Metal expansion joint fatigue times: this parameter can't be read, and the selection will step on pits sooner or later
What exactly is the fatigue number of metal expansion joints? It is not the number written by the manufacturer patting his head, but the complete number of cycles that the bellows can hold under alternating loads. A cycle is to elongate from the original position and back, or compress and back, a complete reciprocating. This number directly determines how long the expansion energy saving lasts. Pipelines with frequent start-and-stop, such as power stations, cement and desulfurization, pay special attention to this parameter-the number of fatigue times is not enough, and it is only a matter of time before the bellows cracks, and it often happens at the juncture when you least want to stop.
What factors are linked to the number of fatigue? Pressure, temperature, single wave displacement, corrugation shape, material thickness, and even welding quality should be mixed with a foot. The same general-purpose corrugated expansion joint may be marked 10,000 times under normal temperature and pressure, but when placed on the steam pipeline, the fatigue life will drop straight down as soon as the temperature rises. There is a common misunderstanding here: many people only stare at the number of fatigue times on the manufacturer's sample, and ignore the actual displacement in their own working conditions. The sample is given the number of times under rated displacement-if you push the expansion joint to the limit, the number of times may shrink to one tenth. Two days ago, a customer asked me with a selection table, saying that his working condition was obviously very mild, why did the expansion joint leak after one year of use. As a result, a look at the installation record showed that the displacement was twice as fast as the design value. This usage was useless for any brand.
How is the number of fatigue determined in the design standard?
It is generally referred to GB/T 12777 in China, and it is often calculated according to EJMA standard internationally. There is a key logic in the standard: the designer first sets the life expectancy, such as 1,000 or 5,000 times, and then inverts the allowable displacement. So you will find a counter-intuitive phenomenon-the lower the number of labeled fatigue times for the same metal expansion joint, the greater the displacement it can absorb. This explains why some large-diameter thick-walled expansion joints look bulky, but the number of fatigue times is not high: they prioritize the ability to absorb large displacements, not long life. When selecting a model, you have to first think about whether your pipeline expands and contracts frequently, or is it basically stable there. If you think the opposite, what you choose is either a waste of money or a mine.
How to estimate the number of fatigue times under actual working conditions?
Give a rough but practical approach. First confirm the thermal expansion of the pipe system, divide by the allowable displacement of a single wave, and get the number of cycles required for each wave. Then check the fatigue curve given by the manufacturer-generally converted from the EJMA formula-corresponding to your actual working pressure and temperature, and find the derating coefficient. For example, a high-temperature axial expansion joint, with a design pressure of 1.0 MPa and a temperature of 400 °C, has 5000 fatigue times on the sample. But if the actual displacement of your pipeline is only half the design value, the number of fatigues may double to more than 10,000. Conversely, if the over-displacement runs, the number of times falls off the cliff. This principle is the same as a spring. The harder you press, the worse the rebound, what do you think?
Selection suggestion: Don't just focus on the number of times of fatigue to buy products
In the final analysis, the number of fatigue times must be put together with displacement, pressure and temperature to see the matching relationship. Frequent pipeline starts and stops? Straight pipe pressure balance type expansion joint or compound hinge transverse type expansion joint is preferred. This kind of structure can disperse stress and extend fatigue life in disguise. Corrosive media? For example, desulfurization flue gas pipeline, the material of corrugated pipe has to be replaced with corrosion-resistant alloy, otherwise it is useless no matter how high the fatigue number is-it is corroded first, so what is the point of the number of times?
Another trick is to pre-stretch or pre-compress during on-site installation, so that the bellows can avoid the stress peak near the zero position during work, which is very helpful to improve the actual fatigue times. Before the selection, throw the working condition parameters to the manufacturer completely, and let them calculate them according to EJMA standards, which is much more reliable than you guess against the sample. Some customers ask with a vague working condition table, saying that the temperature is about two or three hundred degrees-can this vague approach be believed in the calculated number of fatigue times?
The fatigue number of metal expansion joint is not an isolated parameter, it is the result of pressure, temperature and displacement. Understand the logic behind it, so that the selection will not step on the pit.