Inlet butterfly valve is designed for blocking the hydroturbine penstock in the following cases:
1) at each shutdown of a hydraulic unit, thereby preventing the wicket gate wear caused by clearance cavitation;
2) at repairs and inspections of the hydroturbine water passage;
3) In case of emergency:
— in case of a failure of the hydroturbine wicket gate and the hydraulic unit rotor speed increase;
— with decreasing pressure in the oil pressue unit (OPU) being common for a hydroturbine and a butterfly valve.