Specialized in manufacturing compensators, expansion joints, baffle doors

做管道设计的、搞设备采购的，十有八九都问过一句：非金属补偿器哪种最好？

这个问题没法一句话回答。非金属补偿器不是一个“万能神器”，它是一类产品的统称，底下还分好几种。你选错了类型，后面全是麻烦。咱把关键点一条条掰开揉碎说清楚，看完你心里就有数了。

先分清类型：织物纤维、橡胶、四氟，根本不是一回事

非金属膨胀节（也叫织物纤维膨胀节）通常用玻璃纤维、陶瓷纤维这些做蒙皮，耐高温性能很突出，补偿量大，特别适合烟气、粉尘这类工况。你去看脱硫塔进出口、烟囱附近的管道，用的基本都是这种。

橡胶补偿器不一样。它擅长的是减振和吸收位移，能帮你把泵和风机的振动隔掉。但它的耐温和耐压都有限，温度超过100℃就得掂量掂量，压力高了也顶不住。

还有橡胶四氟补偿器，兼顾防腐和柔性，酸碱介质管路上用得比较多。聚四氟乙烯补偿器也是干这个的，耐腐蚀性更强。

所以你看，标着“非金属”不代表万能。先对着工况选类型，这一步走错了，后面花再多钱都白搭。

看工况参数：温度、压力、介质，一个都不能少

选非金属补偿器，最核心的其实是看蒙皮材料层怎么配。而蒙皮怎么配，完全由工况决定。

耐温多少？是连续运行还是间歇？介质里含硫、含酸碱还是带粉尘？压力是负压还是正压？这些参数少一个，厂家都只能按经验猜，猜对了是运气，猜错了现场冒烟。

举个例子。脱硫烟气挡板门后面的非金属膨胀节，所处环境是又湿又酸，还有结露风险。这时候蒙皮就得考虑耐酸腐蚀和防水渗透。你要是把电站锅炉或者水泥窑那边用的波纹膨胀节参数搬过来，照抄一遍直接用，大概率用不长。

蒙皮会不会被吸瘪？有些现场的膨胀节明明是好的，但负压一大，蒙皮直接贴合到内部导流筒上，磨损加剧，没多久就漏了。

补偿量和安装尺寸怎么定？别只看口径

非金属补偿器最大的优势是能多维补偿——轴向、横向、角向都能吸收。但能补偿多少，跟结构形式和纤维层数直接相关。

同样一个DN500的圆形管道，有的非金属膨胀节能吸收50mm轴向位移，有的只能吸收20mm。差距就在蒙皮层数和内部结构设计上。

矩形烟道一般用矩型非金属膨胀节，圆形管道则用圆形结构。这个不难理解，但很多人在安装尺寸上栽跟头。

比如导流筒方向装反了，或者法兰间距没按图纸留够，装的时候硬拉硬拽，补偿量在安装阶段就消耗掉了大半。设备寿命直接打折——不是产品不行，是你装得不对。

看执行标准和生产工艺

非金属膨胀节有国家标准，JB/T 12235-2015，从材料、焊接、检验到出厂试验，都有明确要求。有标准约束的产品，至少底线在那。

但标准的执行情况，得靠眼睛看。拿到产品先看蒙皮有没有做防割层，这层东西在运输和安装时保护蒙皮不被划伤。再看法兰面是否平整、螺栓孔距是否准确——这些细节用肉眼就能判断。

蒙皮强度、密封性这些做没做足，往往就体现在这些小地方。国标不是束缚，是保护。没有标准约束的产品，大部分在看不见的地方偷工减料。

算综合成本：不是越便宜越好，也不是越贵越保险

买非金属补偿器，你买的是补偿能力，不是买一堆金属框架。安装后两年不漏、维护省心，才是真划算。

前两天遇到个客户，图便宜买了非标产品，用了不到一个检修周期就开裂了，更换不说，还耽误了生产。停机一天的损失，比省下的那点采购费多得多。

采购成本+维护成本+停机损失。把这三项摆在一起，你再去对比报价，心里就不慌了。

说到底，非金属补偿器哪种最好？适合你工况的、按国标生产的、细节经得起看的，就是最好的。

Starting from the name of the industry: What is the relationship between compensator, expansion joint and telescoper?

