Specialized in manufacturing compensators, expansion joints, baffle doors

先搞明白：膨胀节疲劳寿命到底在算什么事？

膨胀节的疲劳寿命，说白了就是波纹管能扛多少次伸缩。每一次伸缩，波纹的波谷和波峰处都会产生一次交变应力循环——材料不停被拉过来、压过去，积累到一定程度，裂纹就出现了。

疲劳寿命从来不是材料牌号单方面决定的。同样的SUS304，波纹几何不同，寿命差出两三倍很常见。波高、波距、壁厚、层数，每一个参数都在改变应力分布。位移形式也是变量——纯轴向拉伸和压缩，跟横向剪切叠加轴向，算出来的应力大不相同。压力波动呢？管内压力每波动一次，波纹壁上就多一层环向应力循环。所以你看，通用型波纹膨胀节和外压单式轴向型膨胀节虽然都叫膨胀节，但一个波纹受内压鼓胀，一个波纹被外压压紧，应力状态完全不同，算疲劳寿命的路径自然也不一样。

安全系数取多少才算合理？

GB/T 12777和EJMA的推荐做法，本质上是用一个大倍数安全系数把理论疲劳寿命往下压。EJMA给出的设计疲劳曲线本身就考虑了约10倍的安全系数，GB/T 12777也沿用了这套逻辑。为什么是10倍？因为波纹管的疲劳试验数据分散性很大，同一种设计、同一批材料，做十次试验，寿命可能从800次到3000次乱跳。安全系数就是用来罩住这个散布的。

但不同工况，安全系数不能一刀切。直埋(全埋)型膨胀节埋地之后根本没法检修换管子，失效就是事故，安全系数取到15甚至20都合理。而高温轴向型膨胀节一旦超过材料的蠕变温度范围——比如304不锈钢在425℃以上——失效模式就从纯疲劳转成了蠕变-疲劳交互，这时候拿常温的10倍安全系数去套，就是在赌命。

疲劳寿命次数怎么取？从设计位移到实际工况的换算

很多人在这一步踩坑。样本或图纸上写的“疲劳寿命1000次”，是基于某一个特定循环位移算出来的。打个比方：设计位移是轴向±20mm，寿命1000次，这说的是在±20mm纯轴向位移下能跑1000次。但实际管道系统里，哪来的纯轴向？热胀冷缩带动管系摆动，轴向位移总是带着横向位移一起过来。

把横向位移按波纹管几何特性换算成等效的轴向位移，两者叠加后得到当量轴向位移，再代入疲劳寿命曲线查次数。注意，疲劳寿命和位移之间的关系不是线性的——位移放大10%，寿命可能掉30%。这就是为什么不能用满样本寿命的原因。电站行业用波纹膨胀节得打个七折八折，复式铰链横向型膨胀节因为承受的是角位移组合，折算更复杂，打折幅度往往更大。

工程上最容易被忽略的坑：温度修正和腐蚀余量

你拿常温疲劳曲线去套400℃的烟气管道，那基本是拿命在赌。材料一上高温，屈服强度往下掉，蠕变开始加速，波纹管每循环一次产生的塑性变形更大，裂纹萌生得比室温快得多。

某水泥厂用通用型波纹膨胀节，设计温度350℃，厂家按常温曲线校核给了2000次寿命。结果现场实际运行到420℃，不到500次循环波纹管就裂了。有人还怪厂家质量不行——其实问题出在使用方否定了温度修正系数。腐蚀环境更狠。烟气挡板门、脱硫烟气挡板门里走的是含硫烟气，氯离子和亚硫酸盐在波纹表面打出点蚀坑，这些坑就是疲劳裂纹的天然起点。腐蚀和疲劳叠加，不是1+1=2，是1+1=5。这种环境下安全系数至少翻倍，再配合导流筒隔热、波纹管外部保温等措施，才有得玩。

疲劳寿命次数到底取多少算合适？

落地建议分两档。常规管道系统，比如热力管道、一般化工管线，取设计寿命的70%~80%作为许用循环次数。样本标1000次，你按700~800次制定检修更换周期，留出安全余量应对工况波动。

