Specialized in manufacturing compensators, expansion joints, baffle doors

Find out the difference between single wave and three wave first-structure determines performance

Two days ago, a customer who made steam pipelines called and asked, "Which one is better, single wave or three wave?" I asked him back, "What's the pressure of your pipe? What is the displacement?" He was stunned for a moment and said, "I see that the compensation amount of three waves on the parameter table is greater. Is it right to choose three waves?" Alas, this kind of thinking is horrible. Which is better, single wave or triple wave of metal expansion joint? It's not a question of "who's stronger" at all, but what kind of structure you need for your working conditions.

Single wave, as the name implies, is a ripple; Three waves are three ripples connected in series. The structural differences directly determine their mechanical properties. There are few single wave corrugations, the wall thickness can be made thicker (of course, it is also related to the material and molding process), and the overall stiffness is large. Three waves are equivalent to three springs strung together, and each ripple shares a part of the deformation. The overall flexibility is large, and the compensation amount will naturally go up. However, large flexibility does not mean that everything will be fine-the force between the three ripples is uneven, and the ripples near the ends tend to bear greater stress, which is a flaw of fatigue life.

Therefore, don't just look at the "compensation amount of 100mm" written on the sample and feel that the three waves have won. You have to break it apart and crumble it to see: How many cycles of life is this compensation measured? How much is the design pressure? How hot is the temperature? The combination of these parameters is called the real working condition.

Single wave: Large stiffness and small displacement, suitable for high pressure and small displacement scenarios

The typical application scenario of single-wave expansion joint, let me make an analogy: like the main steam pipeline of a power station boiler, the pressure is always more than ten MPa or even higher, and the temperature is five or six hundred degrees, but the thermal displacement of the pipeline is actually not large-maybe only a few millimeters to ten millimeters. With a single wave at this time, the advantage comes out. The single wave has high stiffness and can resist high pressure without instability; At the same time, because the wave number is small, the displacement of each wave is relatively concentrated, so as long as the design stress is within the allowable range, the fatigue life is easy to guarantee.

We have one on our siteHigh temperature axial expansion jointIt is used a lot in the petrochemical industry, which is a typical single-wave or multi-wave customization. However, in high-pressure scenarios, engineers generally prefer single-wave-with-thick-wall design, which can reduce the overall stress level. In addition,Universal corrugated expansion jointThere are also single-wave models in, which are suitable for the working conditions of "high pressure, small displacement and thick pipeline". It's easy to say, but you have to calculate when you actually choose the modelStiffness and Calculation Formula of BellowsIn that formula, the wave number n is placed directly on the denominator-the fewer the wave number, the greater the stiffness. You taste, you taste.

The structure is simple and there are few leakage points. Because of the many welding parts in three waves (there are usually circumferential welds between waves or formed integrally, but the crest and trough stresses are concentrated), the quality control points in the manufacturing process are also denser. For scenarios such as nuclear power and chemical industry that pursue the ultimate reliability, single wave is a safe choice.

Three waves: good flexibility and large compensation, but fatigue life and stability have a price

Three more waves. Its core selling point is the large amount of compensation. For example, some long-distance thermal pipelines, or scenarios with large installation errors, require the expansion joint to absorb the axial displacement of tens or even hundreds of millimeters. That is when three or more waves (such as four or five waves) come in handy. I've seen aCorrugated expansion joint for power station industryIn the case, the pipe diameter is DN1200, the temperature difference is 200℃, the calculated thermal elongation is 80mm, and the four-wave structure is finally selected. If a single wave is forcibly used, the wall thickness will go up to the sky, and the displacement of a single wave is too large, and the stress will long exceed the limit.

But the cost is also obvious. First of all, the axial stiffness of the three waves is low, and it is prone to columnar instability under high pressure-that is, the ripples bulge like balloons and lose their load-bearing capacity. So the three waves usually need to cooperatetie rodOrguide tubeTo enhance stability. We specifically explained it in the FAQ on our siteFunction of expansion joint tie rod: The tie rod is not used to compensate for displacement, but to restrain the pressure thrust and prevent the expansion joint from excessively elongating or twisting. Nine times out of ten, the three-wave products should be equipped with tie rods, or even designed toStraight pipe pressure balanced expansion jointOrCompound hinge transverse expansion jointTo balance the blind force.

