Signal analysis of an actively generated cavitation bubble in pressurized pipes for detection of wall stiffness drops

• A non-intrusive monitoring method of pressurized pipes is proposed.
• Wall stiffness drops in a steel pipe are modeled by weak reaches (PVC, aluminum).
• The method is based on signal analysis of an actively generated cavitation bubble.
• An accurate estimation of the wave speed reduction was shown.
• Weak reaches' locations were found accurately up to wave reflection ratios of 25%.

Due to the increasing production of volatile new renewable energies as solar and wind, storage hydropower plants have to operate under harsh operation conditions in order to stabilize the electricity grid. As a result, highly transient water pressures occur in pressure tunnels and shafts more frequently. Non-intrusive monitoring techniques are therefore of special interest for these critical infrastructures. The propagation of a pressure wave generated actively by a cavitation bubble was experimentally investigated in a steel test pipe divided in several reaches. A local wall stiffness drop was simulated by replacing steel pipe reaches with less stiff materials as aluminum and PVC. Through the analysis of the pressure wave reflections due to the cavitation bubble explosion, recorded by two hydrophones placed at the extremities of the test pipe, the location of the weak reaches could be detected. An underwater spark generator was developed to produce cavitation bubbles in the pipe resulting in very steep shock waves. This allowed identifying very precisely the wave front and correspondingly the wave speed and the weak reach location. Compared to the wave analysis from water-hammer signals, the active cavitation bubble generation in the pipe is an innovative method that significantly increased the effectiveness of the detection of wall stiffness drops.