How to Repair Metal Expansion Joints? Judge the type of fault first, then decide whether to repair or replace
Two days ago, I met a customer. On the phone, I was anxious to say that their factoryUniversal corrugated expansion jointThere was an air leak and asked if it could be repaired. I asked him to take a video first, and at a glance-there was a trachoma the size of a millet grain on the surface of the bellows, but the whole thing was not deformed, and the tie rod nut was not loose. In this case, repair welding can last for two years. But if the crack has already penetrated three or four waves, or the bellows is directly stuck, then sorry, the cost of repairing is higher than replacing.
SoHow to Repair Metal Expansion Joints?The core is one sentence: first find out the type of fault, and then decide whether to repair it partially or scrap it directly. Don't dismantle it as soon as you come up, and don't change it as soon as you leak, which is a waste of money.
First, find out the root cause of the fault-air leakage, stuck or deformation?
Before maintenance, the most taboo thing is to rely on feeling. You have to at least make a judgment with the naked eye and simple tools. Common faults fall into three categories:
- air leakage: The sound can be heard hissing, or use soapy water to paint welds, bellows peaks and valleys, and watch bubbling. The air leakage point is generally at the base metal of the bellows, the end weld, or the connection between the connection and the flange. LikeCorrugated expansion joint for power station industryHigh operating temperature, high pressure, air leakage is mostly fatigue crack, andMetal Corrugated Expansion Joints in Cement IndustryAir leaks are often due to perforations caused by dust wear.
- Stuck (Displacement Failure): The pipe does not expand or contract when it should be expanded or contracted, or the expansion joint is forcibly pulled into a straight line and cannot be pushed by hand. The tie rod nut is too tight, the guide tube falls off and sticks the bellows, or dust accumulates and scales inside, which will cause stuck. LikeHigh temperature axial expansion jointIf the guide tube in the tube falls off and hits the bellows directly, the movement is not small.
- deformation: The bellows has obvious twisting, uneven wave pitch and local bulging. This is often caused by over-displacement operation. You measure the actual displacement, and then compare it with the factory design value, which is clear at a glance.
If you judge these three situations correctly, it will be easier to handle later. If the judgment is wrong, it will be in vain if the cultivation is done.
2. Local damage of corrugated pipe: if it can be repaired, it will not be replaced, but it depends on the material and thickness
If it is only small trachoma or shallow crack, and the material is austenitic stainless steel of 304 or 316L, with a thickness of more than 0.5mm, it can be repaired. Note: Before repair welding, the damaged area must be polished clean to expose the metallic luster. Use argon arc welding, and the welding wire should be made of the same material or more corrosion-resistant (such as 316L welding wire). It must be smoothed after welding, otherwise it will become a new stress concentration point.
But if that thickness of the bellows is less than 0.3mm (such as someSpecial hose for vacuumOr under low pressurePTFE-lined hose), or the material is special alloys such as Hastelloy and Inconel, which ordinary welders can't handle at all. It is recommended to contact the manufacturer directly. There is another situation: the crack is at the root of the bellows (trough), where the stress is the largest, and the life after repair welding can't be long. It is better to replace it with a new one.
Metal rectangular expansion jointAndLarge diameter thick wall expansion jointMost of the corrugated plates are splicing welds, so attention should be paid to thermal deformation control when repairing welding, otherwise the more they repair, the more they leak.
3. The tie rod nut is loose or the guide tube falls off: adjust the minor problems yourself, don't screw it over
These kinds of problems seem small, but they are self-defeating when they are not handled well. Such asHow to adjust the tie rod nut of expansion joint？ The standard practice is to confirm the design displacement first, and symmetrically loosen the adjustment nut with a wrench to restore the bellows to its free length. Don't screw it unilaterally, otherwise the expansion joint will twist. Many teachers screwed to death as soon as they came up. As a result, the bellows couldn't expand and contract normally and stuck directly.
Let's talk about the deflector falling off. The guide tube itself is fixed to the inner wall of the pipeline by bolts or spot welding. If it falls off, it will fall off and jam the bellows, or even be washed away by the medium and smash the downstream valve. During maintenance, open the manhole and re-weld or bolt the fallen guide tube. Be careful that the weld should not be higher than the surface of the guide tube, so as not to affect the flow of media. LikeDesulfurization flue gas baffle doorIn this kind of equipment, once the guide tube falls off, the slurry pours back into the bellows, and the corrosion rate doubles.