"Is this expansion joint just a telescoper? Can I use a sleeve type instead of a corrugated one?" In fact, in the field of pipe compensation,Compensator and expansion joint are basically the same thing-You see, article 14 of the question and answer on our site also states that expansion joints and compensators actually refer to the same type of equipment in industrial piping systems. But the word "telescoper" is a little ambiguous. In some old drawings, the sleeve compensator is called a telescoper, and some on-site masters collectively refer to all the telescopic tubes as telescopers. So technically speaking,Compensator (expansion joint) is a collective termWhile scaler is more of a colloquial common name, often referring specifically toSleeve type or packing box type。 Don't underestimate this detail. If you choose the wrong model, the pipe may collapse directly when installed.

Classification by material and structure: metal corrugated, rubber, non-woven, sleeve type, what are the characteristics of each?

In the product list on our site, there are dozens of kinds of light expansion joints. Such asUniversal corrugated expansion joint、High temperature axial expansion joint、directly buried expansion joint、External pressure single type axial type…These are metal bellows, which absorb displacement by the elastic deformation of the corrugations. rubber compensator andRubber PTFE compensatorRelying on the flexibility of rubber, it is suitable for scenes with low pressure, large displacement and vibration reduction, such as fan import and export.Non-metallic expansion joint(also called fabric fiber expansion joint) is resistant to high temperatures and corrosion, which is common in flue gas pipelines.Sleeve type pipe expansion joint(that is, the telescoper in some people's mouths) is compensated by the sliding of the inner and outer sleeves. The pressure is high and the axial displacement is large, but the sealing ring is easy to age. AndRotary compensator、Curved tube pressure balance typeThese are specific to operating conditions. At the end of the day, there is no universal compensator, only matching selection.

What to look at when choosing a model? Temperature, pressure, displacement, and medium must not be missing

You use a rubber compensator on the steam pipe? As soon as the high temperature of several hundred degrees goes up, the rubber is directly carbonized. So the high temperature pipeline has to beHigh temperature axial expansion jointOrExternal pressure single type axial type。 If it is desulfurization flue gas, which is highly corrosive, it must bePTFE-lined hoseOrPTFE compensator。 The direction of displacement is also very critical-axial displacement menu, transverse displacement selectionCompound hinge transverse typeBoth axial and transverse,Straight pipe pressure balance typeOrDouble straight pipe bypass pressure balance typeMore appropriate. Don't forget that there arebaffle doorAndIsolation doorSuch ancillary equipment, such asFlue gas baffle door、Round flapper door、Electric plug-in insulation doorThey work with a compensator, but have completely different functions. When selecting the model, fill in the working condition parameters clearly, and don't pat your head.

Don't step on the pits of installation and maintenance

Many people think that the compensator can be installed, but it leaks within two months. There are three common problems: First, the transportation screw was not disassembled before installation-the expansion joint will be fixed by a screw in order to prevent transportation deformation when it leaves the factory, and it must be disassembled or adjusted according to the instructions after installation. Article 13 Q&A specifically talks about this matter. Second, cold-tight pre-stretching was not done, especially high-temperature pipelines, which were not pre-stretched during installation and cracked during operation. Third, the direction of the guide tube is reversed. It is mentioned in the seventh question and answer that the function of the guide tube is to prevent the medium from washing the ripples, and the direction of the arrow must point to the flow direction of the medium. And the packing gland of the sleeve-type telescoper can't be screwed too dead, otherwise the sliding resistance is large and can't be compensated. In terms of maintenance, the rubber compensator should be checked for aging cracks in about three years, and the fatigue life of the metal bellows should be paid attention to. For details, please refer to Article 18 Q&A.

Summary: Different names, the same core functions, but the specific products should be carefully looked at

Back to the original question —Is a compensator the same as a telescoper?Functionally speaking, they are all devices that absorb the thermal displacement of pipelines and reduce vibration and noise, which can be equated. However, when it comes to the procurement link, you report the "telescoper" to the manufacturer, and the other party may give you the sleeve type; You quote "compensator", and the other party may give you bellows. Therefore, it is recommended to clarify the product type directly: for example, "I need an axial corrugated expansion joint with DN200, PN16 and a compensation amount of ±50mm", or "Give me a non-metallic fabric compensator with a temperature of 350℃ and a smoke environment". In this way, you can't go wrong, and you can quickly match the most suitable one from the 29 products on our site. If you are not sure, just throw the working condition parameters over, and we will help you choose-we do this every day, which is more reliable than you patting your head.

