Specialized in manufacturing compensators, expansion joints, baffle doors

How to pick the expansion joint manufacturer? A 10-Year Selection Consultant's Guide to Avoiding Pitfalls

Two days ago, I met a customer. As soon as he came up, he threw me a drawing and asked, "How much is this expansion joint?" I said, don't worry. Is your working condition steam pipe or flue gas pipe? What's the temperature? He was stunned for a moment and said, "Oh, it's just ordinary pipes. Can't you use your general-purpose corrugated expansion energy-saving type?"-I can run into this kind of conversation dozens of times a year. If you choose an expansion joint manufacturer, if you only stare at the price, you will probably roll over. Today, I'll talk to you about the pits I've stepped on and the thunder I've seen in these 10 years of selection.

1. Don't just look at price: product line breadth is the first sieve

Many purchases ask, "How much is your expansion joint per meter"-as soon as I hear this, I know that I am either a novice or just want to be cheap. What is the "universal price" of expansion joints? The same caliber,High temperature axial expansion jointAndThe materials and processes of general-purpose corrugated expansion joints are completely different, and the price difference is three or five times too normal.

Start with the thermal expansion and contraction of a pipe

When the steam pipeline was first started, the temperature soared from room temperature to 500 to 600 degrees, and the tens of meters long pipeline could be forcibly stretched by several centimeters. If you keep it from moving, it is common for flanges to collapse and welds to crack. At this time, the expansion joint comes in handy-to put it bluntly, it is the "breathing valve" of the pipeline, specifically designed to "eat" this deformation. Whether it is a metal bellows or a non-metallic fabric layer, the core principle is the same: it absorbs the displacement through its own elastic deformation, so that the pipe should expand and expand, shrink and contract, and the stress is all released on the expansion joint.

So the first question- -What does an expansion joint do?The most fundamental thing is to compensate for thermal displacement and prevent excessive stress in the pipeline system due to temperature changes. But if you think that is what expansion joints are capable of, you would be underestimating it.

In addition to displacement compensation, I also did three things

The high-frequency vibration of the gas turbine exhaust pipe makes your ears buzz when you stand next to it. Once the rubber compensator was installed in the middle, the vibration was directly isolated by more than half, and the noise was also reduced. When the flue gas baffle door and the expansion joint are used together, the air leakage rate of the desulfurization system can be reduced to the lowest-whether the sealing effect is good or not directly related to the energy consumption and emission of the whole system. To be honest, many times the candidate type is not designed to compensate for displacement at all, but is an expansion joint only because of vibration damping or sealing requirements. For example, textile fiber expansion joint is used in dust removal pipeline, mainly because of its vibration absorption capacity and air tightness. Don't underestimate these "side businesses". In actual working conditions, they are more important than the main business.

Metal, non-metal, rubber-three materials and three tempers

High-temperature steam pipeline in power station industry, the first choiceHigh temperature axial expansion jointOrStraight pipe pressure balanced expansion joint。 Why? High temperature resistance, high pressure, corrugated pipe material must be austenitic stainless steel. Cement industry dust removal pipeline, high temperature also contains dust, with metal rectangular expansion joints easy to accumulate dust and get stuck, this time has to be replacedRectangular non-metallic expansion joint-The fabric fiber layer has good flexibility and is not afraid of dust wear. If you encounter corrosive media, such as pickling tanks or chemical lines, PTFE compensators orPTFE-lined hoseThat's the right answer. Choose the wrong material and pierce it in three months. It's really not to scare you. I have seen a chemical plant that installed ordinary metal bellows on dilute sulfuric acid pipelines, and it leaked in less than two months.

Install that little trick

Two days ago, I met a client.Universal corrugated expansion jointThe screws were all disassembled, and as a result, the bellows collapsed directly during the pressure test-this kind of thing is really uncommon. The role of the tie rod is to limit the bellows from being excessively compressed or stretched during transportation and installation, not for you to disassemble and play with. When the expansion joint of the large tie rod is installed, the pre-tension is not adjusted properly, and the bellows directly exceeds the limit under the hot state, and the life is cut in half. The guide tube is installed backwards, and it is more pitty-the medium directly washes the inner wall of the bellows, and it takes less than a few months to wear out cracks. The specific function of the expansion joint guide tube is to guide the flow medium and avoid high-speed airflow or particulate matter directly hitting the bellows. Therefore, be sure to see the direction of the arrow clearly before installation, which is an indication of the flow direction of the medium, not a display.

