Specialized in manufacturing compensators, expansion joints, baffle doors

先别急着动手——烟道膨胀节焊接到底难在哪？

唉，说到烟道的膨胀节怎么焊接，很多人第一反应就是：不就是焊个接缝嘛，有啥难的？你要是真这么想，那就等着翻车吧。烟道这东西，温度高、介质脏、应力还复杂，你焊的膨胀节要是漏了气，整个系统都得跟着遭殃。前两天还有个客户跟我抱怨，说他们厂里一个高温轴向型膨胀节焊完没两个月就裂了，拆下来一看，焊缝全成渣了。为啥？因为焊之前就没搞明白这块膨胀节到底要扛什么工况。

烟道膨胀节焊接的难点，说白了就三个字：热、腐蚀、应力。高温烟气一冲，普通焊缝的金相组织分分钟就变了；硫化物、氯化物再一泡，裂纹想拦都拦不住。还有管道振动和热胀冷缩产生的位移，焊缝要是没留够韧性，硬碰硬的结果就是撕裂。所以啊，别把它当成普通结构件焊接，你得当成压力容器焊接来对待。

焊前准备里最容易被忽略的那几样东西

很多老师傅一上来就开焊，结果焊完才发现坡口没清理干净、母材厚度不对、或者焊条压根儿就不匹配。你猜怎么着？返工都算轻的，设备报废才叫心疼。

第一，坡口形式和尺寸。烟道的壁厚通常不薄，尤其配电站行业用波纹膨胀节或者大口径厚壁膨胀节的时候，坡口必须开到位。U型坡口还是V型坡口，得根据板厚和焊接位置定。别省那一步，偷懒开个直边，熔深不够，后期必裂。

第二，清洁度。烟道内壁经常附着油污、锈蚀、积灰，不清理直接焊，气孔、夹渣全来了。有个笨办法但最管用：用磨光机把坡口两侧各20mm区域打磨出金属光泽，再用丙酮擦一遍。别嫌麻烦，这一步能省掉一半的返工。

第三，焊材选择。普通碳钢焊条？不是不行，但得看烟气温度。超过400℃的区域，建议用耐热钢焊条，比如R系列。要是烟气里还有腐蚀性成分，那就得上不锈钢焊条，或者考虑衬四氟金属软管那种思路的防腐方案。别忘了，膨胀节本身材质可能是304、316L甚至更高等级的合金，焊接材料必须与母材匹配，否则出现异种钢焊接问题。

焊接工艺怎么选？别死磕一种方法

有人觉得手工电弧焊万能，有人觉得氩弧焊才高级。啧，其实没有绝对的好坏，只有适不适合。

手工电弧焊灵活，现场条件差的时候用得最多。但缺点也明显：效率低，对焊工手法依赖大，而且烟道这种位置，仰焊、立焊很常见，姿势一别扭，成型就难看。

氩弧焊更适合薄壁膨胀节和打底焊，比如通用型波纹膨胀节或者金属矩形膨胀节的对接缝。氩弧焊打底能保证根部熔透、背面成型好，后面用焊条盖面，这种组合很常见。

埋弧焊呢？适合直线长焊缝，比如大型烟道的纵缝。但烟道膨胀节往往是环形接头，埋弧焊未必好使。

打底用氩弧焊，填充和盖面用手工电弧焊。要是条件允许，内壁再做个氩弧焊重熔，把焊缝余高修平，能大大减少烟气冲刷带来的局部腐蚀。至于非金属膨胀节(织物纤维膨胀节)，那压根儿不用焊，但它的金属连接法兰焊接工艺一样得按这个思路走。

焊完了不等于完事了，检验和热处理你得盯住

焊完一看外观挺漂亮，就急着装上去？别急，暗伤还没查呢。

先做无损检测。烟道膨胀节属于承压部件，至少要做液体渗透（PT）或者磁粉（MT）检查表面裂纹。如果压力高或者介质危险，还得上射线（RT）或者超声（UT）。你想想，一个脱硫烟气挡板门后面接的膨胀节，要是焊缝漏了，整个脱硫系统都得停机。

再谈热处理。这个很多人不当回事。烟道壁厚超过20mm的碳钢，或者厚壁的合金钢，焊后必须做消应力热处理。否则内应力积累起来，哪天突然冷启动，焊缝当场就崩。有些高温轴向型膨胀节的制造标准里明确写了热处理参数，你照着做就行。别偷懒，热处理不到位，焊接质量折半。

另外，别忘了尺寸复测。焊完的膨胀节，波高、波距、总长都可能因为热变形而改变。尤其是复式铰链横向型膨胀节这种结构复杂的，焊完之后必须用样板核对几何尺寸。超出公差？那就冷矫正，不行只能报废。