Do you really understand the role of corrugation compensator?

What is the corrugated compensator (also called expansion joint and compensator in the industry) used for? Simply put, it is a flexible joint that absorbs all kinds of displacements and stresses in the pipeline system. Don't underestimate this function. The pipeline can run for tens of meters. When the temperature changes, it expands and contracts. If it is hard, the weld will collapse and the flange will leak, which is a loss of real money. So figuring out its specific role is more important than anything else.

I met a customer two days ago. The steam pipe has been used for two years, and the flange interface began to leak. When I removed it, I saw that the bellows were cracked. After asking, I found out that the displacement was not counted at all in the original selection, so I bought a general-purpose corrugated expansion joint to deal with it. Alas, I have seen many cases of this kind, and I have one feeling: if I don't understand the function of the corrugated compensator, I will have to pay tuition sooner or later.

First role: to compensate for thermal displacement-this is the skill of eating

Steam pipes, hot water pipes and flue gas pipes, the temperature goes up and down, and the pipes stretch and contract. For example, corrugated expansion joints are used in the power station industry, and hundreds of degrees of steam can easily be expanded and contracted by several millimeters to tens of millimeters, which are all absorbed by bellows. Another example is the metal corrugated expansion joint in the cement industry, which is transported by high-temperature clinker and has a larger displacement. Calculate, a 30-meter steam pipe, the temperature difference is 200℃, the linear expansion coefficient of steel is 0.012mm/m·℃, and the optical axial elongation is 72mm. Hard solder dead? The weld stress has already exceeded the yield limit.

When selecting the model, it also depends on axial displacement, lateral displacement or angular displacement. Like a straight tube pressure balance expansion joint, which is specially used to deal with axial expansion and contraction; Curved tube pressure balance type expansion joint, used at the bend of pipe system, absorbs angular displacement and transverse displacement. A few days ago, there was a desulfurization project, and the pipe system turned around. The design institute used three double hinge transverse expansion joints, and all the displacements were eaten, and the site passed at once. So don't use the general-purpose model to deal with all working conditions, that's called laying mines.

The second function: vibration reduction, noise reduction and impact resistance-bellows can also be used as shock absorbers

As soon as pumps, fans and compressors are turned on, vibration is transmitted along the pipeline, which not only annoys the noise, but also loosens the pipe frame and cracks the weld. Corrugated compensator can eat a part of vibration energy, especially rubber compensator and rubber PTFE compensator, which have good flexibility and obvious vibration isolation effect. Metal hoses also do this job, such as the double hinge expansion joint of air-cooled island vacuum pipeline, which has to bear the vacuum negative pressure and absorb the vibration of the pipeline. And guess what? The pipeline on the air-cooled island of the power plant vibrated at a ridiculously high frequency. Without this thing, the welds would crack in fatigue within three months.

I have seen a cement factory where the fan outlet pipe used a non-metallic expansion joint (fabric fiber expansion joint). The original noise was 90 decibels, but it dropped directly below 75 decibels after replacing it. Why? Fabric fiber expansion energy-saving absorbs multi-directional displacement, and at the same time converts vibration energy into heat energy and dissipates it. Tsk, that's a lot easier than adding a soundproof cover.

Third function: solve installation deviation and facilitate maintenance-less quarrel with field workers

It is impossible to install the pipeline on the spot-if it is slightly crooked by a few millimeters, the stress will be great if it is hard connected. Corrugated compensators absorb these installation errors and allow the pipes to connect smoothly. Especially for large-diameter pipes, it is common for flanges to mismatch. I have seen a chemical plant, DN1000 pipeline, which was misaligned by 8mm during on-site welding. It was adjusted by a universal corrugated expansion joint + tie rod, but it was not reworked. Otherwise, cutting and re-welding, the construction period is at least three days.

In addition, during equipment maintenance, such as manual plug-in insulation doors and electric plug-in insulation doors, a section of pipe needs to be removed. The corrugated compensator can provide additional telescopic space, so there is no need to move the pipe. Think about it. As soon as the expansion joint is compressed during overhaul, the space will come out. After replacing it, it will be stretched back, which will save trouble. Without this function, the entire pipe gallery would have to be demolished.

