Corrugated pipe material is the core, so why does stainless steel dominate?
When choosing materials for expansion joints, the first knife has to be cut on the bellows. The bellows is the soul of the expansion joint, which directly determines the life and reliability of the entire compensator. The industrial site has been working hard for so many years. If you look at those general-purpose corrugated expansion joints, nine times out of ten are stainless steel. 304, 316L and 321, these three grades basically cover most conventional working conditions.
304 is cheap, easy to form, and has stable welding performance. It is completely sufficient for pipelines whose temperature does not exceed 400℃ and medium corrosiveness is not strong. With one more molybdenum in 316L, the ability to resist chloride ion pitting corrosion is obviously up, and some chemical media containing salt and halogen have to rely on it. 321 is added with titanium, which is a device that runs repeatedly in the sensitization temperature range of 430℃ to 800℃ in order to resist intergranular corrosion. It is much more reliable to choose 321 than 304. Don't underestimate the differences between these grades. If you choose the wrong one, the bellows may crack for you in half a year.
But then again, stainless steel isn't a master key either. When the temperature is high and the medium is poisonous, it immediately shows its timidity.
How to change materials under high temperature and corrosion conditions?
The power station industry uses corrugated expansion joints, main steam pipelines, reheating sections and high-pressure cylinders to exhaust steam. The temperature in these places is always up to 550℃, and the allowable stress of ordinary stainless steel drops severely, and the creep can't bear it. At this time, you have to change INCONEL alloy or superalloy. Nickel-based alloys such as INCONEL 625 and INCONEL 718 have good high temperature strength, and their oxidation and corrosion resistance are one order of magnitude higher than those of stainless steel. In the case of high-temperature axial expansion joints at the boiler outlet, many of them are corrugated pipes made of INCONEL, and the external temperature-resistant structure can ensure that there will be no problems during the overhaul cycle.
Corrosion conditions are another set of logic. The flue gas of the desulfurization system, the temperature is not high, but the dew point corrosion of sulfuric acid and the stress corrosion cracking of chloride ions are together, and 316L can't hold it. Hastelloy C-276 is a sharp weapon to deal with wet chlorine, hypochlorite and strong oxidizing salt. The expansion joint near the flue gas baffle door and the pipeline compensator upstream and downstream of the desulfurization flue gas baffle door are already the industry standard with C-276. You may ask: Hastelloy is so expensive, is it really necessary to use it all over the line? It's not necessary. It is only used on bellows and components that directly contact the medium. The connecting pipe and flange can be compounded with carbon steel or stainless steel, which can reduce the cost by a large section and still have a long life.
Don't ignore the connection and flange materials
Many engineers only focus on the bellows when selecting materials, ignoring the takeover and flange, which is going to happen. The pipe and flange are the connection boundary between the expansion joint and the pipeline system. If the material does not match the pipeline, the thermal expansion coefficient is inconsistent, and the welded joint will bear alternating stress for a long time, and cracks will be a matter of time. Especially large-diameter thick-wall expansion joints, large pipe diameter, large wall thickness and large rigidity, are more demanding for the material matching of connectors. Your pipe is 15CrMoG alloy steel, and as a result, the expansion joint takes over the pipe for you with Q235B. As soon as the temperature difference comes up, the thermal stress in the weld area may directly tear the base metal.
The directly buried (fully buried) expansion joint is more special. It is buried in the soil, so soil corrosion, groundwater and stray current should be considered. There is still high-temperature medium flowing inside the external pressure single axial expansion joint, and the material selection logic of the outer layer and the inner layer is completely different. The outer wall of the directly buried expansion joint usually needs to be strengthened against corrosion, such as epoxy resin coating or cathodic protection, while the bellows itself may have to be covered with a corrosion-resistant alloy. These details must be determined together with the structural form when selecting materials.
Where are nonmetallic materials mixed with a foot?
Metallic materials are not the only solution. In some cases, non-metallic materials are more suitable. PTFE-lined hose, lined with polytetrafluoroethylene, resistant to strong acid and alkali corrosion, but limited pressure and temperature resistance, generally used in corrosive media pipelines in chemical industry, the temperature does not exceed 150℃. The same is true for PTFE compensator, which relies on the flexibility of PTFE to absorb displacement, but its temperature resistance is far worse than that of metal. Its advantages are low friction coefficient, non-adhesion and strong corrosion resistance.
Non-metallic expansion joints (fabric fiber expansion joints) deal with another dimension of the problem. It uses flexible materials such as ceramic fiber and glass fiber, which can absorb multi-dimensional displacement, insulate heat and silence, and have no metal fatigue problem. Many rectangular non-metallic expansion joints are used in the flue duct and gas turbine exhaust system of thermal power plants. Because of its large cross section, high temperature and complicated displacement, metallic joints can't be done. However, the cost of non-metals is that their life is relatively short, usually they have to be replaced in about five years, and they can't withstand high pressure. Therefore, don't feel inferior when you hear non-metal. You have to find out the media working conditions before selecting the model.
The selection of materials should be looked at together with the structural form
Material selection cannot be separated from the structure. The rotary compensator relies on the relative rotation of the rotary cylinder and the sealing packing to absorb the heat displacement. Its sealing material and cylinder material are not the same as ordinary bellows. There is dynamic friction between the rotating drum and the packing, and the material hardness and wear resistance must match, otherwise the leakage rate will soon exceed the standard.
The composite hinge transverse expansion joint will generate additional bending stress when the bellows is subjected to transverse displacement. At this time, the material of the bellows must have sufficient fatigue life, and usually requires a higher grade of austenitic stainless steel or nickel-based alloy. There are also accessories such as guide tube and tie rod, which look inconspicuous, but the guide tube is directly washed by the medium, and the material must be wear-resistant and temperature-resistant; The tie rod bears the whole pressure and thrust. The strength of ordinary carbon steel has to be checked, and alloy steel has to be used under severe working conditions. Two days ago, I met a customer who said that the expansion joint tie rod was broken. When I saw it, the material was Q235, and the medium temperature was 450℃. It was weird that it kept going.
Pits stepped on in actual combat
Metal corrugated expansion joint for cement industry, used in kiln head, kiln tail and preheater systems. There is a lot of dust there, the temperature fluctuates violently, and there are corrosive gases such as SO2 and NOx. Some manufacturers want to save trouble and use 304 to go on. What is the result? The surface of the bellows is worn by dust, coupled with sulfuric acid dew point corrosion, and it leaks in two or three months. Later, it was replaced with 316L or duplex steel, and a wear-resistant guide tube was installed on the outside of the bellows to solve the problem. The core logic of this matter is that "grinding" is more deadly than "corrosion" in cement working conditions, and the material hardness has to keep up.
The expansion joint near the desulfurization flue gas baffle door is another hardest hit area for rollover. When the baffle door seal wind is opened, the local temperature may soar above 200℃, and the slurry leakage will make the chloride ion concentration soar. 316L still pits in this environment. The safest way is to use C-276 for bellows, or at least 2205 duplex stainless steel. At the same time, consider installing an inner liner to isolate corrosive media. If you take the general-purpose corrugated expansion joint directly, it will start to leak in less than a maintenance cycle, and then the whole flue gas system will be forced to shut down, and the loss will be much greater than the extra material cost.
There is nothing universal about materials, only suitable ones. Put bellows, pipes, flanges, accessories, structural forms and on-site working conditions on a table and consider them all, so as not to be blinded in the future maintenance season. In the final analysis, choosing the material of metal expansion joint is to weigh it with the indicators of temperature, medium, displacement and life. If you calculate one more step, the equipment will be stable for one more year.