Steam Turbine Valve Testing, Inspection and Maintenance to Avoid Turbine Overspeed Events

Steam turbine valves are the most important components before a steam turbine for the admission and control of steam flow. Steam turbine valve configurations can be all in one body or as individual valves for the large units, for the main stop (throttle) and control functions for each section of the steam turbine. Today’s thermal power plants for drum or once-through type supercritical boilers produce very high temperature steam in the final superheat and reheat section of 1055 °F or higher requiring special boiler tube metal alloys, some of which are subject to steam oxidation as in austenitic steels. The oxidation material is mainly in the form of iron oxide or magnetite which is subject to exfoliation. The exfoliated oxide material is carried over to the steam turbine valves and to the high and intermediate pressure turbines. The effect of exfoliated oxide material in valve and turbine components is to cause solid particle erosion (SPE) and therefore shortening the life of those parts due to damage. As time goes by if valves are not tested and do not meet their inspection and maintenance schedule they will build up oxidation material to form scale material called blue blush and eventually they stick and will not shut properly.