A seal assembly for a piston rod including an external seal and an internal seal. The external seal is adapted to be placed on a piston rod facing an external environment, the internal seal is adapted to be placed on the rod facing the cylinder associated to the piston. The external and internal seals define in cooperation with each other a chamber for the containment of barrier fluid. Such chamber has an inlet for the intake of barrier fluid and an outlet for the exit of barrier fluid. The assembly also comprises a recirculation circuit for the barrier fluid, which is placed in fluid communication with the inlet and the outlet to recirculate the barrier fluid from the outlet back to the inlet and a pressurizer device configured to be installed coaxially with the piston rod providing the chamber with a positive pressure with respect to the process environment.