Specialized in manufacturing compensators, expansion joints, baffle doors

Cold draw amount of metal expansion joint? Don't laugh, this is not a question that everyone can answer. When the master of dry pipeline installation sees "cold drawn 50mm" marked on the drawing, most of them will scratch their heads: What does this have to do with the compensation amount? Which way? What happens if you pull too much?

Let's get this straight first.

What is cold drawing amount: Understand the difference between it and compensation amount and pre-displacement

The compensation amount is the total maximum thermal displacement of the pipe from cold to hot, which is the design value and is the part of the deformation that the expansion joint is going to "eat". And what about the amount of cold pull? It is an artificial pre-tension (or pre-pressing) of the expansion joint in the opposite direction of thermal displacement during installation. To put it bluntly, it is to "advance" part of the thermal deformation in advance.

Pre-displacement is the broader term and includes cold drawing and cold pressing. Cold drawing is only the most common type of pre-displacement-for thermal expansion displacement of pipes. Don't be confused by these names: the amount of compensation is "to happen" and the amount of cold pull is "you did it ahead of time". One is the formula in the textbook, and the other is the wrench at the installation site.

Why Cold Draw: What is the true displacement of thermal expansion and contraction?

Some people think that the expansion joint is installed and it's done? Why do you pull it cold?

Settle the account for you. A section of 30-meter-long steam pipeline, the working temperature is 200℃, the ambient temperature during installation is 20℃, and the linear expansion coefficient of carbon steel is calculated according to 12×10⁻⁶/℃:

Δ L =12×10⁻⁶ ×30000mm × (200-20) =64.8mm

More than 63mm. If you install the bellows directly from its natural length, it will have to carry a compression of 65mm when it is hot. The wave pitch is pressed to the limit, and the fatigue life is directly cut in half. What about the cold pull half? 32mm pre-compression +32mm hot compression, the cycle amplitude is small, and the fatigue life is doubled up.

How to determine the amount of cold drawing: calculate step by step according to the temperature, length of pipe section and position of fixed bracket

Δ L = α × L × (T_Work-T_Environment)

α is the linear expansion coefficient (about 12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel), L is the length of the pipe section between the two fixed brackets, and T is the temperature difference. After calculating the total thermal displacement Δ L, 50% to 70% of Δ L is taken as the cold drawing amount, which is a commonly used "cold drawing ratio".

• Steam pipeline, temperature difference> 150℃, cold pull ratio 50% ~65%
• Hot water pipeline, temperature difference 80~150℃, cold pull ratio 60% ~70%
• Low temperature or temperature difference

Of course, this is only an estimation method. The project also depends on the position of the fixed bracket, the direction of the pipeline and the number of elbows. If there are several bends in the same pipe section, the thermal displacement is no longer pure axial, and the cold drawing amount has to be corrected according to the results of stress analysis software. You are marked with the amount of cold drawing on the drawing, and you are done with it, but you know how it came from, and you have a bottom in your heart when checking the calculation.

How to operate on site: pre-compression, tie rod, jack, measurement record

The operation process is not difficult to say, but the details are the worst.

In the first step, remove the expansion joint from the package and check that the set nut on the transport protection tie rod is loose. In the second step, the cold drawing direction is determined according to the drawing. When the pipe heats up, it extends, so the bellows is pre-compressed during installation; Pipes are pre-stretched if they are cold shrunk, such as cryogenic pipes.

The third step is practical execution. If the diameter is small and the cold drawing amount is less than 30mm, just screw the tie rod nut directly. If the diameter is large or the cold drawing amount exceeds 50mm, don't screw it hard. Use a jack and an auxiliary frame to push it. Two people on the side will tighten the flange bolts diagonally at the same time, so that the bellows will deform evenly, otherwise the wave pitch will leak later.

Step 4, record. Don't be too troublesome-ambient temperature, pipe wall temperature, length of corrugated pipe before cold drawing, length after cold drawing, actual cold drawing amount and number of adjustment turns of tie rod are all written into the construction record. Which link is wrong, these data are clues to find the cause.