那些老师傅踩过的坑，你最好提前知道

最后一个环节，聊聊经验。我整理了几个高频翻车现场，你对照着避雷。

• 坑一：焊条不烘干。尤其低氢焊条，不烘干直接焊，焊缝里全是氢致裂纹。烟道振动一来，裂得你措手不及。
• 坑二：焊接顺序乱来。膨胀节的波纹部分特别娇气，要是先焊了端部再焊中部，热收缩把波纹都拉变形了。正确顺序是从中间向两端对称焊，或者分段退焊。
• 坑三：忽略刚性固定。烟道管道本身有支撑，但焊接时膨胀节不能受额外拉力。用拉杆或者临时夹具把膨胀节预拉伸到安装长度，焊好再松开。不然焊完发现拉不动，或者运行时被硬扯坏。
• 坑四：钱没花在刀刃上。有些项目图便宜，用普通碳钢膨胀节替代不锈钢波纹膨胀节，结果焊完一年锈穿。该用直埋(全埋)型膨胀节的地方就别省不锈钢焊丝的钱。

说到底，烟道的膨胀节怎么焊接？别把它当成一刀切的问题。从前期工况评估，到焊材、坡口、工艺、检验，每一步都得较真。你把这些细节摸透了，焊出来的东西才能扛得住高温、顶得住腐蚀、经得起振动。要是心里还没底，翻翻咱们站里那些产品资料——通用型波纹膨胀节、高温轴向型膨胀节、电站行业用波纹膨胀节，每种产品的安装指南里都藏着焊接要点。多看多问，少走弯路。

检修烟道膨胀节怎么安装？师傅手把手教你，别踩坑

前两天有个老客户打电话过来，说他们厂里烟道膨胀节漏气了，换了新的还是不行，问我到底咋回事。我让他把安装过程的照片发过来一看——好家伙，安装方向都搞反了！这玩意儿要是装错了，不漏气才怪呢。

今天就跟大伙儿聊聊检修烟道膨胀节怎么安装。这活儿看着简单，但坑是真不少，我踩过的、见过的，都给你掰扯清楚。

一、扒一扒安装前那些容易忽略的准备工作

很多人觉得换膨胀节嘛，把旧的拆下来、新的怼上去就完事了。屁嘞！准备工作没做好，后面全白干。

工具这块：除了常规的扳手、撬棍、葫芦，你最好备一把游标卡尺和塞尺。为啥？新膨胀节的法兰面有没有变形，你得量一下；安装间隙是不是均匀，塞尺一塞就知道。别嫌麻烦，后面调试的时候你会感谢我的。

材料呢：密封垫片一定要对路。烟道温度高、介质有腐蚀性，普通橡胶垫撑不了多久。像我们常用的非金属膨胀节(织物纤维膨胀节)配的是专用耐温垫片，别贪便宜用通用货。还有螺栓，一定要买全螺纹的，半螺纹的拧不紧。

现场条件这个最容易翻车。旧膨胀节拆之前，先看看周围的管道支架有没有松动，烟道里有没有积灰。前两天一个工地，拆旧膨胀节的时候支架突然垮了，差点砸到人。所以啊，先检查支撑结构，该加固加固，安全第一。

二、拆旧膨胀节的时候，这几个细节不注意，后面有你哭的

拆旧件看着是力气活，其实特别考验细心程度。

首先，标记法兰的安装位置。拿记号笔在旧膨胀节的法兰外缘画个十字线，对应到管道法兰上也画上。这样装新件的时候对中就方便多了，不用重新打镙孔。

其次，注意导流筒的方向。很多膨胀节带导流筒，尤其是电站行业用波纹膨胀节和水泥行业金属波纹膨胀节，导流筒是冲着介质流向的。拆之前看清楚、拍个照，新件装的时候别装反了。

还有一个坑——拉杆和限位装置。拆之前看看拉杆螺母的位置，最好量一下螺杆露出的长度。如果新膨胀节不带预拉伸，你以后还得根据这个数据来调。

三、新膨胀节怎么装？对中、预拉伸、螺栓紧固，一步都不能省

新件到了，别急着往上怼。先核对型号尺寸——我见过把金属矩形膨胀节装到圆形管道上的奇葩事，你说这能不漏？

对中：用你之前画的十字线对齐法兰。如果管道有偏差，用葫芦慢慢拉，别用撬棍硬别。硬别会把膨胀节的波纹管拧出裂纹，特别是高温轴向型膨胀节，材料本来就脆。

预拉伸：大部分烟道膨胀节设计时考虑了热膨胀，所以安装温度下要预拉伸。怎么拉？看产品说明书上的压缩量或拉伸量。比如环境温度20℃，设计温度300℃，厂家给的预拉伸量是5mm，你就得用专用工具（或者简单点用螺栓+垫片）把波纹管拉开5mm再固定。这一步很多人忽略，结果运行时膨胀节被拉坏或压坏。