关键场合或检修困难的部位，控制线要压得更低。空冷岛真空管道双铰链膨胀节在几十米高的管架上，换一个要搭满堂脚手架；直管压力平衡型膨胀节一旦失效，整个管系的盲板力失去平衡，后果是连锁性的。这类产品按设计寿命的50%甚至更低来控制——不到一半寿命就要编制更换计划。原因很简单：疲劳寿命本身就是统计值，有人用到120%还在跑，有人60%就漏了。你把控制线放低，不是保守，是给不确定性留活路。

选型时怎么和厂家沟通疲劳寿命要求？

别只丢一句“给我寿命长一点的”就完事。厂家的设计工程师不是算命先生，你至少得给他这几样东西：介质温度、循环位移（轴向多少毫米、横向多少毫米，测不准就给管系应力分析报告）、循环频率（一天启停几次，一年多少个热循环）、有没有压力波动，以及腐蚀介质种类和浓度。烟气挡板门这种腐蚀性环境，还要单独声明硫化物或氯离子含量。

参数给到位了，厂家才能把安全系数选到合理的档位，再反推出疲劳寿命次数。你什么都不说，对方只能按“常温、纯轴向、无腐蚀”的通用工况给你配一个通用型波纹膨胀节，到手看着挺便宜，上现场跑两圈就露馅。膨胀节疲劳安全寿命安全系数和膨胀节疲劳寿命次数怎么取，答案从来不在样本上，在你自己的工况数据里。

做采购或者搞设备维护的朋友，十有八九都问过这个问题：哪种非金属补偿器好？直接报型号的，十有八九不靠谱。非金属补偿器这个叫法，底下还分好几种呢，工况不一样，选的东西能差出十万八千里去。

先分清一件事：你说的非金属补偿器，到底是哪种？

别急着比好坏，先看清楚自己管道上要装的是哪一类。工业场景里常见的非金属补偿器，大致分三种：织物纤维膨胀节、矩型非金属膨胀节、橡胶补偿器。名字听着都带“非金属”，但结构、材质、适用范围完全是两码事。

织物纤维膨胀节，也叫非金属膨胀节，主体是用多层纤维织物、密封材料和隔热层复合做成的，柔韧性好，能吸收多向位移，特别耐高温。矩型非金属膨胀节呢，说白了就是矩形截面的织物纤维补偿器，专门用在矩形烟风道上。至于橡胶补偿器，主体是橡胶和纤维增强层，靠弹性变形来减振降噪，耐温和耐压都不如前两种。

所以，“哪种非金属补偿器好”这个问题的前提，是你的工况适合哪一类。选错类型，再好的产品也白搭。烟气管道上用橡胶补偿器，没俩月就烤糊了；水泵接口上用织物纤维膨胀节，振动照样把管道震裂。

看工况选结构：温度、压力、介质定生死

温度和压力是硬指标。织物纤维膨胀节通常能扛住几百度甚至上千度的烟气，但承压能力一般，适合低压大直径管道。橡胶补偿器耐温一般不超过100℃，承压倒是能做得高一些，但介质里有油、酸碱、臭氧什么的，橡胶很快就会老化。

所以烟气管道、脱硫系统、除尘风机出口这些地方，为什么更认织物纤维膨胀节？烟气温度高，含硫有腐蚀，还伴有振动和位移，这活儿橡胶干不了。反过来，水泵进出口、冷水机组、柴油机排气口这些低温但有振动的地方，橡胶补偿器就是天然的对口货。它的阻尼特性摆在那儿，减振效果远好过织物纤维层。

别忽略安装位置和位移方向

同一台设备，装在风机出口和装在烟囱入口，对补偿器的要求完全两回事。风机出口气流不稳，有径向位移，还有一定的压力脉动；烟囱入口那一段，主要是轴向热膨胀，还有可能伴随横向风载位移。你要是拿一个只做轴向补偿的型号硬装在需要横向位移的地方，用不了多久就会撕裂。

