How long is the flue plus expansion joint? The reason is simple, but a miscalculation is costly
The flue is filled with high-temperature smoke, with temperatures ranging from tens to hundreds of degrees, and metallic or non-metallic materials will expand. If you try to save trouble without an expansion joint, the pipe will crush the bracket, crack the weld, and even deform the top of the equipment. Two days ago, a power plant customer told me that their 80-meter steel flue, without expansion joint, began to leak smoke from the flange connection after half a year's operation-finally stopped production for maintenance, resulting in a loss of hundreds of thousands. So don't ask "can you not add it?" It is more realistic to ask "how long to add one" first.
So how long is the flue plus the expansion joint? It is not reliable to give a fixed number directly. Because there are three core variables that affect the spacing: temperature, material, and direction.
The higher the temperature, the greater the expansion; Different materials have different linear expansion coefficients; There are elbows in the way that can help
Stainless steel approximately 17×10⁻⁶/°C and carbon steel approximately 12×10⁻⁶/°C. At the same length, stainless steel can expand nearly 50% more than carbon steel. The direction is also particular-the elbow itself can absorb part of the displacement, and the longer the straight section, the more it has to be digested by the expansion joint.
The general rule of thumb is that a metal flue has an expansion joint every 20-30 meters. But this is only for reference. If you really want to set the distance, you have to calculate the expansion:Δ L = α · L · Δ T。 If the calculated expansion exceeds the allowable stress of the pipe, it must be added. The allowable stress generally depends on the design standard. The allowable displacement of the sliding bracket is usually only 50-80mm, and if it exceeds, an expansion joint must be set.
Practical case: 30-meter carbon steel flue, must it be added?
Take an example. A 30-meter-long carbon steel flue, the flue gas temperature is 250℃, the installation temperature is 20℃, and the temperature difference is 230℃. The linear expansion coefficient is 12×10⁻⁶/℃ and substituted into the formula:
Δ L =12 x 10⁻⁶ x 30000 x 230=82.8 mm.
With an expansion amount of 82.8mm, the upper limit of the allowable displacement of the sliding bracket is usually only 80mm. Exceeded the mark! This requires the addition of an expansion joint. What type to choose? If the working condition is clean and the temperature is moderate, useUniversal corrugated expansion jointThat's enough; If it is high-temperature flue gas used in power stations, it is recommended to useCorrugated expansion joint for power station industryWith higher temperature resistance rating. If the length of the flue exceeds 50 meters, don't expect one expansion joint to be all-inclusive-set up multiple sections, and each section should be controlled within 30-40 meters.
But different industries vary greatly, so don't take a scheme everywhere.
Power stations, cement, desulfurization... The selection of flues in different industries is very different
Flue of power stationThe diameter is large, the temperature is high, and there may be dust in the smoke. in common useMetal rectangular expansion jointOrHigh temperature axial expansion jointHigh pressure and temperature resistance requirements.
Cement industryThe flue contains much dust and hard particles, so it is necessary to use wear-resistant type — —Metal Corrugated Expansion Joints in Cement IndustryIn view of this working condition, the wall thickness of the bellows is thickened, and the material is made of corrosion-resistant and wear-resistant alloy.
Desulfurization flueWhat? The temperature is not high but the corrosion is extremely strong. In the acid-alkali environment, the metal bellows will perforate in a few months. This is the time to use itNon-metallic expansion joint (fabric fiber expansion joint)OrPTFE-lined hoseWhich is corrosion resistant and can absorb displacement. If you take the general-purpose model to hard-sleeve corrosion conditions, it will leak in three months. I've seen many such cases.
The easiest pits to step on during model selection: guide tube and bracket thrust
Many people only pay attention to the expansion joint itself and ignore the guide tube. The flue gas velocity in the flue is high. If there is no guide tube, the high-speed particulate matter will directly wash the bellows, and the service life will be greatly shortened. The function of the expansion joint guide tube is to guide the airflow and protect the bellows. This detail must be confirmed before installation. In addition, for straight flues with a length of more than 40 meters, the thrust of the bracket cannot be ignored. The longer the pipeline, the greater the "blind plate force" generated by internal pressure, and the common axial expansion joint can't bear it, so it has to be used at this timeStraight pipe pressure balanced expansion jointTo eliminate the blind plate force, otherwise both the bracket and the expansion joint will fail early.
Installation and debugging: If the screw is not removed, it is equivalent to not being installed
The expansion joint leaves the factory with a transportation fixing screw, which is used to prevent the damage of the bellows during transportation. Many people at the scene forgot this, and directly welded the expansion joint with screw to the pipeline-then the expansion joint you added is useless. Remember: These screws must be removed after installation is complete. How exactly to adjust? You can see our previous Q&A —How to adjust the tie rod nut of expansion joint, with detailed steps. In addition, after flue pressure test, don't forget to check whether the displacement of each expansion joint is within the design range, especially for multi-wave expansion joints. The displacement of each wave joint should be uniform, otherwise it is easy to crack by local overload.
After all, there is no standard answer to how long the flue is plus the expansion joint, but with the formula and case, you can figure it out. Don't rely on experience alone, calculate Δ L first, and then choose according to the working conditions-if you choose the right one, there will be no problems after ten years of operation; If you choose the wrong one, the cost of shutdown maintenance is enough for you to buy more than a dozen expansion joints. This account, do you understand it?