How 'toxic' is aluminum electrolysis flue gas? High temperature, fluoride, dust, three-pronged approach
The non-metallic compensator used in the aluminum and electricity industry is faced with the erosion of flue gas above 300℃, acidic corrosive gas (HF, SO₂) and alumina dust. Some factories try to save trouble and use stainless steel bellows-guess what? Less than half a year, intergranular corrosion perforation, directly scrapped. Under this working condition, the metal compensator often won't last long. Therefore, non-metallic compensators have become the mainstream, but only if the right structure and materials are selected. Not just a piece of cloth can top it.
What exactly does a non-metallic compensator look like?
From our stationNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointThe core is a multi-layer composite structure: the outermost layer is silicone or fluororubber coated fabric, with ceramic fiber or glass fiber insulation layer sandwiched in the middle, and the inner layer has polytetrafluoroethylene (PTFE) membrane orRubber PTFE compensatorThat kind of corrosion resistant lining. The functions of each layer are different-the outer layer prevents smoke from being washed directly, the middle layer is insulated, and the inner layer prevents corrosion. If you take a regular canvas to paste it, it will burn through in three days. Two days ago, I met an aluminum factory to purchase, and I bought general-brand goods cheaply. As a result, the fabric was layered in less than a month, and the smoke came out directly, resulting in a loss of hundreds of thousands of yuan after shutting down production and maintenance.
To put it bluntly, this stuff is like wearing armor: the outer armor resists beating, the cotton armor insulates, and the inner lining is anti-poison. Not even one layer is missing.
Metal or non-metal, who should I choose?
Don't choose in a hurry, look at the working conditions first. The temperature of aluminum electrolytic flue gas pipeline fluctuates greatly, and the pipeline is usually rectangular with large cross section (such as near the flue baffle door). The metal expansion joint has to add several hinges to absorb the three-dimensional displacement, which is expensive and easy to get stuck. Non-metallic expansion joints inherently absorb axial, transverse, and angular displacements, and are lightweight and easy to install. However, the withstand pressure is a shortcoming-if the system pressure exceeds 0.2MPa, you have to consider usingCompound hinge transverse expansion jointOr a structure with a tie rod. In turn, pure non-metallic fabric compensators are more suitable for low-pressure, large-displacement flues.
Then when is it time to bite the bullet on metal? Tsk, for example, the pipeline has vibration, high-frequency pulsation, or needs to bear an internal pressure of more than 0.5MPa, at this time, the metalUniversal corrugated expansion jointOrCorrugated expansion joint for power station industryMore reliable. But then again, the concentration of hydrogen fluoride in aluminum flue gas is always hundreds of ppm, and the corrosion resistance of metal bellows may not be able to withstand it.
Three Parameters Most Easily Overlooked in Type Selection
First, the concentration of hydrogen fluoride in the flue gas composition-if it exceeds 500ppm, the inner layer must be made of tetrafluorine material, such asPTFE compensatorOrRubber PTFE compensator。 Ordinary silicone coating encounters HF, and it will crack brittle in a few days.
Second, the particle size of dust-the particles are too coarse and easy to wear through the fabric. At this time, a guide tube should be installed (refer to ourSpecific Function of Expansion Joint Guide TubeThat Q&A). The guide tube can guide the airflow away and avoid dust directly washing the fabric surface. Some factories didn't pay attention to this, and the compensator was worn out in half a year.
Third, the installation position is in front and rear of the flue baffle door-when the baffle door is closed, the air flow suddenly turns, and the pressure bearing by the compensator will double, so chooseDouble-sealed single-axis circular baffle doorMatching reinforced non-metallic expansion joint. If you take the ordinary model hard, once the baffle door is closed, the compensator will bulge and deform directly.
What are the pits for installation and maintenance?
Many projects forget to care about the non-metal compensator after installing it. In fact, its longevity depends mostly on two things: First, whether the tie rod nut is adjusted in place according to the instructions (seeHow to adjust the tie rod nut of expansion joint), and the second is whether there is dust accumulation on the surface of the fabric. As soon as the dust absorbs water, the acidic substance will corrode the suture. We have seen an electrolytic aluminum workshop, and the compensator was fine after two years of use. As a result, the workshop leaked in the rainy season, and the ash layer turned into dilute sulfuric acid, which broke all the sutures and the whole compensator collapsed.
In addition, if the pipeline vibrates greatly, it is recommended to install itrubber compensatorTo dampen the vibration-ourrubber compensatorAndRubber PTFE compensatorPerforms well in low frequency vibration scenarios. However, it should be noted that rubber is afraid of high temperature. Don't use it if the ambient temperature exceeds 120℃. You still have to use fabric fibers.
National Standard JB/T 12235-2015 is the bottom line
The national standards of non-metallic expansion joints stipulate pressure resistance, temperature resistance, leakage rate and other indicators, but the actual purchase depends on whether the manufacturer has done fatigue test and corrosion resistance test. For example, in our stationHigh temperature axial expansion jointAndCorrugated expansion joint for power station industry, have done 1000 thermal cycle tests before leaving the factory.
To be honest, many small factories haven't even inspected the purchase of raw materials for non-metallic compensators for aluminum and electricity, let alone any fatigue test. What you get may be a layer of silicone brushed with ordinary fiberglass cloth, which saves the cost and life. Therefore, when choosing a supplier, don't just look at the price. JB/T 12235-2015 only gives the basic threshold, and the really good product gives enough margin on this basis.