The life span has to be calculated in detail

The national standard JB/T 12235-2015 for non-metallic expansion joints clearly states that the fabric fiber layer has a fatigue life limit, and it has to be replaced in 3-5 years. If the working condition of metal bellows is stable, it will be fine for 10 years or more. However, if you start and stop frequently or encounter water hammers, it is not unusual for you to crack in two or three years. Don't wait until the pipe bursts before replacing it-regularly check the bellows surface for corrosion points and abnormal displacement, which is stronger than anything else. Sleeve-type pipe expansion joints appear to be simple in structure, but the sealing packing ages quickly and has to be replaced regularly. Generally speaking, the service life of expansion joints is strongly related to four factors: type selection, installation, working conditions and maintenance. Don't expect it once and for all.

So back to the beginning question —What does an expansion joint do?It is more than simply "compensating for displacement". It is also a shock absorber, gasket, noise reducer, and can even save you a lot of equipment repair costs. The premise is that you have to choose the right material, install the right position, and use the right method.

1. First understand the core parameters: what do you think of stiffness, pressure, temperature and compensation?

I have been in this business for 20 years, and I am most afraid that I will fill in "high temperature resistance" directly on the selection list. There are only a few core parameters of bellows expansion joints, but each one hides pits.

stiffness— — Don't be fooled by the word "stiffness". Simply put, it is the ability of bellows to resist deformation. The greater the stiffness, the greater the thrust generated at the same displacement. If you choose an expansion joint with too high stiffness, the pipe bracket and fixing point may directly push through. This site has a special "Stiffness and Calculation Formula of Bellows", which gives the theoretical algorithm. However, when actually selecting the model, it is recommended to ask the manufacturer directly for the measured stiffness value on the sample, which is the real guy.

pressureAndTemperatureIt's a pair of conjoined babies. The pressure level is easy to understand. Which one should I choose between 0.6MPa, 1.0MPa and 1.6MPa? But as soon as the temperature rises, the permissible stress of the material drops. For example, the bearing capacity of 316L stainless steel at 400℃ and room temperature is quite different. Many peers only look at the design temperature, but ignore the "maximum working temperature × duration"-short-term overtemperature may be fine, and long-term operation at the critical point will directly cut the life of bellows in half. So chooseHigh temperature axial expansion jointWhen, not only look at the nominal temperature resistance value, but also confirm whether the number of corrugated layers and wall thickness meet the fatigue life requirements.

Compensation amountIt's a gimmick hit area. Some manufacturers mark "axial compensation ± 50mm", but that is the theoretical value calculated by multiplying the single wave compensation by the wave number. In fact, the pre-stretching amount, safety factor and cold tightness amount have to be deducted. The correct way is to first calculate the thermal elongation of the pipeline, then multiply it by the insurance factor of 1.2~1.5, and finally check it against the "allowable compensation curve" given by the manufacturer. Don't expect an expansion joint to overcome the world. If the compensation amount is too large, consider using itCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointTo share.

2. Don't pat your head when selecting models: high-temperature axial type, general-purpose type, straight pipe pressure balance type, working conditions determine the model

I met a customer two days ago. The temperature of the steam pipeline is 350℃ and the pressure is 1.0MPa. The direction of the pipeline is simple, but it is necessary to go upStraight pipe pressure balanced expansion joint。 Why? Because I heard that "the pressure balance type is good, and there is no blind board force". However, the pressure balance type has complicated structure, high cost and large installation space requirements. For the working condition that there is no strict thrust limit in the straight pipe section, it is purely a knife for killing chickens.

