First, find out first: What counts as "size deviation"? Not all errors should be panicked
Two days ago, a customer called in a hurry, saying that the size of the flue expansion joint installed on the site was wrong, 5mm shorter than the pipe, and asked if he had to return it and redo it. I asked him not to dismantle it in a hurry, and asked: Did you measure the overall length of the expansion joint, or did you only look at the corrugated part? He was stunned for a moment and said, "Just measured the total length outside."
That's the problem. When we engage in flue expansion joints, we must first understand a truth:Not all errors are called dimensional deviations。 The dimensional tolerance of the expansion joint is clearly stipulated in the national standard, such asMetal rectangular expansion jointThe length deviation is typically within ± 5 mm, and can even be relaxed to ± 10 mm for large diameter. The 5mm you measured may still be within the qualified range.
Then what is the "size deviation" to be wary of? See three things:Actual compression/tension of corrugated section, misalignment of mounting interface, clearance between guide tube and pipe。 To put it bluntly, whether it can absorb displacement normally after installation, whether it will leak air, and whether it will generate additional stress is the key. Just staring at the chief was purely asking for trouble.
2. Hands-on measurement: how to use caliper, tape measure and gauge? Give you a practical procedure
Figure out which sizes are worth worrying about, and the next step is to do it. Don't tell me you are going to take a steel tape measure and lay it around the edge of the flange — you can't even believe the measurement. Here, follow this process:
Step 1: Measure the interface size with a caliper or tape measure
Clamp the caliper or tape measure on the inner diameter of the flange connecting surface. Measure four points: up, down, left and right, and take the average value. If the device is square (such as a rectangular expansion joint), then measure the diagonal angles to see if it is rectangular. This step is to confirm whether the interface of the expansion joint and the pipe flange can match. If the deviation exceeds 2mm, the bolt hole may be misaligned. When the screws are tightened, the bellows will be twisted, and the life will be directly discounted by 50%.
Step 2: Measure the length of the corrugated section and straighten it with a tape measure
Expansion joints all have a "free length" when they leave the factory, which is the size of the corrugation when it is not stressed. The actual length you measure now, subtract the free length, and you getAmount of pre-compression or pre-stretching。 For flue gas ductsCompound straight pipe bypass pressure balanced expansion jointOrPTFE compensatorThe pre-compression amount is generally set according to 50% to 70% of the design displacement amount. If the number you measure is too different, it is either not adjusted in place during installation, or it is squeezed and deformed during transportation.
Step 3: Measure the gap of the guide tube and stuff it in with a gauge
This one is the easiest to overlook. The role of the guide tube in ourFAQIt is very clear in-prevent smoke from washing away the ripples. You shove the gauge between the deflector and the inner wall of the pipe and record the evenness. The normal gap is about 5~10mm. If one side is wide and the other side is narrow, it means that the guide tube is crooked or the pipe itself is not round. Once, in a thermal power plant project, one side of the guide tube pushed against the pipe wall, resulting in local wear of the bellows, which leaked in half a year.
Tips:Before measuring, clean the ash and rust on the surface of the expansion joint, especially the flange surface and the edge of the guide tube. Don't be afraid of trouble, this little effort can't be saved.
3. Does the deviation exceed the standard? Don't be in a hurry to dismantle it. Look at the guide tube and the tie rod first
When many people find that the size is wrong, their first reaction is to "remove it and replace it with a new one". Alas, don't be impulsive yet. You spend half a day removing it, and it may be solved by adjusting two parts. Which two?Deflectors and tie rods.
Let's see the deflector first. If the gap between the guide tube and the pipe is uneven, or the guide tube itself is crooked, the measured interface size will definitely deviate. The solution is simple: loosen the fixing bolts of the guide tube (usually spot welding or bolted connection), gently knock them straight with a hammer, and then spot welding them again. After the operation, measure it again, and the deviation may return to the normal range.
