Manual Valves. Devices that regulate the flow of gases or liquids by opening, closing, or partially obstructing passageways. Valves are used in a variety of applications including industrial, military, commercial, residential, and transportation. The valves most obvious in everyday life are plumbing valves such as: water taps, gas control valves on appliances, and safety devices fitted to hot water systems. Most manual valves are operated by a hand wheel or a lever, although some are self operating, controlled by internal mechanisms driven by changes in pressure, temperature, or flow.