Find out first: the expansion joint is fixed, is the pipeline fixed or the expansion joint itself?
Many customers came up and asked, "How to fix the metal expansion joint?", and then planned to hold the expansion joint body with a bracket. This is a mistake of principle. The expansion joint itself is a flexible element used to absorb the thermal displacement of the pipe. If you hold it tightly down, how can it deform? Doesn't that mean pretending for nothing?
What really should be fixed is the pipe. The pipes on both sides of the expansion joint need to be firmly locked by the fixed bracket, so that all the displacements caused by thermal expansion and contraction are concentrated in the "buffer zone" of the expansion joint. On the contrary, the expansion joint body cannot bear any external restraining force, it only needs to expand and contract freely along the displacement of the pipe. In other words, the fixed bracket holds the pipe, the guide bracket holds the direction, and the expansion joint only deforms.
How to arrange the fixed bracket and the guide bracket? Distance, force and position are all special
The layout principle of the fixed bracket, in one sentence: divide the pipe into several independent compensation pipe sections. There can only be at most one expansion joint in each pipe section, and the fixing bracket must be arranged at both ends of the expansion joint, and the distance should be close enough. Why? Because if the fixed bracket is too far from the expansion joint, the friction force of the middle pipeline itself and the blind plate force generated by the medium pressure will be applied to the expansion joint, which will cause the compensation amount to fail at least and the bellows to crack at worst.
Taking the common general-purpose corrugated expansion joint as an example, the distance between the first guide bracket and the end of the expansion joint should be controlled within 4 times the nominal diameter, and the distance between the second guide bracket and the first guide bracket should not exceed 14 times the nominal diameter. Intermediate guide brackets should be set at intervals at the back. The specific distance depends on the stiffness and allowable deflection of the pipe. This is not a head-slapping number, it comes from the pipe system stress analysis. CAESAR II is commonly used in engineering for checking, but you have to know at least these basic limits.
In terms of force, the fixed bracket should be able to withstand the blind plate force generated by the medium pressure, the friction force of the pipeline, the elastic reaction force of the expansion joint and the weight of the pipeline. Blind plate force is a big head. The higher the pressure and the larger the caliber, the greater the force. Therefore, large-diameter thick-walled expansion joints, fixed brackets of the level of corrugated expansion joints used in power station industry, and concrete foundations must be calculated by tons, so they must not be saved.
The tie rod/nut on the expansion joint should be removed or not during installation? How to tune it?
It's a high-frequency problem. The expansion joint leaves the factory with tie rods and nuts, mainly to prevent the bellows from being bruised and strained during transportation and hoisting. Once installed in place, the tie rod nut must be loosened to allow the expansion energy saver to deform freely. If you don't loosen it, the expansion joint will be locked, and the displacement of the pipe will be stuck in the system, and the flange, weld and equipment mouth will all suffer.
When will the tie rod be used? Two scenarios. First, if the expansion joint needs to bear the blind plate force generated by internal pressure, and no pressure balanced expansion joint is designed, the tie rod can be used as a restraining element, but note that the tie rod at this time is not for you to tighten, but is adjusted to the allowable displacement range. Second, the working conditions that require pre-stretching or pre-compression before installation are realized by the tie rod nut. The adjustment method is: first loosen the locking nut, rotate the adjustment nut to compress or stretch the bellows to the specified amount, and then tighten the locking nut. The specific pre-displacement depends on the difference between the installation temperature and the calculated temperature of the pipeline. Don't screw it blindly.
How to adjust the expansion joint tie rod nut, we specifically talked about it in our previous article, and here we emphasize the core-after the adjustment, the locking nut must be tight back, otherwise the vibration during operation will cause loosening and the tie rod will lose its function.
Taboo: Take the expansion joint as a pipe support and hanger. These wrong installation methods should be avoided
The most typical mistake I have seen is that someone welds the pipe bracket directly to the expansion joint bellows, or uses a pipe clamp to pick up the expansion joint body to bear the load. The wall thickness of corrugated pipe is usually only a few tenths to a few millimeters. If you use it as a hanger, who will perforate if it doesn't perforate?
Another type of error is that the guide bracket is installed in the wrong direction. The role of the guide bracket is to limit the radial displacement of the pipe and allow axial displacement. As a result, the snap ring of the guide bracket and the pipe were welded to death on the spot, or the gap was left too small. As soon as the pipe expanded by heat, the guide bracket became a fixed bracket, and the expansion joint was crushed to death. The correct method is: the inner diameter of the snap ring of the guide bracket should be slightly larger than the outer diameter of the pipe, leaving an axial sliding space, but the gap should not be too large, otherwise it will not play a guiding role.
In addition, when installing the expansion joint, don't forcibly use the expansion joint to compensate for the installation error in order to adjust the pipeline counterpart deviation. Corrugated pipe has high requirements for coaxiality, and if it is forcibly installed in misalignment, the local stress will increase exponentially, and it will not take long for fatigue cracking.
Differences in Fixation of Different Expansion Joint Types: What to Pay Attention to for Axial Type, Transverse Type and Pressure Balance Type
Different types of expansion joints, the arrangement logic of fixed brackets and guide brackets is different.
Axial expansion joints, such as general corrugated expansion joints and external pressure single axial expansion joints, mainly absorb axial displacement. The fixed brackets at both ends must firmly lock the pipeline, and the guide brackets must be arranged in the middle according to the aforementioned spacing to ensure that the expansion joint expands and contracts along the axis direction.
Transverse expansion joints, such as double-hinged transverse expansion joints and double-hinged expansion joints for air-cooled island vacuum pipelines, absorb lateral displacement through hinge or universal joint structures, but do not absorb axial displacement themselves. Such expansion joints usually need to be used in groups of two or three. The layout of the fixed bracket should especially consider the bending moment generated by lateral displacement, and the direction of the hinge pin should be consistent with the displacement direction, otherwise the hinge will be pinned and the expansion joint will be wasted.
Pressure balance expansion joints are even more interesting, such as straight pipe pressure balance expansion joints, curved pipe pressure balance expansion joints and compound straight pipe bypass pressure balance expansion joints. They come with their own balanced bellows, which can offset the blind plate force generated by internal pressure, so the force on the fixed bracket can be greatly reduced. However, the guide bracket still cannot be saved, because the balanced expansion joint is more sensitive to lateral displacement and angular displacement, and once deflection occurs, the balancing cavity is easily unstable.
All fixing brackets and guide brackets must be in place prior to installation of the expansion joint. During the pipeline hydraulic test, the fixed brackets at both ends of the expansion joint should bear the blind plate force under the test pressure, which is greater than normal operation, so the welds and anchor bolts of the brackets must be re-checked before the test.