Hydraulic power Unit

Understand the function of the hydraulic power unit

The following video shows an animated basic hydraulic system with its circuit. Ater watching the video you should be able to answer the folling questions:
A1: What is the pumps only function?
A2: When pressure rises up within an hydraulic system?
A3: What is the function of a pressure relief valve within an hydraulic power unit? How does it work?

From the content:

1:58 Hydraulic Power Pack working
2:42 Function of the Pressure relief valve
4:07 Hydraulic Circuit with a cylinder as actor
4:53 Some words about Maintenance

In the following the detailed circuit symbol of a hydraulic power unit with its components:


In the following, the following components are described in detail:
- Hydraulic pumps
- Pressure relief valve
- Filter
- Hydraulic oil
- Pracise: Record the flow characteristic curve