Specialized in manufacturing compensators, expansion joints, baffle doors

Find out one thing first: what will happen if the number of ripples is more or less?

A few days ago, a customer who makes steam pipes asked me, saying that the steam pipes in their factoryUniversal corrugated expansion jointIt leaked after less than half a year of use. When I removed it, I found several cuts at the root of the corrugation. I asked him how to select the type at that time, and he said, "I just matched a regular one according to the diameter of the pipe, and I didn't care too much about the number of corrugations". Tsk, this is the typical problem-the number of ripples is not determined by patting the head, and if there is more or less, something will happen.

Let's put it this way, when the number of corrugations is small, the displacement that a single corrugation has to bear will be large, the stress will be concentrated, and the fatigue life will fall by a cliff. Especially in high temperature and high pressure conditions, such as those used in power stationsHigh temperature axial expansion joint, the number of ripples is not enough, and it will crack in a few months. Conversely, what about more ripples? You think more is safer? Wrong. If there are more ripples, the overall stiffness will decrease, the pressure bearing capacity will follow, and instability will easily occur-if the ripples collapse like noodles, it will be completely wasted.

Therefore, the question of "calculating the number of compensator ripples?" is not a simple addition and subtraction, but to find a balance between the compensation amount, pressure, temperature and fatigue life. To put it bluntly,Number of corrugationsIt's a lever: it can't bear pressure a little to the left, and it can't live a little to the right.

What does the calculation formula look like? These are the core variables. Don't be scared by the formula

When it comes to formulas, many people have big heads. But don't be afraid,Compensator Ripple Number CalculationThe core logic is actually four variables: single-wave compensation amount, total compensation amount, pressure conversion coefficient and temperature correction coefficient. The formula looks like this:

Required number of corrugations = total compensation ÷ (single wave compensation × pressure coefficient × temperature coefficient)

Simple right? However, in practical applications, the "pressure coefficient" and "temperature coefficient" are the most easily overlooked.

Take the pressure coefficient, aExternal pressure single axial expansion jointThe design pressure is 1.0MPa, and the actual operating pressure is only 0.6MPa. If you calculate according to 1.0MPa, the number of corrugations will be too large, which will increase the cost in vain and reduce the stability. Conversely, if you useDirect buried (fully buried) type expansion jointOn, the pressure of underground pipelines fluctuates greatly. If you calculate according to the average value, then wait for an accident.

The temperature coefficient is more pit. Many customers take the single wave compensation amount at room temperature to set the 300℃ steam pipeline, which is definitely not allowed. At high temperature, the elastic modulus of the material decreases, and the single wave compensation ability will be discounted. Such asMetal Corrugated Expansion Joints in Cement IndustryThe flue gas temperature is often five or six hundred degrees, so it is necessary to use heat-resistant alloy and reduce the temperature, otherwise the calculated ripple number is a paper tiger.

Under different working conditions, the number of ripples must be adjusted-give you a few real scenarios

Desulfurization flue gas baffle doorMatching corrugated expansion joint. Desulfurization flue gas has high humidity and low temperature (generally about 70℃), but contains a large amount of acidic condensate. The number of corrugations should not be too small because additional axial displacement due to fouling is to be absorbed? No, the key is to ensure that the spacing between the corrugations is large enough to avoid corrosion by fluid accumulation. In this case,Compensator Ripple Number CalculationAdd an additional corrosion margin-it is usually recommended to reserve 15% ~20% more corrugation than pure displacement calculation, and choosePTFE-lined hoseOrPTFE compensatorTo resist corrosion.

Double hinge expansion joint for air-cooled island vacuum pipeline。 The air-cooled island pipeline has a large diameter and a negative pressure operation. If the number of corrugations is too large and the stiffness is too low, the vacuum degree will deflate the corrugations. At this time, it is necessary to reduce the number of ripples or increase the wave distance. I have seen a project that used a 30-wave expansion joint, and as a result, once the vacuum was pumped, the troughs stuck directly together. Later, it was changed to a design with 18 waves and increased wave thickness, and it was done.

Corrugated expansion joint for power station industry, the main steam pipe. Operating temperature 540 ℃, pressure 10 MPa. Under this condition, the single-wave compensation amount is only about one third of that at normal temperature. The creep fatigue interaction must also be considered in the selection. The number of ripples is generally calculated by the manufacturer with finite element, so don't pat your head yourself. Before, there was a power plant diagram to save money, and I calculated it by myself according to a simple formula. As a result, the number of ripples was reduced by 30%, and the pipe burst in two months.

