Direct acting relief valves have
Features- quick response, low flow, leak free.
Pilot operated - high flow, internal or external leakage. Flatter PQ curve
Unloading valve - e.g. relief valve that switches to minimum pressure for start up.
Setting:
Generally set 15 bar above the working pressure of system.
Back pressure:
Changes in pilot drain or backpressure will generally be added to the set pressure; making a sequence valve.