Specialized in manufacturing compensators, expansion joints, baffle doors

1. 等级划分的核心依据：压力、温度与补偿量

做管道补偿这么多年，被问得最多的就是：“金属膨胀节等级怎么划分？”其实说到底，就三个硬指标：压力、温度、补偿量。这三个参数基本上就锁定了你该用哪个级别的产品。

先说压力。低压（0.1~0.6MPa）和高压（≥2.5MPa）用的波纹管壁厚、层数、波高完全不是一个概念。比如通用型波纹膨胀节，一般设计压力在1.0MPa以内；而电站行业用波纹膨胀节，动不动就上2.5MPa甚至更高，那波纹管必须多层、加强环，甚至要加铠装。压力等级直接决定安全余量和疲劳寿命。

温度更狠。常温下304不锈钢能撑个100℃没问题，但到了600℃以上，你得用高温合金（比如Inconel 625），而且还得考虑蠕变强度。前两年有个水泥厂客户，把高温轴向型膨胀节装在窑尾烟道上，温度标称800℃，结果选了普通304材质，三个月就开裂了——温度等级选错了，等于白干。

补偿量这东西挺有意思。轴向补偿、横向补偿、角向补偿，不同结构形式对应的补偿能力差很远。直管压力平衡型膨胀节主要吸收轴向位移，复式铰链横向型膨胀节专门处理横向位移。补偿量不是越大越好，大补偿量意味着波纹管更长、波数更多，刚度就下来了，容易失稳。所以选等级时，补偿量必须和刚度匹配着看。

2. 从标准看等级：国标、行业标准与厂商自定义

聊到标准，很多采购说“按国标来就行”，但国标其实分得很细。金属膨胀节国标主要是GB/T 12777（金属波纹管膨胀节通用技术条件），但你别指望这一个标准涵盖所有场景。比如水泥行业金属波纹膨胀节，还得参考JC/T 967（水泥工业管道用金属波纹膨胀节）；电站行业用波纹膨胀节，会涉及DL/T 5185（火力发电厂管道用波纹管补偿器设计技术规定）。非金属膨胀节有JB/T 12235-2015，金属和非金属不一样，等级划分逻辑也不一样。

厂商自定义等级就更有意思了。国内一线品牌，比如咱们站上列的这些产品，都有内部代号。比如通用型波纹膨胀节，出厂时按压力分为PN0.6、PN1.0、PN1.6；但同样是PN1.6，不同的厂家在波纹管层数、端管壁厚、导流筒材质上可能有差异。所以只看厂商标的“压力等级”不够，还得看对应的结构细节。

某脱硫烟气挡板门项目，设计院图纸上写的“膨胀节等级：PN1.0，耐温200℃”。结果现场安装时发现，烟气中带大量冷凝酸，304根本扛不住。最后换成衬四氟金属软管，等级从材料角度重新定义——这件事提醒我们，标准等级只是基础，介质腐蚀性、介质相态这些“软参数”才是真正的隐藏等级线。

3. 不同等级对应的典型产品：从通用型到电站/水泥行业专用

分级这事别照搬书本，得看实际应用场景。咱们把产品线拉出来对照一下：

• 低压低温通用场景：通用型波纹膨胀节、金属软管、橡胶补偿器。这些产品设计压力一般≤1.0MPa，温度≤300℃，适用于暖通、给排水、一般工业管道。典型特征是单层波纹管，无导流筒或简单导流筒，价格便宜，换得勤。
• 中压中温公用工程：高温轴向型膨胀节、外压单式轴向型膨胀节、套筒式管道膨胀节。这些产品压力可以做到2.5MPa，温度450℃左右，常用于蒸汽管线、热力管网。注意这里会有导流筒，作用是防止高温介质直接冲刷波纹管——膨胀节导流筒具体的作用，咱们问答里专门讲过，它是保护波纹管免受高速流体磨损的关键部件。
• 高压高温工况：电站行业用波纹膨胀节、水泥行业金属波纹膨胀节、直埋(全埋)型膨胀节。电站主蒸汽管道压力上10MPa，温度超600℃，必须用多层波纹管+外部压力平衡设计。水泥行业回转窑出口烟气温度高且含尘，需要用耐磨导流筒和耐高温合金。
• 特种结构：复式铰链横向型膨胀节、曲管压力平衡型膨胀节、空冷岛真空管道双铰链膨胀节。这些属于“按需定制”等级，没有标准型号，得根据管道应力分析逐一设计。

