1. What exactly is lateral displacement? — — Difference from axial and angular directions
Brothers in this business all know that pipes will expand when heated and shrink when cold. If this displacement is hard to carry, the pipe frame, flange, and even the equipment body will suffer. Metal expansion joints are used to digest these displacements. But there are three kinds of displacement: axial, lateral and angular, which many people confuse at the beginning.
Axial displacement is easy to understand – the tube extends or shortens along the centerline, like you pull a spring. Angular displacement is the angular deflection of the pipe in a certain plane, which is equivalent to bending a chopstick. Lateral displacement refers to the parallel movement of the pipe axis in the direction perpendicular to itself. For example, the middle part of a straight pipe "arched" to the side due to thermal expansion. To put it bluntly, the axial direction is telescoping, the lateral direction is translation, and the angular direction is rotation angle.
In most projects, the pipeline is actually subjected to compound displacement-both axial, transverse and angular. However, the expansion joint specially designed for lateral displacement is completely different from the ordinary axial type in structure. Ordinary axial bellows are as soft as noodles, bend when pressed, and can't bear lateral forces at all; The transverse expansion joint locks the axial deformation of the bellows through the restraining members of tie rods and hinges, and only allows it to swing freely in the direction perpendicular to the axis. Therefore, before selecting, you have to judge: Is the main displacement direction of the pipeline lateral? If so, then you have to take the horizontal route.
2. The mainstream structure of transverse expansion joint: how to choose compound hinge, large tie rod and double hinge
Nowadays, there are three mainstream types of metal expansion joints that can carry lateral displacement on the market:Compound hinge transverse expansion joint、Large tie rod expansion joint、Double hinge expansion joint(That is, the double hinge expansion joint of air-cooled island vacuum pipe). Their principle is to convert axial displacement into lateral swing by adding two sets of hinges or tie rods to the middle pipe section, but the applicable scenarios are very different.
Compound hinge transverse expansion joint: There is a set of hinges at each end, and a straight pipe is in the middle. The pins of the two sets of hinges must be parallel. It can only absorb lateral displacement in a plane (such as horizontal), not axial displacement, nor angular displacement. The advantage is that it is compact and takes up little space, which is suitable for places with tight pipe arrangement. The disadvantage is that it requires high manufacturing accuracy, and if the clearance of the hinge pin is too large, it will easily get stuck if it deviates.
Large tie rod expansion joint: Similar to a double hinge, but replacing the hinge with a tie rod. The pull rod has nuts at both ends, which can be adjusted in length and can withstand the thrust generated by internal pressure at the same time. The advantages of the large tie rod type are cheap and simple to install, but the transverse compensation amount is smaller than that of the hinge type. Many cement industry wind network pipelines and small-diameter pipelines of power stations like to use this.
Double hinge expansion joint: This is a frequent visitor on air-cooled island vacuum pipes and large-diameter thick-walled pipes. It consists of two single hinge expansion joints connected in series through an intermediate pipe, and the pins of the two sets of hinges are perpendicular to each other. This means that it can absorb lateral displacement in any direction (that is, spatial lateral displacement), and the compensation amount can be large. The disadvantage is that the length is long and the weight is heavy, so the pipe support needs to be extra reinforced.
How to choose? Simply put:
-Pipe with lateral displacement in only one direction and limited space → compound hinge
-Limited budget, small pipe diameter, small displacement → large tie rod
-Large displacement, uncertain direction, large pipe diameter → double hinge
3. Calculate the lateral compensation amount: stiffness, pressure and fatigue life must be less
After choosing the structure, it has to count. The three most terrible parameters: bellows stiffness, internal pressure thrust and fatigue life.
Stiffness determines thrust: The smaller the transverse stiffness, the smaller the reaction force of the pipeline, but the wall thickness of the corrugated pipe cannot be infinitely thinned, otherwise the pressure will not be enough. The lateral stiffness of the compound hinge type is usually from tens to hundreds of N/mm, depending on the wave pitch and wave height. When calculating, you need to know the thermal expansion of the pipe X (mm), and then estimate the thrust F = K × X according to the transverse stiffness K in the selection manual. If the thrust is too strong, the pipe rack and the fixing bracket must be strengthened.
Internal pressure thrust is an invisible killer: Many people only count the thermal displacement thrust, forgetting the blind plate force generated by internal pressure. The hinge or tie rod of the transverse expansion joint must be able to withstand the internal pressure thrust, otherwise the bolt will be broken. The tie rod nut of the large tie rod is not adjusted in place, or the strength of the hinge pin is not enough. After installation, it collapses directly. The empirical practice is that the allowable load of the tie rod or hinge should be at least 1.5 times the internal pressure thrust.
