1. What are the most common failure modes of metal expansion joints under high temperature and high pressure working conditions? Let's start with real cases
The expansion joint leaked during the pipeline purging, the whole maintenance plan was completely disrupted, and the construction period was delayed by half a month.
When disassembled, there are obvious corrosion pits on the surface of the bellows. The front half of the guide tube has been washed as thin as paper by high-speed steam, and the crack appears at the bottom of the corrugated valley-a typical fatigue crack superimposed stress corrosion. The reason is simple: only the design pressure was calculated during the selection, but the actual operating temperature was 40℃ higher than the design value. In addition, the steam contained a trace amount of chloride ions, so something happened within a few months.
The most common death methods of corrugated expansion joints under high temperature and high pressure conditions are those: fatigue cracking, stress corrosion, guide tube wear-out and instability bulge. Let's talk about it one by one.
Second, material is the foundation: how to determine the temperature and pressure resistance grade? How to select austenitic stainless steel and superalloy
Many people come up and ask, "Can you withstand 600℃?" This sounds simple, but the relationship between material and temperature is far more than one number.
Conventional austenitic stainless steel (such as 304/316L) begins to precipitate carbides above 425℃, the grain boundary is chromium-depleted, and the corrosion resistance drops like a cliff. Above 550℃, the problems of high-temperature creep and oxidation become prominent. So ask the first question: Is your medium temperature persistently high or a short-term peak? Is there a stop cycle in the operation condition? The damage mechanism of these two working conditions to materials is completely different.
When the temperature exceeds 550℃, superalloys usually have to be considered. The conventional practice is to use Incoloy 800H or higher alloy material to make corrugated pipe, with the surface anti-oxidation coating treatment. However, note that when the materials go up, it means that the cost and delivery cycle are both raised. Don't pursue "the more expensive, the better" as soon as you come up. When the pressure level is the same, the higher the temperature resistance, the more bellows layers usually are, the thicker the wall thickness, and the fatigue life has to be re-checked-this is a pair of contradictions, and we can't just stare at one end.
3. Structural type determines compensation ability: What working conditions correspond to axial type, external pressure single type and straight pipe pressure balance type respectively
After selecting materials, it's time to look at the structure.
Conventional axial expansion joints (that is, we often call universal corrugated expansion joints) are simple in structure and affordable, and are suitable for small-diameter pipes without fixed brackets or acceptable thrust of brackets. However, under the condition of large diameter and high pressure, it has an Achilles heel: the blind plate force generated by internal pressure is too large, and the fixed bracket can't bear it at all.
At this time, it is time to consider external pressure single axial expansion joint. Its bellows work in the external pressure state, and the flow stability and stability are much better than the internal pressure type, which is suitable for high-pressure and large-diameter steam pipes. We have special external pressure single axial expansion joint product information in our station, which is for this purpose.
If the pipeline is complicated, the space is limited, or the equipment interface is not allowed to bear the blind plate force, then a straight pipe pressure balanced expansion joint is necessary. It counteracts the internal pressure thrust by balancing the bellows, so that the force on the fixed bracket is greatly reduced. The cost is that the structure is complicated and expensive, but there are some working conditions where there is really no other option than it.
So you see, structural selection is never a question of "which is better", but rather a question of "which better matches your working conditions".
4. Guide tube, tie rod and connector thickness: These details directly affect the life of the expansion joint under high parameters
Details determine success or failure, and this is not an exaggeration when it comes to expansion joints.
Let's start with the deflector. What is the specific function of the deflector? To put it bluntly, there are two layers: one is to reduce the direct erosion of the inner wall of the bellows by high-speed medium, and the other is to avoid the deposition of particulate matter in the gas-solid two-phase flow in the corrugated valley. Under high temperature and high pressure working conditions, the material of the guide tube should be at least one temperature resistance grade higher than that of the pipeline, and sufficient thermal expansion gap should be left between it and the bellows. In many accidents, the design time gap of the guide tube is small, and the guide tube expands against the bellows at high temperature, directly cracking the wave peak.
Then there's the pull rod. The role of the expansion joint tie rod is not to absorb displacement, but to bear the internal pressure thrust and restrain the excessive deformation of the bellows. How to adjust the tie rod nut during installation is very particular: the lock is too tight, and the displacement compensation function fails; The lock is too loose, and the internal pressure thrust is fully pressed on the fixed bracket. On high parameter pipes, I recommend checking the installation length and pre-stretch value with the manufacturer before installation.
Needless to say, the thickness of the pipe. The expansion joint pipe of high-temperature and high-pressure pipeline usually needs to be thickened and set with a reinforcing ring, otherwise the bellows is not broken, and the weld of the pipe will crack first.
5. Installation and maintenance: pre-stretching, cold tightening, setting of guide brackets, if you don't do it correctly, you will be completely in vain
No matter how good the front selection is, it will still be useless if the installation link goes wrong.
Is the amount of thermal expansion determined after the high temperature pipeline is put into operation? Not. The actual temperature field of the pipeline is unevenly distributed, and the expansion can only be calculated as an approximate interval. Therefore, pre-stretching or cold tightening should be done during on-site installation, and the pre-stretching amount should be calculated according to the pipeline medium temperature and ambient temperature. The bellows should be stretched to the specified length in the cold state, and just returned to the neutral position in the hot state. This seems simple, but it tests the skill of the installation team.
The setting of the guide bracket is even more critical. The distance of the first group of guide brackets on both sides of the expansion joint is usually controlled within 4 times the nominal diameter, and the distance of the second group and the third group is gradually enlarged. Wrong spacing, lateral displacement of the pipe will twist the bellows into a twist. Two days ago, I met a customer. The expansion joint itself didn't have any quality problems, but the guide bracket with too long spacing was destroyed-the bellows column was instable and bulged a big bag.
Therefore, the material, structure, details and installation of high-temperature and high-pressure metal expansion joints are linked to each other. When selecting the model, ask more about several working condition parameters, and keep a close eye on each process during installation, which saves much money than replacing it afterwards.