After the pipeline inspection, when they saw the deformation of the metal expansion joint, many people's first reaction was to take a crowbar and push it back with a jack. Stop for a moment. Deformed expansion joints may not all be repaired, and what can be repaired may not all be repaired-if the judgment is wrong, money will be wasted at the least, and the whole pipeline will suffer at the worst.
Deformation is not that simple: first distinguish whether it is bulging, skewing or pulling
The premise of deformation repair of metal expansion joint is to understand the deformation type first. Bellows bulge, overall skew and ripple cracking all look like "deformation", and the treatment ideas are completely opposite.
The bulge and skew in the cold state are mostly caused by transportation bumps, uneven hoisting forces or forced countermeasures during installation. The bulge and skew in the hot state are often caused by overtemperature of the medium, incorrect calculation of compensation amount or failure of fixed bracket. The same general-purpose corrugated expansion joint, with the same degree of deformation, may be a precursor to fatigue cracks when placed on the main steam pipeline of a power station, and may just be an installation bruise when placed on the dust collection pipeline of the cement industry-the conclusion may be completely opposite. A customer once asked a photo of the skewed expansion joint. After careful inquiry, he found out that it was caused by the settlement of the circulating air duct and pipe support of the cement mill. Then it was not the problem of the expansion joint itself. Just reset the bracket.
The same is not true with the pull crack. Axial cracks appear at the trough of the bellows, which are basically fatigue damage. There is only one word for the safe way: change.
Which transformations can be repaired, and which ones are repaired for nothing
Local depressions, slight distortions, and misalignment of flange joints are the categories that can be attempted to repair. This kind of problem does not hurt the continuity of bellows base metal, and the use function can be recovered after returning to the factory.
But in the following cases, don't bother, just replace them with new ones:
- Cracks appear on the bellows surface, even if there is only one fine axial crack
- Corrosion perforations, especially cracks caused by chloride ion stress corrosion
- The corrugation pitch is obviously uneven, and the difference of adjacent wave pitch exceeds the requirements of the drawing
Intergranular corrosion may occur in austenitic stainless steel if the high temperature axial expansion joint is operated under overtemperature for a long time. The exterior looked as bright as new, and there was nothing unusual in the touch of the hand, but the internal grain boundaries were already crispy. This hidden danger can't be seen with the naked eye, and it's useless to shine it with a flashlight-it can only be confirmed by returning to the factory for metallographic inspection. Fix? There is no restoration value. Keep it? That's a time bomb.
Fixes are not pounding
Onsite cold orthopedics can only handle very minor problems, such as bolt tightening adjustments after a slight warping of the flange. Whenever the deformation of the bellows body is involved, there is only one formal process: remove it and return it to the factory.
What does return to factory restoration do? Re-wave pressure, solution treatment or local repair welding, and then make pressure test and stiffness test according to the original design standard, and issue a report. Some people think "I'll just push back and look the same as before". Alas, the recovery of waveform is not equal to the recovery of material properties of bellows after expansion and buckling deformation. The work hardening zone caused by cold working is still there, and the stress distribution has long changed.
The fatigue life of the repaired expansion joint is usually only about 70% of that of the new product. If the safety factor of the original design is not high, or the working condition itself fluctuates greatly, it will crack again in less than a year after repair. After this account is calculated, the repair cost plus the downtime loss is better than getting a new one in place at once.
If you decide to change, don't step on the same pit a second time
If you decide to replace it, don't look at the old nameplate and copy the model number. It turns out that the universal corrugated expansion joint is frequently deformed, and you have to think about why-it can't absorb so much lateral displacement, or the pressure and thrust are not balanced out.
When communicating with the manufacturer, talk about these things at once: pipeline direction diagram, fixed bracket position, medium name and temperature, design pressure, and hot and cold displacement. The manufacturer will help you calculate the compensation amount and recommend a more suitable structure. For large transverse displacement, use a double hinge transverse expansion joint; If it is necessary to absorb the internal pressure thrust, use a straight tube pressure balanced expansion joint. The same pipeline, with a different kind of force logic, may not have problems for ten years.
I have encountered a case in a cement plant before. The general expansion joint used in the circulation pipeline of raw mill was changed three times a year, and later it was changed to a double hinge horizontal type, which didn't go wrong again for several years. Choosing the right type will work better than any maintenance skill.
Third way beyond repair and replacement
To be honest, expansion joint deformation rarely occurs suddenly with a "bang", and more than 90% of them have precursors. You take pictures once a quarter for archiving, and compare them next time. It works better than any other instrument.
Is the tie rod nut loose? A loose nut indicates that the displacement absorption is not normal; Has the guide tube fallen off? When the guide tube falls off, the high-speed fluid directly washes the inner wall of the bellows, and the wear speed doubles; Is there any water in the insulation layer-water vapor seeps between the bellows and the insulation layer, chloride ions are enriched, and stainless steel will rust.
If you find the problem early, you may just re-tighten the tie rod. When the bellows is dragged and cracked, that is, it is shut down, dismantled and replaced. After the whole process is completed, not only money will be lost, but also the construction period.
Does it really have to be done to repair the deformation of metal expansion joint? First, judge whether it can be repaired and whether it is worth repairing, and then decide whether to repair, replace or adjust the surrounding facilities. This order, never reverse.