If there is a problem with the metal expansion joint, don't rush to replace it with a new one. It is simple to change another, but the cost is high, the downtime is long, and it can actually be repaired in many cases. The point is-you have to figure out what kind of injury it is. Is the bellows fatigued? Or is it cracked by the media? Or was it strained by brutal operation during installation? These three situations are dealt with in completely different ways.
Two days ago, I met a customer who said that a general-purpose corrugated expansion joint on their steam pipeline was leaking. When I removed it, the corrugation was concave but not cracked. This situation does not need to be changed at all, it can be rectified with pressure correction. But if it is another kind-intergranular corrosion cracks appear on stainless steel bellows, then it is useless for you to use pressure correction. After the external shaping is completed, the internal cracks are still there, and they will still leak once pressurized. In this case, you have to repair welding, or directly replace the bellows section. So the first step is not hands-on, but judgment.
Three things to test before recovery. If one thing is missing, it will be in vain
After judging the type of damage, don't rush to get started. Do three things first, otherwise the cultivation will be in vain.
The first piece, wall thickness measurement.Use an ultrasonic thickness gauge to dot the peaks and valleys at multiple points. Don't only measure one or two points. The wall thickness of one circle of bellows may not be uniform, and media erosion and corrosion thinning are often concentrated on one side. Multi-point measurement can be used to find out the real situation. Wall thickness reduction by more than 10% of the design wall thickness? Then don't fix it, just change it.
The second piece, the hydraulic test.The test pressure is 1.5 times the design pressure, and the pressure is maintained for 10 minutes. At this time, my eyes were kept close to see if there was any leakage or residual deformation. Many expansion joints "look okay, but leak when used", that is, the hydraulic test failed.
Third, displacement review.Compare the factory pre-stretch amount, measure the actual length of the current expansion joint, calculate how much displacement it has absorbed, and whether there is any margin. If it exceeds the design compensation amount, it is useless for you to finish repairing the bellows. The thermal displacement of the pipe has not been digested at all, and it will be broken again soon after installation.
Deflector tube. Many people only look at the bellows and forget to check whether the guide tube is worn out by the medium erosion. Once the guide tube is worn out and the high-speed fluid directly washes the inner wall of the bellows, that is the real murderer. You look fine with the bellows on the outside, but the inside has been hollowed out.
Different structures, restoration methods vary greatly
Axial type, hinge type and pressure balance type have different structures, different failure modes and essential differences in recovery means.
The loss of elasticity of axial expansion joint is mostly due to the attenuation of bellows stiffness. How to recover? By pressing the bellows into a predetermined position by re-pre-deforming, that is, cold tightening, a portion of the elastic displacement capacity can be restored. To put it bluntly, it is to "re-teach" the bellows and make it remember its range of elasticity.
What about the double hinge transverse expansion joint? If the hinge pin is worn and the clearance becomes larger, the light reset is useless. Once the clearance between the pin and the bushing exceeds tolerance, the motion trajectory of the expansion joint is deviated. You pull it back in place, and it will deviate back once it runs. In this case, the pin shaft and bushing must be replaced, and the hinge clearance must be repaired to the design tolerance range before it is truly recovered.
Straight pipe pressure balance type expansion joints are the most troublesome. When the middle cylinder is deformed, you can't pull the bellows directly-that will completely mess up the force relationship between the balancing bellows and the working bellows. The correct way is to disassemble it, check the compression and rebound of the balanced bellows and the working bellows respectively, judge which side has a problem, and then deal with it in a targeted manner.
Field repair of weld leakage and bellows crack
Small cracks, length not exceeding one third of the circumferential wavelength, can be repaired with argon arc welding. Before welding, polish the crack out of the groove to expose fresh metal, and do coloring flaw detection after welding. Here's a detail:Avoid arcing on bellows crest。 The crest is where the stress is most concentrated, and starting an arc there is equivalent to planting a new crack source by hand. The grade of the welding rod must match the base metal. If the wrong welding rod is used in the stainless steel corrugated pipe, new cracks will appear immediately in the heat affected zone.
However, if the cracks appear in large pieces and groups, it means that there is something wrong with the material or medium. The most common culprit is chloride ion stress corrosion. At this time, even if you make up all the cracks, it is useless to restore them to the original material-the corrosion conditions haven't changed, and they cracked again not long after. In this case, either lining with a corrosion resistant layer or directly replacing it with a material resistant to chloride ion corrosion. Hard supplement, it can't be made up.
What about corrosion only occurring in local areas? You can cut the damaged bellows section, replace it with a new one, and butt it with a ring weld. However, it should be noted that the newly replaced bellows section must be completely consistent with the wave pitch, wave height, material grade and layer number of the original bellows, otherwise the stiffness will not match and the force will be uneven, and the newly replaced section will become the weakest point.
Performance verification after recovery, and pits for reloading
Finished fixing, don't rush to go online. First do the air tightness test at room temperature, and then use the working medium to do the withstand pressure test. The airtightness test is to check leakage, and the pressure test is to check strength. Each of the two checkpoints is indispensable.
When reloading, there are a few pits to avoid.
First, the tie rod nut must be loosened. The tie rod of the expansion joint is used for fixing during transportation and installation, and must be in a free state during operation. Many expansion joints are "restored and broken" because the user locks the tie rod too dead, turning the axial compensation into a rigid constraint-the force of thermal expansion and contraction is all held in the bellows, so it's weird if it's not bad. The correct way is to let the expansion joint be in a free state after recovery, and only use the tie rod when adjusting the cold tightness.
Second, check whether the fixing bracket and guide bracket are in correct position. If the fixed bracket is not fixed firmly and the guide bracket slips off, the expansion joint will bear additional force outside the design, and this force is continuous and repeated, which has a great influence on the life of the bellows. How to Recover Metal Expansion Joints? Repairing the bellows is only the first step, and if the systemic problem is not solved, it will still break again in the same place.
In the final analysis, the expansion energy saving can't be restored, depending on whether you can accurately judge the type of damage, repair it strictly according to the process, and eliminate the root cause of the damage. These three steps are in place, and the restored expansion joint will be used for another three to five years. If you want to save trouble, just weld and tighten the tie rod no matter what injury, then you might as well buy a new one.