AE Testing of flat-bottom storage tanks

 

Flat-bottom tanks with a storage volume up to 150.000 m³ are used all over the world. The flat bottom can neither be accessed nor inspected from outside. The tanks must be checked at specified intervals since – despite the product loss – an undetected leak can cause significant environmental damages. The conventional inspection method requires emptying, opening and cleaning of the tanks for internal visual inspection.

This takes time during which the tanks are not available. By means of Acoustic Emission Testing the floor of storage tanks can be checked for leakages and active corrosion when full; time-consuming emptying and refilling is not necessary. The information thus gathered is considerably more reliable compared to stationary tests and internal visual inspection.

 
 

To carry out the test, sensors evenly distributed around the circumference must be applied to the exterior wall and, if possible, invasive interior sensors are lowered into the storage product. The measurement takes only a couple of hours during which the tank is kept on its maximum filling level. The metallurgical and physical processes caused by active leakages and/or corrosion emit signals that can be recorded using applied sensors. Based on frequency pattern of the transient signals and the different points in time at which they reach the sensors, the sound source can be localized. TÜV AUSTRIA Group has been using Acoustic Emission since the 1970s and has continuously further developed and enhanced the test method. Since then, hundreds of tanks of up to 100 m in diameter have already been tested. Using frequency-based signal pattern recognition, leakages and other corrosion damages can be detected and registered with a high degree of location accuracy using methods such as threshold crossing and cross correlation.

Location accuracy is ± D/10 (D=diameter) for tanks with many fittings (heaters) and ± D/20 for tanks without fittings. The leak test of flat-bottom storage tanks through Acoustic Emission is widely recognized by authorities and spread throughout Europe.