螺栓紧固：别一次性拧死！先对角预紧，再分两到三次均匀用力。用扭力扳手按厂家给的扭矩值拧，别凭手感。我见过一个老师傅，劲大，把螺栓拧滑丝了，法蓝面都压变形了。你要是没扭力扳手，至少分四次对角拧，最后再检查一遍。

四、装完别急着走人，调试和检查才是见真章的时候

螺栓紧完不代表完事了。你猜怎么着？我有一回装完没检查，第二天运行时拉杆螺母松了，膨胀节直接脱开，烟道漏得一塌糊涂。

第一步：拆除运输固定件。很多新膨胀节出厂时会有临时加固的钢筋或螺栓，装完必须全部拆掉，不然影响位移补偿。

第二步：检查所有螺栓的松紧度。尤其是高温工况，建议装完后24小时再复紧一遍，因为受热后螺栓会松弛。

第三步：检查周围管路是否受力。膨胀节装好后，用手推推旁边的管道，看看有没有憋劲。如果有，说明安装时产生了额外应力，要调整支架。

第四步：试压或试运行。对于脱硫烟气挡板门这类系统，通常要做气密性试验。通低压气体，在法兰连接处刷肥皂水，看有没有气泡。这是最直观的检漏方法。

五、常见翻车现场盘点：膨胀节卡死、漏气、安装方向搞反，你中过几个？

我整理了几个典型的翻车案例，对号入座看看你踩过哪个坑。

• 翻车一：膨胀节卡死，补偿功能失效
  原因：安装时预拉伸量没控制好，或者管道支架卡住了膨胀节的活动端。解决办法：拆开重新调整预拉伸量，检查支架是否灵活。
• 翻车二：法兰处漏气
  原因：垫片没放正、螺栓没拧均匀、或者法兰面有凹坑。补救办法：拆开检查法兰面，必要时车一刀；换用耐温耐腐蚀的垫片；按对角顺序重新紧固。
• 翻车三：安装方向搞反
  这是最低级也最容易犯的错误。尤其是非金属膨胀节，有些有气流方向标志，安装时对着介质流箭头即可。如果没箭头，导流筒一定是朝向介质流入的方向。我见过有人把导流筒装反了，介质直接冲刷波纹管，没几个月就磨穿了。
• 翻车四：忘了拆运输支撑
  有些人把膨胀节装上去，却忘了拆里面的临时支撑螺栓。运行时波纹管无法伸缩，硬生生拉裂。拆包的时候一定要把所有的运输固定件清点出来。

说了这么多，其实核心就一句话：检修烟道膨胀节怎么安装？别图快，按流程走，每一个细节都当成大事来办。你要是自己没把握，我们也有技术支持，随时可以咨询。毕竟这东西装好了能用个十年八年，装砸了三天两头漏，哪个划算你心里有数吧？

In the flue system of power plant, metallurgy, chemical industry and other industries, the expansion joint skin, as the core component of non-metallic compensator, has been in the high temperature flue gas environment for a long time. Its fire-proof performance is directly related to equipment safety and personal safety. So, is the flue expansion joint skin fireproof? The answer is yes-the flue expansion joint skin produced by regular manufacturers has excellent flame retardant performance, and some products can reach Class A fire protection standard. This paper will systematically analyze the fire resistance grade, flame retardant mechanism and selection points of skin materials.

1. Flame retardant properties of skin materials

Is the flue expansion joint skin fireproof depends on the composition of its composite material. The skin of non-metallic expansion joint is mainly made of multi-layer soft composite materials, which has many advantages such as wide temperature resistance range, high pressure resistance, strong corrosion resistance, good flame retardancy, sound absorption and shock absorption, good flexibility, etc。

1.1 Fire resistance of base fabric material

The main substrate of the skin is glass fiber cloth, and glass fiber itself is a non-combustible material. The fiberglass cloth maintains good fire resistance after being coated with silicone rubber or fluororubber. The flame retardant grade of silicone cloth material can reach fire resistance grade A, which meets the requirements of GB8624-2006 and German DIN4102 A1。 Glass fiber cloth is used as a base cloth to produce silicone gel cloth by coating or calendering. It is a high-performance and versatile composite material that can be used for a long time between low temperature-70℃ and high temperature 230℃。

1.2 Combustion performance of silicone rubber coating

The fiberglass cloth coated with high temperature curing silicone rubber has the following behaviors after combustion: smokeless, odorless, quick extinguishing, ash whitening, and longer service life。 This feature makes the skin not burn continuously when encountered with open flame, and has self-extinguishing ability.