What do these examples illustrate? Formulas are dead, working conditions are alive. The same oneCompound hinge transverse expansion jointUsed in thermal pipe gallery and used in chemical pipeline, the number of corrugations may be twice as different.

Selection and Pit Avoidance Guide: Half of these mistakes have been stepped on by customers I have seen

Just look at the total displacement, not the displacement direction. Many people take the total thermal elongation of the pipe and substitute it into the formula, butCompensator Ripple Number CalculationWhen, the single-wave compensation amount is divided into axial, transverse and angular directions. You use an axial expansion joint to absorb the lateral displacement. No matter how many corrugations are, it will quickly distort and break.

Superstition that "the more ripples, the longer the life". Wrong! An increase in the number of corrugations reduces the displacement that a single wave is subjected to, but at the same time reduces the overall stiffness and stability. ForStraight pipe pressure balanced expansion jointThis kind of structure needs to counteract the pressure thrust, the number of corrugations must be strictly controlled. More, the pressure thrust can't be balanced; Less, main bellows overloaded.

Ignore installing pre-stretch/pre-compression. For example, a steam pipe is installed at 20°C and operates at 280°C. If it is not pre-stretched in the cold state, the expansion joint will withstand additional stretching in the hot state. You calculated the number of ripples according to the actual displacement, without considering the amount of pre-stretching, and the result was that the number of ripples was small. The correct way is to deduct the displacement generated by pre-stretching in reverse when calculating the total compensation amount. For example, if pre-stretching is 50%, the total compensation amount will be calculated as 50% of the actual displacement. Can the number of corrugations be halved? No, it can't be halved, because pre-stretching only changes the zero position, and the fatigue life still has to be calculated according to the total displacement-it is easy to be swirled here, so it is recommended to find the manufacturer to confirm.

Mixed formula for non-metallic expansion joints and metallic expansion joints.Non-metallic expansion joint (fabric fiber expansion joint)Andrubber compensatorThe logic of calculating the number of corrugations (actually bands) is completely different, because they depend on the elastic deformation of fabric or rubber, rather than the plastic deformation of metal. You take the formula of metalRubber PTFE compensatorThe result must be wrong.

UnderstandCompensator ripple number calculation?Actually, it's not that mysterious, but you can't be too confident. In case of complicated working conditions (high temperature, high pressure, corrosion, large diameter), it is much more reliable to contact the manufacturer directly to come up with a plan than to calculate it yourself for half a day. We have a library of cases of various working conditions, fromFlue gas baffle doorToSleeve type pipe expansion joint, can be referred to.

Why is there no uniform replacement cycle for compensators? — — The medium, temperature, pressure, working condition and material are all indispensable

After more than ten years of pipeline maintenance, someone always asks me, "Master, how many years does the compensator change? Is there a standard answer?" I usually reply directly: If you have to listen to the standard answer, it means that there is no answer. Why? Because the compensator is too different from the material to the working condition. You take a rubber compensator to fight high-temperature steam, and it will become slag in less than three months; However, if the metal corrugated expansion joint is placed in a room-temperature clean water pipe, it is common to last for ten or eight years.

Media is the number one killer. Corrosive media (such as desulfurization flue gas and acid liquid) will accelerate the pitting of metal bellows, and the rubber compensator will swell and crack when it comes into contact with oil solvents. The temperature is more direct: if it exceeds the design temperature by 50℃, the fatigue life of the metal corrugated expansion joint may be cut in half at 10℃ per liter. Conditions with frequent pressure fluctuations (such as the pump outlet) are more likely to cause stress concentration in the bellows than stable pressure conditions. And the material-the same metal corrugated expansion joint, the resistance to chloride ion corrosion of 304 and 316L is an order of magnitude worse. Therefore, don't expect manufacturers to give you a "shelf life", which is all fooling laymen.

Typical Replacement Years for Metallic Corrugated Expansion Joints, Non-Metallic Expansion Joints, Rubber Compensators, PTFE Compensators

Although there is no unified standard, the experience data of veterans can still be used as a reference:

• Metal corrugated expansion joint(including corrugated expansion joints for power station industry, metal corrugated expansion joints for cement industry, etc.): Under normal working conditions, the design fatigue life is usually 1000~3000 cycles. If it is converted according to one-year start-stop + thermal expansion, it can generally last for 5~8 years. However, in the high-dust and high-temperature environment like the cement industry, corrugated wear or intergranular corrosion begins to appear in three years, so you have to consider changing it.
• Non-metallic expansion joints (fabric fiber expansion joints): Mainly used in smoke air duct, temperature resistant but not pressure resistant. The typical life span is 2~4 years. If the flue gas has a high sulfur content or has local overheating, it is not unusual to burn through in one year.
• rubber compensator: Afraid of aging. The indoor room temperature water supply and drainage system can last 5~6 years. If it is exposed to the sun or exposed to chemicals outdoors, the life span will be directly halved to 2~3 years.
• PTFE compensator: The strongest corrosion resistance, but afraid of high temperature and high pressure. Generally used for strong acid pipelines, the design life is 3~5 years. In fact, because of the creep characteristics of PTFE, it should be closely observed after more than one year.

