Desoldering is not an accident: high temperature, alternating stress and corrosive media join hands to destroy
Metal expansion joint desoldering, to put it bluntly, means that the weld can't bear it. But the reason why I can't bear it is not so simple-it is not a single factor that acts alone, but high temperature, alternating stress, and corrosive medium, which join forces to tear the weld open.
Let's start with the heat. Austenitic stainless steel (such as 304 and 316L), which is commonly used for bellows, begins to decrease significantly in strength above 400℃. At 600℃, the allowable stress is directly cut in half. The weld region is the heat-affected zone, with coarse grains, carbide precipitation, and lower strength than the base metal. Calculate, is the weld the first to suffer when the design is selected according to normal temperature, and the actual operation is under high temperature conditions?
Let's talk about alternating stress. The pipeline is started, shut down, load fluctuates, and the expansion joint is stretched and compressed again and again. Stress concentration occurs in the crest and trough of the corrugation, and the stress coefficient at the root of the weld can reach above 2.0. High-frequency and low-amplitude vibration (such as compressor outlet pipeline) is even more fatal-as soon as the number of cycles is more, fatigue cracks initiate from the weld toe and slowly expand into penetrating cracks.
Plus corrosive media. Sulfide and chloride ions in flue gas, dissolved oxygen and carbon dioxide in steam pipeline will induce intergranular corrosion or stress corrosion cracking in the chromium-depleted zone of weld. Corrosion pit becomes the birthplace of crack, and crack accelerates corrosion, a vicious circle. Alas, these three factors together, desoldering is really not an accident, but a matter of time.
Where are the welding defects hidden? Crests and troughs and annulus are the most easily overlooked
A steam pipeline was installed in a power stationHigh temperature axial expansion jointThe design temperature is 550 °C and the pressure is 1.0 MPa. After just half a year of operation, the inspection found that there was leakage at the circumferential seam at the end of the bellows. After removing it, it was seen that the circumferential seam bead was cracked longitudinally, and the crack length exceeded 100mm.
Why is it a ring stitch? Because the end ring joint is the weakest link of the expansion joint-it connects the bellows and the end tube, and it has to bear both axial forces and bending moments. During welding, defects such as unfused and slag inclusion are easily produced. Coupled with the additional stress during on-site installation, the fatigue life is one order of magnitude lower than that of the bellows body.
In addition to ring seams, crests and troughs are also worth keeping an eye on. The wave crest is the maximum deformation point, and micro-cracks may appear on the surface after multiple expansion and contraction; The trough is easy to accumulate corrosive media, and the local pH value decreases, resulting in pitting. There is another place-the connecting weld between the deflector and the bellows. If the guide tube is unreasonably designed or falls off after wear, the high-speed airflow directly washes the inner wall of the bellows, and it won't take long to wear out.
And guess what? Many on-site inspections only look at whether there is bulge and air leakage in the appearance of the expansion joint, but never open the insulation layer to carefully check the weld area. When you hear the sound of air leakage before dealing with it, it is often already cracked.
Don't rush to repair welding when you find desoldering: find out the root cause first
When many people see the weld cracking, their first reaction is: "Hurry up and weld it!" Wait a minute-before repair welding, ask three questions:
First, is there any problem with design selection? Such asUniversal corrugated expansion jointWhen used on high temperature and high pressure steam main pipe, do the wall thickness, wave number and fatigue life meet the requirements? If the design displacement is too small, the bellows will be over-stretched in actual operation, and the weld will certainly not be able to bear it.
Second, are the materials correct? Can 304 handle 600℃? Should you use Incoloy 800 or Heat Resistant Alloy?
Third, are there any problems with the installation? Is the tie rod nut loose causing the bellows to take extra thrust? Is pipe cold tightening in place? Is the adjacent stent failing?
Different causes correspond to different processing paths. Design problems have to be recalculated and the model is changed; Upgraded material for material issues; Installation problem Just adjust the bracket and tie rod. If you don't know the reason, you will blindly repair welding, and there is a high probability that you will weld and crack, which is a waste of effort.
On-site emergency treatment and replacement judgment: What circumstances can be welded? What circumstances must be changed?