The fourth role: material adaptation under special working conditions-not all pipes eat the same set

Chemical industry runs acid and alkali, with PTFE-lined hose or PTFE compensator, corrosion resistance. Why? PTFE can carry almost all strong acids and alkalis, but ordinary rubber is scrapped in one month. What about high temperature flue gas pipes? It is necessary to use non-metallic expansion joints (fabric fiber expansion joints), which can withstand the temperature up to 1200℃ and can absorb multi-dimensional displacement. In wet desulfurization environment such as desulfurization flue gas baffle door, acid and alkali resistance and leakage prevention have to be considered. Two days ago, in a power plant desulfurization project, a rectangular non-metallic expansion joint was equipped next to the flue gas baffle door, which was both acid-and alkali-resistant and anti-condensation. After two years of use, nothing happened.

The directly buried (fully buried) type expansion joint is specially used for buried pipelines, which takes into account both resistance to soil pressure and compensation for thermal displacement. Some customers use ordinary expansion joints to bury the ground cheaply, but they are crushed by the soil in half a year-that is called the gain is not worth the loss.

Finally, talk about the selection idea: don't let the corrugated compensator take the pot

It is not just buying a corrugated compensator to solve the problem, but it has to be matched according to the medium, temperature, pressure, displacement direction and installation space. For example, large-diameter thick-walled expansion joints are used in high-pressure pipelines, external pressure single axial type is suitable for long straight pipes, and compound hinge transverse type is suitable for pipe systems with many turns. The attachments of the guide tube and the tie rod are not decorations-the guide tube protects the inner wall of the bellows from being washed by high-speed media, and the tie rod limits excessive displacement. Understand these, and the selection will not overturn.

What does a corrugation compensator do?To put it bluntly, it was four words: Keep the life of the dao. If you choose the right one, the pipeline will run as steady as an old dog for more than ten years; The wrong choice will make you doubt your life every three days. Therefore, the next time you choose a model, don't just look at the price. First, calculate the displacement, check the medium and ask about the working conditions-this is a serious business.

Two days ago, a buddy who designed flue gas pipelines called and asked, "How to calculate the resistance of the expansion joint? I calculate the pressure drop according to the straight pipe section. If the pump is selected small, the test run will trip." Alas, this sounds familiar. Too many people have stumbled in the selection of expansion joints-the system pressure drop is quite accurate, but they have slapped their heads in the expansion joints. Today, let's talk about this matter clearly. After you understand it, you can pat your chest and say that you can count.

First, don't rush to set the formula-what problem does the resistance calculation solve?

I calculate this resistance, what is it for?
To put it bluntly, there are two purposes: first, ensure that the head of the pump or fan is sufficient, so that the system will not be pressurized or the flow rate is insufficient; Second, avoid the pipeline stress exceeding the standard due to excessive local resistance, which will tear the expansion joint.

To put it bluntly, the expansion joint is installed in a pipe, and the fluid passing through it will inevitably cause pressure loss. If this loss is ignored, the chosen pump will either trip like the guy above, or the big horse-pulled cart will waste the electricity bill. Calculate it accurately, and you can match an economical and reliable system.

Second, the physical nature of resistance: the expansion joint is not a straight pipe, and the fluid turns a corner here

What is the structure inside the expansion joint? Take the most commonUniversal corrugated expansion jointFor example, the ripples raise one by one, and when the fluid flows through it, it is like driving a car on a washboard road-every wave peak has to change direction, accelerate, and slow down, and energy is lost in the process. This loss belongs to local resistance, which is completely different from the friction resistance along the pipeline.

If you think about it, a straight pipe of the same caliber and a section of corrugated expansion joint, at the same flow rate, the pressure drop of the latter may be 5 to 10 times that of the former. Why? Because the fluid repeatedly "bends" in the ripples, a large number of vortices are created. This kind of local resistance cannot be applied hard by Darcy's formula, but has to be calculated by a special resistance coefficient method.

Then how to quantify? The empirical formula is as follows: Δ P = ξ · (ρ · v²/2), where ξ is the drag coefficient, ρ is the medium density, and v is the flow rate. The difficulty is all about this.

Third, different types of expansion joints, the resistance difference is greater than you think (metal corrugation vs non-metal vs sleeve)

Different types of expansion joints have very different internal geometries, and the drag coefficients can be different by an order of magnitude.

