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The correct installation method of large diameter metal expansion joint? The 3 easiest pits for construction teams to step on

Two days ago, I met a customer. A DN1200 steam pipe in the factory cracked less than a month after the general corrugated expansion joint was installed. When I went to the scene, the installation team screwed the tie rod nut tightly, and the guide tube arrow was installed backwards. Do you think the money was spent unjustly? Large-diameter metal expansion joint, with a diameter of 800mm, has a completely different temper than a small diameter-its own weight is measured in tons, its stiffness doubles, and the thrust of the pipe system is often tens of tons. If it is not served in place, it will leak at least and the bracket will collapse at worst. Today, let's break it up and talk about the three easiest pits for the construction team to step on on this road, and the real correct installation method.

Why Large Diameter Expansion Joints Are More "Delicate" Than Small Diameter

Small-diameter expansion joints (such as DN100 or below) have light weight and low stiffness, so the installation deviation of a few millimeters is not a big problem, and the bellows can "digest" by itself. But what about large diameters? Take the large-diameter thick-walled expansion joint as an example. The bellows section alone may weigh several hundred kilograms. Coupled with the self-weight of the medium, the vertical load of the whole assembly on the pipeline soars. More critically, the larger the diameter, the axial stiffness of the bellows rises nonlinearly (stiffness increases approximately 4 times for every 1-fold increase in diameter), which means that the additional stress generated on the larger diameter may be more than 10 times that of the smaller diameter for the same installation deviation.

In addition, in structures such as external pressure single axial expansion joint, because the bellows is subjected to external pressure under working condition, the requirements for installation coaxiality are more stringent than those of internal pressure type-if the deflection exceeds 1°, the side wall of the bellows may appear wrinkled. To put it bluntly, it is not that large-diameter expansion joints can't be installed, but that they can't stand the "almost" mentality.

5 parameters that must be checked before installation

When the equipment arrives, don't rush to hoist it. First, check these five parameters with the design drawings and product nameplates one by one, and one less item may lay mines.

• Model and displacement:Confirm is the universal type, external pressure single type or double hinge transverse type? Does the design compensation amount meet the actual working conditions? For example, the pre-tension value of directly buried (fully buried) expansion joint has strict regulations, and the difference of 10mm can't be done.
• Pressure Class:The operating pressure of the pipeline must match the nominal pressure of the expansion joint, with special attention being paid to the possible instantaneous overpressure of the steam pipeline.
• Direction of guide tube:The purpose of this thing is to direct the flow of media and protect the bellows from washing. The arrow must point to the direction of the media flow (from upstream to downstream). In the case of the customer, the arrow was installed backwards, and the medium directly washed the root of the bellows, which wore out in two months.
• Tie Rod Status:Is the transport protection tie rod removed? Is the working tie rod nut in the factory preload position? Many construction teams tried to save trouble and removed the nut of the working tie rod. As a result, the expansion joint drooped and deformed under gravity.

Key steps in the installation process

Let's start with pre-stretching. In the correct installation method of large-diameter metal expansion joint, pre-stretching is a step that many masters are prone to make mistakes. Simply put, in order to compensate for the thermal elongation of the pipeline from the cold state to the hot state, the expansion joint needs to be "pulled apart" by a certain distance during installation. The specific stretch amount is given by the design and is generally equal to half the amount of thermal displacement. During operation, use two hoists to pull symmetrically, and measure the wave pitch of the bellows while pulling to ensure uniform deformation. After pulling, install the temporary limit bolt, hold your strength and then go to the butt welding.

Should the tie rod nut be removed or not? According to the situation. For external pressure single axial expansion joint, the working tie rod is a load-bearing part, and the nut must not be removed-removing the bellows will be excessively elongated under the action of internal pressure. For the models with tie rods such as the transverse type of compound hinge and the pressure balance type of curved tube, the angle and limit of the nut have been adjusted when it leaves the factory, and it is only necessary to check whether it is loose on the spot, and it is strictly prohibited to adjust it without authorization.

Where does the deflector arrow go? Bottom line: the arrow points in the direction of the media flow, and the small mouth end (reduced diameter end) of the guide tube must be directed downstream. This point is clearly explained in the question and answer of "The Specific Function of Expansion Joint Guide Tube", with the purpose of forming a gradually expanding channel and reducing resistance. If installed backwards, the medium will form a vortex that washes through the bellows, and the noise and vibration will immediately increase.

Configuration principles of pipe supports and fixed piers

The large-diameter expansion joint weighs too much, and it can't be supported by the pipe itself. The main fixed pier (or pipe rack) must be set to withstand the blind plate force of the expansion joint. How to calculate? For example, for DN1000 and 1.6MPa pipelines, the blind plate force is about 1.6MPa multiplied by the cross-sectional area (about 0.785m²), which is calculated to be 125 tons. If the design bearing capacity of the fixed pier is only 80 tons, the pier will directly crack as soon as the expansion joint is put into operation.

