The ripples are broken, and the pipes are still running. This is the most common state at the scene-the corrugated repair of metal expansion joints, and nine out of ten people wait until they leak before they remember it. It is not impossible to save it if it leaks, but you have to find out what kind of breaking law you are facing.
The high incidence of corrugation damage and its true cause
The corrugated expansion joint, the bad position basically can't run out of the trough, the root and the side of the corrugation. But it's also broken. Power stations and cement industries are completely different things.
The most common killer of corrugated expansion joints used in power station industry is chloride ion stress corrosion cracking. The bit of chlorine in the smoke encounters the tensile stress on the corrugated surface of stainless steel, and the crack goes from the outside of the trough to the inner wall, in the shape of a branch bifurcating, which is just a few thin lines to the naked eye. Once this kind of crack appears, the speed is faster than you think. If it leaks a few points today, it may burst tomorrow. Friends of the power plant should have an impression. When you turn on the insulation during the maintenance period, you can see a dense tortoise crack at the trough position, which basically means that the chloride ion corrosion has not run away.
What about the metal corrugated expansion joint in the cement industry, the bad method is completely different. The cement mill outlet, kiln head and kiln tail those positions, the dust concentration in the air duct is high, the flow rate is fast, the corrugated surface is repeatedly washed by hard particles, and the thinning speed is astonishing. When you touch the crest, it is obviously thinner than other places, and some can even touch the pit. This is called abrasion thinning, not a crack, it is abruptly worn off.
There is also a fatigue crack, which occurs at the wave root position in the transition between the corrugation and the straight edge segment. The pipe expands and contracts repeatedly, and every time the corrugation expands and contracts, the wave root bends once. More times, the metal fatigues, and cracks initiate from the inner and outer surfaces at the same time. This kind of crack is often accompanied by improper selection of expansion joints-for example, where high-temperature axial expansion joints should be used, a general-purpose corrugated expansion joint is installed, and the cycle life can't bear it at all.
Judge whether it can be repaired first: some ripples must be scrapped
Not all damage is worth repairing. The judgment criteria are actually very hardcore, just three.
The crack penetrates but the number is small and the area is small, so it can be mended. What do you mean less? The length of a single crack does not exceed 1/3 of the spread width of the corrugation, or the total number of scattered cracks does not exceed three, and there is no tendency for the cracks to be connected into pieces. This belongs to local damage and can still be used after repair welding.
If the thinning amount of corrugation exceeds 1/3 of the design wall thickness, it is judged to be scrapped. For example, the stainless steel corrugation with a wall thickness of 1.5mm is designed, and the thinnest measured part is only less than 1.0mm. Don't hesitate to change it directly. Even if you fix it, that thin wall won't hold up the pressure cycle and heat cycle of the next overhaul cycle. It is also for nothing to make up, so I will put this here.
The material has undergone intergranular corrosion and is judged to be waste. Intergranular corrosion cannot be seen from the surface, but if the corrugated surface appears slightly orange peel-like, or if you rub it with stainless steel passivation detection solution, it changes color, which means that the grain boundaries are crisp. This kind of corrugation, no matter how high the weld strength is, the base metal itself has fallen apart, and welding it is equivalent to stitching needles on the carrion.
In addition to the corrugation itself, it also depends on the overall state of the expansion joint. If it is found that the attached structures such as tie rods, ring plates and universal hinges also appear deformed or cracked, the design life of the whole expansion joint has come to an end. At this time, repairing ripples would be a crippling if they were repaired.
Correct Posture and Common Rollover Operations for Field Repair
If you are sure you can fix it, how can you fix it? Let's start with the craft.
The corrugated material is austenitic stainless steel, and argon arc welding must be used for welding. The welding wire material should match the base metal, usually 304 or 316L. The current should be controlled in a small specification, and the interlayer temperature should not exceed 100℃. Why control? Because stainless steel is afraid of overheating, when the temperature is high, carbides precipitate, and the corrosion resistance drops directly by a grade. You originally wanted to fix the crack caused by chloride ion corrosion, but after welding, you buried a bigger corrosion danger. What is the picture?
After welding, do you do the hydrogen elimination treatment? For thin-walled bellows, it is actually more important to eliminate the welding residual stress. If conditions permit, use infrared heating plate to partially heat treat, control the temperature at about 400℃, and keep it warm for two hours. No conditions? Then you should at least do pickling passivation, clean the oxide color in the weld area and restore the passivation film. Don't underestimate this step. Chloride ion corrosion likes to find the welding heat-affected zone to start.
This piece of corrugated shaping is the easiest to roll over. After repairing the welded corrugations, because of heat shrinkage, the corrugation spacing will become smaller and the wave height will collapse a little. You take a jack or special shaping tool, and push the wave distance back to the design value. Note that it is pushed back, not deadlifted. The allowable deviation of corrugation spacing is generally ±2mm. Beyond this range, the flexibility of corrugation will change, the compensation amount will not match, and the stress state of the whole pipe system will be completely chaotic.