我能不能把通用型的用在电站项目上？当然不行，别说行业标准不允许，材料本身也扛不住。等级不对，要么漏要么爆，没有中间选项。

4. 选等级时容易踩的坑：材质、刚度与导流筒的影响

材质选型只看压力不看温度。常见错误是PN1.6的不锈钢波纹管，工作温度500℃，觉得没问题——实际上304在425℃以上就开始加速氧化，推荐用321或347。所以选等级时，材质等级必须单独确认。

忽略刚度。波纹管的刚度及计算公式我们站上有详细说明，但很多选型人员只看补偿量。补偿量大了，刚度就小了，管道对设备的作用力反馈变大，可能把设备接头拉坏。刚度这东西，决定了膨胀节能不能“软”到吸收位移又不传递过大推力的程度。有条件的话，让厂家提供刚度值，然后做管道应力计算。

导流筒装反或没装。导流筒的作用除了导流，还能降低流体激振、防止颗粒沉积。但有些项目为了省成本，把导流筒去掉，结果波纹管内壁被高速蒸汽吹蚀，一年就漏了。反过来，导流筒也不能随意加厚——太厚会增加局部应力，导致波纹管疲劳寿命下降。这是个平衡点。

前两天碰到个做燃气轮机的客户，选了真空专用软管用在排烟管道上，等级是够了，但没装导流筒，结果烟气中颗粒物把波纹管磨穿了。唉，这种教训太常见了。

5. 实操建议：怎么根据工况确定膨胀节等级

啰嗦了这么多，来点能直接用的步骤：

第一步：列出工况参数。介质、压力（设计压力/操作压力）、温度（设计温度/操作温度）、位移类型（轴向/横向/角向）和位移量、管道公称直径。
第二步：确定压力等级。按GB/T 12777的表，找到对应DN和设计压力的波纹管层数和波高。注意：高温下许用应力会折减，所以压力等级要按高温工况修正。
第三步：选结构形式。根据位移类型：轴向位移选直管压力平衡型膨胀节或通用型；横向位移选复式铰链横向型；角位移选曲管压力平衡型；大口径低压场合可以考虑套筒式管道膨胀节或多波型。
第四步：核对材质。.温度<200℃，普通304；200~450℃，316L或321；450~650℃，Inconel 600/625；含氯离子介质用双相钢或衬四氟金属软管。
第五步：确认是否要导流筒。介质流速>10m/s且含颗粒，或者介质温度>400℃，必须加导流筒。导流筒材质一般与波纹管相同或更高一级。
第六步：找厂家确认刚度。尤其是管道对设备推力敏感的场景，比如连接汽轮机、压缩机，刚度越大推力越大，可能推歪设备底座。

其实金属膨胀节等级怎么划分？说到底就是这三个维度：载荷（压力+温度+位移）、环境（腐蚀+磨损+冲刷）、功能（安全+寿命）。你把这三点理清楚了，选型就不会出大错。要是还不放心，直接拿着参数找咱们工程师聊，别自己拍脑门定等级——毕竟这东西装上去再拆，费用可不是一点点。

限位过大不只是“拧太紧”那么简单

前两天一个火电厂的哥们跟我吐槽，他们电站行业用波纹膨胀节装好才三个月就漏了。拆下来一看，限位螺母顶死在端板上，波纹管直接被拉成“八”字形。这场景我太熟了——很多人第一反应是膨胀节质量不行，其实根子十有八九在限位上。

限位装置（拉杆、螺母、耳板）设计的初衷是保护波纹管，防止它在运输或安装时被过度拉伸或压缩。但如果你把限位当作死挡，或者安装时预压缩/预拉伸量没算准，等管道一运行，位移方向变了，限位就从“保镖”变成了“枷锁”。结果呢？波纹管实际位移超出设计值，轻则波纹失稳、焊缝开裂，重则整个膨胀节报废，甚至拉脱管道。别问我怎么知道的——干这行十几年，见的比修的都多。

