As hydraulic fluids are in reality not incompressible, but are compressible, in addition to the energy elements above there is the compression energy. To generate the pressure p in a space V o a specific portion of the volume ΔV K must be added:
(in a pump this process takes place continuously or at very short intervals in the individual displacement chambers). If this ΔV is allowed to flow away (e.g. on switching over a directional valve), then the pressure in the still full chamber will be zero.
The compression energy released during this process can be significant. Example: ΔV k = 1.0 L; p = 300 bar; compression energy: 15 · 103 Nm. If this 1 L flows away in 0.5 s, a power of approx. 30 kW is released.
The basis is Bernoulli's equation for ideal, incompressible fluids:
= static pressure + dynamic pressure + geodetic pressure.
This equation yields the related energy element as
The last term ρ · g · h can generally be ignored in hydraulics, as it is mostly only a few tenths of a bar. The kinetic energy also only makes itself apparent where the mass forces of the flowing column of fluid have an effect.