Tsk, when it comes to rollover operation, there is no need for the wild way of directly fighting with ordinary welding machines. Ordinary electrode arc welding has a large heat input, a lot of splash, and the weld seam forming is ugly like a centipede. What's even more terrifying is that someone welds stainless steel corrugations with carbon steel electrodes in order to save trouble. After welding, the surface looks like it is welded, but in fact, an electrochemical corrosion cell is formed between the weld area and the base metal. In less than three months, the weld seam is rotten around. And guess what? It leaks faster after repairing than without repairing. That's not repairing, that's mining the expansion joint.
And even more outrageous, someone patches the corrugation – cut a stainless steel plate, cover the crack, and weld it around the perimeter. In this way, the flexibility of the corrugation is equal to zero. The reason why the expansion joint can compensate the displacement depends on the deformation of the corrugation. You weld a steel plate to it, and that position can't move. All the displacements are pressed onto the adjacent ripples. It won't take long for the ripples next to it to crack.
The completion of the repair does not mean all is well
Finish fixing and be done? Not nearly there. The test must be passed.
Hydraulic test, the test pressure is usually 1.5 times the design pressure. Hold the pressure for 30 minutes, and observe whether there is leakage and sweating on the weld and corrugated surface. However, there is a problem in the hydraulic test-the corrugated expansion joint is a thin-walled part, and the hydraulic pressure will produce a huge axial thrust. During the test, a restraining tie rod must be installed, otherwise the expansion joint will straighten you directly.
The airtightness test is more sensitive. When the compressed air hits the design pressure, soapy water is applied to the weld, and bubbling is leakage. That's a dirty way, but it works. For the repaired ripples, don't save this step. Soapy water costs a few cents. If you really leak, the cost of reworking is tens of thousands of dollars.
Then there is the fatigue life reduction. Too many people ignore this one. The service life of the expansion joint is supported by the number of cycles of the corrugation. Every time it expands and contracts, the corrugation consumes the fatigue life once. The material properties of the heat-affected zone in the area where you repair welding have changed, and the fatigue strength is only 60% to 70% of the original. How to calculate? To be simple, according to the fatigue life formula given in GB/T 12777 standard, multiply the correction coefficient by 0.6, and recalculate the remaining cycle times. Figure out that if it is lower than the number of cycles expected for your next overhaul cycle, then the work is still for nothing.
Take an example. The design fatigue life of a high temperature axial expansion joint is 1000 times. Cracks were found at the 600th service, and repair welding was repaired. The effective life after restoration is not 400 remaining, but 400×0.6=240. If the pipeline has to be moved another 300 times during your maintenance cycle, sorry, the repair plan is not established, just replace it with a new one.
Instead of repairing it over and over again, it is better to calculate the general ledger
Having said all that, finally settle the accounts.
General-purpose corrugated expansion joint, standard products below DN500, the ex-factory price is several thousand yuan. The working hours of repairing a corrugation add materials, and manual welders take 800 to 1,000 a day. Plus inspection, flaw detection and transportation, they also get 3,000 to 4,000 for miscellaneous things. The repair cost is about 40% to 60% of the replacement cost. If it is a non-standard large diameter, such as the high-temperature axial expansion joint on the main steam pipeline of the power station, the replacement price may be 100,000, so it will be much more cost-effective to repair it.
But that's not how it works. You have to consider the remaining life after the repair and the risk of failure again. First repair, saved half the money. The second time it leaks again, do you still fix it? Two more repairs, not to mention the labor cost, who will bear the loss of pipeline shutdown? The rotary kiln of cement plant stops once, and the loss is calculated by hour, which is hundreds of thousands. In comparison, how much does an expansion joint cost?
Standard products, general-purpose types, replace them directly if they are broken; Non-standard customization, large caliber and high parameters are worth repairing.
If you really want to repair it, I also want to remind you: looking for a manufacturer to do offline repair is a whole order of magnitude more reliable than your own on-site welding. Why? The manufacturer has a complete set of process specifications and inspection methods. The bellows should be verified by a fatigue testing machine before leaving the factory. Although the repair parts can't be as good as brand-new parts, it can at least give you a clear remaining life report. No matter how high the level of on-site welders is, it can't replace the process-oriented repair system. What's more, the environment, working conditions and testing conditions of on-site welding are not as clean as those in the workshop.
To put it bluntly, the corrugated repair of metal expansion joints is not impossible, but you have to know what to repair, how to repair it, and how long it can last after repair. Instead of waiting for a leak and then hurrying, it is better to touch the ripples one by one during the maintenance period, repair what should be repaired and change what should be replaced. This thing is like the brake pads on a car. If you have to wait until the iron is ground before changing it, then it is not just a matter of changing the brake pads.