The name of round wear-resistant metal expansion joint sounds rather twisty, but to put it bluntly, it is the metal bellows in the pipeline system that is specialized in carrying wear and absorbing heat displacement. Cement plants, power plants, desulfurization towers, all the media with particulate matter in air ducts and flues are basically inseparable from it. If the selection is not good, the bellows will wear out and leak air, and the whole pipeline will be stopped for emergency repair. If you don't make it right today, just break it apart and talk about it.
What exactly does the round wear-resistant metal expansion joint grind
What the hell is it grinding? Many people think that wear resistance means that the bellows itself should be hard, but it is actually wrong. The core of the circular wear-resistant metal expansion joint is the bellows. What the bellows fears most is not pure friction, but the high-speed flying particles in the medium-dust, mineral powder, coal ash and catalyst powder, which repeatedly wash the troughs and peaks like sandpaper in the airflow. The higher the flow rate, the harder the particles, and the faster the impact angle, the faster the surface of the bellows will erode.
When the flue gas flow rate exceeds 25m/s, the erosion wear rate of ordinary 304 bellows will increase sharply. If there are more than 10mm particles mixed in the medium, it is basically to hit the pipe with a sandblasting machine. What's more hidden is that the wear often starts from the inner wall of the bellows, and there is no problem on the outside. When the air leakage is found, the wall thickness is as thin as paper. Therefore, it is more important than anything to find out the working conditions of the medium composition, particle size and flow rate before selecting.
Structural wear-resistant design: How to match the material of guide tube, inner bushing and bellows
Structural wear relies on three layers of cooperation.
The innermost layer is the guide tube, which is a "shield" directly facing the medium. The guide tube guides the high-speed airflow to the center, preventing it from directly washing the trough of the bellows, and all the wear falls on the guide tube. The material of the guide tube directly determines the life: 16Mn or Q345B is used for general working conditions, and 304 stainless steel is obtained if the temperature is high or the wear is strong; To be harsher, the inner wall of the guide tube is treated with tungsten carbide surfacing, and the hardness can reach above HRC60, and the particle impact resistance is several times higher than that of ordinary steel.
The middle layer is the inner bushing, which cooperates with the guide tube and is responsible for completely separating the medium from the bellows. Sufficient clearance should be left between the inner bushing and the bellows, otherwise the inner bushing will be stuck directly as soon as the bellows expands, and the compensation function will be wasted. How to leave this gap depends on the displacement and diameter of the bellows, which generally varies from 5 to 20mm on one side.
The outermost layer is the bellows body, which is commonly made of 316L or Inconel 625, and its corrosion and fatigue resistance is hard enough. But remember, wear resistance does not depend on the thickness of the bellows wall, but on the front two lines of defense. If you increase the wall thickness of the bellows from 1.5mm to 3mm, the effect is not as practical as adding a guide tube.
How to choose different industries: differences between cement, power station and desulfurization tower
The selection logic of the same round wear-resistant metal expansion joint is completely different when placed in different industries.
In the cement industry, the working conditions are high temperature, high dust and high flow rate. In the kiln head, kiln tail and raw mill, the air duct is full of cement raw powder and clinker particles, and the temperature is always five or six hundred degrees Celsius. At this time, the guide tube must be made of heat-resistant steel, such as 304S or 310S, and the bellows must be selected from high-temperature alloy, plus the outer sheath for heat insulation. Otherwise, as soon as the bellows is overheated, the material strength will drop rapidly, and it will crack in two or three months.
The power station industry, especially the flue duct system, has a large diameter, low pressure and relatively uniform wear, but it has one characteristic: the flue gas contains sulfur and there is corrosion in the low temperature section. At this time, wear resistance is one aspect, and corrosion resistance is more critical. The material of corrugated pipe is much more stable with 316L than 304, and the guide tube can be made thicker, such as 6mm or 8mm. After all, the diameter is large, and the cost of maintenance is too high, so it is thick and worry-free.
The desulfurization tower is the most troublesome side. The flue gas carries gypsum slurry and acid-alkali medium, and wet particles are more difficult to deal with than dry particles. Ordinary carbon steel deflectors can't last a maintenance cycle, so they have to use 2205 duplex stainless steel or lined with ceramics. Corrugated pipes also have to be coated with anti-corrosion, but the coating should not be too thick, otherwise it will affect the flexibility of corrugated pipes. To put it bluntly, the working conditions of desulfurization tower have to consider wear resistance and anticorrosion together, and none of them will work.
The most common pits in installation and maintenance
To be honest, more than half of the expansion joint failures are not due to wrong selection, but to installation and routine maintenance.
Install the guide tube in reverse direction. The design of the guide tube is directional, and the medium flow direction must be consistent with the direction of the bell mouth of the guide tube. When installed backwards, the airflow directly pours into the gap between the bellows and the guide tube, either wearing through the bellows or blowing the bellows to bulge and deform. This is often the case in the factory-to save trouble when taking over the pipe, it was welded without looking at the front and back, and as a result, the air leaked less than a month after starting up.
The deflector is worn out and I don't know how to change it. The deflector is a consumable, and it's meant to be sacrificed. But you have to check its wall thickness regularly to find out the rate of wear. Some factories forget it after installation. It is basically too late to react until the guide tube is worn out and the bellows begins to eat ash directly. The experience value is: every time the machine is shut down for maintenance, take an ultrasonic thickness gauge to measure the thinnest part of the guide tube, record it, and compare it next time to see the wear trend.
Dust accumulation in bellows. Horizontally installed expansion joints, if the medium has a high dust content, dust is easy to deposit in the bellows trough. After a long time, the dust accumulation hardens, and the bellows is stuck, and the compensation amount is all invalid. There are two ways to avoid it: First, try to install the expansion joint vertically during installation. If it can't be vertical, open a drainage and ash discharge hole at the bottom of the bellows; Second, use bellows with outer sheath when selecting, which is not easy to accumulate dust in the trough.
Selection List of Round Wear Resistant Metal Expansion Joints
Finally, I will give you a selection list that can be used directly, and just check it:
- Working condition parameters:Medium composition, particle size and concentration, temperature, pressure, flow rate, displacement amount, all of which must be lacking. If the value is not accurate, the selection is blind.
- Guide tube material:16Mn for non-corrosion at room temperature, 304/310S for high temperature, tungsten carbide for deep wear and surfacing, 2205 or ceramic lining for wet desulfurization.
- Corrugated pipe material:304 is sufficient for conventional working conditions, 316L or Inconel 625 for high temperature or corrosion conditions, and the number of fatigue life times should be clear (generally not less than 10,000 times).
- Structural form:Axial type, transverse type or hinge type, according to the constraint arrangement of the pipe system, don't pat your head.
- Flow Identification:After the goods arrive, check the direction of the guide tube first, and ensure that the flow direction arrow is consistent with the direction of the medium during installation. This note is written in the installation disclosure.
- Maintenance schedule:Include the wall thickness detection of the guide tube, dust cleaning of the bellows and fastener inspection into the daily inspection list, and don't wait for it to be broken before repairing it.
Round wear-resistant metal expansion joint this thing, when choosing a little more serious, the later operation will be less troublesome. If you spend half an hour filling in the working condition parameters clearly, the manufacturer can give you a more durable structure; One more glance at the flow direction arrow when you install will save you an unplanned stop a few months later. In the final analysis, selection is not about whose sample is thick, but about who knows your pipeline better.