矩型非金属膨胀节常见于矩形烟风道，这时候不光要看截面尺寸，还得搞明白设备的热膨胀方向。管道往哪儿伸，补偿器就得能吸收哪个方向的位移。圆形管道呢，轴向还是横向补偿，选型时脑子得清醒。安装空间够不够？法兰连接还是插焊？这些细节，图纸上都得标清楚。

再细看蒙皮材质和层数

织物纤维膨胀节的核心，不是外面那圈金属框，是中间的柔性蒙皮。蒙皮一般分好几层：最外层是耐腐蚀、耐老化的纤维布，中间是隔热层，内层是密封层。层数怎么配，材质怎么选，全看工况。

比如烟气温度600℃，那就得选硅钛玻纤布加高硅氧纤维，再配不锈钢丝网增强。如果介质里含二氧化硫，还得考虑聚四氟乙烯薄膜做防腐蚀层。导流筒要不要加？人家内部有高速含尘气流的时候，导流筒能防止粉尘直接冲刷蒙皮，但加了导流筒又会占用一定的流通面积，还有可能引发振动。这个平衡，只有做过类似案例的厂家才拿捏得准。

选材质别光看耐温表。有的蒙皮标着耐温800℃，实际工况里温度波动大，材料反复热胀冷缩，寿命照样断崖式下跌。所以得问清楚：这个材质在长时间工作温度下的蠕变特性怎么样？有没有做过疲劳试验？

最后才是看厂家

前面几步走完，你已经知道自己需要哪种结构、哪种材质的非金属补偿器了，这时候再去看厂家。不是标着“非金属膨胀节”就能通用的，按JB/T 12235-2015标准做检验，是底线。这个标准对非金属膨胀节的技术要求、试验方法、检验规则都写得明明白白。

除了标准，还得看有没有同行业的实际案例。做过电站锅炉烟风道的，和做过水泥窑尾的，对工况的理解完全不一样。你让他拿两个案例出来，验收记录、运行时间、更换周期，一问便知。

价格这东西，放到最后比。前面结构选错了，材质配低了，买回来装上去就是隐患。你问哪种非金属补偿器好？答案不是型号，是匹配度——工况匹配、结构匹配、材质匹配，再加上一个按标准做事的厂家。这四样凑齐了，你手里那台，就是最好的。

What role does the deflector play in the expansion joint? Why diameter deviation is more critical than you think

The guide tube inside the expansion joint looked like a cylinder, and many people treated it as an ordinary bushing. However, after working in this line for a long time, you will understand that the diameter deviation of the guide tube directly determines that the expansion energy saving can't run a maintenance cycle safely. The primary task of the guide tube is to divert the flow-allowing the medium to pass smoothly through the inside of the bellows, reducing the impact of turbulence and vortices on the corrugations. Secondly, it separates the high-temperature medium from the bellows to avoid the bellows from being directly heated and overheated. Another function is easy to overlook: the guide tube can hold the impurities and welding slag washed down by the tube wall, preventing these things from getting stuck in the gaps of the corrugations.

Therefore, once the diameter of the deflector is ridiculously deviated, trouble comes. The diameter is large, and the gap between the bellows and the inner wall is not enough. As soon as the bellows are compressed, the guide tube directly presses against the corrugated valley, and it will wear out in a few cycles. The diameter is small, the gap is too large, the medium forms a step at the entrance of the bellows, the scour is intensified, and a trench is cut out of the root of the bellows. Do you say this deviation value is critical or not?

Are there any deflector diameter deviations specified in ASME and EJMA standards? Where do the reference values come from

Looking through ASME Volume VIII directly, it only specifies the wall thickness, fatigue life calculation and pressure test requirements of bellows, and does not give specific table-format values for the diameter tolerance of deflectors. The EJMA (American Association of Expansion Joint Manufacturers) standard also lacks a direct provision for baffle tolerances. When many buyers get the drawings, they ask: What is the diameter deviation of ASME expansion joint guide tube? It is indeed the norm that a single citable number cannot be found on standard text.