These years are only reference values, and the details depend on the scene. And guess what? Two days ago, a power plant customer told me that the corrugated expansion joint used in their power station industry has not leaked for 8 years. I said that you are lucky, so hurry up and do a comprehensive flaw detection. The fatigue crack of the corrugated pipe is most likely to suddenly break out in the last year.

Don't wait for the tube to burst before replacing it: What signs in daily inspections indicate that the compensator should be retired?

Many maintenance brothers only stare at whether there is a leak, and only change it if it leaks. That's too late. If one of the following signs occurs, it should be included in the replacement plan:

First, there are significant pits, scratches, or local bulging on the bellows surface.This tends to be a precursor to stress concentration or corrosion penetration. Second, the surface of the rubber compensator is cracked, stiff or partially sticky-indicating that the rubber molecular chain is broken and may explode at any time. Third, the fabric layer of non-metallic expansion joints shows loose fibers, mold, or local ablation, especially black carbonation marks at the flange joints. Fourth, the metal corrugated expansion joint has abnormal sound (clicking sound) during operation, which may be friction between corrugations or the lining cylinder falling off. Fifth, the nut of the tie rod or the expansion joint of the large tie rod is loose and the tie rod is bent-this shows that the pre-stretching is not adjusted properly during installation, resulting in uneven force.

If you areDirect buried (fully buried) type expansion jointIf you can't directly see it, you have to monitor the displacement of the pipeline regularly. If the displacement suddenly becomes large or the return is abnormal, 80% of the bellows is unstable, so dig it out and replace it quickly.

Practical case: Corrugated expansion joint used in power station industry vs metal corrugated expansion joint in cement industry, the replacement cycle is twice the difference?

Tell me the truth. The steam extraction pipeline of a thermal power plant is oursCorrugated expansion joint for power station industry, material 316L, design temperature 350℃, pressure 1.0MPa. Operating records show no more than 20 starts and stops per year and the medium is superheated steam. This set has been used for 7 years. In that year, it was overhauled and dismantled for penetration testing. Only some corrugated roots had slight fatigue cracks. After evaluation, it was decided to use it in the 9th year before replacing it.

On the other hand, a cement factory in a neighboring province used the kiln head pipelineMetal Corrugated Expansion Joints in Cement IndustryThe design temperature is 400℃, but the actual working conditions often exceed 450℃, and the dust concentration is as high as 50g/m³. And the result? In the third year, it was found that the outer wall of the bellows was seriously worn, and the thinnest part was only 1/3 of the original thickness, so it had to be replaced urgently. They are also metal corrugated expansion joints. The power station one can carry 9 years, and the cement one will be wasted in 3 years-more than double the difference. Core reason: medium dust wear + overtemperature. So, don't just look at the name of the product, you have to look at the actual working conditions.

Three major pits in replacing compensator: incorrect matching, reverse installation direction, and failure to adjust the tie rod nut

Changing the compensator looks simple, but there are quite a few people who step on the pit. The first pit: mismatching. Some people used the universal corrugated expansion joint to replace the high-temperature axial expansion joint. As a result, the corrugated pipe was torn by temperature stress in a few days. Different compensators have different compensation directions (axial, transverse and angular), which must be selected according to design. The second pit: the installation direction is reversed. The expansion joint cylinder is generally marked with arrows pointing to the flow direction of the medium. Some people try to save trouble and install it backwards, and the guide tube inside the bellows becomes a throttle, which aggravates the vibration. The third pit: the tie rod nut is not adjusted properly. Large tie rod expansion joint or double hinge transverse expansion joint, the tie rod nut should be pre-tightened and displaced according to the instructions during installation, otherwise the tie rod will be unevenly stressed after starting, which will directly lead to the deflection and instability of the bellows.

How exactly to adjust? You can turn through "How to Adjust the Tie Rod Nut of Expansion Joint" and "Correct Installation Method of Large Tie Rod Expansion Joint" we wrote before, which are quite detailed. All in all, check whether the nameplate parameters are consistent with the scene before changing. Don't reverse when installing it, and don't twist it randomly when adjusting it.

