How Hydrogen pressure increase to 3MPa without compressor in this control?

For this conrol diagram can someone explain me here the setpoint for hydrogen is 3Mpa but the H2 is coming at low pressure and now setpoint is 3 Mpa so in this control how the hydrogen pressure increase to 3 Mpa

https://in.mathworks.com/help/simscape/ug/pem-electrolysis-system.html

Answer 1

[Yifeng Tang]

As hydrogen is produced at the Cathode from the electrolysis process, the pressure in the upstream volume will increase. This control is acting as a pressure relief to keep the pressure on the left of the valve below 3MPa. You may view the evolution of the pressure and hydrogen production mass flow in the Simscape Result Explorer.