Find out what the compensation length is first, miscalculation will cause big problems
Do you know what flue expansion joints are most afraid of? It's not that the temperature is high, it's not that the pressure is high, it's that the compensation length is miscalculated. It's too short, the expansion joint is deadlifted, and the bellows is torn; It's too long, the pipe can't be pushed, the connecting bolts break, and the smoke leaks. I met a buddy from the power station two days ago, and they used the flueMetal rectangular expansion jointBecause the compensation length was less than 20mm, it was leaked after three days of operation. Tsk, that's a distress.
The so-called compensation length, simply put, is the displacement amount that the expansion energy saving "eats in". How much your pipe expands and contracts thermally, the expansion joint has to correspond to how much "expansion and contraction". Calculating this number is not a matter of patting the head. I'll walk you through it step by step below.
Step 1: Calculate the thermal expansion of the pipeline, the formula is very simple
Δ L = α × L × Δ T. Don't panic, let's break it apart.
- Alpha: Linear expansion coefficient of pipeline materials, generally 0.012 mm/ (m·℃) for steel. There is a slight difference between carbon steel and stainless steel, so just check the standard.
- L: Pipe length in meters. You measure the actual distance between the two fixed brackets, don't blindly calculate according to the total length of the drawing.
- Δ T: Maximum operating temperature minus installation temperature. For example, the maximum flue is 400℃, 20℃ during installation, and the temperature difference is 380℃.
A section of carbon steel flue with a length of 30 meters, the temperature rises from 20℃ to 400℃, and the thermal expansion amount =0.012×30×380=136.8 mm. Remember, the calculated value is millimeters.
Tips:If there is insulation in the pipe, the surface temperature may be lower than the smoke, but you have to calculate by the medium temperature, so don't be lazy. Otherwise, as soon as the expansion joint is installed, it will deform within two days.
Step 2: Select the compensation method according to the pipeline layout, don't choose the wrong one
Calculate the amount of thermal expansion, and then choose "how to compensate". The pipeline layout is different, and the selection idea is completely different.
- Straight pipe section without elbow: withUniversal corrugated expansion jointOrHigh temperature axial expansion joint。 This kind of product is specialized in absorbing axial displacement, and your calculated 136.8 mm is its axial compensation requirement.
- There are elbows or limited space: may be consideredCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 They can absorb lateral displacement, or balance out axial forces, protecting equipment.
- Large section rectangular flue: For example, the desulfurization tower outlet is usually usedMetal rectangular expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)。 Non-metallic compensation is large and corrosion resistant, but the pressure cannot be too high.
And guess what? Many people don't look at the on-site layout, but directly apply the standard model. As a result, the expansion joint twists after installing it. Did you say it was uncomfortable or not?
Step 3: Lock the specific model by comparing the product parameter table
After selecting the type, open the parameter table of the corresponding product. Each model has a column for "Compensation Amount", which is usually in "mm". Such asUniversal corrugated expansion jointSome models of may be labeled "Axial Compensation ± 100 mm". If you need 136.8 mm larger than 100 mm, you have to choose a larger size, or use two in series.
Attention! The compensation amount in the parameter table usually refers toSingle wave or single sectionThe rated value is not the sum of the whole expansion joint. Some manufacturers will use the "total compensation amount" mark, you have to see it clearly. TakeHigh temperature axial expansion jointSpeaking, the temperature resistance is above 600℃, the wave number is large, and the total compensation amount may reach 200 mm. But the more wave numbers, the lower the fatigue life. This is a trade-off.
In addition, the pressure level, the connection method (flange or welding), and the presence or absence of the guide tube have to be corrected. Refer to this site'sModel and size of expansion jointQuestions and answers, with detailed comparison tables.
Step 4: Do you want to pre-stretch when installing? Look here
The compensation length is calculated, the model is selected, and there is a crucial step before installation: pre-stretching (or pre-compression).
- Pipe operating temperature higher than installation temperature: The expansion joint should be pre-stretched, the amount of stretch = about 50% of the amount of thermal expansion. In this way, when working, retract a part first, and then continue to expand, maintaining a reasonable stroke.
- Pipe operating temperature below installation temperature: For example, the cold flue, the expansion joint should be pre-compressed.
How to operate it? TakeLarge tie rod expansion jointFor example, when installing, loosen the tie rod nut, push it with a jack or bolt, stretch the expansion joint to the specified length, and then lock the tie rod nut. Notice,Function of expansion joint tie rodIs to control the travel, adjustment method refer to this site Q&AHow to adjust the tie rod nut of expansion joint。
Many on-site masters are afraid of trouble, so they don't pre-stretch directly. And the result? When the expansion joint is hot, the stroke directly exceeds the limit, and the bellows becomes unstable. Oops, all the work for nothing.
Common mistakes to avoid pits: These details hurt many people
After working for so many years, I have seen too many low-level mistakes. Please pay attention to a few:
- Only calculate straight lines and ignore the friction of pipe supports: If the fixed bracket is not designed well, the pipeline will "run" and the actual displacement is larger than the calculated value. When calculating the compensation length, it is recommended to leave a margin of 10% ~15%.
- Wrong product used: For example, there is particulate dust in the flue, but chooserubber compensatorIt was worn out in a few days. Should be usedMetal corrugated expansion jointAdd a guide tube. The function of the guide tube can be seen in the Q&A of this siteSpecific Function of Expansion Joint Guide Tube。
- Forgot to consider cold tight compensation: In some long-distance pipelines, the expansion joint is deliberately displaced in the opposite direction during installation, which is called cold tightness. Can improve fatigue life. When calculating the compensation length, consider it together.
- Order directly after reading the parameter table: Media corrosion without considering actual working conditions. For example, wet desulfurization flue gas, high acidity, using ordinary carbon steelUniversal corrugated expansion jointQuickly pierced, must be used withDesulfurization flue gas baffle doorMatching corrosion-resistant expansion joints orPTFE-lined hose。
To put it bluntly,Flue expansion joint compensation lengthIt is not the end of calculation, but the installation, working conditions and service life should be connected in series. If you follow the above four steps, you will basically not overturn. If you are really unsure, give the parameters directly to the manufacturer and ask them to help check. After all, the equipment is your own, so safety comes first.