Specialized in manufacturing compensators, expansion joints, baffle doors

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.

Two days ago, a customer asked with the drawings, saying that he wanted to change the expansion joint, but he didn't know how to report the parameters. When he asked, he threw over a blurry picture of the scene. I've seen this way too many times. The expansion joint looked like a bellows, but if the parameters were not selected correctly, it would bury a mine if it was installed. Today, break it apart and crumble it into pieces to make it clear-what are the detailed parameters of the expansion joint? Remember the following 7 items, and you can also be a half-expert.

1. Pipeline diameter and connection method: first fix DN, everything else is easy to say

There's no room for negotiation on caliber. DN50 is DN50, DN2000 is DN2000, the wrong measurement is not a joke. Especially large-diameter thick-walled expansion joints, which can't be used when made, and scrap iron can't be sold at a high price.

The flange connection is convenient to disassemble and assemble, but there are more leakage points with more flange surfaces; Welding the connection once and for all, but it can be difficult to cut off the expansion joint during overhaul. Choose flange or welding, ask your on-site maintenance worker's opinion, it is more effective than flipping the design specification.

2. Compensation amount: axial, horizontal and angular, don't just stare at one number

When many people report the parameters, they say "compensation amount 50mm"-axial 50? Horizontal 50? How many degrees in the angular direction? Something completely different.

Axial displacement is the pipeline expanding and contracting along the axis direction, lateral displacement is the pipeline running sideways, and angular displacement is the pipeline turning at an angle. The universal corrugated expansion joint mainly absorbs axial displacement, while the compound hinge transverse expansion joint specializes in dealing with transverse displacement. If it is at the corner of the pipeline, it has to rely on the angular direction.

If you think about it, as soon as the main steam pipe heats up, displacements in three directions tend to exist simultaneously. Only staring at one number report, who will carry the remaining two directions?

3. Pressure and temperature: four numbers, none of which can be less

Design pressure, test pressure, operating temperature, instantaneous temperature-these four items are the lifeblood of expansion joints.

The design pressure is the maximum pressure that the bellows can bear. The test pressure is generally 1.5 times the design pressure, and each unit must be hit before leaving the factory. The working temperature determines what material the bellows uses, instantaneous temperature, how much the temperature can soar when you drive and stop in your factory, you have to tell the truth. Before, I met a cement factory customer, the working temperature was 200℃, and the instantaneous temperature rushed to 450℃. The direct material of 304 bellows was deteriorated, so it was better to change it to 316L. You said that this parameter is not complete, who dares to do it for you?

4. Corrugated material and number of layers: How to choose 304, 316L and Inconel

This piece of material, 304 is cheap, 316L is resistant to chloride ion corrosion, and Inconel alloy carries high temperature. For corrosive medium in desulfurization pipeline, it is correct to choose 316L; The gas turbine exhaust temperature is five to six hundred degrees, and Inconel can withstand it.

The number of layers is interesting. Under the same pressure capacity, multi-layer thin-walled bellows have longer fatigue life and better flexibility than single-layer thick-walled bellows. However, one more layer will make it more difficult to process, and the cost will rise. Many of the corrugated expansion joints used in our power station industry are multi-layered, but the cement industry is uncommon-the working conditions are different.

5. Stiffness and fatigue life: These two determine how long your pipeline can last

The rigidity of the bellows is too great, the thermal deformation of the pipe can't be absorbed, and the force is all transmitted to the support; The stiffness is too small, and it can't hold up the internal pressure. How do you give this number? Generally, it is calculated by the manufacturer according to the EJMA standard, and you will have the measured stiffness value on the parameter table.

There is a concept in the design of fatigue life called "equivalent cycle times". Designing 1000 times does not mean that it can be used 1000 times-if the actual displacement in each operation is smaller than the design value, it may be okay to use it tens of thousands of times; On the contrary, if it exceeds the design displacement, it may crack hundreds of times. This is not a math problem, it is an empirical account.

6. Guide tube, tie rod and pressure balance: accessory parameters really can't be saved

What does a deflector do? When there is high-speed airflow or particulate media in the tube, the corrugations will be directly washed away. A guide tube was added, and the wear and tear were all carried on it, and the bellows were clean.

The function of the tie rod and the nut is to limit the displacement of the bellows, which is used to adjust the pre-deformation during installation, which really needs to be adjusted. But note that the tie rod is not designed to absorb lateral displacement-that's what hinges and gimbals do.

There are also internal and external pressure balance expansion joints, such as straight pipe pressure balance type, whose core function is to eliminate blind plate force. If you choose an ordinary expansion joint, install it near the fixed bracket, and directly overturn the bracket during the pressure test of the pipeline, then you will really cry without tears.