1.3 Fire protection requirements in actual procurement

Judging from the actual procurement technical specifications, the power plant has clear requirements for the fire resistance performance of the skin. It is clearly stipulated in the procurement technical conditions of a large power group that the fire retardant grade of the expansion joint skin should reach UL94-V0 (the highest grade)。 This grade requires the material to self-extinguish within 10 seconds of leaving the fire in a vertical combustion test without combustion drips.

2. Multi-layer structure and fire protection design of skin

The answer to whether the flue expansion joint skin is fireproof also depends on the design of its multilayer composite structure. A typical skin consists of the following functional layers:

Taking the non-metallic expansion joint of the rear flue of a power plant as an example, its skin adopts 8-layer composite structure, which clearly requires flame retardant, high temperature resistance of 200℃, acid corrosion resistance and good seal。 The fireproof and flame retardant grade of zirconium-containing aluminum silicate needle blanket also reaches UL94-V0 standard。

2.1 Special configuration for high temperature operating conditions

For high-temperature flues with temperatures exceeding 400℃, silicone rubber coating alone is no longer enough to guarantee long-term fire safety. At this time, it is necessary to set a heat insulation layer inside the expansion joint, and use high-temperature composite materials such as fluororubber cloth, polytetrafluoroethylene membrane, glass fiber cloth and ceramic fiber cloth in combination to achieve high-temperature resistance, aging prevention and heat insulation。 The ceramic fiber material can withstand high temperatures above 800℃ and is completely non-flammable。

3. Comparison of fire resistance performance of different types of skins

When answering whether the skin of the flue expansion joint is fireproof, you need to distinguish between different materials:

3.1 Silicone rubber skin

• The long-term working temperature is ≤250℃, and it can reach 350℃ in a short time
• Flame retardant grade up to Class A, self-extinguishing off fire
• Good economy, suitable for conventional flue

3.2 Fluororubber Skin

• Temperature resistance 200~300℃, better corrosion resistance than silicone rubber
• Also have excellent flame retardant properties
• Especially suitable for corrosive environment such as desulfurization system

3.3 Composite high temperature resistant skin

• Contains ceramic fiber layer, and the fire resistance temperature can reach 800~1200℃
• Optimum fireproof performance, suitable for high temperature sections such as boiler outlet
• When the flue gas temperature is 1000℃, the outer skin can still work normally

4. Practical significance of skin fire prevention

Whether the skin of flue expansion joint is fireproof is not only a problem of product performance, but also related to:

4.1 Preventing the spread of fire

In the case of a fire in the flue system of power plants and chemical plants (such as carbon deposit combustion and combustible gas deflagration), the non-flammable or flame-retardant skin can effectively prevent the fire from spreading along the flue and buy time for emergency response.

4.2 Eliminate the risk of combustion drip

The UL94-V0 rating requires the material to burn without dripping. Ordinary plastic materials will drip molten matter when burning, which may ignite the equipment below or cause people to burn. However, the silicone rubber/fluororubber composite material is carbonized and does not drip during combustion, which is more safe。

4.3 Meet fire acceptance requirements

The fire protection acceptance of industrial buildings has clear provisions on the fire protection grade of pipeline insulation and sealing materials. The selection of flame-retardant skin is a necessary condition for enterprises to pass fire inspection.

V. Suggestions on selection and use

In order to ensure that the flue expansion joint skin is fireproof to get a positive answer, attention should be paid to the following in the selection and use:

5.1 Confirmation points when purchasing

• Verify that the temperature requirements of the operating condition are met
• View product structure layers and material description

5.2 Installation Precautions

• The skin surface coating shall not be damaged during installation
• When welding the end pipe, cover the skin with asbestos cloth to prevent burning by welding slag
• Ensure that the guide tube is installed correctly to avoid high-temperature smoke directly washing the inner layer of the skin

5.3 Operation and Maintenance

• Check the skin surface regularly for any signs of carbonization, cracking and burning
• If the flame retardant coating is found to be aging and falling off, it should be replaced in time
• Over-temperature operation will accelerate material aging, smoke temperature should be strictly controlled

VI. SUMMARY

Is the skin of flue expansion joint fireproof-regular products have excellent flame retardant properties. The core conclusions are as follows:

• Material is the key: silicone rubber/fluororubber coated glass fiber cloth as the base material, glass fiber is non-flammable, the coating layer is self-extinguishing off fire
• High temperature requires composite structure: working conditions above 400℃ need to be equipped with non-combustible insulation layer such as ceramic fiber, and the fire resistance temperature can reach 800-1200℃
• Required inspection report for procurement: Suppliers shall be required to provide fire rating inspection report during model selection to ensure that products meet fire protection specifications

Therefore, as long as you choose the products produced by regular manufacturers and meet the relevant fire protection standards, the flue expansion joint skin has reliable fire protection performance. However, it should be noted that long-term overtemperature operation will accelerate the aging of materials, resulting in the decline of flame retardant performance. In daily operation, the flue gas temperature should be strictly controlled within the allowable range of the skin, and the aging products should be regularly inspected and replaced.

