Direct acting pressure relief valve, model DPR are guided poppet design, seat type valves. They are used for limiting a system pressure. The valve basically consist of sleeve, seat, poppet, spring and adjusting set screw or rotary hand knob.
The spring pushes the poppet onto the seat. P is connected to the system .The pressure existing in the system acts on the poppet surface area. If the pressure in chamber P exceeds the value set at the spring, the poppet opens against the spring. 
Now hydraulic fluid flows from P to T