Flue displacement is not "move if you want": first distinguish the three kinds of displacement clearly
Everyone who is engaged in engineering knows that the flue looks like a thick steel pipe, but it is actually very temperamental. Once the high-temperature flue gas passes, the pipeline expands and contracts by heat, and it is common for the flue of tens of meters to elongate by more than ten centimeters. Saying "can the flue be displaced?" Sounds like a simple nod-and-shake question, but actually it is clear what kind of displacement it is-thermal displacement, settlement displacement, or installation error.
Thermal displacement is best understood, that is, when the temperature changes, the tube expands and contracts by itself. The troublesome point of settlement displacement is often that the flue passes through the wall, floor, or is connected to large equipment such as dust collector and desulfurization tower. As soon as the equipment foundation sinks, the flue is forcibly deviated. The installation error is the most frustrating. After the on-site pipeline is welded, it is found that it is out of alignment, and it is forcibly pulled and aligned. As a result, the internal stress is all suppressed on the flange.
The properties of these three displacements are completely different. Thermal displacement has a rule to follow, and settlement displacement depends on geological reports and structural settlement observation data. Installation error belongs to man-made problem. You haven't even figured out what kind of displacement it is, and the rest of the compensation scheme is all useless.
How to calculate the displacement amount? Don't slap your head, look at these three numbers first
Calculating the displacement is not about slapping your head to estimate a "almost". Look at the direction of the pipe first. The longer the straight pipe section, the greater the thermal elongation. The calculation formula is Δ L = α × L × Δ T. The linear expansion coefficient α of carbon steel is 0.012mm/ (m·℃), and the flue gas temperature of a 30-meter flue rises from 20℃ to 200℃, and the elongation is calculated to be about 65mm.
Look at the temperature again-there is a pit here. Many people directly substitute the flue gas temperature, ignoring one detail: there is an insulation layer on the outer wall of the flue, and the actual wall temperature is much lower than the flue gas temperature. Whether the wall temperature or the flue gas temperature should be taken during design is directly related to the size of the expansion joint selection. Finally, look at the position of the fixed bracket. The fixed bracket divides the pipeline into independent compensation sections, and the elongation of each pipe section is the amount that the expansion joint needs to absorb.
It is useless for you to only give a total displacement, you have to tell the manufacturer which section is right for which section. The expansion joint is installed between point A and point B. It only absorbs the thermal displacement of the section A-B, and it can't control the other sections.
What to Absorb Displacement: Don't Think of Expansion Joints as Magic Balm
When it comes to choosing a compensator, I am most afraid of a "general" model conquering the world. The flue is mostly rectangular with low pressure, high temperature and large displacement, so it is the first choiceMetal rectangular expansion jointOrRectangular non-metallic expansion joint。
The advantage of non-metallic expansion joint (fabric fiber expansion joint) lies in the large amount of compensation displacement, the transverse displacement of tens of millimeters can be achieved in a single wave, and it is resistant to high temperature and corrosion. The wet and acid working condition of desulfurization flue is particularly suitable. Metal rectangular expansion joints are more suitable for high temperature and need to bear certain pressure occasions, with strong pressure bearing capacity, but relatively small displacement compensation ability.
Some people say it worksUniversal corrugated expansion jointOkay, okay? No way. The circular bellows is fine for use in circular pipelines. It is hard sleeved on the rectangular flue, and the stress is concentrated at the four corners, so the bellows can easily tear. The flue is the flue, don't use the idea of pipe compensator to trap it.
How does the flue gas baffle door cooperate with the expansion joint? The key is "don't hold back"
The function of the flue gas baffle door is to block or regulate the flue gas flow, and it cannot absorb displacement by itself. There are many problems on the spot, that is, both sides of the baffle door are directly welded, and the expansion joint is installed at the other end of the baffle door and the equipment. As a result, the baffle door becomes a fixed anchoring point, and all thermal displacements are pressed on the door frame. Over time, the door panel is deformed and stuck, and it is not closed tightly.
One side of the flue gas baffle door is provided with a fixing bracket, the other side is provided with a guide bracket, and the expansion joint is arranged on the side away from the baffle door. In this way, the force on the baffle door is minimized, and the expansion joint can work normally. Think about it, after all, the flapper door is a device that needs to be moved. If you sandwich it between two large displacements, isn't that causing trouble for yourself?
There is another detail that is easy to overlook —Round Flap Door (Double Seal)This type of equipment should be installed with sufficient maintenance space. Don't install the expansion joint against the baffle door in order to save land. When the bolts can't be twisted, it's too late to regret it.
Common displacement errors in construction: the fixed bracket is not done correctly, and everything is installed in vain
I have seen many construction sites, and the expansion joint is installed, and the selection is correct, but it leaks soon after running. When I removed it, the bellows was twisted out of shape. The reason is simple – the fixing bracket is not done right.
Fixing the bracket is not just finding a steel beam and welding it. What it does is hold the pipe firmly in place and allow the thermal displacement to go in the direction you intend. If the strength of the fixed bracket is not enough, as soon as the pipe is heated, the bracket deforms first, and the expansion joint is forced to absorb the displacement exceeding the design value, and over-compensation, the bellows will soon be fatigued and cracked.
The guide bracket is also particular. The guide bracket ensures that the pipe can only move along the axial direction and does not allow it to run blindly laterally. If the guide brackets are too spaced apart, the pipe will sag under the action of gravity, and the expansion joint bears a bending moment, and this injury is permanent.
And guess what? Some projects even welded the fixed bracket to the expansion joint and directly welded the compensator to death. Then why install expansion joints? It's better to seal it with a blind plate flange.
Selection and Installation Checklist: Leave Maintenance Space for the Project
After so many years of dealing with flue, summarize a few checking points, and you will basically not make a big mistake by following the inspection.
- First, check whether the design displacement direction matches the type of expansion joint: axial type for axial displacement, hinge type or transverse type for transverse displacement, and compound hinge type for angular displacement. If you choose the wrong type, change it directly.
- Check whether the position of the fixed bracket is consistent with the design drawings, whether the solder joint is firm, and whether it is damaged by subsequent construction.
- Verify that the direction of the sleeve in the expansion joint is consistent with the flow direction of the medium-this direction is wrong, the medium directly flushes the bellows, and it won't take long to wear out.
- See if there is any reserved maintenance space around the expansion joint, and whether the outer protective cover of the bellows is pressed down by the insulation layer.
- In the wet smoke environment of non-metallic expansion joints, it is necessary to check whether the skin is aging and cracked, and whether there is condensate water on the surface that penetrates into the insulation cotton.
In addition, when the expansion joint is installed, the tie rod or transportation fixture preset by the manufacturer must be removed after the pipeline hydraulic test and heat insulation are completed. If it is dismantled early, the installation displacement will be consumed in advance, and there will be no margin for trial operation. It is dismantled late, and the expansion joint will not work, which is no different from a rigid pipe.
Can the flue be displaced? It can be done, but first you have to distinguish the type clearly, calculate the displacement clearly, select the correct compensator, and make the bracket solid. The four links are interlocked, and if one is missing, there will be problems. Don't wait until a leak is repaired, when the rework cost will be enough for you to reinstall several sets of expansion joints.