Fault analysis of vacuum pipe fittings

The following problems may occur during the use of vacuum pipe fittings: vacuum degree failure, leakage of vacuum pipe fittings, large vibration of vacuum pipe fittings, etc.

1.  Causes of vacuum failure
   The vacuum degree failure is when the vacuum degree value is lower than the normal value, and the surface of the vacuum pipe fittings sweats, hangs water or freezes, frosts, and so on. The reasons are:
   (1) The vacuum degree naturally fails. Any substance has the characteristics of air leakage and outgassing. Although the leakage and outgassing rate of vacuum fittings is small, over time, vacuum failure will still occur.
   (2) Damage to the outer tube during installation causes internal vacuum leakage.
   (3) The welding or processing problems of the vacuum nozzle, the weak welding of the vacuum pipe fittings, etc., lead to micro-leakage in the later stage, causing the vacuum degree of the vacuum pipe fittings to fail.
   (4) The activation treatment and storage of the adsorbent inside the vacuum pipe fittings are unscientific, resulting in a decrease in the adsorption capacity of the adsorbent in the later stage, which cannot meet the vacuum design service life, and the vacuum fails in advance.
   
   
2. Causes of leakage of vacuum fittings
   (1) Due to cracks in the weak point (expansion joint) of the inner tube, the low-temperature liquid leaks from the rupture of the inner tube into the interlayer of the inner and outer tubes. After the pressure increases, the valve core in the vacuum sealing assembly is punched open and leaked into the air.
   (2) The LNG filling station has environmental factors such as frequent changes in pressure and temperature, fast and unstable liquid flow rate (the vacuum pipe fittings were originally used in the environment of oxygen, nitrogen, and argon in the air separation industry, and the working conditions are relatively stable). Strong scouring, and frequent opening and closing of valves cause water hammer effect, pulse and vibration of submersible pump, especially the pipeline behind the pump is more serious, which reduces the fatigue life of the inner compensator and causes the inner tube to rupture.
   (3) The welding is unreasonable, the temperature of the heat-affected zone of the internal compensator welding is too high, and the service life is reduced, resulting in damage.
   (4) The inspection valve at the front end of the safety valve is not opened, which causes cracks in the pipeline to leak when the pipeline is overpressured.
   (5) The pre-cooling speed of the pipeline is too fast, causing stress damage to the pipeline and crack leakage.
   (6) The design of the internal support structure and compensator of the vacuum pipe fittings is unreasonable, which causes the radial displacement of the inner compensator, and the service life decreases rapidly, resulting in damage.
   
   
3. The reason for the large vibration of vacuum pipe fittings
   The large vibration of the vacuum pipe will cause abnormal sound in the pipeline, and the temperature of the LNG in the pipeline will heat up and increase rapidly. The reasons are:
   (1) The vacuum pipe fitting support frame is damaged, the inner pipe support is unstable, and the pipe vibrates.
   (2) The pipeline enters the working filling state without sufficient pre-cooling.
   Due to the unreasonable structural design of traditional vacuum pipe fittings and the unstable dynamic vacuum data when vacuum pipe fittings are evacuated and sealed, the above common faults must exist, resulting in the shortened service life of vacuum degree; the design of the compensator in the harsh environment of on-site use Unreasonable, unreasonable welding process and other factors will also increase the problem of internal compensator rupture and leakage.