The keyword expansion joint of metal spinning machine is searched every day in the background, but to be honest, half of them are searched wrong. They were actually searching for the expansion joints to be installed on the spinning machine — the kind of metal hoses or corrugated compensators used on the machine spindle, the hydraulic station, the cooling line. The other half asked what they want to say today: the bellows made by spinning with a metal spinning machine, that is, the expansion joint body with spinning as the manufacturing process.
Which of these two things is the mainstream in the industry? Let's not be in a hurry. After reading the comparison of craftsmanship in the future, you will naturally know in your heart.
Find out first: whether it is "expansion joint made by spinning machine" or "expansion joint used on spinning machine"
These two concepts are a hundred and eighty miles apart.
The expansion joint used on the spinning machine is essentially a flexible connector matched to the machine tool, usually a bellows and a metal hose with a small diameter, which absorbs the vibration and thermal displacement of the equipment pipeline. It belongs to a kind of mechanical accessories. Most of these products on the market are welded and formed, and the performance requirements are not harsh.
The expansion joint made by the metal spinning machine is to stick the flat material or tube blank on the main shaft of the spinning machine, and use the rotating wheel to roll and feed it point by point, so that the metal undergoes continuous plastic deformation in cold or hot state, and finally the corrugated cylinder is screwed out. It is an integration of expansion, rolling, and extrusion forming, and the product that comes out is not a fitting, but the core of the large-diameter pipe compensation system-the bellows body.
To put it bluntly, one is a tool and the other is a product.
However, many people in engineering companies can't tell the difference. They often ask if they can make DN600 pipeline compensator with samples of corrugated pipes used in machine tools. In this case, we generally directly suggest that the other party change their thinking: the fatigue life of the spinning expansion joint used in the pipe network requires an order of magnitude higher, and the process route is completely different.
Spinning corrugated pipe, better than stamping and welding? Material streamline and fatigue life have the final say
Hydraulic bulging, stamping and flanging (flap welding), spin forming. The first two were popularized early, but the craft ceiling is very obvious.
Stamping is one die and one cavity, and the trough and peak are pressed out at one time by the concave die and the convex die. It is simple, fast and cheap, but the problem lies in the material-during the stamping process, the thickness direction of the metal sheet will experience violent contraction and bending, and the grain streamline will be forcibly cut off at the trough position. Once the grain streamline is broken, the fatigue resistance of the material under alternating stress drops directly by one gear.
Not to mention welded bellows, the grain boundaries in the heat-affected zone are natural hotbeds of fatigue cracks under the triple blessing of hydrogen charging, grain coarsening and residual stress. Many welded bellows cracked in the trough after less than two years. When they were cut open, they started cracking from the fusion line all the way to the peak.
The spinning process is gradual forming, the rotating wheel goes along the core die step by step, and the metal is partially deformed and gradually accumulated under the rolling of the rotating wheel. Instead of being thrust, the grains are elongated and rearranged along the contours of the corrugations — the material streamlines are not interrupted and the thickness distribution is not abruptly changed. In the same batch of 304L material, the spun corrugated pipe and the stamped welded corrugated pipe were tested for fatigue. One could carry 100,000 cycles, and the other 30,000 wave valleys began to show fine lines.
The data don't fool, and it is common for spinning to have a fatigue life two or three times higher.
What expansion joints can be made by spinning process? From universal corrugated expansion joints to high-temperature axial joints, don't look at the price
The spinning process can not only be made of a single variety. What's amazing about it is that if the core mold is changed, the product will be changed.
The universal corrugated expansion joint is the most mature position of spinning technology. The single-wave and multi-wave structures of DN50 to DN1200 can be rotated, and the waveform can be made into U-shape or ω-shape. With the connector, flange, tie rod and guide tube, it is a set of standard axial compensation units.
Large-diameter thick-walled expansion joints are also the strengths of spinning. The thick-walled cylinder blank can be hot-rotated on the spinning machine, and the metal deformation resistance and rebound are small at high temperature, so it can make corrugated pipes with wall thickness of more than 8mm and super-large nominal diameter. This is common on the main steam pipeline of power stations or the hot air line of steel mills.
