Non-pressure-compensated flow-control valves are used when the system pressure is relatively constant and motoring speeds are not too critical. The operating principle behind these valves is that the flow through an orifice remains constant if the pressure drop across it remains the same. In other words, the rate of flow through an orifice depends on the pressure drop across it.
The disadvantage of these valves is discussed below. The inlet pressure is the pressure from the pump that remains constant. Therefore, the variation in pressure occurs at the outlet that is defined by the work load. This implies that the flow rate depends on the work load. Hence, the speed of the piston cannot be defined accurately using non-pressure-compensated flow-control valves when the working load varies. This is an extremely important problem to be addressed in hydraulic circuits where the load and pressure vary constantly.
Schematic diagram of non-pressure-compensated needle-type flow-control valve is shown in Fig. 1.3. It is the simplest type of flow-control valve. It consists of a screw (and needle) inside a tube-like structure. It has an adjustable orifice that can be used to reduce the flow in a circuit. The size of the orifice is adjusted by turning the adjustment screw that raises or lowers the needle. For a given opening position, a needle valve behaves as an orifice. Usually, charts are available that allow quick determination of the controlled flow rate for given valve settings and pressure drops.
Sometimes needle valves come with an integrated check valve for controlling the flow in one direction only. The check valve permits easy flow in the opposite direction without any restrictions. As shown in Fig. 1.4, only the flow from A to B is controlled using the needle. In the other direction (B to A), the check valve permits unrestricted fluid flow.
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