Air preheater inlet metal expansion joint: how to carry those invisible thermal stresses?
Two days ago, I met an old friend who was doing power station maintenance, and talked about the problems with the expansion joint at the entrance of the air preheater in their factory. He complained, "I obviously selected the model according to the drawing, but after less than a year of use, the bellows cracked. When I removed it, I saw that it was full of dust." This kind of thing is too common in power plants-the metal expansion joint at the inlet of the air preheater, which looks inconspicuous, is the most easily underestimated link in the whole smoke and air system. The problem is not in the expansion joint itself, but in the selection and installation, the working condition is not taken seriously at all.
How ruthless is the working condition of the air preheater inlet? — Find out what it has to face first
The inlet of the air preheater is connected to the flue at the tail of the boiler and the air preheater. The flue gas temperature is generally between 350℃ and 400℃, and some old units can even rush to 450℃. The pressure is not particularly high, usually a few kilopascals, but the key is that there is a lot of fly ash in the smoke-the concentration is often tens of grams per standard cubic meter. What is even more headache is that the air preheater body will have thermal expansion during operation, and the displacement between the pipe box, the shell and the flue will occur in several directions: 20 to 40 mm in the axial direction and 10 to 20 mm in the transverse direction. These displacements do not occur at uniform speed, but fluctuate repeatedly with load changes.
Calculate, high temperature + dusty airflow + multi-directional displacement + frequent circulation, this set of combination punches, ordinary compensation structure can't bear it at all. So why do so many power plants chooseAir preheater inlet metal expansion joint, instead of rubber compensators or non-metallic expansion joints? Because the metal bellows has high temperature resistance, stable pressure and long fatigue life. But only if-you have to choose the right one.
How does the metal expansion joint withstand the double test of high temperature and ash powder? — — Structure, material and corrugation
Let's talk about the materials first. Since it is a high temperature, you definitely can't use a regular 304. The commonly used materials for corrugated expansion joints in power stations are 316L or Incoloy 825, and even more ruthlessly, high-temperature alloys such as GH3030 are used. However, the material alone is not enough, and the wall thickness and waveform of the bellows should also match. If the wall thickness is too thin, it will be easily eroded and perforated by ash particles, and if the wall thickness is too thick, it will affect Generally, the wall thickness of the expansion joint at the inlet of the air preheater is between 1.2 mm and 2.0 mm, depending on the flue gas flow rate and dust content.
structurally,Air preheater inlet metal expansion jointIt is usually rectangular or circular, more rectangular because the flue section is square. Make sure to add a guide tube inside-this thing I specifically checked the FAQ of this site,Specific Function of Expansion Joint Guide TubeMake it very clear: the guide tube directs the dusty airflow toward the center, avoiding the direct exposure of the ripples to particle scour. Without a deflector, it is common for bellows to wear out within a year. In addition, the number of corrugated layers is also particular. If a single layer is not enough, there are multiple layers, and some even four or five layers, with an isolation layer in the middle, which can not only reduce vibration but also improve the pressure resistance.
The most common mistake in model selection: How should the displacement amount, guide tube and tie rod be matched?
I have been to several sites and found that the selection mistakes are mainly concentrated in three places.
First, the amount of displacement is too rough. Many designers only give an axial compensation amount and ignore lateral and angular displacements. In fact, the flue at the inlet of the air preheater often has deflection deformation due to thermal expansion. If there is no compound hinge transverse expansion joint or a structure with universal hinge, the bellows will be screwed and stressed, and the stress concentration will naturally be easy to crack. This site'sCompound hinge transverse expansion jointIs specifically designed to deal with this kind of situation.
Second, the size and fixing method of the guide tube are not attentive. The guide tube is too short to protect; Too long, it can increase local resistance and even cause vibration. The correct approach is to insert the guide tube 50 to 80 mm into the flue, and the thickness cannot be less than 4 mm, otherwise it deforms faster at high temperatures.
