1. Why can't the chimney of the power plant be separated from the metal expansion joint? — — Start with thermal displacement and corrosive environment
The chimney of the power plant looks like a silly, big, black and thick concrete or steel cylinder, but the internal flue gas pipe system is actually "moving" all the time. When the unit starts and stops, the load changes, the flue gas temperature can jump from tens of degrees to hundreds of degrees, and the flue wall expands and contracts accordingly. Don't underestimate the displacement of several millimeters to tens of millimeters. If you hold it hard, the pipeline interface, bracket and desulfurization tower entrance will all have to be cracked.
At this time, it was the turn of the metal expansion joint of the chimney of the power plant. It is installed in the middle of the flue, and the thermal displacement is eaten by the elastic deformation of the bellows, so that the pipe should be extended and contracted, and the stress is not transmitted to the equipment. Speaking of the expansion joint itself, the core is the bellows, which is mostly made of stainless steel, and it has to bear corrosion in temperature and pressure resistance.
However, the flue environment of power plants is different from that of ordinary steam pipes. The flue gas contains sulfide and chloride ions. After wet desulfurization, the humidity is high and the temperature is low. As soon as the condensed water comes out, it is dilute sulfuric acid. In this environment, ordinary stainless steel bellows may have pitting corrosion and stress corrosion cracking in less than two years. Therefore, the metal expansion joint of the chimney of the power plant can't be topped by just taking a general bellows, and the material, structure and diversion mode have to be recalculated.
Second, selection is not patting the head: understand the matching between working condition parameters and expansion joint types
I've seen too many people stumble when it comes to model selection. Just reporting a "flue expansion joint" makes people quote, and the temperature, pressure, displacement and media composition are not mentioned. What is the difference between seeing a doctor and not saying symptoms?
You have to figure out the working parameters first. What is the flue gas temperature? Desulfurization tower inlet or outlet? Is there any dust accumulation? What are the axial and lateral displacements that need to be compensated? Find out these, and then talk about what type of expansion joint to choose.
The most used in the flue of power plants is the corrugated expansion joint used in the power station industry. This kind of product has been optimized for flue gas working conditions. The material, thickness and wave height of the bellows are designed according to the parameters of the power plant. The temperature resistance range is wide, and the corrosion resistance is much stronger than that of the general type. If it is a high-temperature section, such as the flue between the boiler outlet and the air preheater, a high-temperature axial expansion joint is required, which separates the high-temperature medium from the bellows through the combination of multi-layer bellows and heat insulation layer to avoid the bellows from directly bearing thermal shock.
Rectangular flue should choose metal rectangular expansion joint, corner treatment is difficult, weld stress concentration is the most likely to cause problems. As for the self-righteous ones who put universal corrugated expansion joints on power plant flues, I urge you to give up the idea as soon as possible. The working conditions do not match, and the money saved is not enough to change the bellows once.
Third, disassembly of failure cases: corrosion, fatigue, guide tube wear, the problem often lies in the details
After talking about expansion joints for so many years, there are not one hundred but also eighty failed parts that have been dismantled. There are three most common ways to die for metal expansion joints in chimneys of power plants.
The first is corrosion. After wet desulfurization, the flue gas has low temperature, high humidity, high concentration of chloride ions and sulfate ions, and liquid accumulation is easiest at the trough of bellows. And guess what? Corrosion starts at the trough, first pitting, and slowly turns into penetrating cracks. In this case, if you are still using ordinary 304 material, it is basically the life of regular replacement. Switch to nickel-based alloys that are resistant to chloride ion corrosion, or make double-layer structures, which can last longer.
The second is fatigue fracture. Flue vibration, wind pressure pulsation, the stress at the peak of the expansion joint is the largest, and it finally cracks after repeated bending. This is related to the fact that the displacement amount is too conservative during model selection, and it is also related to the fact that the expansion joint is installed too close to the vibration source. The treatment method is to check the fatigue life, and if necessary, install external restraint devices to absorb the vibration.
The third is the deflector tube wear-out. The flue gas is ashy and the flow rate is high, so the guide tube bears the brunt of being washed out. The guide tube wore out, and the smoke directly washed the inner wall of the bellows, which was quickly finished. Therefore, the guide tube itself is a consumable, so the replaceable structure should be considered when designing. Don't weld the guide tube and the bellows to death-otherwise, you have to change the guide tube together with the bellows, and the cost will go up.
4. Practical points of installation and maintenance: cold tightening, tie rod nut, bellows protection, don't let small mistakes destroy large equipment
There are more pits in the installation process than in the selection. Let's talk about cold tightness first. Cold tightening is to pre-pull or pre-press a certain amount during installation, so that the expansion joint just returns to the free state at the operating temperature. When the cold tightness is calculated, the thermal displacement will not die; Without cold tightening, the expansion joint may exceed the allowable displacement as soon as it is started, and the bellows will be directly scrapped. This must be according to the design drawings, and don't make your own decisions on the spot.
The adjustment of tie rod nuts is also a common point of contention. The role of the tie rod is to restrain the bellows and prevent it from over-stretching under the action of internal pressure. When installing, the tie rod nut should be loosened so that the bellows can expand and contract freely; However, before running, the nut must be adjusted in place according to the design requirements to play a limiting role. Many on-site workers try to save trouble, either not loose or tight. In the end, the bellows is either pulled or crushed.
The protection of bellows should not be careless. During electric welding, the welding slag splashed on the bellows is a burn-through point. During transportation and hoisting, the bellows surface is bumped out of pits, which is the starting point of fatigue cracks. Look at those expansion joints that have been used for ten years and those that leak after three years of use. The difference is often not in how good the material is, but whether the bellows is taken seriously when installing it.
From expansion joint to baffle door: cooperative design of supporting equipment in smoke and air duct system
Flue systems don't do everything with expansion joints alone. The expansion joint is responsible for absorbing displacement, and the baffle door is responsible for cutting off the medium. Both must be designed together.
During the maintenance of the desulfurization system, the flue gas channel needs to be completely cut off. At this time, the flue gas baffle door is used, which requires extremely high sealing performance. If the expansion joint is too close to the baffle door, the thrust generated when the baffle door is closed presses directly on the expansion joint, and the bellows will deform beyond the limit. Therefore, when designing, it is necessary to keep enough straight pipe sections between the baffle door and the expansion joint, or add limiting rods on the expansion joint to separate the forces.
For example, when switching the air duct, one channel is closed and the other channel is opened, and the instantaneous pressure fluctuation will be transmitted to the expansion joint. Under this working condition, the dynamic load should be considered in the selection of expansion joint, rather than static displacement only. Therefore, those who design flues in power plants must take the expansion joint, baffle door, fan and bracket as a system to make overall planning, and don't take care of each other. Finally, the interface is full of problems.
After all, the metal expansion joint of the chimney of the power plant is a typical example of "small equipment and university". It is not expensive, but if it breaks down, it will affect the operation safety of the whole unit. Think more about the selection, pay more attention to a few details during installation, and record a few more data during maintenance, which is much more cost-effective than emergency repair afterwards. Everyone who works in power plants understands this truth.