Figure 6.35 illustrates the operation of a pressure-compensated valve. The design incorporates a hydrostat which maintains a constant 1.4kg/cm2 (20 psi) pressure differential across the throttle which is an orifice, whose area can be adjusted by an external knob setting. The orifice area setting determines the flow rate to be controlled. The hydrostat is normally held open by a light spring. However, it starts to close as inlet pressure increases and overcomes the spring tension. This closes the opening through the hydrostat, thereby blocking all the flow in excess of the throttle setting. As a result, the only amount of fluid that can flow through the valve is that amount which a 1.4 kg/cm2 (20 psi) pressure can force through the throttle.
To understand better the concept of pressure compensation in flow control valves, let us try and distinguish between flow control in a fixed displacement pump and that in a pressure-compensated pump. Figure 6.36 is an example of flow control in a hydraulic circuit with fixed volume pumps.
In this system, a portion of the fluid is bypassed over the relief valve in order to reduce flow to the actuator. Pressure increases upstream as the flow control valve, which in this case is a needle valve, is closed. As the relief pressure is approached, the relief valve begins to open, bypassing a portion of the fluid to the tank.
Flow control in a pressure-compensated pump as illustrated in Figure 6.37, is different in that the fluid is not passed over the reUef valve. As the compensator setting pressure is approached, the pump begins the de-stroking operation, thereby reducing the outward flow.
The design of a pressure-compensated flow control valve is such that it makes allowances for variations in pressure, before or after the orifice. In a pressure-compensated flow control valve, the actuator speed does not vary with variation in load.