If the metal expansion joint is broken, don't rush to replace it. Look at these repair methods first

The metal expansion joint plays the key role of thermal displacement compensation, vibration and noise reduction in the pipeline system, but once there is a problem, many people's first reaction is to "remove it and replace it with a new one". Don't rush to do it first. The types of expansion joint damage are different, and the feasibility and method of repair are completely different. Judge right, and the money of a few hundred welding rods can be solved; If the judgment is wrong, the time cost of rework and disassembly can be much higher than that of replacing a new expansion joint.

Judge the type of damage first: corrugation cracking, weld leakage or flange deformation?

Bellows cracking, weld/flange leakage, structural deformation。 These three situations are handled in completely different ways, and something will happen if you get mixed up.

Two days ago, I met a customer who said that the corrugated expansion joint in their cement industry leaked, and the workers went directly to the welding machine to repair the corrugated pipe. As a result, the more the leakage was repaired, the worse it was. Later, when I removed it, I found that it was not the cracking of the bellows body at all, but the sandhole in the end weld. Just such a mistake in judgment, it was hard to weld an expansion joint that could fight for another five years.

Look at the location of the crack, look at the shape of the crack, see if there is deformation。 Is the crack at the crest or trough? Is it going along the grain boundary or across the ripples? Are the flange bolt holes elongated? These details directly determine the subsequent scheme.

Repair of corrugated pipe cracking: Applicable conditions and contraindications of local repair welding

Can the local cracking of the bellows be repaired? Yes, but with a strict premise.

The crack is single, thin and short, located outside the straight edge section of the bellows or the trough arc, and does not extend to the crest. This kind of damage occurs occasionally in the general corrugated expansion joint. If the medium is normal temperature water or low pressure gas, it is feasible to make argon arc welding repair welding with the same material welding wire.

The wave crest position must not be repaired。 The peak is the area where the deformation of the bellows is the most concentrated, and it is repeatedly bent there every time it absorbs heat displacement. The high-temperature annealing zone generated by repair welding will change the grain state of the material, and the welding thermal stress superimposes the original alternating stress, which will crack again in less than a few cycles. There is also a hard rule: if this expansion joint is used in high-temperature and high-pressure occasions such as power stations and chemical industries, don't think about repair welding, just replace it with a new one.

Treatment of weld and flange leakage: Practical points of grinding, repair welding and surfacing welding

Weld leakage and flange sealing surface leakage are relatively "friendly" faults, because they do not involve the fatigue life of the bellows body, and they are almost the same as before after being repaired.

The steps are not complicated, but each step is particular:

• First, clean up the patent leather, rust and oil around the leakage point. It is best to use an angle grinder to make a 5-8mm wide groove near the leakage point-pay attention to polishing the base metal on both sides, and don't just stare at the crack itself.
• During repair welding, welding from both ends of the crack to the middle can reduce the welding stress concentration. The current should be 10%-15% lower than that of normal welding. Don't pursue penetration, but penetration.
• If the leakage of the flange surface is caused by the collision of the sealing surface, it should be repaired by surfacing welding. After welding, the sealing surface must be leveled again by the machine tool. A warning here: when surfacing, avoid the area between the bolt holes, which is the stress concentration area itself. If thermal stress is superimposed, the flange will directly deform for you.

There is another detail of weld leakage that can be easily overlooked-checking for adjacent welds around. If the distance between the two welds is too close, the heat-affected zones of repair welding will overlap each other, so cold welding process or welding at two intervals must be used.

How to choose repair materials: welding electrode of the same material, stainless steel welding wire or temporary plugging agent?

Based on medium and pressureRegardless of the budget.

If it is 304/316L austenitic stainless steel bellows, use stainless steel welding wire of the same grade for argon arc welding, so that the corrosion resistance and strength of the weld can match with the base metal. If you are not sure about the material, use 316L welding wire, which has the best compatibility.

Low hydrogen welding electrodes (such as E5015 and E4315) should be used for leakage points on carbon steel flanges or carbon steel pipes. These welding electrodes have good crack resistance and are suitable for repair welding of stressed parts.

As for the temporary plugging agent, that's for emergency use. Two days ago, a power plant customer asked: Is it OK to repair the micro-cracks of metal expansion joints with epoxy resin plugging glue? My answer is: Emergency repair is ok, but it won't last the next overhaul cycle. The thermal expansion coefficient of plugging glue and metal is an order of magnitude different, and it breaks off as soon as the temperature fluctuates. It is okay to use it on the flange joint surface of non-metallic expansion joint where the pipeline medium is normal temperature and low pressure, but it is purely to cause trouble for yourself when used on metal bellows.

Inspection after repair is completed: hydraulic test, air tightness test and displacement retest, all of which must be missed

Repair welding is done? Not even close. The post-patch inspection is more important than the patch itself.

The first step is the hydraulic test. The test pressure should be 1.25-1.5 times of the design pressure, and the pressure should be kept for at least 30 minutes before the pointer of the pressure gauge drops. This step can control the penetrating leakage caused by welding defects.

The second step is the airtightness detection. Water pressure does not mean that the air tightness is up to standard. Apply compressed air with soapy water to check the weld point by point, or do an ammonia leakage test-this method is sensitive, and microcracks can't be hidden at all. It should be noted that the expansion joint that has been tested for airtightness should be swept with dry compressed air after completion, so as not to let moisture stay in the bellows.

The third step is displacement retesting. The biggest fear of expansion joints is that they are repaired but don't solve the root problem. Measure the axial displacement of the expansion joint under cold and hot conditions respectively, and compare the design compensation to see if it is out of range. If the displacement value obviously exceeds the standard, check whether there is any problem with the pipe support or guide structure, otherwise the repaired bellows will soon be cracked again.

Which damages are not recommended to be repaired: fatigue cracks, intergranular corrosion, severe thinning-straight replacement is more cost-effective than risky repairs

Welds that can be repaired, life that can't be repaired.Some damage types, repairs are simply delaying scrap time.

Fatigue crack is the kind of crack that originates from the surface of corrugated pipe and has shell pattern, and it is directly replaced with a new one. Fatigue damage is an irreversible cumulative process. Repair welding only covers the crack, and microscopic damage has occurred inside the material.

Intergranular corrosion, this kind of damage is gray-black on the surface from the appearance, and cracks can be dug out with nails. In severe cases, the bellows becomes "tofu residue". Stainless steel bellows will be like this when they work in a chloride ion environment for a long time. Intergranular corrosion is the failure of material level, and repair welding is equal to stitching needles on carrion, which must be replaced as a whole.

Don't bother with bellows whose wall thickness is reduced by more than 20% of the design value. For example, the corrugated expansion joint used in the power station industry has a designed wall thickness of 1.5mm and a measured wall thickness of only 1.1mm. This thinning is uniform, thin everywhere, and one place leaks the other.

Judging whether to fix or not, look at three numbers —Residual wall thickness, crack length, working pressure。 If the crack length exceeds 10% of the bellows circumference, replace it directly. If the remaining wall thickness is less than 80% of the design value, replace it directly. If the design pressure exceeds 2.5MPa, don't risk the repair parts. After all, no one wants to bear the responsibility of pipeline bursting.

In the final analysis, the method of repairing metal expansion joints is a job of "judgment taking precedence over craft". It is the real experience accumulated by the masters in chemical industry, electric power and metallurgical industries to fix it simply when it is time to fix it, and to change it decisively when it is time to change it. Next time there is a problem with the expansion joint, go through this idea first, and then decide whether to get started with the welding gun.