Non-invasive cleaning monitoring in pipeline systems using ultrasound: an experimental study

Cleaning is important in the beverage and food industries, where it is conducted in validated, predefined cycles using automated methods known as cleaning-in-place (CIP). Existing sensors monitor cleaning indirectly by measuring operating conditions or detecting deposits suspended in the cleaning fluid. Ultrasound (US) provides a direct approach by detecting deposits that remain on the surface during cleaning. While ultrasound has been used effectively for monitoring fouling beneath planar structures, its application in closed geometries such as pipes is challenging due to complex wave propagation. In this study, a clamp-on US system was developed to investigate the effect of organic deposit removal on propagating ultrasound waves in pipes. The US measurements during pipe cleaning showed that deposits reduced signal amplitude and delayed waveform arrival. Features including energy, time shift, and signal correlation were extracted from the US signals. Deposit removal was quantified by conducting cleaning trials at varying durations and subsequently determining the gravimetric mass of remaining deposits. The results showed that the features were significantly correlated with the amount of deposits left in the pipe. Building on this, an online detection algorithm was implemented to track the slope of the energy feature in real-time. In independent trials, the algorithm consistently monitored cleaning until the onset of a plateau in the energy feature, corresponding to an average residual deposit amount of 0.8 ± 0.5 mg/cm². These findings demonstrate that ultrasound enables direct detection of deposits on pipe surfaces during cleaning and estimation of the near end point of cleaning.