For straight pipe pressure balanced expansion joints, because they can balance the blind plate force by themselves, the fixed pier can be simplified, but it can't be eliminated-at least a guide bracket is needed to prevent pipe twisting. However, the sleeve-type pipe expansion joint needs to be provided with thrust brackets on both sides, otherwise the sleeve will be pushed out.

What happens to "unsteady standing"? Two years ago, there was a power plant, and the metal corrugated expansion joint in the cement industry was installed on the air-cooled island pipeline. Because the support foundation was not anticorrosive, it was corroded and unstable one year later, and the whole expansion joint was smashed down with a 5-meter pipeline. Therefore, the support of large diameter expansion joint should not only calculate the strength, but also the fatigue life and foundation settlement.

Commissioning and Acceptance Criteria

Brush the weld and bellows surface with soapy water, charge to 1.5 times the working pressure, and hold the pressure for 10 minutes without bubbles before it is qualified. Next, measure the displacement: set marking points at both ends of the expansion joint, measure the wave distance with a vernier caliper, and confirm that the actual stretching amount is within ±5% of the design value. Finally record and archive: write down the model number, installation date, pre-stretch amount, measured displacement value and operator's signature-this paper will be a life-saving straw in future maintenance.

The acceptance criteria of rubber compensators and PTFE compensators are different from those of metal expansion joints, which are sensitive to temperature and usually do not make high-pressure airtight, but low-pressure water pressure. Don't get mixed up.

Review of real accidents

Case 1:A chemical plant uses a general-purpose corrugated expansion joint to compensate the steam pipeline. In order to save trouble, the installation team regards the transportation tie rod as a working tie rod and directly welds it to the pipeline. After the pipeline is put into production, the expansion joint cannot be expanded and contracted, and the bellows is forcibly stretched and cracked. Solution? Cut off the heavy welding and lose heavily.

Case 2:The flue gas pipeline of a cement production line adopts external pressure single axial expansion joint, and the guide tube arrow is installed backward during construction. After 3 months of operation, the inner wall of the bellows was washed out with obvious pits by dust, and it was forced to stop for replacement. Later, Party A checked the installation records and found that the direction of the arrow was clearly marked on the construction drawings, but the installer didn't look at it.

Case 3:A power station uses a double hinge transverse expansion joint, and the support foundation is designed according to ordinary pipeline, ignoring the blind plate force. When the fixed pier is put into operation, the position of the fixed pier is displaced by 40mm, and the adjacent weld seam collapses. Fortunately, the inspectors found it in time and caused no casualties. Afterwards, the anchor bolts were made up and the foundation was reinforced, and the construction period was delayed for 45 days.

Alas, to put it bluntly, these things are not technical problems, but questions of responsibility. The correct installation method of large diameter metal expansion joint? In fact, there are three core points: check according to the drawing, construct according to the regulations, and accept according to the standards. Don't be lazy, don't take it for granted, pretend to test one, and be steady.

If the site conditions are complex (such as high temperature, high pressure, large diameter, with corrosive media), it is recommended to directly find the manufacturer's technical support. You are thinking about whether to remove the tie rod and how much to pre-stretch it yourself. It is better to ask clearly by phone. After all, an expansion joint can range from several thousand to hundreds of thousands. Who will carry it if it is wasted?

First, find out what single wave and double wave are: the structural differences of bellows

Don't be fooled by the name. Single-wave expansion joint means that the bellows has only one wave shell, and double-wave is two layers. But by "layer" here we don't mean thickness-it is independent corrugated cavities. You take apart a metal expansion joint and see that the bellows consists of multiple consecutive peaks and troughs, but single wave means that the whole bellows has only one complete node? No, it's biased to understand it that way.

Single wave and double wave in the industry usually refer to expansion jointsNumber of effective nodes of bellows。 But the more accurate names are "single-wave nodes" and "double-wave nodes" — that is, a set of independent corrugated structures. For example, in the general-purpose corrugated expansion joint we often say, the single wave may be a single-layer corrugated pipe, while the double wave is two sections connected in series, with end plates connected in the middle. You see, in our product line, there are both general-purpose corrugated expansion joints and compound hinge transverse expansion joints, which are typical multi-wave structures.

A single wave has only one corrugated cavity to absorb displacement; The double wave has two cavities, separated by an intermediate tube or an end plate, and can be deformed independently. This directly determines that the compensation ability and stiffness of the two are completely not in the same order of magnitude.

2. Core difference: the fluctuation of compensation ability and stiffness

Let's start with compensation. Can a double-wave expansion joint theoretically provide twice the amount of axial compensation as a single wave? Beautiful thought. In fact, after two waves are connected in series, the displacement shared by each node is smaller, but the total compensation amount is indeed larger than that of a single wave-only it is not linear superposition and limited by the stability of the bellows. For example, the general-purpose corrugated expansion joint of DN200 has an axial compensation of about 40mm for single wave and 70-80mm for double wave. But at what cost? Stiffness.