• Metal corrugated expansion joint(For example, those used in the power station industryHigh temperature axial expansion joint、Large diameter thick wall expansion joint): The ripple bulge is obvious, and the fluid turbulence is violent. The resistance can be two or three times different from the one with a guide tube and the one without one. More on that later.
• Non-metallic expansion joints (fabric fiber expansion joints): The interior is generally a smooth rectangular or circular channel, without obvious corrugated bumps, and the resistance is much less than that of metal corrugated expansion joints. However, it depends on the fabric to bear pressure, and the flow rate cannot be too high, usually controlled within 15m/s.
• Sleeve type pipe expansion joint: The displacement is compensated by the sliding of the inner and outer sleeves, and the inner channel is basically straight through, with the smallest resistance, which can be calculated almost according to the straight pipe section. But seals are prone to wear and tear, which is another matter.

So don't take all models with one formula. You're choosingCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointWhen the structure is more complicated, the drag coefficient has to be corrected according to the specific wavenumber and waveform.

4. Calculate it hand-in-hand: empirical formula + key parameter values

Okay, straight to the case. Assume a steam pipeline with nominal diameter DN300, design flow rate of 25m/s and medium density of 0.6kg/m³. Select a steam pipeline with guide tubeUniversal corrugated expansion joint, Single Wave.

Check the drag coefficient ξ. According to industry experience, ξ of single-wave metal corrugated expansion joint with guide tube is generally between 0.5 and 1.2. Take 1.0 for conservative points. Without the deflector, ξ can soar above 3.0.

Calculate the pressure head. ρ ·v²/2 =0.6×25²/2=187.5 Pa.

Δ P = ξ ×187.5=1.0×187.5 ≈ 188 Pa. This is just the loss of a single wave. If you choose a multi-wave expansion joint (such asCompound straight pipe bypass pressure balanced expansion joint), also multiply by the wavenumber, but pay attention to the interwave interference, not a simple linear superposition-usually corrected by the square root of the wavenumber.

The contrast system allows pressure drop. If the outlet pressure of the fan is only 500Pa, an expansion joint will dry out 188Pa, plus the loss of the pipe valve, it is definitely not enough. At this time, either change to a low-resistance type with a guide tube, or increase the diameter to reduce the flow rate.

5. Guide tube, flow rate, medium-those "invisible" factors that affect resistance

Just now, we mentioned the guide tube. What is this thing for?Specific Function of Expansion Joint Guide TubeIt is to let the fluid take a relatively straight path inside the corrugation, so as to avoid direct impact on the root of the corrugation to generate large vortex. With it, the drag coefficient can be reduced to one-third or even lower than without it. So don't be reluctant to give up that cost, especially for high-speed airflow pipelines, the guide tube is a power-saving artifact.

Medium viscosity. Gas and water are very different. The gas has low density and low viscosity, and the resistance mainly comes from eddy loss; Liquids such as water or oil have high density and high viscosity. In addition to local resistance, the friction loss of corrugated wall should not be underestimated. When calculating liquid pipeline, it is recommended to refer toMetal hose pressure standardEmpirical data in, or do CFD simulation directly.

In addition, the higher the flow rate, the better. The flow rate doubles and the resistance becomes quadrupled (because v²). Some people choose small diameter expansion joints to save money, but as a result, the pressure drop is too large, and the pump consumes much more energy than the equipment money saved. Tsk, the gain outweighs the loss.

6. Completion of calculation is not equal to completion-common cases of resistance rollover in engineering

Let's tell me a few real rollover scenes.

Case A: The desulfurization flue of a power plant was usedNon-metallic expansion jointIt would have been fine, but the design flow rate was mentioned above 20m/s, and as a result, the fabric layer was torn by the airflow after half a year of operation. Turning back and calculating, the local pressure drop exceeds the standard, resulting in negative pressure fluctuation and fabric fatigue damage. Later replaced with a deflectorMetal rectangular expansion jointJust steady.

Case B: Steam pipeline of a chemical plant, selectedExternal pressure single axial expansion jointThe pressure drop of the deflector is not counted, and as a result, the system safety valve jumps frequently. Finally, it was found that the corrugated root inside the expansion joint accumulated scale, the actual circulation area shrank by 30%, and the resistance tripled. So regular inspections and cleaning are also important.

