How to install metal expansion joint? Find out these 5 things first, then it's not too late to start
I met a customer two days ago, and the equipment was stopped, and the expansion joint had not been installed yet. When asked, the installation team didn't even look at the product nameplate, and directly took the universal corrugated expansion joint to the pipeline. And the result? The guide tube was installed backwards, and the tie rod nut was not loose. The bellows bulged in less than two hours after the test run. This matter is really not to blame the poor quality of the expansion joint, but the installation link went wrong. How to install metal expansion joint? Don't get started in a hurry, understand these 5 things beforehand.
1. Don't rush to unpack before installation
Check the model, displacement direction and product nameplate first. There are too many kinds of metal expansion joints, such as universal corrugated expansion joints, external pressure single axial expansion joints and double hinge transverse expansion joints, and the installation methods are completely different. If you take the wrong model and install it hard, the compensation amount will fail at least, and the bellows will twist and crack at worst.
Also check that the direction of the guide tube is consistent with the direction of the media flow. The function of the guide tube is to protect the inner wall of the bellows, prevent high-speed fluid from washing and reduce the flow resistance. If the direction is reversed, it is equivalent to not installing. There are generally flow arrows on the expansion joint. If you can't find it, check the product manual. Don't be too troublesome.
2. Understand the arrangement logic of supporting and fixing brackets
The expansion joint itself does not bear the blind plate force of the pipeline, and all depends on the cooperation of the main fixed bracket, the secondary fixed bracket and the sliding bracket. Many people ask "How to adjust the tie rod nut of the expansion joint". Remember: the tie rod nut for transportation should be loosened before installation, but the limiting device must not be removed.
The tie rod itself is not intended to absorb displacement, it is only a temporary constraint for transport and protection. If you directly use it as a permanent support, the expansion joint can't move where it should move, and it is hard to carry where it shouldn't move. All the pipeline stress is concentrated on the bellows, and sooner or later something will happen.
Straight pipe pressure balance expansion joint and curved pipe pressure balance expansion joint can absorb internal pressure thrust, and the force calculation of fixed bracket is different at this time. The former relies on the pressure balance between the bellows to eliminate the blind plate force, while the latter achieves the same effect through the hinge structure. The pipeline is not designed with a bracket, and the expansion joint is installed correctly.
3. Five key actions of on-site installation
Counterpoint, alignment, spot welding, welding, inspection, there are pits at every step.
Corrugated pipe sections are not allowed to force matching. The amount of edge misalignment exceeds the standard will lead to local stress concentration, and the crack in operation starts here. Alignment means that the centerline of the expansion joint and the centerline of the pipe coincide as much as possible. If the deviation is too large, the bending moment of the bellows will exceed the design value.
When welding, be sure to protect the bellows to avoid welding slag splashing and burning the wave shell. This damage is invisible to the naked eye, and stress corrosion cracks come out after several months of operation. Welding splashes also tend to form the starting point of electrochemical corrosion at the trough, and it is too late to make up by then.
If it is a high-temperature axial expansion joint, it should also be paid attention to whether the amount of pre-tension or pre-compression has been adjusted according to the ambient temperature. When the ambient temperature is different from the design temperature during installation, the initial displacement of the bellows has to be adjusted accordingly, otherwise the displacement will not be within the allowable range when it runs to the design temperature.
IV. Installation details under special working conditions
What to do with flue gas, dust and corrosive media? This has to be said separately.
For the metal expansion joint next to the desulfurization flue gas baffle door, make sure that the drainage hole is facing down when installing. The condensed water in the flue gas gathers in the trough, which becomes a hotbed of chloride ion stress corrosion over a long time. The drainage hole is blocked or facing upward, which is equivalent to laying a mine in the bellows.
Metal corrugated expansion joints used in the cement industry often have wear-resistant lining pipes. When installing, the gap between the lining pipe and the inner wall of the pipe should be even, otherwise the medium erosion will wear one side eccentrically. When the gap is small, it gets stuck, and when the gap is large, it wears out, which is all a problem.
When hoisting large-diameter thick-walled expansion joints, lifting lugs or soft slings should be added. It is strictly forbidden to strangle steel wire ropes directly on the bellows. The wall thickness of the bellows is only a few tenths of millimeters to a few millimeters, and the steel wire rope is a pit, which will be the source of fatigue cracks in the future.
5. Completion of installation does not mean acceptance
Suppression, reset and record, one step less of these three steps won't work.
Should the expansion joint be temporarily restrained during hydraulic test? Look at the design. If the test pressure exceeds the rated pressure of the expansion joint, temporary support protection bellows must be installed. Overpressure will cause plastic deformation of the bellows, which is permanent damage and cannot be recovered after the pressure test.
After the pressure test, release the constraint and measure whether the actual displacement of the expansion joint is within the design range. Record the data for measurement, don't use your eyes to estimate. Finally, record and file all the installation date, pre-tension amount, actual displacement and bolt tightening torque-all depends on these data for later problems. When the pipeline system fails, there is no installation record, and even the responsibilities can't be distinguished.
How to install the metal expansion joint, in the final analysis, is to respect the design and work according to the rules. The expansion joint is a "weak person" in the pipeline system. It absorbs displacement by deformation. When you install it, it is unevenly stressed, and it will crack to retaliate against you. It works better than anything to spend half an hour looking at the drawings before installation and ten minutes remembering the data after installation.