根子多半在安装阶段，别急着甩锅给产品

“冷紧量按设计值调整”，但现场工人图省事直接拧到底的有多少？或者选型时选了通用型波纹膨胀节，结果管道实际热位移比设计值大了30%，那限位自然就“卡脖子”了。还有一种常见坑：管道支架松动或地基下沉，导致位移方向偏了，限位装置反而成了主受力点。你猜怎么着？90%的案例通过调整拉杆螺母就能解决，根本不用换新件。

所以别上来就怀疑产品，先问问自己：安装时有没有按膨胀节拉杆的作用搞清楚？拉杆只是运输和保护用的，安装完必须调整到工作位置。膨胀节的螺杆需要拆吗？答案是：运输螺杆必须拆，限位螺母要留间隙。

怎么判断限位是不是真“过大”？别凭手感

拿卡尺量一下波纹管波峰到波谷的实际距离，对比厂家给的允许压缩/拉伸极限值。比如我们本站产品资料里，通用型波纹膨胀节通常有最大压缩量和拉伸量的标注。正常状态下，拉杆螺母和耳板之间应该留有3-5mm间隙，一旦顶死就说明限位过大了。还有个土办法：停机后拆掉一侧螺母，用手推拉膨胀节，如果能轻松活动说明原来限位没问题，如果纹丝不动……那基本可以确诊了。

记住，手感会骗人，但数据不会。膨胀节导流筒的作用也得搞清楚——它负责引导介质流向、保护波纹管，如果限位过大导致波纹管变形，导流筒也容易跟着损坏。

解决方案分三步，别上来就动刀

第一步，松螺母。把限位螺母往回拧，留出设计要求的自由间隙。具体怎么调可以参考我们那篇《膨胀节拉杆螺母怎么调整》的文章。注意松完后要重新检查管道对中，防止偏斜。第二步，如果松完螺母后波纹管已经发生永久变形（波距不均匀），那就得考虑更换了。第三步，对于预拉伸量不足导致的限位过大，可以重新做冷紧：用千斤顶把管道顶到设计位移位置，再把螺母锁死。

要是现场条件不允许搞冷紧，也别硬来。我们还有复式铰链横向型膨胀节或者直管压力平衡型膨胀节，能通过自身结构吸收多向位移，避免单点限位过载。水泥行业金属波纹膨胀节针对粉尘环境做了耐磨处理，高温轴向型膨胀节自带导流筒和保温层——选对型号能省一半的麻烦。

预防永远比补救省心

选型阶段别只看公称通径，要算清楚管道的热膨胀量、安装温度、环境温度，然后对照膨胀节型号及尺寸对照表选对应的型号。比如蒸汽管道，看看蒸汽管道属于什么类别，再选高温轴向型或直管压力平衡型。水泥厂的朋友记得看水泥行业金属波纹膨胀节那页，耐磨设计不是噱头。还有，膨胀节导流筒的作用那篇文章也建议读一下，能帮你避开至少30%的选型坑。

对了，安装时膨胀节拉杆的调整是个技术活。我们网站有大拉杆膨胀节正确安装方法的详细步骤，最后一步一定要把限位螺母调回工作位置。别像那个火电厂的哥们一样，以为拧紧了就万事大吉。

什么时候必须换？别犹豫

波纹管出现肉眼可见的裂纹、鼓包、波距严重不均匀，或者有介质泄漏声——别犹豫，直接换。注意新换的要匹配原有管道的位移量，别又买个大一号的限位回来。我们这里复式直管旁通压力平衡型膨胀节或者曲管压力平衡型膨胀节，都能在有限空间内吸收更大位移，特别适合老管线改造。实在懒得算数据的，可以把现场照片和管道参数发给我们，直接帮你推荐型号——这事儿我们天天干，比你自己摸索靠谱多了。

Calculate the thermal displacement first: temperature change is the starting point, installation temperature is a pit

Metal expansion joint calculation method? The first step is always thermal displacement. The formula is simple enough to recite: Δ L = α × L × Δ T. The linear expansion coefficient α is 0.012 mm/m·℃ for carbon steel, 0.016 for stainless steel and 0.018 for copper. Just note it. The pipe length L is dead, but how to take Δ T? A lot of people are stuck here.

