First, the angular expansion joint is not an "elbow", it is a component specially absorbing the angular displacement of the pipeline
A few days ago, a buddy who designs power plant pipes called me and asked, "What is the difference between your angular metal expansion joint and an ordinary elbow? Can't I turn it even if I weld it with an elbow?"-Alas, I have asked myself this question countless times. Let's start with the conclusion: Not even close.
Ordinary elbows are dead. When the pipeline expands and contracts by heat, the elbow itself will not deform, and the thermal stress is all held in the weld joint and the bracket. Over time, either the weld cracks or the bracket deforms. The angular metal expansion joint is alive-it absorbs the angular displacement in a single plane by the bellows and hinge mechanism. For example, when the pipe turns 5° or 10° horizontally, it follows itself and relieves the stress.
It is also different from the double hinge transverse expansion joint. The double hinge is two sets of hinges plus an intermediate tube, which can absorb lateral displacement and combined displacement. The double hinge expansion joint of air-cooled island vacuum pipeline is more special, which is specialized in dealing with vacuum working conditions and super-large displacement. There is only one hinge in the angular direction, which can only rotate in one plane. If a pipe has both angular and lateral displacement, and you take a single hinge to carry it, it will be difficult for it.
Second, structural disassembly: bellows + hinge + pin, one less won't do
The core is a bellows, with an end pipe welded on both sides, and a hinge plate and pin shaft are sleeved on the outside of the bellows. The bellows is responsible for deformation, and the hinge mechanism is responsible for limiting-limiting axial displacement and allowing only angular rotation. The pin passes through the hole in the hinge plate and determines the maximum number of degrees it can turn.
For the technical source of bellows, you can refer to the corrugated expansion joint or high-temperature axial expansion joint used in power station industry. Parameters such as material, wall thickness and wave number directly affect the life span. The hinge and pin shaft are force bearing parts. The higher the pressure level, the thicker the pin shaft and the thicker the hinge plate. You can usually estimate the approximate weight and wall thickness based on the nominal diameter and pressure rating by looking through the metal expansion joint weight table.
Some people want to be cheap. Take the general-purpose corrugated expansion joint to weld two ear plates and use them as angles. As a result, the bellows has no limit, and it collapses when pulled in the axial direction. Not even one less hinge will do, this is not a cost saver.
3. Focus on these three parameters during model selection: angular displacement, pressure and temperature
How to determine the angular displacement? Not patting the head, but looking at the pipe support layout diagram and thermal expansion calculation. For example, for a 30-meter-long steam pipe, the end of the pipe should be offset by 80mm when calculated by thermal expansion, and the installation point should be 5 meters away from the fixed bracket. The included angle θ ≈arctan (80/5000) ≈0.9°, and then leave some margin. It is enough to choose the angular displacement of 1.5° ~2°. It is too big a waste, but it is too small a death.
The pressure level depends on the pipeline design pressure. Refer to the weight table of metal expansion joints, for example, the angular piece of PN16, the wall thickness is generally 6~8mm; PN25 should be more than 10mm. Don't take a 0.6MPa general purpose model to carry 2.5MPa steam, that is equivalent to dancing on an explosive barrel.
Temperature determines whether to lined with insulation layer or choose the structure of high-temperature axial expansion joint. Ordinary carbon steel bellows are easy to creep above 400℃, so stainless steel or temperature-resistant coating should be used. If the medium temperature exceeds 600℃, it is necessary to refer to the design idea of high-temperature axial expansion joint, add heat insulation layer or adopt multi-layer bellows.
IV. On-site installation: Can the tie rod nut be removed? How is the hinge orientation correct?
Check that the hinge pin is temporarily fixed. When angular metal expansion joints leave the factory, in order to prevent the bellows from displacing during transportation, the pin shaft is usually locked with bolts or lead wire. When you arrive at the scene, don't rush to dismantle it. After the pipeline is in place and the counter welding is completed, remove the temporary fixtures. Otherwise, if it is loose in advance, the bellows will twist around when hoisting, which may be damaged.
Then there's the issue of the tie rod nut. Angular expansion joints often carry several tie rods, which many people think are used to adjust the angle-wrong. The tie rod is simply transport protection against excessive axial tension on the bellows when hoisted. Once the pipe is in place, these tie rod nuts must be removed, leaving none. If you don't remove it, the bellows wants to turn but the pull rod is stuck, which means that you are dancing in tights, and the angular displacement is suffocated to death.
The plane of rotation of the hinge must coincide with the direction of displacement of the pipe. How is this right? It is very simple: look at the displacement direction indicated in the thermal expansion calculation book of the pipe. The hinge pin axis of the angular piece should be perpendicular to the displacement plane. For example, when the pipe swings left and right in the horizontal plane, the hinge pin has to be installed vertically; If it is swinging up and down, the pin shaft is installed horizontally. If you install it backwards, the hinge won't turn at all, which is equivalent to installing it for nothing. We have suffered this loss. The customer installed it backwards on the spot, and after the pipe heated up, the bellows blew up directly.
5. Does it replace each other with double hinge transverse expansion joint and double hinge expansion energy saving of air-cooled island vacuum pipeline?
Can't. This question is often asked, and I have repeatedly explained: the angular metal expansion joint can only absorb angular displacement in a single plane, it has no intermediate tube, and the two end tubes are directly attached to the hinge. The compound hinge transverse expansion joint has two sets of hinges plus an intermediate pipe, which can absorb the transverse displacement (that is, the vertical misalignment of the pipe), and can also combine angular displacement + transverse displacement. The double-hinge expansion joint of air-cooled island vacuum pipeline is more ruthless. In addition to the double-hinge structure, the seal strengthening and anti-instability design are also made for vacuum working conditions, which can absorb a large angular displacement (some to ± 15 °).
What happens if you choose the wrong one? With one angle to absorb the lateral displacement, the bellows will be subjected to additional bending stress, and the life will be directly discounted by three percent. Conversely, using a double hinge to absorb a single angular displacement doubles the cost, and it takes up too much space, so the pipe bracket is not easy to arrange. So don't think about "one thing has many capabilities", just use whatever you should use.
Angular metal expansion joint is not a panacea, but it is the most cost-effective scheme in the scenarios of right-angle turning, Z-shaped pipe and door-shaped compensation. By understanding its temper, you can make the pipeline safe and save money.