Look at the pull rod again. The function of the tie rod is to limit the excessive stretching or compression of the expansion joint, and at the same time, it can also transmit the blind plate force generated by the medium pressure. If you find that the actual length of the corrugated segment is quite different from the design length, nine times out of ten it isTie rod nut not adjusted in place。 How to adjust? Refer to ourFAQ: How to adjust the expansion joint tie rod nut— — Simply put: first loosen the locking nut, then rotate the adjustment nut with a wrench to return the corrugated section to the designed length, and finally tighten the locking nut. When adjusting, pay attention to the nuts on the left and right sides to rotate synchronously, otherwise the pull rod will be unevenly stressed, and it is not possible to screw out the folds of the bellows.
After doing these two steps, re-measure again. If the deviation is still above 5mm, it is necessary to consider whether it is a manufacturing problem or the pipeline reservation deviation is too large.
4. Adjust or replace? Give you a standard of judgment on the spot, so you won't be throwing it in vain
Based on all kinds of cases I have come into contact with, a set of judgment processes have been summarized, and you can just follow it.
- Article 1: Deviation — Leave it alone, just install it. The vast majority of flue gas ducting systems allow this level of error.
- Article 2: The deviation is between 3~8mm, but the guide tube and the tie rod have been adjusted and still remain unchanged-It may be that the pipe flange itself is not horizontal or non-centric. Check the parallelism of the pipe flange with a laser aligner or level gauge. If the pipe is crooked, no matter how many expansion joints you replace, it won't help. At this time, you should adjust the pipe bracket instead of changing the expansion joint.
- Article 3: The deviation is> 8mm, or the bellows has obvious twist and deformation-Don't hesitate, contact the manufacturer directly for exchange. Especially thoseMetal Corrugated Expansion Joints in Cement IndustryOrCorrugated expansion joint for power station industryThese high-pressure and high-temperature occasions have strict requirements for dimensional accuracy, and forced installation will lead to weld cracking or even pipe bursting. Before changing, remember to take photos and leave evidence. By the way, measure the span of the four corner bolt holes of the pipe flange. The manufacturer will rematch according to this.
The guide tube and the pull rod are adjusted, and the deviation is small, but the flue vibration becomes larger after installation。 Tsk, this is often not a problem of dimensional deviation, but the wrong choice of expansion joint-for example, when the flue gas flow rate is too high, you should use one with a guide tubeFlue gas baffle doorOr special structured expansion joints instead of ordinary metal rectangular expansion joints. In this case, you have to find technical support from the manufacturer.
5. How to avoid pits in the next installation? Keep these 3 details in mind, and the size deviation is half less
Finally, let's talk about preventing problems before they happen. When you next purchase and install flue expansion joints, keep an eye on these three points, and you can basically avoid 80% of the dimensional deviation problem.
Detail 1: When ordering, send the actual size of the pipeline to the manufacturer. Don't just look at the drawings.The drawings are theoretical values, and there will be shrinkage and deformation after the pipe is welded. You measure the interface size of the actual pipeline and tell the manufacturer that the other party can compensate for the deviation during production. a great manyDimensional deviation of flue expansion jointIn fact, it is caused by the mismatch between the drawings and the actual objects.
Details 2: Unpack and inspect it immediately after arrival. Don't pile it in the open air and wait for the installation day before disassembling it.Expansion joints, especially non-metallic ones (such asNon-metallic expansion joint (fabric fiber expansion joint)), which is easy to deform due to uneven force during transportation and stacking. You unpack and check in advance, and find that the guide tube is crooked and the ripples are flattened, and there is still time to adjust or change it. Wait until the hoisting day to dismantle it, and find that the size is wrong, and the construction period will be completely delayed.
Detail 3: Install one end first when installing, adjust the pre-compression amount and then install the other end.Don't try to save trouble by firmly aligning two flange surfaces at the same time. The correct way to do this is: first weld one end of the expansion joint to the pipe (or bolt it), then use a jack or tie rod to adjust the interface position at the other end to ensure that the corrugated section is in the designed compression/tension state before fixing the other end. And guess what? Many "dimensional deviations" of on-site feedback are actually deviations caused by the inversion of installation sequence.
Okay, that's it for today. Next time you encounter the wrong size of the flue expansion joint, don't panic, check it in this order: find out the type of deviation → measure with a ruler and gauge → adjust the guide tube and tie rod → judge whether to replace it with a new one → go back and make up the installation details. Most problems can be solved in half a day, and there is no need to remove the good expansion joint and sell it as scrap iron.
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