In pilot operated setup, main relief is combined with and controlled by a smaller self actuated pilot valve. This relief valve uses the process fluid itself, circulated through a pilot valve, to apply the closing force on the safety valve disc. The pilot valve is itself a small safety valve with a spring. The main valve does not have a spring but is controlled by the process fluid from pilot valve. This arrangement allows operation of pilot operated valves with a very narrow margin between set pressure of the relief valve and operating pressure of the protected equipment. Hence these relief valves are particularly used for services where relief valve inlet line pressure drop is high (typically higher than 3% of set point) or when back pressure is high. Allowable back pressure is typically more than 50% of the set pressure