Pipeline vibration, thermal displacement, pressure pulsation... Metal expansion joints do rough work in the pipe system every day. The old masters who run on the scene all know that what the expansion joint fears most is not the corrugated bulge, but the loose bolts. Loosen a bolt, leakage in the slightest, corrugated pipe twisted and scrapped in the worst. But if the bolts are not loose, it really isn't just tightening them.
Metal expansion joint bolts are loose, and the problem is usually not the bolt-find out where the force comes from first
The expansion joint bolts are loose, most of which occur in the early stage of operation or in the condition of drastic temperature fluctuation. When the medium heats up, the pipe elongates, the expansion joint absorbs the displacement, and the bellows generates elastic reaction force. Where is this force going? If the expansion joint has a tie rod, it will be passed to the pipes on both sides through the tie rod bolts. Once the nut of the tie rod bolt is not pressed tightly, the thrust of the bellows directly pours into the weak link, and the bolt begins to jump.
So the loose bolt is just an appearance. The real question is: Do you know how much repeated load this bolt has to bear? The thermal expansion thrust, internal pressure thrust, vibration additional force of the pipe, these forces add up, far more complicated than you think. The bolts are not "shocked loose", but are "gnawed" loose little by little by the repeated stretching-rebound process.
Tie rod, hinge, balanced structure: different expansion joints have different restraint logic on bolts
The expansion joint is not just a corrugation on a pipe. Such asUniversal corrugated expansion jointThe bellows absorbs axial displacement by itself, and the bolts are mainly connecting flanges and pipes, so the force is relatively simple. But it arrivedMetal rectangular expansion jointWith large cross-section, large internal pressure thrust, and extremely uneven force on corner bolts, it is easiest to loosen.
Expansion joints with tie rods, such asHigh temperature axial expansion jointAnd for power stationsCorrugated expansion jointThe tie rod bolt is a load-bearing member. Its design logic is: the tie rod is subjected to internal pressure thrust, and the bellows only absorbs displacement. If the bolt is loose, the thrust does not move the tie rod, and all of it is pressed on the bellows, then the bellows will be "crushed to death" alive.
Hinged expansion joints, such asCompound hinge transverse expansion jointAndDouble hinge expansion joint for air-cooled island vacuum pipelineThe hinge bolt bears the moment generated by lateral displacement, and the bolt should carry the shear and bending moment at the same time. The balanced structure is more particular,Straight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion jointThe thrust of the middle bellows and the balanced bellows should cancel each other out. Once the key bolts are loosened, the balance will be instantly broken, and the bellows will immediately bend to the side.
The core of anti-loosening is preloading force control, not thread glue and spring washer
Many installation teams apply thread glue and add spring washers as soon as they come up, but in fact, the direction is reversed. The premise of bolt loosening is that the initial preloading force is large enough, so that the contact surface of the thread pair generates enough friction to lock itself. Thread glue and spring washer can only assist, but can't save bolts with insufficient actual preloading force.
M20 8.8 grade high strength bolts, the normal preloading force is required to be about 125kN. Use a manual torque wrench to twist it to 400N·m, and the actual preload force may only be 60~90kN. Why? The friction coefficient fluctuates too much. The lubrication state of the thread pair is different, the roughness of the flange surface is different, and the torque coefficient can change from 0.12 to 0.22. For the same wrench, the same batch of bolts, the actual preload force may be one-third different.
Use hydraulic stretchers if possible; If there is no condition, use the torque method, but it is necessary to calibrate the bolt torque-preload force, draw several bolts in each group for actual measurement of torque coefficient, and then calculate the construction torque. When installing, tighten symmetrically in three times, and the final torque value should be reached in the third pass. This step can't be saved.
The most common mistake to be made during on-site installation: Use the adjustment screw as a positioning pin
There is also a category of looseness, which is purely caused by installation errors. Many expansion joints leave the factory with adjusting screws, which are used to adjust the pre-displacement and installation length of bellows. The nut on the screw is loose in the cold state, and the pipe should be loosened or even removed after it is installed in place. As a result, people often screw the nut of the adjusting screw tightly and use it as a positioning pin.
Once it runs, the pipeline expands thermally, and the bellows will be deformed. As a result, it is stuck by the adjusting screw, and all the force is transmitted to the screw. Either the screw pulls and bends or the nut collapses. You think the bolts are loose, but in fact, the structure doesn't leave a way for the bolts to live.
The adjustment screw is not a tie rod. The tie rod is the load-bearing part, and the adjusting screw is the installation auxiliary part. The two are similar in appearance, but the force logic is completely different. After installing the pipe, the nut that should be loosened must be loosened, and the screw that should be removed must be removed. Don't wait until the whole pipeline is shaking before remembering to check it.
What do you see on a hot inspection? Touch, listen and mark the line, and judge whether the bolt is really loose in three steps
How do you tell if the bolt is really loose? Don't put the wrench on as soon as it comes up. Touch first, wear insulated gloves to feel if there is any abnormal local high temperature around the bolts, and loose friction will generate heat. Listen again, stick the listening stick to the nut, the normal bolt is stressed evenly, and the sound is dull; The sound of loose bolt knocking was crisp, with a lingering sound of "empty". Finally mark the line, draw a white paint line on the nut and flange surface, and come back after a few days of running to see if the paint line is wrong. If the misalignment exceeds 2mm, it can basically be judged to be loose.
After completing these three steps, you should know what to do before you start. Don't screw it blindly, let alone wait until shutdown for maintenance to deal with it together. Under high temperature conditions, the thermal relaxation of the bolts is continuous, and the thermal re-tightening should be done in the first week after installation. When tightening, pay attention to waiting for the pipeline temperature to stabilize before operating. Don't screw it fiercely when the temperature climbs, which will easily pull the thread.
After all, the metal expansion joint bolts are not loose, which depends on accurate design, correct installation and diligent inspection. If the design is not reasonable, it will be useless no matter how careful you are. If the installation is not in place, no matter how well the drawings are drawn, there will be accidents. Although the bolt is small, it carries the temper of the whole pipe.