In a fluid flow emergency shutdown valve a valve body is transversely interposed in a fluid line upstream with respect to a flow restricting unit, A valve stem is longitudinally slidably supported by the valve body and referenced to atmosphere at its respective ends, The valve stem supports a valve normally disposed upstream with respect to its seat. The other end portion of the valve stem supports a piston fluid pressure balanced by downstream fluid pressure from the outlet port of the flow restricting unit in combination with the buckling point value of a collapsible pin axially abutting the downstream end of the valve stem, A decrease in downstream fluid pressure below a predetermined value increases the pressure differential across the piston from its upstream to its downstream side which moves the piston to buckle the collapsible pin and move the valve to a closed position.