Does the installation temperature use the ambient temperature or the actual pipe temperature? I'll tell you, directly based on the local extreme temperature difference. For example, outdoor pipelines in the north, -30℃ in winter and +40℃ in summer, Δ T is 70℃. Don't worry about what month it is when it is installed, leave an allowance. The expansion amount of the pipe is considered small, the expansion joint is hard, and the bellows is directly torn. A while ago, a customer used a general-purpose corrugated expansion joint on a steam pipeline. The installation temperature was 20℃, the actual steam was 160℃, and Δ T was only 140℃. As a result, the actual operating temperature of the pipeline fluctuated to 180℃, and the expansion amount exceeded 20%. The bellows cracked after half a year.

The wrong calculation of pressure and thrust causes the expansion joint to be scrapped directly

The thrust generated by the pressure is F = P × A, and this A is the effective area of the bellows. However, the effective area calculation methods of different structures are completely different. For example, the effective area of the general corrugated expansion joint is equal to the circular area corresponding to the average diameter of the corrugated pipe; However, the effective area of the external pressure single axial expansion joint should be calculated according to the inner diameter of the external pressure cylinder, not the bellows itself. There's a big pit here: the blind plate force created by pressure, and the pipe can twist like noodles if it is not withstood by the tie rod or the main fixing bracket.

When calculating pressure thrust, don't forget that the medium may still have impact force. Water hammer effect when steam pipe starts, instantaneous pressure may be twice the working pressure. You calculate the thrust according to the steady-state pressure, but the fixed bracket is not designed enough, and the expansion joint tie rod breaks directly. Last year, in a chemical plant, the straight pipe pressure balance expansion joint was not counted as a water hammer, the fixed bracket was pushed and displaced by 30mm, and the bellows was unstable and scrapped.

Compensation amount, stiffness and fatigue life-how to break the iron triangle?

These three parameters are constrained by each other. The greater the amount of compensation, the softer the bellows (lower stiffness), but the fatigue life will be shortened. The national standard requires that the fatigue life is not less than 1000 times, which is the basic line. In practice, we generally design corrugated expansion joints and high-temperature axial expansion joints for power station industry according to 10,000 times. How to calculate? Reference to EJMA standards, combining wave pitch, wave height and wall thickness.

A bellows with the same caliber DN300 and a wave pitch of 40mm is 30% more stiff than a wave pitch of 50mm, but the fatigue life may double. The wall thickness increases by 0.3mm, the stiffness increases obviously, and the compensation ability decreases. It takes repeated iteration to find the balance point. Two days ago, someone took a double hinge transverse expansion joint and asked for a compensation amount of 100mm. I saw that the wall thickness was only 1.5mm, the wave number was 8, and the fatigue life was only 500 times. It is recommended that the wave distance be widened, the wall thickness be 1.8mm, the wave number be increased to 12, and the life time be mentioned only 8,000 times.

Don't use one algorithm for all models

The calculation ideas of straight pipe pressure balance expansion joint and double hinge transverse expansion joint are completely different. The pressure balanced type counts the thrust cancellation generated by internal pressure-it has two bellows itself, one absorbing displacement and the other canceling the pressure thrust. However, the hinge type should consider the coupling of angular displacement and lateral displacement, not simple superposition. For example, the transverse expansion joint of compound hinge, the angle change between two hinge groups will produce axial and lateral displacement at the same time, and it is necessary to draw a displacement vector diagram to calculate it clearly.

Large diameter thick wall expansion joints are more troublesome. The increase of wall thickness leads to the increase of stiffness and the decrease of compensation ability, but the pressure thrust is large. You have to assume a wall thickness first, calculate the compensation amount, check the fatigue life, and adjust the wall thickness or wave number if it fails, and iterate repeatedly. Some designers try to save trouble and directly use empirical formulas. As a result, the stress of the pipeline exceeds the limit and the vibration is abnormally large.