Secondly, fatigue life. With the same displacement of 10mm, a single wave may experience 100,000 cycles without any problem. Because the displacement of each wave division of the three waves is only 3.3mm, it stands to reason that the life is longer-but in fact, because of uneven stress distribution (edge wave stress concentration), the test data often show that the fatigue safety factor of the three waves is lower than that of the single wave. Therefore, in occasions with frequent pulsating pressure (such as compressor inlet and outlet pipelines), choosing three waves should be extra cautious, and it is best to do a fatigue life check.

How to make a decision on the actual selection? Three dimensions of hard indicators to help you set

Having said all that, how do you choose? I summarize three hard indicators. If you take this to set your working conditions, it will be almost inseparable.

• Pressure dimension: Design pressure> 2.5MPa, preference is given to single wave, especially pipes with larger nominal diameter. The higher the pressure, the more critical the stability. You check itMetal expansion joint weight tableIt will be found that the wall thickness of multi-wave products under the same caliber is often a little thinner than that of single-wave products-that is to reduce the stiffness in exchange for compensation, but it becomes a short board under high pressure.
• Displacement dimension: The required axial compensation amount exceeds the single wave limit compensation amount (usually the maximum single wave can be 20~30mm, depending on the diameter and wall thickness), then multiple waves must be used. However, note that the compensation amount is not greater, the better. If it exceeds 50mm, it is recommended to consider duplex structure or hinge-type expansion joint, such asCompound straight pipe bypass pressure balanced expansion jointInstead of a bunch of wave numbers.
• Temperature/Media Dimension: Under high temperature (> 400℃) environment, the material creep is intensified, the stress relaxation at the peak of multi-wave is faster, and the life decay is obvious. At this time, the stress level of single wave is lower, which is relatively more advantageous. Corrosive media (such as flue gas desulfurization systems) should be consideredPTFE-lined hoseOrPTFE compensatorMore wavenumbers will increase the risk of medium retention, and single waves are easier to clean.

Waste heat power generation pipeline of a cement plant, pressure 1.0MPa, temperature 350℃, displacement 30mm. According to the above three dimensions: the pressure is not high, the displacement is medium, and the temperature is high. At this time, single wave can also be done, but the wall thickness needs to be thickened, which leads to an increase in cost; Three waves can also be done, but it needs to be equipped with a strong tie rod. Eventually we recommended to our clientsMetal Corrugated Expansion Joints in Cement IndustryThe three-wave zone guide tube scheme in China has been used by customers for three years and good feedback. There is no absolute answer, it is all calculated.

Don't ignore the matching: the guide tube, pull rod and installation direction can't be wrong

Choosing the right wave number is only the first step, and the matching accessories are the details that determine success or failure. Guide tube, we have a special article on our stationSpecific Function of Expansion Joint Guide Tube— — It is mainly used in pipelines with high medium flow rate (such as steam and flue gas), so as to prevent ripples from being directly washed by high-speed fluids, and at the same time, it can reduce flow resistance and vibration. Single-wave products are less equipped with guide tubes, because they have great stiffness and are not afraid of erosion; However, three-wave products, especially large-diameter ones, have deflectors that are almost standard. Otherwise, the high-speed airflow will wear through the thin ripples, and it will be impossible to change them.

The adjustment of the tie rod is also critical.How to adjust the tie rod nut of expansion joint？ Remember one sentence: tie rods are used to adjust pre-tension or pre-compression after installation is complete, not to lock up expansion joints. Many on-site workers screw the tie rod to death directly, which is equivalent to turning the expansion joint into a rigid connection, so why do you need it? The correct way to do this is: according to the ambient temperature at the time of installation, calculate the amount of pre-tension required for the pipe, adjust the nut position, and then let the expansion joint expand and contract freely. Because of the low axial stiffness of the three-wave structure, the installation direction is easy to reverse.The direction of the arrow of the expansion joint refers toMedia flow direction, the arrow must be pointed to the fixed bracket or end point, otherwise the deflector will drive and cause vibration.

If the pipeline has large lateral displacement, the three-wave structure is prone to the combined failure mode of "column instability + plane instability". This is the time to considerCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointInstead of simply changing the wave number. Type selection is a systematic project, and wave number is just one of the variables.

