The gate valve, shown in Fig. 1, consists of a gate-like disc, actuated by a screwed stem and handwheel, which moves up and down at right angles to the path of flow. In the closed position, the disc seats against two faces to shut off the flow. Gate valves are not suitable for throttling service because excessive wear, due to wire drawing (erosion), occurs on the gate and gate seats. They are suitable as stop (or isolating) valves, where conditions require either full flow or no flow. They have the advantage that, when fully opened, the resistance to flow is low with a minimum of pressure drop, as the fluid flow moves in a straight line.