Comparative Analysis of wear on Hard and Soft Coated Francis Runners: A Case study of Kali Gandaki ‘A’ Hydro Power Station

Abstract

Abstract Surface coating on Turbine components is a regular practice in hydro power plants to tackle with wear. The Kali Gandaki ‘A’ hydro power station of Nepal having 3x48 MW Francis Turbine units made of CN 13/4 material has been regularly coating its Runners entirely using Tungsten Carbide based alloy through HP-HVOF hard coating process. In 2021 for the first time one of the Runners was partially coated around the blades using elastomeric semi-soft coating material manually via brush application, while the remaining portion of the Runner was hard coated. This study was conducted to compare the wear on the hard and soft coated runners after a similar period of operation under similar circumstances. Both the Runners were found to be heavily worn out at the upper and lower wearing rings, suction side of all blade inlets at the shroud and all blade outlets. The soft coated runner was found to have more erosion and cavitation wear than the hard coated runner. A unique canal-type cavity was found on the hub side wearing ring of the soft coated runner but no such pattern was seen on the hard coated Runner. The sections of the soft coated Runner where the coating was still intact had the parent material fully protected, but the sections with coating completely washed away were found to have deeper and wider cavities showing that the parent material had significantly washed away. The major reason behind the soft coated Runner being more worn out could be the manual application of coating material using hand brush which could have resulted in blade profile being distorted due to which the Runner efficiency might have reduced and thus higher erosion and cavitation occurred. This study could be useful to assess the applicability as well as the benefits and drawbacks of hard and soft coating on Francis Runner.