Abstract: A conduit-break valve for a double conduit (10, 11) has a pressure relief device (28) with a pilot controlled check valve (41, 42) with a control piston (37) which is actuated by the pressure in the space (26) between the double conduits (10, 11) and opens the check valve (41, 42) when a limit value determined by the spring (43) thereof is exceeded. The check valve (41, 42) thereby relieves the pressure in a fluid lock which is formed by a piston and cylinder mechanism (17, 18) the cylinder (18) of which is normally kept closed by the check valve (41, 42) and the piston (17) of which is connected to the main valve member (16) of the main valve, whereby said valve member will be moved to closed position by the action of a pressure spring (19) when the check valve (41, 42) is opened.