The metal expansion joint looked like a circle of corrugated pipe. In fact, what was most afraid of was not the pressure of the pipe, but the invisible cracks on the corrugated pipe. The hydraulic test can find out whether it leaks, but it can't find out whether it leaks soon. How do you say that? If there are surface opening defects in the welds and stress concentration areas of wave peaks and valleys of corrugated pipes, they may not be penetrated in the hydraulic test. However, once it is put into operation, the alternating stresses are superimposed, and it is only a matter of time before the crack propagates. Therefore, the PT inspection of metal expansion joint has become an unbypassable checkpoint before leaving the factory and when shutting down for maintenance.
What exactly is a PT inspection checking?
PT is Penetrant Testing, and its principle is not complicated: capillary action is used to penetrate the permeate into the defects of the surface opening, and then the imaging agent adsorbs the defects, forming visible traces to the naked eye. Surface opening defects were checked-cracks, folds, delaminations, unfused, pores, inclusions. For metal expansion joints, the emphasis is on bellows. Especially near the longitudinal weld and circumferential weld of bellows, there are crests and valleys, which are the positions with the largest deformation. Once there are fine cracks in these places, they are often safe and sound in the hydraulic test. However, in actual working conditions, thermal expansion and contraction plus medium corrosion, the cracks will slowly grow, and finally lead to leakage or even tearing of the bellows.
Before PT inspection, no preparatory work is equivalent to doing it in vain
Many people stumble on PT inspection, not because the detection technique is not good, but because the preparatory work is leaked. The surface cleanliness is not up to standard, the permeate can't enter the defect, and the result is missed inspection. There are often oil, rust and oxide scale on the surface of bellows, which must be cleaned up. Solvent-based or water-based for cleaning agents? It depends on the site conditions, but it must be dried after cleaning, because the permeate and water are incompatible, and the residual moisture will directly block the defect opening.
The crests, troughs, straight edge sections, longitudinal welds and circumferential welds of corrugated pipes must not be missing. Especially the longitudinal weld-corrugated pipe is coiled into a pipe blank by thin plate and then hydroformed. The longitudinal weld bears huge plastic deformation during the forming process, and is the area where micro-cracks are most prone to appear. In order to save penetrant, some manufacturers only check the ring weld, and the peaks and troughs pass by, which is self-deception.
PT Operation Process Dismantling: Every Step is a Pit
Pre-washing, infiltration, cleaning, imaging, observation. But every step is particular.
Pre-cleaning: Use cleaning agent to wash the surface oil and dust clean, and then dry. What is easiest to overlook here is the residue in the bellows trough, and the grooves hide dirt, which should be blown repeatedly with a brush or compressed air.
infiltration: The infiltration time is generally not less than 10 minutes, and it should be extended when the ambient temperature is low. The penetrant should be sprayed evenly, covering the whole detection surface. Some people wait for two or three minutes to spray it in order to be quick. The surface looks wet, but in fact, the defects have not been thoroughly understood, so it dries for nothing. It won't work if you spray too much, excess penetrant will remain, and subsequent cleaning will be difficult.
cleaning: This is the most error-prone step. With water-washing penetrant, the water pressure should not be too large, and the direction should be inclined to avoid washing out the penetrant in the defect. If you use solvent-based, you should wipe it with a clean wipe dipped in solvent, and you can't flush it directly with solvent-once you flush it, the penetrant in the defect will also be flushed away. When wiping, you should follow one direction, not wipe back and forth.
imaging: The imaging agent should be sprayed thin and evenly. Too thick will cover up the defects, and too thin will not be sucked out. The imaging time is generally 7~15 minutes, and then the observation begins.
observe: Look with the naked eye or a magnifying glass under sufficient light, focusing on whether there are red marks precipitating. Note that instead of looking at it immediately after imaging, wait for a period of time for the penetrant in the defect to be fully adsorbed out.
Defect Determination: Not All Traces Will Be Repaired
The traces displayed by PT must first be distinguished whether it is a linear defect or a circular defect. Linear defects refer to displays where the length is greater than the width, such as cracks, unfused, bitten edges; Circular defects refer to pointed pores and inclusions.
The judgment standard in the industry generally refers to NB/T 47013.5 (Non-destructive testing of pressure-bearing equipment) or GB/T 12604.3. For metal expansion joint bellows, linear defects are unacceptable, as long as it is found that no matter how long it is, it must be repaired or judged to be scrapped. Circular defects, depending on size and number, usually allow a small number of small diameter pores, but are also not allowed if they are dense or occur in trough stress concentration areas. Remember that bellows are thin-walled parts, and any crack-like defects may grow rapidly under fatigue load, so you can't take the relaxed standard of ordinary pressure vessels.
PT is not a panacea, it has to be cooperated with other tests
PT can only check surface opening defects, and internal defects it can't work hard. Therefore, the factory inspection of metal expansion joints is often not a solo effort.
For the general-purpose corrugated expansion joint, the working pressure is not high, and it mainly bears thermal displacement. PT is the main surface inspection means, and it is enough to check the unfused and slag inclusion inside the weld joint with radiographic inspection (RT). However, for high-temperature axial expansion joints, the working conditions are harsh-high temperature, high pressure and frequent displacement. At this time, PT + RT alone is not enough. Ultrasonic testing (UT) can check the thickness thinning and buried cracks, and only when they cooperate can they cover the surface and interior.
In addition, PT and RT are not repetitive, but complementary. The surface of PT tube and the inside of RT tube are only complete if both are qualified. Some customers blindly ask to "just be a PT". As long as they see the words "PT qualified" on the report, they are relieved. In fact, whether there is any internal problem or not, PT doesn't count. Conversely, RT is not a substitute for PT, because RT often fails to show the surface micro-cracks generated by bellows during the molding process.
Test report: Not for coping with the inspection, but for next comparison
PT test records can't just write "qualified". The standard practice is to record the inspection site, inspection timing, penetrant model, imaging agent model, ambient temperature, defect description and defect location indication. It is best to draw the actual position mark on the bellows and take a photo to leave the bottom.
Why take it so seriously? Because the expansion joint is a fatigue component, the tiny display this time could be a big crack next time. When shutting down for maintenance, turn out the historical PT records and compare whether there is any change in the same position-the place that was not originally displayed appears, indicating that the stress is concentrated here; It turns out that the place where there is a display shows that it has grown up, indicating that it is expanding. With historical data, there is a basis to judge whether to replace it in advance. Otherwise, it will be like a new test every time, which is equivalent to repairing the equipment with your eyes closed.
Therefore, the PT inspection of metal expansion joint is not a process, but a "physical examination" for the equipment. If it is not done in place, it will directly affect the expansion and energy saving, and it will not be able to complete a maintenance cycle. To be honest, many on-site accidents are buried by the lucky psychology of "never finding out before". The location that should be checked is in place, and the record that should be recorded is clear. This thing can really save lives.