How to solve the thermal displacement of waste heat recovery pipeline? Focus on these three points in the selection of metal expansion joints
The compensator of the export pipeline of the waste heat boiler, which has just been put into operation for half a year, cracked, and the whole line was forced to shut down for emergency repair. I asked them what model they used, and the opposite side hesitated and said that I bought a general-purpose corrugated expansion joint for cheap. Tsk, is this tuition paid unjustly?
Let's do an account first and see how fierce the thermal displacement in the waste heat system is. For every 100℃ increase in the temperature of carbon steel pipe, it will elongate by 1.2mm per meter. Waste heat recovery pipelines are often several hundred meters long. From cold state to operating temperature (for example, from normal temperature to 600℃), a 30-meter pipeline can expand more than 200 millimeters-about the width of an adult's palm. If several turns and several sections of straight pipe expand at the same time, the cumulative displacement can make the pipe frame crooked. And guess what? Many on-site accidents are not design defects at all, but the displacement is not accurately calculated, and a compensator is randomly installed to deal with it.
In the face of corrosive high-temperature flue gas, should we choose a metal expansion joint or a non-metal compensator? The upper temperature resistance limit of non-metallic expansion joints (that is, fabric fiber compensators) is mostly around 400℃, while the waste heat flue gas contains acid, dust and great temperature fluctuation-waste heat of cement kiln and waste heat of steel sintering machine, and the outlet temperature often rushes to 550℃ or even higher. If non-metal can't carry it, you have to use metal corrugated expansion joints. However, there are pits in metals: ordinary stainless steel corrodes extremely quickly in flue gas containing chloride ions and sulfur. What to do? Two ways: one is lined with PTFE (such as the PTFE-lined metal hose of this station), which has good corrosion resistance but limited temperature resistance (generally ≤200℃); Another directly selected heat-resistant alloy, such as Incoloy 800 or 625, can withstand above 600°C. Which to choose depends on the smoke composition analysis report-don't pat your head by feeling.
Don't just stare at the model parameters. If the structure type is wrong, it is equivalent to laying mines on the site. Also metal expansion joints:
- High temperature axial expansion jointIt is only good at absorbing axial displacement, so it is fine to use it in straight pipe sections;
- However, the waste heat pipeline has many turns and limited space, so the axial type alone can't cope with the lateral and angular displacement at all. That's when you have to use itCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint-They can absorb multi-directional displacement at the same time without producing blind plate force. AndStraight pipe pressure balanced expansion joint, suitable for scenarios where large-diameter pipes require balanced pressure.
- If the pipe diameter is large and the pressure is high,Large diameter thick wall expansion jointIs specially designed for this kind of working condition, the wall thickness of the bellows is thickened, and the fatigue life is long.
How to judge which one to use? It's simple-let the design institute take out the pipeline stress analysis report and look at the displacement direction and constraints. Don't guess for yourself, let alone trust a bargain.
Next, let's talk about the three pits of installation and routine maintenance. 80% of early leaks are here:
- Install the guide tube in reverse direction。 The role of the guide tube is to guide the flow direction of the medium and protect the bellows from high-speed particle scour. Once installed backwards, the smoke directly flushes the bellows, which can wear out in a few months. Remember: the direction of the arrow on the guide tube must point to the flow direction of the medium (Article 7 of the FAQ on this site specifically talks about the function of the guide tube of the expansion joint).
- Forgot to adjust tie rod nut。 The tie rod nut is locked when transported and must be loosened to the specified pre-tensioned/pre-compressed position after installation. If it is not loosened, the expansion joint will not work at all, and the thermal stress of the pipe will all be held on the bellows. How to tune it? Refer to the FAQ of this site "How to adjust the tie rod nut of expansion joint".
- Unreasonable design of stent spacing。 Many people think that they can leave the bracket casually after installing the expansion joint. In fact, the spacing of the bracket determines whether the self-weight and thermal expansion of the pipeline can be effectively guided. The spacing is too large, and the pipe sag and crush the compensator; The spacing is too small, wasting money. It is recommended to arrange the fixed bracket and guide bracket reasonably according to the requirements of the Power Pipeline Design Manual and the pipe diameter.
Finally, let's talk about a case, which is very intuitive. We selected the outlet pipeline of the waste heat boiler of a cement factory according to the working conditionsLarge diameter thick wall expansion joint(Material Incoloy 800, lined with deflector), operated for more than two years with zero leakage. However, the factory next door is cheap and uses a certain brandUniversal corrugated expansion jointCracks appeared in less than half a year. The difference is about 20%, but the lifespan is 5 times worse. You can calculate this account yourself: is it cost-effective to spend thousands more dollars at a time to buy peace of mind, or is it cost-effective to stop the machine every six months and lose hundreds of thousands of output?
In the final analysis, the selection of waste heat metal expansion joint should focus on three points:Accurate calculation of displacement, corrosion and temperature resistance of material, and correct selection of structure。 Stop playing with pipeline safety.