Summary: There is no absolute good or bad, only suitable or unsuitable working conditions

Which is better, single wave or triple wave of metal expansion joint? Did you notice that the question itself is not valid. Single wave is an epee without edge, specializing in high pressure and small displacement; Three waves are seven-foot soft swords, which focus on big compensation flexibility. You took the soft sword to cut the iron plate, and the blade collapsed; You use the epee to play tricks, and you break your wrist. So next time someone asks you "which is better" with the parameter table, you will throw him three words: depending on the working conditions.

If you are really unsure, just roughly sieve it according to the three dimensions (pressure, displacement and temperature) I mentioned above, and then find a reliable manufacturer to report the plan. Our product line in our site is very complete, fromMetal hoseToNon-metallic expansion jointToAll kinds of baffle doorsEach has an applicable boundary. Again: there are no perfect expansion joints, only matching designs.

After the on-site installation, it was found that the metal expansion joint had been forgotten to be installed. The pipe has been welded and the hanger has been fixed. You can't tear down the whole pipe, can you? He asked me if I could just cut a cut in the pipe, stick the expansion joint in and weld it.

To be honest, this kind of thing is not unusual at construction sites-the drawings have been changed, the installation sequence has been messed up, or the list has simply been missed. Today's article is dedicated to talking about:The metal expansion joint is missing. Can it be cut and repaired? What can I do to keep it from rolling over?

1. How did the missed installation happen? — — Several common rollover scenarios on the scene

There are various reasons for missing expansion joints, but they can be summarized in three categories:

• Drawing changes were not disclosed: The design was changed to a scheme with expansion joint, and the construction team still worked according to the old drawings. Only after the pipe was welded did it find that something was missing.
• Expansion joint arrived too late: In order to catch up with the construction deadline, the straight pipe section was welded first, thinking of opening and reassembling it later. As a result, it was dragged until it was pressed.
• Pure blindness: Take a real case-the installer of a steam pipeline in a power plantUniversal corrugated expansion jointThinking of it as an ordinary flange joint, he threw it aside and butt-welded the pipe.

In either case, the result is the same: the pipe has formed a rigid connection, and a section must be broken to fit the expansion joint in.

2. Can you cut it? Weigh the risks first-the location of bellows, guide tubes, and welds determine the cutting depth

Don't rush on the cutting machine. Find out what type of expansion joint you are missing first. If it isCorrugated expansion joint for power station industryOrHigh temperature axial expansion jointThere are multiple corrugations inside, and a flow guide tube. When cutting, once the bellows body is cut or the guide tube is injured, it is not a problem of repair welding-the whole expansion joint is scrapped.

• Is the expansion joint already attached to the pipe?If you just miss the entire expansion joint (such as one that should have been installedCompound hinge transverse expansion joint, the result is not installed), that cut is a straight pipe section, the risk is relatively low.
• Medium and pressure of pipeline: Steam, high-temperature flue gas, corrosive media, the welding requirements are completely different.
• Structural form of expansion joint: Expansion joint with guide tube, the guide tube is usually on the medium flow side, and the knife should be cut from the other side when cutting.

Do not blindly cut the knife until you are unsure of the internal structure。 It is best to find the drawings of the model or ask the manufacturer directly.

3. Cut open practical operation: where to cut? How to cut? How much allowance is left?

Suppose you confirm that the missing location is a straight pipe, and the pipe itself is made of carbon steel or stainless steel. Then the operation steps:

• Measurement positioning: The total length of the expansion joint plus the welding allowance at both ends (generally 5-10mm at each end). Like aUniversal corrugated expansion jointWith a length of 300mm, plus weld allowance at both ends, it is necessary to cut off about 320mm of pipe segments.
• Cutting method: Plasma or mechanical cutting, gas cutting is prohibited (unless it is a thick-walled carbon steel pipe and subsequent machining is required). The cutting surface must be perpendicular to the pipe axis with a deviation not exceeding 1mm.
• Bevel treatment: After cutting, make V-shaped grooves at both ends, with blunt edges of 1-2mm and gaps of 2-3mm. It is recommended to use mechanical beveling for stainless steel pipes to avoid carbon pollution.

Fix the pipe before cuttingOtherwise, the tube will bounce off after cutting. Especially for high-temperature pipes, the nozzle displacement after stress release can be frightening.

