Just how 'perverted' is it in the circulating flue?
To be honest, the circulating flue is not a place for "normal people". When you look at the circulating flues of power plants, steel mills and cement plants, there is everything in them: high-temperature flue gas can always move by three or four hundred degrees Celsius, and when the sulfur content is high, ordinary steel can be chewed into slag, and the high-speed ash particles are polished every day like sandpaper-this is the working condition that expansion joints have to face every day.
Two days ago, I met a customer from a cement factory who told me about their lineCirculating flue expansion joint, it cracked less than three months after changing it. I asked him about the working parameters, and he was stunned and said, "Just choose according to the routine". Routine? What is the routine of circulating flue! Large temperature fluctuation, many corrosive media and high ash concentration. These three conditions are put together. If the selection is a little worse, the life span is 30% off.
Regardless of the working conditions, material selection means laying mines for the equipment. Take metal bellows, for example. If austenitic stainless steel meets flue gas containing chloride ions, it is only a matter of time before stress corrosion cracks. However, if the non-metallic fabric is selected, the temperature exceeds the upper limit of its tolerance, and interlayer peeling and bulging are almost inevitable.
Metal Bellows vs Non-Metallic Fabric, Which Is Better For Your Flue?
This is a cliche question, but many people are planted on "only looking at the price". Metal Bellows Cheap? Not really. The general-purpose corrugated expansion joint does have a low unit price, but the displacement and temperature it can carry are limited. If you have a large axial or lateral displacement in the circulating flue, or if the temperature exceeds 400°C, the metal bellows may not be the optimal solution-this is when you should lookHigh temperature axial expansion jointOrCorrugated expansion joint for power station industryThey have made enhanced designs for high temperature and high pressure scenarios, such as multilayer corrugated and heat-resistant alloys.
Conversely, what about non-metallic expansion joints (aka fabric fiber expansion joints)? Its advantages are large amount of compensation, easy installation, and generally better corrosion resistance than metal. However, its shortcomings are also obvious: it is not resistant to high pressure and ultra-high temperature (generally no more than 600℃), and if the ash erosion is serious, the fabric layer is easy to wear out. Therefore, in the circulating flue, non-metals are more used in the low-temperature section or the position where large displacement compensation is needed.
Then how to choose? Don't pat your head, figure out the three numbers:Design temperature, media composition, displacement。 If the temperature exceeds 350℃ and there is corrosive smoke, I recommend giving priorityMetal rectangular expansion jointWith a guide tube, the material goes to 316L or 254SMO. Low temperature, large displacement,Rectangular non-metallic expansion jointMore cost-effective. You have to use a bargain? Okay, then be prepared to change it every six months-it may not be cost-effective to calculate the account.
How do you match expansion joints with flapper doors?
In many projects, the baffle door is next to the expansion joint. But these two things don't work well together, and the failure rate skyrockets. Common scenario: On the flue at the inlet of desulfurization tower, install it firstDesulfurization flue gas baffle doorInstall the expansion joint. What's the problem? When the baffle door closes, there is a huge blind plate force, and if the expansion joint is not designed to withstand enough, the bellows can be squashed or cracked.
Both upstream and downstream of the baffle door are provided with limiting pull rods, or the expansion section has a pressure balanced structure。 Such asDouble-sealed single-axis circular baffle doorOrSingle-axis double-flapper doorThey have good sealing when closed, but the blind plate force is not small, so you have to calculate it clearly. The same is true for the manual plug-in insulation door. Don't think that the force is small because it is manual-the flue section is large, the medium pressure acts on the plug-in board, and the thrust can reach several tons.
The connecting pipe between the expansion joint and the baffle door should not be too short. Allow at least a straight pipe section of 1.5 times the diameter, otherwise airflow turbulence will exacerbate the vibration and deflection of the expansion joint. And guess what? In order to save space, many people put the expansion joint against their face at the exit of the baffle door, and as a result, the guide tube was worn out in three months.
Install it in place and you're done?
Alas, I get angry when it comes to installation. Many users think that the expansion joint can be bought and connected, and they don't read the installation manual at all. In fact, there are at least three pits in the installation link, and the life of stepping on one reduces in half.
First, the direction of the guide tube.What is the specific function of the expansion joint guide tube? It is to guide the flue gas flow and reduce the direct scouring of the bellows. However, the installation direction should follow the flue gas flow direction, and the C end (the outlet end of the guide tube) pointed by the arrow must be facing the medium outflow direction. Install backwards? The guide tube becomes a dustpan for a ride, and the ash is all accumulated in it, and it won't take long to grind through. Specifically, you can refer to the question and answer of "The Specific Function of Expansion Joint Guide Tube" in our station, which has pictures and truth.
Second, tie rod nut adjustment.The function of the expansion joint tie rod is to constrain the displacement range of the bellows, but the tie rod nut is usually in a pre-tensioned state when it leaves the factory to prevent transportation deformation. After installation in place, the nut must be loosened or adjusted as per the design requirements to allow the expansion energy saving to work properly. If you keep tightening it without loosening it, then it is not a compensator, but a rigid connector. The pipe is pushed hard by thermal expansion and cold contraction, so it is weird that it is not bad. Adjustment method? There are detailed steps in "How to Adjust the Tie Rod Nut of Expansion Joint" in our station. Simply put, it is: according to the designed pre-stretching amount, screw the nut to the designated position, and then fix it with a locking nut.
Third, pre-stretch.In cold installation, if the pipe work will be elongated by heat, the expansion joint needs to be pre-stretched to offset this part of the displacement. Many people think that the trouble is saved directly. As a result, the bellows is compressed to the compression limit after heating up, resulting in permanent deformation. Less of this step, the service life of the expansion joint is at least halved. This is not alarmist. Which link goes wrong, and the compensation is more than that installation fee.
If it's broken, don't rush to replace it with a new one. Check these places first
When the expansion joint is broken, the first reaction is often to "replace it with a new one". But you should know that if you find out the reason before starting, you may repair it with another part, saving time and money.
Look at bellows cracks first.If the longitudinal crack along the trough occurs in the metal bellows, it is mostly stress corrosion or fatigue failure. You should check whether the medium contains chlorine or sulfur, and whether the design temperature exceeds the corrosion resistance temperature of the material. If it is only local corrosion, repair welding or lining can be considered; But for the most part, cracks mean that the material has degraded across the board and can only be replaced.
Look at the non-metal layer bulge again.Expansion joints of fabric fibers are bulging, usually due to water vapor entering between the layers or excessive temperature that causes binder failure. If the bulge is not large, it can be partially removed and heat patched with the same material patch, but the premise is that the internal and external pressure difference is balanced. If the bulge is attached to the flange sealing surface, then don't fix it, just replace itNon-metallic expansion joint。
Finally, check the deflector for wear.When you open the expansion joint inspection port, if you find that the wall thickness of the guide tube has been thinned by more than 50%, or there is perforation, it means that the material of the guide tube was not selected correctly or the thickness was not enough in the original design. In this case, change to wear-resistant alloy plate (such as wear-resistant steel NM400) to make a deflector and replace it, which will probably last until the next overhaul. Don't scrap the whole expansion joint at every turn. It only costs a few dollars to replace the guide tube.
At the end of the day,Circulating flue expansion jointNot something high-tech, but it needs to be taken seriously in every detail. Right selection, right installation, right maintenance, it can run steadily for several years; Which link makes do, it will show you the color. Those in this business are all practical experience, don't you believe it? Ask those peers who repeatedly change expansion joints. Nine times out of ten, they have made the wrong choice.