Have you removed the retaining rod on the metal expansion joint? If it has been dismantled, should it be screwed off and thrown aside, or should it be done after carefully looking at the sign on it?
Two days ago, I met an engineer who was doing power plant maintenance. He complained to me that there was a high-temperature axial expansion joint on the site, and the bellows bulged within two months. When the manufacturer sent someone to see it, the first question was: Has the fixed rod been removed? As a result, the four fixed rods were welded on them intact. It was a good expansion joint, but it was "locked" and broken.
Fixed rod, pull rod, hinge: distinguish the role first and then talk about installation
When many people see how many rods are on the expansion joint, they generally call them "pull rods", which is actually a conflation of three different things.
There are roughly three types of rods on metal expansion joints: fixed rods (also called transport rods), limit rods, hinges or pull rods. Among them, the hinge and the tie rod are part of the expansion joint structure, such as the hinge plate on the transverse expansion joint of the compound hinge and the tie rod on the general corrugated expansion joint, which are used to restrict the displacement direction of the corrugated pipe and belong to the working elements. While the fixed rod is a temporary role, it exists for one purpose-to protect the bellows during transportation and installation.
How to tell? Look at the connection method. Both ends of the fixed rod are usually locked with nuts to the ear plate of the end tube or flange, and the rod itself is straight without adjustment clearance. The pull rod is different. There is an adjustment nut on the rod, which can be used to limit the lateral displacement of the bellows by adjusting the nut, which belongs to the operation. If you can't see the difference, flip through the product manual, or ask the manufacturer directly, don't guess.
The retaining rod is not intended to absorb displacement, it is intended to "lock" the bellows
Once the fixed rod is installed, the bellows is locked, and the axial, transverse and angular displacements are all zeroed.
Then why lock it? When transporting and hoisting, bellows are most afraid of accidental impact and deformation. A general-purpose corrugated expansion joint of DN600, the bellows part is a stainless steel thin-walled structure. If the bump is too big during transportation, the bellows may be stretched or twisted. This damage is difficult to see with the naked eye, but the fatigue life has been discounted. Installing the fixing rod is equivalent to putting an insurance on the bellows, so that it can stay in the factory state.
Another overlooked role is to cooperate with hydraulic tests. When the pipeline system is under pressure test, if the pipes on both sides of the expansion joint do not have reliable fixed brackets, the blind plate force generated by internal pressure will directly straighten or deflate the bellows. At this time, the fixed rod can temporarily bear this force to avoid overload of the bellows. But note – this is only a temporary measure, not a long-term design.
Difference between fixed rod and limit rod: one pipe is installed and one pipe is run
This distinction is too critical, and the consequences of mixing it are catastrophic, so I will take it out separately.
The fixed rod, as mentioned earlier, is a positioning rod for factory fashion. After installing the pipeline and completing the pressure test at the site, it will be removed. What about the limit lever? It is part of the long-term operation of the expansion joint, and its function is to limit the displacement of the bellows to not exceed the design value. For example, in high-temperature axial expansion joints, if the thermal displacement of the pipeline exceeds the limit that the bellows can bear, the limit rod can block this displacement and prevent the bellows from being over-stretched or compressed.
Since the limit lever can limit the displacement, can I use a fixed lever instead of the limit lever? The answer is simple – no.
The strength of the fixed rod is calculated according to the transportation conditions, and it does not care about the design displacement at all. You keep the fixed rod as a limit rod, and the bellows will be tightly hooped when it needs to expand, and the internal stress will all be held on the bellows. When the pressure is high and the temperature is high, the bellows will be unstable or ruptured. What's more troublesome is that the fixed rod has no adjustment function, and you don't even have room to fine-tune the clearance.
Timing of installation and removal of fixed rods: Which rods must be kept?
There is no uniform answer to this question, because different types of expansion joints have completely different rod configurations.
