Pressure-compensated flow-control valvesovercome the difficulty causedby non-pressure-compensated valves by changing the size of the orifice in relation to the changes in the system pressure. This is accomplished through a spring-loaded compensator spool that reduces the size of the orifice when pressure drop increases. Once the valve is set, the pressure compensator acts to keep the pressure drop nearly constant. It works on a kind of feedback mechanism from the outlet pressure. This keeps the flow through the orifice nearly constant.
Schematic diagram of a pressure compensated flow-control valve is shown in Fig. 1.5 and its graphical symbol in Fig. 1.6. A pressure-compensated flow-control valve consists of a main spool and a compensator spool. The adjustment knob controls the main spool’s position, which controls the orifice size at the outlet. The upstream pressure is delivered to the valve by the pilot line A. Similarly, the downstream pressure is ported to the right side of the compensator spool through the pilot line B. The compensator spring biases the spool so that it tends toward the fully open position. If the pressure drop across the valve increases, that is, the upstream pressure increases relative to the downstream pressure, the compensator spool moves to the right against the force of the spring. This reduces the flow that in turn reduces the pressure drop and tries to attain an equilibrium position as far as the flow is concerned.
In the static condition, the hydraulic forces hold the compensator spool in balance, but the bias spring forces it to the far right, thus holding the compensator orifice fully open. In the flow condition, any pressure drop less than the bias spring force does not affect the fully open compensator orifice,but any pressure drop greater than the bias spring force reduces the compensator orifice. Any change in pressure on either side of the control orifice, without a corresponding pressure change on the opposite side of the control orifice, moves the compensator spool. Thus, a fixed differential across the control orifice is maintained at all times. It blocks all flow in excess of the throttle setting. As a result, flow exceeding the preset amount can be used by other parts of the circuit or return to the tank via a pressure-relief valve.
Performance of flow-control valve is also affected by temperature changes which changes the viscosity of the fluid. Therefore, often flow-control valves have temperature compensation. Graphical symbol for pressure and temperature compensated flow-control valve is shown in Fig. 1.7.
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