A pressure relief valve is most often used to protect a system from over-pressurisation, but it can also be used to maintain constant upstream pressure or even as a non-return valve in certain installations. It is an in-line valve, and it only opens when the system pressure exceeds the set pressure of the valve.

Pressure regulating valves are also installed in-line. Their job is to maintain the downstream line pressure to the pressure set at the valve, and work by responding to changes in the downstream pressure.