Valve element (3, 6, 10), comprising a frame, at least one inlet and at least one outlet, a passage connecting the inlet and outlet, at least one burst disk (13, 35, 63) or equivalent, which, when unbroken, closes the passage from inlet to outlet, and a piston element (15, 36, 65) provided with a piercing element or the like for piercing the burst disk. The piston element (15, 36, 65) is provided with a passage (17, 39, 67) for a pressure medium, said passage extending axially through the piston element from one first side to one second side, which passage leading through the piston forms a part of the passage of the pressure medium after the burst disk (13, 35, 63) has been pierced.