Expansion joint fatigue life: don't wait for it to be broken to regret it
The little heat expansion and cold contraction in the pipe, which looks inconspicuous, can tear the bellows to pieces over time. To put it bluntly, the fatigue life of the expansion joint is how many times it can withstand repeated pulling-just like bending iron wire, it is okay to bend it once, but it will break after bending it a hundred times. When many projects are accepted, they only look at the pressure resistance without leakage. As a result, the bellows cracked after two or three years of use. It's not a quality problem, it's not fatigue at all.
I met a power plant customer two days ago. TheyCorrugated expansion joint for power station industryFrequent start-and-stop leads to axial displacement exceeding the standard, and the number of fatigue times was not considered in the design, and it leaked after two years of use. Changing a set of labor plus shutdown loses hundreds of thousands. Do you think the money was spent unjustly?
What factors are "killing" your expansion joints?
The first killer is stress concentration. Where the radius of curvature of the bellows crest and trough is small, the stress is several times the average value. The second is medium temperature-high temperatures reduce material yield strength, such asHigh temperature axial expansion jointAbove 600℃, the fatigue life of stainless steel may shrink to one tenth of normal temperature. The third is the displacement combination. Many sites are not simply axial stretching, but lateral deflection plus torsion.Compound hinge transverse expansion jointIf the installation deviation is large, the bellows will bear additional bending moment and the fatigue life will plummet.
Another hidden killer is corrosion.Desulfurization flue gas baffle doorThe nearby expansion joint, the chloride ions in the flue gas will cause pitting of austenitic stainless steel, and the corrosion pit will directly become the source of fatigue crack. And guess what? Some chemical plants usePTFE compensatorAlthough PTFE is corrosion resistant, its fatigue strength is extremely low. The medium contains strong acid and frequent temperature fluctuations, and it cracks in two months.
How to calculate fatigue life in the selection stage?
Don't just look at the rated displacement on the sample, which is measured under ideal conditions. In practice, we should use the EJMA standard formula and consider the safety factor. Such asUniversal corrugated expansion jointThe design life usually requires 1000 times to 10000 times. But if it isDirect buried (fully buried) type expansion jointBecause it cannot be overhauled, the design life is often more demanding, possibly up to 50,000 times.
Number of bellows layers, single layer thickness, wave height, wave pitch. You think the thicker the more durable? Not necessarily. The fatigue performance of single-layer thin-walled bellows tends to be better than that of thick-walled bellows because of the high probability of defects in the heat-affected zone of thick-walled welding.Large diameter thick wall expansion jointBe more careful, the fatigue life decreases when the wall thickness goes up. It is experience, and it is a lesson.
How much life can be saved by installation and maintenance?
Many expansion joints are "playing dead". For example, the tie rod nut of the expansion joint is adjusted to the pre-compressed state when it leaves the factory. After the pipe is installed on site, it must be removed or adjusted to the working position. Otherwise, the bellows will be locked, and the fatigue life will be directly returned to zero as soon as there is thermal expansion.How to adjust the expansion joint tie rod？ Go see our previous Q&A with detailed steps.
There is also the guide tube-if the guide tube is not installed or installed backwards, the high-speed airflow rushes directly to the inner wall of the bellows, scour and vibrate, and fatigue cracks appear quickly. In addition,Non-metallic expansion joint (fabric fiber expansion joint)Andrubber compensatorIt is a flexible material, and its fatigue life is greatly affected by weather and ultraviolet rays. It needs to be checked regularly. Don't wait for embrittlement before replacing.
Tell me a real comparison case
A cement plantMetal Corrugated Expansion Joints in Cement IndustryUsed in kiln head pipeline, the original ordinary single-layer corrugated pipe leaks once every six months. Later, the double-layer structure with deflector was changed, and the tie rod was recalibrated at the same time to control the lateral displacement to the design value. As a result, it was not broken after more than three years of use. Another negative textbook-used in a chemical plantPTFE compensatorThe medium contains strong acid. Although PTFE material is corrosion-resistant, its fatigue strength is very low. The customer did not consider frequent temperature fluctuations, and as a result, it cracked in two months. Therefore, when selecting the model, we must hand over the working condition parameters of medium, temperature, frequency and displacement to the manufacturer, so that they can calculate the fatigue life. Don't pat your head.
There is no new trick to prolong fatigue life, but there are several iron rules
First, don't leave too much margin for the displacement. Rigid design prefers to use several expansion joints to share the displacement rather than let one expansion joint fill up. for exampleCompound straight pipe bypass pressure balanced expansion jointCan effectively absorb axial displacement and balance pressure and thrust. Second, the welding quality determines everything, and the defects of longitudinal and circumferential welds of bellows are the main reasons for early fatigue failure. Third, make sure to do cold tightening after installation, pre-deformation can offset part of the thermal displacement and reduce the actual working stress. Fourth, make regular inspections, focusing on whether there are micro-cracks in welds and troughs, which can be permeation detection or acoustic emission.
Remember, expansion joints are consumables, but scientific selection and maintenance can extend the life from two or three years to more than ten years. Don't wait for it to break before you regret it-that little upfront investment is less than a fraction of the loss of shutdown.