But don't rush to conclusions, although EJMA does not give tolerance values, it gives the recommended clearance range between the deflector and the bellows inner diameter in the design chapter. According to the empirical data of EJMA, the unilateral gap between the outer diameter of the guide tube and the inner diameter of the bellows is usually 6mm to 12mm, depending on the nominal diameter. The larger the diameter, the clearance is appropriately enlarged. This gap is pushed back, which is the lower deviation range of the outer diameter of the guide tube. The domestic supporting GB/T 12777 and HG/T 20205 also follow a similar idea, and the design institute and the manufacturer follow this interval by default.

Therefore, in actual engineering, the diameter deviation of the guide tube is usually determined by the way of "inner diameter as the benchmark and controlling negative deviation". For example: DN1000 universal corrugated expansion joint, the inner diameter of the bellows is 1016mm, the outer diameter of the guide tube is about 1000mm, the deviation is controlled from 0 to-3mm, and the bilateral clearance is about 16mm. For the large diameter thick wall expansion joint of DN2000, the gap must be enlarged to more than 20mm, and the deviation of the outer diameter of the guide tube should be relaxed to 0 to-5mm.

How to control the deflector diameter deviation at the manufacturing end? Measurement location, method, and common deviation ranges

The most common process for rolling guide tubes in manufacturing plants is longitudinal welding of coil plates, and the circumferential direction is inevitably ellipticity. Controlling the diameter deviation does not rely on the accuracy of the plate coiling machine alone-the welding deformation is often greater than the coiling error. It is common that the longitudinal seam shrinks after welding, and the radius of the guide tube shrinks by 2-3mm near the weld.

To control the deviation, the measurement position must be standardized. According to the recommendation of EJMA, the diameter of the guide tube shall be measured at the end and the section not less than 150mm from the end, and the average value shall be taken in two mutually perpendicular directions for each section. This method can effectively avoid the interference of ellipticity. In actual production, inner diameter micrometer and auxiliary support rod are commonly used to measure large diameter guide tubes, and attention should be paid to avoid the weld area when measuring.

The common deviation range is generally classified into three grades in the industry: ordinary grade 0 to-5mm, which is used for general flue ducts and low-pressure pipes; Precision grade 0 to-3mm for high temperature axial type expansion joints and corrugated expansion joints for power station industry; Special Grade (Matching Machining) 0 to-1mm for use in rotary compensators, vacuum-specific and other clearance-sensitive occasions. You will know by comparison, what kind of precision equipment and what inspection methods are used, and the corresponding cost is quite different.

If the deviation is too small, the bellows will be stuck, and if the deviation is too large, it will accelerate the erosion and wear-the field case tells you how to choose the range

The high-temperature axial expansion joint of a power plant, DN1400, with a design temperature of 560℃, has obvious wear on the bellows after more than one month of operation. Disassembly and inspection found that the inner diameter of the guide tube was too large by about 8mm, the medium formed a sudden expansion at the outlet of the guide tube, and the dusty flue gas directly washed the corrugated root. The deflector was later remade with a deviation of 0 to-3mm, and the wear problem disappeared.

There is also a case in a chemical plant. The medium is a slurry with particles, and the expansion joint model is a general-purpose corrugated expansion joint. The outer diameter of the guide tube is enlarged by 1.5mm, and it can't rotate when installed at room temperature. When the corrugated tube is compressed during pressure test, the end face of the guide tube directly bends the corrugated valley. After rework and removal, the ellipticity of the guide tube exceeded the standard, and the long axis direction was nearly 5mm larger. The manufacturer re-rolls it and strictly controls the deviation before it is solved. When the two cases are put together, the truth is clear: the deviation should leave enough gaps, but it should not be large enough to form a step scouring. What about that? Select the deviation gear according to the medium properties and working conditions, take the intermediate value for dusty medium, take the small gap for pure medium, and make the inlet chamfer of the expansion joint of high-pressure and high-speed fluid.

