Step 1: Find out why special expansion joints must be used for vertical flue
Is there a total air leakage at the outlet of your desulfurization tower? Alas, I met a customer two days ago. The vertical section of his desulfurization tower used an ordinary expansion joint, and it cracked in less than three months. Do you think it's irritating? There is a big difference between vertical flue and horizontal pipe-the vertical section not only bears temperature difference displacement, but also tension and compression caused by self-weight, not to mention the vibration caused by flue gas flow. An ordinary expansion joint is like a paper window. It is strange that it can hold up.
So what exactly is the problem? First, the temperature difference displacement. The flue gas temperature at the outlet of desulfurization tower fluctuates greatly, from 150℃ to 80℃ or even lower, and the vertical flue expands and contracts thermally, and the expansion and contraction amount is less than tens of millimeters. Second, vibration. As soon as the fan was turned on, the whole flue shook, and the fatigue life of ordinary metal bellows could not bear it at all. Third, corrosion. After desulfurization, the flue gas has high humidity and strong acidity, which is even worse if it comes with slurry. Therefore, it is necessary to use expansion joints specially designed for vertical flues, instead of just buying a general-purpose type.
Step 2: Don't be blind in the selection, and get stuck according to these 3 parameters
You can't rely on your feelings when it comes to selection. You only have to keep an eye on three parameters: medium temperature, flue section size (rectangular or circular), corrosiveness. Come on, step by step.
Temperature exceeds 150℃？ Don't hesitate to go directly to "Corrugated Expansion Joints for Power Station Industry" or "Non-metallic Expansion Joints (Fabric Fiber Expansion Joints)". The former has good high temperature and pressure resistance, and is suitable for metal flue; The latter is more flexible and is especially suitable for scenarios with large vibrations and displacements. If the temperature is above 200℃, you have to consider the internal insulation layer, otherwise the external skin can't stand it.
Flue section size— Rectangular or circular? A "rectangular non-metallic expansion joint" is preferred for the rectangular flue because the non-metallic material can be made into any shape, has good sealing performance, and is light in weight. Circular flue can use "general corrugated expansion joint" or "corrugated expansion joint for power station industry", depending on the specific pressure level.
Corrosivity— — The desulfurization flue gas contains sulfur and chlorine, which is extremely corrosive. If you still have desulfurization slurry, remember to match the "desulfurization flue gas baffle door" in front of the expansion joint for isolation, so that the medium can be completely cut off during maintenance, otherwise you will have to take risks when changing the expansion joint.
When choosing, don't just look at the temperature, but also look at the pressure. Generally, the flue pressure is not large, but some working conditions are negative pressure, which will cause the non-metallic expansion joint to deflate, so it is necessary to add a "guide tube" or increase the skeleton strength.
Step 3: 2 Things You Must Do Before Installing
The expansion joint has arrived, so don't rush up. Check the direction of the deflector first-the arrow points to the direction of the media flow! And guess what? Once I went to the scene and saw that the workers installed the deflector tube backwards, just like wearing underpants backwards, and the smoke directly rushed to the bellows, which was worn and leaked in a few days. Remember: The role of the guide tube is to protect the bellows from being washed by high-speed media, and the direction must not be wrong.
The second thing, is the tie rod nut removed or not? I specifically wrote about this question in the question and answer (you can search "How to adjust the expansion joint tie rod nut"). Simply put: the transport screw must be disassembled, which is to prevent temporary fixation of deformation during transportation. But you have to keep the adjustment rod! The adjustment rod is used to control the pre-compression or pre-stretching of the expansion joint in the installation state. The specific adjustment method depends on the working condition. The vertical flue generally needs to be pre-stretched, so that the expansion joint has a certain displacement margin when it is cold. If you dismantle it all, the expansion joint will be pulled directly to the limit as soon as you start it, and the life will be discounted.
Step 4: How to position the fixed bracket and the guide bracket?
The bracket layout is the vital gate of the vertical flue. Many people find that the flue is crooked after installing the expansion joint, because the position of the bracket is not calculated properly.
Fixing bracket should be set at the bottom of vertical flue, bearing the weight of the entire flue and the constraints of thermal expansion. What about the top? Install the guide bracket, but the distance from the expansion joint is more than 4 times the tube diameter. What's the meaning? For example, the diameter of the flue is 1 meter, and the guide bracket cannot be more than 4 meters away from the expansion joint. Why? Beyond this distance, the verticality of the flue is easy to deviate after heating. As soon as the expansion joint is deviated, it will begin to suffer from bending moment, and the bellows will deform laterally, which will definitely leak over time.
In addition, if there are a plurality of expansion joints in the middle, a guide bracket should also be provided between each two expansion joints. If the space is limited, the "double hinge transverse expansion joint" can be used to compensate for multi-directional displacement, but the bracket still can't be saved.
The expansion joint itself cannot bear the weight of the pipe, so it must be hoisted well during installation, and it cannot be stressed. Remove the spreader after welding.
Step 5: Don't let it go after it is put into operation
Commissioning is not the end, it is the beginning of maintenance. Do you think that when the expansion joint is installed, everything will be fine? That's naive. Go to the site once a quarter to screw the bolts-it's no joke. The compression bolts of non-metallic expansion joints will be loosened due to vibration and thermal expansion and contraction, and air leakage will occur if they are not tightened. The tie rod bolts of metal bellows should also be checked for loosening.
Does the non-metallic compensator have any bulging and delamination? Are there cracks and corrosion pits in metal bellows? What should I do if I find abnormal noise? Check the sealing of the baffle door first, and don't remove the expansion joint when you come up. There was a power plant before, and when I heard the "hissing" sound, I thought the expansion joint was leaking. As a result, when I removed it, I found that the baffle door was not closed tightly, which wasted time and stopped production. Therefore, check the sequence: ① baffle door seal → ② expansion joint appearance → ③ bolt tightening.
Finally, I'll talk about the experience of an old driver
After working on desulfurization projects for more than ten years, I sincerely suggest that if the vertical flue of your home is rectangular, give priority to "rectangular non-metallic expansion joint" with "double-sealed single-axis circular baffle door". Why? Rectangular non-metallic expansion joint has stronger sealing than metal, and can absorb multi-directional displacement. Only the skin needs to be changed during maintenance, and the whole flue does not need to be moved. Double-sealed single-axis circular baffle door, two seals plus an intermediate purge chamber, zero leakage, people can go in directly during maintenance. This combination saves half the maintenance time compared to the pure metal scheme. Is that the truth? If you choose the right match, you will be worried for the next ten years.
Okay, from selection to installation to maintenance, do you remember these 3 pits (ordinary expansion joints are fine, the transportation screw is not removed when installed/the reverse deflector is installed, and the bracket position is swung at will)? Follow it and keep your desulfurization flue as stable as Mount Tai.