What Are Metal Expansion Joints? From structure to application, explain it at once
Two days ago, I met a customer and asked, "What are the metal expansion joints? Which one is suitable?" This question looks simple, but you really need to answer it clearly-you have to know where it is used, what is flowing in the pipe, and what is the temperature and pressure. Today, let's make it clear according to the three dimensions of structure, industry and special working conditions. By the way, let's talk about whether the metal hose is an expansion joint.
1. According to the structure, what do mainstream models look like?
The core of a metal expansion joint is a bellows, but depending on the pipe direction, displacement type, and pressure grade, more than a dozen structures will be derived. Most commonUniversal corrugated expansion jointTo put it bluntly, it is a section of corrugated pipe in the middle of the flange at both ends, which is suitable for the linear pipe between two fixed brackets, absorbs axial displacement, is affordable and simple to install. But if you run into a high-temperature steam pipe, such as the main steam pipe of a power plant, where the temperature soars to five or six hundred degrees, the general-purpose model is not enough-you have to use it at this timeHigh temperature axial expansion jointIts bellows has many layers and heat-resistant materials. It is usually equipped with a guide tube to protect the inner wall and avoid direct erosion by high-temperature media.
When the internal pressure is high to a certain extent, the tube will bend like noodles, which is called "columnar instability". How to treat it?External pressure single axial expansion jointInstall the bellows upside down, let the medium pressure act on the outer shell, and the inner cavity is basically not pressurized, and the pressure level can easily be above 2.5MPa without instability. Do you think it is fragrant or not?
The pipes don't always have to be straight. Where L-shaped and Z-shaped turns, no one can say with certainty which direction the expansion and contraction amount runs.Compound hinge transverse expansion jointJust do the job-the two bellows are hinged between them, allowing only lateral displacement, angular displacement, and axial force to be carried by the tie rod. By the same token,Straight pipe pressure balance typeAndCurved tube pressure balance expansion jointSpecialized device interfaces. For example, the inlet and outlet of pumps, valves and compressors, if the thermal expansion and contraction of the pipeline transmit the thrust, the sealing ring of the equipment will collapse in minutes. There are two sets of bellows against each other inside the pressure balance type, which digests the blind plate force by itself, and the equipment end basically has zero thrust.
And there's an old-school kind of —Sleeve type pipe expansion jointIt is compensated by the sliding seal of the inner and outer tubes. The cost is low and the compensation amount is large, but the sealing packing needs to be changed regularly, and the maintenance amount is large. Nowadays, long-distance directly buried heating pipes like to useRotary compensatorThe axial displacement is absorbed by a pair of rotating cylinders, the footprint is small, the compensation distance can reach several hundred meters, and the leakage point is much less than that of the traditional sleeve type.
Second, according to the industry, what is the special type?
The set of equipment in the power station industry is always high temperature and high pressure. LikeCorrugated expansion joint for power station industry304 or 316L stainless steel is commonly used for bellows, multi-layer structure, and a guide tube is added in the middle. Why add a deflector? It not only diverts the flow, but also reduces the erosion of the bellows by high-speed media and prolongs the life. The cement industry is different. The dusty hot air runs in the pipeline, and the dust particles are like sandpaper. SoMetal Corrugated Expansion Joints in Cement IndustryThere must be a wear-resistant guide tube, the inner wall must be coated with an anti-corrosion layer, and the wave number must be calculated clearly, otherwise the dust will be stuck. Guess why cement plants still like to use non-metallic expansion joints? Because fabric fibers compensate for greater displacement and are not afraid of wear-but that's another topic.
Desulfurization flue gas system, acidic environment, pH as low as 2-3, ordinary stainless steel will corrode through in a few days.Expansion joint for desulfurization flue gas baffle doorGenerally, high-nickel alloys, such as C276, or lined with PTFE are used. By the way, this site has a specialPTFE compensatorAndPTFE-lined hoseThe outer shell is metal, lined with PTFE, and strong acid and alkali can be used casually.
Have you been exposed to air-cooled islands? Vacuum pipeline, run under negative pressure, and it is finished if it leaks.Double hinge expansion joint for air-cooled island vacuum pipelineUsing hinge structure to limit axial displacement and double sealing ring, the vacuum degree can be kept below 0.1Pa. The general-purpose expansion joint of the same specification will probably deflate in the vacuum system.
3. "Special Forces" under special working conditions
What about large caliber and high pressure? For example, the blast pipe of blast furnace in steel mill has a diameter of 1.5 meters and a pressure of more than 1.0MPa.Large diameter thick wall expansion jointThe wall thickness of the bellows can reach 10mm or even thicker, the single wave compensation is small but the pressure resistance is high, and the reinforcing ring is equipped.
Inspection wells cannot be set for directly buried pipelines, and expansion joints must be buried directly in the soil.Direct buried (fully buried) type expansion jointThe outer cover is covered with an anti-corrosion layer, and the moisture drainage pipe is welded on it to drain out soil moisture and prevent electrochemical corrosion. AndSpecial hose for vacuumWith small wave pitch and low stiffness, it is specially used in vacuum coating machines in electronics and semiconductor industries. WhileMetal rectangular expansion jointLook at the inlet and outlet of the boiler air duct and flue gas desulfurization tower. Those square flues have to use rectangular bellows-the round ones can't fit in.
4. Does the metal hose count as an expansion joint?
Strictly speaking,Metal hose(includingPTFE-lined hose) Like the principle of corrugated expansion joint, it absorbs displacement by corrugated deformation. However, the metal hose emphasizes flexible connection, and its diameter usually does not exceed DN300. It is used to connect the pump inlet and outlet, instrument pipeline, and mainly reduces vibration. However, the corrugated expansion joint is often above DN500, which bears the main force of pipeline thermal compensation. In the actual product list, the two are often juxtaposed, and the displacement, pressure level and installation space are seen when selecting the model. Small diameter, large displacement, need frequent bending, choose metal hose; Large diameter, high pressure, mainly axial compensation, choose expansion joint.
Five, choose a tendon, try combination boxing
Thermal pipes are most commonly usedUniversal corrugated expansion jointAdding a guide tube has low cost and good effect. If the cement industry has to use metal, chooseMetal Corrugated Expansion Joints in Cement Industry； If space allows, non-metallic expansion saves energy to handle large displacements at a cheaper cost. Remember one principle: high pressure, high temperature, strong medium corrosion, preference alloy materialsPressure balance typeOrExternal pressure single type； Large displacement, limited space, upperComplex hinge typeOrRotary compensator。
There are many types of metal expansion joints, and there is no universal model. Take the parameters of the pipeline you have-media, pressure, temperature, displacement direction, installation space-and the answer will come naturally. The product page of this site has the detailed parameters of all models, so you can find them correspondingly.