Pressure takes the role of the hydraulic level. Pressure differences are level differences which we consider to be the causes for hydraulic processes. Alternatively, we may look at processes leading to pressure differences.

Pressure differences in closed hydraulic circuits. The pressure changes from point to point in a closed hydraulic circuit. To make use of this observation, choose a few important points in the system (usually at the inlets and outlets of elements such as pipes, pumps, and tanks). Label pressure differences from point to point (Figure 1) by introducing arrows and symbols delta_p_AB, etc.

We can draw a diagram looking like a “landscape” (Figure 1). When we are back at the origin, the pressure is the same. Therefore, the sum of all pressure differences in a closed circuit must be equal to zero (loop rule, Kirchhoff’s Second Law):

Pressure differences and processes. Pressure differences are associated with many different types of processes and systems.

Processes in an element. Consider elements which are between two chosen points in the above Figure (example: pump and pipe from point A to point C, Figure 2). Identify all possible processes occurring in this element. Each of the processes shares a part of the pressure difference delta_p_AC: