe network in the north was replaced by an ordinary type, and it leaked in two years. After excavation, the bellows had been corroded into a honeycomb shape.

How to predict and prevent in advance? — — Closed loop from design to inspection

Don't wait for the explosion to analyze the compensator failure analysis? That's called remedy. The real reliable approach is to form a closed loop:

• Design phase:Get the detailed stress analysis report of the pipe system and confirm the thermal displacement and pipe thrust. Don't just look at the caliber when selecting a model, but match it accuratelyHigh temperature axial expansion jointOrCurved tube pressure balance expansion jointAnd other special models. Media corrosive? UpperPTFE compensatorOrrubber compensator。 Space constrained? useLarge diameter thick wall expansion jointOrUniversal corrugated expansion jointPaired with limit pull rod.
• Installation phase:Strictly follow the installation instructions. The transport screw must be removed, and the direction of the guide tube must be aligned with the flow direction arrow (Specific Function of Expansion Joint Guide TubeIs to protect the bellows from being washed by the medium, and the wrong direction is equal to white installation). Installation error does not allow the use of the expansion joint hard top, the bracket should be adjusted or addedDouble-sealed single-axis circular baffle doorTo cut off the airflow.
• Operation and maintenance stage:Develop inspection cards and record the displacement, surface temperature and vibration values for each time. If abnormal vibration or large displacement is found, give priority to inspect the bracket andMetal hoseIs it loose or not. For critical partsCorrugated expansion joint for power station industry, it is recommended to do a metal fatigue life assessment once a year.

In the final analysis, the compensator is a treasure if it is designed correctly, but a time bomb if the selection or installation is wrong. Don't always wait for something to go wrong before you pat your thigh. Fill in the holes in front of you first, and it is not difficult to run safely for ten or eight years.

Last month, an engineer at a chemical plant called and said they hadUniversal corrugated expansion jointAs soon as the furnace was started, it screamed harshly, and the workers on the spot were afraid to approach. The problem of abnormal noise of the compensator is not big or small-let it go, which can disturb the people at least, and the fatigue cracking of the bellows and media leakage at worst. Then the question arises: Where did this noise come from?

1. Resonance is the number one suspect

The bellows itself has a natural frequency. If the flow rate or pressure fluctuation frequency of the medium in the pipe is matched with it, it will trigger resonance. Pay special attention toCorrugated expansion joint for power station industryAndHigh temperature axial expansion jointThis kind of metal parts, the resonance under high elasticity is easy to produce high-frequency howling. How to check? It's simple – touch the compensator body with your hand. If the vibration sensation is obvious and the frequency is stable, nine times out of ten it is resonance. At this time, don't rush to change parts. First, adjust the rigidity of the pipe support or change the flow rate of the medium. If necessary, install a damping device outside the expansion joint.

2. Loose guide tube or liner

Many expansion joints are equipped with guide tubes (about the specific function of guide tubes, we have a special article on our station), which is used to protect the bellows from high-speed fluid erosion. Once the deflector weld cracks or the retaining bolts come off, it clatters like a broken gong in the airflow. Especially forFlue gas baffle door、Desulfurization flue gas baffle doorThe rearNon-metallic expansion jointOrrubber compensatorWhen the medium contains particles, the guide tube wears faster. Inspection method: After shutting down, disassemble the insulation layer and observe the interior with an endoscope. If you find that the guide tube has loosened, repair welding or replace the liner directly, don't hesitate.

3. Installation stress or displacement exceeds limit

directly buried expansion joint、Compound hinge transverse expansion jointThese products are mainly used to absorb displacement in a specific direction. If the cold tightening is not in place during pipeline installation or the supports and hangers fail, it will cause the compensator to bear additional stresses outside the design range. At this time, the abnormal noise is often accompanied by a "click" of metal crash, which sounds like a part being hard pulled. What to do? Recalculate the pipeline compensation amount and adjust the tie rod nut (how to adjust it, refer toHow to adjust the tie rod nut of expansion jointThat Q&A). Remember: the tie rod is not a trim piece, it is directly related to the safe life of the compensator.

4. Media Scour and Foreign Matter Stuck

Metal Corrugated Expansion Joints in Cement IndustryAndRubber PTFE compensatorWhen conveying powder or particle-containing wastewater, the hard particles get stuck in the ripple trough and produce scraping sound with the fluid movement. There are worse scenarios: foreign objects such as weld slag and flange gasket fragments enter the pipeline and get stuck inMetal hoseOrPTFE-lined hoseIn the ripples, the sound was like sandpaper grinding iron. Response: Install the filter or check the cleanliness of the previous process. Don't think that changing a new compensator can solve the problem-if you don't catch the source, it will still ring after changing it.

Finally, emphasize: Don't change parts blindly

First, collect the three information of abnormal noise type, working condition (temperature, pressure, medium) and pipeline layout diagram, and eliminate them one by one according to the above reasons. If you are unsure, report the parameters directly to an experienced expansion joint manufacturer-after allRotary compensator、Sleeve type pipe expansion jointThe debugging methods of these specialty models vary greatly. By the way, two days ago, a client asked whyNon-metallic expansion joints (fabric fiber expansion joints)After half a year, the hole was accompanied by a hissing sound. It was found that the flue gas temperature exceeded the design value of 30℃, and it was directly burned through. Therefore, selection and installation are much less worry-free than remediation after the fact. Don't wait until the "quack" sounds on the scene to be anxious. Check the working parameters in advance, which can save 80% of detours.

