What they do:
Directional valves start, stop and control the direction of fluid flow.

How they work:
By moving an internal flow control source in a particular way, a directional valve allows fluid to flow through a passage. The control source could be a poppet (either piston or ball) a rotary or sliding spool.

The control source can be moved manually, mechanically, pneumatically, hydraulically, electrically or certain combinations of each. The flow direction can be one-way (as in a check valve, using a ball to stop reverse flow), two way, three way, or four way. They are sized according to the size of the port or flange connections, rated gallons per minute or with reference to a standard mounting pattern.

Why they are used:
To cause the reverse motion of a cylinder or hydraulic motor (4 way valve). To pressurize and exhaust an actuator port (3 way valve or 4 way with plugged actuator port). As an ON-OFF function used as safety interlock and to isolate and connect various system parts (2 Way). To prevent reverse flow (one way valve).

Usual Sizes and Ratings:
3-10 Gallons Per Minute, 10-20 GPM, 40 GPM, 80 GPM, and 160+ GPM.