What are the working conditions of smoke exhaust pipes? Temperature, corrosive gases, dust and wear are all indispensable
The smoke exhaust pipe looks like a tin tube, but the actual working condition is much dirtier than you think. The flue gas temperature may occur from 150℃ to 900℃. The temperature curves of the raw flue gas in the front section of the desulfurization system of the power plant, the waste gas at the tail of the cement kiln and the blast furnace gas pipeline of the iron and steel plant are completely different from the same order of magnitude. What's more troublesome is the flue gas components-sulfur oxides, nitrogen oxides and chloride ions. When condensed water is diluted sulfuric acid and diluted hydrochloric acid, it is poured directly on the bellows. Counting the dust, the fly ash that is not held by the power plant dust collector and the clinker dust of the cement kiln, the flow rate is slightly higher, and the pipe wall is just like sandpaper polishing.
Pressure fluctuations and pipe vibrations. As soon as the fan is started, the whole pipeline system is shaking, and the corrugated part of the expansion joint bears high-frequency and low-amplitude fatigue load. If you choose 304 just by looking at the temperature, it may crack in less than two months. Therefore, the material of smoke exhaust metal expansion joint is never a single index, but a comprehensive problem superimposed by temperature, corrosion, wear and fatigue.
Inventory of common smoke exhaust metal expansion joint materials: How far can 304, 316L, 321 and Inconel 625 carry them
Don't be in a hurry to record the grade number first, you have to understand a logic first: these materials are not absolutely good or bad, only suitable or inappropriate under the corresponding working conditions. Take 304 as an example. It is an entry-level model in stainless steel. Its temperature resistance range is generally about 650℃, and its oxidation resistance is okay. However, it is afraid of chloride ions. As long as there is a little chloride in the flue gas, stress corrosion cracking will come to your door. It's no problem to use it on a clean hot air duct. Put it in the wet flue gas downstream of the desulfurization tower, that is, it is to carry your life.
The 316L has more molybdenum than the 304, and its ability to resist chloride ion pitting and crevice corrosion is upgraded by a notch. Although the temperature of wet flue gas is not high, it contains sulfur and chlorine, and 316L can stand up in the desulfurization flue gas environment of most power plants. However, note that the high temperature resistance of 316L is actually not as good as that of 304. Don't count on it if it exceeds 500℃ for a long time.
321 is a stabilized stainless steel with titanium added, which is specially designed to deal with intergranular corrosion. When welding or in the sensitization temperature range of 450℃ to 850℃ for a long time, chromium carbide will precipitate near the ordinary 304 weld, resulting in chromium depletion at the grain boundary, which will rot from the weld as soon as it is corroded. The 321 fixes the problem with titanium to hold the carbon in place. Therefore, high-temperature flue gas pipes, boiler flues, 321 are very common.
Further up is Inconel 625, a nickel-based alloy. The temperature resistance of this thing can reach more than 900℃, and it is resistant to chloride ion stress corrosion, high temperature oxidation and fatigue, which is almost omnipotent. But the price is also touching, dozens of times that of the 304. It is used in waste incineration power generation and chemical high-temperature and high-corrosion flue gas occasions, which belongs to the choice of "as long as it is not exploded, the expensive point is worth it".
Material selection can't only look at temperature resistance: how to weigh chloride ion stress corrosion, intergranular corrosion and high temperature oxidation
How many degrees can it withstand? This thinking needs to be changed. Temperature is just the starting point. What really kills people is the combination punch of temperature + medium.
For example, chloride ion stress corrosion, which is particularly insidious in wet smoke. The bellows must have residual stress after forming and installation. As long as the chloride ion concentration in the flue gas exceeds the critical value, and the temperature range of 60℃ to 200℃ is added, the austenitic stainless steel will crack along the grain boundary. The surface still looks good, but the interior is crisp. At this time, you choose 304, even if the temperature is completely within the tolerance range, it will still be scrapped. 316L, Inconel 625 is because of the high content of molybdenum and nickel, and the resistance to chloride ions is obviously improved.
Besides, intergranular corrosion mostly occurs in the maintenance stage of furnace shutdown. When the equipment stops, the sulfide and moisture in the flue gas form dew on the metal surface. If the material is unstable, the grain boundary will be corroded first. That's why 321 and 316L are more popular than 304 in smoke systems-their anti-sensitization ability is there.