Guide tube and tie rod nut are not decorations

The function of the guide tube is to reduce the erosion of the medium, especially for high temperature and high speed steam. But it affects the effective area and the pressure loss – the inner diameter variation must be taken into account in the calculation. Many people directly calculate according to the inner diameter of the bellows, ignoring the space occupied by the guide tube. As a result, the actual pressure loss is more than 30% greater than the calculated value. The adjustment of the tie rod nut directly determines the direction of pre-deformation, and if it is installed backward, the expansion joint will fail in advance. How to adjust the expansion joint tie rod? The principle is: when cold tight, the tie rod nut compresses the bellows, so that it has a pre-stretch in the installed state, so that when the working temperature rises, the bellows can be both compressed and stretched, and the service life is longer.

Actual Combat Checklist: Don't wait for an accident to regret it

After getting the medium temperature, pressure, pipe diameter and direction diagram given by the customer, follow this order: first draw the displacement vector diagram to clarify the axial, transverse and angular displacements; Calculate thermal displacement and pressure thrust; Then the structure type is selected according to the working conditions-high temperature axial type for high temperature, large diameter thick wall expansion joint for large diameter, and transverse type of compound hinge for absorbing multi-directional displacement; Finally, check the fatigue life, which is not less than the design life.

Two days ago, a customer used the double straight pipe bypass pressure balance expansion joint on the steam pipeline. As a result, the lateral displacement was not counted. After the thermal expansion of the pipeline, the lateral offset of 15mm was produced, and the expansion joint was hard, so the bellows leaked after three months. You say it was wrong or not? The lateral displacement is only 15mm, and it can be solved by adding a double hinge transverse expansion joint. Why save that money?

Metal expansion joint calculation method? To put it bluntly, there are three parameters: thermal displacement, pressure thrust and fatigue life. If one is wrong, it will wait for rework. Selection is not a chance, it is calculated.

1. Why insulate the metal expansion joint? Not just to save energy

Many people think that the insulation layer for the pipe is just to save some steam money. However, in the metal expansion joint, the significance of heat preservation is far more than energy saving. If you think about it, the core function of the expansion joint is to absorb the thermal displacement of the pipe-the bellows has to expand and contract freely to work. If the temperature is too high, the creep strength of the bellows material (such as 304 or 316L stainless steel) will drop dramatically. For example, when 304 stainless steel is at 550℃, the allowable stress is only about 1/3 of the normal temperature. At this time, the heat insulation is not in place, and the bellows may be fatigued and cracked in advance before the design life.

A more insidious problem is condensation corrosion. If the insulation layer of steam pipe is damaged, the local temperature drops below the dew point, and the water vapor condenses into acidic water, which directly corrodes the bellows wall. I have seen a corrugated expansion joint used in a power station industry in a power plant. Because the outer iron sheet of thermal insulation was not sealed well, the rain seeped in and stuffed, and it rusted out in two years. Therefore, insulation is not only to save coal and gas, but also to protect the expansion joint from heat damage and chemical corrosion. To put it bluntly, if the insulation is not done well, it is only a matter of time before the expansion joint is scrapped.

Second, how to choose insulation materials? Don't let the heat "burn through" your protective layer

When choosing insulation materials, first look at the temperature resistance grade. Don't listen to the manufacturer's bragging that "composite silicate felt can withstand 1000℃", you have to leave an allowance in actual working conditions. For steam pipes (usually ≤350℃), rock wool or aluminum silicate fiber wool is enough, with a density of more than 120kg/m³. However, if it is flue gas and high-temperature gas, such as the kiln tail flue gas connected by the metal corrugated expansion joint in the cement industry, the temperature can reach 600~900℃. At this time, ceramic fiber blanket or aerogel felt must be used. Aerogels have low thermal conductivity and are three times more insulating than rock wool at the same thickness, but they are expensive-you have to calculate the full life cycle cost.

In addition, it should be noted that the insulation material cannot contain chloride ions that are harmful to stainless steel. The chlorine content of some inferior rock wool exceeds the standard, and chloride will precipitate when exposed to water, which will lead to stress corrosion cracking of bellows. I recommend that the chloride ion content test report be required at the time of purchase (standard general requirement ≤25ppm). Let's talk about the actual details: the thicker the thickness of the insulation layer, the better. After the thickness exceeds the critical point, the outer layer dissipates heat and increases the cost. Generally, 100~150mm for steam pipeline and 200~250mm for high-temperature flue gas. When you are not sure, you can find the manufacturer to calculate the heat loss. Don't slap your head.

