What occasion must you use metal thick-walled expansion joint? — — Hard demand under large pipe diameter, high pressure and high temperature working conditions
Two days ago, I met a customer who was doing power station renovation. When I came up, I asked, "Can your large-diameter thick-walled expansion joint, DN2000, carry 6.4MPa and 500℃?" I asked him how long he had used the original ordinary corrugated expansion joint and he said it started leaking in three months. Tsk, that's a typical choice of the wrong type.
Large diameter (above DN1000), high pressure (starting at 2.5MPa, high can reach above 10MPa), high temperature (going up at 400℃). For example, the main steam pipeline, the hot air pipeline, the boiler outlet of the power plant, and the head and tail of the cement kiln have large thermal displacement and high pressure. Ordinary bellows have thin walls and small wave pitch, so they become fatigued after a few stresses. At this time, you have to look at the thick wall-the wall thickness is generally more than 3mm, some even 8mm and 10mm, and the wave distance is correspondingly increased, the transition between peak and trough is more gentle, the stress concentration is small, and the fatigue life can be stretched to thousands or even tens of thousands of times.
So don't listen to people blindly saying "it's all the same". The working conditions are wrong, and they are an order of magnitude worse.
See the structure clearly: What is the difference between thick-walled expansion joints and ordinary corrugated expansion joints? — — The details of wall thickness, wave pitch and reinforcing ring determine the life
Structural differences, to put it bluntly, are three things: wall thickness, wave distance and reinforcement circle.
The wall thickness of ordinary corrugated expansion joints is usually 1.0~2.5mm, and the wave pitch is compact. It relies on the superposition of multi-layer thin plates to bear pressure. But the thick-walled ones are either single-layer thick plates, or multiple layers but each layer is thickened. When the wave distance is enlarged, the deformation of each wave is more uniform, and it will not be violently folded at the wave root. If you look at our large-diameter thick-walled expansion joints, there are often reinforcing rings at the troughs-this thing is equivalent to "hooping an iron ring" on the bellows to prevent the troughs from bulging outward under the action of internal pressure. The material, thickness and welding process of the reinforcing ring are all particular. Just weld twice, and the pressure will be desoldered as soon as it comes up.
Deflector tube. The guide tubes of thick-walled expansion joints are typically thicker than ordinary models and must be consistent with the flow direction of the medium. The function of the guide tube (there is a special article on this in our station) is to divert the medium and prevent the vortex from washing the bellows. The direction of the arrow cannot be wrong when installing, and the wrong bellows is directly exposed to high-speed airflow, and the wear rate is frighteningly fast.
Don't just look at the caliber when selecting the model: how to match the pressure level, compensation amount and fatigue life? — — Take large-diameter thick-wall expansion joint as an example to explain the parameters
Many purchases say "I want DN1400" first, and then ask the price. I said wait, what about your working parameters? What is the pressure rating? How much is the design compensation? How many times is required for fatigue life? If these three parameters don't match, it will be a ticking time bomb if you buy it back.
For example, a large-diameter thick-walled expansion joint of DN2000 is used in the main steam pipeline of a power station, with a working pressure of 6.4MPa, a temperature of 540℃, and an axial compensation requirement of 150mm. If you select the type of ordinary bellows, with a wall thickness of 2.5mm and a wave pitch of 60mm, the fatigue life may be only 300 times-while the national standard requires at least 1,000 times for power station pipelines. If the wall is thick, the wall thickness should be 5mm, the wave distance should be pulled to 80mm, and two reinforcing rings should be added, so the fatigue life can reach 2000 times. At what cost? Doubled in weight and 40% more expensive, but at least nothing happened.
Therefore, don't just look at the caliber when choosing a model. The pressure grade determines the wall thickness and the number of layers; The compensation amount determines the wave number and wave height; Fatigue life determines the structural form. These three restrict each other, you have to use professional expansion joint selection software or check the standard curve to match. When we make plans here, we generally ask customers to provide pipe drawings or at least provide the spacing and thermal displacement of the fixed brackets at both ends, so that they can be calculated accurately.
Installation and guide tube: It is better not to install the wrong thick-walled expansion joint-tie rod adjustment, guide tube direction, fixed bracket and other pits
And guess what? The most outrageous installation accident I have ever seen is to install the deflector of the expansion joint backwards. It is common sense that the arrow points to the flow direction of the medium. As a result, the worker thinks that "the arrow is the same everywhere" and directly welds it backwards. After one month's operation, the inner wall of the bellows is washed by the medium like it has been beaten with sandpaper, and it is directly scrapped.
