First, find out first: what exactly is the metal expansion joint carrying for the pipeline? — — Causal relationship between displacement, stress and fatigue crack
When the pipeline is heated by 50℃ and 100℃, how much does it elongate every 10 meters? The linear expansion coefficient of carbon steel is about 12×10⁻⁶/℃, and the 10-meter pipe is elongated by 12 mm at a temperature rise of 100 ℃. If this displacement has nowhere to be digested, all the force will be pressed on the fixed bracket, pipe holder and flange-bracket deformation, flange leakage or small matter, and the bellows being forced out of cracks is the big trouble.
The essence of metal expansion joint is to absorb the displacement by the flexibility of thin-walled bellows, and convert the displacement energy of rigid pipeline system into the deformation energy of bellows. However, every time the bellows absorbs displacement, it completes a low-cycle fatigue cycle. According to GB/T 12777 standard, the design fatigue life is generally considered as 1000 times, and it may only be a few hundred times in high temperature and high pressure conditions. Therefore, if you find cracks in the bellows during maintenance, don't rush to scold the quality-calculate how many times this expansion joint has started and stopped and how many times the temperature has fluctuated in recent years, and you often know it in your heart.
2. Preparation work before maintenance: Don't rush to dismantle it. The nameplate parameters, operating conditions and original installation records are all indispensable
Many people overhaul metal expansion joints, and when they come up, they unload the tie rods and bolts. Guess what? After disassembly, I found that this one in my hand was pre-stretched, and a large part of the corrugated pipe with loose nuts was retracted on the spot-the originally set compensation amount was completely messed up.
Three things must be obtained before construction starts. First, the nameplate parameters: nominal diameter, design pressure, design temperature, compensation amount and fatigue life are recorded item by item. Second, operating conditions: what is the medium and whether there are corrosive components; What is the actual operating temperature and pressure, and whether the fluctuation frequency is high or not-this information determines whether the bellows should be replaced as a whole or treated locally. Third, the original installation record: how much pre-tension is left, where is the initial position of the tie rod nut, these data are not clearly checked, and you have no reference at all when reassembling.
If the information is incomplete, it is equivalent to a blind man touching an elephant. The time saved in this step will be doubled and returned later in the pressure test and debugging stages.
3. Core maintenance skills: how to see, measure and judge whether bellows, guide tubes, tie rod nuts and welds should be replaced
The bellows is the core of the core. During maintenance, take a bright light flashlight close to the trough area and photograph it one by one, focusing on whether there are the following points:
- Cracks in troughs and crests: Axial cracks as thin as hair tend to appear in troughs – where stress is greatest. Immediately after discovery, permeation test (PT) was performed to confirm the depth.
- Corrugated surface pits and wear marks: Localized pits usually suggest deflector failure or media band particle scour. If the wear part is lower than the base metal by more than 10%, replace the parts directly.
- Whether the wave pitch is uniform: Use a vernier caliper to measure the distance between adjacent crests. If the wave pitch deviation of the same set exceeds 10%, it indicates that the expansion joint has been overstretched or compressed, and the waveform has failed.
- Color change: The surface of stainless steel bellows is blue or dark brown, indicating that the operating temperature exceeds the design value. In this case, even if there is no crack, the metallographic structure of the material may have deteriorated, so it is recommended to replace it.
The inspection of deflectors is often neglected, but many bellows perforation accidents are caused by it. The guide tube is on the inlet side of the medium flow direction, and the aperture diameter is usually slightly smaller than the inner diameter of the bellows, and its function is to prevent high-speed fluids and particles from directly washing the bellows. Reach in and touch it during maintenance-if the leading edge of the guide tube has been worn out, warped or fallen off, the bellows behind it has actually been subjected to direct washing. In this case, the guide tube must be replaced and the bellows must be checked for wear.
Tie rods and nuts can't be spared either. Whether the surface of the tie rod is bent, whether the thread is bitten, and whether the nut can be rotated with bare hands-these problems are particularly common in the transverse expansion joint of compound hinge and the pressure balance expansion joint of straight pipe. If the thread still can't rotate after being coated with anti-rust grease, spray loosening agent to soak it first, and don't smash it with a sledgehammer-once the rod is bent, the stress state of the whole expansion joint will change completely.
Don't just look at the appearance. The circumferential weld between the bellows and the end pipe and the fillet weld of the tie rod ear plate must be tested by magnetic particle (MT) or penetration (PT). Old maintenance masters often say a saying: cracks are too late when you see them with the naked eye. Any transverse crack in the weld basically declares that this expansion joint should be scrapped.
