Why is there always a problem with the flue expansion joint of desulfurization tower? — — Starting from the working condition and environment
Anyone who has done desulfurization operation and maintenance knows that the flue expansion joint is one of the most prone components in the whole system. Why? To put it bluntly, the environment is too bad. Although the flue gas temperature after wet desulfurization has dropped to 50 or 60 degrees, it contains a large amount of acidic condensate (sulfuric acid, hydrochloric acid), coupled with the repeated washing of dust particles, so the expansion joints of general materials can't bear it at all.
Two days ago, I met a friend from a power plant. The ordinary metal rectangular expansion joint they used had intergranular corrosion and cracked welds in less than two years. Later replaced withNon-metallic expansion joint (fabric fiber expansion joint)As a result, the fabric layer was deflated because the negative pressure working condition was not considered in the type selection. You say it was wrong or not?
There is even more ruthless-the design of the inlet and outlet flue of some desulfurization towers is unreasonable, and the thermal displacement directly exceeds the absorption capacity of the expansion joint. In this case, whetherHigh temperature axial expansion jointStillCompound hinge transverse expansion jointHe couldn't bear it hard. Therefore, figuring out why is ten times more important than rushing to change parts.
How to judge if the expansion joint is broken? Don't ignore these signals
Are small cracks, slight deformations, a big problem, you might ask? Yeah, and very much. Expansion joint failure never happens suddenly. It will give a series of warning signals. The key is that you have to understand it.
- Visible leakage: Black or yellow water stains around the flue, or even white smoke. Don't think it's just a lax seal, the inside may have corroded and perforated.
- abnormal noise: When running, you hear a "hissing" sound, which is mostly a gas ejected from the crack at high speed. Once this kind of sound appears, it means that the problem is not small.
- Deformation and bolt loosening: Bulging or collapsing of metal bellows, or loose flange connecting bolts. EspeciallyUniversal corrugated expansion jointWhen the lateral displacement exceeds the limit, the bellows will soon break by fatigue.
- Wet staining of insulation layer: If the outside of the expansion joint is wrapped for insulation, wet stains are the most intuitive signal-the inside has leaked, but the surface is invisible.
I suggest you do a special inspection at least once a quarter, with a flashlight from different angles, focusing on the welds and flange roots. Don't be too troublesome, wait until the leak is big before dealing with it. If you shut down for a day, you will lose hundreds of thousands.
Fix or replace? Comparison of Cost and Effect of the Two Schemes
Many people's first reaction is to "fix it first". But are repairs really a bargain? Let's settle the account.
Fix scheme:For example, patching the damaged part of the non-metal expansion joint, or repair welding the metal bellows. The cost of restoration is about several thousand to ten or twenty thousand, but the lifespan after restoration usually does not exceed one year. Moreover, it is necessary to shut down during the repair process, and the shutdown loss ranges from tens of thousands to hundreds of thousands per day. When it comes down to it, the total cost of a repair may be higher than a new one.
Replacement scheme:Direct replacement of the same modelRectangular non-metallic expansion jointOrrubber compensatorThe cost may be 20,000 to 30,000 to 50,000 to 60,000, but the service life of regular products is generally 3 to 5 years. If you select that right material (such as for acidic environmentPTFE compensatorOrPTFE-lined hose), it is not a problem to use it for 6 to 8 years. On average, every year, the cost of replacing a new one is lower.
Moreover, some of the damage can't be repaired at all. For example, if the metal bellows has penetrating cracks or plastic deformation, forced repair welding will only make the stress concentration more serious and crack faster next time. In this case, replacement is the only option. Therefore, my suggestion is very direct: don't worry about repairing it. Unless a small area of non-metallic layer is damaged and the skeleton is not damaged, it is a more worry-free way to replace it directly.
Choosing the right expansion joint works better than repairing it a hundred times
In the final analysis, the problems of expansion joints are mostly wrong selection. The working condition of the flue of the desulfurization tower is special, so you can't just take an expansion joint and push it up.
Look at the media first. The flue gas after wet desulfurization contains a large number of acidic droplets, which ordinary carbon steel bellows can't bear at all. At this time, either chooseNon-metallic expansion joint (fabric fiber expansion joint)Using its corrosion resistance and vibration damping properties; Either choosePTFE-lined hoseThe tetrafluorine layer can effectively isolate acid liquid. If the temperature exceeds 200℃, you have to useHigh temperature axial expansion joint, with high temperature resistant alloy.
Look at the displacement. The thermal displacement of flue system is large, and it tends to be multi-directional displacement. A single axial expansion joint may not be sufficient and needs to be usedCompound straight pipe bypass pressure balanced expansion jointOrCurved tube pressure balance expansion jointTo eliminate blind force. In addition, the direction of the guide tube directly affects the service life, and the direction of the arrow must be consistent with the flow direction of the medium during installation (please refer toSpecific Function of Expansion Joint Guide TubeOne article).
Finally, don't forget the matching onesFlue gas baffle doorAndDouble-sealed single-axis circular baffle door。 Many leakage accidents actually start from the lax seal of the baffle door, and the smoke backwards corrode the expansion joint. So the overall system design is fundamental.
Four Key Details of Construction and Maintenance
If you choose the right product, installation and maintenance can't keep up with it. Talk about a few places that are easy to overturn in practice:
- The tie rod nut should be adjusted in place during installation.Many masters forget to loosen the tie rod nut for transportation after installing the expansion joint. As a result, the expansion joint could not expand and contract freely, and the bellows was directly pulled and cracked. The correct thing to do is to remove the dowel bolts after installation, or follow theHow to adjust the tie rod nut of expansion jointDescription, reserve the pre-stretch value.
- Gaskets must be corrosion resistant.Ordinary asbestos gaskets are used at flange joints, which will be corroded by acidic condensate in less than half a year. Always use PTFE gaskets orRubber PTFE compensatorThe matching flange pad.
- Don't wrap the insulation too dead.Some operation and maintenance personnel are afraid of heat dissipation, so they wrap the expansion joint in heat insulation. However, non-metallic expansion joints need to be ventilated and drained, and if they are wrapped up, they will accelerate aging. The right thing to do is to keep the bottom drain holes and set vents outside the insulation.
- Check the deflector regularly.Once the guide tube falls off or corrodes, the high-speed dusty flue gas will directly scour the bellows body, and the life of the bellows will decrease by a cliff. Every time the machine is shut down for maintenance, the manhole should be opened to check the state of the guide tube.
Having said so much, it is actually a sentence: the "small accessory" of the flue expansion joint of the desulfurization tower, if you attach importance to it, it will not cause you chaos; You ignore it and it can cost you hundreds of thousands. Don't wait for it to be missed before you rush to change it. Do a good job of selection and maintenance in advance. This account is worth it.