The specifications of metal expansion joints are complete, but the axial, transverse and angular directions are distinguished first-choosing the wrong type is equal to white installation
The same metal expansion joint, how can there be more than a dozen series? Don't be in a hurry to blame the manufacturer for random listing. First, I want to understand one thing: Where does the pipeline displacement go? The structures of bellows corresponding to axial, transverse and angular displacements are completely different.
Axial displacement is absorbed by the expansion and contraction of the bellows itself. Universal corrugated expansion joints, high-temperature axial expansion joints and external pressure single axial expansion joints all do this. What about lateral displacement? If you use the axial type to carry it, the bellows will be stressed laterally and will crack in two or three years. At this time, it is necessary to rely on the compound hinge transverse expansion joint or the compound straight pipe bypass pressure balance expansion joint, and use the hinge and tie rod to convert the transverse force into rotation. Angular displacement is more troublesome, and it can't be handled by one bellows alone. Usually, two bellows and intermediate connectors are combined, such as curved tube pressure balance expansion joint.
Therefore, the complete specifications of metal expansion joints are not for you to pick randomly, but to first judge which kind of displacement the working condition belongs to. If you choose the wrong type, installing it is a hidden danger, which is equal to white clothing.
From general-purpose corrugated expansion joints to high-temperature axial, external pressure single and double hinges... Most specifications are not decorations, but forced out by working conditions
Why are there so many models on the market? To put it bluntly, no expansion energy saving takes all working conditions. The main steam pipeline of the power plant has a temperature of more than 600 degrees and a pressure of more than ten MPa. Ordinary bellows can't handle it at all, so high-temperature axial expansion joints have to be used. The bellows must be made of high-temperature alloys such as Inconel 625. The duct of cement industry has heavy dust and serious wear, so the guide tube and wear-resistant bushing must be added inside the expansion joint. The temperature of the desulfurization flue gas system is not high but corrosive. At this time, you may need rectangular expansion joints or non-metallic expansion joints, but metallic ones may not be suitable.
Taking the products of this station as an example, the large-diameter thick-walled expansion joint is prepared for high-pressure and large-diameter pipes, the directly buried (fully buried) expansion joint is used for buried pipelines, and the double-hinged expansion joint of air-cooled island vacuum pipeline is specially matched with the vacuum pipeline of air-cooled units. Behind each model are a bunch of lessons from practical cases. The specifications are complete because the working conditions are inherently complicated.
How to choose with complete specifications? Displacement, pressure, medium temperature, three hard indicators are indispensable
The first step of selection is to find out three numbers first: displacement, nominal pressure and medium temperature. The displacement determines the wavenumber and structural form of the bellows; Pressure determines the number of layers and wall thickness of the bellows; Temperature determines the material and the need for insulation. If these three numbers are not implemented, it is nonsense to talk about type selection.
For example, a DN500 steam pipe has an operating temperature of 350 DEG C, a pressure of 1.6 MPa, a pipe length of 30 meters and a thermal expansion of about 120 mm. In this case, if you take a general-purpose corrugated expansion joint, the allowable displacement of a single corrugated pipe may only be 60mm. Is it enough? Clearly not enough. Either two general types are connected in series, or double tie rod structure is used, or a single axial expansion joint is directly pressed externally, which has good internal pressure capacity and large displacement. But if you only report a "DN500" to the manufacturer, how can they match it for you?
Many engineers pick these three parameters very fine, but instead plant them in the details: What is the medium? With or without corrosion? Is the pipeline overhead or buried? Is there any vibration? These will affect the structure selection. Therefore, don't feel troublesome about selecting the type. First, fill in the working condition table.
In power plants, cement and desulfurization flue gas systems, the same metal expansion joint has complete specifications, but the requirements are very different
The expansion joint of power plant is most afraid of high-temperature and high-pressure steam leakage. Therefore, corrugated expansion joints used in power station industry usually require multi-layer corrugated pipes, plus liquid leakage indicators, and even online monitoring. In the cement industry, the diameter of the air duct is always two or three meters, the temperature is not high but the dust wear is serious. At this time, the metal rectangular expansion joint thickened wall guide tube is more effective than anything else, and the dust cleaning port has to be considered.
Desulfurization flue gas systems are a different story. The flue gas temperature is not high, but it contains sulfide. After wet desulfurization, there is slurry scaling, and ordinary stainless steel will be corroded. At this time, either choose corrosion-resistant alloys, or simply give up metal and use non-metallic expansion joints (fabric fiber expansion joints). There are special desulfurization flue gas baffle doors and rectangular non-metallic expansion joints in our products. With the flue gas baffle doors, the whole system is closed-loop.
You see, the same "metal expansion joint specifications are complete", and the parameters that different industries pay attention to are not in the same dimension at all. Take the idea of power plant to choose the expansion joint for cement, and it won't take long to be worn out.
Complete specifications are just a ticket, and the real effort lies in the details of non-standard customization and deflectors and pull rods
The standard model on the sample book looks fine, but it will be a problem if it is installed on site. Why? The expansion joint is not a standard part, it is a "semi-custom" product. The size of the tube at both ends, the field space, the connection method, whether there is a guide tube, and how to arrange the tie rod are the keys to decide whether it can be used.
The role of the deflector is underestimated by many people. It protects the inner wall of the bellows and reduces media erosion and corrosion. In the high-temperature pipeline, the guide tube can also reduce the direct contact between the bellows and the high-temperature medium, which is equivalent to a heat barrier. Without a deflector, bellows life may be half shorter. What about the pull rod? It is not an ornament. The role of the expansion joint tie rod is to constrain the bellows to bear the thrust generated by the pressure, while not affecting the axial displacement. The hinge and tie rod in the transverse expansion joint of the compound hinge bear all the transverse force, and how to adjust the nut is particularly important.
Let's talk about non-standard customization. The electric furnace flue of steel mills and the flue gas pipelines of waste incinerators have all kinds of strange working conditions, and the standard models can't be installed at all. If the manufacturer only copies according to the sample, the selection will definitely flip. What is really reliable is to calculate according to the on-site interface size, displacement direction and ambient temperature, and change the length of the guide tube, the number of bellows layers and the angle of the tie rod on the basis of the standard series. The metal expansion joint has complete specifications. To put it bluntly, it is just an admission ticket. The customization ability and on-site experience behind it are the real kung fu.
Why are the specifications so complete, or the wrong choice? Because the selection depends not on turning over the sample, but on thoroughly understanding the working conditions. Displacement, pressure, temperature, medium, space, none of these five elements can be leaked. Next selection, first write these parameters clearly, and then go to the manufacturer to chat. Don't come up and ask "I want an expansion joint for DN500", then the gods can't help you.