When signing the technical agreement with the supplier, the deviation of the guide tube diameter must be written in to avoid discord in acceptance

The following four items are clearly written in the technical agreement, which can save ten quarrels during acceptance.

First, give the benchmark size. Make clear whether the inner diameter or outer diameter of the guide tube is the reference, and indicate the design nominal size and deviation range. Don't just write "conforming to EJMA". EJMA doesn't give a value, and everyone will hold their own opinion when the time comes. Write directly: the outer diameter of the guide tube is D =1000mm, the deviation is 0/ -4mm, and the ellipticity is not more than 1/2 of the diameter tolerance.

Second, specify the measurement method. Note that the measurement position is 150mm from the end face, and each section is measured in two vertical directions, except for the weld area. If it is not written clearly, the supplier will take a caliper on the end face and hand it in for inspection, and the measured data can't reflect the true ellipticity at all.

Third, the gap range is determined. According to the one-sided clearance of 6-12mm recommended by EJMA as the acceptance basis, and specify the maximum clearance limit. This is particularly important-the design time gap is 8mm, and as a result, the guide tube is made 13mm smaller, and the scene looks fine. In fact, the risk of scour has doubled.

Fourth, clarify the end treatment requirements. The inlet end of the guide pipe must be chamfered or rounded, and the outlet end should be burred smoothly. The chamfer radius R shall not be less than 2mm. If conditions permit, the hard surface treatment of surfacing welding shall be done. The reason for writing this one is simple, the outlet end burrs and steps are the starting point of bellows wear.

These items are written into the agreement, so suppliers dare not fool them, and inspectors are worried. If you have signed the contract but have not written these, the factory inspection report requires that the measured data be filled in, and then the gap mark on the final assembly drawing is checked. After this set of actions is completed, you will have a bottom in your heart.

How to arrange the bracket of the expansion joint? This question is asked almost every few days. Most of the people who asked were that the pipeline stress calculation had just been completed, and the result was stuck in the bracket step-the bellows model was selected and the compensation amount was calculated. As a result, the bracket layout was wrong, and the expansion joint was selected for nothing.

Fixed brackets and guide brackets. The fixed bracket is responsible for dividing the pipe system into several independent pipe sections, each section can digest its own thermal displacement, and prevent force from being transmitted randomly to equipment, valves or weak points. The transverse displacement of the guide bracket tube ensures that the tube goes in a straight line when it expands and contracts, and the bellows is not screwed as a universal joint. With the cooperation of the two, the bellows can honestly do the expansion and contraction it should do. Mixed? The bellows cracked when twisted, the fixed bracket was crooked, and the thrust pushed to the place where it shouldn't have been pushed. There were many accidents.

How to fix the distance of the fixed bracket? Don't slap your head

The fixing bracket is the anchoring point of the pipe system. When the spacing is large, the thermal elongation of the pipe between the two anchor points exceeds the absorption capacity of the expansion joint; The spacing is small, the number of brackets is too large, and the cost can't be suppressed. How does that count? Looking at the type of expansion joint, compensation amount and pipeline stress calculation results, all three are indispensable.

The spacing between the fixed brackets of the universal corrugated expansion joint (axial type) is usually divided according to the natural compensation ability of the pipe system. The natural compensation is not enough, so it is considered to absorb with expansion joints. The hinge-type and compound-hinge transverse expansion joints with tie rods are different-their compensation ability comes from angular displacement and transverse displacement, and the spacing of fixed brackets should be checked according to the length of reverse thrust arm. To put it bluntly, the bellows is subjected to lateral thrust and bending moment in addition to axial force. When the bracket distance changes, the force arm changes.

Someone asked, are there any experience points? Yes, but don't use experience points as a master key. For small-diameter steam pipes below DN300, the distance between fixed brackets is 10 meters and 20 meters. Large-diameter high-temperature pipeline, five or six meters have to be set up. Why? The expansion amount is large, the wave number of the bellows is large, the stiffness is low, and the spacing is slightly enlarged, and the middle pipe section sinks. Calculate the pipe stress, and there are any answers.

