Pressure compensated valves use an adjustable restrictor together with an addi­tional valve that opens and closes in order to maintain a constant pressure drop across the restrictor.

Figure 1 shows this type of valve whereby as flow passes from the A to the B ports the pressure drop, (P2 - P3) across the manually adjustable rotary restrictor creates a force on the spool against that of the spring. If the force from the pressure drop exceeds the value set by the spring the spool valve closes and further restricts the outlet flow to the B port.

Figure 1. Pressure Compensated Valve  

With the inlet being supplied from a controlled pressure source, P,, the supply flow will be reduced in order to maintain this pressure at a constant value. Either a pressure relief valve or a pressure compen­sated pump could be used to perform this action. 

Figure 2. Pressure Compensated Flow Control Valve  

is kept constant with changes in the outlet pressure which provides a flow that, unlike the simple restrictor, is independent of the outlet, or load, pressure. This valve is a closed loop control in that the pressure drop is fed back onto the spool to control its position. However, the flow itself is controlled by the pressure supply control system. A typical pressure compensated valve is shown in Figure 2 the variable restriction being set by the control screw on the top of the valve.

The control method employed in the pressure compensated valve is used extensively in many other types of valve for a variety of hydraulic circuit functions.