Consequences of mistaken cold drawing amount: bellows instability, bracket exceeding limit, nozzle leakage

Two years ago, there was a project of thermal pipe network, the general-purpose corrugated expansion joint of DN600. During the installation, the workers didn't draw it cold, so they welded it directly. Three months into operation, the bellows cracked at the trough, and the steam blew out far away. When I removed it, I saw that the wave pitch had been pressed together, and the bellows was completely unstable. When the manufacturer sent an engineer to the scene, the first thing he asked was: Has the cold drawing been done?

Even if the amount of cold pull is too large, it won't work. The pre-compression is excessive, and the bellows is close to the yield limit in the cold state. Before it is put into production, the fatigue life has been burned by a large part. The axial force bearing by the fixed bracket will also exceed the design value, and there have been cases where the guide bracket has fallen off.

Bellows instability, bracket force exceeding limit, nozzle leakage-any one of these three things is a shutdown accident.

Cold Drawing Quantity and Matching of Different Types of Expansion Joints

Different structures of expansion joints have different concerns in cold drawing operation.

Universal corrugated expansion jointMost commonly, the structure is simple, just adjust the tie rod nut in place directly when cold drawing. Be careful not to install the deflector in the opposite direction, and the medium should flow in the direction of the arrow.

External pressure single axial expansion jointThe bellows is on the outside of the guide tube and has a protective sleeve on the outside. When cold drawing, you can't see the state of the bellows from the outside, only the scale mark on the outer cylinder. The advantage of this structure is that the bellows is not directly washed by the medium, but you must look at the marking line when pulling it in cold, and don't forget to lock the nut after pulling it in place.

Straight pipe pressure balanced expansion jointThe most particular. It has a balancing bellows and a working bellows inside, which relies on the self-balancing of internal pressure and thrust. If the cold drawing amount is only calculated according to the thermal displacement, and the force on the balance end is not considered, the actual displacement of the bellows after installation will not match the design value. It is suggested that when this model is cold drawn, the bellows on both sides should be pre-displaced at the same time, and only 20% ~30% of the cold drawing amount should be adjusted each time, and the operation should be alternated so that the pressure balancing system should always be in a symmetrical state.

AndCompound hinge transverse expansion jointIt absorbs lateral displacement by the change of hinge angle. There is no such thing as axial cold pulling, but the rotation direction of hinge should be found according to the drawings when installing, and the reason is similar.

Okay, that's it. The formula of this set of cold-pulled things is not complicated, but the difficulty lies in that someone has to be serious in every link. Next time you go to the construction site and see that the bellows is crooked before installation, don't be surprised, that may be the right cold drawing site.

Let's talk about the conclusion first: temperature compensation, let's talk about the score situation

Give the answer directly-whether temperature compensation is required for non-metal tensile non-destructive testing depends on what your testing purpose is. If you just want to compare the consistency of the quality of the same batch of products relatively, there is little difference between compensation or not; However, if you want to take the data measured at room temperature to check the design load and do safety assessment, the data will be wasted without temperature compensation.

This is not a big deal. After doing this for a long time, you will find that the dispute on the spot is often not whether there is a problem with the material itself, but that the testing conditions do not match the actual working conditions, and no one recognizes the data.

Tensile properties of non-metallic materials, how sensitive are they to temperature? — Starting with fabric fibers and rubber

Let's take the common things as an example. The tensile strength, elastic modulus and elongation at break of glass fiber cloth, fluororubber and silicone rubber coatings used in fabric fiber expansion joints are completely different at 20℃ and 150℃.

Take rubber as an example. As soon as the temperature rises, the movement of molecular segments intensifies, the hardness decreases, and the tensile strength drops considerably. Some formulas have a strength of only about 60% of normal temperature at 100℃. For materials such as PTFE (polytetrafluoroethylene), the fluctuation of mechanical properties caused by temperature changes cannot be ignored. It is hard and stiff at room temperature. As soon as the temperature approaches its upper limit of use, softening and creep will all come.