Four, welding this step is the most fatal-welding material, preheating, interlayer temperature, preventing bellows from overburning

What are you most afraid of when welding? I am afraid that your bellows will be baked after welding. The wall thickness of the bellows inside many expansion joints is only 0.3-0.5mm, which is close to the straight pipe section. If the welding heat is transmitted, the bellows will directly anneal or even burn through.

• Welding material selection: After the pipe material is determined, the welding materials must match. For example, 304 stainless steel pipe with ER308 welding wire, carbon steel with ER50-6. It should be noted that there are some expansion joints in this station such asHigh temperature axial expansion jointInconel alloy may be used, and the welding material should be selected separately.
• preheating: The thickness of carbon steel pipe exceeding 20mm needs to be preheated at 150-200℃. Stainless steel is generally not preheated, but must be preheated when the ambient temperature is below 0℃.
• Interlayer temperature: The stainless steel is controlled below 150 °C to prevent sensitization. Carbon steel does not exceed 300°C.
• Anti-overheating measures: Wrap wet asbestos cloth around the expansion joint body, or blow compressed air toward the bellows side to cool it. When welding, try to use small current, fast welding, multi-layer and multi-pass.

In addition,Specific Function of Expansion Joint Guide TubeIs to protect the bellows from high-speed media scour. If the guide tube is misaligned with the inner wall of the pipe during welding, it will cause the medium to directly impact the bellows, and the life of this expansion joint will be greatly reduced.

5. Post-welding inspection and pressure test-no leakage does not mean that it can carry displacement

Don't be in a hurry to shout OK when the welding is done. First do the appearance inspection: the weld surface has no cracks, no pores, and the biting depth does not exceed 0.5mm.

Then doStress testing: Hydraulic pressure test is carried out at 1.5 times of the design pressure, and the pressure is held for 30 minutes. But hydraulic testing carries risks – for bellows, the stress caused by hydraulic pressure may exceed the design value. It would be safer toAir pressure test(1.1 times the design pressure, use compressed air or nitrogen), but pay attention to the safe distance.

Most easily overlooked isDisplacement compensation capability test。 After the expansion joint is installed, it must be checked whether it can still expand and contract normally. On the spot, you can use a jack or manual pull rod to simulate the thermal displacement of the pipeline to see whether the bellows is stuck and whether the guide tube collides. If the expansion joint tie rod is screwed to death during welding, or the bellows is skewed due to welding deformation, then the expansion joint will be wasted.

6. What should I do if I can't save it? — — Replace the expansion joint with a new one or reinstall it?

• The bellows has been burned out during welding
• The pipe is too misaligned and the bellows is twisted after being forcibly installed
• The medium is corrosive, and the heat treatment cannot be guaranteed by field welding

Don't take it hard at this time. The best way to save money isReassemble a section of short section + flange: Cut off a section of the original welded pipe, weld flanges at both ends, and connect them with a metal expansion joint with flanges in the middle-such asUniversal corrugated expansion jointThere's a flange. The cost does not increase much, but the on-site welding risk is avoided.

If the pipe has been completely welded and there is no margin, it is only necessary to remove the entire pipe and repurchase the pipe section with the expansion joint. Don't feel like a waste. Compared with the pipe cracking and leaking in the future, this little loss is nothing.

How to weld the metal expansion joint when it is missing?There is no unified answer to this question, but as long as you figure out the structure, choose the right process, and take precautions, you can save it in most cases. If it doesn't work, it is better to admit defeat and replace it with a new one than to have an accident.

1. National standard system of metal expansion joints: who has the final say between GB/T and JB/T?

Two days ago, I met a purchaser for a power station project. When I came up, I asked, "Does your metal expansion joint meet the national standard?" I said yes, and he immediately asked, "Which is the mandatory standard, GB/T 12777 or JB/T 10617?" This question is a good question, but many people actually confuse one thing- -GB/T 12777 "General Technical Conditions for Expansion Joints of Metal Corrugated Pipe" is a product standard, while JB/T 10617 "Expansion Joints of Metal Corrugated Pipe" is more an industry standard for design, manufacture and inspection.The two are not a substitute relationship, but complementary. GB/T 12777 specifies the terms, classification, size series, technical requirements, test methods, etc. of expansion joints, which belong to the basic general standard; JB/T 10617 is more detailed, such as more specific clauses on bellows raw materials, forming process and welding requirements. Which one has the final say, you ask? In actual selection,The national standard is the bottom line, and the industry standard is the implementation rules.If someone takes "conforming to the national standard" as a universal shield, you have to ask: "Is it GB/T 12777 or JB/T 10617 specifically? Has the fatigue life test been done?"-Alas, many small factories can't even produce the type test report.

