TANKSERV: Acoustic Emission Testing of Aboveground Storage Tanks
The floor of an Aboveground Storage Tank (AST) remains the most difficult part of the vessel to inspect for corrosion damage. Generally, this section of the tank is inaccessible from the outside for conventional Non Destructive Testing (NDT) and the costs of opening the tank and preparing it for an internal inspection including cleaning, purging and loss of production availability, creates a significant financial burden for the tank owner/operator. An advanced NDT method with the capability of assessing the condition of the tank floor without opening the tank is acoustic emission (AE) monitoring. TECHKNOWSERV is an industry leader in providing inspection of ASTs using the AE technology, with back up NDT using ultrasonic spot inspections and c-scans other conventional NDT methods such as Magnetic Flux Leakage (MFL).
Figure 1: Acoustic emission testing of above ground storage tanks.
Acoustic Emission Testing
Acoustic emissions are stress waves produced by sudden release of the energy in stressed materials or products. Classic acoustic emission sources include defect-related deformation processes such as fatigue crack growth and plastic deformation that releases elastic energy, some of which is converted to stress wave propagation in and on the surface of structure being tested. Acoustic emission testing of tank floors differs from the traditional steel fatigue context. In this scenario, the acoustic emission sources are leaks, corrosion product breakdown, and possible electrochemical activity.
Piezoelectric sensors mounted on the external surface of the shell of the tank near its base, detect these stress waves and output an electrical waveform that contains information about the source of emission. Most AE tests are carried out under controlled stimulation using systematically increasing mechanical or thermal loading. In the case of AST floors, the fluid load in the tank is used to excite the emission and monitoring is carried out over a prescribed time interval. The remote monitoring capability allows the whole volume of the structure to be inspected globally and non-intrusively as the emissions travel from the active defect through the liquid in the tank to the remote sensors on the outside of the tank. It is not necessary to scan the structure point by point or scanning the structure looking for local defects. This leads to major savings in testing time and the global AE inspection is used to identify areas with structural damage or deterioration and other NDT methods are then used to identify and fully characterize them, if necessary, in terms of shape, orientation and size.
The TANKSERV procedure involves AE monitoring of the tank for a period of one hour or longer to identify damage and prioritize these tanks for maintenance and/or complementary NDT. A ring of AE sensors is mounted equidistant around the outside circumference of the tank wall at approximately one meter up from the bottom, and are monitored in the frequency range of 20 – 100 KHz. Following a period of conditioning of 12 to 24 hours to minimize ambient noise during which valves are closed and heaters/agitators turned off, the tank floor is monitored for the fracture of corrosion products and leakage that are active during the test period. Time-of-arrival differences at the sensors in the array on the outer shell are used to compute the location of the emission. Each channel has the capability to measure amplitude, adjust thresholds, filter signals outside the range of interest, and store digital records of all AE signals. Example test results are shown in Figures 2 and 3. In Figure 2, each red square is an event that has been located on the tank floor through triangulation. Triangulation occurs when 3 or more sensors detect an acoustic emission hit above the required threshold. The results are searched though in a 1m x 1m grid to identify clusters of acoustic emission events. The density of the event cluster is used to assess the corrosion severity and leak potential. Figure 3 is the same data presented in a 3-D view with event quantity displayed in the vertical axis. During this test a cluster of acoustic emission events were detected in the middle of the tank floor.