Abstract: A valve for closing a main fluid flow through a main flow passage having a valve stem with a valve head for engaging a valve seat to close the passage, the valve stem carrying at the end remote from the valve head a piston which can be actuated by a closing fluid against spring action to move the valve head (48) into closed position, the valve stem being hollow and is open at the valve head end and at the opposite end has openings opening into the interior of the hollow valve stem, and the piston is mounted for axial movement on the valve stem to allow further movement towards the valve seat against spring action when the valve head has engaged the valve seat and the valve stem has come to a halt, said further movement relative to the valve stem opening a passage into the openings in the valve stem, whereby closing fluid will be injected into the main flow passage at one side of the valve seat through the valve stem.