Instead of struggling with cycles, build a preventive maintenance plan-with a simple ledger template

Stop staring at the problem of "change in a few years". Instead of passively waiting, it is better to take the initiative to set up a ledger. I suggest every plant has at least monthly inspections, quarterly cosmetic inspections, and annual non-destructive testing (penetration or magnetic particle testing) during overhaul. The ledger template is simple: equipment number, installation location, model (such as metal corrugated expansion joint/rubber compensator/non-metal expansion joint), installation date, last replacement date, inspection status (normal/abnormal), and remarks. The actual service life curve can be deduced by recording the abnormalities found in each inspection.

You giveDesulfurization flue gas baffle doorNext to the compensator to make a ledger, three months found that the rubber compensator appears micro-crack, then next time to replace it will be preferredRubber PTFE compensatorOrPTFE-lined hoseAnd has stronger corrosion resistance. This is the real way to solve the problem.

At the end of the day, compensator replacement cycle? There is no standard answer, but there is a scientific approach to management. Don't believe the nonsense of "not changing for ten years", and don't wait for it to blow up before regretting it. Make the inspection detailed and the ledger solid, and the compensator will naturally tell you when it is time to change.

What to do if the bellows breaks? Close the valve first and then check. Don't rush to replace it with a new one

Two days ago, I met a customer. The corrugated expansion joint of the steam pipe was broken, and the steam on the spot was like a sauna. What was his first reaction? Take a wrench to screw the flange bolt and try to crush the leak to death. And the result? As soon as the bolt was tightened, the crack was directly torn open, and the whole section of the pipe almost collapsed. Alas, we have seen too many of these things.

To be honest, 90% of people did the first step wrong when it came to bellows rupture. Either scrambling to block, or rushing to call for a new one. Today, let's make it clear how to rectify this situation.

First, don't panic when you find a leak-90% of people make the first step wrong

As soon as the leak comes out, people instinctively panic. But you have to understand: Bellows rupture is usually a local failure and will not blow up instantly. The right first step is —Valve closing, pressure relief, isolation。 Whether it is a gas bellows or a metal hose for steam, first close the upstream and downstream valves, discharge the medium, and let the system pressure return to zero. Don't screw bolts or touch damaged parts under pressure. The pressure will tear big cracks and even hurt people.

After closing the valve, don't rush to dismantle it. Ask yourself three questions: What media is leaking? What's the temperature? How high is the pressure? Gas leakage should be prohibited from any fire source, high-temperature steam must wait for cooling, and corrosive media must be protected. If you want to use this information later, write it down first.

2. Investigation of the cause of rupture: fatigue, corrosion, or forgot to remove the tie rod during installation?

When many people see that the bellows is broken, they directly decide that it is a quality problem. Tsk, not necessarily. I've seen more than half of the early ruptures, which are good things done when installed-such asExpansion joint tie rod nut not adjusted in place, or more commonly: the tie rod is not removed at all.

You asked what will happen if the tie rod is not removed? The expansion joint should have absorbed the displacement, but it was locked by the tie rod. The thermal expansion force of the pipe was fully pressed on the bellows, so it was strange that it didn't crack. So troubleshooting step one: check the tie rod and screw. Someone asked in the question and answer of this site, "Does the screw of the expansion joint need to be disassembled?" The standard answer is: the transport tie rod of the axial expansion joint must be disassembled or loosened after installation, unless it is a tie rod with limit function. How many scenes have you seen where the pipes are connected to SAIC and the tie rods are still screwed?

In addition to installation issues, there are several reasons for the crack:

• Fatigue failure— — The pipeline starts and stops frequently or vibrates, the bellows expands and contracts repeatedly, and fine cracks appear, which slowly develop into perforations. Such cracks are usually distributed transversely along the trough.
• Corrosion perforation— — The medium contains chloride ions or acidic gas, and the material of the bellows is wrong. For example, the gas pipeline is made of ordinary stainless steel, which is quickly pitted through. PTFE-lined hoses or PTFE compensators are more resistant in such situations.
• Stress corrosion cracking— — High temperature + chloride environment, stainless steel bellows will crack in a few days. Common in power stations or chemical industries.

Look at the crack shape. Fatigue cracks resemble hair strands, subtly parallel; Corrosion holes are irregular round holes; Stress corrosion cracks are like branches bifurcating. If you are not sure, cut a section and send it to metallographic analysis, and you will pay dozens of dollars to report it.