So you see, the mechanical properties of non-metallic materials are a curve that changes with temperature, not the single point at normal temperature.

What do standards say? Is there a clear answer in JB/T 12235-2015

JB/T 12235-2015 "Non-metallic Expansion Joint" standard, it has requirements for the tensile property test of laminated composite materials and non-metallic elastic materials. The standard specifies the test methods, such as sample preparation, loading rate and test environmental conditions. However, the standard does not give a ready-made correction formula for temperature compensation.

Why? Because there are many grades and mixed formulas of non-metallic materials, each manufacturer's fabric fiber and rubber system are different, so it can't be converted with a uniform coefficient. The standard is to give this initiative to the tester: when you measure, you have to record as much ambient temperature as it is-the implication is that you have to take responsibility for this temperature difference yourself.

To put it bluntly, the standard gives the methodological framework, but it doesn't answer the engineering judgment for you.

Actual scene: What should I do if the pipeline operating temperature vs the detection ambient temperature is 20℃?

Two days ago, I also met a customer. The rectangular non-metallic expansion joint used in their project had a designed operating temperature of 180℃. The on-site equipment was shut down for maintenance, removed and sent to the laboratory for tensile non-destructive testing. As soon as the laboratory air conditioner was turned on, the ambient temperature was 22℃. When the test report came out, the data was pretty, and the breaking strength was completely up to standard. But the problem is-180℃ and 22℃, the difference is 158℃.

The tensile strength measured at room temperature of 22℃ and the actual bearing capacity at high temperature of 180℃ are not the same at all. If you use this data to prove that this expansion joint still has enough safety margin at 180℃, then this report is not convincing. The difference of 20℃ is already obvious, not to mention this temperature difference of hundreds of degrees Celsius at every turn.

Therefore, the closer the detection conditions are to the real working conditions, the more the data can explain the problem. Can't do it? That also has to quantify the effect of temperature clearly, not when it doesn't exist.

In what cases can you not compensate? In what circumstances must compensation be made?

Let's start with the situation that you can't do it. The first is the factory comparative inspection of the same batch of products-everyone measures it together at room temperature, and the measured values are horizontally comparable. Anyway, the benchmarks are the same. The second is routine arrival acceptance. You mainly want to confirm that the product is not damaged in production and transportation, and whether there is any obvious deterioration. The benchmark judgment at room temperature is enough.

However, in the following cases, it is recommended to make temperature correction honestly:

• The design safety margin under high temperature working conditions is inferred by using the detection data, such as non-metallic expansion joints (fabric fiber expansion joints) or rubber compensators on flue ducts and desulfurization systems, and the operating temperature is generally above 100℃;
• In failure analysis, it is necessary to correspond the normal temperature detection data with the temperature working condition at the time of failure to find out the real reason;
• For life assessment or fatigue performance analysis, temperature directly affects the creep and relaxation behavior of materials;
• It involves horizontal comparison and selection and replacement between different suppliers and different material systems-the temperature response characteristics are different, and it is easy to choose wrong if you are not on the same benchmark.

There are also products such as PTFE-lined metal hoses and PTFE compensators. The mechanical attenuation of PTFE materials at high temperatures should not be underestimated. If the temperature change correction has been done or not, the results can be an order of magnitude worse.

Three implementable suggestions for inspectors and design institutes

First, the ambient temperature and the sample temperature must be marked on the test report. Don't underestimate this line of words. If there is a dispute, this is the basis. There is no compulsory unified temperature compensation formula in JB/T 12235-2015, so make the condition record work solid.

Second, if conditions permit, put the sample at the target operating temperature for pretreatment before testing, and then test it after constant temperature for a period of time. Even if it can't be completely isothermal loaded, it is a big step towards real working conditions.

Third, when the design institute selects non-metallic expansion joints, if the material testing data obtained is measured at room temperature, don't directly put it into the high-temperature working condition. The performance data of the material system at the corresponding temperature level should be requested from the manufacturer, or the calibration calculation after temperature correction should be requested. This is particularly critical for fabric fiber expansion joints and rubber compensators.

