How bad is the working condition of converter dust removal flue?
When the converter is blowing, the flue gas temperature can instantly soar from normal temperature to above 1600℃, and after passing through the vaporization cooling flue, it enters the dust removal system, and the temperature still fluctuates back and forth between 800~1000℃. Think about it, how much of a test is this scenario of drastic temperature changes for pipe connections? What's more, a large amount of corrosive media such as iron oxide dust, CO and SO2 are entrained in the flue gas. In this environment, the expansion joint has to withstand high temperature, thermal cycling, dust scour and chemical corrosion at the same time-which is not something that ordinary bellows can cope with.
Specifically, the hard indicators faced by the expansion joint of the converter dust removal flue include:
- High temperature resistance: The design temperature should consider at least the instantaneous peak value to 1200℃, and the long-term operating temperature should also be selected according to 600~800℃;
- Fatigue life: The smelting cycle of the converter is as short as 30 minutes and as long as 45 minutes, with dozens of thermal expansion and contraction cycles every day, tens of thousands of times a year, and the fatigue life of the bellows must meet the standard;
- Abrasion resistance: The flow velocity of dust-containing air flow is usually 15~25m/s, the hardness of dust particles is high, and the material and structure of the guide tube directly determine the life;
- Corrosion resistance: The acidic gas in the flue gas will form condensed acid liquid below the dew point temperature, which will cause pitting and stress corrosion cracking to the metal.
To put it bluntly, the first step in model selection is not to turn through the product manual, but to clarify the working condition parameters: temperature fluctuation range, pressure grade, media composition, displacement (axial, transverse, angular), and interface size. These data are not clear, and the rest is all wasted.
Common Failure Cases: What's the Problem?
Two days ago, I met the equipment minister of a steel mill, complaining that the metal rectangular expansion joint on their site leaked in less than half a year on average. Take it apart, and there are three typical failures:
- Fatigue cracking of bellows-Fine cracks appear near the weld, gradually expanding until perforated. The reason is that the number of thermal cycles exceeds the design value, or the wall thickness of the bellows is selected thin.
- Guide tube wear down— — The material of the guide tube is ordinary Q235, which is holed by dust in less than three months. The flue gas directly washes the bellows body and accelerates the failure.
- Weld corrosion— — Acid dew point corrosion leads to the preferential destruction of the welding bead, especially at the connection between stainless steel bellows and carbon steel flange, and no anti-corrosion treatment is done after different steel is welded.
The root causes of these problems are actually very concentrated: either the wear-resistant design of the guide tube is neglected during model selection, or the material grade is low, or the additional stress is caused by improper adjustment of the tie rod during installation. Many users have chosen the universal corrugated expansion joint for cheap pictures, and the result is not worth the loss.
What kind of expansion energy saving holds?
According to the particularity of converter dust removal flue, there are three commonly used schemes, each with its own applicable scenarios.
1. High-temperature axial expansion joint
This product uses multi-layer bellows structure, and the material is usually Inconel 625 or 316L high-temperature stainless steel, which can withstand temperatures up to 1100℃. Large axial compensation, suitable for thermal expansion and cold contraction of straight pipe section. However, the disadvantage is that the adaptability to lateral displacement and angular displacement is weak, and the price is on the high side. It is more suitable to be used in the branch pipe section of converter for primary dust removal.
2. Non-metallic expansion joint (fabric fiber expansion joint)
Also called non-metallic compensator, it is made of composite materials such as silica gel cloth, fluorine tape and glass fiber cloth. The upper limit of temperature resistance is generally about 600℃, but it has excellent flexibility, can absorb axial, lateral and angular displacement at the same time, and has good vibration and noise reduction effect. Reference JB/T 12235-2015 for standard. The most critical point: Its resistance to dust scour is actually better than metal bellows – because the deflector (usually in wear-resistant ceramic or high-chrome cast iron) can be replaced independently, and the fabric layer itself is not afraid of minor wear. For the converter secondary dust removal pipeline (the operating temperature is usually below 400℃), it is a very mature scheme to replace the metal rectangular expansion joint with the non-metal expansion joint.