Third, selection and replacement are not patting the head-how to pick in high temperature, high pressure and large displacement occasions

After finding out the reason, here comes the key point: what kind to change? Don't go to the hardware store and just buy one. The selection must match the actual working conditions, otherwise it will be a short-lived ghost.

• Temperature: Ordinary general-purpose corrugated expansion joint temperature resistance is only a few hundred ℃, more than 500 ℃ can be usedHigh temperature axial expansion jointWith insulation inside. If the medium exceeds 1000℃? Forget about it, use non-metallic expansion joints (fabric fiber expansion joints), lined with refractory fibers.
• pressure: High-pressure steam pipes (such as above 10MPa) must be made of multi-layer stainless steel bellows, which can't be carried by a single layer. At this timeStraight pipe pressure balanced expansion jointOrExternal pressure single axial expansion jointIt is more reliable and can balance the internal pressure and thrust.
• displacement amount: Large displacements don't expect to rely on one expansion joint alone. The pipeline has many turns and large thermal expansion, so considerCompound hinge transverse expansion jointOrRotary compensatorThey can absorb angular displacement or torsion and reduce the burden on the bellows.
• Medium corrosivity: Acid, lye? choosePTFE-lined hoseOrPTFE compensator, PTFE corrosion resistance is almost universal. Flue gas pipes contain sulfur?Desulfurization flue gas baffle doorAddNon-metallic expansion jointCombinations of are common.

Cement plant clinker conveying pipeline, temperature 400℃, pressure 0.5MPa, axial displacement 80mm. useHigh temperature axial expansion jointAdd the guide tube, note that the direction of the guide tube must follow the flow direction of the medium (it is said in the question and answer of this site that "the direction of the arrow of the expansion joint refers to"), and it will be blown directly if it is installed backwards.

Fourth, after changing it, it's over? These installation details will last you three to five years longer

If you choose the right one, it will still be useless to install a rollover. Let's talk about some of the easiest pits to step on:

• Check the tie rod before installation: The transport rod must be loosened or removed, but the limit rod must be adjusted to the predetermined position. TakeLarge tie rod expansion jointFor example, you have to adjust the nut to the design length according to the drawing, otherwise the compensation amount will not match.
• Cold-tight pre-deformation: When the pipe temperature is high, the installation should be pre-stretched or pre-compressed. For example, in steam pipelines, the expansion joint is stretched for a section at ambient temperature, and the bellows returns to the neutral position when the pipeline is thermally expanded, doubling its life. The amount of pre-deformation has to be calculated, don't screw it by hand feel.
• Don't install the guide tube backwards: The function of the guide tube is to protect the inner wall from being washed, and the arrow points to the flow direction of the medium. Installed backwards, the medium directly collides with the bellows and wears out in less than a year.
• Don't be lazy about supporting hangers: The bellows cannot bear excessive weight, and a guide bracket must be set nearby, so that the pipe can only move axially, and cannot twist or sag.

And guess what? Many factories put on new ones and leaked less than two years after running. Looking back, it was checked that the support and hanger were not fixed properly, and the pipeline sag and the expansion joint was fractured. Details determine longevity, don't believe it.

5. Several heart-breaking questions: Can you weld? How many years will it last? When should I change it?

Q: Can the bellows be welded if it is broken?
A: Basically not. The wall of the bellows is thin (1~1.5mm), and it penetrates as soon as it is welded. Moreover, the material near the welding bead becomes brittle, and cracks appear again soon. The only exception is that only a small trachoma is broken and the medium is low pressure and normal temperature. It can be made up by argon arc welding with a small current, but this is an emergency measure and must be replaced afterwards. Formal practice: change components directly, don't repair them.

Q: How many years does it generally last?
A: There is no way to give dead numbers. For example, the national standard requirement of gas bellows (household use) is not less than 8 years, and the industrial design life is usually 10~15 years. However, it depends on the actual working conditions-it may take less than 5 years under high temperature and high pressure, but it can be used for 20 years at low temperature and normal pressure. In the question and answer of this site, someone asked "the service life of expansion joint", and the core is one sentence: look at the number of fatigue times and working cycle. If it starts and stops once a day, the expansion joint with less wavenumber may crack 10,000 times; If it runs steadily all the time, it will last much longer.

Q: When should I take the initiative to change?
A: Don't wait for a leak before replacing it. During the inspection, it is found that there are corrosion pits, discoloration (overburning), surface cracks on the surface of the bellows, or the guide tube falls off or the tie rod is loose and deformed, so it is time to arrange replacement. Also, during the overhaul, take a measuring tool to check whether the wave pitch of the bellows is uniform. If a certain wave pitch is compressed or stretched to the limit value, it means that it has reached the end of life.

