Find out the concept first: radial displacement is transverse displacement, what is the running method of pipeline "lateral running"
The pipe expands under heat and extends along the axis of the pipe, which is called axial displacement. However, many pipelines are not straight, with elbows, branch pipes and equipment interfaces. Thermal expansion will produce a component force perpendicular to the axis at the corner, and the pipeline will "run laterally". This lateral offset is radial displacement, which is also called lateral displacement in engineering. Note that the unit of radial displacement is millimeters (mm) and the direction is perpendicular to the pipe axis, usually denoted by Y.
Give an example and you'll see. A 10-meter-long steam pipeline, the medium temperature is 200℃, the linear expansion coefficient of carbon steel is about 0.012mm/ (m·℃), the temperature rise is calculated according to 180℃, and the total elongation is about 21.6mm. If there is a 90° elbow in the middle of the pipeline, this 21.6mm elongation will translate into lateral displacement on the other side of the elbow. The numerical value may not be large, but the torque generated by pipe supports and hangers and equipment nozzles may not be small.
There are four kinds of pipe displacements, axial, transverse, angular and combined-don't let the expansion joint "wear multiple jobs"
Axial (telescoping), lateral (translation), angular (deflection), combination (the above exists simultaneously). The first thing to do when choosing an expansion joint is to find out the actual displacement pattern of the pipeline.
The main axial displacement between the two fixed brackets in the straight pipe section can be solved by choosing the general corrugated expansion joint or the external pressure single axial expansion joint. However, if there is a turn in the pipeline direction, such as a Z-shaped pipe section, the thermal elongation between the two fixed brackets will form lateral displacement and angular displacement at the turn. At this time, if you hard-install a general-purpose expansion joint to let it bear the lateral displacement, the bellows will soon appear instable or fatigue crack. When the expansion joint is designed, each displacement form corresponds to different force models, and its indiscriminate use is equivalent to making it wear many jobs, which will cause problems sooner or later.
How to land in radial displacement selection: what working conditions do the compound hinge transverse type, rotary compensator and large tie rod transverse type manage respectively
After it is clear that there is radial displacement, how to choose? Three directions:
It has two bellows, connected by a hinge in the middle, which absorbs large lateral displacements but almost no axial displacements. It is suitable for pipe sections with short spacing between two fixed brackets and need to absorb large lateral displacement, such as pump inlet and outlet pipelines and near the nozzle of heat exchanger. It should be noted that the hinge structure restricts axial expansion and contraction, and the pressure thrust is borne by the hinge, so the force on the pipe support will be significantly reduced.
Its structure is that the pull rod connects the two ends of the pipe, the pull rod bears the internal pressure thrust, and the bellows only bears the transverse displacement. Suitable for pipelines with larger diameter and higher pressure, because the pressure thrust is digested by the tie rod, and the fixing bracket does not need to be designed to be particularly bulky. It is common in the main steam pipeline of power plants and the process pipeline of chemical plants.
The principle is to absorb the thermal displacement of the pipeline through the rotation of the rotating cylinder, the required space is small, the friction torque is small, and it is suitable for overhead pipelines and pipe galleries where the space is tight. When selecting a rotary compensator, pay attention to the upper limit of medium temperature-the sealing packing inside it has a temperature resistance limit, and the seal will fail if it runs at overtemperature.
The easiest pits to step on at the installation site: the tie rod nut is not decorated, temporary fixation is not removed in advance, and cold tightening is not necessary for every project
Pits installed on site, I've seen more than selection errors.
First, tie rod nuts are not decorative. When the large tie rod transverse expansion joint leaves the factory, the tie rod nut is in a locked state, and the bellows is pre-compressed or locked at the installation length. Some people thought that the tie rod was redundant, so they disassembled it directly. As a result, as soon as the pipe heated up, the bellows suffered all the pressure and thrust, and it cracked in a few days. The tie rod nut should be adjusted in place according to the design drawings, loosened during operation and locked during installation. If the order is reversed, the consequences will be serious.
Second, do not dismantle it in advance for temporary fixation. The temporary supports and positioning bolts used for transportation and installation must wait until the whole pipeline system is installed and the hydraulic test is qualified before they can be removed. Early dismantling causes the bellows to be in a state of stress for a long time, and it has not been put into production yet, so the fatigue life has been discounted.
Third, cold tightening is not necessary for every project. Cold tightening is to allow the expansion joint to bear a reverse displacement in advance when installed at normal temperature, which can reduce the actual stress in the working state. However, it needs to accurately calculate the difference between the operating temperature of the pipeline and the temperature of the installation environment, and it is only worth doing if the temperature difference exceeds a certain range. There is no need for some small temperature difference pipes to be cold and tight at all, which will increase the difficulty of installation. Whether it is necessary to do it, the design drawings will be clearly written, and don't make your own decisions on the spot.
Don't just look at displacement, stiffness, pressure, and fatigue life are meaningful when combined
Some people choose expansion joints and place orders only by looking at whether the displacement is enough. This is the most typical take-for-granted thinking. Expansion joints have rigidity – the more rigid the bellows, the greater the reaction force generated under the same displacement, and the greater the load on the pipe support and equipment nozzle.
Design pressure (MPa), design temperature (℃), displacement (mm), stiffness (N/mm), fatigue life (times). They restrict each other: when the displacement increases, the fatigue life will decrease; When the pressure rises, more bellows layers or larger wall thickness are required, but the increase in thickness will lead to an increase in stiffness; The stiffness and displacement determines the elastic reaction force, which directly affects the design of the fixed bracket.
For example, a certain working condition requires a radial displacement of 50mm, a fatigue life of 1000 times and a design pressure of 1.6MPa. According to this, you chose a double hinge transverse expansion joint, and the displacement and pressure are satisfied, but the elastic reaction force is more than 20 tons, and the bracket can't bear it at all. In this case, it is necessary to adjust the bellows parameters, increase the wave number to reduce the single wave displacement, or use double hinge structure to share the load. Every step is a trade-off. Which step is neglected, the scene has to pay for it with real money.
Therefore, the radial displacement of metal expansion joint can not be finalized by just looking at a numerical value, but involves multiple dimensions of product structure, pipeline stress and installation process. Whether it is good to design or purchase, it is much more cost-effective to figure out the above problems before starting.