Third, the easiest pit to step on during construction: the wave section, guide tube and tie rod can't be wrapped

This is the core problem of metal expansion joint insulation. Many people directly wrap the thermal insulation cotton on the bellows, and then pack it with iron sheet-it's done, and the expansion joint is directly wasted. Why? Because the axial expansion and contraction and lateral displacement of the bellows require space. If the insulation layer is stuck between the peaks and valleys, once the pipeline expands thermally, the bellows will be hardly supported by the insulation material, which will reduce the compensation amount at the light and squeeze the corrugated shell into deformation at the worst.

Correct practice: There must be an air gap between the insulation layer and the bellows.How to do it specifically? First, make an independent layer of detachable thermal insulation box (also called thermal insulation cover) outside the expansion joint, make the box body with stainless steel sheet, fill the inside with thermal insulation cotton, and keep a gap of 50~80mm between the box body and the outer wall of the bellows. This insulates without restricting movement. Don't forget the deflector-it is installed inside the expansion joint and is responsible for deflecting the medium and protecting the bellows. The guide tube itself does not need heat insulation, but during construction, do not stuff the heat insulation material into the annular gap between the guide tube and the bellows, otherwise the guide tube will deform, the medium vortex will intensify, and even cause vibration.

And tie rods and nuts. The tie rod on the general corrugated expansion joint and the large tie rod expansion joint is used to bear the internal pressure thrust and cannot be wrapped in the insulation layer. Because the tie rod needs to be checked regularly for looseness (to see if the nut is displaced), if the package is dead, the inspector can't see it at all. The correct way is to expose the pull rod and adjustment nut, and the insulation box only covers the main section of the bellows. Similarly, the reinforcing ribs on the rectangular expansion joint must also leave a heat dissipation gap.

4. What should I do if the insulation layer fails? Look at these three signals for daily inspection

The insulation layer will not break suddenly, and it will wear out slowly. Keep an eye on three points in daily inspection:

• Abnormal increase in surface temperature:Sweep the outer wall of the insulation box with an infrared temperature measuring gun. If the temperature somewhere is more than 20℃ higher than the adjacent temperature, it means that the insulation cotton has fallen off or water has entered. Especially the corrugated section of the high-temperature axial expansion joint is most prone to hot spots.
• Deformation or bulging of outer sheet:If the stainless steel plate used in the insulation box bulges, water may accumulate in it. The expansion of water vapor under heat will push the iron sheet up and aggravate corrosion at the same time. Seeing this situation, quickly take it apart to check whether the insulation cotton is wet.
• Insulation box fixing loose:The expansion joint vibrates when it runs, and the hoops and bolts will slowly loosen. During the inspection, push the insulation box with your hand. If the shaking is obvious, tighten it again.

Expansion joints on steam pipes, recommended once a month; High-temperature flue gas (such as near the desulfurization flue gas baffle door) is best seen every Monday because of the high risk of corrosion. If the insulation layer is found to be damaged, don't drag it, and replace it immediately when the machine is shut down for maintenance. Otherwise, the little labor cost you save won't be enough to pay for an expansion joint.

5. Comparison of insulation schemes under different working conditions: steam, flue gas and high-temperature gas are different

The insulation strategies of different media vary greatly. Let's go through them one by one:

Steam pipeline:The medium temperature is generally 250~400 ℃, and the pressure is medium and low. Insulation scheme recommended: rock wool + stainless steel insulation box, thickness 100~150mm. Note that the steam pipeline has hydrophobic requirements, and a drainage hole (a small hole of Φ 6mm is enough) should be left at the bottom of the insulation box to avoid the accumulation of condensed water. If the external pressure single axial expansion joint is used, the external pressure cylinder also needs to be insulated, but the guide sleeve should be ensured to slide freely.

Flue gas pipeline:Flue gas baffle doors and expansion joints commonly found in power stations and cement industries. The flue gas temperature fluctuates greatly (200~700℃), and contains sulfur and dust. The insulation material must be acid-resistant ceramic fiber with a thickness of more than 200mm. It is recommended to use a double-layer structure during construction: the inner layer is high-temperature resistant ceramic fiber blanket, and the outer layer is waterproof aerogel felt. Non-metallic expansion joints (fabric fiber expansion joints) that are in direct contact with smoke usually do not need external insulation, but rely on their own multilayer insulation structure. However, the metal rectangular expansion joint must be wrapped, and a rain cover should be added outside the insulation box-because the smoke will condense due to the decrease in temperature and corrode the bellows.