In the installation or maintenance of flue system, the welding of expansion joint is the key process to determine the sealing performance and service life. Unqualified welding quality will lead to leakage, deformation and even early failure of the expansion joint. However, many on-site construction workers have misunderstandings about how to weld the expansion joint of the flue-the expansion joint is welded equally with the ordinary flue pipe, which causes serious consequences. This paper will systematically explain the standard welding method of flue expansion joint from four aspects: welding preparation, process parameters, operation steps and quality inspection.

I. Preparation before welding

Before discussing how to weld the expansion joint of the flue, an important principle must be clarified first: it is strictly prohibited to pass welding current through the flexible components of the expansion joint. Whether it's a metal bellows or a non-metal skin, the welding current can cause irreversible damage to it.

1.1 Ground Wire Location Selection

• The ground wire must be clamped on the flue body on the same side as the part to be welded and must not be connected to the other side across the expansion joint.
• For metal bellows expansion joints, the grounding wire cannot be clamped on the bellows crest, otherwise the arc will ablate the thin wall of the bellows.

1.2 Protection of flexible elements

• Non-metallic expansion joint: When welding end pipe or flange, the skin surface must be completely covered with asbestos cloth or fire blanket to prevent welding slag from splashing and burning the fabric layer.
• Metal expansion joint: Cover the bellows trough with thick cardboard or rubber sheet to prevent welding slag from embedding in the corrugated gap.

1.3 Group-to-Size Review

• Before welding, measure whether the actual length of the expansion joint is consistent with the designed cold length (non-metallic expansion joint needs to be pre-compressed by 5% ~8%).
• Check the concentricity of the pipes (or equipment interfaces) at both ends; the deviation shall be ≤3mm.

2. Welding process of metal expansion joint

There are usually two ways to connect the expansion joint of metal bellows with the flue: flange connection and direct welding. How to weld the expansion joint of the flue is mainly aimed at direct welding.

2.1 End pipe welding

Both ends of the metal expansion joint come with end pipes (short joints), and only butt welding of the end pipe and the flue pipe is necessary during construction.

Welding process parameters (taking carbon steel flue, thickness 8mm as an example):

• Welding method: Manual arc welding (SMAW) or CO₂ gas shielded welding (GMAW)
• Electrode/Wire Model: E5016 (J506) or ER50-6
• Electrode diameter: φ 3.2mm (base) → φ 4.0mm (filling cover)
• Welding current: 110~130A (φ 3.2), 160~190A (φ 4.0)
• Interlayer temperature: ≤150℃

2.2 Groove Form

• When the wall thickness of the flue is ≤6mm, the groove may not be opened, but the gap of 2~3mm should be ensured.
• When the wall thickness is> 6mm, a V-shaped groove should be opened with an angle of 60° ~70° and a blunt edge of 1~2mm.

Key point: The root of the weld must be welded through, but the internal guide tube must not be burned through. There is usually only 10~15mm gap between the guide tube and the inner wall of the bellows, and excessive welding current will break down the end tube and damage the guide tube.

2.3 Welding sequence and deformation control

• Symmetric segment welding is adopted: for circular flue, segment according to clock point (12 o'clock → 6 o'clock → 3 o'clock → 9 o'clock), each segment is 50~100mm long, and welding is applied alternately.
• For rectangular flue, welding should be applied from the midpoint of the long side to both ends, then the short side, and finally the corner.
• After each weld, tap the weld and the heat affected zone with a wooden hammer to release the welding stress.

2.4 Prohibited Matters

• No welding (including spot welding, repair welding) shall be performed on the bellows. The wall thickness of the bellows is only 0.8~2.0mm, and the welding will immediately burn through or cause stress cracking.
• When welding the end pipe to the flue, do not clamp the ground wire to the bellows or the opposite flue.

3. "Welding" related procedures in the installation of non-metallic expansion joints

The non-metallic expansion joint itself has no welding parts, and its connection to the flue relies on flanges and bolts. However, during installation, it is necessary to weld flanges or angle steel platen frames at the end of the flue. This part of the welding also requires caution.

3.1 Flange welding

How to weld the expansion joint of the flue For non-metallic expansion joints, it refers to welding the flange ring or frame used to compress the skin.

