First, why is the pipeline system of steel plant inseparable from non-metallic compensator? — — From sintering, ironmaking to steelmaking, every high-temperature flue gas pipeline is shouting for help
When you walk into a steel mill, the first thing you feel is not the coldness of steel, but the heat wave coming to your face. Sintering machine, blast furnace, converter, continuous casting... every link is dealing with high-temperature flue gas. The temperature of flue gas always soars to 800℃, 1000℃, or even higher. Pipelines expand and contract by heat, and the displacement is frightening-tens of millimeters are common, and hundreds of millimeters are not unusual. Without a compensator, the pipe weld can crack for you, the equipment vibrates like a screen chaff, and the flange gasket leaks with a sound.
Metal compensator? In some cases, it can indeed carry it, but under the working conditions of sintering flue gas, ironmaking wastegas and steelmaking secondary dust removal, metal bellows are often corroded and riddled with holes. Why? Because there is not only high temperature in the smoke, but also SO₂, HCl, water vapor, dust particles... These things combined are like a blunt knife cutting meat. Non-metallic compensators are here for this problem. It is not afraid of corrosion, can absorb multi-directional displacement, and can also greatly reduce vibration. To put it bluntly, non-metallic compensators are almost the only reliable choice for high-temperature flue gas pipelines in steel plants, especially those with low pressure, large diameter and frequent maintenance.
Second, the core principle of non-metallic compensator: isn't it just a cloth bag? — — Structural disassembly: skin, insulation layer, frame and guide tube perform their respective duties
"Isn't the non-metal compensator just a cloth bag? Why can it carry a thousand degrees?" I directly laughed — a cloth bag? Can your cloth bag hold up to 1000℃ without burning through? The structure is not that simple.
Standard non-metallic expansion joints (also called fabric fiber expansion joints) consist of four parts:
Skin (loop band): This is not a piece of cloth, but a lamination of multiple layers of composite materials. The outermost layer is a silicone rubber coated fiberglass cloth, which is temperature resistant and airtight; With PTFE film and ceramic fiber paper sandwiched in the middle, each layer has its own task. I have seen some factories make the skin into vacuum blister, and the pressure resistance can reach more than 0.1MPa.
Thermal insulation: The first line of defense directly facing high-temperature smoke. Commonly used ceramic fiber felt, aluminum silicate wool, the thickness ranges from 50mm to 200mm. The function of the insulation layer is simple: separate the skin from the internal heat source to ensure that the skin temperature does not exceed 300℃. If the insulation is not selected properly, the skin can age at a rate that can make you collapse.
frame: Generally, it is a rectangular or circular flange welded from carbon steel or heat-resistant steel, and both ends are connected to pipes. Not only does the frame have to withstand the thrust of the pipe, but it also has to provide support for the insulation and skin. Many factories design sealing grooves on the frame and install rubber sealing strips to prevent smoke leakage.
guide tube: This component is easily overlooked, but its function is the same as the specific function of the expansion joint guide tube: guiding high-temperature flue gas to flow through the center, avoiding direct scour of the insulation layer and skin. The guide tube is usually made of heat-resistant stainless steel (such as 310S), and the length should be 50~100mm into the inside of the pipe, otherwise the smoke will roll back and burn the edge.
Therefore, the term "cloth bag" is wrong. It is a multi-layer composite, engineering calculated flexible compensation system. The choice of material, thickness, number of layers, and even the layup angle of each layer directly affect the service life.
III. Metal vs. Non-metal: Who is the better solution for high-temperature pipelines in steel plants? — — Don't just look at pressure resistance, the key is corrosion resistance and displacement absorption capacity
When many engineers mention compensators, their first reaction is metal corrugated expansion joints. Indeed, metal expansion joints withstand high voltage, which can reach several MPa or even tens of MPa. However, the pressure of flue gas pipelines in steel plants is usually only a few kilopascals to tens of thousands of pascals, that is, 0.0 MPa level. When you use metal bellows, it is more likely to have problems-under high temperature, metal bellows will experience stress corrosion cracking, especially flue gas containing chloride ions, which has a surprisingly short life.
On the other hand, looking at the non-metallic compensator, its pressure resistance is indeed inferior to that of metal, but it is completely sufficient in low-pressure flue gas systems. Moreover, it is inherently "flexible" and can absorb axial, transverse and angular displacements at the same time-metal bellows can achieve multi-directional compensation through complex hinge and tie rod structures, and non-metals can be done by their own flexibility.