High temperature gas (e.g. hot air, inert gas):Temperatures may exceed 900°C. In this case, it is recommended to use aerogel composite insulation structure, or simply make internal insulation-spray high-temperature insulation coating on the inner wall of the expansion joint. However, the internal insulation will affect the installation of the guide tube, so it needs to be specially designed. For example, the double hinge expansion joint of air-cooled island vacuum pipeline, the medium is a mixture of high-temperature steam and air, and anti-freezing should be considered outside, and the thickness of insulation layer may exceed 300mm.

Finally, there is no universal "universal insulation scheme". When selecting, be sure to send the medium, temperature, pressure and environmental conditions of your pipeline to the expansion joint manufacturer, and ask them to help calculate the insulation thickness and structure-don't copy other people's schemes by your own feeling. After all, how to insulate metal expansion joints is ultimately an engineering problem, not a mathematical problem.

Last summer, the desulfurization flue of a coastal power plant was shut down for maintenance. The reason was dumbfounded-the flange gasket was aging, and the bolt preloading force was uneven. The flue gas came out of the gap, and the whole line was forced to land the load. Lao Wang, who is in charge of the maintenance, told me, "The corrugated pipe material cost a lot of money, and the result was planted in the connection method." Indeed, non-metallic expansion joints (fabric fiber expansion joints, rubber compensators, rectangular non-metallic expansion joints) are good at carrying heat expansion and cold contraction in flue ducts and desulfurization systems, but if the connection method is not selected correctly, none of them can run away from air leakage, running and shutdown.

1. Flanged connection: the most common pit, often the deepest

Flange connection is the most commonly used way of non-metallic expansion joints, none of them. Regardless of the siteNon-metallic expansion joint (fabric fiber expansion joint)StillRectangular non-metallic expansion jointThe mating flanges will be equipped when they leave the factory. But when installed on site, three details are prone to rollover.

flange surface flatness— — I have seen the on-site pipe flange tilt up by 3mm after welding, and the workers firmly plugged the asbestos gasket flat. As a result, it leaked one week after it was put into operation. Correct practice: the flatness error of the matching flange is ≤1mm/m, otherwise it should be leveled with sealant or machining.

bolt preload force-Many people think that "the tighter it is screwed, the less it leaks", which may be right for metal pipes, but it is a disaster for non-metal expansion joints. The loop tape (fabric fiber layer) is pressed too tightly by the bolt, the edge stress is concentrated, and it tears in a few months. This site'sRectangular non-metallic expansion jointClearly mark the torque value in the installation manual. For example, the preload force of M16 bolts should be controlled at 80-100N·m. Don't rely on the feel.

Gasket selection— — The flue gas temperature is high and contains sulfur, so ordinary rubber gaskets can't bear it.Rubber PTFE compensatorCommonly used integral flange + PTFE gasket, corrosion resistance and good fit. But note: the gasket should not be too thick, otherwise the bolt will not be compressed firmly; It should not be too thin, otherwise the compensation joint will be easily displaced when vibrating. According to JB/T 12235-2015 standard, the gasket compression ratio is controlled at 25%-30% to be the most stable.

Used in a cement plantNon-metallic expansion joint (fabric fiber expansion joint)Connecting the cyclone outlet, the workers tried to save trouble and screwed all the flange bolts to the maximum torque. As a result, the ring belt tore a hole along the bolt hole, and the smoke was sprayed with dust for half an hour. Later, I changed the preload wrench and never happened again.

2. Connection between clamp and buckle edge: save money? Temperature resistance passes first

Many people like to connect low-pressure large-diameter pipelines with clamps or buckles, which are cheap and convenient to disassemble. But here's the paradox: You think clamps save money and hassle? Wrong, the temperature resistance rating does not match the fastener material, and it will come loose in a few months.

