Metal expansion joints do dirty work in the pipeline system-absorbing thermal displacement, isolating vibration, compensating for installation errors, and no one remembers it when it is done, smashing the whole factory to stop. Dislocation deformation is one of the most common failure modes at the scene, but many people confuse it with normal displacement, and only regret it when the bellows is worn out and the medium is sprayed out.
Misalignment deformation is not equal to normal displacement: First clarify the definition and judgment criteria
The normal displacement is the axial expansion and contraction, transverse deflection or angular displacement within the design range of the expansion joint, with uniform waveform and controllable stress. Misalignment deformation is the permanent geometric distortion caused by the expansion joint in the non-design direction or over-limit state-the bellows tilts to one side, the wave pitch is uneven, and the center line of the receiving pipe is no longer aligned.
Take the general-purpose corrugated expansion joint as an example. The design parameters indicate the axial displacement of ± X mm and the transverse displacement of Y mm. Once the measured data exceeds 1.2 times of the allowable value, or the deformation does not rebound after shutdown, it indicates that plastic deformation has occurred. Another trick is to see if there are scratches or oxidized bands in the trough, which are marks left by the rubbing of the bellows against the guide tube or external components during displacement.
Five common reasons: Installation error, improper selection, and over-limit operation. Don't ignore one
Let's talk about installation errors first. The fixed bracket is not welded firmly, the spacing between the guide bracket is too large, and the cold tightness value is incorrectly calculated. Such low-level errors are not uncommon in small and medium-sized projects. What's more hidden is that some construction teams use the expansion joint as the pipeline adjustment margin-if the counterparts are not aligned, they will deadpull the expansion joint to make up. As a result, they have already taken the job with the initial misalignment after installation.
Improper selection is even more common. The thermal displacement on the pipeline layout diagram is a combined vector. Some people only count the axial direction regardless of the transverse direction, and take the straight pipe pressure balance expansion joint to absorb the multidimensional displacement, which is not crooked. On the other hand, although the transverse expansion joint of the compound hinge can absorb large transverse displacement, if the hinge pin is stuck and the tie rod nut is not loosened during installation, the displacement is all stuck on the bellows, and it will still be misaligned for you.
There are two cases of over-limit operation. One is that the operating temperature exceeds the design value, and the creep of the bellows material is accelerated; The other is the instantaneous impact load brought by water hammer and air hammer of pipeline, and the peak stress far exceeds the fatigue design curve. There is a third situation that is easily overlooked-the stress level is selected low. When the system tested the pressure, the bellows was stuffed into a bulge, and the operation behind it looked normal, but in fact, the hidden danger of misalignment had already been buried.
Piping system stress imbalance (such as adjacent supports and hangers falling off) and foundation settlement. The misalignment deformation caused by these two kinds of problems often manifests as overall tilt, rather than local waveform abnormality, and the observation range needs to be enlarged during diagnosis.
What are the consequences of misalignment deformation: from media leakage to fatigue fracture of bellows
The most direct consequence of misalignment deformation is that the effective cross-sectional area of bellows changes, the medium flow is blocked, and the pressure loss increases. With the misalignment, the curvature radius of the crest and trough on one side of the bellows is enlarged, and the stress concentration coefficient in this area increases exponentially. The fatigue life plummets-the expansion joint with the original design life of 3,000 times may crack 300 times after the misalignment exceeds the standard.
The leak is the second step. The wall thickness of bellows is inherently thin, and the local thinning and micro-cracks caused by misalignment will gradually penetrate the wall thickness, and high-temperature steam and corrosive media will be ejected from the cracks. If the pipe is filled with smoke or dusty gas, the problem is even bigger-the dust will accelerate the wear of the bellows, creating a vicious cycle.
Don't forget about adjacent devices. The misalignment deformation of the expansion joint is often accompanied by a large reaction force, which will be transmitted to the interface flange of pump, compressor, baffle door and other equipment. Last year, in a power plant project, the expansion joint near the baffle door of desulfurization flue gas was misaligned, which forcibly cracked the shell of the baffle door, caused the flue gas to leak out, and the environmental protection ticket was directly opened to the company.
