High-temperature non-metallic compensator for steelmaking: How to choose the "flexible joint" of high-temperature pipeline?
Two days ago, I met the equipment supervisor of a steel mill. As soon as I came up, I complained: "The metal expansion joint used in the flue of our hot blast stove cracked and leaked in less than half a year. If I replace it for two days, I can't bear the cost." I asked him if he had tried a non-metallic compensator? He was stunned for a moment. "That thing can hold up to a thousand degrees?" Tsk, I've met too many people with this question. Today, we'll break it up and break it down.
"Hardness" of High Temperature Pipeline in Steelmaking Line: Double Test of Thermal Expansion, Cold Contraction and Corrosion
The temperature of high-temperature pipelines in steelmaking, such as the outlet of hot blast furnace, flue and dust removal system, always soars to more than 1000℃. Think about it, a steel pipe tens of meters long, from cold to hot, can expand by tens of millimeters casually. If it is hard to carry, it is common for pipe welds to crack and flange to leak. Even more cruel is the sulfide and chloride ions in the flue gas, which are extremely corrosive. Metal Expansion Joints In this environment, thin-walled bellows are corroded and perforated within a few months-a double crit of physics and chemistry.
Why does a non-metallic compensator hold thousands of degrees? — — Material and Structure Disassembly
When many people hear "non-metal", their first reaction is that rubber compensator, which is soft and waste when baked at high temperature. Wrong. The core material of high-temperature non-metallic compensator for steelmaking is high-temperature resistant fabric fiber layer + ceramic fiber felt + fluororubber or silicone rubber sealing layer. There is also stainless steel wire mesh or steel wire reinforcement on the outside. Like the one on our stationNon-metallic expansion joint (fabric fiber expansion joint)The inner layer is made of high aluminum ceramic fiber blanket, and the temperature resistance grade is directly dried to 1260℃. Coupled with the multi-layer composite structure, it can absorb the displacement in the three-dimensional direction, and is not afraid of thermal expansion, contraction and vibration. Moreover, the natural flexibility of non-metallic materials allows it to absorb greater radial and axial displacements, which metal bellows can't catch up with.
Don't compare high temperature with metal compensators, but the comprehensive life and cost performance
Don't metal expansion joints also withstand high temperatures? Like the one on our stationHigh temperature axial expansion jointThe nickel-based alloy can also withstand 1000℃. But the problem is: first, the price is so expensive that it hurts; Second, in the flue gas containing sulfur and chlorine, stress corrosion cracking is difficult to prevent. Although the temperature resistance of non-metallic compensator is not as good as that of some metals, the actual working layer temperature is far lower than the internal flue gas temperature through multi-layer thermal insulation design. The price of a non-metallic compensator may only be 30% ~50% of the metal of the same specification, but its life is longer when the working conditions are right-this is cost-effective rolling. Think about it from another angle, is it true that you spend half the money on twice the life?
Record of type selection: temperature, pressure, displacement, a wrong parameter will be useless
Tell me a real case. A steel mill selected a rectangular non-metallic expansion joint for desulfurization flue, but it bulged and torn in less than three months. The engineer complained about poor quality. Later, I took a look at the parameters: the flue gas temperature design was written as 350℃, and in actual operation, the peak value rushed to 500℃. The upper temperature resistance limit of the silicone rubber sealing layer they selected was only 300℃. Over-temperature leads to coking of the sealing layer and embrittlement of the fiber layer, which is strange if it is not bad. Therefore, it must be clear when selecting the model:Peak operating temperature(not an average value),Maximum working pressure(including negative pressure),Three dimensional displacement(axial, transverse, angular displacement). Also consider the media composition-oil-containing, acid-containing, dust-containing flue gas, and add anti-corrosion coating or fluororubber seal when selecting materials. On our stationRubber PTFE compensatorIt is specially designed to deal with corrosive media, and its acid and alkali resistance is first-class. In addition, note that the guide tube (lining tube) of the non-metallic compensator should not be omitted, which can not only protect the fabric from airflow washing, but also reduce the local high temperature. Regarding the function of the guide tube, let's standAnswered in detail。
Installation and maintenance: The sequence of screwing determines compensator life
Don't laugh, it's true. When the non-metallic compensator is installed, if all the flange bolts on one side are screwed to death first, and then the other side is screwed, huge prestress will be generated inside the compensator in advance, resulting in wrinkles in the fabric layer and misalignment of the sealing layer. Correct practice: First, pre-tighten all bolts symmetrically, and then use a torque wrench to gradually tighten to the specified torque in diagonal order 3~4 times. In addition, the limit screw for transportation must be removed after installation-this detail I asked ten customers, and six forgot to remove it. If you don't dismantle it, the compensator can't be freely expanded and contracted, which is equivalent to being installed for nothing. What about maintenance? Regularly check the skin surface for bulging and cracking, and check whether the beading bolts are loose. Also, if you find water under the compensator, deal with it immediately-water soaking will accelerate the hydrolysis of the fabric, and the life span will be cut in half directly.
Real case: The non-metallic compensator for hot blast furnace flue in a steel mill has not been replaced once in three years
Last year, I visited a customer, a steel mill in Hebei Province. The flue diameter of the hot blast stove was 1.8 meters, and the temperature often reached 1100℃. They used to use metal rectangular expansion joints, which were changed twice a year on average, and each shutdown cost hundreds of thousands. Later, we replaced it with our custom-madeRectangular non-metallic expansion joint, thickened ceramic fiber layer + double-layer fluororubber seal. By this year, it has been running for three full years, and it has not been changed once. There was an overhaul in the middle. When I opened it, there was some dust in the inner layer. After cleaning it, it continued to serve. Their equipment minister told me, "If I knew this worry-free, I would have changed it three years ago." You see, choosing the right product not only saves money, but also reduces downtime.
In the final analysis, the high-temperature non-metallic compensator for steelmaking is not a metaphysics. It relies on reasonable material combination and structural design to solve the displacement absorption and sealing problems of high-temperature pipelines. Remember the three elements of selection: temperature, pressure and displacement, and don't exceed the limit; Don't save trouble with installation, and the bolts should be twisted symmetrically; Maintenance look regularly, standing water can't have. If you choose according to this line of thinking, you will basically not step on pits.