Clamps are mostly used in circular pipes, such as the flue section behind the flue gas baffle door. However, the bolts and spring sheets of the clamps will creep at high temperatures-when the temperature exceeds 200℃, the elastic modulus of ordinary carbon steel buckles decreases by 30%. A stainless steel 304 clamp was used in a desulfurization project. Half a year later, the bolts were loosened due to thermal cycle, and acid fog emerged at the interface. Later, it was replaced with a 310S clamp to stabilize.

This site'sRectangular non-metallic expansion jointThe structure of buckling edge + strip is mostly used: the edge of the expansion joint band is flanged, embedded in the flange groove, and the outside is fixed with an entire strip. This design is particularly friendly to the four corners of a rectangular pipe, and there will be no case where the clamp can't press the R corner. However, if the material of the strip is not selected well, it will also roll over-ordinary galvanized strip will rust and wear in the wet desulfurization environment for three months. It is recommended to use 316L or FRP composite strips.

Have you ever used a failed clamp case on your project? Chat in the comments section.

3. Hot vulcanization bonding and integral molding: Don't be superstitious about the word "sticky"

Rubber non-metallic expansion joints, such as this station'srubber compensatorAndRubber PTFE compensatorThe connection method is commonly bonding or integral vulcanization molding. However, many purchases are biased by sales talk: "We use imported glue, which sticks firmly!"-Not enough bonding strength? That's because you didn't follow the standards.

High temperature flue gas section (> 120℃) must use hot vulcanization process。 The essence of vulcanization is that rubber molecules are crosslinked with reinforcing materials to form chemical bonds, and the peel strength can reach more than 50N/mm. In the normal temperature section (≤80℃), cold adhesion and special primer can be used, and the peel strength can also reach ≥25N/mm as required by JB/T 12235-2015.

I met a customer who used a cold-stick rubber compensator in a hot air duct (180℃) of a power plant, and the bonding surface degummed after two weeks. During the maintenance, it was discovered that the cold adhesive glue softened at high temperature and couldn't hold it at all. So,Not all non-metallic expansion joints can be bonded— — When selecting the model, be sure to ask clearly: What is the medium temperature? Is it frequent start and stop?

Integral molding is to directly mold rubber or PTFE into a compensator with flanges, such as this station'sPTFE compensator, the flange and the corrugated body are formed at one time, there is no bonding interface, and the reliability is higher. But the cost is also high, and it is not suitable for oversized sizes (such as rectangular sections above 5 meters).

4. Special structure: pipe welding and composite seal

In some operating conditions, the standard connection method is simply not sufficient. For example, double seal structure-learn from this siteRound Flap Door (Double Seal)The idea that the non-metallic expansion joint can also be made into two layers of seal inside and outside. The inner layer is covered with fabric fiber skin to absorb displacement, and the outer layer is protected by metal deflector + temperature-resistant sealing strip. This structure is commonly found in high temperature fan inlets and outlets that require zero leakage.

Such asHigh temperature axial expansion jointNon-metal alternative-welded to the pipe with stainless steel through pipe, covered with multiple layers of skin (silicone cloth + ceramic fiber + stainless steel wire mesh), the design logic of layered overlap is simple: each layer is responsible for a function-the innermost layer is insulated, the middle layer is pressurized, and the outer layer is corrosion-resistant. The weld position must be staggered to avoid thermal stress concentration. After one project did so, it was extended from quarterly maintenance to biennial maintenance.

In addition,Double hinge expansion joint for air-cooled island vacuum pipelineAlthough it is mostly used in metal pipes, its hinge compensation concept can also be transplanted to non-metals: the universal joint structure allows the displacement in three directions, and the composite gasket is added at the connecting flange, which has a very stable effect.

To summarize – there is no best way to connect, only the most suitable. If the medium temperature is lower than 150 ℃ and the pressure is slightly positive, the flange connection is sufficient; Large-diameter low-pressure buckling strip; Vulcanization or integral molding for high temperature; If you want zero leakage, go to double seal. This site'sNon-metallic expansion jointCovering fabric fiber, rubber, PTFE and other materials, the connection methods can be customized from flange, clamp to vulcanization bonding. The key is to select according to parameters-the temperature, pressure and displacement are clearly reported, and we will help you match the scheme without running.