After so much, the core is one sentence: don't panic when the bellows breaks, isolate it first, then check it, then select it, keep an eye on the installation details, don't weld it blindly and don't make do. Whether the pipe in your hand is used for a long time or not depends on whether these gates are strictly guarded or not.

How to determine the compensator temperature range? Understand these points before choosing a model

Two days ago, I met a customer, and I asked: Can your high-temperature axial expansion joint be used in 600℃ steam pipeline? I asked him back: What's the steady-state temperature? Is there an instantaneous overtemperature? He said he didn't know, but it was 600 on the nameplate anyway. And the result? After checking the process parameters, the actual operating temperature fluctuation range is 450~620℃, and there is a cold start once a day. In this working condition, the design temperature of 600℃ can't bear it at all-the fatigue life of bellows drops directly from a cliff. So, the compensator temperature range is determined? You can't just look at the nameplate.

1. Why is the temperature range the first factor in selection?

The high-temperature axial expansion joint used in a power plant has a design temperature of 600℃, but it leaked after half a year of operation. After checking, the actual temperature fluctuation of the medium exceeded the upper limit. It's not that the design temperature is not enough, but that the designer didn't take the temperature cycle into account. Compensators are not universal parts, and temperature directly determines three things: the bellows material (304 or 316L or Inconel 625), the structural thickness (every 0.5mm increase in thickness may double the fatigue life), and the overall fatigue life. The permissible stress of austenitic stainless steel decreases by about 30% ~40% for every temperature increase of 100℃. If you think about it, choosing the wrong temperature is equivalent to planting a ticking time bomb in the system.

2. Temperature limits of different materials

Metal hoses and corrugated expansion joints mainly rely on stainless steel to support the surface. The maximum temperature resistance of 304 is about 450℃. If this number is exceeded, oxidation and peeling will begin; 316L can carry it to about 500℃, but pay attention to chloride ion corrosion; The Inconel 625 can withstand more than 800℃, but the price is five or six times that of the 304. Non-metallic expansion joints (fabric fiber expansion joints) rely on coatings and thermal insulation layers, and usually have temperature resistance in the range of 300~600℃, but it depends on the bonding process of fiber substrate and silica gel/PTFE coating. Don't think about the rubber compensator. When it exceeds 120℃, it starts to age and become sticky. The rubber PTFE compensator can barely last to 150℃, but its life is not long. What about PTFE-lined hoses and PTFE compensators? PTFE temperature resistance is-200~260℃, and highly toxic gas is decomposed when it exceeds 260℃, so extra caution should be taken when used in high-temperature flue gas containing acid-condensation corrosion when the temperature is low, and material collapse when the temperature is high.

Three hard indicators for determining the temperature range

Don't think that knowing the maximum temperature of the medium is done. The first is the operating temperature — the temperature of the medium during normal operation, such as the steady-state temperature of a steam pipe. The second is the design temperature-it must contain a safety margin, usually 10~20℃, but don't forget the temperature difference impact during cold start-up. The third is the temperature cycle frequency-frequent heating and cooling will make the fatigue life drop off by a cliff. I have seen a project that uses a general-purpose corrugated expansion joint. The working condition is to start and stop once a day, and the temperature difference is 400℃. As a result, it cracks after 6000 cycles. The standard design life can be 10,000 times. Temperature difference impact is the killer, especially when the wall thickness of the pipe is large, the thermal stress generated by the temperature difference between the inner and outer walls may directly tear the bellows.

Fourth, what to do under special working conditions?

Compensators behind flue gas baffle doors often face acid-containing dew point corrosion. The compensator matched with the desulfurization flue gas baffle door should be temperature resistant (the flue gas temperature is usually 120~160℃) and corrosion resistant (dilute sulfuric acid, chloride ion). At this time, PTFE compensator or PTFE-lined hose is more reliable-the PTFE layer isolates the corrosive medium, and the metal layer provides strength. But don't forget, if the flue gas temperature exceeds 260℃, PTFE will be wasted, and you have to use non-metallic expansion joints (fabric fiber expansion joints) plus acid-resistant coating instead. In addition, the metal corrugated expansion joint used in the cement industry (metal corrugated expansion joint in the cement industry) faces the problem of high temperature dust wear. The temperature itself may not be high (400~600℃), but the dust particles scour the wall thickness of the bellows, and the wear rate is positively correlated with the temperature-the higher the temperature, the softer the material and the faster the wear. Therefore, it is necessary to leave a margin for the wall thickness or add a guide tube when selecting the type.