Second, working condition determination standard: which specification should power station, cement and flue gas desulfurization be compared with?

National specification for metal expansion joints under different working conditions? The emphasis is completely different. For example:Corrugated expansion joint for power station industryIt mainly takes high-temperature steam or hot water, with high pressure and large pipe diameter. At this time, you have to stare at the Technical Regulations on the Design of Steam and Water Pipelines in Thermal Power Plants DL/T 5054, which clearly requires that the fatigue life of the expansion joint is not less than 1000 times, and there are strict regulations on the thickness of the guide tube and the wall thickness of the nozzle. We sold a batch to a thermal power plant, and the other party directly took the terms of DL/T 5054 to accept it one by one. If the wave pitch deviation of the bellows exceeds 0.5mm, it will be returned.

Look at the cement industry again. The expansion joint on the cement production line is full of dust and high-temperature gas in the medium, which is not corrosive but abrasive.Metal Corrugated Expansion Joints in Cement IndustryIt is usually carried out according to JB/T 10617, but the key is to thicken the guide tube and choose wear-resistant lining. The national standard does not mandate the material of the guide tube, but it can be actually used. The 304 guide tube will be worn out in three months, so it has to be replaced with 316L or wear-resistant coating.

Flue gas desulfurization (FGD) systems are even more complex. The flue gas after wet desulfurization contains corrosive media such as sulfuric acid and sulfurous acid, and the temperature is about 80℃ but the humidity is extremely high. At this timeDesulfurization flue gas baffle doorAnd matching expansion joints, non-metallic expansion joints or metal expansion joints lined with F4 should be used. The standard should refer to the industry specifications such as Technical Conditions for Non-metallic Expansion Joints for Flue Gas Desulfurization Engineering. I have seen a case: a chemical plant installed a general-purpose corrugated expansion joint on a desulfurization pipeline for a cheap picture. As a result, it was perforated and leaked in three months, and millions were lost when it was stopped for maintenance. So, don't take the universal standard set for special working conditions, death.

3. Key clauses that cannot be avoided in model selection-pressure, temperature, displacement and fatigue life

Selecting a metal expansion joint is essentially to find a balance among four parameters:Design pressure, design temperature, compensated displacement, fatigue life.GB/T 12777 provides a lot of formulas, but in actual purchasing, you only have to focus on three points:
First,Fatigue life。 According to the standard, the fatigue times of expansion joints used in steam pipe network should not be less than 1000 times (calculated according to the maximum displacement). If it is a pressure pipeline, some design institutes require 3000 times or even 7,000 times. What do you think? Look directly at the manufacturer's type test report, which will indicate the test conditions and results. Don't trust verbal promises.

Second,displacement direction。 Is the pipe axially telescopic or laterally offset? Or angular displacement? Different displacement types correspond to different structures:Universal corrugated expansion jointMainly absorbs axial displacement;The double hinge transverse expansion joint is suitable for absorbing transverse displacement;Curved pipe pressure balance expansion joint is used for large diameter pipeline and needs to balance blind plate force. You've chosen the wrong structure, and no matter how good the standard is, it's useless.

Third,Pressure thrust。 On high-pressure pipelines, the bellows expansion joint will produce a huge blind plate force (the force is equal to the pressure multiplied by the effective area of the bellows). If this force is not balanced by the tie rod or the main fixing bracket, the pipe and equipment will fly directly out. SoLarge tie rod expansion joint、Pressure balanced expansion jointThese have special fatigue stress calculation chapters in the national standards.

