Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 63

The following chapters of the book focus on the analysis of hydraulic systems operating in steady‐state conditions. Hence, after the presentation of the basic equations for hydraulic resistance and conservation of mass, it is now appropriate to provide the reader with the general approach that can be used to model a flow network.

A flow network can be defined as any collection of elements (valves, cylinders) and sources (pumps). The network interconnections are the fluid conveyance elements.

According to the approach also presented by Merritt [32], the flow and the pressure distribution within a network must satisfy three constraints:

1 Flow–pressure relationshipEvery element of the circuit is characterized by a flow–pressure relationship. The simplest example is the case of the hydraulic resistance that can be used to describe pipes, fittings, and certain hydraulic valves, previously shown in Eqs. (3.38) and (3.39). The next chapters will present also relations for other elements, such as pumps, motors, and linear actuators.

2 Flow lawThe flow law applies at any junction of pipes, and it was already presented as a direct consequence of the conservation of mass principle:(3.41) Equation (3.41) can be seen as the equivalent of the Kirchhoff's current law in the electric domain. Essentially, this equation states that the sum of the flow rates entering a junction has to be equal to the sum of the flow rates exiting it (Figure 3.16a).

3 Pressure lawThe pressure law can be seen as the equivalent of the Kirchhoff's voltage law in the electric domain. It states that the overall pressure drop around any closed circuit has to be null:(3.42)

Figure 3.16 Graphical representation of flow law (a) and pressure law (b).