Third, the adjustment of the tie rod. Many projects don't move after installation.Function of expansion joint tie rodIt is to limit the bellows to work within the rated displacement range and prevent over-stretching. After the installation is completed, the tie rod nut must be loosened to the reserved position of the design. How to adjust it can be seen on our stationHow to adjust the tie rod nut of expansion jointThat Q&A. However, the situation that often occurs at the scene is that the nut is screwed to death, or the tie rod is not installed when it is simply installed, and as a result, the bellows is directly fatigued and broken. To put it bluntly, this problem is that there is not enough responsibility, but the price is tens of thousands of dollars.
Installation and routine maintenance: You thought it was done by installing it? If these steps are not done well, they will still leak
Installing this step, there are two pits that are particularly easy to step on. One is that the current is too large during welding, which burns through the bellows wall. The walls of the bellows are very thin, and the base metal will be damaged by welding stress and splash. The correct way is to weld the connectors (such as flanges or connectors) first, and then assemble the bellows assemblies after cooling and connect them with bolts. If on-site welding is necessary, low current, multi-layer and multi-pass welding must be adopted, and penetration testing must be done immediately after welding.
Another is clearance management for bushings and deflectors. There should be enough gap between the guide tube and the inner wall of the flue, generally 10 to 15 mm, to prevent the guide tube from being squeezed and deformed when the flue is thermally expanded. But the gap is too large, and the airflow will flow backwards to flush the bellows. This balance needs to be calculated based on the actual pipe diameter and temperature, and it is not recommended to pat the head.
What about routine maintenance? Don't expect to do it once and for all. It is recommended to open the manhole and check it once every minor repair cycle: see whether the guide tube is deformed or falling off, whether there is pitting or wear on the corrugated surface, and whether the tie rod bolts are loose. If you find that the bellows has a local corrosion pit with a depth of more than 0.3mm, consider replacing it in advance-wait until there is an air leak before replacing it, and the downtime loss is greater.
Failure case analysis: From cracking to stuck, most problems are in the early stage
Tell me two real examples.
600MW unit of a power plant in the north,Air preheater inlet metal expansion jointThe bellows root cracked after 14 months of operation. Disassembly analysis shows that the design displacement is only 30mm in the axial direction, while the actual pipeline has a lateral displacement of 15mm because of the loose support. In addition, the guide tube is seriously worn, and holes have been worn out at the edge, and the high-temperature flue gas directly washes the inside of the corrugation, which accelerates the fatigue. Root cause: the yaw of the actual direction of the pipeline was not considered when selecting the type, and the anticorrosion coating of the inner part fell off.
Another factory used a non-metallic expansion joint, which leaked after half a year's use. Later, it was replaced with a metal rectangular expansion joint-but the tie rod was not adjusted when it was installed, and the bellows was always in a compressed state. After a year, it was stuck at the minimum length, which could no longer compensate for the displacement, and the whole flue was pulled and deformed. This is typicalDo you need to remove the screw of the expansion jointThat kind of problem, it didn't release in time after installation.
You see, all the negligence in the early stage will be recovered ten times a hundred times later.
Why are metal expansion joints preferred over other compensation methods for power station entrances?
Someone asked, the rubber compensator is cheap and the non-metal expansion joint is corrosion resistant, so why can't it be used? In fact, it can also be used, but the score working condition. The rubber is directly carbonized in the high temperature situation above 350℃ at the inlet of the air preheater. Although the fabric fiber layer of non-metallic expansion joint can withstand the temperature to 400℃, it is afraid of erosion and pressure fluctuation, and once the ash powder absorbs moisture and scales, it will soon lose its flexibility. The advantage of metal expansion joint is that it is resistant to high temperature, pressure and wear at the same time, and it can also cope with complex displacement combinations through structural design (such as multi-layer corrugation, with baffle and with guide tube).
Provided by this siteCorrugated expansion joint for power station industryIt is developed for this type of application and will be pre-stretched before leaving the factory to ensure accurate absorption of thermal displacement at installation temperature. If you are using a small unit or auxiliary flue, chooseUniversal corrugated expansion jointEnough too; However, the main flue, large diameter, high temperature and high pressure must be dedicated power station-grade products.
Don't just stare at the expansion joint itself. Pipeline supports, hangers and limiting devices, these supporting designs can't keep up, no matter how goodAir preheater inlet metal expansion jointIt's also useless. Thermal stress is invisible, but the consequences are visible-air leakage, rising energy consumption, shutdown for maintenance, none of which are cost-effective.