Bellows stiffness is inversely proportional to the number of nodes (approximate). The overall axial stiffness of a double wave is about half that of a single wave. And guess what? Many engineers think that the low stiffness of double waves is a good thing-soft. But it is too soft. If the pipeline system produces blind plate force under pressure, the double-wave expansion joint may be directly pressed into a "twist". Our common high-temperature axial expansion joints must use single-wave or reinforced structure in high-temperature working conditions, because the double-wave stiffness is not enough and it is easy to become unstable.

Fatigue life. The deformation of each node of double wave is small, and the fatigue life is longer in theory, but the premise is that the design is reasonable. If the wave pitch and wall thickness are not well matched, the double wave will be broken first because of local stress concentration.

3. Practical application scenarios: Where are single waves used and where are double waves required?

Two days ago, I met a purchaser in the cement industry and asked me whether to choose single wave or double wave for metal corrugated expansion joints in the cement industry. I asked directly: Is your pipe a straight section or a bend? What is the medium temperature? Is there any vibration?

• High pressure pipeline: For example, the main steam pipeline of the power station has high pressure, high temperature (> 500℃), high stiffness requirements, and single wave cooperation with reinforcing ring is more reliable. Many corrugated expansion joints used in our power station industry are single-wave reinforced types.
• Occasions where precise guidance is required: For example, straight pipe pressure balance expansion joint, single wave structure is easier to control deflection.
• Space-constrained short tube: The length of double wave is more than twice that of single wave, and only single wave can be used when the installation space is insufficient.

• Large displacement compensationFor example, for long-distance thermal pipelines, the single-wave compensation is not enough, so double or even multiple waves must be used. At this time, it is necessary to match a tie rod or hinge to limit the blind plate force-such as the transverse expansion joint of compound hinge, which is a typical example of double wave plus hinge.
• Low pressure large diameter pipeline: For example, flue gas pipes, low pressure and low double wave stiffness are not a problem, but can better absorb axial and lateral displacements. Our metal rectangular expansion joints and non-metal expansion joints are commonly used in this type of scenario.
• Occasions requiring vibration damping: The double wave has great flexibility and better absorption effect on mechanical vibration. For example, the air-cooled island vacuum pipeline will use double hinge expansion joints.

4. Record of type selection and pit stepping: I thought double wave insurance, but the result was that the stiffness was not enough to directly deform

There's a real case. Steam pipeline of a chemical plant, design temperature 350℃, pressure 1.6MPa, pipe diameter DN300. The original single-wave expansion joint was leaked after several years of use, and the maintenance worker said, "Change to a double-wave one, and the compensation amount is larger and safer". As a result, less than three months after installation, the pipe began to click. After a closer look, the middle of the double-wave expansion joint was bulged-without adding a tie rod, the blind plate force directly pushed the bellows into a "beer belly".

Double-wave expansion joints without tie rods or hinge constraints, the axial thrust generated under pressure can elongate or compress the bellows beyond the design value. In our product information, if the universal corrugated expansion joint is made into double wave, it will definitely be equipped with a tie rod or limit device as standard. You go to the question and answer about the function of the expansion joint tie rod-the tie rod is used to bear the pressure thrust and not let the bellows overload.

Another pit is an occasion with a high frequency of thermal cycling. Because of the low stiffness of double wave, the deformation of each node is large, and fatigue cracks are easy to occur under high frequency vibration. Therefore, the selection does not mean that "the more wavenumber, the better", but must be calculated in combination with the displacement, pressure, temperature and number of cycles of the system. If you are not sure, go directly to the double straight pipe bypass pressure balanced expansion joint or the curved pipe pressure balanced expansion joint-these structures come with their own balancing function and do not transmit pressure thrust to the pipe.

5. Details to be paid attention to during installation: different wave numbers, the configuration of guide tube and tie rod changes accordingly

Install single-wave expansion joints, and the guide tube is usually made on the inside to protect only one wave node. But the double wave is different-the guide tube length must cover the gap between the two nodes, otherwise the medium directly flushes the intermediate tube and quickly perforates. We often encounter customers who buy double-wave expansion joints by themselves and forget to match the guide tube. As a result, it leaks after half a year. The specific function of the expansion joint guide tube is mentioned in the question and answer. It can also reduce flow resistance and prevent vortex.

If a single-wave expansion joint needs a tie rod, it is generally symmetrical 4 or 6. Because of the long length of double wave, the number of tie rods should be large, and the preloading force of each tie rod should be uniform. How to adjust the tie rod nut when installing? It must be adjusted to the design length when cold, leaving the amount of pre-compression. Does the screw of the expansion joint need to be removed? Remember: the transport screw is to prevent deformation during transport and must be removed after installation, otherwise the expansion joint will not work.

Different wave numbers have different weights. In the weight table of metal expansion joint, the weight of double wave under the same diameter is 1.6-1.8 times that of single wave-not twice, because the proportion of structural parts such as intermediate tube and end plate. If you design your pipe support without considering extra weight, it may not hold up. Therefore, before selecting the model, you must get the size, weight and stiffness data, and don't pat your head.

