Servo valve endurance test for water-hydraulic systems in ITER-relevant conditions

ITER Divertor maintenance equipment operates in the vacuum vessel in elevated temperature and under considerable radiation load. The heavy Divertor assemblies are lifted and transported using servo valve systems, such as Cassette Multi-functional Mover (CMM). Systems are powered with water hydraulics, using demineralized water as a pressure medium. Operations have not been tested in ITER-relevant environmental conditions and over projected duty cycles. As the hydraulic medium is rather aggressive and there are no servo valves designed for demineralized water, the greater than 2000-h operational time was considered a potential issue. Hence, a project was undertaken to ascertain the component compatibility with the environment and pressure medium, and their robustness over the required operational period. Irradiation of components was not considered at this phase of technology validation. A heated test chamber was constructed to emulate the projected maximum ambient temperature of 50 °C in the Divertor area. Test routines and measurements were specifically tailored to monitor the operational parameters of the servo valve. After the 2188-h test the servo valve parameters remained within the limits promised by the manufacturer. Pressure gain decreased and hysteresis increased but remained within the allowable limits. These changes did not have significant effect on the joint angle tracking error.