4. Specification details neglected in installation and inspection: guide tube, tie rod, arrow direction

In terms of installation specifications, GB/T 12777 is actually clearly written, but the on-site construction team often ignores three details:
Direction of guide tube。 What is the direction of the arrow on the expansion joint? The answer isDirection of media flow。 The role of the guide tube is to guide the medium smoothly through the bellows, preventing vortices and erosion. If it is installed backwards, the guide tube will become a "baffle", and the airflow will impact the bellows, ranging from abnormal noise to tearing. There is a case of returning goods in our warehouse, that is, the cement factory installed the guide tube backwards, and the edge of the guide tube cracked after half a year.

How to adjust the tie rod nut?When expansion joints leave the factory, tie rod nuts are typically pre-tightened to limit accidental deformation during transport. However, after installation in place, if the pipe is installed cold, the nut must be loosened, otherwise the expansion joint will not be able to expand and contract freely. Many sites remove the tie rod directly-that's wrong! The function of the tie rod is to bear the blind plate force. If the tie rod is removed, the bellows may be pushed into a "balloon" by pressure. Correct practice: Adjust the nut clearance according to the drawing requirements, and allow the amount of pre-tension or pre-compression when cold.

Weld flaw detection。 According to the national standard, the circumferential weld between the bellows and the connecting pipe and the longitudinal weld of the bellows itself need to be inspected by radiation or ultrasonic according to different pressure levels. But how many small factories do you know that skip this step directly? We have seen a project in which the weld seam was not detected, and it leaked after half a year of operation. Therefore, when purchasing, it is best to ask forWeld flaw detection report, at least the reports of filming (RT) or magnetic particle (MT).

V. Metal vs. non-metallic expansion joint: What is the difference between national standards? Don't mix things up

Many people think that expansion joints are the same, and metal and non-metal are different materials. Wrong.The national standard for non-metallic expansion joint is JB/T 12235-2015While the metal expansion joint is GB/T 12777 and JB/T 10617, the two standard systems are completely different.
Non-metallic expansion joints (e.g.Rectangular non-metallic expansion joint、Fabric fiber expansion joint) Mainly depends on the skin materials (silicone cloth, fluororubber, polytetrafluoroethylene, etc.) to bear pressure and seal, and the bellows skeleton generally plays a supporting role. Its advantages are large compensation, corrosion resistance, good vibration isolation, but low pressure resistance (usually no more than 0.1MPa), and fast skin aging. The metal expansion joint is just the opposite: it has high pressure and temperature resistance, but the displacement compensation is relatively small and the rigidity is large.
When selecting, do not mix: flue gas pipeline, dust pipeline, low pressure and large displacement working conditions, preference is non-metal; High pressure, high temperature, metal sealing is preferred. If you install the metal expansion joint on the low-temperature wet flue gas pipeline after desulfurization, wait for it to be corroded by sulfuric acid; In turn, if a non-metallic expansion joint is attached to a steam pipe, the skin will burn through directly-this is common sense, but there are still purchasers who make this mistake.

6. Avoiding procurement pits: How to use normative clauses to judge whether products are compliant

When purchasing metal expansion joints, don't just look at the quotation and delivery date. Teach you to recruit quick judgment:
First, check the number of bellows layers and materials.GB/T 12777 recommends the number of bellows layers with different pressure grades. For example, above 2MPa, multi-layer bellows (such as 3 or 4 layers) are usually used to improve pressure resistance and flexibility. Some manufacturers take single-layer bellows as multi-layer, and their service life is several times worse.
Second, check the size and weight.We have aMetal expansion joint weight tableYou can compare it. If the wall thickness, wave height, wave distance and standard value reported by the manufacturer deviate too much, there is a high probability of cutting corners. For example, the general-purpose corrugated expansion joint has a nominal diameter of DN500 and a standard weight of about XX kg. If the weight of the quoted manufacturer is 20% less, it can basically be determined that the wall thickness has been thinned.

National Code for Metal Expansion Joints? It is not placed there to watch, but is actually used to ensure safety. If you buy the wrong expansion joint, you may cause enough parking losses to buy dozens of qualified products.

What exactly is the length of the expansion joint used on the fan duct? This question has been asked by almost every purchase. But let's be honest, there's no standard answer. Because the length of fan expansion joint is not determined by patting the head, it depends on the pipe diameter, displacement, working pressure, temperature, and even installation space. For example, for pipes of the same diameter, if the fan outlet temperature is high and the thermal expansion is large, the number of corrugations needed will be more, and the length will naturally be longer. Conversely, if it is only used to dampen vibration, maybe a short section is enough.

