Go around the site and you'll find a lot of pipes fitted with bellows expansion joints, with two or more long screws running through the ear plates and sandwiching the bellows in the middle. This thing looks simple, just a few screws and a few nuts, but if you really use it wrong, the bellows will show you what it means to "straighten" in minutes.
Metal expansion joint telescopic tie rod, not ordinary bolt. Ordinary bolts are used to tighten, and tie rods are used to limit positions-this positioning is not clear, and the subsequent installation, debugging and maintenance have to deviate.
What force does the tie rod carry on the expansion joint
Blind board force. The internal pressure of the pipe acts on the end face of the expansion joint, creating an axial force that attempts to push the pipe away from the flange connection. How big is this force? The pipeline of DN500 has a pressure of 1.0MPa and a blind plate force of almost 19.6 tons. The bellows can't bear this thrust by itself, so at this time, you have to rely on the tie rod to pull the pipes on both sides.
The second duty of the tie rod is to protect the bellows from being straightened. Bellows are designed to allow axial compression and stretching, but once displacement exceeds the limit, the corrugations undergo plastic deformation, either "bulging" or "straightening". The function of the tie rod is to make the bellows work within the set displacement range, and the displacement amount is limited by the tie rod nut.
So what is the essence of a tie rod? It is a fence that delineates the range of motion for the bellows. The fence is, and the bellows are dangling in the safe zone; If the fence is gone or failed, the bellows is a streaking thin-walled piece, which will be wasted in two strokes.
Looking at configuration from structure: Different pulling methods solve different problems
On the transverse expansion joint of the compound hinge, the tie rod is a part of the hinge structure. The hinge pin bears the moment caused by the lateral displacement, and the tie rod ensures that the bellows does not generate additional axial displacement. Such structures are typically used in pairs and absorb lateral displacement by angular displacement.
Curved tube pressure balance type expansion joint is more particular-it uses two sets of bellows, one set to absorb displacement, and one set to balance blind plate force. The middle pressure balance bellows cancels out the blind plate force itself, and the tie rod mainly plays a restraining role to prevent the balance bellows from over-compressing. The advantage of this structure is that the expansion joint itself does not transmit the blind plate force to the fixing brackets at both ends, and the fixing brackets can be made smaller.
The structure of the expansion joint of the large tie rod is that on the basis of the general corrugated expansion joint, a tie rod and an ear plate are added, and the two ends of the tie rod are connected with spherical washers, which allows the expansion joint to generate transverse displacement within the length range of the tie rod. Such expansion joints are typically used to absorb large lateral displacements in piping systems, and the tie rod needs to withstand all blind plate forces and lateral reaction forces. Large tie rod horizontal type is very common in hot air duct and smoke air duct system, and kiln head and kiln tail are more used in cement industry.
The most critical part of installation: pre-adjustment, locking and removal timing of tie rod nuts
When the expansion joint leaves the factory, the tie rod nut is usually locked. This state is called "transportation state" or "installation state", and the purpose is to prevent the bellows from displacing during transportation and hoisting. Many people ignore this point, and when the equipment arrives at the site, they don't adjust it, and directly weld the pipeline. As a result, the expansion joint can't be expanded or contracted at all, and the stress of thermal expansion and contraction of the pipeline is all held in the bellows.
First, check the installation length of the expansion joint according to the drawing, and confirm the initial position of the tie rod nut. After the pipeline is installed in place, adjust the nut according to the design requirements. The purpose of adjustment is to keep the bellows at a reasonable initial compression amount-generally about half of the compensation amount, leaving a margin in both the thermal expansion and cold contraction directions.
Here is a key action, which many people make wrong: after the tie rod nut is adjusted, the nut on one side should be locked, and the other side should be left with a gap. The side with the gap is the direction in which the bellows expands and contracts freely. If both sides are locked, the expansion joint becomes a rigid connection, and the bellows is deformed by force, which can be reduced in fatigue life at least and directly torn at worst.
What about the timing of the demolition? If the expansion joint is designed with pressure thrust in mind, such as the curved tube pressure balance type, the tie rod can be removed or adjusted to working condition when the system is in operation. However, if it is a general-purpose type and a large tie rod type, the tie rod bears the blind plate force and cannot be disassembled casually. Specifically, whether to dismantle or not, there are instructions on the design drawings, so just work according to the drawings.
The most controversial question on the spot: Will the screw be disassembled or not? Will the nut lock be too dead to fail the expansion joint?
The question of "whether the screw should be disassembled or not" can be argued all morning in the maintenance team. Some people say that the bellows can be freely expanded and contracted only by removing them, while others say that it will hold up the pipe if it is not removed. And guess what? It makes sense on both sides because the expansion joint types are not the same.
Straight pipe pressure balance type expansion joint, the tie rod can be removed during operation-it relies on its own pressure balance bellows to offset the blind plate force, and the tie rod is only a temporary fixture for installation. However, if it is not disassembled after installation, it will not affect the operation, and the tie rod will no longer bear the axial force.
The lateral type of the big tie rod is different. The tie rod is the core load-bearing part. Do you try to dismantle it? The bellows was immediately pushed into a straight tube by the blind plate force, and the cracking of the weld seam was considered light.
So don't ask "whether the screw is disassembled or not", ask "what kind of structure is this expansion joint". If you can't figure it out, turn through the design drawings, which mark the working state of the tie rod.
Let's talk about the nut lock being too dead. If the nuts on both sides are all locked, the axial displacement of the bellows is completely limited, the expansion joint loses its compensation function, and the thermal stress of the pipe has no outlet, which will be transferred to the elbow, tee and valve. However, the blind plate force that the tie rod must bear is still there, so although the bellows can't expand and contract axially, it will be forced to produce radial deformation-it will be fatigue crack over time.
How to start maintenance and overhaul
Daily inspection focuses on three aspects: whether the tie rod is bent and deformed, whether the nut is loose, and whether the ear plate and pin are worn. The deformation of the tie rod is an abnormal signal, indicating that the expansion joint has been subjected to transverse force or blind plate force beyond the design, and the status of the pipe support and the fixed support need to be checked immediately.
The inspection frequency depends on the working conditions of the pipeline system. For high-temperature and high-pressure steam pipelines, it is recommended to check them once in every maintenance cycle; Generally, once a half year is enough for normal temperature pipelines. The bellows surface does not need additional maintenance, but the threaded part of the tie rod should be kept clean to prevent rust and sticking. Use molybdenum disulfide grease for lubrication, instead of ordinary engine oil. Carbonization of engine oil in high temperature environment will accelerate wear.
If you find that the tie rod is bent, don't knock it straight and continue using it. Tie rod is a load-bearing part, and the load-bearing capacity of the tie rod that has undergone plastic deformation has decreased, so there are potential safety hazards when it is continuously used. The correct thing to do is to replace the original fittings while checking that the bellows has also been deformed. If the bellows has obvious permanent deformation, the tie rod is changed for nothing-the bellows has to be changed if it should be changed.
When the hydraulic test is done, the fixed brackets at both ends of the expansion joint must be reliably fixed, and the blind plate force under the test pressure is all borne by the pull rod and the fixed bracket. There was a project that didn't lock the tie rod nut tightly during the pressure test. As a result, the pipe was pushed out directly and the bellows was torn in half. This kind of thing, once is enough.
The metal expansion joint telescopic tie rod looks inconspicuous, and the matter of the pipe is the safety of the whole pipe. Don't be too long-winded. It is more useful to take a look at the stress of the tie rod and the position of the nut than to memorize the instructions ten times.