Alas, in the final analysis, the difference between single wave and double wave of metal expansion joint? The core is to weigh the amount of compensation and stiffness. There is no universal model, only the one that is best suited to the working conditions. Before the next selection, calculate the thermal expansion of the pipeline first, then check the corrosion margin, and finally look at the pocket budget-double wave is usually 30%-50% more expensive than single wave. Don't choose a single wave to save money, which leads to insufficient compensation, and don't blindly go on a double wave for "safety", which results in stiffness failure. If you are really unsure, look through the product information on our website.

Why can the metal expansion joint "carry" the thermal expansion and contraction of the pipe? — — Core Principle and Structural Dismantling

Think about it, how much can a steam pipe several hundred meters long expand when its temperature rises from normal temperature to five or six hundred degrees? Dozens of millimeters are light. Hard carry? The welds and brackets of the pipeline have to be scrapped. At this time, the metal expansion joint plays a decisive role. What does it rely on? By the bellows.

The bellows is the heart of the metal expansion joint, which is, to put it bluntly, a set of annular corrugated thin-walled metal tubes. It can produce elastic deformation in axial, transverse and even angular directions, and "eat" the thermal displacement of the pipe. Bellows alone is not enough, but also accessories such as upper end pipe, guide tube and tie rod. For example, our commonUniversal corrugated expansion jointThe end pipe is responsible for connecting the pipeline, the guide tube protects the bellows from being washed by high-speed media, and the tie rod limits the displacement direction-if this thing is installed backwards, it will suffer a big loss later.

Structurally, according to different compensation directions, it can be divided into axial type, transverse type, hinge type and so on. LikeExternal pressure single axial expansion joint, bellows bear external pressure, better stability, suitable for large-diameter pipes. AndCompound hinge transverse expansion joint, with two sets of bellows and hinge structure, specially absorbs transverse displacement. Therefore, metal expansion energy saving to withstand thermal expansion and contraction relies not on brute force, but on delicate corrugated elastic deformation.

Temperature resistance, pressure resistance and fatigue resistance-three hard indicators determine what harsh applications it can be used in

Q: What are the features of metal expansion joints? The most direct answer is: it can work stably under high temperature, high pressure and high frequency alternating working conditions. But these three indicators are not isolated, they contain each other.

Let's start with temperature resistance. General 304 stainless steel bellows, the long-term working temperature is about 400℃. When you encounter the high temperature of seven to eight hundred degrees such as power plant boilers and cement kiln tails, you have to use itHigh temperature axial expansion jointThe material is changed to Incone 625 or GH3030, and then lined with thermal insulation. And guess what? In our stationCorrugated expansion joint for power station industryAndThe metal corrugated expansion joint in cement industry is specially designed for this kind of scene, and the thickness of insulation layer and the number of corrugated layers have been optimized.

In this part of pressure resistance, the wall thickness, wave height and number of layers of bellows directly determine the upper limit of pressure bearing. Such asLarge diameter thick wall expansion jointThe wall thickness of a single layer can reach more than 2mm, and the design pressure starts at 1.6MPa. But when the stress is high, the fatigue life will be discounted-there is a balance.

Fatigue resistance is the real "invisible killer". The pipeline starts and stops, steam hammers and vibrates every day, and the bellows expands and contracts back and forth. According to the standard, the fatigue life of general bellows is about 1000~3000 times. But this is only a theoretical value. In actual use, corrosion, stress concentration and installation deviation will greatly shorten the life. Therefore, the real test of quality is the comprehensive performance of these three hard indicators.

How to choose between different working conditions? Universal type, high temperature type and pressure balance type all have their own unique skills

When it comes to model selection, don't expect one product to conquer the world. Let's take a look at the three most typical scenarios:

Scenario 1: Normal temperature, low pressure, ordinary pipeline.For example, heating water systems, usingUniversal corrugated expansion jointThat's enough. Simple structure, affordable price, axial compensation amount 20~50mm, easy to do. Of course, if the pipe has a lateral displacement, it can be consideredDuplex hinge transverse type expansion joint.

Scenario 2: High temperature, corrosion, large displacement.For example, desulfurization flue and high-temperature fan outlet. Here the medium temperature is high and there is an acidic condensate. Conventional bellows can't hold it, so you have to use itHigh temperature axial expansion jointAdd corrosion resistant coating. In addition,Desulfurization flue gas baffle doorUsed in conjunction, it can cut off smoke and facilitate maintenance. Also pay attention to large-diameter pipes, rectangular cross-sectionsMetal rectangular expansion jointMore suitable, we have special products in our station.

Scenario 3: High pressure, strictly limited displacement.For example, steam main pipe, pump outlet. The thrust generated by the internal pressure of the pipeline is huge. If the axial expansion joint is directly installed, the pipe frame must be made like a bunker. At this timePressure balanced expansion jointIt comes in handy-canceling the internal pressure thrust from each other through two bellows, without the need for extra fixing brackets. Breaking it down, there areA straight tube pressure balance type expansion joint,Curved tube pressure balance expansion joint、Double straight pipe bypass pressure balance type expansion joint. Their core difference lies in the arrangement of bellows, which should be determined according to the direction of pipeline and space size.

