A needle valve is a mechanical device that starts, stops, or regulates the circulation of liquids or gases using a tapered valve stem as the closure element. A valve is a device installed in a machine, system, or instrument to open, close, or regulate fluid flow through piping. Valves and instrumentation include equipment used in industrial facilities or machinery to regulate, permit, or prevent fluid flow, as well as devices used to measure and control different variables in these processes.