High-temperature axial expansion joints are also typical applications of spinning processes. When spinning with heat-resistant alloys such as Inconel 625 or Haynes 214, the deformation characteristics of the material at high temperature are completely different from those at normal temperature, but the gradual cooling of spinning can just control the tendency of grain growth, and the structure of the corrugated pipe is more uniform. With the external pressure cylinder, the external pressure single axial expansion joint can be formed, and the high temperature displacement absorption of the pipe system depends on it.
So don't just listen to the quotation when choosing a model. The same DN500 expansion joint is 30% cheaper for stamping and welding, but the number of corrugations increases and the fatigue life is not up to standard. Used on critical circuits, the money saved is simply not enough for the shutdown loss of later pipe change.
Install spinning expansion joints, guide tube orientation, pre-deformation amount, tie rod nut and other details are most likely to overturn
When the product arrives at the site, installation is the highlight. These three questions are asked almost weekly.
The first is the guide tube orientation.The function of the guide tube is to reduce the erosion of the inner wall of the bellows caused by the dielectric vortexSo its open end must be directed toward the medium flow direction. When installed backwards, not only will there be no diversion effect, but also an additional turbulent dead zone will be formed at the trough, and dust and condensate will all accumulate at the bottom of the bellows-the corrosion rate is visible to the naked eye.
The second is the amount of pre-deformation. The heat compensation margin should be left when the pipeline is cold loaded, so the spinning expansion joint will generally be pre-deformed by cold drawing or cold pressing before leaving the factory. When the installation team got the goods, it went directly to the pipeline without reading the pipeline displacement calculation book, and the pre-deformation amount was wasted. As soon as the pipeline heats up, the compensation stroke directly exceeds the limit.
The third is the tie rod nut. The pull rod and nut that come with the expansion joint are used for transportation positioning and installation adjustment, not for running and fixing. After installation, the locking nut must be loosened to the distance marked on the drawing to allow the bellows to expand and contract freely. If it is not loosened, it is equivalent to the bellows being locked, the compensator becoming a rigid joint, and the thermal expansion stress of the pipeline is all pressed on the fixed brackets at both ends-as a result, the bracket is deformed or the equipment flange is biased. To tell the truth, a large proportion of installation accidents are caused by this problem.
There are also compound hinge transverse expansion joint and straight pipe pressure balance expansion joint and other varieties with tie rods. When installing, we should also pay attention to whether the hinge direction of the tie rod is consistent with the transverse displacement direction. If it is not consistent, the bellows will bear additional torque.
How to check if the spinning expansion joint is broken? Trough crack, stiffness change, intergranular corrosion, three-step localization
The failure of the expansion joint in service, no matter how accurate the selection is, will be encountered over a long time. The three-step troubleshooting method helps you locate the problem.
The first step is to look at the location and direction of the trough crack. The fatigue crack originates from the outer surface of the trough and is parallel to the direction of the wave peak. It is mostly because the cyclic displacement exceeds the limit, and the displacement estimation is wrong when selecting the type. If the crack spreads outward from the inner wall of the trough, has many branches, and has blue or gray oxides, it is basically stress corrosion cracking-check the medium for chloride ions or hydrogen sulfide.
The second step is to measure the stiffness. The stiffness curve of the spinning expansion joint is determined when it leaves the factory. After several years of operation, if the hand-pressed bellows feels significantly softer or harder than when it was new, it means that the material has undergone plastic damage or work-hardened saturation. The wave pitch becomes irregular and the ripples collapse, which is also an intuitive signal of the change of stiffness. In this case, even in the case of spin molding, it can no longer be expected to continue to carry displacement.
The third step is to check the intergranular corrosion. Many 304 and 316L bellows look intact on the surface after several years of use, but they crack at a light knock, and the fracture is like rock sugar slag-this is typical intergranular corrosion. In essence, the grain boundary is chromium-depleted during welding or hot processing. Although the spinning process is much better than that of stamping and welding parts, it can't escape if the carbon content of the material itself is high or it runs in the sensitization temperature range for a long time. In this case, check two factors: Is the base metal low-carbon grade? Has the medium temperature been swaying between 450°C and 850°C for a long time?
Alas, to be honest, the expansion joint looks inconspicuous, but when there is a real problem, the stable operation of the whole pipeline has to be stopped. Spinning process solves part of the life problem, but the selection, installation and maintenance of these three levels, which one can not save the mind.