What are the main differences and working principles between Pressure relief and pressure reducing valves?

1.)Pressure relief valves are used for safey when high pressure happen.

2.)Pressure Reducing valves are used to regulate or develop pressure for the system

pressure relief is normally closed valve open only if the pressure goes more than a certain adjust pressure in the hydraulic circuit ,and pressure reducing valve is normally open valve installed in the flow way to reduce the pressure at an area so close to the pump

Pressure relief valves are used in hydraulic system to limit the maximum system pressure to a specific set pressure, if this pressure reached the pressure relief valve as activated and feeds the excess flow back to the tank.

by the other hand the pressure reducing valve dose not affect the system pressure (Pump pressure)  but it controls the valve  outlet pressure to a value lower than the system pressure.

relief valves are use to bring back the excess pressure at the required pressure of the system to the tank , while pressure reducing valves are use to develop the the pressure of the unit not exceeding to the required pressure of the relief valve.

Pressure relief valves guards or protects the hydraulic system from excessive pressure by dumping excess pressure back to the tank and a pressure reducing valve reduces the system pressure after the valve therefore it differentiate the pressures before and after the pressure reducing valve

A Pressure relief valve acts as a Safety Device. It Releases/ Vents all the Fluid in Process Line when the Fluid Pressure exceeds the Set Pressure to avoid any Hazard due to over pressurization. 

A Pressure reducing Valve on the other hand is a Pressure regulating device. A Pressure reducing valve functions to maintain the fluid Pressure at the downstream of valve equal to its Set pressure. 

any hydraulic system should contains a relief valve that used as (safety) for the system ..when the pressure goes more than relief valve pressure it opens the way and let the oil back to the tank (normally)

but , pressure reducing valve used as (controller) for the system. Not in all hydraulic system should be installed depends on applications

Pressure reducing and pressure regulating valves are essentially the same though pressure regulating valves are considered to be more accurate in maintaining the set pressure. Also, the term pressure reducing valve is more commonly associated with liquids while pressure regulating valve or more correctly regulators are use only for gases. Both these types of valves control the flow of the liquid or gas through pipes and tubes. Pressure relieve valves are valves that will vent a device at a preset pressure. They are typically safety devices. The fluid is vented to an overflow area that may simply be the open environment. These types of valves are controlled by springs or are designed as rupture plates that fail at a predetermined pressure. Pressure release valves are manual valves. They can be combined with pressure relief valves. They require manual intervention and will release excess pressurized fluid in a safe controlled manner. 

pressure relief valve is used for safety of any hydraulic system which ensure system pressure is under control,if pressure is increased then required pressure pressure relief valve relief that pressure and control all system pressure.

and pressure reducing valve is used as system pressure controller which ensure that system pressure in under control as we set pressure in reducing valve. 

if this both used in system then we can make system fully controllable and we can ensure  hydraulic system pressure and equipment health always good.

The pressure relief valve act as a safety valve for the whole system to reduce the over pressurization which could damage the entire system and will allow only set pressure to be maintained in the system whiles

the pressure reducing valve acts as a safety device for a particular devices whose pressure is always lower than the system pressure and ensures that the lower set pressure is maintained even when the main system pressure goes up.