To be honest, once a metal expansion joint is desoldered, it is rare to repair welding. Unless the following conditions are met: the crack length does not exceed 10% of the total length of the weld, is located at a non-critical location (such as a trough instead of a crest), the medium is non-toxic and non-flammable, and the pressure temperature is in the normal and low pressure range. Moreover, repair welding must be carried out by certified welders according to the welding process evaluation, and non-destructive testing must be done after welding.
But more often than not, especially on high-temperature and high-pressure pipelines, they have to be replaced with new ones when desoldering is encountered. refer toUniversal corrugated expansion jointAndStraight pipe pressure balanced expansion jointMaintenance standard: If there are penetrating cracks in the bellows body, the wall thickness is thinned by more than 30% due to mutual friction between corrugations, or the weld crack extends to the base metal, all of them cannot be welded, and they will be directly scrapped and replaced with new ones. Don't feel bad about that little money-the pipe burst causes downtime and personnel injury, and the loss is dozens of times the price of expansion joints.
In case of emergency, temporarily seal the leakage point with a hoop or leakage plugging belt, reduce the pressure and run, and change the spare parts immediately when they arrive. Don't put the welding rod up in order to catch the construction deadline. That's digging a hole for yourself.
Anti-desoldering from source: Avoid these pits when selecting
Rather than fighting fires afterwards, it is better to avoid minefields when selecting models. Some experiences for reference:
- wall thickness: Don't just look at the nominal pressure, but calculate the fatigue life according to the actual working conditions. Under high temperature conditions, the wall thickness should be appropriately increased to compensate for the decrease of strength, but it should not be too thick-too thick will increase the stiffness and affect the compensation amount. Usually, the fatigue life of thin-walled bellows of 0.5-1.5mm is better.
- wavenumber: The more the wave number, the smaller the single wave displacement, the lower the stress and the longer the life. But too many wave numbers can increase length and cost, and can be unstable. It is generally selected according to the displacement amount and the allowable stress balance.
- Guide tube design: This thing not only diverts flow, but also prevents high-speed media from directly washing the bellows. The material of the guide tube should match the temperature of the medium, with a thickness of at least 3mm, and a sufficient gap (generally 3-5mm) between it and the bellows. If the medium contains particulate matter (such as smoke), the end of the guide tube should be wear-resistant.
- Heat treatment process: Austenitic stainless steel will generate residual stress after welding. For thick-walled or complex structures, solid solution or stabilizing heat treatment must be carried out to relieve stress and prevent intergranular corrosion. Ignoring heat-treated expansion joints, weld zone life is at least 30% off.
Pay attention to these details when selecting, and the probability of subsequent problems can be reduced by 80%.
Don't let maintenance become a decoration: What points should you look at for inspection?
Many people think that expansion joints are "maintenance-free"-just put them on and leave them alone. As a result, I didn't remember it until I missed it. In fact, regular inspection is very simple. Just make a list and check it:
- Expansion joint tie rod nut: Is it loose? A loose nut can cause the tie rod to fail and the bellows to take extra thrust. Refer to the previous question and answer "How to adjust the expansion joint tie rod nut", and use a torque wrench to re-tighten according to the design value.
- guide tube: If the medium has solid particles, the guide tube wears fastest. Disassemble and check the inlet end of the guide tube. If it is worn out or deformed, replace it immediately, otherwise the next step is to grind the bellows.
- Corrosion pit around weld seam: Use magnifying glass or penetration detection to see whether there is pitting and crack in the heat affected zone of the weld. Especially ring seams and troughs, the parts prone to water accumulation focus on.
- Bellows appearance: Are there any bulges, depressions, and uneven spacing of ripples? These are signs of instability or fatigue.
- Pipe support and limit device: If the bracket sinks or the limit bolt falls off and the actual displacement of the expansion joint exceeds the design value, it is not far from de-welding.
Frequency, high-temperature and high-pressure pipelines are recommended once a month, and general working conditions are at least once a quarter. Keep inspection records well to track trends. If you find small cracks, deal with them in time, and don't wait for the pipe to burst before regretting it-at that time, it will not be something that can be solved by "changing an expansion joint".