• High temperature axial expansion joint— — It is suitable for straight pipe sections with high temperature and large axial displacement. The temperature resistance can reach above 600℃, but it can only absorb axial displacement and cannot bear lateral or angular displacement. Note that its guide tube is standard to prevent high-temperature media from directly washing the bellows.
• Universal corrugated expansion joint— — It is often called universal type, which can absorb axial, transverse and angular displacement, and is suitable for pipelines with small and medium diameter, low pressure and temperature no more than 300℃. High cost performance, but the fatigue life is relatively short, so don't use high-frequency vibration occasions.
• Straight pipe pressure balanced expansion joint— — It is mainly used in places where the fixed bracket cannot bear the blind plate force, such as the inlet and outlet of steam turbine and the outlet pipeline of pump. It can balance the thrust generated by internal pressure, but it needs to reserve enough space when installing, and the two bellows need to act synchronously, which requires high welding and assembly accuracy.

Apart from that, there areCurved tube pressure balance expansion jointFits at the elbow,Double straight pipe bypass pressure balance typeSolve the problem of large compensation amount. I just want to tell you that there is no standard answer to model selection, only the working conditions determine the model. Don't save trouble, calculate the design parameters clearly, and then flip through the product library of this site. Each model has the corresponding application scenario description.

3. The three most common pits at the installation site: the direction of the guide tube is reversed, the pre-tension is forgotten, and the tie rod nut is randomly adjusted

If you choose the right model, it will still be useless if you mess up the installation. There are countless pits on the scene, and these three are the most typical ones.

The first pit: the direction of the guide tube is reversed.The function of the expansion joint guide tube is to protect the bellows and avoid direct scouring of the medium. However, many masters think that it is a flow direction sign when they look at the arrow-wrong, the direction of the arrow is the flow direction of the medium, and the open end of the guide tube must face the direction of the incoming flow. If it is installed backwards, the guide tube becomes a gas collecting chamber, and the medium forms a vortex in the tube, which intensifies the vibration, and the bellows is worn out in minutes. Regarding this point, there is a diagram in the question and answer of "Specific Function of Expansion Joint Guide Tube" on this site. It is recommended to take a look before installation.

The second pit: forgot to do the pre-stretch.For axial bellows expansion joints, if pre-tensioning (or cold tightening) is not done during installation, the expansion joints may be directly pulled to the limit after the pipeline heats up, and the bellows may be plastically deformed and scrapped. The amount of pre-stretch is generally half the amount of design compensation. For example, the design compensation amount is 80mm, and the pre-stretching amount is 40mm. How to pull? Use the pull rod and the nut to cooperate, don't pull hard, it is easy to hurt the ripples. Note: Remove the limit screw for transportation after pre-tensioning, otherwise the expansion joint will not work. Some expansion joints (such asExternal pressure single axial expansion joint) Comes with pre-stretching device, confirm before installation.

The third pit: the tie rod nut is randomly adjusted.What is the function of the tie rod of the expansion joint? It is clearly written in the question and answer-limit the excessive expansion and contraction of the bellows and prevent instability. However, some on-site masters think that tightening the nut means fixing, and loosening it means releasing. As a result, the tie rod is adjusted to a dead limit, and the expansion joint completely loses its compensation ability. The correct way is to adjust the nut to the design position according to the drawing before installation (usually with a gap), and then fine-tune it according to the actual needs after installing the pipe. EspeciallyLarge tie rod expansion jointThe tie rod is not a load-bearing part, but a limiting part. Don't use it as a hard support. There are also expansion joints with screws, asking "Do the screws need to be removed"? The answer is: the transport screw must be disassembled, and the tie rod screw remains but the position is adjusted.

4. Industry application comparison: how to set up corrugated expansion joints for power stations, metal corrugated expansion joints for cement industry and desulfurization flue gas baffle doors

Different industries, working conditions vary greatly, and the emphasis of type selection and installation is completely different.

Power station industry: The main steam pipeline, reheating pipeline and water supply pipeline are all large-diameter, high-pressure and high-temperature expansion joints, such asCorrugated expansion joint for power station industryAndDouble hinge expansion joint for air-cooled island vacuum pipeline。 The calculation of pipeline thrust in power stations is extremely strict, and compound hinges or pressure balance types are often used to release the thrust on steam turbines and boilers. Special attention should be paid to when installing: preheating before welding, and slow cooling after welding to prevent stress cracking in the heat-affected zone. Moreover, power station pipelines often have vertical sections, so guide brackets should be set to prevent the expansion joint from twisting.

