The expansion joint leaks, which is upsetting to watch and more troublesome to deal with. But don't rush to dismantle it, first find out where the water came from. The location of the leakage point is different, and the reason is completely different from the modification of the method.
First distinguish the leakage point: bellows, weld, flange or connection
Get a leaky expansion joint and turn around it first. Water stains, rust, crystalline salt, will help you guide the way. There are only four common leakage points: bellows body, circumferential weld, flange sealing surface, and the connection between pipe and pipe.
Bellows leaks, usually with pinholes, cracks, or corrosion pits in the walls. Weld leakage mostly occurs on longitudinal seams or annular seams. If you look closely, you can find pores and unfused. Flange leaks, usually at the gasket position, with wet marks or dripping. As for the leakage of the pipe, it is often where the pipe butts with the expansion joint, and there is a problem with the weld or thread. And guess what? Many users report for repair and say that "the expansion joint is leaking", which is actually a flange leak, which has nothing to do with the half dime of the bellows.
Corrugated pipe leakage: corrosion, fatigue and stress corrosion cracking are the main causes
The bellows is the heart of the metal expansion joint. If it leaks, the problem is usually more serious. Three conditions are the most common.
The first is corrosion. There are chloride ions, sulfide in the medium, or improper chemicals are used during cleaning, and the inner wall of the bellows will pitting and intergranular corrosion. 304 stainless steel is very fragile in chlorine-containing environment, so you have to change 316L or higher grade material at this time.
The second is fatigue. The pipeline expands and contracts repeatedly, and every time the bellows expands and contracts, the corrugated root bears alternating stress. The number of cycles is more, and the crack will initiate from the trough and slowly penetrate the wall thickness. In particular, it should be noted that the constraints underground are more complicated than those above ground, and the fatigue life may be much lower than the design value.
The third is stress corrosion cracking. If the tie rod nut is adjusted too tightly and forcibly matched during installation, the bellows will take on duty with extra stress. If you encounter corrosive media again, the crack will break through in a few days. Here is a data: the 304 bellows of the circulating water pipeline of a power plant leaked after less than eight months. After metallographic analysis, it shows transgranular cracks and typical stress corrosion. Later, it was replaced with 254SMO material resistant to chloride ions, and the problem was solved.
Leaky welds and flanges: improper installation, loose bolts and gasket failure
Weld leakage is most likely a problem of welding quality. In the ring seam between the bellows and the connecting pipe, if the current is too large during welding, the welding wire is damp, or the slag is not thoroughly cleaned, there will be pores and slag inclusions. This kind of leakage point, all defects must be dug out before repair welding, otherwise it will be repaired in vain.
Flange leakage, don't blame the gasket yet. Check the bolts first. Have you ever encountered this situation: tightening the bolts a few turns, it didn't leak at the time, and then it leaked again two days later? That's because the flange is unevenly stressed, one side is crushed to death, and the other side is still open. The correct way to do this is to tighten diagonally, adding to the specified torque in two or three times. Gasket failure should be distinguished whether it is aging, creep or corroded by the medium. The rubber gasket quickly hardens in front of high-temperature oil products, so it can be held by replacing it with a metal winding pad or a PTFE pad.
The expansion joint was in use, and the weld between the connector and the pipe cracked. This is often due to the lack of pipe frame support in the pipeline system, and the expansion joint is regarded as a force bearing member. The expansion joint compensates for displacement, it is not responsible for load-bearing. The addition of sliding brackets is different.
Non-metallic expansion joint water leakage: fabric layer aging, skeleton corrosion and lax sealing
Don't think that only the metal expansion joints will leak. The non-metallic expansion joint (fabric fiber expansion joint) leaks, and the precursor is more concealed. It generally has several layers of structure: fluororubber or silicone coated cloth as airtight layer, ceramic fiber as heat insulation layer, and the outermost is stainless steel wire mesh and skin. Water seeped out, indicating that the airtight layer was broken.
Airtight layer aging is a common occurrence. The flue gas temperature fluctuates greatly, and the coated cloth alternates hot and cold repeatedly, and it begins to become brittle and cracked in about two years. If you notice powdery shedding on the surface of non-metallic expansion joints, that's a sign of aging. At this time, when it rains again, water seeps in along the fibrous layer.
Don't ignore skeletal corrosion either. The frames and platens of non-metallic expansion joints, if stainless steel, will still rust in sulfur-containing flue gas. If the platen bolts are rusty and the skin can't be pressed tightly, water will seep from the edges. The treatment method is to clean the bolts regularly, apply anti-jamming agent, and change to high-strength alloy bolts if necessary.
The problem of lax sealing mostly lies in the installation link. The sealant between the skin and the flange is not evenly applied, or the wave shape of the pressure plate is wrong, which will leave gaps. When installing, it must be pressed tightly according to the manufacturer's torque sequence. Don't try to save trouble and only screw a few points.
Treatment plan and prevention: explain from type selection, material to installation and maintenance
Water leaks are not done once and for all by plugging them. You have to hold it down from the source.
In the selection stage, first see the working conditions clearly. What is the medium? What's the temperature? How stressful? Is it corrosive? If it is a flue gas pipeline, it has to use non-metallic expansion joints (fabric fiber expansion joints) to absorb three-dimensional displacement, which is resistant to high temperature and has large compensation. If it is a steam pipe, metal corrugated expansion joints are more suitable, such as general purpose corrugated expansion joints or high temperature axial expansion joints, which can withstand higher pressures and temperatures. Wrong choice, it is only a matter of time before the water leaks.
In terms of material, for media with high chloride ion content, don't use 304, use 316L or 254SMO. When the temperature exceeds 400℃, stainless steel bellows should consider high-temperature creep. At this time, heat-resistant alloy can be selected. If the medium is particularly dirty and there are many particles, add a guide tube to avoid direct flushing of the bellows.
In terms of installation and maintenance, remember three things: First, the expansion joint tie rod nut, adjust it according to the requirements after installation, some need to be removed, some need to be loosened, don't screw it. Second, the flange connection should be symmetrically tightened and the moment should be uniform. The third is to make regular inspections, touch your hands and see, and find rust spots or wet marks, so deal with them as soon as possible. Small leaks do not make up, big leaks suffer.
In case of urgent work and no suitable spare parts on hand, can you temporarily block it? You can use pipe hoops and rubber plates to hoop the leakage point of the bellows first, but that is only an emergency and can't last for a month. What is really reliable is to replace the leaky expansion joint. Even if it is just a weld leak, it is recommended to replace it as a whole. Welding quality is such a matter, the site conditions are difficult to guarantee, and the risk of repair is much greater than that of replacing it with a new one.
Expansion joint leaks are no trivial matter and involve pipeline safety and production continuity. Distinguish the leakage points, see the working conditions clearly, choose the right products, and standardize the installation. After this set of combination boxing is played down, the probability of water leakage can be reduced by more than half. If you are annoyed by this matter, go through the above steps first, and there is a high probability that you can find out the root cause of the disease.