Let's talk about corrosion. SO₂ in the sintering flue gas encounters water to generate sulfurous acid, and HCl in the converter flue gas encounters water to generate hydrochloric acid. Metals in this environment, even stainless steel, can not escape pitting and intergranular corrosion. Fluororubber, silicone rubber and polytetrafluoroethylene for non-metallic skin are naturally resistant to these acids. And guess what? Many steel plants use non-metallic compensators in front and behind the desulfurization flue gas baffle door, and the effect is much better than the metal one.
At the end of the day, selection is not about who is stronger, but who is better suited. Metals are strong in pressure resistance, while non-metals are strong in corrosion resistance, vibration reduction and large displacement absorption.
IV. The three most easily overlooked details when selecting non-metallic compensators in steel plants-temperature, pressure and medium. Which one is the fatal injury?
When choosing a non-metallic compensator, many people only ask "what is the temperature"-but temperature is not the only parameter. I have seen a case with my own eyes: the customer said that the flue gas temperature was only 200℃, and as a result, the skin was powdered less than half a year after installation. After checking for a long time, it was found that the medium contained a trace amount of tar, which penetrated into the thermal insulation layer for a long time, resulting in the decrease of thermal insulation performance, and the actual temperature of the skin far exceeded 200℃. So,Media compositionMore deadly than temperature. Flue gas containing oil, tar and organic solvents will destroy silicone rubber and fluororubber, so polytetrafluoroethylene coating or perfluorororubber skin must be selected.
The second pit isPressure fluctuation。 The flue gas pipelines of iron and steel plants often have negative pressure conditions. For example, when the dust removal system is started, the fan pumping force soars, and the pipeline instantly becomes negative pressure. If the non-metallic compensator lacks a rigid supporting frame or the skin is not thick enough, it will be deflated and collapse. Under this working condition, stainless steel support rings or reinforcing ribs must be installed inside.
Third easy to miss isLength of guide tube。 As mentioned earlier, the guide tube should extend into the inside of the pipe. However, during many on-site installations, it was found that the pipe flange and the compensator flange did not match, and the workers knocked the guide tube short with a hammer-as a result, the flue gas directly washed the insulation layer and burned through in a few months. This detail is inSpecific Function of Expansion Joint Guide TubeIt is specifically emphasized in the question and answer, but it is always ignored in actual operation.
Therefore, in addition to paying attention to temperature and pressure when selecting the model, we must know the media composition, pressure fluctuation range, and the installation and protection measures of the guide tube. Any detail that goes wrong is fatal.
5. Installation and maintenance: Should the tie rod be disassembled? How to install the direction of the deflector? — Tell you the pits in those frequently asked questions and answers in advance
The non-metallic compensator is shipped to the site, and the first thing you see is a couple of tie bars on the frame. Many masters asked: What is this pull rod for? Do you want to dismantle it? And the answer is:It must be dismantled!The tie rod (screw) is a protective device to prevent excessive displacement of the compensator during transportation and hoisting. After installation in place, be sure to loosen the nut and take out the screw, otherwise the compensator will not be able to expand and contract freely, which is equivalent to white installation. Regarding the function of the expansion joint tie rod and how to adjust the expansion joint tie rod nut, we have detailed questions and answers on our station. Here is another emphasis: if the tie rod is not removed, the compensator will be broken when the pipeline is displaced, and even the flange will be deformed.
The installation direction of the guide tube is also particular. The guide tube usually has arrows pointing to the direction of the medium flow. If the arrow is marked backwards, the guide tube will not guide the flow, but will lead the smoke to the skin. Carefully check the drawings before installation, and the pipeline flow direction is consistent with the site. If there is a risk of backward flow in operating conditions (such as a sudden shutdown of the fan), the deflector may need to be designed to be bidirectional or equipped with a check device.
Metal frame on non-metal compensator, how to do rust-proof treatment? Many people think that there is paint on the outside anyway, no problem. However, the environment of steel plant is humid and acidic gas, and ordinary rust-proof paint will rust in half a year. It is recommended that the frame be hot-dip galvanized or epoxy zinc-rich primer + polyurethane top coat, guaranteed to be stainless for at least 3 years. In addition, regularly check the skin surface for bulging and cracking, and whether the insulation layer has fallen off-these can be visually inspected without spending a lot of money.
Non-metallic compensators are not complex devices, but they require more installation and maintenance than metallic ones. Remove the tie rod, install it in the right direction, and do a good job in anti-corrosion. These three things have been done, and the life span is at least twice as long.
The production pace of iron and steel plants is fast and the downtime cost is high. Choosing and installing the right compensator is an insurance policy for the pipeline system. Next time you encounter "what to do with thousands of degrees of smoke", you should know the answer.