Mode of operation: hydraulic section The emergency stop valve is opened hydraulically by oil and closed by spring action. The opening operation s initiated by starting device. For this purpose, trip oil (start-up oil F) flows into the space behind the piston. As it overcomes the action of the spring, the piston moves in the direction of the spring cap and is eventually pressed against the cap to form a seal. Controlled by the starting device, the trip oil pressure E now builds up ahead of the piston disc.