Why do you need metal spherical expansion joints in your piping system? — — The nemesis of angular and lateral displacements
I met a customer two days ago. The steam pipe ran for half a year, and the weld cracked three times. After checking and checking, the problem lies in the angular displacement caused by the thermal expansion of the pipe-the twisting force at the corner of the pipe, and the ordinary corrugated expansion joint can't withstand that multi-directional torsion at all. At this time, we have to ask for the metal spherical expansion joint.
To put it bluntly, this stuff is tailor-made for angular and lateral displacements. If you think about it, the pipeline elongates at high temperature. If the pipeline goes in a complicated direction, it will also produce bending and twisting in addition to axial expansion and contraction. Corrugated expansion saves energy and absorbs axial displacement, but it is easy to become unstable when encountered with large angle deflection. The metal spherical expansion joint rotates by the spherical surface, and the maximum rotation angle can be 15 ° or even larger, and it can absorb the displacement in multiple directions at the same time. It is almost the only reliable choice in large-diameter pipelines of power stations, metallurgy and chemical industry.
The structure and principle of metal spherical expansion joint: spherical rotation and sealing, what is the difference between corrugated expansion joint and corrugated expansion joint?
A ball shell and a ball head, with a sealing ring sandwiched between them. The ball head can rotate in any direction in the spherical shell, just like a human joint. Sealing ring is the key, commonly used polytetrafluoroethylene or graphite, temperature and pressure resistance. The greater the fluid pressure, the tighter the seal-this is called a self-tightening seal, and it is cleverly designed.
Compared with the corrugated expansion joint, the fundamental difference lies in the deformation mechanism. The corrugated pipe is elastically deformed by the corrugation, and the metal spherical expansion joint slides by the spherical surface. Therefore, its compensation ability does not depend on the fatigue life of materials, but on the degree of freedom of structures. The disadvantage is also obvious: the ability to compensate for axial displacement is almost zero, so it must be used with other compensators. There are also universal corrugated expansion joints and straight pipe pressure balance expansion joints on our website. They each have a stall. Don't expect spherical expansion to save energy.
Four parameters are selected: nominal diameter, pressure level, rotation angle and connection mode
When it comes to model selection, you may flip if you look at one parameter less. Article 1: Nominal diameter. This matches the inner diameter of the pipe, but be careful that the internal flow section of the sphere should not be reduced too much, otherwise the pressure loss will cause you a headache. Article 2: Pressure rating. Don't just look at the design pressure, you have to count the hydraulic test pressure and impact pressure. Can the sealing ring bear it? For high-pressure applications, it is recommended to choose the model with metal seal. Article 3: Turning angle. A common mistake made by customers is to mark the "maximum corner" but not give the design corner. How many degrees do you want it to move in actual operating conditions? Leave some allowance, don't push to the head as soon as you come up. Article 4: Connection method. Flanged connections are most common and also available with welding or threading. We have metal hoses and PTFE-lined hoses on our website. The connection method can refer to that set of standards, but the flange thickness of the spherical expansion joint should be calculated separately, because the end of the sphere is stressed more.
3 common mistakes made at the installation site: fixed bracket, pre-stretching, guide frame, don't miss one
I have seen too many sites, and it leaks after three months of installation. First error: The fixing bracket is not in the right position. The metal spherical expansion joint itself does not absorb axial force, so a fixed bracket must be set near it to guide axial thrust to the bracket. If the bracket is located remotely, pipe deformation will force the ball joint to rotate over the limit. Second mistake: Didn't do pre-stretching. Some construction teams save trouble and install cold directly. After the pipeline heats up, it produces axial elongation, and the spherical expansion joint itself has no axial compensation ability. As a result, the ball head and spherical shell are hard-topped, and the sealing ring is scrapped in advance. The correct way is to pre-pull or pre-press according to the design displacement during installation, so that it returns to the middle position at operating temperature. Third error: Missing guide frame. When the ball joint turns, the pipes on both sides will swing with it. If the guide frame is not added to limit the lateral offset, other equipment in the pipe gallery will suffer. The guide frame is installed at a certain distance from the ball joint. For specific data, check our websiteCorrect installation method of expansion joint of large tie rodWhere detailed calculations are available.
Comparison of brothers in the same door: Metal spherical vs rotary compensator vs duplex hinge expansion joint, how to pick?
All three are experts in angular displacement compensation, but they are applicable to different scenarios. The rotary compensator relies on the combination of two rotary joints and connecting rods, which can absorb large axial displacement and angular displacement, but its structure is complicated and requires high installation space. We have rotary compensators on the station, which are suitable for bi-directional compensation of long straight lines. Compound hinge transverse expansion joint (which is what you often call hinge expansion joint) consists of two hinges and bellows, which can only rotate in one plane and cannot bear torsional load. However, the metal spherical expansion joint can withstand three-dimensional arbitrary angular displacement, but the axial displacement is almost zero. So, if your pipe is mainly bending in a horizontal plane, it is more economical to use a hinged expansion joint; If the pipeline wants to go around the column, there are deflections in all directions up, down, left and right, then go directly to the spherical expansion joint. Or, you look for the double straight pipe bypass pressure balance expansion joint on our website, which takes into account both axial and angular directions, but the cost is high, so you have to settle the account.
Routine maintenance: lubrication cycle, seal replacement, thread inspection-don't wait for a leak to repair
Maintenance seems simple, but it is easiest to ignore. Lubrication cycle: If the sealing ring is made of polytetrafluoroethylene, there is actually no need for additional lubrication; In the case of graphite packing, it is recommended to replenish the grease every six months and tighten the gland bolts. Seal replacement: Don't worry if you find a leak, it may be that the bolts are loose. If it still leaks after tightening, disassemble and change the sealing ring. Note that the replacement cycle depends on the medium temperature and pressure, and steam pipes are usually replaced every 2-3 years. Thread inspection: The connecting bolts and tie rod nuts at both ends of the ball joint should be inspected once a month, especially in the first three months of production. The thermal cycle will make the stress of the bolts relax. In addition, we have a question and answer on our websiteDo you need to remove the screw of the expansion jointThe screw of the spherical expansion joint is used for limiting the position. Don't disassemble it randomly, otherwise the sphere will disengage.
The metal spherical expansion joint can be used for ten years, but it will leak after three months of smashing. The key is not to be lazy when selecting the model, and don't save the process when installing. If you are not sure, turn through the one on our stationService life of expansion jointAndMetal expansion joint weight tableCompared with it, it is much better than reworking afterwards.