Two days ago, I met a customer. The corrugation of the expansion joint of a high-temperature steam pipeline in their factory was obviously a little crooked, but it didn't leak, so I wanted to last until the maintenance period. As a result, it didn't last until it broke in the middle of the night, and the whole line stopped. This is not an exception.
Three variations, distinguish first and then judge
Do you want to find out the deformation and scrapping standard of metal expansion joint? The first step is not to measure the dimensions, but to distinguish the deformation properties. Elastic deformation, can rebound after displacement recovery, this does not constitute scrapping; Plastic deformation is permanent deformation, and the wave distance can't go back if it is elongated. However, slight plasticity may not fail immediately, so we must continue to observe it. The real danger is instability deformation-the bellows buckles laterally or distorts in the plane under the action of pressure, and the corrugations seem to have been screwed, which is basically directly judged. GB/T 12777 and EJMA specifications give design guidelines, telling you how to calculate when designing, but on-site judgment depends on specific measurement and observation.
Hard indicators for on-site disposal judgment
On-site judgment on several hard indicators, reach any one directly change:
- The wave pitch variation of bellows exceeds the design value by more than 15%, or the wave height has obvious local depressions/bumps;
- Cracks, penetrating corrosion, pitting and perforation appear on the bellows surface;
- Signs of cracking in the connection or end welds.
These do not meet the condition of "making do with it". Many masters can make a rough judgment just by looking at the shape of the ripples-the troughs are squeezed together, the peaks collapse, and the ripples are obviously asymmetric-but it is more reliable to measure the wave distance with a caliper and check the gap between the troughs with a gauge.
Invisible deformation is more dangerous than thought
Pressure-induced plane instability and column instability. The manifestation of plane instability is that a certain wave bulge of the bellows is convex, as if it is blown up alone; Column instability is the whole bellows bending to one side. These two instabilities don't leak immediately, but the bellows have lost their ability to absorb displacement, and continued service will only accelerate fatigue fracture. In this case, it is recommended to replace it directly regardless of whether it leaks or not. If the lateral displacement exceeds the limit, which causes the end flange to deflect, it is necessary to combine the tie rod and hinge structure to judge whether it is damaged jointly-for example, the single hinge of the transverse expansion joint of the compound hinge is worn out, and the whole structure is stressed incorrectly.
Different working conditions have different scrap scales
The scrap scale corresponding to different working conditions varies greatly. In the high-temperature creep environment, even if the deformation of bellows is less than 15%, the cumulative creep deformation continues to increase, so the remaining life should be considered. Uniform thinning to the lower wall thickness limit in corrosive media is more dangerous than simple deformation. There are also stainless steel bellows that are prone to stress corrosion cracking in chloride ion environment-this kind of crack is often penetrating, and the appearance deformation may not be obvious, but it has been wasted. So don't just focus on the size, the material and media environment should be looked at together.
Pits that are easy to step on in practice
Someone screwed the tie rod nut to "correct" the deformation. As a result, the displacement that should be freely absorbed became a binding force, and the force distribution of the bellows changed completely, accelerating the failure. Others forcibly fix the bellows by welding steel plates on the outside-this operation will change the original flexible compensation path of the bellows. It looks like it is fixed, but it is actually creating a new stress concentration point. In addition, the guide tube wears out or falls off. Although the bellows itself is not deformed, the medium directly washes the trough, which will soon reduce the leakage, which is also an indirect signal of scrapping. In daily inspection, priority is given to the outlet of the guide tube, the low point of the bellows and the trough position near the bracket, which have the most problems.
Finally, give a bottom line
If your measured deformation data does not meet the standard, or you are unsure whether it is unstable, then it will be disposed of as scrapped. The cost of replacing a metal expansion joint is much lower than the cost of downtime for maintenance caused by pipe leakage. Especially for high-temperature axial expansion joints and straight pipe pressure balance expansion joints used in key pipelines, once the bellows fails, the consequence is not as simple as leakage. Don't wait until you miss it before you regret it.