Reciprocating wear damage of grooved non-grooved flat surfaces for grid valves of steam turbines

Grid valves are used in extraction type steam turbines for the steam extraction in different sections of the turbine before the last stage. This valve is based on the relative rotation of two holed disks by means of an actuator, steam is extracted when the holes of these disks are fully or partially overlapped. When the valve is closed the holes of rotating disk are overlapped to a grooved flat surface of the statoric disk. If these grooved parts are too worn the valve does not rightly close steam extraction. This study investigates the wear damage mechanism of the grooved parts of the statoric disk. Research was based on experiments where grooved and non-grooved flat surfaces were kept in contact and worn by relative reciprocating motion. Test rig was characterized by free approaching of contact surfaces and self-positioning of specimens. Specimens may be disassembled, in order to make the surface topography measurements by means of optical instrument based on focus variation to evaluate the wear volume, and subsequently reassembled to continue the wear process. Friction force and relative displacement was continuously measured during the processes in order to display hysteresis loops and consequently evaluate the friction coefficient. Relative displacement was measured in the vicinity of contact surfaces directly on the specimen in order to avoid the inclusion of test rig component deformations in relative displacement measurements. The choice of wear process parameters was based on operating conditions of grid valves, three temperatures, two normal loads, one sliding amplitude (500 micrometers), two frequencies (5, 50 Hz) and two sliding distances (300, 500 m). This set of process parameters were used to study the wear and the friction of five different materials based on different thermochemical treatments (nitriding and carbonitriding) and coatings (ENP, CrC, and two Stellites) of the steel 39NiCrMo3.