If it is not fixed in place, the expansion joint is equivalent to white installation-first understand why the metal expansion joint needs to "control the displacement"
The core role of the metal expansion joint is to absorb the thermal displacement of the pipe, but it is not "free to expand and contract". You have to understand a truth: the expansion joint itself is very thin, and the wall thickness of the bellows is usually only 0.5mm to 3mm. If there is no external constraint, the internal pressure thrust will directly straighten it, resulting in bulging, instability and even rupture of the bellows. Two days ago, a power plant customer sent a photo. They installed itUniversal corrugated expansion jointBecause the fixed bracket is not set, the whole expansion joint is twisted into an S-shape like a snake, and the pipe can't align, so it can only be cut off and re-welded in the end. Therefore, the word "controlling displacement" is the underlying logic of fixing the expansion joint-not only to let the bellows expand and contract freely in the design direction, but also to limit its lateral offset or torsion.
How to set fixed bracket and guide bracket? Different expansion joint type vary greatly
The fixed bracket is a must-have "dead center" near the expansion joint, which catches the pipe firmly and ensures that thermal displacement only occurs on this side of the expansion joint. The guide bracket is responsible for guiding the pipe to move in a straight line to prevent the bellows from being twisted. But different types of expansion joints have completely different setting rules- -
- Universal corrugated expansion joint: Most widely used, main absorption axial displacement. The fixed bracket is generally arranged at one end, and the other end is arranged with a guide bracket, and the spacing is controlled within 4 times the nominal diameter. For example, for DN200 pipes, the spacing between guide brackets should not exceed 800mm.
- High temperature axial expansion joint: The design temperature of this goods can be up to 600℃, and the bellows usually have multiple layers and insulation layers. The fixed bracket must be located on the side close to the heat source, and the spacing between the guide brackets should be reduced to 3 times the nominal diameter. Why? Under high temperature, the material strength decreases, and the bellows is more likely to become unstable, so the support and hanger have to be "more diligent".
- Straight pipe pressure balanced expansion joint: This thing itself has a balance ring, which can withstand internal pressure thrust. In theory, the main fixing bracket can be cancelled. But don't be too happy-in order to save money and space, many projects really don't have fixed brackets, and as a result, the pipes sway back and forth like a pendulum when they run. My suggestion is to set up at least one auxiliary fixing bracket, or useCompound straight pipe bypass pressure balanced expansion jointIts bypass structure can naturally counteract the thrust, and the requirement for the bracket is lower.
Should the tie rod and the limit screw be disassembled? How to adjust the tie rod nut so that it doesn't get stuck or fail?
This question is asked the most by people at the engineering site. The role of the tie rod is to limit the excessive deformation of the expansion joint during transportation and installation, while acting as a "safety rope" to prevent excessive displacement in working condition. But many people screw the tie rod nut to death directly after installation, or simply remove it-both are pits.
Before installation, adjust the tie rod nuts to the initial spacing specified by the design. Like a certainCompound hinge transverse expansion jointThe manufacturer marked "the cold tightness value is 20mm", which means that the nut is screwed to the position where the screw is exposed 20mm during installation. After the pipe heats up in place, the nut will naturally fit the tie rod seat. At this time, if you tighten it again, the bellows will not be able to expand and contract, waiting for it to be cracked.
As for "dismantling or not"-the tie rod is a permanent piece, don't dismantle it. But the limit screw can be removed! The limit screw only temporarily fixes the bellows during transportation, and it must be removed after installation, otherwise it is equivalent to you welding the expansion joint to death. Remember the mantra: "Leave the pull rod, remove the limit, and initially adjust the nut to half open."
Pre-stretching and pre-compression: "Turn back arrow" operation for fixing expansion joints in high temperature pipes
When encountering high temperature conditions (steam pipeline, flue gas pipeline), the expansion joint must be pre-stretched or pre-compressed. For example:High temperature axial expansion jointThe designed compensation amount is 100mm, the operating temperature of the pipeline is 500℃, and the elongation amount is 80mm. If you install it directly without pre-stretching, the bellows will be stretched from 0 to 80mm during operation, and the life will be greatly reduced. The correct way is to pre-stretch the expansion joint to 50% of the design value (that is, pull out 40mm) during installation, so that the actual displacement range becomes-40mm to +40mm during operation, and the bellows always works within the elastic range.
