A tie rod is not an ornament: first understand what it does on the expansion joint
Many on-site masters regard the tie rod on the expansion joint as a permanent structural part, and leave the tie rod on it after welding the pipeline. As soon as the pipeline heats up, the bellows can't move, and the tie rod can carry all the thermal displacement. Over time, either the tie rod bends or the bellows cracks. This is really not a big deal.
The role of the tie rod on the expansion joint, to put it bluntly, is a temporary constraint during transportation and installation. The general corrugated expansion joints and external pressure single axial expansion joints produced in our station will be equipped with tie rods or limit screws when they leave the factory. There are two purposes: one is to prevent the corrugated pipe from being crushed or stretched and deformed during transportation, and the other is to resist the pre-displacement during installation. When the welding and pressure test of the pipeline system are completed, the mission of the tie rod is over.
Since it is going to be dismantled, why doesn't the manufacturer dismantle it directly and then deliver it? The reason is very simple. Without a tie rod, the expansion joint with a larger diameter bumps on the transport vehicle several times, and the bellows will bump and deform, especially big guys such as large-diameter thick-walled expansion joint and straight-pipe pressure balance expansion joint. Without the tie rod constraints, who dares to ship naked?
Installing it without removing it is equivalent to letting the expansion joint dry for nothing: which scenarios must remove the tie rod
After the pipeline is put into operation, should the expansion joint bear thermal displacement? To bear it, the tie rod must be dismantled.
- Steam pipeline: When the medium temperature is above 200℃, the thermal elongation of the pipeline is basically absorbed by the axial expansion joint. If the pull rod is not disassembled, it is equivalent to locking the expansion joint.
- Hot water heating pipeline: start and stop frequently, and repeated temperature changes require the bellows to expand and contract freely. Keeping the tie rod will accelerate the fatigue of the bellows.
- Flue gas pipeline: Flue gas pipeline in desulfurization of power plant or cement kiln tail. Although the temperature fluctuation is not as violent as that of steam, the bellows need a certain flexible buffer under the action of wind dynamic load.
Some expansion joint tie rods have loose nuts when they leave the factory, and they think it's okay at the scene. In fact, once the pipeline is pressed after installation, the bellows is compressed, and the tie rod begins to stress. We have encountered many customers sending photos asking: Does the screw of the expansion joint need to be removed? The answer is straightforward – as long as the bellows need to work, they have to be dismantled.
In the pipeline design, double hinge transverse expansion joint or double straight pipe bypass pressure balance expansion joint are used. Some tie rods are used as permanent components to bear loads and resist internal pressure thrust. In this case, "permanent pull rod" will be clearly marked on the design drawings, and the font is eye-catching. It is better than anything else to look at the drawings before construction.
Look at these three points before demolition: pipeline status, medium temperature, and support condition
Don't rush to copy the wrench before you start removing the tie rod. Confirm three things first, and don't move if one is missing.
First, pipeline status. The pipeline system must have completed the hydraulic pressure test or air tightness test, pass the weld flaw detection, and pass the acceptance of the support. Are the fixing brackets on both sides of the expansion joint welded firmly? Is the guide bracket spacing correct? These basic conditions are not up to standard, and removing the tie rod is equivalent to letting the expansion joint streak naked.
Second, the medium temperature. The internal temperature of the pipe must be lowered, or in other words, the expansion joint does not need to be compensated by pre-displacement. There is a common misunderstanding here: some people think that the pipeline has not been supplied, the temperature is normal temperature, and there is no pressure to remove the tie rod. In fact, it also depends on whether the pipeline is in a cold and tight state. If the pipeline is cold tightened and the expansion joint has been pre-tensioned or preloaded for a certain displacement, extra care should be taken when removing the tie rod. It is best to set temporary supports at both ends of the pipeline.
Third, support situations. Check for temporary supports or hangers near the expansion joint. Directly buried (fully buried) expansion joints and sleeve pipe expansion joints have special support requirements in the installation stage, and the tie rods can't move before these auxiliary supports are dismantled. In short, removing the tie rod is not a matter of loosening a few nuts, but to confirm that the whole pipeline system has "stood firm", and then remove the temporary constraints.
Practical steps: Full sequence from checking the nut to removing the tie rod
The order in which the tie rod is removed is wrong, which is prone to problems. Especially the expansion joint of multi-tie rod structure, such as the transverse expansion joint of compound hinge, has tie rods at all four corners, which must be dismantled symmetrically, otherwise the bellows is unevenly stressed, and it is a trivial matter to tear the insulation cotton next to it when the bullet is opened, but it is a big matter to hurt people.
- Step 1: First check whether the nuts at both ends of each tie rod have rust. If there is rust, spray loosening agent first. Take a torque wrench to test the tightness of the nut. Don't tighten it when you come up.
- Step 2: If it is multiple tie rods, loosen them sequentially in diagonal order. Loosen the nut of each tie rod for half a turn to one turn first, so that the force of the bellows is slowly released. Don't withdraw all the nuts of a certain one at once.
- Step 3: After all the tie rod nuts are loosened in place, remove them completely one by one. Put the unloaded tie rods, nuts and gaskets according to the numbers. Some project completion data need to record the status before and after dismantling.
- Step 4: Check the appearance status of the bellows. After the tie rod is disassembled, visually check whether there are foreign bodies, scratches and deformations in the bellows trough. Under normal conditions, the wave pitch of the bellows should be uniform.
During the dismantling process, if you hear abnormal noise from the bellows or find abnormal displacement of the expansion joint, stop the operation immediately to find out the cause. In this case, most of the pipe supports and hangers are stuck, and hard disassembly will damage the bellows.
Don't rush to finish work after dismantling: what to pay attention to when resuming pre-displacement and trial operation
After the pull rod is disassembled, many people think that the work is done, but in fact, there are still two steps left.
The first step is to restore the pre-displacement. Some expansion joints are pre-displaced in order to compensate for the cold shrinkage of pipelines during installation. After the tie rod is removed, the expansion joint naturally springs back or shrinks to its free length. At this time, measure the actual length with a ruler to see if it is consistent with the cold tightness value on the design drawing. If the deviation is too large, it means that there is a problem with the force of the pipeline, and it is necessary to adjust the bracket and try again.
The second step is commissioning observations. In the process of pipeline system and medium heating, focus on observing whether the deformation amount of bellows of expansion joint meets the expectation. If possible, make several marking points on the bellows, and compare the measured thermal displacement with the design value. Take the corrugated expansion joint used in the power station industry as an example. The design compensation amount generally has a safety margin. However, if the measured displacement exceeds the design value, it means that there is a problem with the pipeline layout or fixed bracket, and it has to be stopped for inspection immediately.
Finally, a reminder, don't throw the removed tie rod. In the future, when hydraulic test, maintenance, or pipeline is stopped for heat preservation, the tie rod can be temporarily installed to lock the expansion joint to protect the bellows from accidental damage.