First, find out how "toxic" the desulfurization flue is? — — Working condition analysis
Don't tell me, everyone in power plants knows that the environment of desulfurization flue is simply not for people. When the temperature is low, it can reach 40 or 50 degrees, and when it is high, it can reach 160 or 70 degrees. The key is that it is full of sulfur dioxide and sulfuric acid mist. The flue expansion joint is soaked in acid every day, and it has to carry the heat and cold expansion of the pipe. Think about it, if you choose the wrong model and the slurry leaks every three days, the maintenance worker will have to wear acid-proof suits to plug the leak. The picture is sour to think about.
So the first step, don't rush through the product manual. First pull out the data of your site: What is the maximum operating temperature? How high is the acid concentration of the medium? Is the pipeline positive or negative? Is there any amount of displacement that needs to be compensated? Especially the rectangular flue, whose four angular stresses are concentrated, is the most prone to cracking.
Two days ago, I met a customer who said that the non-metallic expansion joint bulged after less than half a year. After asking, I found out that the temperature condition was 150℃, but the actual operation often exceeded 200℃, and the fabric layer was directly carbonized. Alas, this is the consequence of not figuring out the working conditions.
Action checklist:
- List the maximum/minimum temperature, pressure range of the flue
- Confirm media composition (especially chloride ion and sulfide content)
- Measure axial, transverse and angular displacements of pipes
- Record flue section shape (circular or rectangular)
Second, don't be blind in the selection: metal or non-metal? Rectangular or circular?
A lot of people ask "Which expansion joint is durable?" as soon as they come up – crap, scoring occasions!
Metallic and non-metallic. Metallic, like the one we stand onUniversal corrugated expansion joint、Corrugated expansion joint for power station industry,Metal rectangular expansion jointThe advantages are high voltage resistance and long life, but the disadvantages are also obvious-fear of acid corrosion. Even if you use 316L stainless steel, it will still be rusty for you under the corrosion of sulfuric acid dew point of wet desulfurization. Therefore, metal expansion joints are generally used on the original flue gas side (dry flue gas), or where pressure is required.
What about nonmetallic ones, likeNon-metallic expansion joint (fabric fiber expansion joint)、Rectangular non-metallic expansion joint、rubber compensatorThe advantages are corrosion resistance, large compensation amount and cheap price. The disadvantage is that it cannot withstand high pressure and the fiber layer is prone to aging. However, on the net smoke side (wet smoke), non-metallic expansion joints are basically standard. And guess what? In order to save money, many power plants also put metal on the clean flue gas side. As a result, it leaked in less than two years, and the labor cost of changing it once was more expensive than the product itself.
Let's talk about shape. The circular flue has a ready-made circular baffle door, and the expansion joint also matches well, likeSingle-axis double-flapper doorOrDouble-sealed single-axis circular baffle door, all for matching use. Rectangular flue, can only use rectangular expansion joint or non-metal. However, the stress at the rectangular corner is large, so the belt must be selected during designguide tubeOtherwise, the medium washes the bellows, and the life is directly discounted by 50%.
How to choose? Here's a formula:
- Raw flue gas, high pressure, high temperature → metal (general corrugated expansion joint or high temperature axial expansion joint)
- Clean flue gas, low pressure, strong corrosion → non-metal (fabric fiber expansion joint or rubber PTFE compensator)
- Rectangular section → preference is given to non-metal, or use metal rectangular expansion joint + guide tube
- Requires a large amount of compensated displacement → Non-metallic or compound hinge transverse type expansion joint
By the way,expansion joint tie rodYou can't play around. The role of the tie rod is to prevent the bellows from becoming unstable, and it is not used to deadpull the pipe. Many people tighten the tie rod nut tightly, and as a result, the bellows can't move at all, and the force of thermal expansion and contraction of the pipe is all transmitted to the equipment. For details, refer to the answer of "How to Adjust the Tie Rod Nut of Expansion Joint" in our station, which says: After installation, the nut must be loosened to leave a gap.
3. Three details that are easiest to overturn during installation (with pit avoidance operation)
After choosing the model, you think it's done? Wrong! Installation is the hardest hit by rollovers. I have been in this business for so many years, and I have seen too many examples of "the product is fine, but it leaks when it is pretended". You must go through the following three items before loading the car.
Detail 1: Install the guide tube in reverse direction
guide tubeThe function is to guide the flow of media and protect the bellows. Many construction sites don't distinguish the direction of import and export. Only after welding did they find that the guide tube was facing the opposite side of the incoming flow direction. As a result, the medium directly washed the bellows and wore it out in a few months. Remember: The direction of the arrow of the guide tube must coincide with the direction of the media flow. Expansion joints are generally marked with arrows, if not, ask the manufacturer for a drawing.
