Two days ago, I received a phone call from a customer, saying that the expansion joint of the returner leaked after less than a maintenance cycle. When I removed it, I saw that the bellows wore out, the guide tube fell off, and the lining was half broken. The other end of the phone is still complaining that the working conditions on the site are too bad, saying that no matter how good the expansion joint is, it can't hold it. But when we checked the original selection parameters, we found that it was not the case at all-when selecting the model, we thought of the working conditions simply.
What exactly is the returner metal expansion joint carrying?
First, break up the real working conditions of the returner and crush them clearly. The internal medium of the returner of circulating fluidized bed boiler is a mixture of high-temperature circulating ash and flue gas, and the temperature is generally between 850℃ and 950℃, and some can even break through 950℃. The concentration of solid particles is high, the flow rate is fast, and the erosion and wear on the wall surface are very serious. At the same time, remarkable thermal expansion displacement will occur in the process of starting and stopping furnace and load change, and this displacement is often not a single direction, but a complex displacement superimposed in axial, transverse and angular directions.
That is to say, the metal expansion joint of the returner has to face the triple test of high temperature, high wear and thermal displacement at the same time. This is not an occasion for ordinary expansion energy savings to cope with. If you install a general-purpose corrugated expansion joint on it, the corrugated pipe material can't withstand high-temperature creep, and the guide tube has no wear-resistant design, so it won't take long for problems to happen. This logic has to be understood when choosing a model.
Doorway in structural design: guide tube, wear-resistant lining, corrugated pipe material
The core of the structural design of the metal expansion joint of the returner lies in the guide tube and wear-resistant lining. The function of the guide tube is to guide the high-temperature dusty flue gas to pass smoothly, so as to avoid the medium directly washing the bellows. However, the guide tube itself is also in a high temperature and high wear environment, so the material and wall thickness are particular. Generally, heat-resistant alloy steel should be selected, and the thickness should not be too thin, otherwise it will be worn out soon. Wear-resistant lining is usually a high-temperature castable cast inside the expansion joint, which can effectively reduce the temperature of the metal wall surface and resist the erosion of solid particles.
The choice of corrugated pipe material is equally critical. Austenitic stainless steel is usually used for general-purpose corrugated expansion joints, but under the high temperature environment above 850℃, the creep limit of 304 and 316 drops very rapidly, and excessive plastic deformation and even instability will occur during long-term operation. It is recommended to choose superalloys such as Inconel 625 or GH3030 for returner conditions. Although the cost has gone up, the life span is completely different.
In terms of structural type, why is the high-temperature axial expansion joint more suitable? Because it is specially designed for working conditions with large axial displacement and high temperature, the bellows adopts a multi-layer structure with moderate thickness of each layer, which not only retains flexibility, but also improves pressure and temperature resistance. However, the general-purpose corrugated expansion joint often only considers the pressure-thrust balance, and has limited resistance to high temperature and wear. To put it bluntly, the general-purpose type is for conventional pipelines, and special products have to be used in such occasions as returning devices.
Four Parameters Easily Overlooked in Type Selection
Next, let's talk about type selection, which is where the metal expansion joint of the returner is easiest to overturn.
The first is the temperature margin. Many selection personnel directly select materials at the operating temperature of 850℃, but do not consider the possibility of furnace temperature fluctuation and local overtemperature. At the time of selection, leave at least a margin of 80℃ to 100℃ for the temperature, and check the strength of the material according to 950℃ or even 1000℃, otherwise you will be dumbfounded in case of emergencies.
The second is fatigue life. Some people only look at how much displacement the bellows can absorb, but not the number of fatigue times. The fatigue life of bellows is exponentially related to the displacement. If the displacement increases by 20%, the life may decrease by half. The thermal expansion of the returner occurs repeatedly. Starting and stopping the furnace once a day is a cycle, and it is hundreds of times a year. When designing, the displacement should be controlled within the range corresponding to the allowable fatigue life. Usually, the displacement-life curve will be given on the bellows product sample, so just check it.
The third is the stiffness calculation. The bellows stiffness K directly determines the reaction force of the expansion joint on the pipe and the returner body. The greater the stiffness, the greater the reaction force, which will affect the force of the whole system. The formula for stiffness calculation is K =1.7 Dm E δ ³ n/ (4 B³), where Dm is the mean diameter of the bellows, E is the elastic modulus, δ is the wall thickness of a single layer, n is the number of layers, and B is the wave height. It can be seen from this formula that the influence of wall thickness on stiffness is cubic. If the wall thickness is slightly thicker, the stiffness will rise sharply, so multi-layer thin-walled structure is more reasonable than single-layer thick-walled structure. When selecting the type, do not only look at whether the bellows can absorb displacement, but also check the influence of stiffness on the boundary pipe.