Back to the title question-does non-metallic tensile non-destructive require temperature compensation? The answer is not a simple "yes" or "no", but you have to clarify what the data is used for first. When used in the right place, room temperature data is also valuable; If you use the wrong scenario, no matter how beautiful the numbers are, you can't support the safety margin.

Find out what non-metallic compensator is first: compared with metal compensator, where is it stronger and weaker?

The core structure of non-metallic compensator, also called non-metallic expansion joint (fabric fiber expansion joint), is flexible fabric band, thermal insulation filler layer and metal frame. It is different from the metal corrugated compensator: metal absorbs displacement by the deformation of the corrugated thin wall, while non-metal absorbs displacement by the flexibility of the fabric band.

First, the amount of compensated displacement is large, especially the lateral and angular displacement, which is often several times that of metal; Second, the thrust is small, because the stiffness of the fabric loop is extremely low, and the load requirement on the pipe support is low; Third, it has good corrosion resistance and can resist corrosive media such as sulfide and chloride ions in flue gas. There are also shortcomings-poor pressure bearing capacity, which can generally only be used in low pressure or even slightly negative pressure conditions, and fabric belts are consumables, and their life is not as good as that of metal bellows.

Therefore, the use of non-metallic compensator is not to replace metal, but to make up the shortcomings of metal. The following working conditions are its home field.

Application 1: Large-diameter low-pressure pipes such as air ducts and flues of power plants can be easily compensated by rectangular cross sections

The air ducts and flues of power plants are often two meters, three meters or even larger in diameter, and many of them are rectangular in cross section. Metal corrugated compensator for this occasion? Material and molding costs are ridiculously high, and metal bellows are sensitive to cross-sectional shape, making rectangular bellows extremely difficult to manufacture.

Nonmetallic compensators don't have this concern. Its ring belt is flat, and it is made into a rectangular cross section by the frame. This is how rectangular non-metallic expansion joints come from. Large diameter and low pressure just avoid the short board of non-metal pressure difference, and give full play to its advantages of large compensation and small thrust.

And guess what? A rectangular flue of DN4000, the heat displacement may reach fifty or sixty millimeters, and the non-metallic compensator can easily absorb it. What about the metal ones? Honestly calculate the fatigue life.

Use 2: How can fabric fibers and rubber materials hold high temperature and corrosive smoke environment?

Flue gases and corrosive gases are another typical type of operating conditions. The temperature of the flue gas at the inlet of the desulfurization tower may reach 150℃, as well as corrosive components such as SO₂, SO₃ and HCl, and dust washing in some places.

The band material of the non-metallic compensator is a composite structure such as glass fiber, polytetrafluoroethylene and silicone rubber, which can withstand the temperature above 200℃, and its corrosion resistance is much better than that of metal. Metal bellows are prone to stress corrosion cracking in chloride ion environment, but non-metallic bands become more and more solid.

In addition, the insulation layer of the non-metallic compensator can be designed as a multi-layer structure, which can not only compensate for displacement, but also reduce heat loss. This kind of structural design flexibility, metal compensator can't compare.

Application 3: Vibration and noise reduction of equipment interface, and "fault tolerance" function of installation deviation

The inlet and outlet of rotating equipment such as pumps, fans and compressors can't avoid vibration. If the pipes are connected rigidly, vibrations are transmitted directly to the piping system, and both noise and stress are large.

The fabric band of the non-metallic compensator itself has damping effect, which can absorb part of high-frequency vibration. A non-metal compensator is installed at the outlet of the fan, and the vibration transmission rate can be reduced by an order of magnitude. Rubber compensator and rubber PTFE compensator are the same way, which is suitable for vibration reduction of fluid media pipelines.

There is also a particularly practical use – fault tolerance. During on-site installation, the pipeline alignment deviation is several millimeters or even more than ten millimeters, which is normal. Hard connections can't be installed, so soft connections are much more convenient. The non-metallic compensator can absorb the installation deviation, which saves the user the time of field adjustment, which is considerable.

