Abstract: Many chemical reactions involving high pressures are carried out in reactor or distillation vessels. As a safety feature, the vessel is usually connected into an alarm system designed to shut it down if an overpressure condition occurs. During a routine safety check of the alarm system the vessel is isolated from the system by manually closing a valve. Following the safety check, the valve must again be opened to connect the vessel into the system. Some situations may require that a valve be left in closed position following a safety check, or some other operation.The valve of this invention can only be closed or opened with a key member which engages the valve stem. The key is an integral part of the equipment carried by the technician who makes the safety check, so that the act of disengaging the key from the valve stem ensures that the valve is left in either open or closed position, as desired.