Pilot-Operated Relief Valves:

Pilot-operated relief valves are designed to manage pressure by the use of a small passage to the top of a piston which is linked to the stem such in which system pressure closes the major relief valve. While the small pilot valve opens then pressure is relieved from the piston and system pressure under the disk opens the major relief valve. Like pilot valves are classical solenoid operated, along with the energizing signal originating from pressure measuring systems.