| A gate valve is a linear valve where a gate (or wedge) is pushed down across an orifice to cut off the flow. These are used in applications where the flowing media is erosive since the configuration of elements puts nothing in the flow path that would be eroded.
Of course, since a flat piece of steel placed against the end of a pipe is not going to provide you a tight seal, circular valve seats are placed on either side of the gate to prevent leakage. The seats can be metal, an elastomer such as EPDM or even just a Buna O ring. The key design configuration is preventing the gate from damaging the seats during the up and down motion. This damage would be galling in the case of metal seats or scratches or tearing in the case of soft seats. To prevent this, the use of a triagular or trapezoidal gate has been used.
Gate valves come in sizes ranging from 1" NPS up to the large wedge gates used for flood control on dams. They come in a spectrum of materials ranging from brass to carbon steel to stainless steel to nobler materials such as Monel or Hastelloy.
The valves can be manually operated or automated by use of a cylinder that is either pneumaticly or hydraulicly powered. Electric actuators can also be used. | 