Application 4: Special applications in desulfurization, dust removal and other systems, used with baffle doors and other equipment

Flue gas baffle door, electric plug-in type insulation door, manual plug-in type insulation door of desulfurization system, these devices will have thermal displacement in the pipeline when switching and starting and stopping. The baffle door body is rigid and cannot absorb displacement, so it must be digested by the peripheral compensator.

Here non-metallic compensators are pretty much the only option. The temperature of desulfurization flue gas is not high but it is extremely corrosive, and the cross section of pipeline is often rectangular, so the space is limited. The non-metal compensator can be made into a flat shape, connected on both sides of the baffle door, and used in cooperation with the desulfurization flue gas baffle door, which just meets the technical requirements.

The same goes for dusting systems. The fabric ring belt compensator at the inlet and outlet of the dust collector can not only compensate the displacement, but also block the vibration transmission and protect the body of the dust collector.

Pits to be avoided in model selection: pressure, temperature, displacement, none of which can be taken for granted

Finally, talk about selection. What are non-metallic compensators most afraid of? Stress. Some people think that the fabric loop is soft anyway, so the pressure should not matter, right? Wrong. Under positive pressure, the ring belt will bulge, and the skeleton will be subjected to additional pressure load, which will tear if the design is not good.

Temperature is also a red line. Silicone rubber bands and fluorine rubber bands have different temperature resistance grades, and they will age and crack if they exceed the upper limit. The displacement is more accurate, and the deformation of the ring belt must be controlled within the safe range, otherwise creep fatigue will shorten the life.

The usefulness of non-metallic compensators?In the final analysis, it is born for specific working conditions such as large diameter, low pressure, high corrosion, large displacement and vibration reduction. When selecting the model, give the working condition parameters to the manufacturer and let the engineer help you check it-don't take it for granted.

I met a customer two days ago. The non-metallic expansion joint on the flue gas pipe was used for less than a year, and the skin leaked. When it was removed, the surface of the silicone cloth was cracked in a large area, and the reinforcing layer was exposed. After asking around, I realized that when I selected the model, I looked at the temperature resistance data and ignored the small amount of SO₂ in the medium. This thing turns into dilute sulfuric acid when it encounters condensed water, and ordinary silicone cloth can't hold it at all.

So,Material of non-metallic compensator skin?This problem is by no means just flipping through the samples and picking the one with the highest temperature resistance. If the skin is chosen wrong, it can be replaced in advance at least, and production can be stopped at worst. Today, break up these five questions and explain them clearly to help you avoid detours.

1. The skin is not a layer of cloth: the structure determines the lower limit of performance

Many people think that the skin is just a piece of cloth, and it is done by holding the medium. If it is so simple, why divide it into non-metallic expansion joints (fabric fiber expansion joints) and rectangular non-metallic expansion joints?

Temperature resistant layer, reinforcing layer, heat insulating layer, outer protective layer。 The temperature-resistant layer directly contacts the medium and is responsible for blocking high temperature and corrosion; The reinforcement layer is a skeleton, usually made of glass fiber or steel wire mesh, which bears the pressure tension; The heat insulation layer is made of ceramic fiber felt or the like to prevent heat from transmitting outside; The outer protective layer shields the whole structure from wind and rain and prevents mechanical damage.

Each of these four levels does its own work, and one is indispensable. The inner layer burns through, and the enhancement layer follows; The outer layer is damaged and watered, and the inside is no matter how good it is, it is useless. Therefore, when choosing skin, don't just stare at a certain layer, but look at the overall structural design and matching between layers.

2. Silicone cloth, fluororubber cloth and polytetrafluoroethylene film: how to compare the three mainstream materials

The most common skin surface materials in the market are these three kinds, each with its own temper.

Silicone clothTemperature resistance-60℃ to 250℃, good flexibility, cheap price, it is a cost-effective choice for general working conditions. However, it is afraid of strong acid and alkali, oil, especially corrosive flue gas containing sulfur. The client mentioned above is planted on this.