Step:

1. First, spot weld the flange ring and fix it at the end of the flue to ensure that the flange surface is perpendicular to the axis of the flue, and the deviation is ≤2mm/m.
2. Adopt segmented jump welding method: 100mm per weld, skip 100mm, and repair welding after cooling. Prevent flange deformation and warpage caused by continuous welding.
3. The weld should be continuous, pore-free, and the weld slag should be ground to smooth, without sharp bumps-otherwise it will puncture the skin.

3.2 Plate Bolt Seat Welding

Some non-metallic expansion joints need to be welded with nut plates or bolt seats. Care should be taken when welding:

• Remove splashes after welding and re-pass the thread with a tap to prevent weld slag from clogging the thread.
• The position of the bolt seat must correspond to the hole position of the skin pressure plate, and the deviation shall be ≤1.5mm.

3.3 Deflector welding

The guide tube is welded on the inside of the flue, which belongs to a concealed process, and is especially important:

• The fixed end of the guide tube shall be continuously welded with the inner wall of the flue, and no spot welding shall be allowed.
• The free end is strictly prohibited from welding and must be kept in a free sliding state.
• Before welding, confirm the installation direction of the guide tube: the bell mouth or overlap end faces the incoming flue gas.

Welding quality inspection

After completing the construction of how to weld the expansion joint of the flue, the following inspections must be carried out:

4.1 Appearance inspection

• There are no cracks, pores, slag inclusions and biting edges on the weld surface (depth ≤0.5mm).
• The weld residual height is ≤3mm, and the transition is smooth.

4.2 Dimensional inspection

• The parallelism of the flanges or end tubes at both ends of the expansion joint after welding is ≤3mm.
• The overall length variation of flue shall be within the design allowable range (the non-metallic expansion joint shall be kept in the pre-compressed state).

4.3 Sealability Test

After completion of welding and before heat insulation, conduct air tightness test:

• Methods: Apply soapy water to the weld, and fill the flue with compressed air to 1.1 times the working pressure (but not exceed the pressure resistance limit of the expansion joint).
• Criteria: No bubbles are continuously generated as qualified.
• For non-metallic expansion joints, it is also necessary to check the flange pressure plate bolts for leakage.

4.4 Non-destructive testing (according to design requirements)

• The butt welds of important flues (such as denitrification inlet and absorption tower inlet) shall be subjected to 20% ~100% radiographic inspection (RT) or ultrasonic inspection (UT) according to NB/T 47013 standard, and are qualified for Class II.

V. Common welding problems and prevention

6. Welding matching of different flue materials

Special Note: When welding dissimilar steel, the ground wire must be clamped on the carbon steel side to avoid the tendency of intergranular corrosion in stainless steel.

Welding Safety and Protection

1. Fire prevention: combustible gas (gas, VOCs) may remain in the flue. Gas detection and hot fire ticket must be carried out before welding.
2. Anti-scalding: Bellows and skin will heat up under welding heat radiation. The surface of non-metallic skin should not be exposed to high temperature (> 80℃) for a long time, and wet asbestos cloth can be used to cool down.
3. Ventilation: When welding in the flue, forced ventilation and dust mask must be worn.

VIII. Summary

The core principle of how to weld the expansion joint of the flue can be summarized as three sentences: "protecting flexible parts, controlling deformation by sections, and strictly checking and sealing":

• Protect flexible parts: Cover with asbestos cloth (non-metal) or protective plate (metal) before welding, the grounding wire is strictly prohibited from crossing the expansion joint, and the arc shall not touch the bellows or skin.
• Sectional deformation control: Symmetric sectional jump welding method is adopted to control the interlayer temperature and prevent flange warping or bellows instability.
• Strict sealing inspection: Air tightness test must be carried out after welding, and non-destructive testing must be carried out for important welds.

For non-metallic expansion joints, the so-called "welding" is actually the welding of the flange frame and the guide barrel, and the skin itself is not involved in the welding. Workers must distinguish the process differences of different types of expansion joints, and avoid directly applying the welding method of metal expansion joints to non-metallic products. Correct welding process is the first guarantee for the long-term reliable operation of expansion joints, and any negligence may lead to premature failure of equipment or even safety accidents.

In flue system design, the expansion joint not only needs to absorb thermal displacement, but also produces a significant "pressure thrust" due to internal medium pressure. If this thrust is neglected in the design stage, it may lead to the failure of the fixed bracket, flue deformation and even the instability of the expansion joint itself. Therefore, mastering the calculation formula of flue expansion joint thrust is the key link to ensure the safety of flue structure. This paper will systematically explain the thrust source, calculation formula and engineering application examples of metal expansion joint and non-metal expansion joint.