Detail 2: Cold draw pre-deformation not done
Sometimes the temperature difference of desulfurization flue is large, and the ambient temperature during installation may only be 10℃. When it runs to 150℃, the expansion amount is tens of millimeters. If the bellows is in a free state when installed, it will be compressed to the limit after operation, and the fatigue life will be greatly reduced. The correct way to do this is to calculate the amount of pre-tension or pre-compression based on the design temperature of the pipe. For example, if the flue is thermally elongated by 100mm, the expansion joint should be pre-pulled by 50mm during installation (see the design drawings for specific values). This operation is called cold pulling, and many installation teams skip it directly because it is troublesome. As a result, it deviates and leaks in the later stage.
Detail 3: Fixing bracket and guide bracket are not left properly
The expansion joint can only absorb displacement and cannot withstand thrust. In some construction sites, the pipeline is directly welded to death, or the spacing of the brackets is incorrect, resulting in the expansion joint being forcibly twisted. Correct practice: Fixing brackets and guide brackets must be set on both sides of the expansion joint. Especially for rectangular flue, four corners should be provided with guide limit plates to prevent excessive lateral displacement.
Tips:After installing the expansion joint, remember to check that the tie rod nut has been loosened. Someone in the FAQ of our station asked, "Does the screw of the expansion joint need to be disassembled?" The answer is very clear: after the installation is completed, the locking nut on the limit screw must be loosened, otherwise the expansion joint will not work.
Fourth, what to do if something goes wrong with it? Troubleshooting and emergency treatment
After the equipment is put into operation, it will encounter something wrong sooner or later. Don't panic, judge the type of fault first.
Fault 1: Bulging and slurry leakage on the surface of the expansion joint
The non-metallic expansion joint is bulging, mostly because the inner fabric is torn. The reason could be excessive temperature, hard particle scour in the media, or scratched layers of fabric during installation. Emergency treatment: If it is only a small bulge, it can be temporarily blocked with acid-resistant cement and replaced as a whole when the machine is shut down. If it is a big bulge and leaking slurry, don't think about it, just stop and change it.
Fault 2: Metal bellows leakage
Metal bellows leaks, which are commonly seen at welds or troughs. Weld leakage may be due to poor welding quality, and trough leakage is mostly due to stress fatigue. Emergency treatment: After shutdown, use repair welding, but note that repair welding must be heat treated, otherwise the heat-affected zone is more likely to crack. If it is a large area crack, replace it with a new one directly. From our stationLarge diameter thick wall expansion jointAlthough the wall is thick, it can't withstand long-term fatigue.
Fault 3: Expansion joint stuck, pipe vibration
In this case, first check whether the tie rod nut is not loose, or it is loose and tightened again. Check again if the fixing bracket is loose. If all is fine, it may be that the compensation amount is not enough when the expansion joint is selected, which leads to the bellows being crushed to death. At this time, you can only stop the machine for adjustment, or install expansion joints with large compensation amount.
5. Maintenance, you really can't wait until it leaks before repairing it-daily check list
To put it bluntly, expansion joints are consumables. Whether metal or non-metal, there is a lifespan. But when it is well maintained, it will not be a problem to use it for two or three years. I suggest you check by this frequency:
- Weekly: Inspect once to see if there are any traces of bulging, corrosion and liquid leakage on the surface. Especially for non-metallic ones, touch the fabric layer to see if it has hardened and fluffed.
- Monthly: Check whether the tie rod nut is loose (but don't screw it out), check whether the guide tube has been washed and perforated, and check whether the bracket bolts are corroded.
- Quarterly: Measure the actual expansion and contraction amount of the expansion joint once, and compare it with the design value. If the actual displacement exceeds it, it means that there may be a problem with the pipe support, or the thermal expansion increases due to the peeling of the insulation layer.
- per annum: When the machine is shut down for overhaul, remove the expansion joint and complete inspection of the interior and exterior. Especially for metal, the wall thickness should be measured to see if it has been thinned. Non-metallic, if the fabric layer is cracked, change it directly.
By the way, there's a small detail:Arrow direction of expansion jointIt is not a display, it refers to the direction of the medium flow. If the expansion joint is removed and put back in during equipment maintenance, be sure to pay attention to the direction not to reverse. There is a special article on this in the FAQ of our station. I suggest turning it out and taking a look.
Desulfurization flue expansion joint, selection is the foundation, installation is the key, and maintenance is the guarantee. Following the steps above, I don't dare to say that it will be used forever, but it is still fine to take three years of detours.