Give me a rough range first, so I have the bottom of my mind. For small and medium-sized fans (DN100~ DN500), the length of common metal expansion joints (such as general corrugated expansion joints) is generally between 200mm ~600mm. If it is a non-metal expansion joint (fabric fiber expansion joint), because the material is soft and the compensation angle is large, the length may be shorter, and 150mm ~400mm is more common. However, for large-diameter fans (DN1000 or above), such as induced draft fans in power stations or cement industries, the length of expansion joints may be 800mm or even more than 1m, especially for high-temperature axial expansion joints or structures with guide tubes.

How to set the length? The core is to look at two parameters: axial compensation and fatigue life

Each bellows has a designed compensation amount. For example, a single wave can compensate 10mm, and you need to compensate 40mm, so you have to have 4 waves, plus the pipes and flanges at both ends, and the length will come out. But be aware that the longer the bellows, the better – too long can cause instability, or the space won't fit. Therefore, when selecting the type, the wave number is usually calculated according to the displacement first, and then the pressure level is combined (such as the external pressure single axial expansion joint or the straight pipe pressure balance type), and finally the total length is determined.

Structural form is the premise, length is only the result

Two days ago, I met a customer, and I had to copy the length of others. As a result, the vibration of the fan exceeded the standard after installation. Why? He bought a double hinge transverse expansion joint, but the pipeline on site went in different directions, and the required transverse displacement direction was inconsistent. The length was the same but the structure did not match. So length is only the result, structural form is the premise. For example, the length of double hinge expansion joint for air-cooled island vacuum pipeline is different from that of ordinary axial type. Another example is the rubber compensator, which is shorter in length, but can only be used in low pressure and normal temperature occasions.

Don't forget the deflector

The expansion joint length also includes a flow guide barrel. For example, the corrugated expansion joint used in the power station industry often has a guide tube inside, and the length of the guide tube will occupy a part of the effective length. If the flow rate of the pipeline medium is high, the design of the guide tube is unreasonable, which will generate vortex and noise instead. Therefore, when asking for the length, it is best to report the working conditions (temperature, pressure, medium and displacement direction) to the manufacturer together and let the manufacturer calculate. Don't expect one length to take all fans.

Compensator deformation? Don't be in a hurry to say "broken."

Has this ever happened to you? Not long after the pipeline was running normally, the compensator was bulging, the bellows were twisted into twists, and the rubber joints were bulging like balloons. Most people's first reaction is "the quality of this compensator is not good". But I want to say that 90% of the deformation has nothing to do with the product itself, but the stress is not released. General-purpose corrugated expansion joints, rubber compensators, non-metallic expansion joints... There are all kinds of strange deformation forms, and there are only a few reasons at the root. Today, let's take a look at them one by one.

1. Working condition design deviation-the selection parameters are far from the site

High-temperature pipeline selects common general-purpose corrugated expansion joint. Once the temperature rose, the ripples crept directly-not a day or two, but slowly collapsed. There are also insufficient pressure levels, and when the medium impacts, it bulges or even bursts. And guess what? A standard expansion joint is used in the steam pipeline of a chemical plant, and the design temperature is 350℃. In actual operation, it often rushes to 420℃. After three months, the wall thickness of bellows is thinned by 40%. When selecting the model, you must keep an eye on the medium temperature, pressure, displacement, especially the applicable temperature range of high-temperature axial expansion joints and non-metallic expansion joints. Don't be too troublesome, list the media parameters clearly and check them against the product manual-this step can't be saved.

Let's talk about stress. The pressure rating should be high rather than low when selecting the model. For example, steam pipe water hammer, instantaneous pressure can soar to double the design value. Do you think choosing a 1.6MPa one is enough? It may actually rush to 3.2MPa. If the directly buried (fully buried) expansion joint is not insulated, the repeated freezing and thawing of moisture in the soil can also make the ripples crack-this is another "selection bias": the environmental conditions are ignored.

2. Improper installation and constraint setting-the number one killer of human factors

The fixed supports are not welded firmly, the guide brackets are too spaced apart, and the tie rod nuts are screwed randomly-these operations are the number one human factors of deformation. Two days ago, I met a customer. The flue gas pipeline of the power plant was equipped with a compound hinge transverse expansion joint. As a result, the guide bracket was installed inclined, the transverse displacement of the compensator was locked, and the bellows was twisted into a twist. At the scene, the installer didn't look at the drawings at all, and the bracket was crooked by five centimeters.

