Isolating device for calibrated safety valve

An isolating device for positioning in a circuit upstream of a safety valve, comprising a valve member which can sealingly bear on a valve seat formed in said circuit, neutralization means which normally maintain the valve member remote from its seat, said means being actuatable to bring the valve member in sealing engagement with its seat when it is necessary to isolate the safety valve and calibration means acting on the valve member when it is sealingly engaged with its seat to enable it to move away from the latter when the pressure upstream of the isolating device exceeds a predetermined value, said neutralization means comprising a mechanical system controlled by a motor, said mechanical system comprising a first member integral with the valve member and a second member which moves in accordance with the displacement direction of the valve member under the action of said motor, a connection with clearance being provided between said members in order to normally permit the second member to act on the first member so as to keep the valve member remote from its seat and to permit the first member to move with respect to the second member when the valve member moves away from its seat counter to the calibration means, when the pressure upstream of the isolating device exceeds the predetermined value after starting up the motor, each of said members comprising a part in the form of a piston sealingly received in a bore so as to define a first tight chamber filled with hydraulic fluid which communicates with a second tight chamber into which is introduced a given volume of compressed gas defining the calibration means.