For straight pipe pressure balanced expansion joints and curved pipe pressure balanced expansion joints, they come with their own tie rods and hinges, these rods can never be removed. Remove them, and the pressure balancing mechanism fails, and all the blind plate force on the pipe will be transferred to the bellows and pipe frame. You just need to remove the rods that are marked "Remove Before Installation" or "Transport Secure".
For the transverse expansion joint of compound hinge, the hinge plate is a load-bearing member and cannot be moved. Its fixing rod is usually mounted between the hinge plates and disassembled after installation.
Then when will it be demolished? Two time points: first, after the pipeline is installed and welded; Second, before the hydraulic test of the system. Pay attention to the order-first remove the fixing rod, then do the hydraulic test. If the pressure test is carried with a fixed rod, the result of the pressure test cannot reflect the true stress state of the bellows at all. During pressure test, the pipeline needs to deform freely, and the fixed rod will lock the bellows, which means that you are not measuring the expansion joint, but a section of rigid pipeline.
After removing the fixed rod, the nuts and gaskets on the rod should be kept well, and some project completion documents require that they be returned to the original factory. Don't throw it at the scene. When you go back for acceptance, you can't find all the accessories, which is another trouble.
Selection Myth: Use fixed rods instead of pipe racks to fix?
This is the most dangerous operation I've ever seen.
The design of the pipe bracket in some projects is unreasonable, and the main fixing bracket is not set on both sides of the bellows. The installer looked at several poles on the expansion joint, and his mind turned: This pole is quite strong. Can it be used as a bracket? Keep it undisassembled and save money on two brackets.
Absurdly wrong.
The fixed rod is not a structural part, it has not undergone long-term fatigue strength check. When the pipeline is running, the thrust generated by thermal expansion changes repeatedly, and the threaded connection of the fixed rod will gradually loosen under this alternating load. After loosening, it is a potential leakage point. More seriously, after the fixed rod locks the bellows, the bellows can't absorb the displacement of the pipeline, and the whole stress system collapses. The first bad thing is the bellows.
In the calculation of pipeline force, the calculation of thrust of fixed support is an indispensable item. The internal pressure thrust (blind plate force) and stiffness reaction force of bellows expansion joint must be borne by the main fixing bracket, which is not small. For example, a DN800 pipe has a working pressure of 1.0MPa and a blind plate force of about 55 tons. With 55 tons of force, how many M20 fixed rods do you expect to hold? Just think about it and you know it's unrealistic.
Project case: Bellows failure caused by not removing the fixing rod
Finally, a real accident.
A steam pipeline in a chemical plant, pipeline specification DN350, design temperature 280°C, working pressure 1.6MPa, uses high-temperature axial expansion joint. When the equipment was installed, the construction team didn't read the instructions and didn't know that the fixed rod should be disassembled. The whole pipeline is equipped with three expansion joints, and all six fixed rods are not disassembled.
After the system heats up, the pipeline begins to thermally expand, and the axial displacement is about 40mm. Normally, this displacement should be absorbed by the three expansion joints. But the fixing rod locks the bellows, the displacement has nowhere to release, and all the thermal stress is transferred to the pipe support and valve. As a result, less than three months later, the bellows of one of the expansion joints developed circumferential cracks at the trough, and steam leaked. After shutting down for maintenance, after removing it, there were obvious traces of plastic deformation on the surface of the bellows-it was forcibly pressed and repeatedly extruded under thermal stress.
Check the records, the team responsible for installation has not installed expansion joints with fixed rods before. They thought that this rod was a design stiffener, and it would be safer not to dismantle it. The manufacturer's installation instructions clearly say "Remove the transportation fixed rod before pressure test", but the workers didn't read it at all.
Do you think this is a man-made disaster? Yes.
Therefore, the metal expansion joint fixing rod this thing, say it is simple, it is a bolt rod; Say it is complicated, it is related to the safety and longevity of the whole pipeline. Install it in place when installed, and disassemble it according to the instructions when disassembled. Don't dismantle what should be dismantled, and the bellows will show you the color sooner or later.