In addition, for buried pipelinesDirect buried (fully buried) type expansion joint, directly buried in the soil, the outer layer of antiseptic is particularly well done. Vacuum pipes are usedSpecial hose for vacuumAnd high sealing requirements.

Exactly how long do metal expansion joints last? Influencing factors and measured data

As soon as this question is thrown up, many customers will ask. To be honest, there is no standard answer, but you can give a rough range of judgment.

Look at the design criteria first. GB/T 12777 stipulates that the fatigue life of bellows is calculated according to the number of cycles. Under normal working conditions, the design life is 1000 times, but in actual tests, many high-quality products can run more than 3000 times. We tested a batch beforeUniversal corrugated expansion jointWith an axial displacement of 30mm at a normal temperature of 0.6MPa, the crack appeared only after 3500 cycles.

• Temperature:Fatigue life drops by about half for every 100°C rise. At 400℃, the life may only be 20% of normal temperature.
• Pressure fluctuations:Frequent pulse pressure is more harmful than stable pressure. For example, the water hammer of the steam pipe may make the local stress of the bellows exceed the limit at one time.
• Corrosive media:Chloride ions and sulfides will accelerate stress corrosion cracking. In the desulfurization and denitrification system, if the material selection is wrong, it may leak in half a year.

Therefore, the actual service life can range from a few months to more than a dozen years. The key is to choose the right model in the early stage and install it correctly in the later stage. Such asrubber compensatorThe life span is relatively short, generally 3~5 years; However, if the metal expansion joint is stable in working condition and properly maintained, it will be used for 8~10 years without any problem.

Details that are easy to overlook in installation and maintenance-you can't go wrong with tie rods, deflectors, and arrows

A while ago, there was a customer who finished installingLarge tie rod expansion jointLater, it was found that the compensation was not enough, and the pipeline was still crooked. After investigating for a long time, it turned out that the tie rod nut was locked too dead, and the bellows couldn't move at all. Tsk, this kind of low-level mistake is actually quite common.

The function of the tie rod, as mentioned in the frequently asked questions and answers-"The function of the tie rod of the expansion joint" makes it very clear: it only bears the internal pressure thrust, and does not limit the displacement of the bellows. When installing, the tie rod nut should be reserved for adjustment clearance, and cannot be screwed to the end. How to tune it? The article "How to Adjust the Tie Rod Nut of Expansion Joint" gives the specific steps: first pre-tighten to the end, and then loosen the thread length corresponding to the displacement.

Look at the deflector again. Many people think it is just a shield, but it is not. As explained in "The Specific Function of Expansion Joint Guide Tube", the guide tube has two key functions: first, drainage, allowing the medium to pass smoothly and reducing vortex erosion; The second is to protect the inner wall of the bellows from being worn by particulate matter. Note when installing: the direction of the arrow must be consistent with the flow direction of the medium-the article "The direction of the arrow of the expansion joint refers to" specifically said that the arrow is the flow direction mark. If the guide tube is installed backwards, it becomes a throttle device, and the bellows will be impacted.

• Before on-site welding, be sure to remove the expansion joint or protect it, and the welding slag splash will burn through the bellows.
• When hoisting, you can't directly strangle the bellows with ropes, but use soft belts or lift from the end pipe.
• After installation, remember to dismantle the limit screw for transportation-"Does the screw of the expansion joint need to be dismantled" answers this question. If it is not dismantled, the expansion joint will be completely locked, which will not compensate.

To be honest, the structure of metal expansion joints is not complicated, but the details determine success or failure. Choose the right model, install it in the right direction, and adjust the tie rod, and it can serve safely for many years. If these details are not paid attention to, the failure rate skyrockets.

Finally, if you are choosing a model for a pipe, you might as well take a look at our siteMetal hose、PTFE-lined hose、Rotary compensatorAnd other product series, there is always an adapted scheme.

Two days ago, I met the project manager of a steel mill, and talked about the hot air duct in their factory recently-the weld cracked, the bracket deformed, and the machine was shut down every three days for emergency repair. I asked him: What compensator is installed in your pipeline connection section? He was stunned for a moment and said, It's just an ordinary metal expansion joint. I told him directly, switch to a non-metal one. In fact, many people have similar confusion: Why do steel mills use non-metallic expansion joints? Isn't it just a cloth bag? Really not. Today, let's make it clear what this thing is used for in steel mills, how to use it, and how to choose it so that it won't step on pits.