4. What circumstances must be scrapped? Fatigue crack, corrosion perforation, excessive displacement
Don't be reluctant. In some cases, forcibly repairing is planting mines.
- Fatigue crack: If the crack extends more than 3 waves along the bellows axial direction, or the circumferential crack length exceeds 1/4 of the circumference, it means that the material has been fatigued and will crack again soon after repair welding. Direct replacement, recommendedExternal pressure single axial expansion jointOrCompound hinge transverse expansion jointThis type of structure has better fatigue resistance.
- Corrosion perforationPerforations caused by pitting and intergranular corrosion often have only a small hole on the surface, but the surrounding metal has been embrittled. Use magnetic particle detection or coloring flaw detection. If you find dense micro-cracks, don't fix them. Especially for chemical pipelinesPTFE compensatorOnce the inner liner is perforated, the media will corrode the outer metal.
- Excess displacement: The actual operating displacement exceeds the design value by more than 120%, and the bellows has produced plastic deformation. Such asDirect buried (fully buried) type expansion jointOver-stretched, the wave pitch is uneven, and repair welding is useless at this time, because the bellows has lost its elasticity.
By the way,Service life of expansion jointGenerally, the design is 1000~2000 full displacement cycles, but many factories have not broken after ten or eight years of use, because the actual displacement is far less than the design value. If the limit is exceeded frequently, the life span will fall off a cliff.
5. How to accept after maintenance? Pressure test and displacement rebound are indispensable
It's not finished after repairing, you have to verify that it still works. Two steps:
The first step, the pressure test.Carry out hydraulic pressure test according to 1.25 times of the design pressure, hold the pressure for 10 minutes, and check whether there is any leakage on the surface of welds, flanges and bellows. Note: It must be thoroughly blow dried after the hydraulic test, esp.Double hinge expansion joint for air-cooled island vacuum pipelineThis kind of vacuum equipment, when the residual moisture is vacuumed, it will freeze and damage the bellows.
The second step is the displacement rebound test.Loosen the tie rod nut, push and pull the bellows by hand, see if it can smoothly expand and contract to the designed displacement, and whether it can rebound automatically after releasing. If the rebound fails to reach its original position, it means that there is a jamming inside, or the bellows has become unstable. ForCurved tube pressure balance expansion jointFor this type of balanced structure, it is also necessary to check whether the tie rod of the pressure balancing ring moves synchronously.
After these two steps, the maintenance is qualified.
6. Keeping an eye on a few details daily can save half of the trouble
After all, maintenance is a passive remedy, rather than prevention. If you keep an eye on the following points, the failure rate will be reduced by at least half:
- Deflector Wear Inspection: Open the manhole every six months, especially the flue gas system and pulverized coal pipeline. When the deflector is worn to less than half the original thickness, replace it in advance, don't wait for it to fall and smash the bellows. LikeNon-metallic expansion joint (fabric fiber expansion joint)Although there is no metal bellows, the deflector is equally afraid of wear.
- Periodic preloading of tie rod nuts: Not for you to tighten it, but to check if it is loose or rusty. ForLarge tie rod expansion jointEvery three months, use the torque wrench to re-tighten according to the design value to prevent the nut from loosening due to vibration.
- Displacement monitoring: Mark the scale on the expansion joint and record the amount of displacement at each shutdown. If you find that a displacement suddenly becomes larger or smaller, it means that the pipe support may be offset. Let's sayStraight pipe pressure balanced expansion jointIf the displacement is abnormal, there is a high probability that the main fixation bracket is loose.
- Heat preservation and corrosion protection: Expansion joints for outdoor installation, especiallyHigh temperature axial expansion jointAfter the insulation layer is damaged, rainwater enters, and the corrosion rate is astonishing. Regularly check the sealing condition of the insulation layer and repair the damaged areas. Also, don't let the corrosive liquid drip directly onto the bellows.
Okay, so aboutHow to Repair Metal Expansion Joints?I was detailed enough. You go back and compare your own expansion joint, look at the type of fault first, and then decide whether to repair or replace it.