Mechanism: a gate lifted clear of the bore, not throttled across it
A gate valve differs from quarter-turn designs "in that the closure element, called the disc, gate, or obturator, rises on the base of a stem or spindle out of the waterway and into the valve top, called the bonnet, by means of multiple turns of the spindle or stem." Because the gate lifts clear rather than rotating into the flow, "an open gate valve offers about the same amount of resistance to flow as a section of pipe of the same port size" — which is also why "gate valves are generally bad choices for regulating flow or operating in any orientation other than fully open or fully closed." The catalogue's solid- and flexible-wedge families close on the classic wedge principle: "the slightly angled disc mating with the same angled valve body seats" affects "a tight closure," seated "by applying torque to push the disc firmly into the seats." Stem arrangement is the other mechanical choice — "the motion of a gate valve's disc upon the stem dictates whether the stem rises during opening or threads into the disc," defining rising-stem and non-rising-stem (NRS) styles, with the outside-screw-and-yoke (OS&Y) rising-stem version "preferred for corrosive environments because the threads are outside the fluid containment area."
