1. What is a metal hinge expansion joint? How is it different from ordinary corrugated expansion joint?
Many people will be confused when they hear "hinge expansion joint" for the first time-expansion joint is expansion joint, why add a hinge? Actually, the difference is big. The common axial expansion joint can only absorb the expansion and contraction of the pipe along the axis, while the hinge expansion joint is specially used to absorb the angular displacement. To put it bluntly, when the pipeline needs to turn, or the equipment interface is not in a straight line, the bellows can be "twisted" by the hinge structure, instead of pushing it head-on. Among the products in our station, such as "double hinge expansion joint of air-cooled island vacuum pipeline" and "double hinge transverse expansion joint" all belong to this category. One hinge is responsible for swinging in one direction, and the combination of two hinges can realize transverse displacement. This thing is used a lot in the smoke and air ducts of power plants and cement plants, especially in scenes where the space is limited and the big tie rod can't be installed.
Second, how do hinges and tie rods work together?
The core of the hinge expansion joint is the pair of "ears"-the hinge plate and the pin that connect the ends of the bellows, allowing the tube to swing in the plane of the hinge, but limiting displacement in other directions. You may ask: Then who's going to take the pressure thrust? The answer is a pull rod. Note that the hinge expansion joint is usually equipped with a "big tie rod" or a "circumferential tie rod", which is different from the limit tie rod on the ordinary expansion joint. The function of the tie rod is to bear the thrust generated by the medium pressure, so that the bellows is only responsible for deformation without additional force. When installing, how to adjust the tie rod nut? Don't screw around. When leaving the factory, the manufacturer will generally pre-adjust it to an intermediate position. After on-site installation, the transportation fixtures should be removed (some screws need to be removed!), and then the nut should be adjusted according to the actual displacement to ensure that the pull rod does not block the normal swing of the bellows. If the adjustment is wrong, either the bellows will tire ahead of time, or the tie rod will not break.
Third, the easiest pit to fall into when selecting the model
Many purchasers think that the hinge expansion joint is to change the structure, and the parameters are right. Wrong. The first thing is to look at the load-bearing capacity of the hinge-this thing has to carry not only the self-weight of the pipe, but also the pressure thrust. For example, although our "straight pipe pressure balance expansion joint" and "curved pipe pressure balance expansion joint" also compensate for displacement, their principles are different. The hinge type relies on the hinge to transmit thrust, while the pressure balance type relies on its own structural balance. The pipeline thrust must be calculated clearly when selecting, especially in large diameter and high pressure occasions. Second, look at the temperature. The hinge pin and ear plate are usually carbon steel or stainless steel. If the medium temperature exceeds 400℃, the high-temperature creep of the material must be considered, and the alloy steel should be used. Third, look at the displacement form. Single hinge should be used for single plane angular displacement, and compound hinge or universal hinge should be used for multi-plane angular displacement. There is a "compound hinge transverse expansion joint" in the station to solve the large transverse displacement. Don't make it hard with a single hinge.
4. Those "hidden rules" at the installation site
I met a customer a while ago, and the hinge expansion joint leaked within half a year after installing it. In the past, the installation direction was reversed-the split pin of the hinge pin was misaligned, resulting in uneven force on the bellows. You say it was wrong or not? So be sure to check the direction of the arrow on the expansion joint before installation (that arrow represents the direction of media flow, not the direction of installation), and check whether the hinge pin is lubricated. In addition, the role of the expansion joint guide barrel is often neglected. If the medium contains dust (such as flue gas from the cement industry), the guide tube prevents particles from abrasing the inner wall of the bellows. The direction of the guide tube must also be consistent with the flow direction of the medium, otherwise it is equivalent to not being installed. Finally, after the expansion joint tie rod nut is adjusted, remember to screw the locking nut to death, otherwise it will loosen after long vibration.
5. Don't rush to change it if it is broken. See if it is a minor problem first
The most common "false fault" of hinge expansion joint is the abnormal noise caused by the loosening of the tie rod nut, so it's okay to tighten it. Secondly, the hinge pin is worn. If the clearance is too large, just change the pin shaft. There is no need to dismantle the whole set. If you really want to change the bellows, pay attention to matching the original factory parameters, and don't just replace it with a general-purpose corrugated expansion joint-the number of corrugated layers, wave height and wave pitch are wrong, and the angular displacement compensation amount will change. Service life, under normal working conditions (temperature below 300℃, no obvious corrosion), it is no problem to use it for 10 years. However, if it is a desulfurization flue gas pipeline containing acidic condensate, the life will be directly discounted in half. It is recommended to regularly check whether there is pitting corrosion at the trough of the bellows. In the station, the "desulfurization flue gas baffle door" and "rubber PTFE compensator" are used together, and the effect is better.
6. A summary
When choosing a hinge expansion joint, don't just look at the price, but whether the hinge can hold the thrust, whether the tie rod will interfere, and whether the installation direction is correct. If you are really unsure, go directly to the manufacturer for the calculation book. Every product in our station can provide detailed displacement parameters and installation guidance, including double hinge of air-cooled island vacuum pipeline and horizontal type of double hinge. Don't guess yourself.