Metal Expansion Joint Cracks: Is It Fatigue or Selection Pit? This is how veterans investigate
The old brother who does pipe maintenance has probably seen cracks in metal expansion joints. As soon as the crack came out, my heart skipped a beat-I had to stop the machine again, I had to change parts again, and the cost went up. But whose pot is the crack anyway? Is the material not good? Or did you not get the design right? Or did the guys slip their hands while installing them? Don't hurry to shake the blame, let's tear it down one by one.
Where do the cracks come from? Don't just blame the materials, design, installation and working conditions can't run away
Two days ago, the equipment supervisor of a cement plant spat with me, saying that they were newly installedMetal Corrugated Expansion Joints in Cement IndustryAfter less than half a year of use, the bellows cracked at the root. The manufacturer said it was fatigue failure, and they suspected that the material was cut corners. I asked him to send the working parameters and installation records-tsk, the problem was that there was not enough pre-stretching during installation, and the vibration frequency of the pipeline just collided with the natural vibration frequency of the expansion joint. The bellows was in resonance for a long time, so it was weird that it didn't crack.
The generation of cracks is often the result of the superposition of three aspects:
- Design side:When selecting the model, only the pressure is calculated but not the displacement, or an inappropriate structural form is used. For example, hard on high-temperature pipesUniversal corrugated expansion jointInstead of adding thermal insulation, the thermal stress directly tears the bellows open.
- Installation side:The welding stress at the end is not eliminated, the cold drawing amount is not executed according to the drawing, and the direction of the guide tube is installed backwards... These "low-level errors" can directly cut the life of the expansion joint by half.
- Working condition end:Media corrosion, sudden temperature change, water hammer impact, external vibration, each can be the "last straw" of cracks.
So don't scold the material when you look at the crack-even if you useLarge diameter thick wall expansion jointIf the working conditions do not match the crack, the crack will still crack.
Cracks wait for no one: Leaks are just the beginning, how horrible is the ripple effect of a system downtime?
When the crack first appears, it may just seep a little media. You only see a hair-like line when you shine a flashlight for a long time. At this time, many on-site masters will think, "It's okay, I can still hold it for a while." Alas, this kind of fluke kills people.
Cracks can grow rapidly along the stress concentration zone, especially under alternating loads-a small crack today may become a through joint tomorrow. If the medium leaks out, if the pipeline is filled with high-temperature steam, toxic gas or corrosive liquid, it is the cause of safety accidents. What's more, once the expansion joint fails, the thermal displacement compensation of the whole pipeline system will be broken, and the adjacent brackets, valves and equipment will have to be carried hard, or maybe a"Domino"The destruction of the type. And guess what? I saw a power station project last year, aCorrugated expansion joint for power station industryThe crack was not treated in time. Three days later, the whole pipeline was disconnected, and it was shut down for repair for a whole week, resulting in a loss of seven figures.
Therefore, cracks are never "minor problems", it directly threatens system security.
Is it reliable with the naked eye? Three Kinds of Earth Methods and Instrument Methods for Detecting Cracks in the Field
The experienced master shines the side of the flashlight on the bellows, sweeps it along the bottom of the corrugated valley, and can see slight hair lines. But this is only effective for surface cracks, and it is eye-consuming.
- Penetration detection (local method):Spray the suspected area with kerosene or special colorant, wait for a few minutes to wipe it off, and then spray the developer. A red line will appear at the crack. Simple operation, extremely low cost, can be done in the warehouse.
- Magnetic particle detection:Suitable for ferromagnetic materialsMetal expansion jointIt can detect surface and near-surface cracks, and its sensitivity is higher than that of permeation method, but it requires equipment and cleaning the coating.
- Ultrasonic thickness measurement + TOFD:For the parts that have been thinned but have not been cracked, make a circle with an ultrasonic thickness gauge, and the abnormal wall thickness area is the potential crack area. If coupled with TOFD (diffraction time difference method), even small internal defects can be found out.
