Testing Above Ground Storage Tanks
What Exactly is High Frequency Acoustic Emission Testing?
AET can better be described as another form of seismology i.e. micro seismology if you will. Just as the movement of the earth’s crust in the form of an earthquake releases energy as a shock wave detectable by low frequency sensors, so objects/ structures themselves under test by AET release energy detectable by sensors placed at strategic locations. The main difference between seismic and AET sensors are the frequency spectrum used. The seismic industry uses low frequencies between 0 to 10 Hz that would create problems if used in noisy environments. AET uses frequencies between 20 and 400 kHz (in some cases even higher), which allows the monitoring of equipment subject to high noise from pumps and liquids fl owing in piping etc. Another difference is the distance used between sensors. In the seismic industry the sensors are placed many miles away from the source. Typical AET sensor spacing is in the order of 10 to 15 feet apart.
When conducting a Tank Bottom AE test using the ultra-low frequency AE sensors there are a lot of restrictions and limitations. For example, the tank has to be quiet without any liquids flowing for minimum of 12 hours prior to the test. Also the presence of any extraneous noise nearby prevents the technician from collecting meaningful data. There are several in plant tanks that AE testing was not possible.
The use of HFAE to collect tank bottom data has solved this problem, however this is restricted to tanks less than 60 feet (18m) in diameter. Any tanks above this size low frequency is a requirement. A large number of tanks that are less than 60 feet have been tested and the results verified and AE has been proven to be a very good screening allowing the owner to prioritize the internal inspection. Use of this method to grade the tanks to meet with RBI programs is also possible.