Fluorine rubber clothTemperature resistance-20℃ to 200℃, chemical corrosion resistance is one grade higher than silica gel cloth, and it has good resistance to acid, alkali and oil. The disadvantage is that it is easy to harden at low temperatures, has poor elasticity and is more expensive. It is much more reliable than silica gel cloth when used in desulfurization flue gas and chemical medium pipelines.

Polytetrafluoroethylene membrane (PTFE)It is almost corrosion-resistant ceiling, can carry almost all strong acids and alkalis, extremely low friction coefficient, and does not adhere to dust. However, the temperature resistance is generally about 180℃, the mechanical strength is poor, and it is easy to tear, so it is usually used on glass fiber cloth or fluororubber cloth, rather than as a skin alone.

Is there anything else? Yes. LikePTFE compensatorIt is all-PTFE structure itself, but that belongs to another class of products, so I won't go into it here. When choosing the skin of the non-metallic compensator, directly throw the working condition parameters to the manufacturers, and let them give the scheme according to the matching of these three types of materials, which is less trouble than yourself.

3. Select the skin according to the working conditions: high-temperature flue gas, corrosive medium and high-temperature dust are different

Not long-winded, just say the conclusion.

Flue gas pipeline at the tail of boiler and cement kiln in power stationThe temperature is often above 200℃, the dust is large, and there may be sulfur corrosion. In this working condition, the inner layer of the skin must be laminated with fluororubber cloth or PTFE glass fiber cloth, the reinforcing layer must be made of stainless steel wire mesh, and the heat insulation layer must be thickened to more than 80mm. Mentioned in the information on the siteCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAlthough the main body is a metal bellows, the matching non-metal skin is the same routine.

Desulfurization system, wet dust removalThis kind of corrosive medium environment, the temperature is not high but the pH is very fierce. PTFE or fluororubber cloth is the first choice for the skin layer, and attention should be paid to the problem of condensate accumulation-even if the medium temperature is low, the inner material must be able to withstand the long-term soaking of acid and alkali liquid.

Dry dust removal, pneumatic conveyingIn high-temperature dust conditions, wear is more deadly than corrosion. The inner layer of the skin should be made of fluororubber cloth with good wear resistance, and a layer of wear-resistant protective layer should be added between the heat insulation layer and the reinforcing layer. Otherwise, the dust will wear the skin out in a few months like sandpaper.

Selection is not single-point optimal, but system matching. You go to the manufacturer with the working condition and ask: Which skin structure do you recommend for this working condition? Those who know the business naturally know which plan to use.

4. Composite lamination process: why is the life of the same material several times worse

It is also silicone cloth + glass fiber cloth + ceramic fiber felt. The products of Factory A will be used for 5 years, and the products of Factory B will leak for 2 years. What's the difference? Craft.

Instead of layers of cloth simply sewn together, the skin is created byHigh temperature vulcanization, rolling composite, interlayer bondingWait for the process to be made into a whole. The bond strength between layers is not enough, and it will delaminate, bulge and crack after several times of thermal expansion and contraction. The vulcanization temperature and time control are not in place, and the temperature resistance of silicone cloth is directly discounted.

When you look at the skins that have problems, nine times out of ten, it is not the wrong material choice, but the rough composite process. The most typical is to use ordinary glue to stick between layers, which is not used. Genuine manufacturers will use special high-temperature adhesives to control the process parameters in the whole process of internal mixing, calendering and vulcanization, and each batch has a test record.

So don't just compare the thickness and weight of the sample, and ask one more question: What glue do you use for composite? Is there a temperature resistance aging test report? This is more real than anything else.

5. Installation and routine maintenance: Don't wait for a leak to regret it

No matter how good the skin is, it will be for nothing if it is not installed properly.

First, the pipe flange must be flat, and the bolts should be evenly tightened diagonally. If the local pressure is too tight, the skin will be pressed out of marks or even fractured. Second, do not stretch or twist the skin when it is installed. The reserved displacement is adjusted from the factory, so do not move on the spot. Third, sealant and gasket should be added to the place of riveting or bolting, which is the most easily overlooked leak point.