What about high temperature oxidation? Over 700℃, a dense chromium oxide protective film will be formed on the surface of stainless steel. However, once the temperature fluctuates drastically, this film is easy to peel off, peel off and regenerate, and chromium is repeatedly consumed, and the material will fail quickly. At this time, it is necessary to rely on aluminum and titanium in nickel-based alloy to form a more stable oxide film. Therefore, material selection is a triangular trade-off, not as simple as looking up the table and filling in the temperature.
Material boundaries between metal and non-metal expansion joints in smoke exhaust systems: when metal must be applied
Some smoke exhaust pipes, temperature and corrosion you are worried about, is it easier to directly put on non-metal expansion joints? It depends on the scene. The non-metallic expansion joint is made of fabric fiber plus rubber/fluoroplastic composite layer, which is sometimes more resistant to temperature and corrosion than metal, but it has a flaw: it can't withstand high pressure and large axial force. If the flue gas pipeline is running under negative pressure or low pressure, it is extremely cost-effective to use non-metallic expansion joints for rectangular cross-section and large-size pipelines. It can also absorb multi-directional displacement and reduce vibration and noise.
However, if it is high temperature and high pressure, there is impact airflow, or the pipeline needs to bear large thrust and reaction force, then metal expansion joints must be installed. For example, the non-metallic fabric layer can't stand the pressurized parts of the main flue of the power station, the inlet and outlet of the desulfurization tower and the outlet of the fan. In addition, if there are sharp hard particles in the medium, the non-metallic skin is easy to wear out, and the metal bellows can at least carry it for a while by thickness and hardness.
Select non-metals at low pressure and large diameter, and select metals at high temperature, high pressure and high wear. When you look at the corrugated expansion joint products in the power station industry, they are basically metal structures, which is the truth.
Actual case: Why should the material of the expansion joint behind the baffle door of desulfurization flue gas in power plant start with 316L?
Two days ago, I talked to a friend who worked on the maintenance of a power plant. He said that the expansion joint behind the baffle door at the entrance of the desulfurization tower in their plant, the 304 used to be used before, leaked in less than a year. After removing it, the surface of the bellows is full of pitting pits, and the weld position is black, which will crack as soon as it is broken. Later, it was changed to a 316L, which is still in use for more than three years now.
Why do you have to start with 316L? Because the position behind the desulfurization flue gas baffle door happens to be the intersection area of the original flue gas and the clean flue gas, the flue gas temperature is not high but the humidity is high, and it contains sulfur, chlorine and gypsum slurry particles. Occasionally, the baffle door is switched, and there will be cold air backwards, repeated condensation-evaporation, and constant enrichment of chloride ions. In this case, the molybdenum content of 316L provides the most basic pitting resistance. If you go further upstream, the flue gas temperature exceeds 400℃, and the high temperature strength of 316L is not enough, then you have to use 321 or higher grade alloy. This is the field experience, don't take the temperature range in the book hard.
Three questions must be asked to the manufacturer before purchasing: the number of waveform layers, the material of the guide tube, and the process of welding seam
Is it done when the material is determined? Far from it. The same 304 grade, the life of expansion joints made by different manufacturers can be three times different. When you go to the manufacturer with the drawings, at least ask these three things clearly.
First, how is the number of waveform layers determined? Multilayer bellows are more flexible and have longer fatigue life than single layers, but if they are not handled well between layers, they will hide corrosive media instead. You have to ask what standard the manufacturer calculates the fatigue life according to, and whether the actual working pressure and the number of temperature cycles are considered.
Second, what material is used for the guide tube? The guide tube is the key part to protect the inner wall of the bellows. If it is too thin or made of the same material as the bellows, the dust in the smoke can quickly wear it down. Many expansion joints fail not because the bellows are broken, but because the guide tube is broken first, resulting in high-temperature flue gas directly washing the bellows. It is appropriate to use a thicker wear-resistant material for the guide tube and leave a reasonable clearance with the bellows.
Third, what is the weld seam treatment process? Longitudinal weld and circumferential weld of corrugated pipe are weak links. Have you done solution treatment after welding? Has 100% NDT been performed? Weld surplus height grinding and smoothing? These are directly related to the sensitivity to stress corrosion. Some small factories don't even pickle after welding, and the color of the weld area has changed. Do you dare to use this on flue gas pipelines?
To put it bluntly, material selection is only the first step. By figuring out these three questions, the later maintenance will not become an infinite cycle of disassembly and installation. If the material of the smoke exhaust metal expansion joint is not selected correctly, it is not the equipment that collapses, but you who runs on the scene every day.