Find out first: What is the essential difference between it and axial type and horizontal type?
When encountering the problem of pipeline displacement, many people's first reaction is the upper axial expansion joint. But when I really arrived at the scene, I found that the pipe would not only stretch and shorten, but also sway left and right and twist up and down. At this time, the function of universal metal expansion joint comes out. From the structural point of view, the axial expansion joint has only one group of bellows, and the axial displacement is absorbed by the compression and stretching of bellows; Transverse types (such as double hinge transverse expansion joints) have a hinge at each end, forcing the bellows to swing only laterally. And what about the universal type? Usually, two sets of mutually perpendicular hinges are installed at both ends of the bellows, or a universal ring structure is used to allow the bellows to rotate and deflect freely in any direction. To put it bluntly, it is to rub together both horizontal and angular abilities.
For example, the compound hinge transverse expansion joint in our factory limits the axial displacement by two hinge groups, and only allows the tube to move laterally; The curved tube pressure balanced expansion joint counteracts the pressure thrust through a set of balanced bellows, while absorbing axial and transverse displacements. However, the universal type is more "free"-it does not restrict the direction, and can absorb any displacement from any direction. It is not a single-function contestant, but an all-around compensator.
The "three mountains" of pipeline displacement: how can a universal type hold it at the same time?
Axial expansion and contraction, lateral offset, angular torsion. The ordinary axial type can only carry the axial direction, the transverse type can only carry the transverse direction, and the angular type can only carry the angular direction. However, the actual working conditions are often "three-pronged approach". For example, the corrugated expansion joint is used in the power station industry. When the main steam pipeline starts and stops, the temperature soars from normal temperature to 500 to 600 degrees. The pipe not only becomes longer, but also laterally offsets and twists due to uneven settlement of the pipeline support or installation error. At this time, if you use an axial hard brace, the bellows will quickly be twisted or laterally unstable. The universal expansion joint separates the bearing of axial and angular displacements by hinge or universal joint structure-the bellows body mainly absorbs axial expansion and contraction, while the hinge system absorbs lateral and angular displacements, each performing its own duties.
The environment of metal corrugated expansion joints in the cement industry is harsher. In addition to high temperature, there is a lot of dust in the kiln tail pipe and preheater pipe. The lateral displacement often reaches tens of millimeters, and the angular torsion is several degrees. Because the universal type does not need to rely on the bellows itself to bear the transverse force like the general corrugated expansion joint, and the hinge shares most of the bending moment, the fatigue life of the bellows is significantly improved. A customer used the universal type on the clinker conveying pipeline, and there was no problem for three years, while the axial type used before leaked in one year.
Which places must be gimbal? Don't take the universal stiff brace
Ordinary general-purpose corrugated expansion joints (such as general-purpose corrugated expansion joints) have limited application scope, but when encountering complicated displacement, hard bracing is accelerated scrapping. In the following scenarios, the universal type is almost just needed:
First, the flue gas pipeline. The boiler outlet and the desulfurization tower inlet and outlet, the pipeline direction is complicated, the temperature fluctuates greatly, and the displacement direction is irregular. It is more necessary to compensate accurately near the smoke baffle door, otherwise the door panel is stuck. Second, the steam line. Especially in the air-cooled island vacuum pipeline system, the height difference of the pipeline is large, the span is long, and the vacuum state cannot be leaked. Why is the double hinge expansion joint of air-cooled island vacuum pipe designed as double hinge? Because the double hinge construction allows the pipe to deflect in any direction in the horizontal plane while limiting axial stretching-this both compensates for thermal displacement and prevents the bellows from being pulled off. Third, the desulfurization system. The slurry pipeline not only changes in temperature, but also attaches vibration and corrosion. Universal type with guide tube (refer to the specific function of expansion joint guide tube) can effectively prevent slurry from washing the bellows. If you use the ordinary axial type in these occasions, it is guaranteed that there will be problems in less than half a year.
Can you replace it with a general-purpose type? I say you calculate the lateral displacement-don't think about it if it exceeds 10mm, the rigidity of the bellows will destroy the pipe support. Eventually, he replaced it with a curved tube pressure balance expansion joint, which is also a product with universal characteristics.
Three pits that are easy to step on in model selection
Pit 1: Which do you choose between tie rod and hinge?
The tie rod is mainly used to withstand the internal pressure thrust and prevent the bellows from being straightened. The hinge is used to guide the direction of displacement. In the universal type, a hinged structure is usually employed because the tie rod limits the angular displacement. The role of the expansion joint tie rod is to limit excessive stretching, but if you need a large angle deflection, the tie rod gets in the way instead. When adjusting the expansion joint tie rod nut, pay attention to the reservation stroke. The hinges are more flexible.
Pit 2: Do you want the deflector?
Look at the media. If the medium flow rate is high (> 15m/s) or contains particles (cement clinker, desulfurization slurry), be sure to add a guide tube. The guide tube allows the medium to flow smoothly through the inner wall of the bellows, avoiding direct flushing. We have specifically written about the specific function of the expansion joint guide: it can also reduce the fluid resistance and prevent the bellows from vibrating. However, note that the guide tube will occupy a part of the diameter, so the effective cross-sectional area should be calculated when selecting the type.
Pit 3: How to calculate the pressure level?
Many people directly choose the model according to the design pressure, ignoring the fluctuating pressure and water hammer. The fatigue life of bellows is directly related to the number of pressure fluctuations. It is necessary to calculate the maximum working pressure and the number of cycles, and then compare it with the metal expansion joint weight table or the allowable fatigue curve provided by the manufacturer. Don't be greedy for cheapness and choose a low grade. If you leak it later, you will lose money.
Don't do stupid things when installing
There are three most common mistakes when installing universal expansion joints:
First, is the screw disassembled or not? When leaving the factory, there are usually transport screws (also called limit screws) at both ends of the expansion joint to prevent the bellows from shaking during transportation. It must be removed after installation, otherwise the pipe cannot be displaced freely. Does the screw of the expansion joint need to be removed? The answer is clear: it must be disassembled, and not disassembled means using the expansion joint as a rigid pipe. Second, where does the arrow point? The arrow mark on the expansion joint represents the flow direction of the medium. Press the arrow to install to ensure the correct orientation of the guide tube. If installed backwards, the deflector can block the flow of the medium, causing vortices and scouring. Third, what is the difference between the installation of directly buried expansion joint and sleeve expansion joint? The direct burial type should be prefabricated with insulation layer, while the sleeve type should pay attention to the compression degree of sealing packing. Universal expansion joints are not usually used for direct burial, but if there are special requirements, refer to the installation specifications of direct burial (fully buried) expansion joints.
The service life depends on the working condition, don't expect it once and for all. Even if the right universal type is selected, if the design displacement is exceeded during operation, frequent start-and-stop or medium corrosion, the bellows will crack by fatigue. Check the bellows surface and hinge wear regularly, and replace it in time if there is any leakage.