Guided Wave Monitoring of Industrial Pipework - Improved Sensitivity System and Field Experience

Low frequency guided wave inspection using the torsional, T(0,1), mode is routinely used in the petrochemical and other industries for the detection of corrosion patches, the detection threshold being typically around 5% cross section loss, though better sensitivity is obtained on simple pipe configurations not suffering from general corrosion. It has been shown in a blind trial that switching to a permanently installed system operating in SHM mode can improve the sensitivity to about 1% cross section loss and this is very attractive in corrosion monitoring applications. Later work has shown that the detection limit could be reduced to below 1% cross section loss if the compensation for environmental changes, particularly temperature, could be improved. This paper presents a new temperature compensation method involving both overall signal stretching, analogous to the well-known baseline stretch technique, and a further, location-by-location adjustment; this gives significant further improvements in performance. A commercial permanently installed monitoring system giving both local thickness measurements at the transducer location and long-range monitoring for corrosion over 10 s of metres from the transducer position is described. The system enables frequent measurements to be taken, the results being delivered to the operator via a wireless link. The benefits of the frequent readings enabled by the automatic data collection and transmission are discussed. Initial results presented here indicate that this enables defects as small as 0.1% cross section loss to be detected.