- Tie rod adjustment problem: How to adjust the expansion joint tie rod nut? The FAQ in our station has detailed answers-generally, the tie rod is in the state of "installation length" when leaving the factory, and the nuts at both ends are locked to prevent the bellows from compressing during transportation. After installation in place, adjust the nut position according to the design compensation amount to leave space for thermal expansion. Pre-stretching of thick-walled parts is particularly important because the wall thickness is rigid, and the compensation amount is insufficient without pre-stretching. How much exactly? According to the cold tightness value on the design drawings, generally screw with a wrench, pay attention to the synchronization of both sides, don't screw off.
- Do you want the screw disassembled?This question is asked by many newbies. Does the screw of the expansion joint need to be removed? The answer is: if it is a transport screw (for temporary fixation), it must be removed after installation, otherwise the bellows cannot be expanded or contracted. If it is a permanent tie rod (such as a tie rod with a large tie rod expansion joint), it cannot be removed, it is responsible for withstanding pressure thrust. The difference is simple: see if there are lock nuts at both ends of the screw and adjustment threads on the screw-that kind is a permanent tie rod; If it is just a thin screw that passes through the flange at both ends of the bellows, it is generally a transport screw and disassemble it.
- Fixed bracket: Fixed brackets must be set on both sides of the thick-walled expansion joint, otherwise the pipeline thrust will push the expansion joint everywhere. The bracket should be able to withstand the blind plate force generated by internal pressure, and the calculation formula is: thrust = pressure × cross-sectional area. 6.4MPa, DN2000 pipeline, the thrust exceeds 200 tons, the support is not strong enough, and it collapses directly.
FAQ Disassembly: How to adjust the expansion joint tie rod nut? Do you want the screw disassembled? - -Thick-walled parts should pay special attention to pre-stretching
I just mentioned the pull rod adjustment, so I will say a few more words in depth.
How to adjust the expansion joint tie rod nut? The operation steps are actually not complicated: first confirm the installation length (generally marked when leaving the factory), then loosen one end of the nut, and rotate the adjustment nut on the tie rod with a wrench to elongate or compress the bellows to the cold tightness value required by the design. Note that thick-walled parts are easy to bite to death because of their high rigidity, so it is recommended to apply some molybdenum disulfide grease. Once the adjustment is complete, tighten the locking nut to prevent loosening.
Does the screw of the expansion joint need to be removed? Again: the transport screw must be disassembled! Remove it before installation. But if it is a tie rod on the expansion joint of a large tie rod, it is a structural part and cannot be disassembled. In addition, there is another situation: the expansion joint used on the vibration pipeline, the tie rod sometimes assumes the function of limiting position, and it can't be disassembled. The criterion is one sentence: if the bellows will expand and contract at will after it is removed, it is the transportation screw; If the system is unbalanced after removal, it is a permanent part.
From power stations to cement: application cases of metal thick-walled expansion joints in different industries-practical choice of corrugated expansion joints for power distribution stations industry and metal corrugated expansion joints for cement industry
Finally, let's talk about a few real cases to help you understand what to choose for what occasion.
Power station industry: Main steam pipeline, reheating steam pipeline, water supply pipeline, high temperature, high pressure, generally using corrugated expansion joint for power station industry. However, if it is the working condition of large diameter (above DN1800) and high pressure (above 10MPa), the bellows of ordinary power stations can't bear it, so it has to be upgraded to a large-diameter thick-walled expansion joint. For example, the main steam pipeline of a 660MW unit in a power plant uses our large-diameter thick-walled expansion joint, with a wall thickness of 6mm, a wave pitch of 100mm, and a reinforcing ring. The fatigue life design is 1,500 times, and it has been operated for 6 years without any problems.
Cement industry: The hot air ducts, tertiary air ducts and flue gas ducts at the head and tail of the kiln on the cement production line have large temperature fluctuations and large dust content in the medium. Metal corrugated expansion joints in the cement industry usually require wear-resistant guide tubes, and thick-walled models are more common. Because of the severe dust washing, ordinary bellows wear out in a few months. Replace with a thick wall, the guide tube is thickened to 8mm, and the wall thickness of the bellows is 4mm. With wear-resistant lining, the life can be prolonged by more than three years.
Not all working conditions require thick walls, but once high pressure, high temperature and large pipe diameter are involved, don't choose ordinary models for cheap. Thick-walled expansion joints are expensive, but worry-free. The maintenance cost of an accident is enough to buy several.