If the operating conditions during direct actuation become too unfavourable (e.g. due to excessive pressure, excessive flow rate), indirect actuation will be chosen, which generally uses a smaller directly controlled actuation element to control the larger device.
Since actuation in such cases is always performed in opposition to a spring (which then determines the size from a design perspective), indirect actuation enables designers to produce smaller devices (Figure I 5 a). Another advantage is the more favourable p/Q behaviour associated with pressure valves, with pressure increasing more gradually as the flow increases (b) (pilot actuation). In addition, the main device and the operating element (generally a pilot valve) can be separated from each other, making remote controls an option.
Figure I 5: Effects of indirect actuation on the size of devices (a) and the p/Q characteristic curve (b)