And guess what? These rollovers all have one thing in common: the expansion joint is regarded as "part of the pipe" in the design stage, and the resistance is not counted separately at all. In fact, the expansion joint is a local resistance member in the pipeline, which must be listed separately in the pipeline hydraulic calculation table.

How is the resistance of the expansion joint calculated?Remember this formula: Δ P = ξ · (ρ V²/2), then honestly check ξ, set the flow rate, and check the system. Don't slap your head, don't be lazy. Understand these steps, and selection is no longer metaphysics.

How does the wave pitch of the bellows change? Full interpretation from mold to application

A few days ago, a customer who made power station pipelines called and asked, "How did you adjust the wave pitch of your bellows? I see 20mm marked on the drawing. Can you change it to 15mm for me?" This problem is actually quite typical-many people think that the wave pitch is a fixed size, and the manufacturer can set as much as he wants. Today, let's break this matter apart and explain it clearly, from mold to manufacturing to type selection, explain it clearly at once.

Wave length is not randomly determined: first understand what this parameter is for

Wave distance, to put it bluntly, is the axial distance between two adjacent peaks (or troughs). It directly determines one of the core capabilities of the bellows-how many waves can be stuffed into a unit length. The more wavenumbers, the greater the amount of compensation; The larger the wave pitch, the higher the stiffness of the individual waves.

But this stuff isn't a patter on the head. A high-temperature axial expansion joint is used in steam pipeline, and a metal corrugated expansion joint in cement industry is used in kiln tail pipeline. The logic of wave pitch is completely different. The former has to bear high temperature and high pressure, and the wave distance is too small to easily concentrate stress; The latter is mainly to absorb thermal displacement, and a larger wave pitch will be more resistant to fatigue.

So you see, the wave pitch is essentially a "performance adjustment knob". Wherever the knob is twisted, the stiffness, compensation amount and fatigue life all change accordingly. Then how is this knob twisted in the manufacturing process?

Manufacturing: How to Control Wave Pitch in Hydroforming and Mechanical Forming

Hydroforming and mechanical forming (also called roll forming). The wave length control logic is completely different.

Hydroforming: The pipe blank is filled with high-pressure oil, and the pipe wall is pushed up by oil pressure to form waves. The wave pitch is determined by the axial spacing of the mold-the groove engraved on the mold, the groove pitch is the wave pitch. Because of the uniform oil pressure and full wave crest, hydroforming is suitable for small wave distance and multi-wave number products, such as universal corrugated expansion joints and metal hoses. If you want to change the wave pitch under this process, you have to change the mold or adjust the spacer ring on the mold. So once shaped, the wave distance is basically locked.

Mechanical forming: Roll the pipe wall by a pair or several pairs of rollers, and gradually press out the corrugations. The wave pitch is controlled by the ratio of the feed speed and the rotation speed of the roller. The advantage is that it can be adjusted flexibly-the same set of molds, adjusting the speed ratio can enlarge or reduce the wave distance. The disadvantage is that the material requirements are high, and the stainless steel thin-walled pipe is easy to wrinkle. Many large-diameter thick-walled expansion joints and external pressure single-type axial expansion joints are mechanically formed, because such products often need customized wave pitches to match specific compensation requirements.

The bellows will have a rebound during the molding process. There will be a difference of 0.3-0.8mm between the actual wave distance and the mold size, and experienced masters will count the rebound amount in advance in the mold design. After you receive the product, measure it, and the wave distance can generally be controlled within ± 0.5mm.

When the wave pitch changes, the performance changes completely: the relationship between stiffness, compensation amount and fatigue life

Let's start with stiffness. The larger the wave pitch, the larger the bending radius of the single-wave metal material, and the stronger the ability to resist deformation – the stiffness is naturally high. The reason is very simple, just like a spring, the larger the lap gap, the harder it is to press. For straight pipe pressure balance expansion joint, which relies on stiffness to balance the thrust, the wave pitch is often designed to be too large. On the other hand, the transverse expansion joint of the compound hinge needs a large amount of compensation, and the wave pitch has to be small, so that the unit length can accommodate more waves.