7. Implementation standards and special working conditions: Act according to rules and do not step on red lines

For metal corrugated expansion joints, look at GB/T 12777 in China, and EJMA for design calculation. Non-metallic expansion joints follow JB/T 12235-2015, and there are rigid regulations on materials and test methods.

There are more rules for special working conditions. Flue gas baffle door and expansion joint in desulfurization system, corrosion resistance is the first priority; The expansion joint of the cement kiln tail, high temperature and wear resistance is the core; The vacuum double hinge expansion joint used in air-cooled pipelines of power stations should also consider the vacuum degree. Like a mountain, professional manufacturers are used to which industry, you better find deep cultivation in this field.

After so much verbose, the core is one sentence: What are the detailed parameters of the expansion joint? Pipe diameter, compensation amount, pressure and temperature, number of material layers, stiffness life, accessories, standards-seven things, each falling on the paper, the manufacturer can make something reliable for you. When you select the model next time, sort out these seven parameters before calling, and the efficiency will at least double. "

After many people get the expansion joint, the first thing they ask the manufacturer is: Where does this thing fit? Honestly, this question hit the nail on the head. Installing the expansion joint in the wrong position is more fatal than choosing the wrong model. If the selection is wrong, the service life will be shorter at most, and the position will be wrong. The bellows will be stretched, twisted and unstable, ranging from abnormal noise in the pipeline to tearing the bellows and leaking the medium. We have done pipeline compensation schemes all the year round, and have seen too many such cases. Today, we will break up the location and make it clear.

Which section of the pipeline should the expansion joint be installed: first distinguish the fixing bracket and the guide bracket

To find out where the expansion joint is installed, two roles must be recognized first: the fixing bracket and the guide bracket. The fixing bracket is the anchoring point of the pipe, it locks the pipe firmly and does not allow displacement in any direction. The guide bracket only limits lateral displacement and allows axial sliding. The mission of the expansion joint is to absorb the displacement caused by thermal expansion and contraction between these two types of brackets.

The expansion joint must be located between the two fixed brackets and mounted close to one of the fixed brackets. Leave enough straight pipe segments on the other side to cooperate with the guide bracket to allow the pipe to expand and contract in a predetermined direction. In other words, the expansion joint is not installed wherever you want, it is a link in the pipe support system. You set the fixed bracket first, and the position of the expansion joint will be set by 70% to 80%.

Is the expansion joint close to the fixed bracket or the device end? Force differences between two typical layouts

Pipe layouts typically come in two typical forms. One is that the expansion joint is close to the fixed bracket, which is called "single axial layout". At this time, the expansion joint bears pure axial compression or stretching, and the bellows has the most uniform force and the longest service life. LikeHigh temperature axial expansion jointUnder this layout, the maximum compensation ability can be exerted.

The other is that the expansion joint is close to the equipment end, such as connecting sensitive equipment such as steam turbines, pumps and fans. In this case, the expansion joint has to not only absorb the thermal displacement of the pipe, but also bear the additional displacement caused by the thermal expansion of the equipment. The stress state is much more complex, and sometimes it is necessary to useCurved tube pressure balance expansion jointOrCompound hinge transverse expansion jointTo defuse. Which one you choose depends on whether the equipment can withstand pipe thrust.

Selection of connection position under different working conditions: How to determine high-temperature steam, large-diameter flue gas and corrosive medium

Different working conditions have different ideas for location selection.

High-temperature steam pipelines, such as the main steam pipeline of power stations, have a temperature above 400℃. At this time, the expansion joint should be as close to the fixed bracket as possible, and enough guide brackets should be set in the middle straight pipe section. If the straight pipe section is too long, the bellows will be unstable, and if the straight pipe section is too short, the fixed bracket will bear excessive blind plate force.Corrugated expansion joint for power station industryThe installation spacing is generally controlled at 20~40 meters for a compensation unit, depending on the pipe diameter and medium temperature.

Large-diameter flue gas pipelines, like the inlet and outlet flues of desulfurization towers, have low pressure but large cross-section, and are usually rectangular structures. At this time, where to install the expansion joint depends more on the drop point of the flue support and the arrangement of the baffle door. Generally, it is recommended to set a fixed point on both sides of the flue, and the expansion joint is placed in the middle section, so that the displacement in both directions is concentrated in the middle.

What about corrosive media pipes? The position of the expansion joint must avoid dead angles and fluid accumulation sections. When the medium flow rate is low, corrosive liquid is easy to accumulate at the bottom of the bellows, so in addition to selecting the correct position, it is also necessary to selectRectangular non-metallic expansion jointOr a structure with a guide tube to ensure smooth passage of the medium without staying in the bellows area.