1. How "tossing" is the pipeline system of steel mills? — — Temperature, displacement and corrosion

Steel mill pipeline is not an ordinary pipeline, it is in "deep water" every day. Take the blast furnace hot blast stove as an example. The supply air temperature is often above 1000℃, and the temperature difference can reach 800 or 900 degrees when hot and cold alternate. Pipe thermal expansion and contraction, a pipe section more than ten meters long, the thermal displacement may exceed 100 mm. That's not the end, what's running in the pipeline? High-temperature flue gas, sulfur-containing waste gas, dust particles, and some media are also highly corrosive. In this environment, ordinary carbon steel pipes are riddled with holes by corrosion in a few months. Not to mention the additional displacement caused by the settlement of the pipe support foundation and the vibration of the fan. Think about it, in such a torturous environment, if there is no reliable compensation means at the pipeline connection, the whole system will fall apart.

Second, what is the use of non-metallic expansion joints in steel mills? — — Dismantling three core functions

What can non-metallic expansion joints (also called fabric fiber expansion joints, we have a special product introduction in our station) do? Break it apart and say three things:

1. Absorb multi-dimensional displacement and do not produce thrust.Although the metal expansion joint can also compensate for displacement, it will generate elastic reaction force, which requires high pipe support. The band of non-metallic expansion joint is made of multi-layer flexible material composite, which can easily absorb axial, transverse and angular displacement, and has almost no resilience. For example, a section of hot air duct expands by 80mm due to temperature increase, and the non-metallic expansion joint directly "swallows" this displacement without transmitting force to the equipment flange.

2. Vibration isolation and noise reduction to protect equipment.The fans and dust collectors in steel mills vibrate heavily, and the metal hard connection will directly transmit the vibration to the pipeline. After a long time, the bolts will loosen and the welds will crack. The non-metallic expansion joint itself is a flexible body, which is equivalent to a soft joint, and can filter out 70% to 80% of high-frequency vibration. In a previous case, a steel mill installed a non-metallic expansion joint at the outlet of the induced draft fan, and the vibration amplitude of the pipeline was directly reduced by 60%, and the life of the equipment was obviously extended.

3. Corrosion resistance, high temperature resistance, adapt to harsh media.The ring belt material of the non-metallic expansion joint can be customized according to the working conditions-fluororubber is resistant to strong acids, silicone rubber is resistant to high temperatures, and PTFE is resistant to corrosion. For the sulfur-containing wet flue gas in the desulfurization flue, the temperature is not high but the corrosion is strong, and the rubber PTFE compensator is much more reliable than the metal expansion joint. Metal expansion joint Once the bellows is corroded and perforated, the whole system has to leak. Even if the non-metal ring belt is partially damaged, it is easy to repair and does not need to be replaced as a whole.

III. Comparison of metal expansion joints: Why prefer non-metals in flue gas and hot air ducts of steel mills?

Hey, are metal expansion joints useless? Of course not. Metal expansion joints (such as the general corrugated expansion joints and high-temperature axial expansion joints in our station) are still the first choice in high-pressure and high-purity media applications. However, in the low-pressure, large-diameter and high-temperature flue gas pipelines of steel mills, the advantages of non-metals are too obvious.

For the same caliber compensator, the price of non-metal tends to be only 1/3 to 1/2 of the price of metal. Look at the installation again: the metal expansion joint requires high installation accuracy, and the direction of the guide tube and the adjustment of the tie rod nut (refer to the content of how to adjust the tie rod nut of the expansion joint in our Q&A) are wrong. The non-metal expansion joint is simple to install, just tighten the flange connection, and be careful not to twist when installing the ring belt. Another point: once the metal expansion joint is welded to the pipeline, it must be hot cutting for maintenance and replacement, while the non-metal expansion joint only needs to remove the flange bolts to pull out the ring belt and replace it with a new one. Steel mill maintenance time is money, which makes people in the equipment department love it.

Of course, non-metals also have shortcomings: the pressure bearing capacity is low, generally no more than 0.1MPa. The flue gas pipeline of steel mills is originally slightly positive or negative pressure, which is just the right way. If you use it to connect to the steam pipe, it won't work. It must be emphasized here that the selection should not be messed up, but should be done according to the working conditions.

4. Typical application scenarios of steel mills: from blast furnace hot blast furnace to desulfurization flue, one is indispensable

Let me list a few of the application scenarios of non-metallic expansion joints in steel mills:

• Blast furnace hot blast furnace area: On the hot air main pipe and branch pipe, the temperature is 800~1200℃, and high-temperature resistant non-metallic expansion joints (such as aluminum silicate fiber + ceramic fiber cloth composite band) are used to compensate for thermal displacement and insulate heat at the same time.
• Desulfurization and denitrification flue: The flue gas temperature after wet desulfurization is 50~80℃, but it contains a large amount of SO₂ and HCl, which is extremely corrosive. At this time, with rubber tetrafluorine compensator or polytetrafluoroethylene compensator, the fluoroplastic layer can withstand strong acid corrosion, and has good flexibility, which can absorb the displacement of flue due to temperature difference.
• Dust removal system pipeline: The inlet and outlet of the bag dust collector, the flue gas has high dust content and large temperature fluctuation. The inner wall of the non-metallic expansion joint is smooth, does not accumulate dust, and can also absorb fan vibration. Some steel mills use rectangular non-metallic expansion joints for large rectangular flues, specifically matching square sections.
• Gas pipeline: Blast furnace gas and converter gas pipelines, the temperature is not high but contains CO, and the sealing requirements are high. The multilayer sealing structure of non-metallic expansion joints has extremely low leakage rate and will not be damaged by water hammer.