The fourth is the direction of displacement. The thermal displacement of the returner is not purely axial and has a horizontal lateral component. If the type is selected only according to axial displacement, the bellows will bear extra transverse shear force in actual operation, which will easily lead to excessive local stress of the bellows. It is best to find out the displacement direction in the selection stage, and if necessary, choose the compound hinge transverse expansion joint or straight pipe pressure balance expansion joint to deal with it.
Common pits for installation and O&M
There is no problem in the selection, but if the installation and operation and maintenance are not done well, all the previous efforts will be wasted.
Let's start with pre-deformation. When the metal expansion joint of the returner is installed in the cold state, it needs to be pre-stretched or pre-compressed in the opposite direction of the hot displacement, so that the bellows can work near the equilibrium position in the hot state. Many installation teams don't do this step because it is troublesome. As a result, the bellows exceeds the allowable displacement in the hot state, and fatigue cracks soon.
Let's talk about the guide bracket. The guide brackets around the expansion joint should be set strictly according to the design drawings, and the spacing and position should not be changed indiscriminately. The role of the guide bracket is to ensure that the pipe is displaced in the intended direction, and if it is missing or incorrectly set, the expansion joint will bear bending loads outside of the design. In reality, many field guide brackets are installed in a mess, and the expansion joints are displaced and twisted, so they can only be replaced completely.
Welding slag. When welding the returner pipeline, if the sundries such as welding slag and welding electrode head are not cleaned up, they will fall into the bellows trough, which will jam the bellows during operation, affect the displacement absorption, and even directly scratch the surface of the bellows, becoming the starting point of corrosion and fatigue. This seems to be a minor problem, but it actually causes a lot of failures.
Inspection is also particular. Most inspectors of power plants only look at whether there is air leakage and abnormal noise in the appearance, and rarely measure the displacement of the expansion joint. In fact, the displacement is an important index to judge the working state of the expansion joint. Use stainless steel ruler or vernier caliper to regularly measure the distance between the peaks and valleys of the bellows, and compare it with the initial record to find whether there is any abnormal deformation. It is recommended to make a displacement record every maintenance cycle to form trend data, so that the problem can be exposed early.
Review of failure cases
Tell me a few real cases.
In the first case, the metal expansion joint of the returner of a power plant was worn and leaked after half a year's operation. Disassembly and inspection found that the guide tube had been worn out, and the surface of the bellows was directly washed out of the holes by circulating ash. The reason is that the material of the guide tube is ordinary 304 stainless steel, which is not wear-resistant and the wall thickness is not enough. Once the guide tube fails, the bellows is directly exposed to the high-speed dusty air flow, and the wear rate is very fast. During on-site investigation, first listen to the sound of air leakage, and then check the surface temperature of the expansion joint with an infrared thermometer. If there is a local low temperature area, it means that the internal guide tube has been damaged and the airflow is short-circuited. The remedy is to replace the expansion joint with the wear-resistant liner and check whether the material and thickness of the guide tube meet the requirements.
In the second case, the bellows became unstable. The bellows showed obvious lateral bending and uneven wave pitch during operation. The reason is that the combined effect of internal pressure and displacement is not fully calculated during the type selection, the effective area of bellows is large, and the pressure thrust exceeds the column stability limit of bellows. In addition, the number of layers of the bellows is not enough, and the yield strength of the material decreases at high temperature, and the ability to resist instability is further weakened. The discovery of this problem is usually found in the abnormal appearance of the bellows during inspection. It is best to stick it with a ruler to check whether the wave peaks are on the same straight line. The only remedy is to shut down and replace the bellows, and improve the number of layers and material grade.
In the third case, the guide tube fell off. This is the most dangerous, falling deflector debris falls into the returner and may block the return channel. The reason is that the connecting weld between the guide tube and the expansion joint body produces thermal fatigue cracks under high temperature cycle, and finally breaks and falls off. This kind of problem is difficult to find in advance during daily inspection, but the state of the guide tube can be monitored by setting a temperature measurement point outside the expansion joint-if the guide tube falls off, the airflow distribution suddenly changes, and the surface temperature will fluctuate abnormally. The way to prevent problems before they occur is to design the connection structure of the guide tube well in the production stage, so that the weld seam is fully penetrated, and if necessary, the rib plate is added to reinforce it.
The metal expansion joint of the returner is complicated and simple. Carry high temperature, wear resistance, absorb displacement, and don't leak air, just these four things. However, each piece has to be practical and detailed in the selection stage, otherwise there will be problems after operation, and an expansion joint will be cheap to replace, but no power plant can bear the loss of shutdown.
After working for so many years, I have seen at least hundreds of failure cases of the expansion joint of the returner, and most of the problems can be traced back to an oversight or a take for granted in the selection stage. Therefore, there is a saying that I want to give to all colleagues who are engaged in the maintenance of boiler auxiliary machines: it takes an extra hour to select the model, and the operation will be worry-free for a whole year.