How the expansion joint is connected to the pipe: welded, flanged or sleeved? Position Determines Joint Form

The connection method is not randomly determined, but is directly related to the spatial conditions of the installation position.

The welding connection is the most reliable and suitable for high temperature and high pressure pipelines, but it requires welding conditions and flaw detection means at the site. The flange connection is convenient for loading and unloading, and is suitable for pipelines that need regular maintenance and cleaning, such as desulfurization flue and sewage treatment pipe. The sleeve connection is not welded, without flange, and is sealed by packing. It can be selected in compact spaceSleeve type pipe expansion joint。

If the expansion joint is installed in a narrow pipe gallery with limited access space, preference should be given to flange or sleeve connections. If the position is at the end of the overhead pipeline, which is convenient for hot work, welding is a better choice.

Common installation error: What happens if the expansion joint is installed near the elbow and the distance between the two expansion joints is insufficient

Install the expansion joint near the elbow. The elbow is where the pipe produces lateral displacement and bending moment, where the expansion joint will bear additional lateral forces and bending stresses, and the bellows will soon fatigue crack. The correct way is to set fixed brackets at both ends of the elbow, and the expansion joint is placed on the straight pipe section away from the elbow.

The two expansion joints are too close to each other. Some designers put two expansion joints next to each other in order to save space in the pipeline. This will lead to the middle pipe section becoming a floating unit, the two expansion joints push each other, the displacement distribution is uneven, one is overloaded and the other is idle. The specification requires that the length of the straight pipe section between the two expansion joints, usually not less than four times its nominal diameter, and that an intermediate fixing bracket be provided to distribute the displacement.

Practical recommendations for type selection and positioning: Final installation point determined by combining the design of fixed bracket and manufacturer's data

The positioning of this matter, in the final analysis, depends on the layout plan of the fixed bracket. When making compensation design, first clear the direction of the pipeline, set the positions of all fixed brackets, and then calculate the compensation amount needed for each compensation section according to the thermal expansion amount and allowable span. Then compare the displacement parameters provided by the manufacturer to determine the specific installation point of the expansion joint.

Are temporary supports installed during the pipeline pressure test? Is the amount of pre-stretch adjusted to ambient temperature? Is there a protective sleeve on the outside of the bellows? These details can affect the quality of the installation. As a reminder,Direct buried (fully buried) type expansion jointThe installation method is completely different from the overhead pipeline and cannot be mixed.

Which position the expansion joint is connected to the pipeline is not worked out by the construction team on the spot, but it has to be calculated clearly in the design stage. If the location is selected correctly, there will be no problems with the pipeline for twenty years; Wrong choice, rework is a small matter, safety accidents are a big deal. It is recommended to send the pipe drawing to the manufacturer before purchasing, and let the technicians help you review the positioning. Many problems can be avoided in the drawing stage.

Bellows can absorb thermal expansion and contraction, but there is a premise. It does not rely on the material deformation to carry hard, but relies on the corrugated structure to transform the axial displacement, transverse displacement and angular displacement of the pipeline into its own elastic deformation. Note that the bellows mentioned here is not the kind of gas hose, but the corrugated expansion joint commonly used in industrial pipelines, also called corrugated compensator. You asked, "Can the bellows play the role of thermal expansion and contraction?" When it comes time to select the type, the expression of this sentence has to be changed. The accurate statement is that "the bellows can compensate for the displacement caused by thermal expansion and contraction". One word difference, the logic is completely different.

Why can bellows "telescope"? The key is the corrugated design

A corrugation is equivalent to a spring, and multiple corrugations are connected in series, and the overall stiffness is reduced. When the pipe expands under heat, the corrugations are compressed; When the pipe cools and shrinks, the corrugations are stretched. The essence of this process is elastic deformation, so repeated displacement does not destroy the corrugation itself-provided the amount of displacement is controlled within the design range. There is a key parameter here called bellows stiffness. The lower the stiffness, the smaller the reaction force generated when compensating for displacement, and the smaller the thrust on the pipe support. But this does not mean that the lower the stiffness, the better. Too low the stiffness will cause the bellows to become unstable, so it is done when designing.

It is not enough to have ripples alone, and the structure should be selected according to the working conditions

Similarly, the bellows used in different pipes vary greatly. For high-temperature steam pipes and flue gas pipes, the selection is completely different. For example, when corrugated expansion joints are used in power station industry, the basic problem of "whether they can absorb displacement" is not considered, but the comprehensive performance under high temperature, high pressure and large displacement; The metal corrugated expansion joint in cement industry should be wear-resistant and corrosion-resistant, and other materials can't hold it in the dust environment. If the medium is strongly corrosive, it must be lined with PTFE hose or PTFE compensator. Ordinary stainless steel bellows may not last a maintenance cycle under this working condition.