5. Selection misunderstanding: Don't regard "instantaneous temperature" as "continuous temperature"

Two days ago, I met a customer who said that the rotary compensator should be used in 800℃ steam pipeline. I asked him to adjust the process curve. In fact, it was just instantaneous overtemperature during driving, and the steady state was only 550℃. This kind of working condition does not need to go to Inconel at all, 316L plus guide tube is enough. Under instantaneous overtemperature, although the bellows can be tolerated for a short time, frequent instantaneous high temperature will lead to accelerated intergranular corrosion of materials. Therefore, the temperature range should be clearly confirmed with the process engineer: is it continuous or intermittent? Is there emergency overtemperature protection? Don't ask me how I know-there was a project before that I didn't ask clearly, so I chose a high-temperature axial expansion joint of Inconel 625. As a result, the actual temperature at the site was less than 500℃, which cost three times more.

6. Finally, give a self-examination list

Before the next selection, go through these four items yourself:
① Media type (steam, flue gas, corrosive gas?)
② Maximum/minimum operating temperature (note that it is not the design temperature, but the actual long-term operating temperature)
③ Temperature change rate (℃/min) -this data is not provided by many manufacturers, but it affects the fatigue life calculation
④ Expected number of cycles (including cold start and load fluctuation)
Take these four items to ask the manufacturer, and the quotation given by the other party will have reference value. If you are still worried, ask us directly for the temperature test method in the national standard JB/T 12235-2015 for non-metallic expansion joints, and you can compare it yourself. Compensator temperature range determined? If you understand these four points, you will basically not step on a pit.

Wrong compensator selection? Don't worry. Look here for the remedies

Last week, a power plant customer called in an extremely anxious tone- -one of their high-temperature steam linesUniversal corrugated expansion jointSteam leakage began after less than three months of operation. When removed, dense cracks appear on the surface of the bellows, which is typical of overtemperature aging. When selecting the model, the medium temperature is regarded as normal temperature, and the actual steam temperature is 450℃, far exceeding the upper limit of 350℃ that the general-purpose model can bear. This kind of thing is too common on engineering sites, but don't panic, most selection errors have room to be remedied, provided you have to figure out where it went wrong first.

Section 1: What is wrong in the selection error?

• Temperature rollover: Use the general-purpose type as a high temperature, such as usingUniversal corrugated expansion jointTo carry high-temperature steam, the corrugated pipe material accelerates its creep at excessive temperature, and it will crack in a few months.
• Pressure rollover: The pressure level is selected low, and the pipeline is directly blasted as soon as it is pressurized-there is no regret medicine for this mistake, but it is more common to only look at the design pressure without considering the test pressure when selecting the type. The operating pressure is normal, and it collapses during the hydraulic test.
• Displacement type confusion: putAxial expansion jointHard-mounted on pipes requiring lateral displacement, or withExternal pressure single axial expansion jointDeabsorbing the angular displacement, the bellows quickly becomes twisted and deformed.
• Miscalculation of medium corrosion: Conveying sulfur or chlorine-containing media, but with ordinaryMetal hose, perforated lining after a few months. The right thing to do is usePTFE-lined hoseOrPTFE compensator。

These mistakes will not burst the pipe immediately, but fatigue cracks, leaks, and even pull-off will come to your door after hundreds of hours of operation. On-site inspection to see more: Is there any abnormal vibration in the pipe? Is there any bulging or rust on the bellows surface? Is there any abnormal noise at the flange connection? These small signs can help you "confirm the diagnosis" in advance.

Section 2: After diagnosis, remedy according to the situation

Not all mistakes have to be disassembled and reinstalled, and handling them according to situations can save a lot of money and time.

Case 1: Displacement Calculation Deviation
For example, the designed axial displacement is 20mm, and the actual thermal expansion of the pipe is 25mm, but it is still within 80% of the rated displacement of the bellows. At this time, you can adjust Expansion joint tie rod nutTo fine-tune the amount of pre-compression. For the specific adjustment method, refer to our previous"How to adjust the tie rod nut of the expansion joint"The core principle is to loosen the locking nut and rotate the tie rod to change the initial length of the bellows so that it works within the service interval. If the displacement deviation is too large, a limiting device can be installed to prevent over-stretching.

Case 2: Liner damage due to media corrosion
Conveying highly corrosive media but using ordinary metal hoses? Don't think about it, just change it. Remove the old piece and replace it with one of the corresponding material-such as PTFE-lined hoseOrPTFE compensator。 Note that the thickness of the liner should match the medium concentration and temperature, otherwise it will leak again after a few months. If you are not sure, send us the media composition list and temperature and pressure to help you check the corrosion resistance data.

