How bad is the working condition of the aluminum plant? Why can't traditional metal compensators hold up?
The environment of aluminum factory, to put it bluntly, is high temperature, strong corrosion and dust. The flue gas temperature near the electrolytic cell casually soared above 150℃, and the stainless steel bellows couldn't bear it when it encountered acidic gases containing hydrogen fluoride and sulfur dioxide, plus alumina dust. Two days ago, I met the equipment supervisor of an aluminum factory. The metal compensator they used to use had intergranular corrosion in less than half a year, the weld cracked, and the air leakage made the workshop full of acid fog. Under this working condition, the metal expansion joint is either corroded or thermal fatigue failure, which can't sustain the overhaul cycle at all.
Why? Traditional metal compensators (such as general-purpose corrugated expansion joints and high-temperature axial expansion joints) absorb displacement by deformation of stainless steel thin-walled bellows, but the corrosiveness of fluoride ions and chloride ions in aluminum plant flue gas to austenitic stainless steel is destructive. Even if you choose 316L or even 254SMO, stress corrosion cracking is a matter of time. Coupled with the scaling and dust accumulation on the inner wall of the pipeline, the compensation ability of metal bellows is greatly reduced-you can't dismantle the pipeline every six months to replace the compensator, can you?
What exactly is the structure of insulated non-metallic compensator? How to choose materials?
Insulated non-metallic compensator is essentially a fabric fiber compensator, but it is specially designed for aluminum factory to make insulation and corrosion resistance strengthening. Structurally, it is composed of multiple layers of flexible materials: the innermost layer is a chemically resistant fluoroplastic liner (such as polytetrafluoroethylene or modified PTFE), the middle is a high-temperature resistant glass fiber or ceramic fiber reinforcement layer, and the outer layer is added with a weather-resistant silicone rubber or fluororubber sealing layer. The key is an extra layer of insulation-with high-purity mica sheet or ceramic fiber paper to cut off the current and prevent electrochemical corrosion caused by leakage of the electrolyzer.
How to choose materials? It depends on the temperature and gaseous composition. Generally, the temperature of electrolytic flue gas pipeline in aluminum plant is between 100~250℃, and the acid gas concentration is high. Just choose the PTFE compensator (that is, the PTFE compensator in this station's product), which has a temperature resistance of 260℃, and fluoride ions can't penetrate. If the temperature of the desulfurization system is low but the wet desulfurization carries a large amount of sulfate radicals, it must use perfluorororubber (FFKM) or tetrafluororubber composite structure. Remember, never use a normal rubber compensator, which won't last more than three months in acidic smoke-the rubber will expand, age, become sticky, and eventually break apart.
Compared with metal compensator and rubber compensator, what is it better? A set of measured data speaks
Comparative test was done on the desulfurization clean flue of the same aluminum plant. The metal compensator (316L bellows) was used for 8 months, corroded and perforated, and the leakage rate exceeded 3%; The rubber compensator has been used for 5 months, the surface is cracked, and a large bulge bulges at the flange connection; However, the insulated non-metallic compensator, which runs continuously for 24 months under the same working condition, has a good appearance and the air leakage rate is controlled within 0.5%. Do you think the gap is big?
The axial compensation amount of insulated non-metallic compensator can reach ±50mm, the transverse compensation amount is ±30mm, and the angular displacement is ±5°, while the axial compensation of metal bellows of the same size can only be ±20mm, and the stress concentration will occur every time it expands and contracts. The fatigue life of non-metals easily exceeds 100,000 times – metal bellows tend to crack in 10,000 times in corrosive environments. In weight, non-metal is more than 60% lighter than metal of the same caliber, and can be lifted manually without large hoisting equipment during installation. The most important thing is the insulation performance: the insulation resistance is greater than 1000m Ω, which can completely cut off the stray current corrosion pipeline caused by the leakage of the electrolytic cell to the ground.