What about the amount of compensation? Total compensation amount = single wave compensation amount × wave number. When the wave distance is smaller, the wave number increases at the same length, and the total compensation amount increases. However, the amount of single wave compensation is not fixed-the single wave with large wave distance can be pulled more open, so the two should be calculated comprehensively. Take our commonly used bellows as an example: if the wave pitch is changed from 20mm to 16mm, the total compensation can be increased by about 30%, but the fatigue life may be reduced by 15%-20%. How to choose? Depends on the working conditions.

Fatigue life is the most sensitive piece. When bellows expand and contract frequently (such as double hinge expansion joint for air-cooled island vacuum pipe), if the wave pitch is too small, the stress concentration at the wave root will increase, and cracks often start there. I handled a case in which the customer temporarily changed the wave pitch by two millimeters, but it leaked after eight months of use-it was replaced with the original design wave pitch, and it was fine for three years. Therefore, when making non-standard customization, don't just focus on the compensation amount, the fatigue life is the ceiling.

Practical Selection: How to Select Wave Distance under Different Working Conditions

Give you a few specific scenarios, and you will understand by comparing them:

• High temperature and high pressure steam pipeline(Corrugated expansion joint for power station industry): The wave pitch is recommended to be 18-25mm. This range can take into account both stiffness and fatigue life, and it is more stable with multi-layer corrugated structure.
• Corrosive media transport(Lined with PTFE hose and PTFE compensator): The wave pitch should be too large, 22-30mm. Because the lining layer is brittle, the wave distance is too small to break the PTFE layer easily.
• Large displacement and low frequency working condition(Direct buried expansion joint, rotary compensator): The wave pitch can be as small as 12-15mm. It mainly depends on the compensation amount of the wave number pile. Anyway, it only acts dozens of times a year, so fatigue is not a problem.
• Smoke duct system(Non-metallic expansion joint, rectangular non-metallic expansion joint): In fact, metal bellows are not used in this occasion, but if you use metal rectangular expansion joint in the flue, the wave pitch will be larger-more than 30mm, because soot particles are easy to accumulate in the trough, and if the wave pitch is too small, it will fail.

In addition, to be honest, many customers only look at the diameter and pressure when selecting models, and don't mention the wave distance at all. The wave distance column on the sales bill is often blank. But if you think about it, the price difference between the wave pitch of 18mm and 22mm of the same general-purpose corrugated expansion joint can be 20%, and the performance is completely different. Therefore, the next time you inquire, it is best to tell the manufacturer your "displacement + working frequency" directly, and the manufacturer can help you calculate the appropriate wave distance.

How to deal with wave pitch abnormality during installation and maintenance

Finished installing on site and found that something was wrong with the wave distance? Don't panic, first judge whether it is caused by manufacturing deviation or installation stress. Take a ruler and measure the wave distance at three different positions:

• If the deviation is within ±0.5mm, normal, rebound influence.
• If the deviation exceeds 1mm and is concentrated on one side, there is a high probability that there is a problem with the pipeline alignment during installation, and the bellows is pulled off. At this time, you have to loosen the expansion joint tie rod nut and readjust the axial position of the pipe. For specific adjustments, you can turn our previous article "How to Adjust the Expansion Joint Tie Rod Nut".
• If the whole bellows is axially compressed and the wave pitch generally becomes smaller, it means that the cold drawing amount of the pipeline is not done correctly, or the medium temperature exceeds the design value. After parking and cooling down, observe it. If the wave distance can't be restored, you have to change parts-forcibly running with illness will accelerate fatigue cracking.

It is found that the wave pitch is significantly larger during maintenance, usually because plastic deformation (over-stretching) has occurred. This is basically hopeless, just replace it with a new one. Remember to add a limit tie rod (such as the tie rod structure in the expansion joint of the large tie rod) to the bellows when selecting the model next time to prevent accidental over-displacement.

The more accurate the wave distance is, the better. Some high-end customers take vernier calipers for acceptance, and they have to be stuck to a whole millimeter, which is actually unnecessary. As long as within the design tolerance, priority is given to ensuring that the compensation amount and stiffness meet the standard-after all, bellows is a "flexible element", and being too real may turn out good things.

Regarding the wave distance, if you still want to talk about specific working conditions, call our technical department directly. We have more than 20 kinds of drawings of expansion joints, from high-temperature axial type to directly buried type, from metal hose to PTFE compensator, and how to determine the wave pitch has ready-made case reference. Don't worry, don't fool you into choosing the most expensive, only help you choose the most correct one.