Compensator tie rod bent? Don't be in a hurry to change. Find out these points before you start

When you see that the compensator tie rod is bent at the scene, your first reaction is usually "it's over, you have to replace it with a new one". However, veterans who have worked in pipeline maintenance for several years know that tie rod bending is often the appearance, and the root mostly lies in installation or selection. Direct replacement treats the symptoms but not the root cause, and it may bend next time.

Why do tie rods bend? Three common "real murderers"

Inaccurate estimation of blind plate force of pipeline。 Blind plate force is the thrust generated by the medium pressure acting on the closed end of the blind plate or elbow. If this force is small, the tie rod will have to carry the axial force exceeding the design value, so it is strange that it does not bend. Especially in the steam pipeline of power station or the high-temperature air duct of cement industry, the blind plate force will soar when the pressure fluctuates, and the tie rod can't stand it at all.

Fixed bracket failure。 The fixed bracket is used to restrict the displacement of the pipe. If it is loose, slipped or directly pulled apart, the thermal displacement of the pipe will all be thrown to the compensator. At this time, the role of the tie rod is equivalent to a "temporary bracket", but when designed, it is only responsible for transmitting thrust, and does not bear transverse bending moment-one bend is permanent deformation.

Wrong selection。 For example, you useUniversal corrugated expansion jointIt mainly absorbs axial displacement. As a result, the field actually needs to absorb lateral displacement, so the tie rod becomes a "*". I have seen a cement factory install the general-purpose type on the outlet air duct of the rotary kiln. The thermal expansion of the duct is oblique, and the tie rod is bent into a bow shape after two weeks. This kind of mistake, it is useless to change the tie rod, it has to be changedCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointThat's fine.

Can it be fixed if it is bent? Look at three points

Judging whether it can be repaired, don't just rely on the naked eye. Three key points:

• Degree of bending: The bending angle is less than 5 degrees, and the deformation amount does not exceed 10% of the diameter of the tie rod. You can try to correct it. Beyond this range, the internal lattice of the tie rod has slipped, and the strength is greatly reduced.
• Whether the material is fatigued: The fatigue limits of ordinary carbon steel tie rods and 304 stainless steel tie rods are different. If the equipment runs for more than 20,000 cycles, or micro-cracks have appeared on the surface of the tie rod, don't repair it and replace it directly.
• Is there any associated damage to the compensator body: Focus on bellows. When the tie rod is bent, the bellows is often squeezed off or pressed against the guide tube. You take apart the insulation and light it with a flashlight to see if the corrugated valley floor has any scratches, bulges or penetrations. As long as the bellows is injured, fixing the tie rod is a waste of effort.

If all three points are qualified, you can try to adjust the nut fit correction. How exactly to operate? Refer to what we talked about earlierExpansion joint tie rod nut adjustment method— — First, loosen the nut to a free state, slowly push it back with hydraulic top or special correction tool, then pre-tighten the nut diagonally, and apply force to the design torque three times.

Don't do these two stupid things

"The pull rod is bent. Can I use the torch to roast it red and break it straight?" I directly advised him not to take any risks. High temperature will cause irreversible changes in the mechanical properties of steel-for example, carbon steel will decarbonize when it exceeds 650℃, and 304 stainless steel will precipitate chromium carbide when it stays in the sensitization temperature range (450-850℃) for too long, resulting in intergranular corrosion. It suddenly breaks in subsequent use, that's no joke.

Some people directly smash it with a hammer and hardtop it with a jack. This trick is the easiest to crack the bellows. The bellows wall is thin (usually 0.5-2mm), and it will crack when the local impact force is large. And guess what? The crack may be as thin as a hair at the beginning. After a few days of running, the medium flushes, and it perforates and leaks directly. If something like this happens on the steam pipeline of the power station, the loss of one day's shutdown will be enough for you to buy dozens of sets of compensators.

Change the tie rod? Don't just stare at that pole

If you decide to change it after evaluation, don't just focus on the tie rod itself. There may be other compensators in the same system-for example, you are usingCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint, their tie rod construction is completely different. The pull rod of the compound hinge type is in the form of hinge pin shaft, and two groups of hinges should be processed at the same time when disassembling and assembling; The tie rod of curved tube pressure balance type connects two bellows and the balance ring. If it is disassembled incorrectly, the bellows will become unstable. Check whether the guide tube has been displaced and the sealing surface has been worn. Many times, you don't know that the guide tube is offset. The airflow directly washes the inner wall of the bellows and wears it out in half a year.

The last heartbreaking truth

Maintenance of tie rod is only an emergency means, and the fundamental solution is to recalculate the thermal displacement and thrust of pipe system. EspeciallyCorrugated expansion joint for power plant industryAndMetal Corrugated Expansion Joints in Cement IndustryIn this kind of system with complicated working conditions, a single tie rod bending often means that the whole bracket system needs to be adjusted-it may be that the stiffness of the bracket and hanger is not enough, or it may be that the cold tightness is given wrong. At this time, it is much more useful to find a manufacturer for on-site diagnosis than to mess around with you. With 3D laser scanner and finite element analysis software, the manufacturer can give a rectification plan in half a day. If you change ten tie rods yourself, you might as well re-weld the fixing bracket once.