Routine maintenance is not so mysterious, just regular inspections. Check the skin surface every quarter for any cracks, aging, bulging, and dust accumulation that causes local overheating. Don't wait until the smoke comes out before shutting down the machine. That will cost you a lot.

To put it hard, in order to save money in some projects, the material of the skin of the non-metallic compensator is kept to the lowest level, and the installation and maintenance are not paid attention to. In the end, if something goes wrong, the product is blamed. This account is actually not cost-effective at all.

When choosing a skin, remember three sentences:Material matches working conditions, process determines life, installation and maintenance guarantee bottom line。 These three are achieved, and the non-metallic compensator can be used for more than ten years. If you can't do it, then wait for it to be changed every two years.

Find out what you want to measure first: 4 core sizes

Two days ago, I met a customer. On the phone, I said anxiously that the expansion joint was leaking and I needed to replace it with a new one. I asked him what the diameter of the pipe was, and he was stunned for a long time and said, "About one meter". Tsk, this "probably" is in trouble. How to measure metal expansion joints? It's really not a matter of just taking a tape measure and measuring it casually.

Before measuring, figure out what you are going to measure. There are only four core sizes:

• Diameter (DN)— — The inner diameter of the pipe, which is the first factor in the selection. The common ones are DN50, DN100, DN200, etc., according to the nominal diameter.
• Installation length— — The actual length between the end faces at both ends of the expansion joint. This directly determines whether you can fit it.
• displacement amount-Amount of axial compression, stretching, and lateral offset. This is the core parameter of the expansion joint, which will be described in detail later.
• Ripple parameters— — Wave number, wave height, wave pitch, inner and outer diameter of bellows. These determine the stiffness and compensation capacity of the expansion joint.

How to choose a measuring tool: Don't measure the inner diameter of the corrugation with a tape measure

Many on-site masters are used to walking the world with a tape measure. Tape measure the installation length is fine, but measure the corrugated inner diameter? The error can reach more than 5mm. The bellows is a thin-walled piece, and once the tape measure is pulled, the tension of the ruler itself can make the reading too large.

Install length and diameter, either with a tape measure or laser rangefinder; The corrugated inner diameter must be with vernier caliper or special inner diameter gauge; Vernier caliper for wave height and wave distance; If you need a precise corrugation profile, use a dedicated template or projector.

For 90% of on-site measurements, a vernier caliper plus a tape measure is enough. Don't be superstitious about laser rangefinders. The dust in the pipeline well is large and the light is poor, so the laser is not good to use. For what occasions do you use lasers? Large-diameter pipelines, above DN500, can't be reached by a tape measure, so it is most reliable to measure the installation length with a laser rangefinder.

Teach you step by step: standardized operation process

How to measure the metal expansion joint to be standard? Give you a set of operating procedures:

The first step is to measure the path.If the expansion joint is installed, measure the inside diameter of the flange or the inside diameter of the pipe directly. If it is removed, measure the inside diameter of the bellows. Note that measure in three directions and take the minimum value. Why? Bellows ellipticity is very common, and you can't fit it when you take the maximum value.

The second step is to measure the installation length.The distance between the end faces of the flanges at both ends is measured at 4 points evenly along the circumference, and the average value is taken. If the difference between the four points exceeds 5mm, it means that the expansion joint is crooked or the flange surface is uneven. At this time, it has to be corrected before measuring.

The third step is to measure the corrugation parameters.Use a vernier caliper to measure wave height (the vertical distance from crest to trough) and wave pitch (the distance between two adjacent crests). Count the wave numbers. There is a common mistake here-someone included the end edge wave, and the result was that the number of waves was 2 more, and the selection was directly wrong.

Step 4, record the nameplate information.If the nameplate is still there, write down the model, nominal diameter, design pressure, design temperature, and displacement on it. This information is much more accurate than you can measure yourself.

