Improper design selection, no matter how long the life is, it will be useless
The life of metal expansion joints is never calculated from the moment of leaving the factory, but is doomed from the moment of selection. Corrugated material, wall thickness, wave number, compensation amount, these parameters look boring, but in fact, each of them sets an upper limit for life. You take 316L to carry the smoke containing chloride ions. Even if the manufacturer patts his chest and says that it can be used for ten years, it will actually show you a look in two or three years. On the other hand, the cost is saved where the superalloy should be used, and the life is shorter.
Talking about wall thickness, some people think that the thicker the more durable, which is a misunderstanding. If the bellows is too thick, its flexibility will be poor. When compensating for displacement, the stress will be concentrated, and the fatigue life will also fall. The same is true for the design compensation amount. If you let the expansion joint work near the ultimate displacement for a long time, it is overdrawn every day. When selecting a model, leaving a margin of 20% ~30% looks like a waste, but in fact, it is an insurance for life. To put it bluntly, if you choose the wrong model, all the later efforts will be to wipe your ass off the wrong.
Work conditions are the real judge
Design life is printed on the factory certificate, but that is the number under ideal working conditions. How long it actually takes depends on the number of temperature, medium and pressure cycles. For every 100℃ increase in temperature, the fatigue life of corrugated materials may decrease by more than half. If the medium is corrosive, it is more complicated-stress corrosion cracking often comes faster than pure fatigue, especially in chloride ion environment, and austenitic stainless steel is the easiest to fall for.
Many people don't take stress cycles seriously. The damage to the expansion joint caused by the steam pipeline starting and stopping several times a day is completely different from the damage caused by continuous and stable operation. Fatigue life is calculated by number of cycles, not years. You let an expansion joint with a design life of 10,000 cycles experience dozens of pressure fluctuations every day, and that one or two years of scrapping is not surprising at all. On the other hand, if the working condition is stable, it is common to use it for ten or eight years.
So you see, asking how long the life of a metal expansion joint can be, it's better to ask how ruthless your working conditions are first.
Concealed pit in installation
The selection is right, the working conditions are also figured out, and the installation is still useless. There are three most common pits: the cold tightness value is not calculated correctly, the direction of the guide tube is installed backwards, and the tie rod nut is not loose. Cold tightness value, to put it bluntly, is pre-stretching or pre-compression, which can make the expansion joint in a more reasonable stress range when working. If the calculation is wrong, either the compensation amount is insufficient during operation, or the initial stress exceeds the standard, and the life is directly discounted in half.
Install the guide tube in the reverse direction, and the consequences are more direct. The guide tube is to allow the medium to flow smoothly and reduce turbulence and erosion. If the direction is reversed, the medium will directly flush the bellows. It is strange that it will not wear out after a long time. As for the tie rod nut, that is used to hold the bellows in place during transportation and installation, and must be loosened when installed in place. And guess what? Some projects leaked half a year after they were put into production. When they were disassembled, the tie rod nut remained intact, the bellows couldn't deform freely at all, and all the displacements made the pipeline hard. This pot, the expansion joint does not carry.
Don't wait for a leak to change
Many people maintain expansion joints, just two words: wait for leakage. Only when the medium is sprayed out do I think of replacing it. At that time, not only do I have to buy a new expansion joint, but I also have to calculate the downtime loss. When these signs appear, bellows crack, stiffness decrease and abnormal deformation, the life has actually entered the countdown.
Cracks could not be seen with the naked eye in the early days. It is necessary to regularly check the corrugated valley bottom for corrosion spots and fine intergranular cracks. How to find stiffness drop? Touch the bellows, or look at the displacement feedback, and it feels softer and more deformed than when it was newly installed, that is, the material is fatigued. Abnormal deformation is more intuitive-uneven corrugated pitch, local bulging, and permanent skew, any of which means that the bellows is no longer working.
What about that? Be diligent. Disassemble it during overhaul, and check several points with a thickness gauge at ordinary times. Don't be too troublesome, wait until it leaks before replacing it. The cost is more than three times that of normal replacement.
Practical means of extending life
To prolong the life of metal expansion joints, to put it bluntly, there are three things: choose the right one, install the right one and maintain it well. Choosing the right is what I said earlier. Don't pat your head on the material, wall thickness and compensation amount. The right installation is the cold tightness value, the direction of the guide tube and the tie rod nut. Don't be sloppy. It is more direct to maintain it well-make regular inspections, record the displacement, check whether there is any abnormal noise, and observe whether there is any rust on the surface.
Which is more reliable, the experience of the master or the suggestion of the manufacturer? The master has seen thousands of scenes and knows which locations are prone to problems; Manufacturers know more about design and manufacturing, and know where the limit of this expansion joint is. Both sides listened, but ultimately according to the actual working conditions. For a practical example: the expansion joint used in the desulfurization flue of a power plant, the manufacturer said that the design life was eight years, and the master said that three years of this environment was enough. What was the result? The bellows cracked in two and a half years. The reason is not the poor quality, but rather that the residual chloride ions after desulfurization are much higher than the design expected. Therefore, regular inspection is not a formality, but a correction of your expectations with data.
Finally, let's be honest. There is no standard answer to the life of metal expansion joints, but you can minimize the uncertainty through the three links of type selection, installation and maintenance. Don't believe the nonsense of "lifetime maintenance-free". There is no such thing as lifetime for industrial equipment. Every time you shut down for maintenance, look at the expansion joint once more, which is more useful than if you change ten brands.