Fatigue life cannot be made do: For each cycle of lateral displacement, the bellows trough experiences a bending stress. When designing, the formula in EJMA standard is commonly used, and the number of displacement cycles N is considered. For example, the power station pipeline starts and stops frequently, and N may reach more than 7,000 times, so the fatigue safety factor should be at least 15. The air-cooled island, which runs all the year round, is displaced once a day, 365 times a year, and its design life is 7,300 times in 20 years. It is calculated that if the life is not enough, either increase the wave number or change to a more flexible wave pattern.
4. Several pits that are easiest to overturn at the installation site-are the tie rod nuts adjusted? Can the screw be removed or not?
Two days ago, I encountered a project, and the installation team screwed the tie rod nut on the expansion joint of the big tie rod to death. As a result, as soon as the pipeline heated up, the bellows bulged directly. And guess what? They used the limit nut as a connecting bolt.
The tie rod nut of the large tie rod expansion joint is used to adjust the amount of pre-tension, not to lock the bellows.Installation usually requires first adjusting the tie rod to the installation length (the product is marked when it leaves the factory), leaving the bellows in a free state or a small amount of pre-stretching. After the pipeline is installed in place and the pressure test is completed, loosen the locking nut to the designated gap to leave displacement space for the bellows. For specific adjustment methods, please refer to the Q&A article "How to Adjust the Expansion Joint Tie Rod Nut" we wrote earlier.
Can the screw be removed or not?During the transportation of double hinges and double hinge expansion joints, the manufacturer will lock the hinges with temporary screws to prevent transportation bumps. After installation, these temporary screws must be removed! If it is not disassembled, the hinge can't rotate at all, and the lateral displacement makes the bellows hard to carry, and it will crack in a few cycles. Last year, there was an air-cooled island in a power plant. Because the temporary screw was not removed, it leaked after three months of operation.
Lubrication of hinge pins。 If the pin of the double hinge expansion joint is rusted and stuck, the lateral compensation will fail. Remember to apply molybdenum disulfide grease between the pin and the sleeve when installing, and check that the clearance is even.
5. Actual scene: Which horizontal type should be used for power station pipelines, cement wind networks and air-cooled islands
With all that said, let's look directly at the case.
Power station pipeline (main steam, reheating and heating section): Large pipe diameter, high temperature (above 550℃), high pressure (more than ten MPa), the displacement is mainly axial, but the lateral displacement will also occur when the cold and hot states are switched. This type of working condition is preferredCompound hinge transverse expansion jointOrDouble hinge expansion joint。 Why? Because the corrugated pipe material is usually made of Inconel 625 or austenitic stainless steel, which is resistant to high temperatures, but the wall thickness of the corrugated pipe is thick and the axial stiffness is large, if the universal axial expansion joint is hard, it is likely that it will not be pushed. With the hinge, the bellows only bear the lateral swing, and the stress distribution is more reasonable. It is recommended to consult the product page of "Corrugated Expansion Joints for Power Station Industry" on our website, which has specific selection parameters.
Cement wind network (high temperature fan outlet, kiln tail pipe): The medium is dusty hot air, the temperature is 300~400℃, the pipe diameter is medium (DN500~ DN2000), and the displacement is mainly horizontal and transverse. The on-site pipeline of cement plant is bent and winding, and the space is tight. The expansion joint of large tie rod is the most cost-effective. However, note that a guide tube needs to be installed inside the expansion joint of the large tie rod to prevent dust from washing the bellows. There are large tie rod models with guide tubes in our materials of Metal Corrugated Expansion Joints in Cement Industry.
Air cooling island (vacuum duct): This is the main battlefield of the double-hinged expansion joint. The diameter of the air-cooled island exhaust pipe is very large (above DN3000), and it is in vacuum condition (negative pressure). The axial thrust is very small, but the lateral displacement caused by thermal expansion is very large. Sometimes the lateral compensation amount of a pipe section can reach 300mm. Ordinary compound hinges can't bear such a large displacement, so double hinges must be used. The two sets of hinges are perpendicular to each other, which can compensate for lateral displacement in any direction. We have a special product line of "Double Hinge Expansion Joint for Air-cooled Island Vacuum Pipe", which is designed for this scenario. Special attention when installing: The pipe bracket must be a sliding bracket and cannot restrict lateral movement.
Whichever you choose,Metal expansion joint transverseCompensation schemes must be combined with pipeline stress analysis (CAESAR II or AutoPIPE), and don't rely on experience. Get the hard data of pipe system thrust, displacement and cycle times, and then finalize the model according to the selection manual, so as to ensure that there will be no accidents for ten years.