Section 1: How to select raw materials-the basic logic of sheet, tube blank and bellows
Making expansion joints is the same as cooking. If the ingredients are not good, no matter how good the craft is, it will be useless. The core of metal expansion joint is corrugated pipe, and the raw materials of corrugated pipe are two kinds: plate and pipe blank. Don't underestimate this choice, it directly determines how much pressure and temperature the expansion energy saving can carry.
Sheets are commonly used in coil welding of stainless steel sheets, such asUniversal corrugated expansion joint、Corrugated expansion joint for power station industryThese. The thickness ranges from 0.3mm to 3mm, and the materials are mainly 304, 316L, Inconel 625. If you encounter the high chloride ion environment of desulfurization system, you have to replace it with 254SMO or duplex stainless steel, otherwise it will corrode you in a few months.
What about the tube blank? Generally used for small diameter or thick wall working conditions. LikeSpecial hose for vacuum、Large diameter thick wall expansion jointTake the seamless steel pipe directly to bulge, the wall thickness is uniform, the welding is less, and the fatigue life is a section longer than that of plate coil welding. And guess what? With the same expansion joint of 300mm diameter, the tube blank can carry 30% more cycles.
There is also a hard index in material selection-intergranular corrosion. Austenitic stainless steel is prone to chromium depletion when welded at 450-850℃, so it is necessary to control the carbon content or use stabilizing elements. Two days ago, I met a customer, and I bought 304 plates for cheap as the expansion joint for desulfurization flue gas baffle door. As a result, it leaked in half a year. This is the typical material selection rollover.
Section 2: Bellows forming-hydraulic, rolling and mechanical bulging, when should each of the three methods be used
The material is set, and the next step is to make the flat plate corrugated. At present, there are three mainstream forming methods: hydraulic, rolling and mechanical bulging. How to choose? Look at the diameter, wall thickness and batch size.
HydroformingIt is a general method, suitable for small and medium caliber (DN50-DN600). When the mold is clamped, high-pressure oil is filled inside, and the tube blank or plate is pressed into corrugations. The advantages are accurate corrugation shape, smooth surface and uniform stress distribution. MostlyUniversal corrugated expansion joint、High temperature axial expansion jointThat's how it works. The disadvantages are also obvious-the mold is expensive, and you have to reopen the mold once you change the specification.
Roll formingIt relies on rotating rollers to press out corrugations in circles, which is suitable for large-diameter thin-walled parts. Such asMetal rectangular expansion joint、Rectangular non-metallic expansion jointThe metal frame has high roll forming efficiency and low mold cost. But the accuracy is slightly worse, and the consistency of corrugation depth is not as good as hydraulic pressure.
Mechanical bulging(also called bulging machine) is a violent play: push out from the inside with a multi-petal mold. FitLarge diameter thick wall expansion joint、External pressure single axial expansion jointThese have a wall thickness of more than 2 mm. Why? Because the hydraulic pressure is more than 5MPa, it is difficult to control, and the mechanical thrust is more stable. However, the equipment investment is large, and small factories can't play.
Don't think everything will be fine when shaping is done. Cold work hardening in the molding process will make the material brittle, and if the residual stress at the root of the corrugation is not removed, it will crack after a hot cycle on the pipeline. So the next section must talk about heat treatment.
Section 3: Heat Treatment and Surface Treatment-Stress Relief and Corrosion Prevention, None of which can be saved
After the shaped bellows, the internal stress is frightening. Without heat treatment, if you make friends with it, it may give you brittle fracture at any time. What about that? Solid solution treatment is standard-the bellows is heated to 1050-1100°C and then quickly cooled (water quenching or air cooling). This allows the carbide to dissolve again and restore the plasticity of the austenite structure.
LikeHigh temperature axial expansion jointIt is used for the main steam pipeline of power station with a design temperature of 650℃. If the solution treatment is not in place, carbide precipitation will occur in the bellows at high temperature, resulting in intergranular corrosion and a sudden drop in fatigue life. In fact, the prevailing standards in the industry (such as GB/T 12777) require that every formed bellows must be sampled by batches for metallographic inspection.
What about the finish? Pickling passivation is a routine operation. The welding oxide scale is washed off with a mixture of nitric acid and hydrofluoric acid, and then a dense passivation film is formed on the surface. This is the first line of defense against corrosion. Especially forDesulfurization flue gas baffle doorThe matching expansion joint has high sulfur dioxide concentration in flue gas, serious acid dew point corrosion, and the passivation effect directly determines the life. There is a rule in our factory: All products exported to coastal projects must be treated with a layer of sandblasting to ensure adhesion.
Alas, in order to save costs, some small factories cut corners in pickling time, and as a result, the products are covered with rust spots three months after they leave the factory. This is a counter-example of treating "Li Zi" as "face".
Section 4: Assembly and welding-end tube, guide tube, tie rod, ring plate, how to make a complete expansion joint
Corrugated pipe is only the core part. To become a complete expansion joint, it is necessary to install accessories such as end pipe, guide tube, tie rod and ring plate. Each part has its part.