Scenario 3: Insufficient pressure level
This is the worst thing. Don't make do with it-the high-pressure pipe blew up is no joke. The machine must be stopped, the compensator removed, and replaced with the same model but with a higher pressure rating. For example, the original one was 1.0MPa Straight pipe pressure balanced expansion joint, the actual system pressure is 1.6MPa, then replace it with 1.6MPa or 2.5MPa specification. By the way, check the pipe brackets and foundations for damage.

Section III: Temporary Emergency Measures

Before the spare parts arrive, one day of on-site shutdown may be hundreds of thousands of losses. Can you carry it? Yes, but only to "delay".

• Install external protective sleeve: LearnSleeve type pipe expansion jointThe structural principle is that an outer sheath is welded with a steel plate and sleeved at the leakage point, and a high-temperature-resistant sealing gasket is filled inside to temporarily seal the leakage. Note that this can only be used at low pressure and low temperature. Don't try it at high pressure.
• Reduce operating parameters: Pressure reduction and temperature reduction operation. For example, when the steam pipeline drops from 10MPa to 6MPa, and the temperature drops from 450℃ to 350℃, the stress energy of the bellows drops greatly. But remember to support the longest maintenance cycle (generally no more than three months), and then it must be changed.
• Handle flue gas baffle door sealing is not tight: If yesDesulfurization flue gas baffle doorOrDouble-sealed single-axis circular baffle doorWrong selection leads to lax sealing, you can temporarily replace the sealing packing, or adjust the bolt pre-tightening force of the compression mechanism. However, temporary measures treat the symptoms rather than the root cause, and the baffle door with corresponding specifications has to be changed for a long time.

Section 4: Pit Avoidance Guide during Re-selection

Each experience leads to wisdom. Before the next selection, honestly list the working condition parameters clearly, don't be too troublesome. Parameters that need to be noted include:

• Media composition: Does it contain sulfur, chlorine, acid and alkali? These direct decisions are made usingMetal hoseStillPTFE-lined hoseOrPTFE compensator。
• Temperature range: Maximum, lowest, normal operating temperature. High temperature separationHigh temperature axial expansion jointOrExternal pressure single axial expansion joint； For low temperature (such as air-cooled island vacuum pipe)Double hinge expansion joint for air-cooled island vacuum pipeline。
• Pressure rating: Design pressure and test pressure are two different things, according to the test pressure when selecting the model.
• Type and amount of displacement: axial, lateral, angular, the magnitude of each displacement. For example, if the pipeline needs large lateral displacement, you can chooseCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。
• Pipe arrangement: For buried pipelinesDirect buried (fully buried) type expansion joint； Used in cement industryMetal Corrugated Expansion Joints in Cement Industry； For vacuum pipeSpecial hose for vacuum。

Filling in a selection form for these parameters and sending them to the manufacturer is 10,000 times better than remediating them afterwards. Don't use "probably" or "almost" to make up, data accuracy is the lifeblood.

Section 5: The most hassle-free way-find a manufacturer to do technical review

And guess what? Many customers sent us the parameters, but when they installed them, they found that they chose the wrong one-it turned out that the "medium temperature" was written as normal temperature in the parameter table, which was actually high temperature. In fact, before ordering, send the pipe drawing, media parameters and installation position photos, and our technical support will help you review the selection. Especially for these special operating conditions:

• Flue should be usedNon-metallic expansion joint (fabric fiber expansion joint)StillRectangular non-metallic expansion joint？ It mainly depends on the temperature, pressure and whether corrosion resistance is needed. The temperature resistance of fabric fibers generally does not exceed 250℃, and rectangular non-metals can be made higher.
• Double-sealed single-axis circular baffle doorAndElectric plug-in insulation doorHow to choose? The former is used when the smoke and air system needs to be sealed and isolated, and the latter is suitable for working conditions where the material needs to be completely cut off. The professionals can see the pits in the pipe arrangement at a glance.
• High temperature and high pressure large diameter pipeline, forLarge diameter thick wall expansion jointStillSleeve type pipe expansion joint？ The pressure resistance mechanism of the two is different. The former depends on the elasticity of bellows, while the latter depends on sliding seal. The wrong choice will lead to seal failure.

In the final analysis, compensator selection is not a puzzle, and the four dimensions of temperature, pressure, displacement and medium are indispensable. It doesn't matter if you are wrong this time. Follow the above scheme to remedy it step by step. Next time, give all the parameters and ask a professional to review it again, so you won't step on the pit again.