Screening of Corrosion in Storage Tank Walls and Bottoms Using an Array of Guided Wave Magnetostrictive Transducers

Above ground storage tanks are used to store various fluids and chemicals for many industrial purposes. According to API standard 653, the structural integrity of these tanks must be regularly assessed. The U.S. EPA requires each operator to have a Spill Prevention, Control and Countermeasure Plan (SPCC) for aboveground storage con-tainers. The accepted practice for inspection of these tanks, particularly the tank bot-toms, involves removing the tank from service, emptying the tank, and inspecting di-rectly from the interior. The required inspection operations are hazardous due to the chemicals themselves as well as the requirement to operate within confined spaces. An inspection from outside the tank would have significant cost and time benefits and would provide a large reduction in the risks faced by the inspection personnel. Guided wave (GW) testing is one promising candidate for screening of storage tanks walls and bottoms from the tank exterior due to the ability of GWs to propagate long distances from a fixed probe location. The lowest order transverse-motion guided wave modes (e.g., torsional vibrations in pipes) are a good choice for long-range inspection because this mode is not dispersive; therefore, the wave packets do not spread out in time. A common weakness of guided wave inspection is the complexity of report generation in the presence of multiple geometry features in the structure, such as welds, welded plate corners, attachments and so on. In some cases, these features cause generation of non-relevant indications due to mode conversion. Common non-relevant indications are described in this paper. Another significant challenge in applying GW testing is development of probes with high enough signal amplitudes and a relatively small footprint to allow them to be mounted on relatively short tank bottom extensions. In this paper a new generation of magnetostrictive transducers will be presented. The transducers are based on a reversed Wiedemann effect and can generate shear hori-zontal mode guided waves over a wide frequency range (20 – 150 kHz) and with SNR in excess of 50 dB. The recently developed SwRI MST 8x8 probe contains an array of 8 pairs of individual magnetostrictive transducers (MsTs). The data acquisition hard-ware allows acquisition using Full Matrix Capture (FMC) and analysis software re-porting of anomalies based on Total Focusing Method (TFM) image reconstruction. This novel inspection package allows generation of reports containing high accuracy corrosion mapping information. Case studies of this technology on actual storage tanks walls and bottoms will be presented together with validation of processing methods on mockups with known anomalies and geometry features.