Common Errors Demonstration

Once I went to the scene, I saw a master put a tape measure on the ripples to measure the length. Ripples have peaks and troughs, and he measures the length of the arc between the peaks, which is several centimeters longer than the actual installation length. This kind of error is particularly hidden. After testing, I took it to correct the model. If I didn't match it, I thought it was the manufacturer who did it wrong.

The most difficult thing to measure is actually the displacement

The previous ones are easy to handle, but the real test of kung fu is the measurement of displacement. Why? Because the displacement is divided into cold state and hot state, the data of the two states are completely different.

How to measure cold pre-stretch?Before the expansion joint is installed, it is generally necessary to pre-stretch or pre-compress. Universal corrugated expansion joints and axial expansion joints usually leave the factory with transport rods to hold the bellows in a predetermined position. When installing, what you want to measure is the change in the amount of displacement from the natural state to the installed state. Specific method: After installation, adjust the tie rod nut, and measure the distance change of the flanges at both ends of the bellows with a vernier caliper. Stop when you pull to the design value, and the nut locks.

How to estimate hot operation data?This really can't be measured directly-the pipe is so hot that you can't touch it. How do you measure it? It can only be calculated. The thermal expansion coefficient of the pipe material is multiplied by the length of the pipe and multiplied by the temperature difference, and the theoretical thermal displacement is calculated. For example, a 20-meter-long carbon steel pipe, with a temperature difference of 200℃, has a thermal expansion of about 20×0.012×200=48 mm. This means that the expansion joint has to absorb at least 48mm of axial displacement.

What about that? The reliable way is to look at the running traces. After the expansion joint is installed, if there are strains or scratches on the bellows surface, or the tie rod bolts are bent, it means that the actual displacement exceeds the design value. Conversely, if there is dust accumulation on the bellows surface but not locally, the boundary of that clean area is often the actual displacement range. Earth methods are sometimes more accurate than instruments.

Don't hurry to place an order after testing: How to convert data into model

After measuring for a long time, the data is all written in the notebook, and then what? Just take the size and buy it? Wrong. The expansion joint is not a standard part, not that you can buy it in stock by quoting a DN200 with a length of 500mm. It is a non-standard part designed according to working conditions.

After you get the data, you have to do three things:

First, check the displacement amount.Multiply the measured displacement by the safety factor (usually 1.2~1.5), and report it to the manufacturer. For example, if you calculate that the thermal displacement of the pipeline is 48mm, then you have to report the compensation requirement of about 60mm. Why? The actual working condition is more complicated than the calculated working condition, with vibration, installation error and occasional working condition fluctuation.

Second, check the medium and temperature.The diameter and length are just the shell. What really determines the material and number of layers of the bellows is the medium, pressure and temperature. Stainless steel bellows for high-temperature steam pipelines, PTFE-lined hoses may be used for corrosion-resistant media, and flue gas pipelines should consider wear resistance, so guide tubes must be added. It is also the expansion joint of DN200, room temperature air and 500℃ flue gas, and the material and thickness of the corrugated pipe used are different from one thousand miles.

Third, choose the right structural form.General corrugated expansion joints are used for straight pipe sections, compound hinge transverse expansion joints are used for transverse displacement, and straight pipe pressure balanced expansion joints may be used for large displacement and high pressure. If the structure is chosen wrong, no matter how accurate the measurement is, it will be useless.

There is one of the worst things here-the expansion joint is installed cold during measurement, but you have to tell the manufacturer the displacement direction under hot working conditions when selecting the model. Is axial displacement compression or tension? Is there a lateral offset? If this information is not provided, the manufacturer can only do it according to the most unfavorable working conditions, and the products made are big and expensive, so you can't complain yet.

In the final analysis, how to measure the metal expansion joint is the measurement of the size and the understanding of the working conditions. Dimensional measurement is a basic skill, and what really opens the gap is whether the field data can be converted into accurate selection parameters. Only after measuring the right amount and asking about the working conditions can you buy the right expansion joint.