End tubeIt is the connecting pipe section at both ends, and the material matches the bellows. When welding, use argon arc welding as the base, and choose the same material or a higher grade of welding electrode. Note: If the welding parameters are wrong, for example, if the heat input is too large, the bellows will be partially annealed, and that place will become the weak point of strength.
guide tubeMany people don't understand the role-there is a question on the Internet about "the specific role of the expansion joint guide tube". To put it bluntly, it is to reduce the direct erosion of the bellows by the medium and reduce the flow resistance. For particle-containing flue gas systems, the thickness of the guide tube is at least 4mm, and the material is wear resistant 16Mn. Without the guide tube,Metal Corrugated Expansion Joints in Cement IndustryIn less than a year, the bellows were worn out by the powder.
tie rodAndring plateA part belonging to the constraint class. The tie rod limits the maximum displacement of the expansion joint and prevents excessive stretching or compression; The ring plate is used to fix the bellows and disperse the force. LikeCompound hinge transverse expansion joint、Curved tube pressure balance expansion jointThe thread adjustment amount of the tie rod must be strictly according to the drawing, otherwise the compensation direction will be messed up.
Weld the end pipe and bellows first, then install the guide tube, and finally put up the tie rod and ring plate. If the tie rod is installed first and then welded, the welding deformation will deviate the tie rod hole position, and finally the bolts can't be screwed. Isn't it silly? There are really factories that have done this before.
Section 5: Pressure Testing and Testing-What are the Passes and Standards Before Leaving the Factory
Before the finished product leaves the factory, it must pass three passes: appearance inspection, pressure test and air tightness test. However, it is directly reworked or scrapped.
Appearance inspectionMost basically, see if there are scratches, pits and oxide scales on the surface. Is the corrugation pitch even? According to GB/T 12777, the pitch deviation of a single corrugation should not exceed ±1mm. This is clear when measured with a caliper.
Pressure testThere are two types: strength test and seal test. The strength test is pressurized to 1.5 times the design pressure, and the pressure is held for 10 minutes to see if the bellows is permanently deformed. The seal test is pressurized to 1.1 times, and then the leak is detected with soapy water. LikeDouble-sealed single-axis circular baffle doorFor this kind of product, the leakage rate requires ≤0.1% rated flow, and the standard is stricter.
Airtightness testingFor occasions where there is a gas requirement, such asCorrugated expansion joint for power station industryFor main steam pipeline, helium mass spectrometry must be performed for leak detection with a sensitivity of 1×10⁻⁴ Pa·m³/s. If you do the math, this accuracy means less than a gram of gas is leaked a year.
Fatigue life testing. Although it is not necessarily complete inspection after leaving the factory, each batch of products must be sampled for cycle test. Such asHigh temperature axial expansion jointIt is required to cycle more than 1000 times at the highest operating temperature without leakage. If it is unqualified, the whole batch will be scrapped, and the cost will be borne by yourself.
Section 6: Effects of Different Working Conditions on Process-Power Station, Cement, Desulfurization, High Temperature, Where is the Production Detail Difference
The same expansion joint, used in power stations and used in cement plants, the production process can be worlds different. Why? Different working conditions, different failure modes.
Power station industry: Main steam pipe, reheat pipe, high temperature and pressure. Typical products likeDouble hinge expansion joint of air-cooled island vacuum pipeNot only have to withstand high temperature and high pressure, but also compensate for the vacuum negative pressure. In the process, Inconel 625 bellows must be used, and after molding, solid solution + stabilization heat treatment is done, and nickel-based welding wire is used for welding. Moreover, the guide tube should be made into a streamlined shape to prevent eddy current caused by steam impact and vibration.
Cement industry: The dust is large and the temperature fluctuates drastically. Such asMetal Corrugated Expansion Joints in Cement IndustryMounted on the kiln tail exhaust pipe, the dust content is up to 50g/Nm³. At this time, the bellows material is 304, but the guide tube must be lined with wear-resistant lining (ceramic patch or cemented carbide). In terms of molding, because the diameter is large (more than 2 meters), it can only be molded by rolling, and the cost of hydraulic mold can't bear it.
Desulfurization system: The flue gas of wet desulfurization contains a large amount of chloride ions and sulfate radicals, which is extremely corrosive. LikeDesulfurization flue gas baffle doorFor the matching expansion joint, the material must be 254SMO or super austenitic stainless steel. The surface treatment can't just be pickling, but also PTFE coating or PTFE lining-you can see it on our sitePTFE-lined hose、PTFE compensatorSuch products. Heat input must also be controlled during welding to prevent corrosion in the sensitized zone.
High temperature operating condition: Temperatures exceeding 600°C, such asHigh temperature axial expansion jointFor heating furnace outlets. At this time, the bellows material can only use superalloy (such as GH3030), and the forming process must be cold forming, because hot forming will destroy the strengthening phase of superalloy. After welding, aging treatment should be done to stabilize the organization.
See clearly, huh? A set of processes only corresponds to one type of working conditions. Don't expect a "one pot stew" design to take all items. Therefore, before choosing an expansion joint, find out what is flowing in your pipe, how high the temperature is, and whether there is corrosion. This is much more important than struggling with price.