Lo and behold, from the hot end to the cold end, from the dry process to the wet process, non-metallic expansion joints cover nearly all nodes of a steel mill's flue gas system. Isn't it a bit of a surprise?

5. Pay attention to the selection: temperature grade, media composition and installation position directly affect the life span

If you choose the wrong model, no matter how good the product is, it will be useless. Focus on three parameters when selecting:

First, temperature.The non-metallic expansion joints have limited temperature resistance. Conventional silicone rubber belt can withstand 200 DEG C, fluorine rubber 250 DEG C, and ceramic fiber layer can be above 1000 DEG C. However, it should be noted that the actual operating temperature should be at least 50℃ lower than the material limit, otherwise the aging will be accelerated. For example, a steel mill used a ring belt with a temperature resistance of 400℃ on a flue gas pipeline at 450℃, and as a result, it was brittle and cracked in three months. Degraded usage is a common mistake.

Second, a medium component.Oily smoke can not use rubber ring belt, oil will swell the rubber. Flue gas containing chloride ions must be isolated by a fluoroplastic layer. The desulfurization flue has a large amount of moisture, so it is necessary to choose a PTFE compensator that resists hydrolysis. In the product information of our station, there are desulfurization flue gas baffle doors and matching compensators specifically for desulfurization conditions, which can be used in combination.

Third, the installation location.If the expansion joint is mounted on a horizontal pipe, consider whether dust accumulates. On the vertical pipeline, it is necessary to prevent the ring belt from bearing and cracking. Some positions need to install a guide tube (refer to the specific function of the expansion joint guide tube in the question and answer) to prevent high-speed airflow from directly washing the ring belt. In addition, the layout of pipe brackets is also critical-fixed brackets must be set on both sides of the expansion joint, and the spacing of guide brackets is also required. These details are in our Q&AAs mentioned in, the tie rod is mainly to prevent over-stretching, and it is also applicable to non-metallic expansion joints.

6. Just put it on and you're done? Those pits for installation and routine inspection

When installing non-metallic expansion joints, there are several pits that must not be stepped on:

• Pit 1: Forced counterpart.Pipe installation deviation is large, and expansion joints are used to "level". Isn't that the same as hard twisting the loop tape into a twist? Correct practice: Adjust the pipe bracket to ensure that the expansion joint is installed in the free state and the amount of pre-deformation does not exceed 10% of the design value.
• Pit 2: The loop belt is installed backwards.Some non-metallic expansion joints have directionality (for example, the guide tube faces the flow direction of the medium). If the guide tube is installed backwards, it can't play a protective role, and the airflow directly washes the inside of the ring belt.
• Pit 3: The bolts are tightened too tightly.The torque of the flange connecting bolt is too large, which will press the ring belt out of dents and destroy the sealing surface. It should be evenly tightened diagonally, and the torque should be according to the manufacturer's requirements.
• Pit 4: Neglect insulation.The exterior of the non-metallic expansion joint on the high-temperature pipe must be wrapped with an insulation layer, otherwise the outer surface temperature of the ring belt will be too high, which will accelerate the aging. The insulation layer should also leave an inspection port to facilitate daily inspection.

What do you see for daily inspections? Look at the surface of the ring belt for bulging, cracking, aging and powdering. Second, see if the flange bolts are loose. Third, see if there is any abnormal noise-the ring belt rubs against the pipe wall or the displacement of the guide tube will make a sound. Four, see if there is a drip. The maintenance cycle of the steel mill is long, and the inspection frequency is recommended once a week. Use an infrared thermometer to measure the surface temperature of the ring belt. Compared with the initial record, we should be vigilant if the temperature difference exceeds 20℃.

After so much verbose, it is actually a sentence: steel mills use non-metallic expansion joints not because they are cheap, but because they are indeed more suitable than metallic ones in the scenarios of flue gas, hot air and corrosive media. If you choose it right, install it right, and maintain it well, it will not be a problem to use it for three or five years. If you are still unsure which one to choose, flip through the product pages of rectangular non-metallic expansion joints, rubber compensators and PTFE compensators in our station, or ask the technology directly.

First, find out which four kinds "four kinds" refer to?

"What are the four kinds of metal expansion joints?" In fact, strictly speaking, according to the structural form and compensation function, there are four mainstream categories:Axial, transverse, angular and pressure balanced。 But this is not random-each type corresponds to a specific pipe displacement scenario. Such asUniversal corrugated expansion jointMainly absorbs axial displacement,The transverse expansion joint of the compound hinge is specialized in treating the transverse displacement,Curved tube pressure balance type expansion energy saving balance internal pressure thrust,Sleeve-type pipe expansion joint is good at absorbing large axial displacement and resisting high pressure. To put it bluntly, if you choose the wrong type, the pipeline will go wrong sooner or later.