When selecting the model, we can't only look at "can absorb displacement", but also look at fatigue life

The bellows absorbs thermal expansion and contraction repeatedly, and each expansion and contraction produces stress concentration at the trough. The design must be calculated according to the actual number of cycles, and it is not just a general-purpose corrugated expansion joint that can be used. For example, directly buried (fully buried) expansion joint and external pressure single axial expansion joint are optimized structures for specific working conditions, and their fatigue life is not the same order of magnitude as that of ordinary type. The bellows of directly buried pipelines are buried in the soil and cannot be repaired, so the structure must be reliable; External pressure type corrugated pipe bears external pressure, has better stability, and is suitable for high-pressure pipeline. If you choose the wrong structure, the displacement will be absorbed, and the life may only be one tenth of the design value.

Another category is easily overlooked: non-metallic expansion joints

Many people think that only metal bellows can compensate for thermal displacement. In fact, fabric fiber expansion joints and rubber compensators are more flexible in low-pressure and large displacement scenarios. In particular, the rectangular non-metallic expansion joint, which is used on the flue gas pipeline, can not only absorb thermal expansion and contraction, but also isolate vibration and noise, and be corrosion resistant. Why? Because the elastic modulus of non-metallic materials is low, the reaction force generated under the same displacement is small, and it can be made into a rectangular cross section to match the shape of the flue. So back to the question at the beginning, "Can bellows do the role of thermal expansion and contraction?" — the phrase itself implies a presupposition, as if only bellows can do the job. When it comes to the selection stage, you still have to know that in addition to bellows, there are non-corrugated structures such as sleeve pipe expansion joints and rotary compensators that can also compensate for displacement, but the applicable scenarios are different.

Final reminder: Installation and constraints determine whether bellows can function properly

Tie rods, hinges, duplex structures, these are not decorations. For example, the transverse expansion joint of the compound hinge can only absorb the transverse displacement. If you take it hard to carry the axial displacement, the bellows will be scrapped soon. For example, the expansion joint of the large tie rod, whether the screw should be disassembled during installation, and how to adjust the nut, all of which directly affect the compensation effect. Choose the right type, install it in the wrong position, and still have problems. Alas, two days ago, I met a customer who used the double hinge transverse expansion joint as a general-purpose type. As a result, the bellows bulged directly when the pipeline was tested for pressure. Therefore, if the working condition analysis is not in place, it will be useless to change any compensator.

Can bellows play the role of thermal expansion and contraction? The answer is simple: yes, but it also depends on how you choose and install it. Understand this layer, and you won't be led by the parameter table when selecting a model.

Find out first: What is the fatigue life of expansion joints?

The fatigue life of the expansion joint, to put it bluntly, is how many times the bellows can carry the expansion and contraction. With each expansion and contraction, an alternating stress cycle will occur at the trough and peak of the corrugation-the material is constantly pulled over and pressed over, and when it accumulates to a certain extent, cracks appear.

Fatigue life is never determined unilaterally by material grade. It is common for the same SUS304 to have different corrugation geometries and a difference in life by two or three times. Wave height, wave pitch, wall thickness, number of layers, every parameter changes the stress distribution. The displacement form is also a variable-pure axial tension and compression, which is very different from the transverse shear superimposed on the axial direction, and the calculated stress is very different. What about pressure fluctuations? Every time the pressure in the tube fluctuates, there is an additional layer of circumferential stress cycle on the corrugated wall. So you see, although the general-purpose corrugated expansion joint and the external pressure single axial expansion joint are both called expansion joints, one corrugation is swollen by internal pressure and the other corrugation is compressed by external pressure. The stress state is completely different, and the path of calculating fatigue life is naturally different.

What is the reasonable safety factor?

The recommended practice of GB/T 12777 and EJMA is essentially to push down the theoretical fatigue life with a large multiple of the safety factor. The design fatigue curve given by EJMA itself considers about 10 times the safety factor, and GB/T 12777 also follows this set of logic. Why 10x? Because the fatigue test data of corrugated pipe is very dispersed, the life of the same design and the same batch of materials may jump from 800 to 3,000 times after ten tests. The safety factor is used to cover this spread.

