Why does the expansion joint in the desulfurization system break faster than elsewhere?
In the maintenance season of power plant desulfurization system, you go around the flue gas baffle door, and at least five or six of the ten metal expansion joints are running with injuries. It is not that the scene is not paid attention to, but the working conditions in this place are too tricky.
Slurry corrosion is the number one killer. The net flue gas from the wet desulfurization tower is not high in temperature but close to saturation in humidity, and it also carries a large number of gypsum slurry droplets. The slurry dripped down the walls of the flue, where the troughs that encountered the corrugated expansion joints accumulated — the troughs, which were originally designed to absorb displacement, now became reservoirs of corrosive fluid. The concentration of chloride ions is slightly higher, and the bellows of 304 stainless steel will not last for three years before pitting pits.
Chloride ion stress corrosion is more insidious. It is not visible on the surface like uniform corrosion, but it drills along the grain boundary, perhaps with only a fine line on the surface, and the inside is already cracked. Once such cracks appear, they are basically sentenced to death. In addition, the desulfurization system starts and stops frequently, the flue gas temperature fluctuates high and low, and the bellows repeatedly expands and contracts, so fatigue cracks appear along the trough position of stress concentration.
And wear and tear. The expansion joint near the smoke baffle door and the guide tube were washed away by dusty smoke every day. After being worn out, the slurry directly impacted the bellows body. The corrosion rate, tsk tsk, was five years higher than normal working conditions in one year.
Diagnosis before maintenance, don't guess with naked eyes
Two days ago, I met a customer who said that the expansion joint was leaking, and took a photo so that we could judge whether we could fix it. In the photo, the surface of the bellows is all ashy, so you can't see the situation clearly. I said you should take a thickness gauge to measure it first. He was not happy, saying that the old master knew it at a glance. And the result? After removing it and measuring it, only 0.4mm is left in the thinnest part of the wall thickness, and there is a hole when you poke your finger.
This job is similar to doing a gastroscopy for a patient. You have to use instruments, and you can't just ask whether it hurts or not. Three steps:
See if there are pitting pits on the bellows surface.Shine it obliquely with a flashlight, and the pitting pit will reveal small dark circular spots in the light. Dense, continuous pitting corrosion indicates that the material is selected low or the chloride ion concentration exceeds expectations.
Second, feel whether there is any bulging deformation in the peaks and troughs of the waves.Put on gloves and touch them in the direction of the ripples. You feel that there is a local bulge or depression, which is mostly due to overpressure or instability. This deformation is irreversible, and the original fatigue life cannot be restored by repair welding.
Third, check whether the guide tube is worn out.The wear of the lining guide tube is invisible to the naked eye, so it has to be seen through the interface gap with an endoscope. Wear-down is usually on the downstream side of the medium flow direction, where the vortex is most intense.
These three steps alone are not enough. If you want to judge the nature of the crack, you have to do penetration detection. Intergranular cracks and fatigue cracks have different strike and morphology-the former is a network along the grain boundary, and the latter is a typical transgranular parallel line. Only when you figure it out can you decide whether to make up or replace it.
Can it be repaired or should it be replaced? It's a matter of settling accounts
Weld cracks can be repaired, but it depends on the material. Can the electrodes of 316L and 254SMO be the same? If the weld material is selected wrong, the corrosion resistance of the weld is lower than that of the base metal, and it will continue to leak after repairing.
Fatigue cracking of the bellows body, or chloride ion stress corrosion-don't bother to mend this. It is also a white supplement, and it will leak after two months. The high temperature of welding will make the nearby grains coarse, and the stress corrosion sensitivity will be higher. At this time, direct replacement is more cost-effective than repeated welding. After all, the loss of one shutdown is enough to buy several expansion joints.
Then what situation is worth fixing? The deflector tube is worn out, so just change it; The tie rod bolt is loose, just pre-tighten it again; If the surface of the bellows is slightly scratched and there is no crack, measure the wall thickness after polishing, and it can be used if it is thick enough.
The material properties of the bellows itself cannot be compromised. If the corrugated pipe body is cracked, unstable or thinned by more than 30% of the design wall thickness, all the machines will be replaced. Don't worry about money, security is the biggest cost.
Replacement installation pits, nine times out of ten, are in pre-displacement and tie rod locking
Desulfurization pipeline is usually a large-diameter thin-walled pipe, and the expansion joint is not tightened when installed, and the blind plate force of the pipeline can crook the bracket. I saw a scene where the installer didn't install pre-displacement without trouble. On the first day of trial operation, the bracket was crooked, the expansion joint was forcibly stretched, and the bellows was directly twisted into a twist.
In addition, if you use the expansion joint matched by the electric plug-in insulation door or the desulfurization flue gas baffle door, remember to remove the transportation rod on the bellows before driving. I have seen this no less than three times-I forgot to dismantle it after installation, and as soon as I started the pipe displacement, the tie rod forcibly tore the bellows. The transport tie rod is to protect the bellows when shipped, not for operation. Such a large character is written on the sign, that is, someone doesn't read it.
Installation direction of expansion joint. The medium flow direction arrow should be in the direction marked on the expansion joint, and the sealing effect of the guide tube is completely lost if it is installed backwards. The slurry is backed into the gaps between the bellows, and the bellows can be scrapped in a few months.
For daily inspection after maintenance, it is enough to keep an eye on three points
First, check whether the drainage port is blocked. The drainage port is blocked, and the condensed water can't be discharged. It accumulates in the trough and soaks, and the corrosion rate doubles. Take a wire for every inspection, for ten seconds.
Second, listen to the ripple knot to see if there is any abnormal howling. When running on the grid, listening to the expansion joint, there is a sharp hissing sound indicating that there is airflow washing the leakage point, and the sound is completely different from normal working conditions.
Third, see whether there is white crystalline salt on the surface of the bellows of such equipment as external pressure single axial expansion joint. The salt in the smoke was analyzed to crystallize, which was a precursor to leakage, and the smoke was seeping out with slurry.
Under normal working conditions, it is not a problem to use metal expansion joints for ten or eight years, but in desulfurization environment, five-year inspection is the bottom line. It would be better if you could glance inside the bellows with an endoscope every year when the machine is shut down for maintenance-when visible cracks appear on the surface of the bellows, the corrosion depth will already exceed the safe value.
Desulfurization metal expansion joint maintenance is essentially a race against time. Accurate diagnosis, change when you need to change, standardized installation, regular inspection, every step is done in place, and the expansion joint is not so easy to drag back. I am afraid of taking a chance, dragging small problems into big failures, and finally not even having time to shut down for maintenance-that is the real trouble.