2. Disassembly one by one: the core characteristics of these four types of products

1. Universal corrugated expansion joint-the "old scalper" of linear pipeline

The structure is the simplest, and it absorbs axial displacement by elastic deformation of bellows. Straight line pipes suitable for power stations and cement industries. Inside our stationUniversal corrugated expansion jointAndHigh temperature axial type expansion joints all belong to this category. Note: It is only axial, don't expect it to handle lateral or angular displacements.

2. Compound hinge transverse expansion joint-specialized in treating "non-straight line" pipelines

The lateral displacement compensation is realized by two sets of bellows and hinge structures, and the key is that no blind plate force is generated. Inside our stationThe lateral expansion joint of double hinge is a typical representative. A steam pipe that turns 90 degrees? Large lateral displacement? It would be right to go.

3. Curved tube pressure balance expansion joint-the "pressure reducing valve" of pump and valve equipment

There is a pressure balancing element inside, which can eliminate the thrust of the medium pressure on the fixed bracket. Pump, valve, compressor inlet and outlet sensitive equipment, must use this. Inside our stationCurved tube pressure balance type expansion joint specializes in this. If the ordinary axial type is used, the thrust of the bracket is too large, and the equipment interface will crack sooner or later.

4. Sleeve-type pipe expansion joint-the "tough guy" of high temperature and high pressure

The displacement is absorbed by the relative sliding of the inner and outer sleeves, and the pressure resistance is extremely strong, but the sealing requirements are high. Inside our stationSleeve type pipe expansion joints are suitable for high temperature and high pressure steam pipes, such as main steam pipes. The disadvantage is that it cannot absorb lateral displacement and can only slide axially.

3. Pits that are easy to step on when selecting models: don't confuse types

Two days ago, I met a customer, and the expansion joint should be selected for the steam pipeline, which should be universal-as a result, the pipeline on site turned 90 degrees, and the lateral displacement was very large. Universal type can't solve it at all, and it is necessary to upper double hinge transverse type or angular type. Do you think this pit is injustice or not?

① Displacement direction of pipeline(axial, transverse, or angular);② Operating pressure and temperature(Decide whether to choose bellows or sleeve type);③ Whether it is necessary to balance the pressure(Pressure balanced type is preferred when pump and valve equipment is available).

In addition, don't forget to look at the media-choose corrosive mediaPTFE-lined hoses orPolytetrafluoroethylene compensator, do not use common metal expansion joints. Otherwise, the bellows will be corroded through in a few months.

4. Practical application scenarios: from power stations to cement plants, each shows its magical abilities

Power station industry

The main steam pipeline has high temperature and high pressure, which is usually usedSleeve type pipe expansion joint orExternal pressure single type axial type expansion joint. Inside our stationThe corrugated expansion joint used in the power station industry is specially designed for this kind of working condition. In addition, the air-cooled island vacuum pipe has to be usedAir-cooled island vacuum pipe double hinge expansion joint, this thing can absorb multi-dimensional displacement.

Cement industry

The axial displacement and vibration of the high-temperature flue gas pipeline and the inlet and outlet of the fan are large,Universal corrugated expansion joints andHigh temperature axial type expansion joints are used the most. Inside our stationMetal corrugated expansion joints in cement industry have served many production lines, and some have not been changed for 8 years.

Desulfurization system

The flue gas contains sulfur and is corrosive, and it needs to be isolated. At this time, it is often matchedFlue gas baffle doors andDouble-sealed single-axis circular baffle door, expansion joint is selectedNon-metallic expansion joints orPTFE-lined hose. Don't use metal, it won't withstand corrosion.

5. Installation and maintenance: Details determine life

Many customers bought expansion joints, but they failed early because of improper installation. For example, the direction of the arrow of the expansion joint refers to the flow direction of the medium. If the bellows is installed backwards, it will be squeezed. There is also the adjustment of the tie rod nut-the tie rod and nut for transportation should be loosened to the designated position after installation, otherwise the expansion joint will not be able to displace normally. For specific adjustment methods, please refer to the "How to adjust the tie rod nut of expansion joint"and"Correct installation method of expansion joint of large tie rod》。

In addition,expansion joint guide tubeNor can it be ignored-it protects the bellows from being washed by high-speed media, especially must be retained under high-flow conditions. And guess what? Some customers disassembled the deflector in order to save money, and as a result, the bellows wore out in half a year.

In short, if you choose the right type, install it correctly, and check it regularly, the metal expansion joint will last for 10 years without any problem. If you are really unsure, go directly to the product information in our station:Universal corrugated expansion joint、Compound hinge transverse expansion joint、Curved tube pressure balance expansion joint、Sleeve type pipe expansion jointEach model has detailed parameters. Don't pat your head and choose, look at the working conditions more.