Abstract: An automatically and independently operating safety device for triggering a total or partial closure of a guard or safety valve member when the velocity of a fluid circulating in a conduit attains a predetermined dangerous value. The device senses the static and total pressures of the fluid within the conduit and supplies these pressures which control the movement of a piston and rod wherein the movement of the rod in response to fluid velocities which are dangerously high will initiate the actuation of the safety valve.