Always check the adjustment method of the expansion joint tie rod nut before installation. Many products leave the factory with locked rods to protect transportation and installation. After installation, it should be loosened or removed according to the instructions, otherwise the compensator will not work properly. Does the screw of the expansion joint need to be removed? The answer is: it depends on the working conditions. If it is an axial type, the transport screw is usually removed; If it is a hinge type, the tie rod nut should be adjusted to the design clearance. Don't think tightening is done.

3. Media corrosion and overtemperature and overpressure-invisible knife

Acid, alkali, salt spray or sulfide in smoke can directly chew through the bellows. Although PTFE-lined hoses and PTFE compensators are corrosion-resistant, they fail when the temperature exceeds 180℃. Tsk, many people think that "PTFE is all-powerful". As a result, the flue gas temperature rushed to 250℃, and fluoroplastics directly softened and deformed.

The water hammer in the steam pipe is more concealed. The instantaneous pressure of the water hammer can soar to twice the design value, and the bellows has no time to release, which leads to direct plastic deformation. This is not the worst-if the directly buried (fully buried) expansion joint is not insulated, the moisture in the soil will freeze and thaw repeatedly, the bellows will become brittle at low temperature, and micro-cracks will appear in a freeze-thaw cycle. You say it was wrong or not?

4. Mechanical vibration and fatigue damage-resonance is a chronic poison

The vibration frequency of the pump, fan and air-cooled island equipment matches the natural frequency of the compensator? Once it resonates, the bellows bends tens of thousands of times a day, and it will crack within three months. The double hinge expansion joint of air-cooled island vacuum pipeline is especially important for fatigue life. Many designers only consider thermal displacement and ignore vibration frequency. And the result? After the high-frequency displacement accumulates, the appearance does not change when you look at it, but in fact, the micro-cracks are already covered.

How to solve it? Manufacturers are required to provide stiffness calculation formula and fatigue life curve during model selection. When installing, add vibration damping bearings in the pipe system, or replace with rubber compensators to absorb vibration. Rubber PTFE compensator also has a certain vibration reduction effect, but attention should be paid to the medium temperature.

5. Manufacturing defects and transportation damage-no one can save it when the source is rotten

When the bellows is formed, the wall thickness is uneven, the corrugation roundness is not enough, or the stiffness test is not done before leaving the factory, and the original shape is directly revealed when installed. Also, it was flattened and bumped during transportation, and it was welded without inspection on the spot. It would be strange if there was no accident. A large number of large-diameter thick-walled expansion joints were purchased in a certain project. After the arrival, the workers didn't unpack them because they were troublesome, and directly hoisted and welded them. As a result, all three bellows had pits and all cracked after two months of operation.

When purchasing, ask for the stiffness calculation formula and material report. When receiving the goods, take a caliper to measure the wave distance and total length. Don't be too troubled. A copy of the metal hose size comparison table, expansion joint model and size must be prepared on site. If you fail the acceptance, you will return the goods directly. Don't take chances.

Prevention Guide: Three Steps

• Type selection check: The medium temperature, pressure, displacement amount and environmental conditions (freeze-thaw, corrosion) are matched one by one. High temperature axial expansion joint for high temperature, PTFE-lined or PTFE-lined compensator for corrosion, rubber compensator or damped for high frequency vibration.
• Installation Specifications: The fixed support and guide bracket shall be constructed according to the drawing. The tie rod nut is operated according to the adjustment method of the tie rod nut of the expansion joint, and the transport screw is disassembled when it is necessary.
• Periodic inspection: Is there any crack, bulge or corrosion on the surface of the bellows? Direct burial type to check whether the insulation layer is intact? The vibration frequency of air-cooled island double hinge expansion joint is measured every six months.

Causes of compensator deformation? Complicated is also complicated, and simple is also simple-it is nothing more than the five lines of type selection, installation, medium, vibration and quality. If you stomp on each of them, the probability of deformation will be reduced by 90%. Remember: compensators are not consumables, they are life-savers. Don't wait until something goes wrong to regret it.