Cement industry: The high-temperature fan outlet, kiln tail smoke chamber and decomposition furnace inlet and outlet of cement production line, the temperature is 350~500℃ all the year round, with large dust content and serious medium wear. You can't use ordinary stainless steel bellows at this time, you have toMetal Corrugated Expansion Joints in Cement IndustryIn particular, attention should be paid to the lining wear-resistant layer and multi-layer corrugated design. In addition, such occasions are usually associated withRectangular non-metallic expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)Used on flue gas pipelines, non-metallic materials are corrosion resistant and have high fault tolerance.

Desulfurization flue gas system: The flue gas after wet desulfurization has low temperature but acid corrosion, so it must be usedDesulfurization flue gas baffle doorMatching expansion joint. The most common thing here isDouble-sealed single-axis circular baffle doorAndRound Flap Door (Double Seal), cooperate withrubber compensatorOrPTFE-lined hoseTo isolate corrosive media. Note: The rubber compensator is not resistant to high temperatures and can only be used for clean flue with smoke temperature below 80℃; If the flue gas temperature fluctuates greatly, it is recommended to usePTFE compensator。 The length of the connection between the baffle door and the expansion joint should be sufficient, otherwise the expansion joint will be squeezed and deformed after closing.

5. There is a way to maintain: What is the impact on the service life of the expansion joint? Replacement cycle of rubber compensator and metal hose

Expansion joints are not permanent pieces, but they vary greatly in life. Main factors affecting longevity-by importance:

• Large temperature cycle-frequent start-stop process harms the bellows the most, because the number of thermal stress cycles directly determines the fatigue life.
• Medium corrosion-the chloride ion content exceeds 100ppm, 316L can't bear it, must be replacedPTFE-lined hoseOr higher grade stainless steel.
• Installation error-the initial stress caused by misedging and forced matching of the pipeline is equal to overdrafting the life every day.
• Vibration-Pipe vibration will accelerate the stress concentration at the crest of the bellows, which is prone to cracking.

thatrubber compensatorHow often do you change? Depending on working conditions. The rubber compensator used for desulfurization flue is subject to acid-alkali corrosion and ultraviolet aging, and generally needs to be inspected in 2~3 years. If you find that the rubber surface is hardened, cracked, or even delaminated, replace it immediately. WhileMetal hose(includingSpecial hose for vacuum) The replacement cycle is slightly longer, and it can be used for 5~8 years when there is no obvious corrosion, but the premise is to regularly check the outer surface of the bellows for pitting or wear marks.

Every overhaul cycle, open the insulation layer and visually check the bellows for pits, cracks and scratches; Touch the guide tube with your hand to see if it is loose; Whether the loosening spacer for the tie rod nut is still there. Don't wait until a leak is replaced, by which time you may have damaged nearby brackets and equipment.

To put it bluntly, the bellows expansion joint is worth ten years if it is selected correctly, but it will be scrapped if it is installed wrongly for ten days. I hope these experiences can help you step on fewer pits at the scene.

Universal expansion joint and ordinary expansion joint, from the appearance of how to distinguish at a glance?

In the dry plumbing business, who hasn't seen a few expansion joints yet? But when they really arrived at the scene, the universal type and the ordinary type were mixed together. At first glance, it was really easy to be confused. Two days ago, a customer sent a photo and asked, "Don't these two have corrugated pipes? Why is the price difference double?" Well, today, I'll make this matter clear from the appearance. You don't care if you are new or veteran, you can judge it in seconds after reading it.

First, look at the bellows structure first-one section or two sections?

That's the most straightforward difference. Common expansion joints, such as our commonUniversal corrugated expansion jointOrAxial type expansion joint, bellows is a complete, from end to end. You look down the tube and there is only one set of ripples, without any disconnect in the middle.