The spacing and position of the guide brackets is more specific than you think

The guide bracket is too close to the expansion joint, and the bellows cannot absorb the lateral displacement; Too far, the pipe is unstable. How to handle this sense of proportion? The distance between the first guide bracket and the expansion joint is generally calculated according to 4D (D is the nominal diameter of the pipe), and the second guide is controlled according to 14D. For example: In the pipeline of DN200, the first guide bracket is about 800mm away from the expansion joint, and one is set every 2.8 meters behind it. However, note that this is only the recommended value of the general axial expansion joint-specific to the products of a certain manufacturer, the rigidity, pressure thrust and fatigue life of the bellows are different, so it is best to check the limit value in the product sample.

The role of the guide bracket is to give the tube a sliding constraint so that the thermal displacement goes in the intended direction. The pipe cannot swing laterally at the guide bracket, but it should be free to expand and contract in the axial direction. The most common mistake at the site is that the guide bracket is used as a fixed bracket-the angle steel is welded to death, the axial direction can't move, the thermal expansion and contraction forces are all suppressed on the bellows, and the pipe wall is pushed to bulge. On the other hand, the distance between the guide brackets is enlarged, the pipe shakes laterally, one side of the bellows is stretched and the other side is compressed, and the fatigue life drops directly from a cliff.

The bracket arrangement logic is completely different for different expansion joints

The single axial expansion joint is guided by the pipeline itself, and the bracket arrangement is the most conventional; But what about replacing it with a double straight pipe bypass pressure balanced expansion joint? It comes with its own balance structure, the thrust on the fixed bracket is small, and the bracket can be made lighter. The pressure balance expansion joint of curved tube is suitable for the elbow, and it can balance the internal pressure thrust itself, so the load of the fixed bracket is mainly friction.

It is an iron rule that hinged expansion joints must be used in pairs. Single hinges can only absorb angular displacement, and pairs can form lateral displacement compensation. The bracket must withstand the internal pressure thrust-the hinge structure can balance the internal pressure blind plate force, but the external load must be carried by the bracket. It would be a joke for someone to take the bracket scheme of the universal expansion joint to set the rotary compensator. The rotary compensator absorbs the displacement by the packing seal and the rotation of the rotating cylinder. The bracket layout pays attention to making the pipe rotate around the rotation center, which is completely different from the force logic of the bellows expansion joint.

The Three Most Common Mistakes in Real Cases

First, the fixing bracket is not welded firmly. Two days ago, I met a customer. During the pressure test of the pipeline, the whole bracket was pushed away, the bellows was pulled in a straight line, and the compensation amount was all gone. Check the reason, the welding length between the bracket and the embedded parts is not enough, and the height of the weld seam also shrinks. The internal pressure thrust is hundreds or thousands of kilograms, and the welding joint is not solid, so it is strange that the glue is not opened.

Second, the spacing between the guide brackets is too large. The waste heat power generation pipeline of a cement plant, DN500, and the guide brackets are set at an empirical spacing of 8 meters, nearly double that of 14D. After three months of operation, the bellows was twisted and deformed, and the lateral displacement pushed the bellows out of the pit. When I remove it, I see that the trough is full of fatigue cracks.

The third is to ignore cold tightness or pre-displacement. After the pipe system is cold tight, the bellows will have an initial displacement. The bracket was welded according to the position after cold tightening. As a result, as soon as the operating temperature rose, the displacement direction was opposite to the design, and the initial force of the bracket exceeded the standard. The squeak is considered light, and it is not uncommon for the bellows to be squeezed out of the tie rod limit.

So how to arrange the bracket of the expansion joint? There is no unified formula, but there is a main line: first distinguish the responsibilities of the fixed bracket and the guide bracket, and then determine the spacing and position according to the type of expansion joint, compensation mode and pipeline stress calculation result.

Find out first: What exactly does the "delivery status" of the expansion joint mean?

