Enhanced Cleaning Efficiency through Repeated Pigging: Visualization and Mechanistic Analysis in Water Distribution Pipelines

Abstract

Pigging is widely employed to remove deposits from water distribution pipelines, yet technical guidelines for determining the optimal number of passes remain insufficiently established. This study investigates the mechanistic reason why repeated pigging provides superior cleaning performance compared with a single pass by integrating visualization experiments and computational fluid dynamics (CFD) analysis. A serrated scraper pig, consisting of two rows of circumferential serrated rubber pads with a Shore hardness of 60D, was tested in a laboratory-scale recirculating system using a 100 mm diameter transparent acrylic test pipe and artificial sediments designed to simulate strongly adhered pipeline deposits. Direct visualization showed that the second pigging pass achieved a sediment removal efficiency of 37.55%, which was approximately five times higher than that of the first pass (7.46%) and substantially greater than that of conventional flushing (0.55%). Transient CFD simulations (pimpleFoam solver with the γ – Re θ transition model coupled to the shear stress transport (SST) k – ω turbulence model) clarified the physical basis for this enhancement. During the first pass, sediment accumulation within the serrated pad gaps suppresses bypass flow development. After these gaps are partially cleared, accelerated annular flow develops more effectively during the subsequent pass, increasing the local wall shear stress and skin-friction coefficient in the noncontact regions. This combined action of mechanical scraping and hydrodynamic shear explains why the major cleaning effect appears primarily after the first pass. The main practical implication is that at least two pigging passes are recommended for the effective removal of strongly adhered deposits. These findings provide a mechanistic basis for optimizing pigging protocols in water distribution pipeline maintenance.