What the hell is open welding all about? Understand the working conditions of the boiler flue first
This place of boiler flue is simple and complicated, but it is really complicated. The flue gas temperature is high, often several hundred degrees, and the temperature difference can be several hundred degrees when the furnace is started and shut down. In addition, the flue gas carries sulfur, moisture and various corrosive substances, so the thermal expansion and contraction of the pipeline is particularly large. The expansion joint is used to absorb these displacements. As a result, you find that the weld is cracked. Is your first reaction "the quality of this thing is not good"? Don't worry, don't give the blame to the manufacturer yet.
When the flue expansion joint is welded, to put it bluntly, the weld can't bear it. But there is more than one reason why you can't bear it. It may be that the design is not correct, it may be that the installation leaves hidden dangers, or it may be that it has already reached its lifespan. We have to check one by one, otherwise it will still crack if we replace it with a new one, which will be called wrongful.
Common Open Welding Reasons: Thermal Stress, Corrosion, or Installation Backing Pan?
Heat stress-the most hidden killer
The boiler starts and stops frequently, the expansion joint extends and contracts, and the thermal stress acts repeatedly. If the displacement of the expansion joint is not accurate when selecting the type, or the pipeline constraint is too dead, then the weld is the hardest hit area of fatigue fracture. Especially for corrugated expansion joints used in power station industry, the original design life is quite long, but the actual working conditions are greatly different from the design. For example, if it runs at overtemperature, it will be a matter of time before the weld cracks.
Corrosion-slowly gnawing away the weld
The sulfur in the flue gas encounters water vapor, generating sulfurous acid and sulfuric acid. The low-temperature parts are prone to condensation, and acid corrosion comes. Because of the stress concentration and the material may change during welding, the weld area is more prone to gnawing. If you look at the open welding place, if there are corrosion marks at the fracture, it is probably acid dew point corrosion. At this time, it may be more suitable to use metal rectangular expansion joints or non-metallic expansion joints (fabric fiber expansion joints) -non-metallic ones are not afraid of corrosion, but their temperature resistance is limited, depending on the working conditions.
Install a backing pot? Nine times out of ten, he didn't run
The pipeline is misaligned, and it is forcibly pulled and welded; The pre-stretch was not done or done wrong; The bracket is not positioned correctly, causing the expansion joint to bear additional forces that it should not. If you think about it, the expansion joint was originally designed to compensate for displacement, but it turned out to be a stress-bearing member, so it is strange that the weld does not crack. In order to save trouble, some people weld the deflector in the wrong direction, and the flue gas directly washes the weld-alas, I have seen too many cases of this kind.
What should I do if the welding is open? Temporary disposal and thorough maintenance options
Don't panic if you find that the welding is open. If the leakage is not serious and the flue gas does not emit much, you can temporarily repair it with electric welding-but note that before repair welding, you must confirm that the boiler has been stopped or the flue gas temperature has dropped, so safety comes first. When repairing welding, try to drill a crack stop hole at both ends of the crack, then polish it clean, and repair it with a welding electrode of the same material. But to be honest, repair welding is an expedient measure, which treats the symptoms rather than the root cause.
For complete repair, either replace the entire expansion joint, or cut off the cracked bellows section and re-weld it. Don't just think about saving money when changing, choose the right model. For example, in the high-temperature section, you can look at the high-temperature axial expansion joint, which has a high temperature resistance level; If the space is limited, the external pressure single axial expansion joint may be considered. In another case, if the flue is square, it has to use a metal rectangular expansion joint or a rectangular non-metallic expansion joint, depending on the temperature and degree of corrosion.
How to choose the expansion joint to avoid this kind of thing? Type selection recommendations
The matter of selection, to put it bluntly, is "prescribing the right medicine". High temperature and low pressure, preference is given to metal corrugated expansion joints; The temperature is not high but there is corrosion, and non-metallic expansion joints (fabric fiber expansion joints) or PTFE-lined hoses are very fragrant; If it is high temperature and corrosive, it is necessary to use stainless steel plus lining coating. For example, corrugated expansion joints used in power station industry are often lined with corrosion-resistant materials.
In addition, the amount of displacement must be calculated clearly. Axial displacement, lateral displacement, angular displacement, how many are each, don't just give an approximate. Some expansion joints have their own tie rods, such as compound hinge transverse expansion joints or curved tube pressure balance expansion joints, which can limit unnecessary displacement and protect welds. There is also the guide tube, which not only prevents scour, but also reduces the smoke resistance. When selecting the model, remember to confirm the material and direction of the guide tube, and don't install it backwards.
If there is a baffle door in the flue, such as a flue baffle door or a circular baffle door (double seal), the connection between the expansion joint and the baffle door should also consider thermal expansion. It is best to use the same expansion joint brand to ensure matching.
What points should we pay attention to in usual maintenance? Preventing problems before they happen
Don't wait until the welding starts to remember to check it. During the usual inspection, look more to see if there is any discoloration, bulging and leakage around the weld. Especially in winter, the expansion joint is most prone to problems because of frequent start-and-stop and large temperature difference. Check whether the guide tube falls off at least once a month, because once the guide tube falls off, the flue gas directly impacts the bellows, and the weld cracks quickly.
And drainage issues. Acid water is easy to accumulate at the bottom of the flue. If the drainage is not smooth, the expansion joint will be soaked in acidic liquid for a long time, and the weld will corrode rapidly. You can add traps, or clean them regularly. In addition, brackets and hangers can't be ignored, see if they are loose and deformed, and don't let the weight of the pipe press on the expansion joint.
Finally, don't buy three-nothing products cheaply. The general-purpose corrugated expansion joint or straight pipe pressure balance expansion joint of regular manufacturers have factory inspection reports, and the material and weld flaw detection are guaranteed. You spend time maintaining it, so it is better to choose the right product at the beginning, which is much worry-free.
All in all, there are many reasons for welding of boiler flue expansion joints, but most of them can be avoided through reasonable selection, standardized installation and regular maintenance. Next time you encounter it again, don't change it at every turn. Check it in these directions first, and you may be able to save a lot of money. Do you think that's the case?