Every month, two to three ripple peaks and troughs were sampled by osmotic method and recorded. Don't wait for a leak to remember to check it-that's called hindsight.
The underlying logic of crack prevention: the thicker the selection is not the better, the matching of working conditions is the king
The larger the wall thickness of the expansion joint, the more durable it is. Which is wrong. The core of bellows is flexible compensation ability. The larger the wall thickness, the greater the stiffness, which reduces the ability to absorb displacement, and it is easy to produce high stress concentration at the root. You have to put a high-temperature and high-pressure steam pipeHigh temperature axial expansion jointThe wall thickness is increased by two millimeters, and as a result, the displacement compensation is not enough, and the bellows is directly cracked. Who do you ask for reasoning?
- Displacement Amount:Axial, lateral, angular displacement, regardless of which, leave a 20% safety margin when selecting.
- Fatigue life:The design requires at least 1000 cycles, and the actual working conditions are matched according to more than 3000 cycles.
- Medium and Temperature:Corrosive media selectionPTFE-lined hoseOrPTFE compensatorSelect high-grade heat-resistant steel for high-temperature occasions.
- Vibration and shock:If there is a vibration source, consider adding damping or choosingCompound hinge transverse expansion jointSuch structures can constrain vibration.
To put it bluntly, selection is a "match game", not a stacking contest.
What to do if there is a crack? Welding repair? Don't make trouble, most cases can only be changed
Someone at the scene tried to save trouble, and took a welding gun to fix the crack of the metal expansion joint twice. What's the difference between putting a band-aid on a bleeding artery? Welding heat input will destroy the cold hardened layer of the bellows, and new welding stress will be generated. After welding, there will be no leakage. After a few days, there will be another crack next to the welding bead. If it is austenitic stainless steel bellows, welding is easy to cause intergranular corrosion, which is more dangerous.
Once penetrating cracks or obvious thinning of metal expansion joints appear, they should be dismantled and replaced with new ones。 Don't think about fixing. In order to rescue, three conditions must be met at the same time: the crack is non-penetrating, the length is less than 10mm, and it is not located in the maximum stress area at the root of the bellows. Even so, it is necessary to use argon arc welding at low temperature, and do stress relief treatment after welding. But to be honest, with this effort and cost, it is better to just change it to a new oneUniversal corrugated expansion jointOrDirect buried (fully buried) type expansion jointAnd worry-free.
Avoid pits from the source: Keeping an eye on these 4 parameters when selecting metal expansion joints can reduce the crack rate by half
After working in this business for more than ten years, I summed up a set of "four-piece pit avoidance set"-if you catch these four parameters to death, the crack rate will be reduced by at least 50%:
- Pressure Class:Don't just look at the design pressure, but count the peak pressure of the water hammer impact. Many cracks are caused by instantaneous overpressure.
- Compensation amount:The actual thermal expansion of the pipeline is certain, certain and must be reviewed by the manufacturer. Many people estimate by patting their heads, but as a result, the compensation amount of the expansion joint bought back is not enough, and the crack is forced out.
- Number of corrugated layers:The fatigue life of multi-layer thin-walled bellows for high-pressure applications is one order of magnitude higher than that of single-layer thick-walled bellows.
- End structure:Try to choose one with a guide tube. The guide tube can protect the bellows from high-speed media erosion and wear. This isSpecific Function of Expansion Joint Guide TubeIt was repeatedly emphasized in.
Don't be superstitious about low prices. A reliableExternal pressure single axial expansion jointAlthough it is 10% more expensive, it can last for five years without cracking; Bargains leak after one year's use, and the labor and downtime loss of replacing them once is enough for you to buy three good ones.
Cracks are not terrible. What is terrible is that we don't know where cracks come from, how to prevent them and how to treat them. If you check this way, your plumbing system will be a lot less worried.