1. Why do you need to calculate the thrust of the expansion joint

The expansion joint is installed in the flue, and when there is pressure (positive or negative) acting inside, the pressure creates an axial force on the effective area of the bellows or skin. This force will be transmitted to the fixed brackets at both ends and will also act on the expansion joint body.

The core value of the thrust calculation formula of flue expansion joint lies in:

• Determining the structural dimensions and anchoring mode of the fixed bracket
• Check the pressure stability of the expansion joint itself
• Prevent flue interface cracking or expansion joint inversion due to thrust exceeding limit

The consequences of ignoring the thrust calculation are often disastrous: a power plant did not calculate the pressure thrust of the metal expansion joint, which led to the flue fixing bracket at the outlet of the induced draft fan being pushed 30mm away from the foundation, and the flue welds cracked in many places.

2. Thrust calculation of expansion joint of metal bellows

2.1 Sources of Pressure Thrust

Under the action of internal pressure, the effective area of the metal bellows expansion joint will produce an axial expansion force. The magnitude of this force is proportional to the pressure value and the effective area of the bellows.

The basic form of the calculation formula of the thrust of the flue expansion joint (metal bellows) is:

F_p = P × A_eff

Among them:

• F_p — — Pressure thrust, unit: N
• P — — Working pressure in flue (gauge pressure), unit: Pa (Note: the thrust direction is opposite under negative pressure)
• A_eff-Effective area of bellows in m²

2.2 Determination of effective area A_eff

The effective area of the bellows is not equal to the cross-sectional area of the flue because the corrugated structure of the bellows makes its pressure-bearing area between the inner diameter area and the outer diameter area. The following methods are commonly used in engineering to obtain:

Method 1: Check the product sample
The A_eff value is given directly in the technical parameter sheet provided by the manufacturer.

Method 2: Empirical Formula
For standard U-bellows:

A_eff ≈ (π/4) × (D_m) ²

Where D_m is the mean diameter of the bellows = (D_in + D_out) /2, D_in is the inner diameter, and D_out is the peak outer diameter.

Method 3: Inverse calculation by stiffness method
For installed expansion joints, it can be calculated back from the length change under pressure:

A_eff = K × Δ L/P

Where K is the axial stiffness of the bellows (N/mm) and Δ L is the elongation under pressure (mm).

2.3 Corrections in actual operating conditions

Metal bellows expansion joints are usually equipped with tie rods or hinges. The role of the tie rod is to withstand the pressure thrust, thus protecting the bellows. Therefore, the thrust calculation needs to distinguish between two cases:

Key conclusion: For metal expansion joints with tie rods, the pressure thrust is balanced by the tie rods themselves and is not transmitted to the external bracket. However, the design strength of the tie rod must be able to withstand F_p (usually taking 1.5 times the safety factor).

2.4 Calculation Examples

Known:

• Circular flue diameter DN1200mm, metal bellows expansion joint
• Inner diameter D_in =1200mm, crest outer diameter D_out =1320mm
• Operating pressure P = +5000Pa (5kPa positive pressure)
• Try to calculate the pressure and thrust and judge whether the tie rod needs to be installed

Calculation:

1. Average diameter D_m = (1200+1320) /2=1260mm =1.26m
2. Effective area A_eff = π/4× (1.26) ² =1.247 m²
3. Pressure thrust F_p =5000×1.247=6235 N ≈ 636 kgf

Conclusion: If the free expansion joint is used, the fixed bracket at both ends needs to bear a thrust of about 636kgf, which must be included in the design of the bracket. If the type with tie rods is adopted, it can be easily withheld by 4 M16 tie rods (each with a bearing capacity of about 3000kgf).

Thrust calculation of non-metallic fabric expansion joint

A non-metallic expansion joint has a different source of thrust than a metal. Because the fabric skin is so soft that it can barely withstand pressure thrust, the thrust is all borne by the external metal frame and platen.

The calculation formula of flue expansion joint thrust (non-metal) is:

F_p = P × A_duct

That is, the effective area is directly taken as the internal cross-sectional area of the flue (instead of the average area of the bellows).

For rectangular flue:

A_duct = W × H

For circular flues:

A_duct = π/4× D²

3.1 Thrust transmission path of non-metallic expansion joint

The thrust of the non-metallic expansion joint is not borne by the skin, but is transmitted through the following path:

1. The flue gas pressure acts on the end face of the flue
2. The end plate transmits force to the flange connected to the expansion joint
3. The flange is transmitted to the outer metal frame by a platen bolt
4. The frame is then transmit to the flue fixing bracket by a pull rod or bracket

Therefore, for non-metallic expansion joints, installation must ensure that the platen bolts have sufficient strength and pre-tightening force to prevent internal pressure from blowing off the skin.