However, the safety factor cannot be one-size-fits-all in different working conditions. After the directly buried (fully buried) expansion joint is buried, it is impossible to repair and replace the pipe at all. The failure is an accident, and it is reasonable for the safety factor to be 15 or even 20. Once the high-temperature axial expansion joint exceeds the creep temperature range of the material-for example, 304 stainless steel is above 425℃-the failure mode changes from pure fatigue to creep-fatigue interaction. At this time, it is gambling on your life to take the safety factor of 10 times of normal temperature.

How to take the number of fatigue life? Conversion from design displacement to actual working condition

Many people step on pits at this step. The "fatigue life 1000 times" written on the sample or drawing is calculated based on a specific cyclic displacement. For example: the design displacement is ±20mm in the axial direction, and the life span is 1000 times, which means that it can run 1000 times under ±20mm pure axial displacement. But in the actual pipeline system, where does pure axial direction come from? Thermal expansion and contraction drive the pipe system to swing, and the axial displacement always comes with the lateral displacement.

The transverse displacement is converted into the equivalent axial displacement according to the geometric characteristics of the bellows, and the equivalent axial displacement is obtained after the two are superimposed, and then substituted into the fatigue life curve to check the number of times. Note that the relationship between fatigue life and displacement is not linear-a 10% magnification of displacement may result in a 30% drop in life. This is why the full sample life cannot be used. The corrugated expansion joint in power station industry has to be discounted by 30% or 20%. Because the transverse expansion joint of compound hinge bears the combination of angular displacement, the conversion is more complicated, and the discount range is often larger.

The most overlooked pits in engineering: temperature correction and corrosion margin

If you take the fatigue curve at room temperature to set the flue gas pipe at 400℃, it is basically a gamble with your life. As soon as the material is at high temperature, the yield strength drops, the creep begins to accelerate, the plastic deformation produced by the bellows every cycle is greater, and the crack initiation is much faster than the room temperature.

A general-purpose corrugated expansion joint used in a cement plant has a design temperature of 350℃. The manufacturer has checked the life of 2000 times according to the normal temperature curve. As a result, the bellows cracked in less than 500 cycles after the actual operation reached 420℃. Some people also blame the manufacturer's poor quality-in fact, the problem lies in the user's negation of the temperature correction coefficient. Corrode the environment harder. The flue gas baffle door and the desulfurization flue gas baffle door are sulfur-containing flue gas, and chloride ions and sulfite make pitting pits on the corrugated surface, which are the natural starting points of fatigue cracks. Corrosion and fatigue are superimposed, not 1+1=2, but 1+1=5. In this environment, the safety factor is at least doubled, and with measures such as the heat insulation of the guide tube and the external heat insulation of the corrugated pipe, it can be played.

What is the appropriate number of fatigue life times?

Landing recommendations are in two gears. For conventional pipeline systems, such as thermal pipelines and general chemical pipelines, 70% ~80% of the design life is taken as the allowable cycle number. The sample is marked 1000 times, and you will set the maintenance and replacement cycle according to 700~800 times, leaving a safety margin to cope with the fluctuation of working conditions.

In critical situations or parts where maintenance is difficult, the control line should be pressed lower. The double hinge expansion joint of air-cooled island vacuum pipeline is on a pipe frame tens of meters high, and it is necessary to build a full hall of scaffolding to change it; Once the straight pipe pressure balance expansion joint fails, the blind plate force of the whole pipe system loses balance, and the consequence is chain. Such products are controlled at 50 percent or less of the design life-and replacement plans are required for less than half the life. The reason is simple: the fatigue life itself is a statistical value. Some people are still running when they use 120%, while others leak 60%. You lower the line of control, not conservatively, but to leave uncertainty alive.

How to communicate fatigue life requirements with manufacturers during model selection?

Don't just drop "Give me something that lives longer" and be done. The design engineer of the manufacturer is not a fortune teller. You have to give him at least these things: medium temperature, cyclic displacement (how many millimeters in the axial direction and how many millimeters in the transverse direction, if the measurement is not accurate, give the stress analysis report of the pipe system), cycle frequency (how many times a day to start and stop, how many thermal cycles a year), whether there is pressure fluctuation, and the type and concentration of corrosive medium. In this corrosive environment, sulfide or chloride content should be declared separately.

Only when the parameters are in place can the manufacturer select the safety factor to a reasonable gear, and then deduce the fatigue life times. If you don't say anything, the other party can only match you with a universal corrugated expansion joint according to the general working conditions of "normal temperature, pure axial direction and no corrosion". It looks quite cheap when you get it, and it will be revealed after running two laps on the scene. How to take the safety factor of fatigue life of expansion joint and the number of times of fatigue life of expansion joint? The answer is never in the sample, but in your own working condition data.