A gate valve has a flat closure element that slides into the flow stream to stop the fluid. They are one of the most commonly used valves, and are primarily used to permit or prevent the flow of liquids. The distinct feature of a gate valve is that the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is desired. Gate valves are primarily designed for on-off services. They are best used in systems which require infrequent use of the valve. They are designed for full-area flow to minimize the pressure drop and allow the passage of a pipe-cleaning pig. Since most of the flow change occurs near the shutoff, the relatively high fluid velocity causes disk and seat wear and eventual leakage if the valve is used to regulate flow.