1. What exactly is the welded metal corrugated expansion joint? — — Essential difference from ordinary bellows
Many people confuse welded metal corrugated expansion joints with rubber compensators and non-metal expansion joints. Isn't it just a telescopic pipe? Tsk, not even close. The core of the welded metal corrugated expansion joint is that bellows-a component that is hydraulically or rolling formed with multiple layers of thin-walled stainless steel (such as 304, 316L, or even Inconel), and then welded together with end tubes, tie rods and deflectors. The key lies in the word "welding": the connection between the end pipe and the bellows is not screwed on by a clamp or flange, but by argon arc welding, and the strength can carry tens of MPa. The pressure of ordinary bellows (such as gas bellows) is only a few kilograms, and the welded metal corrugated expansion joint is always up to MPa, and the temperature can reach 500 to 600 degrees, which is not of the same order of magnitude at all.
Universal corrugated expansion joint, high temperature axial expansion joint, external pressure single axial expansion joint... These products are inseparable from welding process. Compared with PTFE metal hose, it is more resistant to high temperatures, but especially corrosive occasions have to be lined with PTFE. Therefore, the welded metal corrugated expansion joint is essentially a high-pressure and high-temperature "transformer"-absorbing the deformation of thermal expansion and contraction of the pipeline, and at the same time holding the internal pressure without leakage or explosion.
Second, where is this thing mainly used? — — Hardcore scenes of power stations, cement and chemical industries
Two days ago, I met a buddy who was doing power station operation and maintenance, saying that the expansion joint on their main steam pipeline had been used for five years, and the bellows cracked and almost stopped. I said, are you using corrugated expansion joints used in the power station industry? He nodded. The main steam pipeline of the power station has a temperature of 560℃ and a pressure of more than ten MPa. The ordinary rubber compensator has long been melted. Welded metal corrugated expansion joint is standard in this-it absorbs axial displacement and lateral displacement, cooperates with the guide tube to guide high-speed steam, and avoids flushing the bellows.
What about the cement industry? Preheater outlet, high temperature, large dust, cement industry metal corrugated expansion joint comes in handy. And guess what? Some on-site selections didn't pay attention to the direction of the guide tube, and as a result, the corrugated pipe was stuck by backward material flow, which was scrapped in three months. Chemical industry is more complicated, such as desulfurization flue gas pipeline, which is extremely corrosive. It is necessary to use desulfurization flue gas baffle door with expansion joint, or consider lining PTFE compensator. These scenarios do not have welded metal corrugated expansion joints, and the pipe must be pulled and cracked.
Third, don't pat your head when selecting a model. These points must be stared at (pressure, temperature, compensation amount, material)
I have seen too many cases of "patting the head" when it comes to model selection. The customer said, "Give me a DN200". As a result, the working pressure was 2.5MPa, the temperature was 300℃, and the compensation amount was 50mm in the axial direction. He took the universal corrugated expansion joint to hard it, and it leaked after half a year. What to do?
Pressure:The design pressure must cover the maximum operating pressure of the pipeline system, including the hydraulic test pressure. Don't just look at nominal pressure, the allowable pressure under fatigue life calculation is the key. Like directly buried (fully buried) expansion joints, the covering pressure should also be considered.
Temperature:The upper limit of material temperature resistance should be clear. Austenitic stainless steel 304 can carry about 650℃, but creep should be considered when it exceeds 400℃. Higher temperatures with Inconel 625 or Hastelloy, double the cost but safety first. High-temperature axial expansion joints are specialized in this kind of work.
Compensation amount:Axial, lateral or angular? Pipe layout decision. For example, L-shaped pipeline, the natural compensation is not enough, so it is necessary to use compound hinge transverse expansion joint or curved pipe pressure balance expansion joint. The greater the compensation amount, the better. The safety factor should be reserved in design, which is generally less than 80% of the calculated value.
Material:Medium is key. Got chloride? Use 316L or duplex stainless steel. Flue gas contains sulfur? With C276. There is also a special anti-corrosion coating on the smoke baffle door. Here's a sentence: PTFE-lined hoses and PTFE compensators are also good in strong corrosive media, but their temperature resistance is not as good as that of welding.
Fourth, the pits stepped on during installation-the draw rod, guide tube and screw
At the installation site, I ran no less than a hundred times, and there are only a few common pits. The problem of tie rod ranks first-the function of expansion joint tie rod is to restrain the bellows from being over-stretched or compressed, but many people don't adjust the nut after installing it, which either locks up and can't compensate, or is too loose to lose the restraint. The correct method is: before installation, adjust the tie rod nut to the set length according to the manufacturer's guidance, and after the pipeline is fixed, remove or loosen the screw for transportation to a free state. Someone in the FAQ asked "Does the screw of the expansion joint need to be disassembled"-depending on the situation: the transportation screw must be disassembled, otherwise the expansion joint can't move.
The deflector is another minefield. What is the specific function of the expansion joint guide tube? Guide the medium to flow smoothly and prevent high-speed fluid from washing the bellows. The direction of the arrow must be consistent with the flow direction of the medium. If it is installed backwards, the guide tube will become an obstacle, and the bellows will be eroded and perforated. Also, the guide tube can't bear the weight of the pipe, so don't use it as a support.
As for pre-stretching or pre-compression, it depends on the ambient temperature and pipe design at the time of installation. For example, the steam pipeline is installed in a cold state and needs to be pre-stretched to compensate for thermal expansion; Cement pipe flue gas pipe has high temperature but quick heat dissipation, and sometimes it needs to be pre-compressed instead. Get it backwards, and the pipe bursts as soon as it is hot.
5. How many years did it go wrong? — — Investigation ideas of fatigue, corrosion and instability
The equipment was in use for three or five years and it broke down. Don't be in a hurry to change, check out the reason first. The three most typical categories:tired、corrosion、Instability。
Fatigue:The number of repeated expansion and contraction of the bellows exceeds the design life. Have you calculated the actual number of cycles? Some pipelines start and stop once a day, 365 times a year, and they are calculated as 1,000 times during design. As a result, they crack in less than five years. Check with the naked eye to see whether there are transverse cracks at the peak of the bellows, and use colored penetration or magnetic powder for non-destructive testing.
Corrosion:Stress corrosion cracking is a horror film-the bellows surface does not thin significantly but suddenly breaks. It is more common in chloride-containing environments (such as coastal chemical plants). Check the bellows fragments for metallographic analysis to see if there are dendritic cracks. Wrong material choice? Hurry up to change Hastelloy or lined with PTFE.
Instability:The plane instability is manifested by the distortion between the bellows valleys, while the columnar instability is the bending of the whole section. Usually because of overpressure or compensation amount exceeding the limit. For example, if the pressure balance expansion joint (straight pipe pressure balance type, compound straight pipe bypass pressure balance type) is improperly adjusted during installation, the pressure will become unstable as soon as it comes up. Troubleshooting method: Measure the wave pitch of the bellows to see if it is uniform, and then check the design pressure and actual working conditions.
The maintenance of welded metal corrugated expansion joints is not "not bad or not". It is recommended to check the appearance once a year and do non-destructive testing every three years. Especially in power stations and chemical projects, the failure of an expansion joint may cause chain parking, so the loss is not a matter of a few dollars.