Take 10 minutes to figure out where the leak is
Hey, the metal expansion joint is leaking. This matter is big or small. But don't rush to dismantle it and replace it with a new one-take 10 minutes to determine the specific location and cause of the leak. And guess what? 90% of emergency replacements are actually wasted money.
Bellows body cracked, weld cracked, flange sealing surface leaked. The way to tell is simple: take a flashlight and see if the medium is seeping out of the corrugated folds, dripping out of the butt weld, or just popping out of the flange gasket. Different leakage points are handled in different ways. For example, if the sealing surface leaks caused by loose flange bolts, just tighten it diagonally with a wrench, without moving the expansion joint itself at all.
The first thing after discovering a leak: emergency isolation, don't blindly block
When a leak occurs, the first reaction is often to "plug it quickly". But it is very risky to plug leaks under pressure. What circumstances can be welded? What circumstances must be downtime? Let me tell you a practical case: a customer of a home appliance station had a pinhole-sized trachoma on the bellows of a high-temperature axial expansion joint. They wanted to spot weld it, but as a result, after a dozen welding arcs, the bellows was directly torn by stress concentration, and the whole pipeline was forced to shut down for two days. Loss or not?
If the medium is water, steam or low-pressure gas, the leakage point is less than 2mm and the corrugated pipe is made of austenitic stainless steel, it can be temporarily treated with a pressure plugging fixture. However, if the medium is high-temperature flue gas, corrosive gas, or the pressure exceeds 0.5MPa, the machine must be stopped and relieved before operation. Remember, safety always comes first, don't lose the whole system just to save half an hour of downtime.
Partial repair or overall replacement? Look at three conditions
After figuring out the leakage, the next step is to decide whether to repair or replace it. There are three bases for judgment: leakage location, severity and medium working condition.
- Local perforation of bellows: If the diameter of the perforation does not exceed 5mm, and there is no obvious thinning and fatigue crack in the surrounding metal, stainless steel can be used for repair welding. However, note that the oil stain and oxide scale must be cleaned before repair welding, and the welding electrode should be made of stainless steel welding electrode of the same grade as the base metal. After welding, local heat treatment should be done to relieve stress. Hey, there is a threshold for this job. It is recommended to find a professional welder to do it.
- Weld cracking: If the circumferential weld or longitudinal weld is cracked, it is basically impossible to repair it. Because the weld is the weakest link of the expansion joint, the heat affected zone will embrittle again after repair welding, and it will crack before long. In this case, just replace it with a new one.
- Under what circumstances must I replace it with a new one?— — When corrugated pipes have large-scale corrosion thinning, fatigue cracks (for example, metal hoses have been used for more than ten years), or the medium temperature exceeds 550℃ (for example, corrugated expansion joints used in power station industry run at high temperature for a long time), at this time, repair welding is to treat the symptoms but not the root cause. Especially for general-purpose corrugated expansion joints and high-temperature axial expansion joints, the specific fatigue life has been considered when designing, and the risk of overdue service is extremely great.
Don't step on the pit when changing the model: Why is it faster to change the same model?
Many customers think that if it is broken, they will buy the exact same one and replace it, which will save trouble. And the result? It leaked again after less than half a year of use. Why? Because the working conditions have changed!
Your original pipeline was hot water at 80℃, but later it was replaced by steam at 160℃. The general-purpose corrugated expansion joint of the same model is made of 304 stainless steel, and the upper temperature resistance limit is about 450℃, but the pressure resistance grade is different. The allowable stress of stainless steel will decrease at high temperature. If the displacement has not changed, the stress bearing by the bellows will exceed the standard. This is the reason why changing to the same model will break faster.
Therefore, when changing the selection, three parameters must be recalculated:Medium temperature, working pressure, pipeline thermal displacement。 In addition, you have to consider the accessory configuration: for example, is the guide tube intact? The function of the guide tube is to divert the medium and prevent high-speed erosion. If the guide tube is corroded and broken, the newly replaced bellows will soon leak due to erosion. In addition, how to adjust the tie rod nut? The function of the expansion joint tie rod is to limit the over-stretching or over-compression of the bellows. Before installation, the nut position should be pre-adjusted according to the design requirements (refer to the "How to adjust the expansion joint tie rod nut" we talked about earlier). Omitting these details is equivalent to laying mines in the new expansion joint.
Routine maintenance is ten times worry-free than post-event remediation
Having said so much emergency treatment before, in fact, what I want to emphasize most is: don't wait for it to leak before repairing it. If routine maintenance is in place, the probability of leakage can be reduced by 80%.
- Installation phase:Make sure to pre-stretch or pre-compress the metal expansion joint when installing it (depending on the type you buy). For example, the directly buried expansion joint is installed directly without pre-compression. As soon as the pipeline heats up, it begins to "grab the stroke", and the bellows quickly fatigues. Don't ask why, it's common sense in mechanics.
- Inspection of pipe supports and hangers:Many leaks are not a problem with the expansion joint itself, but rather a sinking of the pipe support or a loose thrust frame, causing the expansion joint to take extra thrust that it shouldn't have. It is more effective to inspect the hanger once a month than to stare at the bellows.
- Regular inspection of bellows appearance:Use the naked eye or magnifying glass to see the corrugated surface for corrosion pits, scratches and cracks. For corrugated pipes matched with high temperature axial expansion joints or flue gas baffle doors, it is recommended to permeation test (PT) every six months, and the remaining life of micro-cracks is assessed immediately.
- Log the run log:Write down temperature changes, pressure fluctuations, and vibration data for each start and stop. Hey, no one looks at these data at ordinary times, but once leaked, it is the most important basis for you to judge the cause of failure.
After all, what to do with a leaking metal expansion joint? Not by one-time "divine operation", but by daily careful maintenance and scientific selection. I hope this article will help you avoid detours.