In turn, cryogenic pipes (such as air-cooled island vacuum pipes) need to be pre-compressed. You may ask: Who will do this operation? Generally, the on-site construction team does not dare to make random adjustments. It is suggested that the expansion joint manufacturer preset the length of the tie rod before leaving the factory, and install it directly at the site. Like oursDouble hinge expansion joint for air-cooled island vacuum pipelineWhen leaving the factory, the tie rod is locked according to the cold tightness value provided by the customer, and the installer just takes over the pipeline, which saves a lot of worries.
How to fix special scenes? Direct burial, air-cooled island vacuum pipeline, large-diameter thick-wall expansion joint each has its own special
Let's startDirect buried (fully buried) type expansion joint。 This thing is buried in the soil and can't be inspected on a daily basis. The fixing method is completely different from the above-ground pipeline-it must be "triple fixing": a fixed pier with thrust block is set at both ends of the expansion joint, and a layer of anti-corrosion and waterproof hoop is added in the middle. In order to save trouble, many construction units directly use backfill soil for compaction. As a result, the pipeline sinks and the bellows are flattened. Remember: the fixed pier of the directly buried expansion joint must be higher than the center line of the pipe, and the concrete above C30 should be poured, otherwise the soil creep will push the fixed pier askew.
Air-cooled island vacuum pipeThe pressure is close to negative pressure, but the displacement is large (the temperature difference is more than 50℃). The conventional bracket scheme is easy to make the pipeline unstable, so it is better to use double hinge structure with suspension bracket.Double hinge expansion joint for air-cooled island vacuum pipelineIt comes with two hinges to allow the pipeline to rotate in the plane. The fixed bracket only needs to be set at the corner of the pipeline, and the ordinary hanger can be used for the straight line segment.
Large diameter thick wall expansion jointThe difficulty of (DN1000 or above) lies in self-weight. The bellows themselves are heavy and the guide brackets must bear lateral loads. It is recommended to add a rolling support bracket under the expansion joint. The bottom plate of the bracket is polytetrafluoroethylene plate, and the friction coefficient is only 0.04, which does not affect the axial displacement. We did a steel mill project, DN1400Large diameter thick wall expansion jointIt is by this kind of bracket that the sagging problem is solved.
Common fixed rollover scenes: the bracket position is wrong, the tie rod is not adjusted properly, and the guide spacing is too large.
Of a chemical plantCurved tube pressure balance expansion jointWhen installing, the fixing bracket is set directly above the expansion joint instead of both ends. As a result, when running, the expansion joint bears additional bending moment, and the bellows cracks in half a year. Correct position: The fixing bracket should be 1 times the diameter of the pipe from the end of the expansion joint, neither too far (causing the pipe to become unstable) nor too close (affecting the free expansion and contraction of the expansion joint).
The tie rod nut is screwed dead. There is a garbage incineration project, and the workers finish installing itExternal pressure single axial expansion jointAfter twisting the tie rod tightly, thinking that he was "firm". As soon as the boiler heats up, the bellows can't expand, so the tie rod seat is pulled and deformed directly. Finally, the expansion joint was replaced with a new one, and it was stopped for three days, resulting in a loss of more than 100,000 yuan.
The guide brackets are too spaced apart. According to the national standard, the maximum spacing of guide brackets is 16 times the outer diameter of pipes, but this is for general pipes. The spacing of guide brackets near the expansion joint must be reduced to less than 4 times the outside diameter, otherwise the pipe bending will twist the bellows. Let's not pay tens of thousands of dollars for expansion joints just to save a few brackets.
How to fix metal expansion joints? To put it bluntly, there are three things-control the displacement (fixed bracket + guide bracket), adjust the tie rod (leave the margin for the nut, remove the limit screw), and treat special scenes (pre-stretching, anti-corrosion, bracket). If you follow these points, basically there will be no major problems.