How to Repair Metal Expansion Joints? Judge the type of fault first, then decide whether to repair or replace

Two days ago, I met a customer. On the phone, I was anxious to say that their factoryUniversal corrugated expansion jointThere was an air leak and asked if it could be repaired. I asked him to take a video first, and at a glance-there was a trachoma the size of a millet grain on the surface of the bellows, but the whole thing was not deformed, and the tie rod nut was not loose. In this case, repair welding can last for two years. But if the crack has already penetrated three or four waves, or the bellows is directly stuck, then sorry, the cost of repairing is higher than replacing.

SoHow to Repair Metal Expansion Joints?The core is one sentence: first find out the type of fault, and then decide whether to repair it partially or scrap it directly. Don't dismantle it as soon as you come up, and don't change it as soon as you leak, which is a waste of money.

First, find out the root cause of the fault-air leakage, stuck or deformation?

Before maintenance, the most taboo thing is to rely on feeling. You have to at least make a judgment with the naked eye and simple tools. Common faults fall into three categories:

• air leakage: The sound can be heard hissing, or use soapy water to paint welds, bellows peaks and valleys, and watch bubbling. The air leakage point is generally at the base metal of the bellows, the end weld, or the connection between the connection and the flange. LikeCorrugated expansion joint for power station industryHigh operating temperature, high pressure, air leakage is mostly fatigue crack, andMetal Corrugated Expansion Joints in Cement IndustryAir leaks are often due to perforations caused by dust wear.
• Stuck (Displacement Failure): The pipe does not expand or contract when it should be expanded or contracted, or the expansion joint is forcibly pulled into a straight line and cannot be pushed by hand. The tie rod nut is too tight, the guide tube falls off and sticks the bellows, or dust accumulates and scales inside, which will cause stuck. LikeHigh temperature axial expansion jointIf the guide tube in the tube falls off and hits the bellows directly, the movement is not small.
• deformation: The bellows has obvious twisting, uneven wave pitch and local bulging. This is often caused by over-displacement operation. You measure the actual displacement, and then compare it with the factory design value, which is clear at a glance.

If you judge these three situations correctly, it will be easier to handle later. If the judgment is wrong, it will be in vain if the cultivation is done.

2. Local damage of corrugated pipe: if it can be repaired, it will not be replaced, but it depends on the material and thickness

If it is only small trachoma or shallow crack, and the material is austenitic stainless steel of 304 or 316L, with a thickness of more than 0.5mm, it can be repaired. Note: Before repair welding, the damaged area must be polished clean to expose the metallic luster. Use argon arc welding, and the welding wire should be made of the same material or more corrosion-resistant (such as 316L welding wire). It must be smoothed after welding, otherwise it will become a new stress concentration point.

But if that thickness of the bellows is less than 0.3mm (such as someSpecial hose for vacuumOr under low pressurePTFE-lined hose), or the material is special alloys such as Hastelloy and Inconel, which ordinary welders can't handle at all. It is recommended to contact the manufacturer directly. There is another situation: the crack is at the root of the bellows (trough), where the stress is the largest, and the life after repair welding can't be long. It is better to replace it with a new one.

Metal rectangular expansion jointAndLarge diameter thick wall expansion jointMost of the corrugated plates are splicing welds, so attention should be paid to thermal deformation control when repairing welding, otherwise the more they repair, the more they leak.

3. The tie rod nut is loose or the guide tube falls off: adjust the minor problems yourself, don't screw it over

These kinds of problems seem small, but they are self-defeating when they are not handled well. Such asHow to adjust the tie rod nut of expansion joint？ The standard practice is to confirm the design displacement first, and symmetrically loosen the adjustment nut with a wrench to restore the bellows to its free length. Don't screw it unilaterally, otherwise the expansion joint will twist. Many teachers screwed to death as soon as they came up. As a result, the bellows couldn't expand and contract normally and stuck directly.

Let's talk about the deflector falling off. The guide tube itself is fixed to the inner wall of the pipeline by bolts or spot welding. If it falls off, it will fall off and jam the bellows, or even be washed away by the medium and smash the downstream valve. During maintenance, open the manhole and re-weld or bolt the fallen guide tube. Be careful that the weld should not be higher than the surface of the guide tube, so as not to affect the flow of media. LikeDesulfurization flue gas baffle doorIn this kind of equipment, once the guide tube falls off, the slurry pours back into the bellows, and the corrosion rate doubles.

