Centrifugal Slurry Pump (CSP) is one of the most important fluid transportation equipment, widely applied in processing plants. We conduct Reliability Growth Analysis (RGA) on two CSPs, i.e. 352PU342 and 352PU343, in a mineral processing plant at Western Australia (WA), both of which are repairable systems and work in duty-standby mode, to evaluate their maintenance performance during an impeller material transition from metal to rubber. Three Units of Measures (UoMs), i.e. Operating Time (OT), Cumulative Mass (CM), and Effective Work (EW), are employed to accurately quantify the service life of CSPs, the last one of which is constructed by hydraulic modeling and firstly applied in RGA on CSP with embedding the working mechanism, and therefore can better represent the service life. Laplace Test is first conducted and reveals the improving performance trends for the CSPs during the impeller transition. Then, two Goodness-of-Fit (GoF) hypothesis tests, i.e. Cramér-von Mises (CVM) Test and Common Beta Hypothesis (CBH) Test, are implemented in sequence to validate that the failure and overhaul data of some of the combinations between impeller and UoM fits Power Law Model, in which the wear-out failure patterns of the CSPs are identified. Finally, economical life models are developed to find out the optimum overhaul interval, i.e. economical life, and the associated minimum average life cost per unit service life for each qualified combination, based on which the overhaul policy simulations of the qualified combinations over an actual operating time frame are compared to figure out the one incurring the lowest total cost, i.e. achieving more than 96.5% cost reduction compared to the current overhaul policy. This study paves the way for massively applying RGA on other critical repairable systems in processing plants, and further lays a solid foundation for Preventative Maintenance Optimisation (PMO), to unlock the hidden substantial value of the existing assets and comply with the relevant Australian Standard (AS).