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The unloading valve is installed on the hydraulic block of the hydraulic actuator. Its main function is to allow the pressure oil in the lower chamber of the actuator’s piston to be released rapidly through the unloading valve, when an emergency shutdown is required due to a malfunction in the unit and the emergency trip device causes a loss of pressure in the AST oil; the valve then closes under the force of the spring. Working principle: The rapid unloading valve contains a cup-shaped spool; the lower part of this spool is connected to the high-pressure oil circuit beneath the piston of the hydraulic actuator. High-pressure oil enters the upper part of the spool through the throttle orifice at the input port, via the emergency shutoff oil circuit. Since the needle of the regulating needle valve completely blocks the passage there, the oil pressure at the upper part of the spool becomes equal to the emergency shutoff oil pressure. Therefore, the force of the oil pressure acting on the upper part of the spool, combined with the spring force, is greater than the force of the high-pressure oil acting on the lower part of the spool; as a result, the spool is pressed against the base, and the flow of high-pressure oil to the return/inlet port is blocked. When the emergency shutdown device activates and causes a loss of pressure in the AST oil, the oil pressure above the spool is almost zero. Given that the spring’s stiffness is not high, the high-pressure oil at the lower part of the spool overcomes the spring force and pushes the spool open. This allows the high-pressure oil circuit to connect to the return line and flow back to the tank, causing the pressure oil beneath the piston of the hydraulic actuator to drop rapidly, thereby quickly closing the steam inlet valve. The adjusting needle valve can be used for manual unloading.