Cryogenic Ammonia Tanks
| Inspecting or
testing a cryogenic ammonia tank is a major operational
challenge. The low operating temperature and need to
maintain the integrity of the insulation system limits
the options for conventional (ultrasonic) on-stream
examinations. Shutting a tank down for an internal
inspection is very expensive especially if there is no
backup storage available. Cleaning and decontamination is
costly and results in the need to dispose of large
quantities of waste. Once shut down, the tank may be inspected by numerous conventional NDE methods and the problem becomes one of scale as many of these tanks are very large. More importantly, research on stress corrosion cracking of cryogenic ammonia tanks shows that although SAC does not initiate at low temperatures it will continue to propagate, albeit slowly, once started. The highest probability for initiation of stress corrosion cracking occurs when a tank is commissioned. Even a small amount of residual oxygen can cause stress corrosion cracking to start. Thus shutting down for an internal examination brings the prospect of introducing environmental damage, even if none existed before, in addition to the high cost. As you can probably tell, this is leading up to something... Acoustic emission monitoring has been applied to many of these tanks as a means of assessing the tank's condition while it remains in operation. Indeed, several tanks have AE sensors mounted permanently and are tested every three years or so. This is not to say that this is a simple application of AE, it isn't, but results show that defects are detectable by this means. Most of the defects detected have been welding flaws but some of these have been significant and required repair. |
To conduct a test, AE sensors are
mounted on the tank –preferably directly. Waveguides
are used in some cases in order to minimize disturbance
to the insulation. However, the wave guide tip must be
well bonded or coupled to the tank wall to avoid signal
loss. Once the sensors are hooked up to the data
acquisition system, the ammonia liquid level is raised
from 90% to 105% of the maximum operating level – a
standard "Monpac" in-service loading. Difficulties with this type of test include high noise from filling (often liquid ammonia is sprayed down from the roof) and noise from ice where moisture has built up under the insulation. We work with operations personnel to find the optimum (least noisy) filling conditions, but ice can produce high amplitude signals that are very problematic. Noise from ice can be recognized with careful and time consuming analysis of the data. The real problem is that it may mask more significant, but lower amplitude signals from flaws. Matrix is currently involved with an AE signal characterization research program at the University of Texas in Austin. The program is being led by Dr. T. J. Fowler and a solution to the problem of ice and other noise in cryogenic tanks may well come from this work. Meanwhile, AE remains a valuable tool for periodic testing of cryogenic ammonia tanks to detect any general deterioration and highlight areas in need of inspection. In several cases, Matrix has conducted automated ultrasonic examinations on stream to confirm AE test results and characterize any defects found |