It's not simply the four words "do a good job of packing" and it's done. The delivery status of the expansion joint is a complete set of technical definitions: whether the bellows is pre-deformed, where the tie rod nut is locked, whether the direction of the guide tube is correct, how to protect the flange sealing surface, and whether the transportation tool is assembled or disassembled. These details are bundled together, which determines whether you can hoist and weld directly after receiving the goods, or you have to squat on the site to do a bunch of pretreatments first.

Two days ago, I met a customer and received a batch of general-purpose corrugated expansion joints. I opened the wooden box and welded it. Halfway through the welding, it was found that the bellows was crushed to death, and the tie rod was still locked tightly-the manufacturer issued the transportation protection state, which should have loosened the nut and adjusted the pre-deformation before installation. And the result? As soon as the welding heat comes up, the bellows bulges directly. Who takes the blame? The delivery status was not clearly written in the contract, so the manufacturer sent it in the safest way, and no one understood it on the spot. The final construction period was delayed for one week.

What are the common delivery states?

Free Condition, Pre-Stretched/Pre-Compressed Condition, With Limit Tie Rod Condition, Overall Delivery and Part Delivery。

• Free state: The bellows has no artificially applied displacement and the length is equal to the design length. Most general-purpose corrugated expansion joints are sent by default, because they are safest and are not afraid of accidental collisions during transportation.
• Pre-stretched/pre-compressed state: According to the cold tightness of the pipe, the bellows is pre-elongated or compressed for a section, and then locked with tooling. It is commonly used in corrugated expansion joints in power station industry, or straight pipe pressure balance expansion joints that need to absorb large displacements.
• State with limit tie rod: The tie rod nut is locked to limit the bellows from shaking during transportation. But there is a pit here-some tie rods are for transportation protection, and they must be disassembled after installation; Some tie rods are working components, such as the tie rods of external pressure single axial expansion joint and double hinge transverse expansion joint, which are used to restrain axial force or distribute transverse displacement. If you dismantle them, you will have a big problem.
• Block Delivery vs Part Delivery: Small calibers are usually delivered as a whole; Large diameter thick wall expansion joints or non-metallic expansion joints may be shipped separately and reassembled on site. When the parts are delivered, flange bolts, gaskets and fabrics have to be counted separately, and one less is troublesome.

Why does the manufacturer send free state by default? Because the free state has the widest adaptability surface, the site can be adjusted according to the actual cold tightness value. However, some working conditions must be pre-deformed: for example, the design temperature of the pipeline is 500°C, the installation temperature is 20°C, and the thermal expansion is very large. If the bellows is not pre-stretched during delivery, the bellows may be pushed to the limit after operation, and the life will be greatly reduced.

How is pre-stretching and pre-compression achieved? Is the tie rod disassembled or not?

By tie rod nuts, or special tooling. The manufacturer uses a jack or screw to pull the bellows to the specified displacement before leaving the factory, and then locks it with a tie rod nut. On delivery, the tie rod has a lead seal or a yellow warning label that says "loosen nut before installation" or "strictly prohibited removal". But you must not just look at the label, you must look at the structure.

How to identify? Remember one thing:There are usually only one or two transportation protection rods, which are slender and directly connected to the flanges or connecting pipes at both ends, and the bellows cannot move at all after the nut is tightened; Working tie rods tend to be multiple uniformly distributed rods with spherical washers or spherical bearings that allow the bellows to generate lateral displacement.Take a practical example: the tie rod of the transverse expansion joint of a compound hinge. If you remove it, the hinge structure will lose its constraint, and the transverse displacement will twist the bellows. However, the short tie rod that comes with the general corrugated expansion joint from the factory, if it is not disassembled after installation, the expansion joint cannot be deformed at all, which is equal to a rigid short tube.

How to check the delivery status during acceptance?