What about the rubber compensator? The only advantage is that it is cheap. However, under the working conditions of aluminum factory, the temperature resistance of rubber compensator is only 80℃ (it will age if it exceeds it), and the pressure resistance is generally 0.1MPa. Non-metallic expansion joints (fabric fiber expansion joints) can achieve a pressure of 0.3MPa without any problem, and there are mature schemes for temperature resistance of 280℃. According to the general ledger, although the initial investment of non-metals is 30~40% more expensive than that of rubber, its service life is 4~5 times that of rubber, and the comprehensive cost is low.
Which places in an aluminum plant need it most? Electrolysis flue gas, purification pipeline, desulfurization system
At a glance, it can be seen that the places where aluminum plants need insulating non-metallic compensators are all key nodes where high temperature, corrosion and dust coexist:
- Flue gas collection pipeline in electrolysis workshop: The connecting pipe section from the upper hood of the electrolyzer to the purification system is high in temperature, contains hydrogen fluoride and alumina dust, and often operates under positive pressure. Here, the traditional metal rectangular expansion joint or circular baffle door seal is easy to leak air, and the insulated non-metal compensator can not only compensate the thermal displacement, but also prevent the corrosive gas from escaping.
- Dry purification system pipeline: After the adsorption reaction of alumina, the flue gas temperature is about 140 ℃, with a large amount of abrasive dust. The non-metallic expansion joint is used for pipeline connection, the inner fluoroplastic layer is wear-resistant and corrosion-resistant, and the outer fabric can also absorb vibration-the pipeline vibration problem can also be solved by the way.
- Desulfurization system (wet/semi-dry): The temperature of the inlet flue is high, contains SO2 and HCl, and the wet flue gas of the outlet flue carries acidic condensate. The biggest headache here is chloride ion stress corrosion of metal compensator and low temperature brittleness of rubber compensator. Insulated non-metallic compensators with PTFE liners can handle it perfectly. In particular, the inlet and outlet of desulfurization tower need to absorb axial and transverse displacement at the same time, and the flexible advantage of non-metal is reflected.
In addition, such as roasting furnace flue gas pipeline and anode assembly workshop dust removal pipeline, as long as there is corrosive gas and thermal displacement, it is worth replacing with non-metal.
What to pay attention to during installation and routine maintenance? Avoid these 3 pits and double your life
Before installation, verify pipe alignment. Non-metallic compensators cannot withstand torsional and shear forces. If the pipe is misaligned and hard-fitted, the flange surface will warp and the seal will fail in minutes. Therefore, when installing, it should be leveled with gaskets, the bolts should be tightened symmetrically, and the torque should not exceed the design value-if it is tightened too hard, the non-metallic layer will be crushed.
- Pit 1: Ignore the deflector.There is much dust in the pipeline of the aluminum plant. If there is no guide tube on the inner wall of the compensator (many metal expansion joints in this station have guide tubes, and non-metallic ones have to be matched), the high-speed dusty airflow directly washes the non-metallic layer, and it can be worn out in half a year. Stainless steel deflectors must be installed to keep the airflow away from the flexible material.
- Pit 2: Blocked drain.There will be condensed acid water in the wet flue gas pipeline, and drainage holes are generally reserved at the bottom of the non-metallic compensator. If the drainage hole is blocked and the acid accumulates to soak the non-metallic layer, no matter how good the material is, it will rot. It is recommended to open the drain valve regularly for inspection, at least once a quarter.
- Pit 3: Misuse of tie rod fixation.Some people are worried about the expansion and deformation of non-metallic compensators, so they use tie rods to restrict it. Remember, the essence of a compensator is to "let it move", and forcibly fixing it is equivalent to forcing it to fail. The tie rod can only be used for temporary limitation during transportation and installation. At least 30% of the stroke margin must be loosened before commissioning, so that the flexible layer has free compensation space.
One is whether there is any leakage on the flange surface, and the other is whether there is any crack in the outer silicone rubber. Once the outer layer is bulged or torn, stop and replace it quickly-don't think about making do with a patch. Corrosive gas will drill in through the breach, and the inner layer will age quickly, and then the whole compensator will be scrapped.