Valves are throttling and shut-off fittings in pipeline systems, e.g. in buffer systems. They are categorized: by type of shut-off mechanism as seat valves, slide valves, ball valves, needle valves, etc.; by type of actuation as manual shut-off valves, solenoid valves, pneumatic and hydraulic valves, etc.; by function as non-return valves, check valves, pressure reducing valves, overflow valves, etc.