COMBATING THE SILT EROSION OF HYDRO TURBINE COMPONENTS IN THE HIMALAYAN BELT : NEW DUPLEX STAINLESS STEEL COMPOSITIONS WITH ENHANCED SILT EROSION RESISTANCE

摘要

In hydro-turbines, runner blading and stationary guide vanes have been traditionally manufactured from a CA-6NM type martensitic stainless steel corresponding to the nominal chemistry of 13% Cr and 4% Ni. While this steel has excellent mechanical & physical properties and it also gives a robust response to manufacturing, fabrication, and reclamation/maintenance operations, it is prone to acute abrasion & erosion in the silt loaded water encountered in the Himalayan region. A major maintainability issue facing hydro power plants operating in the Himalayan region is the acute silt abrasion of runner & stationary components which leads to dramatic reduction in efficiency & operational reliability of these units. The usual approaches adopted to solve the silt erosion problem have included deposition of wear resistant hard coatings using thermal spray techniques such as High Velocity Oxygen Fuel (HVOF), D-Gun, Plasma Gun, etc., modification of surface chemistry of the turbine components using plasma nitriding and other surface modification techniques, as well as use of composite coatings derived from polymeric matrices. Very limited success has been attained with HVOF based WC coating in Co matrix. However, cost effective permanent solutions continue to evade researchers. In the present work, an approach based on developing new silt erosion resistant stainless steels with phase constitution significantly different from the traditionally used martensitic stainless steels has been attempted. Results of silt erosion test studies undertaken on a specially designed silt erosion test rig, which accurately simulates the actual two phase fluid flow conditions encountered in hydro turbines, indicate that the new duplex grade stainless steels have only 35-50% of the silt erosion rate of the traditional martensitic stainless steel grade. It is conjectured that with further optimization of the duplex stainless steel chemistry and thermal treatment protocols, additional enhancement in the silt erosion resistance can be attained leading to significant improvement in maintainability of Indian hydroelectric projects operating in the silt prone Himalayan belt.