Why is the tightening of metal expansion joint bolts the life and death line in the installation process
Let's talk about two on-site accidents first, both of which happened in real life.
In a pipeline of a chemical plant, the metal expansion joint bolts were not tightened in order. Only a few bolts on the front were tightened, but the bolts on the back were still loose. As soon as the system heats up, the medium is sprayed out of the flange gap, and the whole device is shut down in an emergency. After overhaul and disassembly, the gasket has been flushed out of a gap. In another power plant, workers find torque wrenches troublesome, so they directly screw them with afterburners, and each bolt depends on the feel. As a result, the flange was partially pressed out of dents, the bellows was also twisted, and the expansion joint was directly scrapped. That pipeline is still in high-temperature steam condition, so replace a high-temperature axial expansion joint, and add construction to the delivery date, resulting in a six-digit loss.
The matter of tightening the metal expansion joint bolts looks simple, but it is actually the life and death line of the installation link. Screw too loose leakage medium, screw too tight to crush the flange-the middle degree, you have to use rules to get the card, not by feeling.
Something You Have to Think About Before Fastening
Bolt material, spacer status, torque wrench.
The bolt material is not matched casually. High-temperature dielectric pipelines, such as corrugated expansion joints used in power station industry, bolts must use at least 35CrMoA or 25Cr2MoV, and check the heat treatment state when leaving the factory. Ordinary carbon steel bolts will creep and slack above 400℃. If you tighten them in place again, you will return them all after running a thermal cycle.
Spacer this problem is more negligible. Many expansion joints come with their own flanges and gaskets when they leave the factory, but some sites continue to use the old gaskets in order to save money. The old gasket has been pressed out and permanently deformed. If you use the same moment to tighten it, the preload force will not be reached at all. Change it when you need to, and don't save money on metal-wound gaskets.
Is a torque wrench necessary? Let me tell you this, the deviation of the torque twisted out by ordinary wrenches can be plus or minus 40% by hand feel. The same M16 bolt, requiring 200Nm, you feel "quite tight", but it may actually only be 120Nm, or it may have soared to 280Nm. The former leaks when driving, while the flange of the latter deforms directly. Do you think you have to use a torque wrench?
How exactly does the fastening order go?
It's not just twice twisting it diagonally. Corrugated expansion joints, general corrugated expansion joints and compound hinge transverse expansion joints have different force characteristics, and the rhythm of sequential and fractional pressurization is also particular.
Universal corrugated expansion joint, if the flange is round, according to the star order, that is, screw one bolt apart, and load it three times: the first time to 30% of the target torque, the second time to 60%, and the third time to 100%. Be careful to go through it again in sequence every time to ensure that each bolt is stressed evenly.
Compound hinge transverse expansion joint, usually with a tie rod or hinge, where the stress points are concentrated in a specific direction. When tightening, tighten the bolts close to one side of the hinge first, and then tighten the outside, otherwise the bellows will be pulled off. The metal rectangular expansion joint of the rectangular flange is more troublesome, and the four corner bolts should be marked separately, in diagonal order, spreading from the middle to both sides. Don't be bothered. If you screw it in the wrong order, the flange sealing surface will warp. Take a gauge to measure it. The gap is a few tenths of a millimeter. It will be a matter of time before it leaks.
Four common mistakes
The first kind of mistake: screwing it to the end at once.Direct on the torque wrench to the target value in one breath. This way the first bolt bears the full load, and by the time the last bolt is screwed, the first one has been loosened because of the flange deformation. The correct thing to do is to load in split portions, at least ten minutes apart, to allow the gasket and flange to deform sufficiently.
The second wrong: only tighten the front and not the back.The site space is limited, and some bolts are blocked by pipes or brackets, so workers skip them. As a result, only half of the bolts on the sealing surface on that side were stressed, and when the pressure was high, they were directly pricked open.
The third mistake: forgetting to leave a heat tight margin.Under high temperature conditions, the thermal expansion coefficients of bolts and flanges are different, and the preload force will drop after the temperature rises. Therefore, when tightening at room temperature, we should consciously give a little more pre-tightening force, or wait until the system heats up to the working temperature before doing heat tightening. This should be especially paid attention to for high-temperature axial expansion joints.
Fourth mistake: Take the tie rod nut as a fastening bolt.Some expansion joints have tie rods, and the tie rod nut is used to adjust the displacement, not to press the flange. Some people try to save trouble and directly screw the tie rod nut to press the flange tightly. As a result, the tie rod is deformed by force, and the displacement compensation function of the bellows is completely wasted. How to adjust the tie rod nut, that's another set of rules, don't mix it up.
Tightening after 200 hours of operation is the real test
It is not over to load and drive. The real test is the re-tightening after 200 hours of operation. Because the bolt is under the multiple effects of temperature, vibration and gasket creep, the preload force will definitely fall. How much drop? Here are the data: After the high-temperature axial expansion joint runs under 300 ℃ steam for 200 hours, the residual preload force of the bolt may only be about 60% of the initial value. The corrugated expansion joint used in the power station industry is even more ruthless after the alternating heat and cold, because the number of heat cycles is large, and the bolt relaxation speed is faster.
When should I make up the screw? When shutting down for maintenance, draw a line on the bolt and flange with a marker. If the mark of the bolt is obviously displaced relative to the flange, it means that it has been loosened. Twist it again in the same order and torque. When is it time to change the spacer? If it is found that the torque has not been applied in place during re-tightening, the bolt has been screwed close to the yield point, indicating that the gasket has been thinned or damaged. Don't screw hard at this time, change the gasket, and check the flange surface for scratches by the way. Think about it from a different angle, how much does a spacer cost? The flange is scrapped, and the whole expansion joint has to be removed and returned to the factory, which is really painful.
Metal expansion joint bolt tightening, in the final analysis, is to do things according to the rules. If you understand the rules, accidents can be avoided. Before the next installation, calibrate the torque wrench, check the material of the bolts, remember the order, and repair it again after 200 hours. You skip these steps, and sooner or later the expansion joint will be shown to you with leaks.