3.2 Calculation Examples

Known:

• Rectangular flue, width 1500mm, height 1200mm
• Operating pressure P = -8000Pa (8kPa negative pressure, i.e. suction)
• Trial calculation of the thrust to be withstood by the fixed bracket

Calculation:

1. Flue cross-sectional area A_duct =1.5×1.2=1.8 m²
2. Thrust F_p = P × A_duct = (-8000) ×1.8= -14400 N (negative sign indicates directional inward contraction)
3. About 1469 kgf in absolute

Conclusion: The fixed bracket must withstand a tensile force of about 1470kgf (because the negative pressure is inward suction). This value is required for anchor checking during bracket design.

4. Elastic reaction force generated by temperature load

In addition to the pressure thrust, the expansion joint also produces an elastic reaction force when absorbing thermal displacement. This force also needs to be factored into the total load.

Calculation formula of elastic reaction force:

F_e = K × Δ L

Among them:

• K-axial stiffness of the expansion joint (N/mm), supplied by the manufacturer
• Δ L — — Thermal displacement absorbed after actual installation (mm)

For metal bellows expansion joints, the K value is usually 100~500 N/mm; For non-metallic expansion joints, the K value is small (typically

The total thrust (acting on the fixed bracket) is:

F_total = F_p + F_e (When the metal expansion joint has no tie rod)
F_total = F_e (when the metal expansion joint is equipped with a tie rod, the pressure thrust is balanced by the tie rod)
F_total = F_p (non-metallic expansion joint, elastic reaction force can be ignored)

V. Precautions in engineering application

5.1 Thrust direction under negative pressure

When the flue is under negative pressure (such as behind the induced draft fan), the direction of thrust is opposite to the positive pressure, which is the "suction" of inward contraction. At this time, the fixed bracket needs to be subjected to tension instead of pressure. Many engineers only focus on positive pressure thrust and ignore negative pressure suction, resulting in insufficient pull-out ability of the bracket and being pulled out of the foundation.

5.2 Effect of temperature change on thrust

For metal expansion joints, if cold pre-compression/pre-stretching is not performed at the design temperature during installation, the actual Δ L will deviate from the design value, resulting in more than expected elastic reaction force F_e. For example, if the thermal elongation is designed to be 40mm, if it is not pre-compressed during installation, the actual Δ L may reach 2 times the design value and F_e may double, which may lead to overload of the fixed bracket.

5.3 Introduction of safety factors

Regardless of pressure thrust or elastic reaction force, when finalizing the bracket load, the safety factor shall be multiplied by:

• Normal operating load: safety factor 1.5
• Extreme working conditions (e.g. start-stop, failure): Safety factor 1.2 (check according to material yield strength)

That is:

F_design ≥ F_total ×1.5

5.4 Coupling Effect of Multiple Expansion Joints

When multiple expansion joints are arranged in series on the same section of flue, the forces on the fixed bracket are not simply superimposed. A pipe flexibility analysis is required because the stiffness of the expansion joints interacts with each other and the displacement distribution may not be consistent with the initial design. At this point it is recommended to use CAESAR II or AutoPIPE software for simulation.

6. Quick table look-up for thrust calculation

To facilitate engineering site estimation, the following table gives the pressure thrust F_p (non-metallic expansion joint) at ±5kPa pressure for common flue sizes:

Instructions for use: The values in the table are approximate values (converted to 9.8 N/kgf). In practical application, for the expansion joint of metal bellows, the effective area A_eff should be calculated instead of the flue cross-sectional area A_duct, and the value will be slightly lower.

Common Mistakes and Avoidance

VIII. Summary

The calculation formula for flue expansion joint thrust varies depending on the type of expansion joint:

• Metal bellows expansion joint: thrust F_p = P × A_eff (A_eff is the effective area of bellows). When there is no pull rod, F_p is carried by the supports at both ends; When there is a tie rod, it is balanced inside the tie rod, and the bracket only bears the elastic reaction force F_e = K × Δ L.
• Expansion joint of non-metallic fabric: Thrust force F_p = P × A_duct (A_duct is the internal cross-sectional area of flue), and elastic reaction force can be ignored. The thrust force is all transmitted from the external metal frame to the fixed bracket.

Correct calculation of expansion joint thrust is an indispensable step in flue structure design. In engineering practice, the process of "first distinguishing the types of expansion joints, then selecting the correct effective area, and finally counting the elastic reaction force and safety factor" should be strictly followed. For complex pipeline systems, it is recommended to use professional stress analysis software for overall calibration. Through scientific calculation and reasonable type selection, serious accidents such as fixed bracket damage, expansion joint inversion and flue cracking can be effectively avoided.