• nameplate: Model, nominal diameter, pressure, displacement, factory number. Focus on whether there is a label "pre-stretch xx mm" or "cold tightness xx mm".
• Certificate of Conformity: Material, welding process, inspection date. Compare the contract to see whether the bellows material is 304 or 316L. Don't find the wrong delivery after a few days of rust.
• appearance: Whether there are bruises, scratches and pits in the bellows, whether the flange sealing surface is bumped, and whether the direction of the guide tube is consistent with the medium flow direction (the direction of the arrow must point to the medium flow direction). Flexible products such as non-metallic expansion joints and rubber compensators also depend on whether the fabric or rubber has wrinkles and cracks. They are easier to deform during transportation, so they must be laid flat or hung, and cannot be stacked.

Whether the tie rod nut is loose is also a key check item. If the nut is loose during transportation and the pre-deformation amount runs away, you have to measure the actual length of the bellows and compare it with the installation length marked on the nameplate. If the difference exceeds 5mm, don't hard install it, contact the manufacturer to readjust it.

Relationship between delivery status and installation status

Don't think that you can weld it directly when you receive the goods. Especially for corrugated expansion joints used in power station industry, pipelines often have strict cold tightness requirements. For example, the main steam pipeline has a designed cold tightness value of 30mm. When the manufacturer delivers the goods, it is pre-stretched by 20mm. When the ambient temperature changes during on-site installation, you may have to pull another 10mm. If there is no adjustment allowance reserved at the time of delivery, or the tie rod has been locked to the limit, then the scene is blind.

Look at the cold tightness table of the design drawing first, then measure the actual length of the arriving expansion joint, and then correct the cold tightness value according to the installation temperature. This process is called "secondary cold tightening". Some expansion joints have adjustment marks when they leave the factory, such as engraving lines on the tie rod to tell you how much displacement each turn of the nut corresponds, which is for the field micro-tuning.

Status of delivery clause written into the purchase contract

To avoid scrambling, these five articles must be clearly written in the contract:

• Delivery status: free state or pre-stretched/pre-compressed state, what is the amount of pre-deformation in mm.
• Nature of tie rod: whether it is a transportation protection tie rod (removed during installation) or a working tie rod (strictly forbidden to remove), and whether there is any mark.
• Shipping protection: with or without additional support, anti-collision wooden frame, bellows sheath.
• Packing Method: Wooden Case, Naked Package, Pallet. If the large-diameter thick-walled expansion joint is naked, the outside of the bellows must be wrapped with a protective pad, and the bolts on the flange surface should be coated with rust-proof oil.
• Storage period: Non-metallic expansion joints and rubber compensators are stored for more than 6 months, the rubber may age and must be turned over or deflated; If metal expansion joints are stored outdoors for more than one year, check the bellows for rust.

Don't bother, these terms would rather be written in detail than slapped on the head on the spot. And guess what? Many procurement disputes, finally check the contract, it says "delivery according to the manufacturer's standard"-what is the manufacturer's standard? Of course, people come by the most hassle-free.

Attached: Common delivery status comparison table

• Free stateApplicable to universal corrugated expansion joint and rotary compensator; Delivery length = design length; The site needs to be adjusted by itself.
• Pre-stretched state: Applicable to corrugated expansion joints and straight pipe pressure balance expansion joints for power station industry; Delivery Length> Design Length; Retest according to the requirements of cold tightness on site.
• Pre-compressed stateApplicable to working conditions with large axial compensation requirements; Delivery length
• With limit tie rod (transport protection): Suitable for long distance transportation; Must be loosened and removed before installation.
• With working tie rodApplicable to external pressure single axial type expansion joint and double hinge transverse type expansion joint; It is strictly prohibited to remove, but the nut can be adjusted to adjust the displacement distribution.
• Overall deliverySmall-diameter expansion joint, metal hose; Install as soon as it arrives.
• Parts deliveryLarge diameter non-metallic expansion joint, rectangular expansion joint; It needs to be assembled on site, and the parts list should be checked.

In what state the expansion joint is delivered, this question should not be remembered on the day of arrival. Before signing the contract, throw the real needs of your site to the manufacturer-free state or pre-stretch, how to deal with the tie rod, and the packaging is not in place. It is much more reliable to say it once than to call for help when you arrive at the scene.