3. Metal rectangular expansion joint
Rectangular cross-section, commonly used in the reduction of diameter or bend of dust removal pipe. Has the advantages of simple manufacture and low price, and has the disadvantages that the guide tube is easy to wear and stress concentration at the welded part of the corrugated plate. If you have to choose a metal rectangular expansion joint, the manufacturer must be required to provide a thickened guide tube (at least 6mm or more) and add a protective layer outside the bellows.
When the temperature is lower than 400℃ and the dust content is large, non-metallic expansion joints are preferred; If the temperature exceeds 600 DEG C and the displacement is mainly axial, use a high-temperature axial expansion joint; In the middle section or where the space is limited, the double hinge transverse expansion joint or the curved tube pressure balance expansion joint can be considered to share the stress.
Installation and maintenance: Don't step on these five pits
If you choose the right equipment, it will still be useless if the installation and maintenance are messed up. Share a few common pits in actual combat:
- Improper adjustment of tie rod nut-The tie rod nut must be loosened before installation to allow the expansion joint to be in a free state. Some workers try to save trouble and not loosen the nut, which causes the bellows to fail to expand and contract normally and tear quickly. Correct practice: After installation is in place, adjust the tie rod nut as per design requirements, pre-stretch or pre-compress into place, and then lock tight. The role of the expansion joint tie rod is to restrain the bellows from failure, not to fix the pipe.
- Install the guide tube in reverse direction-The direction of the arrow of the guide tube must coincide with the direction of the medium flow. Install backwards, the airflow directly flushes the bellows, and the life is directly reduced in half. This detail will be overlooked by many field personnel.
- Ignore cold or hot tightening— — For the high-temperature axial expansion joint, it is necessary to pre-stretch or pre-compress according to the ambient temperature at the time of installation, so as to ensure that the displacement is within the design range at the operating temperature.
- Too long inspection period— — It is recommended to check the wear of the guide tube, whether there are cracks on the surface of the bellows, and whether the flange connecting bolts are loose at least once a month. If the guide tube is found to be partially worn out, replace it in time, and don't wait until air leaks before dealing with it.
- Blindly selecting general-purpose products— — Don't choose a general-purpose corrugated expansion joint in order to save hundreds of dollars. The dust removal condition of the converter is bad and must be customized.
Real Case: Secondary Dust Removal Transformation of Converter in a Steel Plant
Last year, there was a project in which the secondary dust removal pipeline of a steel mill converter was originally made of metal rectangular expansion joints, with an average life of only 8 months. Frequent air leakage leads to insufficient negative pressure in the dust removal system and fume overflow. We later suggested that they replace it with the one on our websiteNon-metallic expansion joints (fabric fiber expansion joints)。 It ran for 16 months after the modification and remained intact, doubling its life span. The key point is:
- The non-metal expansion joint is equipped with a wear-resistant guide tube, and the replacement cost is low;
- The fiber layer can absorb multi-directional displacement without generating additional stress;
- The overall weight is light, and the requirements for pipe support are reduced.
Calculated an account: 50,000 yuan was invested in renovation, which reduced the downtime and maintenance time by 7 days every year, and the benefit of increasing production far exceeded the investment. So sometimes it's not that you can't choose the expensive one, but that you have to calculate the full life cycle cost.
In the final analysis, the selection of the expansion joint of the converter dust removal flue is a systematic project. It is suggested that the working condition data should be handed over to professional expansion joint manufacturers in the early stage of the project, so that they can recommend the scheme according to the displacement, temperature, pressure and medium. Don't pat yourself on the head, and don't just compare prices. After all, an expansion joint is broken, which may affect the normal operation of the entire dust collection system.