How to diagnose misalignment deformation on site: Key points of observation, measurement and data comparison
The first step is to look. Stand on the front side of the expansion joint, use visual inspection to judge whether the axis of the bellows is a straight line and whether the wave pitch is uniform. The misalignment deformation is manifested by the bellows "falling" to one side, like grass blown askew by the wind. Then check the attachments such as tie rods, hinges and universal rings for bending deformation and weld cracking. These are the paths for transmitting loads. If they can't bear it first, the bellows will suffer.
The second step is quantity. Use a vernier caliper to measure the spacing between adjacent wave peaks, select four azimuths evenly distributed on the circumference, and compare the average value with the factory data. If the wave distance on one side is significantly larger than the other side, it indicates that there is an angular displacement or a lateral misalignment. Measure the coaxiality and parallelism deviation of the connecting flanges at both ends of the expansion joint by laser rangefinder or pull wire method, and record the real-time values at ambient temperature and medium temperature.
The third step is to check the historical data. Turn up the recent maintenance records to see if there is an increasing trend in displacement. One excess may be an incidental event, but gradual increment means that the structure of the pipeline system has changed-such as loosening of supports and hangers, settlement of pipe gallery or cold tightening failure of pipeline. Data doesn't lie, trends are evidence.
If conditions permit, do a stress-strain test, attach strain gauges to the peaks and troughs of the bellows, and measure the strain distribution in the running state. This is much more reliable than hand calculations, saving big money with a small amount.
Prevention and treatment strategies: Key actions from selection and installation to routine maintenance
In the selection stage, the multi-dimensional displacement should be calculated clearly. Run the pipeline stress analysis software, take out the combined values of axial, transverse and angular displacements, and compare them with the expansion joint selection manual. For transverse displacement, choose double hinge transverse expansion joint or rotary compensator, consider external pressure single axial expansion joint for limited space, and use high-temperature axial expansion joint or large-diameter thick-walled expansion joint for high-temperature large-diameter pipeline-if the product is selected correctly, there will be many fewer things later.
First, whether the pre-tension/pre-compression value of the expansion joint when it leaves the factory is kept intact, and whether it is changed by external forces during transportation and hoisting; Second, the installation quality of the fixed bracket and the guide bracket, the spacing of the bracket shall be constructed according to the design drawings, and the expansion joint itself shall not be used as a guide; The third is the coaxiality of the pipeline at both ends of the expansion joint. It is strictly forbidden to use the expansion joint to forcibly correct deviation when flange connection.
Don't just focus on the bellows surface for routine maintenance. Loose tie rod nuts are a common problem-the nut is too loose and the displacement constraint fails; Too tight, the displacement is all pressed on the bellows. The torque range given by the manufacturer should be strictly implemented, and it should be checked once every time the machine is shut down for maintenance. Pay attention to whether there is any debris accumulation around the expansion joint and moisture intrusion caused by damage to the insulation layer. Austenitic stainless steel bellows is most afraid of chloride ion stress corrosion. The environment is humid and the medium contains chlorine, so cracks come faster than expected.
To deal with misalignment deformations that have occurred, evaluate the severity first. Slightly misaligned and the bellows surface is intact, which can be corrected by adjusting the tie rod and restoring the bracket constraint. Moderate misalignment accompanied by uneven wave pitch, it is recommended to remove and return to factory for calibration or replace bellows assembly. Severe misalignment-waveform instability, surface cracks, media leakage-don't hesitate anymore, just replace the whole expansion joint. Hang on hard, and once the bellows burst, the overhaul cost is three to five times that of replacing it ahead of time.
The expansion joint is not the cheaper the better, nor is it the more expensive the better, and it is the last word that it is suitable for working conditions. If you have frequent misalignment deformation of the expansion joint in the pipeline system in your hand, sort out the failure photos, operating parameters and design drawings together, and ask the manufacturer for a selection review. Most problems lie in the disconnect between the design input and the actual working conditions.