A valve is a mechanical device which, by opening and closing, enables the flow of fluid in a pipe or other vessel to be controlled. Common valve types are generally named after the shape or mode of operation of the movable element, e.g., cone, or needle, valve; gate valve; globe valve; poppet valve; and rotary plug cock. Self-acting valves include: safety valves, usually spring-loaded and designed to open at a predetermined pressure; nonreturn valves, which permit flow in one direction only; and float-operated valves, set to shut off a feeder pipe before a container overflows.