These graphs compare the pressure/flow characteristics of orifice and capillary restrictors. The characteristic impedance of an orifice restrictor does not vary linearly, so it will not always match the system impedance. Therefore some reflections are possible under certain operating conditions. Capillary restrictors, however, have linear characteristic impedances, so they can be sized to match a systemâ€™s impedance for any combination of flow and pressure.
Pressure transients in hydraulic systems are usually caused by valve operations that then create pump ripple and water-hammer effects. All of these can lead to pipeline failures, reduced operating efficiencies, noise, and vibrations. Solving this problem often involves expensive active elements such as relief valves, rupture discs, and controls valves, or else passive-but-complicated devices such as surge tanks and accumulators. A simpler, passive method of controlling transients can be accomplished using line restrictors. They are often effective for damping pressure transients in systems with long fluid lines.
Sizing these restrictors takes a certain amount of care and experience. But the task can be simplified by using a method of matching impedances—a technique common in electrical, acoustic, and mechanical systems, but not widely applied to hydraulics.