4. Identification of common "false faults": Which abnormal noises and deformations don't need to be replaced, and can be solved by adjusting the clearance of the tie rod nut or baffle door
Let's start with the abnormal noise. When the pipeline is running, there is a "gala gala" sound near the metal expansion joint. Many people's first reaction is that the bellows is cracked. In fact, at a certain flow rate, airflow excitation will be generated in the annular cavity between the guide tube and the bellows, making an abnormal noise similar to metal collision. The treatment method is not necessarily changing parts-check whether the guide tube is loose, adjust or fix the guide tube, and the abnormal noise can often disappear.
Let's talk about deformation. Upright mounted external pressure single type axial type expansion joint, over time the lower part of the housing looks a little bulging. This is probably not the broken bellows, but the poor hydrophobicity, the accumulation of condensed water in the external pressure chamber, and the formation of steam and water impact after heating. Repair the drain valve and drain the water smoothly, and the bulge will recover by itself. If you remove it directly and replace it with a new one, the new one will still drum.
The expansion joint on the flue gas pipeline shook during operation, and the maintenance worker checked around and found no problem. Finally, it was found that the blade gap of the circular baffle door (double seal) next to it was too big, and the airflow pulsed. Adjust the clearance of the baffle door, and the expansion joint side immediately becomes quiet. Therefore, the most valuable skill in overhauling metal expansion joints is not to change parts, but to judge whether they should be changed.
V. Reassembly and pressure test acceptance: how to keep the pre-stretching amount, how much pressure to hit in the hydraulic test, and how to reset the damping and limiting devices, so as not to leave future troubles
Clean the pipe flange surface and replace the sealing gasket before reassembling. The tightening sequence of the bolts is alternated diagonally, and the wave pitch change of the bellows is monitored at the same time-if the wave pitch begins to compress significantly in the middle of screwing, it means that the flange surfaces are not parallel, and you have to stop to adjust the gasket. Forced tightening will make the bellows bear extra torque.
The setting of the pre-stretching amount is calculated according to the difference between the installation temperature and the operating temperature. For example, the operating temperature of the pipeline is 200℃, and it is installed at room temperature. The carbon steel pipeline elongates by about 42 mm every 10 meters. If this expansion joint is installed between two fixed brackets, the pre-stretching amount is generally about 50% of the total thermal elongation. In actual operation, according to the design drawings, if the drawings are lost, the compensation amount on the nameplate will be inferred backwards: the compensation amount is 100mm, and half of the pre-stretching is 50mm. Pre-stretching should be completed by the pull rod. After stretching in place, lock the nut and mark it. During subsequent maintenance, you can see whether there is any displacement at a glance.
The hydraulic test pressure shall be 1.5 times the design pressure, but shall not exceed 90% of the yield strength of the bellows material at the test temperature. Note: The test water must be clean, and the chloride ion content should be controlled below 25mg/L-austenitic stainless steel bellows are very sensitive to chloride ion stress corrosion, and many bellows crack after one or two years of operation, that is, chlorine-containing tap water is used in the hydraulic test, and the residual chloride ions open the crack source on the surface of the bellows. Many manufacturers have suffered losses in this detail, so don't be negligent.
The reset of the damping device and the limit rod shall be carried out according to the original installation record. The gap specified in the design must be left between the nut of the limit rod and the ear plate, and it cannot be locked or suspended-the expansion joint loses its compensation function if it is locked, and if it is suspended, it means that there is no limit protection. The hinge bolt of the transverse expansion joint of the compound hinge and the universal hinge of the straight pipe pressure balance expansion joint also need to check whether the pin shaft rotates flexibly and whether the bolt reaches the specified torque one by one.
Passing the pressure test is only the beginning, and there is a closing action: drain the pressure test water, clean the pipeline, re-check the wave pitch of the bellows and the position of the tie rod nut, and confirm that there is no residual displacement due to the pressure test. Finally, clean the external surface of the expansion joint and apply a layer of anti-rust oil. After all this, this expansion joint is really overhauled. Looking back at the whole process-from checking data, disassembling and inspecting, judging to reinstalling, every step is to save insurance for the next safe operation. The skills of overhauling metal expansion joints, to put it bluntly, are: confirm what should be confirmed in place, measure what should be measured accurately, and clean the place where the hand is changed, without leaving a fluke.