How old is the bolt in the expansion joint?
When many people see the bolts on the expansion joints for the first time, the idea that pops up in their minds is-this is an ordinary fastener for tightening and preventing loosening, which is no different from flange bolts. If you think the same way, you really think of the expansion joint simply. In the circle of metal expansion joints, bolts are serious functional parts, not for soy sauce. Tie rod bolts, limit bolts and connecting bolts each carry their own jobs. If they are screwed in the wrong position or the wrong torque, the bellows will lose their temper.
Two days ago, I met a customer. A universal corrugated expansion joint was used on the steam pipeline. Less than half an hour after installation, the corrugated pipe bulged directly. When disassembled, the tie rod bolts were not adjusted in place at all, and the internal pressure thrust was all carried by the bellows. You say it was wrong or not? So let's break these bolts out today-who should do what, how, and don't let them "stand in the wrong team".
The worst job: Resisting internal pressure thrust
After the pipeline is pressurized, the medium pressure will generate a huge blind plate force at both ends of the expansion joint. How big is this force? A simple calculation: DN500 pipeline, 1.0MPa pressure, blind plate force close to 20 tons. If all the 20 tons of force is pressed on the bellows, the bellows will be straightened, bulged and even torn in minutes. So you look at the tie rods on straight tube pressure balance expansion joints and curved tube pressure balance expansion joints, which are as thick as forearms, which are used to be stiff against internal pressure.
The tie rod bolts directly transmit the blind plate force to the pipe support by connecting the end plates at both ends, and the bellows can only compensate for the displacement with peace of mind, so there is no need to worry about the businesses of mental pressure. If the material of the tie rod bolt is chosen wrong-for example, the steam pipe is made of ordinary carbon steel, which creeps loosely at high temperature-the consequence will be the "sudden death" of the bellows. In some working conditions, the tie rod will be equipped with double nuts and anti-loosening washers. If the nut loosens itself in the vibration environment, it will be a safety accident.
Installation: Pre-stretching and pre-compression all depend on it
In order to counteract thermal expansion when cold installing pipes, the expansion joints need to be artificially stretched or compressed a certain distance during installation. How to tune it? All depends on the tie rod nut as the adjustment wrench. The nut is tightened and the bellows is compressed; Loosen the nut (while adjusting the other side) and the bellows is stretched. This initial deformation amount directly determines that the expansion energy saving cannot run to the design compensation amount at the operating temperature.
Transport retaining bolts. When the manufacturer leaves the factory, delivery bolts (also called transportation fixing bolts) will be installed at both ends of the expansion joint to hold the bellows tightly to prevent transportation bumps and damages. Many on-site installers don't take this seriously, and the pipelines are not disassembled after being welded. As a result, the expansion joint becomes a rigid connection. During thermal expansion and contraction, the bellows does not move in place, and the pipeline stress is all suppressed and exploded elsewhere. So remember: the delivery bolts must be removed, and after removal, they should be put away. Don't lose them, and they may be used for the next overhaul. Many scenes in this pit have suffered losses, so you have to be careful.
Limiting and guiding functions: Bolts are tracks
Equipment such as double hinge transverse expansion joint and single-axis double baffle door rely on bolts to restrict the bellows from deforming only in a specific direction. For example, if only the lateral displacement is allowed and the axial stretch is not allowed, the bolt becomes a "track". Without this limit bolt, the bellows will twist randomly after being heated, and the equipment interface will easily be broken.
Another example is the high-temperature axial expansion joint. There is usually a row of limit bolts next to the guide tube, and their function is to prevent excessive compression of the bellows. Under high temperature, the rigidity of the bellows decreases. If the compression amount exceeds, the bellows will become unstable and bend laterally, which will produce abnormal noise at least and be scrapped directly at worst. The limit bolt is the last safety, physically blocking the end plate from continuing to approach. In some designs, a limit sleeve will be placed on the bolt to accurately control the compression gap, and the accuracy is millimeter-level.
Details of easy rollover in selection and maintenance
The bolt does not match the medium and temperature, and it is the largest rollover. In the desulfurization flue gas baffle door, the medium contains sulfur and high humidity, and ordinary bolts will corrode into twists in a few months. At this time, you have to use duplex stainless steel or Hastelloy bolts, which are expensive but life-saving. On steam pipelines, the creep relaxation of the bolts will occur at high temperatures (> 400°C), the preloading force will gradually disappear, and the tie rods will become decorations. Therefore, heat-resistant alloy steel (such as 25Cr2MoVA) should be used in high-temperature working conditions, and heat tightness should be reviewed after installation.
Excessive preload force is also a common pit. Once, the customer tightened the limit bolt tightly, and as a result, the bellows was dented, and the compensation amount was directly cut in half. The correct way is to construct according to the torque value given in the design document, or use the marking method-draw lines on nuts and bolts after screwing to the designated position, and observe whether the lines are misaligned during operation, which is clear at a glance.
To summarize in one sentence
The life of the expansion joint depends half on the quality of the bellows and half on whether the bolts are screwed correctly. From tie rod bolts that resist internal pressure, to adjustment bolts that install pre-adjustments, to limit bolts that limit displacement, each bolt has its own mission. Next time you see bolts on expansion joints on site, don't think of them as passersby-they're the "heroes behind the scenes" who really carry the pressure.
By the way, if you are selecting a model or encountering installation problems, you may wish to compare the product information of this site, such as straight pipe pressure balance expansion joint and compound hinge transverse expansion joint, and see their tie rod bolt configuration and material description. If you screw it right, the expansion joint can work obediently for twenty years; Wrong screw, bellows calls you to lose money.