4. What circumstances must be scrapped? Fatigue crack, corrosion perforation, excessive displacement

Don't be reluctant. In some cases, forcibly repairing is planting mines.

• Fatigue crack: If the crack extends more than 3 waves along the bellows axial direction, or the circumferential crack length exceeds 1/4 of the circumference, it means that the material has been fatigued and will crack again soon after repair welding. Direct replacement, recommendedExternal pressure single axial expansion jointOrCompound hinge transverse expansion jointThis type of structure has better fatigue resistance.
• Corrosion perforationPerforations caused by pitting and intergranular corrosion often have only a small hole on the surface, but the surrounding metal has been embrittled. Use magnetic particle detection or coloring flaw detection. If you find dense micro-cracks, don't fix them. Especially for chemical pipelinesPTFE compensatorOnce the inner liner is perforated, the media will corrode the outer metal.
• Excess displacement: The actual operating displacement exceeds the design value by more than 120%, and the bellows has produced plastic deformation. Such asDirect buried (fully buried) type expansion jointOver-stretched, the wave pitch is uneven, and repair welding is useless at this time, because the bellows has lost its elasticity.

By the way,Service life of expansion jointGenerally, the design is 1000~2000 full displacement cycles, but many factories have not broken after ten or eight years of use, because the actual displacement is far less than the design value. If the limit is exceeded frequently, the life span will fall off a cliff.

5. How to accept after maintenance? Pressure test and displacement rebound are indispensable

It's not finished after repairing, you have to verify that it still works. Two steps:

The first step, the pressure test.Carry out hydraulic pressure test according to 1.25 times of the design pressure, hold the pressure for 10 minutes, and check whether there is any leakage on the surface of welds, flanges and bellows. Note: It must be thoroughly blow dried after the hydraulic test, esp.Double hinge expansion joint for air-cooled island vacuum pipelineThis kind of vacuum equipment, when the residual moisture is vacuumed, it will freeze and damage the bellows.

The second step is the displacement rebound test.Loosen the tie rod nut, push and pull the bellows by hand, see if it can smoothly expand and contract to the designed displacement, and whether it can rebound automatically after releasing. If the rebound fails to reach its original position, it means that there is a jamming inside, or the bellows has become unstable. ForCurved tube pressure balance expansion jointFor this type of balanced structure, it is also necessary to check whether the tie rod of the pressure balancing ring moves synchronously.

After these two steps, the maintenance is qualified.

6. Keeping an eye on a few details daily can save half of the trouble

After all, maintenance is a passive remedy, rather than prevention. If you keep an eye on the following points, the failure rate will be reduced by at least half:

• Deflector Wear Inspection: Open the manhole every six months, especially the flue gas system and pulverized coal pipeline. When the deflector is worn to less than half the original thickness, replace it in advance, don't wait for it to fall and smash the bellows. LikeNon-metallic expansion joint (fabric fiber expansion joint)Although there is no metal bellows, the deflector is equally afraid of wear.
• Periodic preloading of tie rod nuts: Not for you to tighten it, but to check if it is loose or rusty. ForLarge tie rod expansion jointEvery three months, use the torque wrench to re-tighten according to the design value to prevent the nut from loosening due to vibration.
• Displacement monitoring: Mark the scale on the expansion joint and record the amount of displacement at each shutdown. If you find that a displacement suddenly becomes larger or smaller, it means that the pipe support may be offset. Let's sayStraight pipe pressure balanced expansion jointIf the displacement is abnormal, there is a high probability that the main fixation bracket is loose.
• Heat preservation and corrosion protection: Expansion joints for outdoor installation, especiallyHigh temperature axial expansion jointAfter the insulation layer is damaged, rainwater enters, and the corrosion rate is astonishing. Regularly check the sealing condition of the insulation layer and repair the damaged areas. Also, don't let the corrosive liquid drip directly onto the bellows.

Okay, so aboutHow to Repair Metal Expansion Joints?I was detailed enough. You go back and compare your own expansion joint, look at the type of fault first, and then decide whether to repair or replace it.