A full lift safety valve for a pressurised vessel to be safeguarded containing water and air, pressurised steam, fluid or hot oil, the full lift safety valve comprising a pressure inlet body, a pressure discharge body, a clapper, a guide rod, a pressure setting cylinder, an inner spring, an outer spring, a diaphragm reservoir, optionally a water reservoir and a command board wherein • pressurised water and air from the pressurised vessel to be safeguarded are taken from the pressure input body where they enter through a metal pressure transfer pipe and sent directly to the diaphragm reservoir, or • pressurised steam, fluid or hot oil coming from the pressurised vessel are taken from the pressure input body where they enter through a metal pressure transfer pipe and sent directly to the water reservoir and water equivalent to the pressure is transferred from the water reservoir to the diaphragm reservoir, wherein • the guide rod to which a diaphragm (17.1) in the diaphragm reservoir is attached is lowered equivalently to the pressure in excess of the limit, • the guide rod contacts a clapper flange (4.4) in the pressure inlet body and lowers and removes it from a clapper contact site (5.3) for some distance, and pressurised fluid is discharged into the external environment in the pressure discharge body, • wherein the safety valve further comprises: • a probe (18.4) adapted to sense high heat that may occur in the pressurised vessel to stop the operation of the pressurised vessel, • a stop switch adapted to stop the operation of the pressurised vessel when the pressure in pressurised vessels exceeds the set limit, the safety valve being characterised by comprising an additional pressure unit, wherein when the guide rod contacts the clapper flange (4.4) in the pressure inlet body, it lowers and removes it from the clapper contact site (5.3) for some distance, and a small amount of pressurised material is discharged into the external environment through an inlet hole of a unit pressure setting rod (5.1.5) in the additional pressure unit in the pressure discharge body, thus providing a proportional lift that discharges the maximum flow rate envisaged at an initial stage, wherein the pressurised fluid filling into the additional pressure unit withdraws a reservoir bottom cover (5.1.3) from a reservoir lower cover sitting site (5.1.7) to compress a reservoir spring (5.1.2) in additional pressure unit and as a result of the increase of excess pressure over the limit accumulating in the additional pressure unit, the clapper flange (4.4) gets further away from the clapper contact site (5.3), and thus, the pressurised fluid entering from the increasing gap is discharged into the external environment through the additional pressure discharge channels (5.1.4) in the additional pressure unit, to provide a full lift that discharges larger amount of pressure into the external environment. • pressurised water and air from the pressurised vessel to be safeguarded are taken from the pressure input body where they enter through a metal pressure transfer pipe and sent directly to the diaphragm reservoir, or • pressurised steam, fluid or hot oil coming from the pressurised vessel are taken from the pressure input body where they enter through a metal pressure transfer pipe and sent directly to the water reservoir and water equivalent to the pressure is transferred from the water reservoir to the diaphragm reservoir, • the guide rod to which a diaphragm (17.1) in the diaphragm reservoir is attached is lowered equivalently to the pressure in excess of the limit, • the guide rod contacts a clapper flange (4.4) in the pressure inlet body and lowers and removes it from a clapper contact site (5.3) for some distance, and pressurised fluid is discharged into the external environment in the pressure discharge body, • wherein the safety valve further comprises: • a probe (18.4) adapted to sense high heat that may occur in the pressurised vessel to stop the operation of the pressurised vessel, • a stop switch adapted to stop the operation of the pressurised vessel when the pressure in pressurised vessels exceeds the set limit,