In hydraulic systems, it is often necessary to use varying operating pressures. A system which works perfectly with a high rate of efficiency can be achieved only if it is possible to adjust the pressure precisely to the load in question.
You often see systems in which most of the pump station oil flow passes through a pressure relief valve which is set to an unnecessarily high pressure This is because for a short time there is a need for high pressure. because, time, pressure at a certain time in the cycle of the machine in question. The rate of efficiency of such systems is very low, and the integrated components are subject to a high level of wear because the pressure drop creates a high surface pressure between the moving parts.
In order to counter this problem, a system is often constructed from several pumps which are connected in such a way that the volume flow delivered depends on the operating pressure in question. In a large system, the pump may have a variable displacement, and its regulator is then able to operate at several different operating pressure.